High-performance p-type multicrystalline silicon (mc-Si): Its characterization and projected performance in PERC solar cells
Altermatt, P. P., Z. Xiong, Q. X. He, W. W. Deng, F. Ye, Y. Yang, Y. F. Chen, Z. Q. Feng, P. J. Verlinden, A. Y. Liu, D. H. Macdonald, T. Luka, D. Lausch, M. Turek, C. Hagendorf, H. Wagner-Mohnsen, J. Schon, W. Kwapil, F. Frühauf, O. Breitenstein, E. E. Looney, T. Buonassisi, D. B. Needleman, C. M. Jackson, A. R. Arehart, S. A. Ringel, K. R. McIntosh, M. D. Abbott, B. Sudbury, A. Zuschlag, C. Winter, D. Skorka, G. Hahn, D. Chung, B. Mitchell, P. Geelan-Small, T. Trupke
Solar Energy 175 (SI), pp 68-74 (2018)
abstractRecent progress in the electronic quality of high-performance (HP) multicrystalline silicon material is reported with measurements and modeling performed at various institutions and research groups. It is shown that recent progress has been made in the fabrication at Trina Solar mainly by improving the high excess carrier lifetimes tau due to a considerable reduction of mid-gap states. However, the high lifetimes in the wafers are still reduced by interstitial iron by a factor of about 10 at maximum power point (mpp) conditions compared to mono-crystalline Cz wafers of equivalent resistivity. The low lifetime areas of the wafers seem to be limited by precipitates, most likely Cu. Through simulations, it appears that dislocations reduce cell efficiency by about 0.25% absolute. The best predictors for PERC cell efficiency from ingot metrology are a combination of mean lifetime and dislocation density because dislocations cannot be improved considerably by gettering during cell processing, while lifetime-limiting impurities are gettered well. In future, the material may limit cell efficiency above about 22.5% if the concentrations of Fe and Cu remain above 1010 and 1013cm−3, respectively, and if dislocations are not reduced further.
Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates
Banerjee-Ghosh, Koyel, Oren Ben Dor, Francesco Tassinari, Eyal Capua, Shira Yochelis, Amir Capua, See-Hun Yang, S. S. P. Parkin, Soumyajit Sarkar, Leeor Kronik, Lech Tomas Baczewski, Ron Naaman, Yossi Paltiel
Science 360 (6395), pp 1331-1334 (2018)
abstractIt is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.
Interface-assisted sign inversion of magnetoresistance in spin valves based on novel lanthanide quinoline molecules
Bedoya-Pinto, Amilcar, Sara G. Miralles, Saul Velez, Ainhoa Atxabal, Pierluigi Gargiani, Manuel Valvidares, Felix Casanova, Eugenio Coronado, Luis E. Hueso
Advanced Functional Materials 28 (16), pp 1702099/1-9 (2018)
abstractMolecules are proposed to be an efficient medium to host spin-polarized carriers, due to their weak spin relaxation mechanisms. While relatively long spin lifetimes are measured in molecular devices, the most promising route toward device functionalization is to use the chemical versatility of molecules to achieve a deterministic control and manipulation of the electron spin. Here, by combining magnetotransport experiments with element-specific X-ray absorption spectroscopy, this study shows the ability of molecules to modify spin-dependent properties at the interface level via metal-molecule hybridization pathways. In particular, it is described how the formation of hybrid states determines the spin polarization at the relevant spin valve interfaces, allowing the control of macroscopic device parameters such as the sign and magnitude of the magnetoresistance. These results consolidate the application of the spinterface concept in a fully functional device platform.
Exchange coupling torque in ferrimagnetic Co/Gd bilayer maximized near angular momentum compensation temperature
Bläsing, R., T. P. Ma, S. H. Yang, C. Garg, F. K. Dejene, A. T. N'Diaye, G. Chen, K. Liu, S. S. P. Parkin
Nature Communications 9, pp 4984/1-8 (2018)
abstractHighly efficient current-induced motion of chiral domain walls was recently demonstrated in synthetic antiferromagnetic (SAF) structures due to an exchange coupling torque (ECT). The ECT derives from the antiferromagnetic exchange coupling through a ruthenium spacer layer between the two perpendicularly magnetized layers that comprise the SAF. Here we report that the same ECT mechanism applies to ferrimagnetic bi-layers formed from adjacent Co and Gd layers. In particular, we show that the ECT is maximized at the temperature TA where the Co and Gd angular momenta balance each other, rather than at their magnetization compensation temperature TM. The current induced velocity of the domain walls is highly sensitive to longitudinal magnetic fields but we show that this not the case near TA. Our studies provide new insight into the ECT mechanism for ferrimagnetic systems. The high efficiency of the ECT makes it important for advanced domain wall based spintronic devices.
The role of inhomogeneities for understanding current-voltage characteristics of solar cells
Breitenstein, O.
IEEE Journal of Photovoltaics 8 (6), pp 1429-1435 (2018)
abstractAll solar cells show more or less inhomogeneous electronic properties. This holds in particular for multicrystalline silicon cells, where local differences of the lifetime of more than an order of magnitude exist. This contribution explains how these inhomogeneities can be imaged and quantified, and the physical origins and the efficiency degradation potential of J01-, J02 -, and ohmic inhomogeneities are reviewed. It is found that J02 and ohmic currents are always highly localized, in contrast with J01 currents. Hence, for describing most of the area of silicon solar cells, a one-diode model is sufficient, but J02 and ohmic currents reduce the efficiency at low illumination intensity. Moreover, the physical origins of known prebreakdown phenomena are reviewed and a new breakdown type dominating in monocrystalline silicon cells is proposed.
Lock-in thermography with depth resolution on silicon solar cells
Breitenstein, O., H. Straube, K. Iwig
Solar Energy Materials and Solar Cells 185, pp 66-74 (2018)
abstractLock-in thermography (LIT) is the standard method for imaging and evaluating leakage currents in solar cells. For usually applied lock-in frequencies in the order of 10 Hz, silicon solar cells are considered to be thermally thin. Hence, depth-dependent investigations, as they are performed in non-destructive testing and failure analysis of ICs, were not performed until now by LIT. In this contribution two special LIT investigation and evaluation methods are introduced, which have the potential to judge whether some recombination occurs at the top, in the middle, or at the bottom of a Si solar cell. Such investigations can be useful to evaluate e.g. metal induced recombination or the influence of crystal defects in multicrystalline solar material on the emitter or backside recombination. The methods are tested at a cell containing a diamond scratch in the emitter and backside recombination at the Ag back contact.
Lock-in thermography for analyzing solar cells and failure analysis in other electronic components
Breitenstein, O., S. Sturm
Proceedings 14th Quantitative InfraRed Thermography Conference, Quantitative Infrared Thermography, pp 235-242 (2018)
abstractLock-in thermography (LIT) is a dynamic variant of infrared thermography, where local heat sources are periodically pulsed and amplitude and phase images of the surface temperature modulation are obtained. If used in electronic device testing, this method enables the localization of very weak local heat sources below the surface. This contribution reviews the basics and application of LIT for local efficiency analysis of solar cells and for failure analysis in other electronic components like bare and encapsulated integrated circuits. In both application fields LIT has established as a reliable and easy-to-use standard method for failure analysis.
Vanadium oxide bandstop tunable filter for Ka frequency bands based on a novel reconfigurable spiral shape defected ground plane CPW
Casu, E. A., A. A. Müller, M. Fernández-Bolanos, A. Fumarola, A. Krammer, A. Schuler, A. M. Ionescu
IEEE Access (Special Section: Tunable devices for modern communications: materials, integration, modeling, and applications) 6, pp 12206-12212 (2018)
abstractThis paper proposes and validates a new principle in coplanar waveguide (CPW) bandstop filter tuning by shortcutting defected ground plane (DGS) inductor shaped spirals to modify the resonant frequency. The tunable filter is fabricated on a high-resistivity silicon substrate based on a CMOS compatible technology using a 1 μm x 10 μm long and 300 nm thick vanadium oxide (VO2) switch by exploiting its insulator to metal transition. The filter is designed to work in Ka band with tunable central frequencies ranging from 28.2 GHz to 35 GHz. The measured results show a tuning range of more than 19 %, a low insertion loss in the neighboring frequency bands (below 2 dB at 20 GHz and 40 GHz in on/off-states) while a maximum rejection level close to 18 dB in off-state, limited by the no RF-ideal CMOS compatible substrate. The filter has a footprint of only 0.084 · λ0 x 0.037 · λ0 (where λ0 represents the free space wavelength at the highest resonance frequency) thus making it the most compact configuration using CPW DGS structures for the Ka frequency band. In addition, a more compact filter concept based on the Peano space filling curve is introduced to increase the tuning range while minimizing the DGS area.
A reconfigurable inductor based on vanadium dioxide insulator-to-metal transition
Casu, Emanuele A, Andrei A. Muller, Matteo Cavalieri, A. Fumarola, Adrian Mih Ionescu, M. Fernández-Bolanos
IEEE Microwave and Wireless Components Letters 28 (9), pp 795-797 (2018)
abstractThis letter introduces a reconfigurable planar square-coil-shaped inductor exploiting as the tuning mechanism the insulator-to-metal transition (IMT) of a vanadium dioxide (VO2) switch placed in the interwinding space in an unprecedented manner. The VO2 thin-film bar-shaped switch is electrically connected to provide a temperature-selective current path that effectively short-circuits a part of the inductor coil changing the inductance of the device. The inductor is fabricated on a high-resistivity silicon substrate using a CMOS-compatible 2-D planar low-cost technology (four photolithography steps). The design, optimized to work in the 4-10-GHz range, provides measured inductances at 5 GHz of 2.1 nH at 20 °C and 1.35 nH at 100 °C with good stability in the entire frequency band (4-10 GHz) resulting in a reconfiguration ratio of 55%. The quality factor (Q-factor) at 7 GHz is about 8 at 20 °C (off state) and 3 at 100 °C (on state), outperforming tunable inductors employing VO2 with 2 orders of magnitude higher Q-factor and a smaller footprint. This represents an advancement for the state of the art of 2-D CMOS-compatible inductors in the considered frequency range.
Tunable RF phase shifters based on vanadium dioxide metal insulator transition
Casu, Emanuele A, Nicolo Oliva, Matteo Cavalieri, Andrei A Mueller, A. Fumarola, Wolfgang A Vitale, Anna Krammer, Andreas Schueler, M. Fernández-Bolanos, Adrian M. Ionescu
IEEE Journal of the Electron Devices Society 6 (1), pp 965-971 (2018)
abstractThis paper presents the design, fabrication, and electrical characterization of a reconfigurable RF capacitive shunt switch that exploits the electro-thermally triggered vanadium dioxide (VO2) insulator to metal phase transition. The RF switch is further exploited to build wide-band RF true-time delay tunable phase shifters. By triggering the VO2 switch insulator to metal transition (IMT), the total capacitance can be reconfigured from the series of two metal-insulator-metal (MIM) capacitors to a single MIM capacitor. The effect of bias voltage on losses and phase shift is investigated, explained, and compared to the state of the art in the field. We report thermal actuation of the devices by heating the devices above VO2 IMT temperature. By cascading multiple stages a maximum of 40 ° per dB loss close to 7 GHz were obtained.
Asymmetric Josephson effect in inversion symmetry breaking topological materials
Chen, Chui-Zhen, James Jun He, M. N. Ali, Gil-Ho Lee, Kin Chung Fong, K. T. Law
Physical Review B 98 (7), pp 075430/1-5 (2018)
abstractTopological materials which possess topologically protected surface states have attracted much attention in recent years. In this work, we study the critical current of superconductor/inversion symmetry breaking topological material/superconductor junctions. We found surprisingly that, in topological materials with broken inversion symmetry, the magnitude of the critical Josephson currents | Ic+(B) | at fixed magnetic field B is not the same for critical currents | Ic(B) | flowing in the opposite direction. Moreover, the critical currents violate the | Ic±(B) | = | Ic±(−B) | relation and give rise to asymmetric Fraunhofer patterns. We call this phenomenon asymmetric Josephson effect (AJE). AJE can be used to detect inversion symmetry breaking in topological materials such as in quantum spin Hall systems and Weyl semimetals.
Synthesis and morphology of semifluorinated polymeric ionic liquids
Chen, S. B., A. Funtan, F. Gao, B. Cui, A. Meister, S. S. P. Parkin, W. H. Binder
Macromolecules 51 (21), pp 8620-8628 (2018)
abstractPolymeric ionic liquids (POILs) are important materials in the field of ionic liquid gating, requiring the precise synthesis of new POILs with tailored structural variability and defined nanoscaled structure. In the current contribution, using reversible addition-fragmentation chain-transfer polymerization (RAFT) technique, the homopolymerization of three imidazolium-based acrylates with different counterions is reported, namely 1-[2-acryloylethyl]-3-methylimidazolium bis(trifluoromethane) sulfonamide (APMIN(Tf)2), 1-[2-acryloylethyl]-3-methylimidazolium hexafluorophosphate (APMIPF6), and 1-[2-acryloylethyl]-3-methylimidazolium tetrafluoroborate (APMIBF4), to afford the respective poly(ionic liquids (POILs). All polymerizations display pseudo-first-order kinetics and a rapid growth of P(APMIN(Tf)2), P(APMIPF6), and P(APMIBF4), yielding homopolymers with controlled molar mass, revealing a strong influence of the counterions on the polymerization rate, increasing in the order of BF4θ < PF6θ < N(Tf)2θ. As a direct determination of molecular weights via size exclusion chromatography (SEC) of the POIL homopolymers from RI and UV detectors was not successful, we developed an alternative strategy to generate accurate, "normal" SEC peaks of POILs using RAFT copolymerization technique: APMIN(Tf)2) was copolymerized with a semifluorinated monomer 2,2,2-trifluoroethyl acrylate (TFEA), allowing to study the influence of the comonomer feeding ratio on the resulting SEC signals of P(APMIN(TO2-co-TFEA) copolymers. We found that when the feeding molar ratio of TFEA is adjusted to 0.77, symmetric SEC peaks from the resulting P(APMIN(Tf)2-co-TFEA) copolymers are obtained. Furthermore, copolymerizations of TFEA with the other two IL monomers, APMIPF6 and APMIBF4, are also performed to afford P(APMIPF6-co-TFEA) and P(APMIBF4-co-TFEA) copolymers. Moreover, the propensity of the so obtained POIL random copolymers P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA) to grow a new block (polypentafluorostyrene, PPFS) is explored, intending to generate the fluorinated POIL triblock copolymers P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA), P(APMIPF6-co-TFEA)-b-PPFS-b-P(APMIPF6-co-TFEA), and P(APMIBF4-co-TFEA)-b-PPFS-b-P(APMIBF4-co-TFEA), respectively. The morphology and size of such semifluorinated POILs are investigated using transition electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS), revealing the aggregated nanoparticles from P(APMIN(Tf)2-co-TFEA) due to the mesoscale organization of the ionic "multiplets". Significantly larger and crowded globular objects/aggregates are formed from the chain-extended POIL triblock P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA) copolymers under the same conditions.
Gating effects of conductive polymeric ionic liquids
Chen, Senbin, Falk Frenzel, B. Cui, F. Gao, Antonella Campanella, Alexander Funtan, Friedrich Kremer, S. S. P. Parkin, Wolfgang H Binder
Journal of Materials Chemistry 6 (30), pp 8242-8250 (2018)
abstractPoly(ionic liquid)s (POILs) belong to one of the most promising materials class in electrochemistry. In this study, we investigate POILs as a gating material within a field-effect transistor, additionally describing their glassy dynamics and charge transport properties. Four different imidazolium-based POILs have been investigated, ranging from homopolymers with varied counterions, i.e. POIL 1: P(APMIN(Tf)2) poly(1-[2-acryloylpropyl]-3-methylimidazolium bis(trifluoromethane)sulfonamide) and POIL 2: P(APMIPF6)poly(1-[2-acryloylpropyl]-3-methylimidazolium hexafluorophosphate, to semifluorinated random copolymers, i.e. POIL 3: P(APMIN(Tf)2-co-TFEA) (TFEA: 2,2,2-trifluoroethyl acrylate), and finally to semifluorinated triblock copolymers, POIL 4: P(APMIN(Tf)2-co-TFEA)-b-PPFS-b-P(APMIN(Tf)2-co-TFEA) (PPFS: polypentafluorostyrene). Their glassy dynamics and charge transport mechanism are investigated by broadband dielectric spectroscopy (BDS), differential scanning calorimetry (DSC) and alternating current chip-calorimetry (ACC). The gating effects of these POILs are studied in detail, showing for the first time a reversible phase transition between thin films formed from the brownmillertite phase SrCoO2.5 and the perovskite phase SrCoO3 by use of such POILs, being especially pronounced for POIL 1: P(APMIN(Tf)2) homopolymer displaying gate voltages (VG) of 3-4 V and a gating time of  ∼  4 h. In the case of the POIL 3, P(APMIN(Tf)2-co-TFEA) as a random copolymer, higher VG (-8/+5 V) and a longer gating time ( ∼  16 h) are revealed. Phase transition between SrCoO2.5 and SrCoO3 could not be observed from POILs 2 & 4 even using very large gate voltages (-10/+8 V) for a much longer time (48 h), indicating that primarily charge density and charge-carrier mobility are decisive in ionic liquid gating.
Combining nanostructuration with boron doping to alter sub band gap acceptor states in diamond materials
Choudhury, Sneha, Benjamin Kiendl, Jian Ren, F. Gao, Peter Knittel, Christoph Nebel, Amelie Venerosy, Hugues Girard, Jean-Charl Arnault, Anke Krueger, Karin Larsson, Tristan Petit
Journal of Materials Chemistry A 6 (34), pp 16645-16654 (2018)
abstractDiamond is a promising metal-free photocatalyst for nitrogen and carbon dioxide reduction in aqueous environment owing to the possibility of emitting highly reducing solvated electrons. However, the wide band gap of diamond necessitates the use of deep UV to trigger a photochemical reaction. Boron doping introduces acceptor levels within the band gap of diamonds, which may facilitate visible-light absorption through defect-based transitions. In this work, unoccupied electronic states from different boron-doped diamond materials, including single crystal, polycrystalline film, diamond foam, and nanodiamonds were probed by soft X-ray absorption spectroscopy at the carbon K edge. Supported by density functional theory calculations, we demonstrate that boron close to the surfaces of diamond crystallites induce acceptor levels in the band gap, which are dependent on the diamond morphology. Combining boron-doping with morphology engineering, this work thus demonstrates that electron acceptor states within the diamond band gap can be controlled.
Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures
Cui, B., P. Werner, T. P. Ma, Xiaoyan Zhong, Zechao Wang, J. M. Taylor, Y. Zhuang, S. S. P. Parkin
Nature Communications 9, pp 3055/1-8 (2018)
abstractThe controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO2.5 and the oxygen ordered phase perovskite SrCoO3. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds similar to 30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.
Nickel precipitation in light and elevated temperature degraded multicrystalline silicon solar cells
Deniz, H., J. Bauer, O. Breitenstein
Solar RRL 2 (9), pp 1800170/1-4 (2018)
abstractMulticrystalline silicon passivated emitter and rear contact (PERC) solar cells often show light and elevated temperature-induced degradation (LeTID) followed by a regeneration process. In previous works, it has been found that this degradation/regeneration process can be described by a model including diffusion of still unknown recombination-active point defects to the cell surface. The diffusion coefficient of this unknown species is compatible with interstitial Ni diffusion in Si. In this work, scanning transmission electron microscopy (STEM) and highly sensitive energy dispersive X-ray analysis (EDX) results from altogether 14 positions taken from two cells fabricated from neighboring wafers are evaluated, one of the cells remaining virgin and the other one LeTID degraded. In two of the degraded samples thin Ni-containing platelets are found, which are most probably nickel silicide platelets. This result is an indication that Ni can be involved in the LeTID degradation and regeneration process.
Linking symmetry, crystallography, topology, and fundamental physics in crystalline solids
Derunova, E., M. N. Ali
In: Symmetry (group theory) and mathematical treatment in chemistry, pp 67-85 IntechOPen, 2018
abstractIn this chapter, we briefly introduce the evolution of symmetry as a mathematical concept applied to physical systems and lay the mathematical groundwork for discussion of topological physics. We explain how topological phases, like the Berry phase, can be obtained from a gauge symmetry of a quantum system. Also, we introduce numerical tools (e.g., Chern numbers, Wilson loops) for topological analysis of chemical solids based on the crystal structure and corresponding electronic structure.
Adaptive modulation in the Ni2Mn1.4In0.6 magnetic shape-memory Heusler alloy
Devi, P., Sanjay Singh, B. Dutta, K. Manna, S. W. D'Souza, Y. Ikeda, E. Suard, V Petricek, P. Simon, P. Werner, S. Chadhov, S. S. P. Parkin, C. Felser, D. Pandey
Physical Review B 97 (22), pp 224102/1-6 (2018)
abstractThe origin of incommensurate structural modulation in Ni-Mn based Heusler-type magnetic shape-memory alloys (MSMAs) is still an unresolved issue in spite of intense focus on it due to its role in the magnetic field induced ultrahigh strains. In the archetypal MSMA Ni2MnGa, the observation of "nonuniform displacement" of atoms from their mean positions in the modulated martensite phase, premartensite phase, and charge density wave as well as the presence of phason broadening of satellite peaks has been taken in support of the electronic instability model linked with a soft acoustic phonon. We present here results of a combined high-resolution synchrotron x-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using a (3+1)D superspace group approach, which reveals not only uniform atomic displacements in the modulated structure of the martensite phase with physically acceptable ordered magnetic moments in the antiferromagnetic phase at low temperatures, but also the absence of any premartensite phase and phason broadening of the satellite peaks. Our HRTEM studies and first-principles calculations of the ground state also support uniform atomic displacements predicted by powder diffraction studies. All these observations suggest that the structural modulation in the martensite phase of Ni2Mn1.4In0.6MSMA can be explained in terms of the adaptive phase model. The present study underlines the importance of superspace group analysis using complementary SXRPD and NPD in understanding the physics of the origin of modulation as well as the magnetic and the modulated ground states of the Heusler-type MSMAs. Our work also highlights the fact that the mechanism responsible for the origin of modulated structure in different Ni-Mn based MSMAs may not be universal and it must be investigated thoroughly in different alloy compositions.
Synthetic antiferromagnetic spintronics
Duine, R. A., K. J. Lee, S. S. P. Parkin, M. D. Stiles
Nature Physics 14 (3), pp 217-219 (2018)
abstractSpintronic and nanomagnetic devices often derive their functionality from layers of different materials and the interfaces between them. We discuss the opportunities that arise from synthetic antiferromagnets consisting of two or more ferromagnetic layers that are separated by metallic spacers or tunnel barriers and have antiparallel magnetizations.
Chiral domain wall motion in unit-cell thick perpendicularly magnetized Heusler films prepared by chemical templating
Filippou, P. C., J. Jeong, Y. Ferrante, S.-H. Yang, T. Topuria, M. G. Samant, S. S. P. Parkin
Nature Communications 9, pp 4653/1-10 (2018)
abstractHeusler alloys are a large family of compounds with complex and tunable magnetic properties, intimately connected to the atomic scale ordering of their constituent elements. We show that using a chemical templating technique of atomically ordered X'Z' (X' = Co; Z' = Al, Ga, Ge, Sn) underlayers, we can achieve near bulk-like magnetic properties in tetragonally distorted Heusler films, even at room temperature. Excellent perpendicular magnetic anisotropy is found in ferrimagnetic X3Z (X = Mn; Z = Ge, Sn, Sb) films, just 1 or 2 unit-cells thick. Racetracks formed from these films sustain current-induced domain wall motion with velocities of more than 120 ms−1, at current densities up to six times lower than conventional ferromagnetic materials. We find evidence for a significant bulk chiral Dzyaloshinskii-Moriya exchange interaction, whose field strength can be systematically tuned by an order of magnitude. Our work is an important step towards practical applications of Heusler compounds for spintronic technologies.
Giant anomalous Hall effect in a ferromagnetic kagome-lattice semimetal
Filippou, P. C., J. Jeong, Y. Ferrante, S.-H. Yang, T. Topuria, M. G. Samant, S. S. P. Parkin
Nature Communications 14 (11), pp 1125-1131 (2018)
abstractMagnetic Weyl semimetals with broken time-reversal symmetry are expected to generate strong intrinsic anomalous Hall effects, due to their large Berry curvature. Here, we report a magnetic Weyl semimetal candidate, Co3Sn2S2, with a quasi-two-dimensional crystal structure consisting of stacked kagome lattices. This lattice provides an excellent platform for hosting exotic topological quantum states. We observe a negative magnetoresistance that is consistent with the chiral anomaly expected from the presence of Weyl fermions close to the Fermi level. The anomalous Hall conductivity is robust against both increased temperature and charge conductivity, which corroborates the intrinsic Berrycurvature mechanism in momentum space. Owing to the low carrier density in this material and the considerably enhanced Berry curvature from its band structure, the anomalous Hall conductivity and the anomalous Hall angle simultaneously reach 1,130 Ω−1 cm−1 and 20%, respectively, an order of magnitude larger than typical magnetic systems. Combining the kagome-lattice structure and the long-range out-of-plane ferromagnetic order of Co3Sn2S2, we expect that this material is an excellent candidate for observation of the quantum anomalous Hall state in the two-dimensional limit.
Injection intensity-dependent recombination at various grain boundary types in multicrystalline silicon solar cells
Frühauf, F., P. P. Altermatt, T. Luka, T. Mehl, H. Deniz, O. Breitenstein
Solar Energy Materials and Solar Cells 180, pp 130-137 (2018)
abstractIf the ratio of two open circuit photoluminescence (Voc-PL) images taken at two different light intensities is displayed, some grain boundaries (GBs) may show up as bright lines. This indicates that these special GBs show distinct injection intensity-dependent recombination properties. It will be shown here that this results in an apparent ideality factor of the light emission smaller than unity. The effect is reproduced with numerical device simulations using a usual distribution of defects in the band gap along grain boundaries. Quantitative imaging of this apparent luminescence ideality factor by PL imaging is complicated by the lateral horizontal balancing currents flowing at open circuit. The local voltage response of an inhomogeneous solar cell at different injection levels under open circuit is modelled by Griddler simulations, based on PL investigations of this cell. The evaluation of Voc-PL images at different illumination intensities allows us to conclude that the apparent luminescence ideality factor at the special GBs is about 0.89, whereas in the other regions it is between 0.94 and 0.95. Reverse bias electroluminescence showed no pre-breakdown sites, and hyperspectral PL imaging showed only in one of the investigated GBs particular defect luminescence. TEM investigations of two GBs, one showing distinct injection intensity-dependent recombination and the other one showing none, revealed that the investigated special GB is a large-angle GB whereas the GB not showing this effect is a small-angle GB.
Improved Laplacian photoluminescence image evaluation regarding the local diode back voltage distribution
Frühauf, F., O. Breitenstein
Solar Energy Materials and Solar Cells 174, pp 277-282 (2018)
abstractLaplacian photoluminescence-based local diode voltage evaluation was shown recently to lead to correct mean values of the local saturation current density in mm-sized regions, but local maxima in the positions of recombination-active grain boundaries appear overestimated. It is shown here by 2-D device simulations that this effect is at least partly due to the influence of the local diode back voltage, which is caused by the voltage drop at the bulk and back contact resistances. Visually the image of this back voltage appears like a blurred copy of the local diode current density. It is shown in this work that indeed the diode back voltage can be simulated in good approximation by blurring the diode current image, which comes out of the Laplacian evaluation, multiplied with an effective vertical bulk resistance. The corresponding point spread function can be obtained e.g. by device simulation. An iterative procedure is proposed leading to self-consistent results for the diode current density and the diode back voltage. If this method is applied to simulated local cell data, the assumed distribution of the saturation current density is retrieved accurately. Applying this method to measured photoluminescence images leads to a better correspondence to non-linear Fuyuki PL evaluation results than the previously performed direct evaluation of the local diode voltage data. Remaining differences will be discussed.
Bidirectional non-filamentary RRAM as an analog neuromorphic synapse, part II: Impact of Al/Mo/Pr0.7Ca0.3MnO3 device characteristics on neural network training accuracy
Fumarola, A., S. Sidler, K. Moon, J. Jang, R. M. Shelby, P. Narayanan, Y. Leblebici, H. Hwang, G. W. Burr
IEEE Journal of the Electron Devices Society 6 (1), pp 169-178 (2018)
abstractNeuromorphic computing embraces the "device history" offered by many analog non-volatile memory (NVM) devices to implement the small weight changes computed by a gradient-descent learning algorithm such as backpropagation. Deterministic and stochastic imperfections in the conductance response of real NVM devices can be encapsulated for modeling within a pair of "jump-tables." Such jump-tables describe the full cumulative distribution function of conductance-change at each device conductance value, for both weight potentiation (SET) and depression (RESET). First, using several types of artificially constructed jump-tables, we revisit the relative importance of deviations from an ideal NVM with perfectly linear conductance response. Then, using jump-tables measured on improved non-filamentary resistive RAM devices based on Pr0.7Ca0.3MnO3 [see companion paper], we simulate the effects of their nonlinear conductance response on the training of a three-layer fully connected neural network. We find that, despite the relatively large conductance changes exhibited by any Pr0.7Ca0.3MnO3device when either potentiating from its lowest conductance state or depressing from its highest conductance states, neural network training accuracies of > 90% can be achieved. Highest accuracies are achieved by programming both conductances on each timestep ("fully bidirectional"), with the improved conductance on/off ratio of Al/Mo/PCMO resulting in marked improvements in training and test accuracy. Further accuracy improvements can be obtained by tuning the relative learning rate for potentiation (SET) by a factor of 1.66x with respect to depression (RESET), to offset the slight asymmetry between the average size of the associated SET and RESET conductance changes. Finally, we show that the bidirectional programming of Al/Mo/PCMO can be used to implement high-density neuromorphic systems with a single conductance per synapse, at only a slight degradation to accuracy.
Highly asymmetric chiral domain-wall velocities in Y-shaped junctions
Garg, C., Aakash Pushp, See-Hun Yang, Timothy Phung, Brian P. Hughes, Charles Rettner, S. S. P. Parkin
Nano Letters 18 (3), pp 1826-1830 (2018)
abstractRecent developments in spin orbit torques allow for highly efficient current-driven domain wall (DW) motion in nanowires with perpendicular magnetic anisotropy. Here, we show that chiral DWs can be driven into nonequilibrium states that can persist over tens of nanoseconds in Y-shaped magnetic nanowire junctions that have an input and two symmetric outputs. A single DW that is injected into the input splits and travels at very different velocities in the two output branches until it reaches its steady-state velocity. We find that this is due to the disparity between the fast temporal evolution of the spin current derived spin orbit torque and a much-slower temporal evolution of the DMI-derived torque. Changing the DW polarity inverts the velocity asymmetry in the two output branches, a property that we use to demonstrate the sorting of domains.
Reversible phase transformation phenomenon in titanium dioxide films: Evidence beyond interface-nucleation and dissolution-precipitation kinetics
Gautam, Subodh K., Jitendra Singh, D. K. Shukla, E. Pippel, P. Poddar, Fouran Singh
Acta Materialia 146, pp 253-264 (2018)
abstractThe re-crystallization kinetics and rutile to anatase reversible phase transformation (PT) in nano crystalline titanium dioxide (TiO2) are reported. Initially, an amorphous TiO2 film is used for the present study and in situ isothermal annealing dependent nucleation and growth kinetics of anatase and rutile phase is studied at low temperature ( ∼  523 K) and well explained using Johnson Mehl AvramiKolmogorov (JMAK) model. The anatase nanocrystallite (NCs) transformation into rutile phase is reported with isothermal annealing for longer time and temperature dependent annealing in lower temperature range 523 K-673 K and explained using interface-nucleation mechanism. Furthermore, the thermodynamic stability of rutile NCs and lattice stress-induced reversible PT in nano-sized rutile TiO2 are confirmed in moderate temperature range (623 K- 973 K) and well explained using x-ray diffraction, micro-Raman spectroscopy and near edge x-ray absorption fine structure spectroscopy studies. However, annealing at higher temperature (1123 K- 1323 K) induces the growth of anatase NCs and their natural transform into rutile phase are explained by well-known dissolution precipitation mechanism. Activation energy of rutile PT is quantified and found higher for dissolution-precipitation mechanism than that for interface nucleation at earlier stage. Thus, overall PT kinetics at different temperature range is well understood by invoking in three step mechanism: 1) early stage anatase-to-rutile transformation is dominated by interface-nucleation, II) then intermediate stage reversible rutile-to-anatase PT and, Ill) at later stages, anatase-to-rutile PT is controlled by dissolution precipitation mechanism.
The role of ionic liquid breakdown in the electrochemical metallization of VO2: An NMR study of gating mechanisms and VO2 reduction
Hope, M. A., K. J. Griffith, B. Cui, F. Gao, S. E. Dutton, S. S. P. Parkin, C. P. Grey
Journal of the American Chemical Society 140 (48), pp 16685-16696 (2018)
abstractMetallization of initially insulating VO2 via ionic liquid electrolytes, otherwise known as electrolyte gating, has recently been a topic of much interest for possible applications such as Mott transistors and memory devices. It is clear that the metallization takes place electrochemically, and, in particular, there has previously been extensive evidence for the removal of small amounts of oxygen during ionic liquid gating. Hydrogen intercalation has also been proposed, but the source of the hydrogen has remained unclear. In this work, solid-state magic angle spinning NMR spectroscopy (H-1, H-2, O-17, and V- 51) is used to investigate the thermal metal insulator transition in VO2, before progressing to catalytically hydrogenated VO2 and electrochemically metallized VO2. In these experiments electrochemical metallization of bulk VO2 particles is shown to be associated with intercalation of hydrogen, the degree of which can be measured with quantitative H-1 NMR spectroscopy. Possible sources of the hydrogen are explored, and by using a selectively deuterated ionic liquid, it is revealed that the hydrogenation is due to deprotonation of the ionic liquid; specifically, for the commonly used dialkylimidazolium-based ionic liquids, it is the "carbene" proton that is responsible. Increasing the temperature of the electrochemistry is shown to increase the degree of hydrogenation, forming first a less hydrogenated metallic orthorhombic phase then a more hydrogenated insulating Curie- Weiss paramagnetic orthorhombic phase, both of which were also observed for catalytically hydrogenated VO2. The NMR results are supported by magnetic susceptibility measurements, which corroborate the degree of Pauli and Curie-Weiss paramagnetism. Finally, NMR spectroscopy is used to identify the presence of hydrogen in an electrolyte gated thin film of VO2, suggesting that electrolyte breakdown, proton intercalation, and reactions with decomposition products within the electrolyte should not be ignored when interpreting the electronic and structural changes observed in electrochemical gating experiments.
Sweeping plane dependence of the percolation-induced colossal anisotropic magnetoresistance in spatially confined manganite films
Jeon, J., J. Jung, K. H. Chow
Physical Review B 98 (14), pp 144441/1-6 (2018)
abstractThe angular dependence of the colossal magnetoresistance as the magnetic field is swept in different planes is investigated in a tensile strained La-Pr-Ca-Mn-O microbridge. For the in-plane sweep, the rotation of the magnetic field drives percolation/depercolation of electron conduction channels and in the spatially confined system produces abrupt resistance jumps. As the magnetic field is swept out of the plane of the film towards the direction normal to the film's plane, the relative volume fraction of the ferromagnetic phase is reduced. This leads to depercolation of the conduction channels. This process is reversible when the magnetic field is swept back into the plane of the film.
Modification of Dzyaloshinskii-Moriya-interaction-stabilized domain wall chirality by driving currents
Karnad, G. V., F. Freimuth, E. Martinez, R. Lo Conte, G. Gubbiotti, T. Schulz, Stephan Senz, B. Ocker, Y. Mokrousov, M. Klaeui
Physical Review Letters 121 (14), pp 147203/1-6 (2018)
abstractWe measure and analyze the chirality of Dzyaloshinskii-Moriya-interaction (DMI) stabilized spin textures in multilayers of Ta | Co20F60B20 | MgO. The effective DMI is measured experimentally using domain wall motion measurements, both in the presence (using spin-orbit torques) and absence of driving currents (using magnetic fields). We observe that the current-induced domain wall motion yields a change in effective DMI magnitude and opposite domain wall chirality when compared to field-induced domain wall motion (without current). We explore this effect, which we refer to as current-induced DMI, by providing possible explanations for its emergence, and explore the possibility of its manifestation in the framework of recent theoretical predictions of DMI modifications due to spin currents.
Understanding phase-change materials with unexpectedly low resistance drift for phase-change memories
Li, C., C. Hu, J. Wang, X. Yu, Z. Yang, J. Liu, Y. Li, C. Bi, X. Zhou, W. Zheng
Journal of Materials Chemistry C 6, pp 3387-3394 (2018)
abstractThere is an increasing demand for high-density memories with high stability for supercomputers in this big data era. Traditional dynamic random access memory cannot satisfy this requirement due to its limitation of volatile and power-consumable data storage. Multi-level cell phase-change memory (MLC PCM) based on phase-change materials possesses a higher storage density, and is considered to be the most promising candidate. However, a detrimental resistance drift exists commonly in phase-change materials, and it destroys the stability and greatly limits the development of MLC PCM. Here, we propose a completely new strategy to suppress resistance drift by exploring its microscopic mechanism via combinations of theoretical calculations and experiments. We have found, for the first time, that resistance drift originates from the change in electron binding energy induced by structural relaxation and is proportional to the reciprocal of the dielectric coefficient according to the hydrogen-like model. On this basis, we propose to reduce the resistance drift by increasing the thermal stability of the dielectric coefficient. Two series of experiments prove the effectiveness of our new strategy. The resistance drift exponent of phase-change films is significantly reduced to 0.023 using our strategy, which is lower by half than the best result (0.050) reported previously. Interestingly, the films also show improved storage properties. These results not only unravel the fact that the stability and storage function of phase-change films can be simultaneously improved by modification of dielectric properties but also pave the way for future material design for stable MLC PCM.
Carbon-tailored semimetal MoP as an efficient hydrogen evolution electrocatalyst in both alkaline and acid media
Li, Guowei, Yan Sun, Jiancun Rao, Jiquan Wu, Anil Kumar, Qiu Nan Xu, Chenguang Fu, Enke Liu, Graeme R. Blake, P. Werner, Baiqi Shao, Kai Liu, S. S. P. Parkin, Xianjie Liu, Mats Fahlman, Sz-Chian Liou, Gudrun Auffermann, Jian Zhang, Claudia Felser, Xinliang Feng
Advanced Energy Materials 8 (24), pp 1801258/1-7 (2018)
abstractThe electrolysis processes such as hydrogen evolution reaction (HER) require high efficient catalysts with robust surface stability. A high conductivity is also necessary to speed up the charge transport between the catalyst and the electrolyte. Recently, the observation of exceedingly high conductivity in the topological semimetal MoP, has provided a model catalyst to investigate the correlation between the electrical transport and the electrocatalytic activity for the HER. Thus, MoP is encapsulated in a Mo, P codoped carbon layer (MoP@C). This composite material exhibits outstanding HER performance, with an extremely low overpotential of 49 mV at a current density of 10 mA cm−2 and a Tafel slope of 54 mV dec−1 in an alkaline medium. In addition, electron transport analysis indicates that MoP exhibits high conductivity and mobility due to the existence of triple-point fermions and a complex Fermi surface. Furthermore, the presence of P-C and Mo-C bonds at the interface between the carbon layer and the MoP particles modulates the band structure of MoP@C and facilitates fast electron transfer, accumulation, and subsequent delocalization, which are in turn responsible for the excellent HER activity.
Orbital origin of extremely anisotropic superconducting gap in nematic phase of FeSe superconductor
Liu, Defa, Cong Li, Jianwei Huang, Bin Lei, Le Wang, Xianxin Wu, Bing Shen, Qiang Gao, Yuxiao Zhang, Xu Liu, Yong Hu, Yu Xu, Aiji Liang, Jing Liu, Ping Ai, Lin Zhao, Shaolong He, Li Yu, Guodong Liu, Yiyuan Mao, Xiaoli Dong, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Youguo Shi, Jiangping Hu, Tao Xiang, Xianhui Chen, Zuyan Xu, Chuangtian Chen, X. J. Zhou
Physical Review X 8 (3), pp 031033/1-10 (2018)
abstractThe iron-based superconductors are characterized by multiple-orbital physics where all the five Fe 3d orbitals get involved. The multiple-orbital nature gives rise to various novel phenomena like orbitalselective Mott transition, nematicity, and orbital fluctuation that provide a new route for realizing superconductivity. The complexity of multiple-orbital physics also requires us to disentangle the relationship between orbital, spin, and nematicity, and to identify dominant orbital ingredients that dictate superconductivity. The bulk FeSe superconductor provides an ideal platform to address these issues because of its simple crystal structure and unique coexistence of superconductivity and nematicity. However, the orbital nature of the low-energy electronic excitations and its relation to the superconducting gap remain controversial. Here, we report direct observation of the highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor by high-resolution laser-based angle-resolved photoemission measurements. We find that the low-energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the single dxz orbital. The superconducting gap exhibits an anticorrelation relation with the dxz spectral weight near the Fermi level; i.e., the gap size minimum (maximum) corresponds to the maximum (minimum) of the dxz spectral weight along the Fermi surface. These observations provide new insights in understanding the orbital origin of the extremely anisotropic superconducting gap in the FeSe superconductor and the relation between nematicity and superconductivity in the iron-based superconductors.
Effect of hydrogen implantation on the mechanical properties of AlN throughout ion-induced splitting
Mamun, M. A., K. Tapily, O. Moutanabbir, H. Baumgart, A. A. Elmustafa
ECS Journal of Solid State Science and Technology 7 (4), pp P180-P184 (2018)
abstractThe ability to transfer bulk quality III-N thin layers onto foreign platforms is a powerful strategy to enable high-efficiency and low-cost optoelectronic devices. Ion-cut using sub-surface defect engineering has been an effective process to split and transfer a variety of semiconductors. With this perspective, hydrogen-implanted AlN samples were annealed in air at temperatures ranging from 300 °C to 600 °C for 5 min to study the influence of pre-layer splitting treatments on the nanomechanical properties. There is a clear dependence of the hardness on implanted hydrogen implantation fluence. We observe that the as-implanted hardness increased from 18 GPa for the virgin reference sample to  ∼ 25 GPa for the highest fluence of 3 x 1017 H cm−2 prior to annealing. In the case of reference single crystalline Si samples, a significant drop in the hardness and elastic modulus is observed in the H implantation-induced damage zone subsequent to thermal annealing, while for crystalline epitaxial AlN samples with 0.5 x 1017 and 2.0 x 1017 H implant fluences, the hardness increases and peaks until the thermal annealing temperature reaches 350 °C and subsequently begins to drop thereafter for higher annealing temperatures. However, for the 1.0 x 1017 H implantation fluence the hardness continues to increase with increasing thermal annealing temperature.
Noncollinear antiferromagnetic Mn3Sn films
Markou, A., J. M. Taylor, A. Kalache, P. Werner, S. S. P. Parkin, C. Felser
Physical Review Materials 2, pp 051001(R)/1-6 (2018)
abstractNoncollinear hexagonal antiferromagnets with almost zero net magnetization were recently shown to demonstrate giant anomalous Hall effect. Here, we present the structural and magnetic properties of noncollinear antiferromagnetic Mn3Sn thin films heteroepitaxially grown on Y:ZrO2 (111) substrates with a Ru underlayer. The Mn3Sn films were crystallized in the hexagonal D019 structure with c-axis preferred (0001) crystal orientation. The Mn3Sn films are discontinuous, forming large islands of approximately 400 nm in width, but are chemical homogeneous and characterized by near perfect heteroepitaxy. Furthermore, the thin films show weak ferromagnetism with an in-plane uncompensated magnetization of M = 34 kA/m and coercivity of μ0Hc = 4.0 mT at room temperature. Additionally, the exchange bias effect was studied in Mn3Sn/Py bilayers. Exchange bias fields up to μ0HEB = 12.6 mT can be achieved at 5 K. These results show Mn3Sn films to be an attractive material for applications in antiferromagnetic spintronics.
Sublimable chloroquinolinate lanthanoid single-ion magnets deposited on ferromagnetic electrodes
Miralles, Sara G., Amilcar Bedoya-Pinto, Jose J Baldovi, Walter Canon-Mancisidor, Yoann Prado, Helena Prima-Garcia, Alejandro Gaita-Arino, Guillermo Minguez Espallargas, Luis E. Hueso, Eugenio Coronado
Chemical Science 9 (1), pp 199-208 (2018)
abstractA new family of chloroquinolinate lanthanoid complexes of the formula A+[Ln(5,7Cl2q)4], with Ln = Y3+, Tb3+ and Dy3+ and A+ = Na+, NEt4+ and K0.5(NEt4)0.5+, is studied, both in bulk and as thin films. Several members of the family are found to present single-molecule magnetic behavior in bulk. Interestingly, the sodium salts can be sublimed under high vacuum conditions retaining their molecular structures and magnetic properties. These thermally stable compounds have been deposited on different substrates (Al2O3, Au and NiFe). The magnetic properties of these molecular films show the appearance of cusps in the zero-field cooled curves when they are deposited on permalloy (NiFe). This indicates a magnetic blocking caused by the interaction between the single-ion magnet and the ferromagnet. X-ray absorption spectroscopy confirms the formation of hybrid states at the molecule/metal interface.
Bidirectional non-filamentary RRAM as an analog neuromorphic synapse, part I: Al/Mo/Pr0.7Ca0.3MnO3 material improvements and device measurements
Moon, K., A. Fumarola, S. Sidler, J. Jang, P. Narayanan, R. M. Shelby, G. W. Burr, H. Hwang
IEEE Journal of the Electron Devices Society 6 (1), pp 146-155 (2018)
abstractWe report on material improvements to non-filamentary RRAM devices based on Pr0.7Ca0.3MnO3 by introducing an MoOx buffer layer together with a reactive Al electrode, and on device measurements designed to help gauge the performance of these devices as bidirectional analog synapses for on-chip acceleration of the backpropagation algorithm. Previous Al/PCMO devices exhibited degraded LRS retention due to the low activation energy for oxidation of the Al electrode, and Mo/PCMO devices showed low conductance contrast. To control the redox reaction at the metal/PCMO interface, we introduce a 4-nm interfacial layer of conducting MoOx as an oxygen buffer layer. Due to the controlled redox reaction within this Al/Mo/PCMO device, we observed improvements in both retention and conductance on/off ratio. We confirm bidirectional analog synapse characteristics and measure "jump-tables" suitable for large scale neural network simulations that attempt to capture complex and stochastic device behavior [see companion paper]. Finally, switching energy measurements are shown, illustrating a path for future device research toward smaller devices, shorter pulses and lower programming voltages.
Reduction of thermal conductivity in nanowires by combined engineering of crystal phase and isotope disorder
Mukherjee, S., Uri Givan, Stephan Senz, M. de la Mata, J. Arbiol, O. Moutanabbir
Nano Letters 18 (5), pp 3066-3075 (2018)
abstractNanowires are a versatile platform to investigate and harness phonon and thermal transport phenomena in nanoscale systems. With this perspective, we demonstrate herein the use of crystal phase and mass disorder as effective degrees of freedom to manipulate the behavior of phonons and control the flow of local heat in silicon nanowires. The investigated nanowires consist of isotopically pure and isotopically mixed nanowires bearing either a pure diamond cubic or a cubic-rhombohedral polytypic crystal phase. The nanowires with tailor-made isotopic compositions were grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purities better than 99.9%. The analysis of polytypic nanowires revealed ordered and modulated inclusions of lamellar rhombohedral silicon phases toward the center in otherwise diamond-cubic lattice with negligible interphase biaxial strain. Raman nanothermometry was employed to investigate the rate at which the local temperature of single suspended nanowires evolves in response to locally generated heat. Our analysis shows that the lattice thermal conductivity in nanowires can be tuned over a broad range by combining the effects of isotope disorder and the nature and degree of polytypism on phonon scattering. We found that the thermal conductivity can be reduced by up to  ∼ 40% relative to that of isotopically pure nanowires, with the lowest value being recorded for the rhombohedral phase in isotopically mixed 28Six30Si1−x nanowires with composition close to the highest mass disorder (x  ∼  0.5). These results shed new light on the fundamentals of nanoscale thermal transport and lay the groundwork to design innovative phononic devices.
Reply to "Comment on 'Instability of the topological surface state in Bi2Se3 upon deposition of gold' "
Polyakov, A., C. Tusche, M. Ellguth, E. D. Crozier, K. Mohseni, M. M. Otrokov, X. Zubizarreta Iriarte, M. Garcia Vergniory, M. Geilhufe, E. V. Chulkov, A. Ernst, H. L. Meyerheim, S. S. P. Parkin
Physical Review B 98 (13), pp 136202/1-4 (2018)
abstractIn the Comment on our publication [Phys. Rev. B 95, 180202(R) (2017)], R. A. Gordon claims that our main conclusion is not valid, namely that gold atoms deposited in situ on the (0001) surface of single-crystalline Bi2Se3 reside in substitutional sites, i.e., replacing bismuth atoms within the topmost quintuple layer (QL). Based on x-ray absorption near-edge (XANES) spectra and a re-evaluation of extended x-ray absorption fine structure (EXAFS) data above the Au LIII edge, R. A. Gordon concludes that Au resides in a twofold environment as a result of an interface reaction leading to an Au2S-type local structure, in which gold adopts an Au(I) state and is linearly coordinated by selenium atoms. In this Reply, we will confirm the results published in the original paper and their interpretation that Au atoms reside in the substitutional site.
Pressure-induced superconductivity and topological quantum phase transitions in a quasi-one-dimensional topological insulator: Bi4I4
Qi, Yanpeng, Wujun Shi, P. Werner, Pavel G. Naumov, Walter Schnelle, Lei Wang, K. G. Rana, S. S. P. Parkin, Sergiy A. Medvedev, Binghai Yan, Claudia Felser
npj Quantum Materials 3 (4), pp 4/1-6 (2018)
abstractSuperconductivity and topological quantum states are two frontier fields of research in modern condensed matter physics. The realization of superconductivity in topological materials is highly desired; however, superconductivity in such materials is typically limited to two-dimensional or three-dimensional materials and is far from being thoroughly investigated. In this work, we boost the electronic properties of the quasi-one-dimensional topological insulator bismuth iodide βBi4I4 by applying high pressure. Superconductivity is observed in βBi4I4 for pressures, where the temperature dependence of the resistivity changes from a semiconducting- like behavior to that of a normal metal. The superconducting transition temperature Tc increases with applied pressure and reaches a maximum value of 6 K at 23 GPa, followed by a slow decrease. Our theoretical calculations suggest the presence of multiple pressure-induced topological quantum phase transitions as well as a structural-electronic instability.
Thermopower and unconventional Nernst effect in the predicted Type-II Weyl semimetal WTe2
Rana, K. G., F. K. Dejene, N. Kumar, C. R. Rajamathi, K. Sklarek, C. Felser, S. S. P. Parkin
Nano Letters 18 (10), pp 6591-6596 (2018)
abstractWTe2 is one of a series of recently discovered high mobility semimetals, some of whose properties are characteristic of topological Dirac or Weyl metals. One of its most interesting properties is the unsaturated giant magnetoresistance that it exhibits at low temperatures. An important question is the degree to which this property can be ascribed to a conventional semimetallic model in which a highly compensated, high mobility metal exhibits large magnetoresistance. Here, we show that the longitudinal thermopower (Seebeck effect) of semimetallic WTe2 exfoliated flakes exhibits periodic sign changes about zero with increasing magnetic field that indicates distinct electron and hole Landau levels and nearly fully compensated electron and hole carrier densities. However, inconsistent with a conventional semimetallic picture, we find a rapid enhancement of the Nernst effect at low temperatures that is nonlinear in magnetic field, which is consistent with Weyl points in proximity to the Fermi energy. Hence, we demonstrate the role played by the Weyl character of WTe2 in its transport properties.
Higher-order topological insulators
Schindler, Frank, Ashley M. Cook, Maia G. Vergniory, Zhijun Wang, S. S. P. Parkin, Andrei B. Bernevig, Titus Neupert
Science Advances 4 (6), pp eaat0346/1-8 (2018)
abstractThree-dimensional topological (crystalline) insulators are materials with an insulating bulk but conducting surface states that are topologically protected by time-reversal (or spatial) symmetries. We extend the notion of three-dimensional topological insulators to systems that host no gapless surface states but exhibit topologically protected gapless hinge states. Their topological character is protected by spatiotemporal symmetries of which we present two cases: (i) Chiral higher-order topological insulators protected by the combination of time-reversal and a fourfold rotation symmetry. Their hinge states are chiral modes, and the bulk topology is Z2-classified. (ii) Helical higher-order topological insulators protected by time-reversal and mirror symmetries. Their hinge states come in Kramers pairs, and the bulk topology is Z-classified. We provide the topological invariants for both cases. Furthermore, we show that SnTe as well as surface-modified Bi2Tel, BiSe, and BiTe are helical higher-order topological insulators and propose a realistic experimental setup to detect the hinge states.
Temperature dependence of luminescence from dislocated Ge on Si substrate
Schwartz, Bernhard, Manfred Reiche, Martin Kittler
Materials Today: Proceedings 5 (6), pp 14712-14721 (2018)
abstractFor mono-crystalline Ge the indirect luminescence intensity declines upon growing temperature from 80 to 300 K, whereas for dislocated Ge structures the opposite behavior occurs. These findings are comparable to earlier observations on Si. The drop of the luminescence in dislocated material upon lowering temperature was attributed to the increase of the competing non-radiative recombination due to shallow dislocation states. In opposition to the indirect luminescence, the character of the direct Ge luminescence, i.e. incline of intensity upon growing temperature, is not converted by dislocations. The measured behavior of the direct peak position of Ge, in the temperature range between 80 and 300 K, is in accordance with calculated dependence and reflects the direct bandgap energy. The observed red shifts in dislocated Ge structures are shown to be produced by tensile strain, bandgap narrowing and by the Sn-content of GeSn quantum wells, respectively. A direct influence of dislocations could not be observed. Satisfying understanding of the existing temperature behavior of the indirect Ge peak position is on embryonic stage.
Electronic properties of wurtzite GaAs: A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire
Senichev, A., P. Corfdir, O. Brandt, M. Ramsteiner, S. Breuer, J. Schilling, L. Geelhaar, P. Werner
Nano Research 11 (9), pp 4708-4721 (2018)
abstractIII-V compound semiconductor nanowires are generally characterized by the coexistence of zincblende and wurtzite structures. So far, this polytypism has impeded the determination of the electronic properties of the metastable wurtzite phase of GaAs, which thus remain highly controversial. In an effort to obtain new insights into this topic, we cross-correlate nanoscale spectral imaging by near-field scanning optical microscopy with a transmission electron microscopy analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer. Thus, spatially resolved photoluminescence spectra could be unambiguously assigned to nanowire segments whose structure is known with lattice-resolved accuracy. An emission energy of 1.528 eV was observed from extended zincblende segments, revealing that the dispersed nanowire was under uniaxial strain presumably due to interaction with its supporting substrate. These crucial information and the emission energy obtained for extended pure wurtzite segments were used to perform envelope function calculations of zincblende quantum disks in a wurtzite matrix as well as the inverse structure. In these calculations, we varied the fundamental bandgap, the electron mass, and the band offset between zincblende and wurtzite GaAs. From this multi-parameter comparison with the experimental data, we deduced that the bandgap between the Γ8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the Γ7-A bandgap.
Anomalous Hall effect in Weyl semimetal half-Heusler compounds RPtBi (R = Gd and Nd)
Shekhar, Chandra, Nitesh Kumar, V. Grinenko, Sanjay Singh, R. Sarkar, H. Luetkens, Shu-Chun Wu, Yang Zhang, Alexander Komarek, Erik Kampert, Yurii Skourski, Jochen Wosnitza, Walter Schnelle, Alix McCollam, Uli Zeitler, Juergen Kuebler, Binghai Yan, H. -H. Klauss, S. S. P. Parkin, C. Felser
Proceedings of the National Academy of Sciences of the United States of America 115 (37), pp 9140-9144 (2018)
abstractTopological materials ranging from topological insulators to Weyl and Dirac semimetals form one of the most exciting current fields in condensed-matter research. Many half-Heusler compounds, RPtBi (R = rare earth), have been theoretically predicted to be topological semimetals. Among various topological attributes envisaged in RPtBi, topological surface states, chiral anomaly, and planar Hall effect have been observed experimentally. Here, we report an unusual intrinsic anomalous Hall effect (AHE) in the antiferromagnetic Heusler Weyl semimetal compounds GdPtBi and NdPtBi that is observed over a wide temperature range. In particular, GdPtBi exhibits an anomalous Hall conductivity of up to 60 Ω−1·cm−1 and an anomalous Hall angle as large as 23%. Muon spin-resonance (μSR) studies of GdPtBi indicate a sharp antiferromagnetic transition (TN) at 9 K without any noticeable magnetic correlations above TN. Our studies indicate that Weyl points in these half-Heuslers are induced by a magnetic field via exchange splitting of the electronic bands at or near the Fermi energy, which is the source of the chiral anomaly and the AHE.
Growth and electrostatic/chemical properties of Metal/LaAlO3/SrTiO3 heterostructures
Vaz, Diogo Cast, E. Lesne, Anke Sander, Hiroshi Naganuma, Eric Jacquet, Jacobo Santamaria, Agnes Barthelemy, Manuel Bibes
Journal of Visualized Experiments (JoVE) 132, pp e56951 (2018)
abstractThe quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.
Directly photoexcited Dirac and Weyl fermions in ZrSiS and NbAs
Weber, C. P., L. M. Schoop, S. S. P. Parkin, R. C. Newby, A. Nateprov, B. Lotsch, B. M. K. Mariserla, J. M. Kim, K. M. Dani, H. A. Bechtel, E. Arushanov, M. N. Ali
Applied Physics Letters 113 (22), pp 221906/1-5 (2018)
abstractWe report ultrafast optical measurements of the Dirac line-node semimetal ZrSiS and the Weyl semimetal NbAs, using mid-infrared pump photons from 86 meV to 500 meV to directly excite Dirac and Weyl fermions within the linearly dispersing bands. In NbAs, the photoexcited Weyl fermions initially form a non-thermal distribution, signified by a brief spike in the differential reflectivity whose sign is controlled by the relative energy of the pump and probe photons. In ZrSiS, electron-electron scattering rapidly thermalizes the electrons, and the spike is not observed. Subsequently, hot carriers in both materials cool within a few picoseconds. This cooling, as seen in the two materials' differential reflectivity, differs in sign, shape, and timescale. Nonetheless, we find that it may be described in a simple model of thermal electrons, without free parameters. The electronic cooling in ZrSiS is particularly fast, which may make the material useful for optoelectronic applications.
Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2
Yang, H., Marcus Schmidt, Vicky Suess, Mun Chan, Fedor F. Balakirev, Ross D. McDonald, S. S. P. Parkin, Claudia Felser, Binghai Yan, P. J. Moll
New Journal of Physics 20, pp 043008/1-7 (2018)
abstractThree-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac-fermion excitations.
Symmetry demanded topological nodal-line materials
Yang, S.-Y., H. Yang, E. Derunova, S. S. P. Parkin, Binghai Yan, M. N. Ali
Advances in Physics: X 3 (1), pp 1414631/1-35 (2018)
abstractThe realization of Dirac and Weyl physics in solids has made topological materials one of the main focuses of condensed matter physics. Recently, the topic of topological nodal line semimetals, materials in which Dirac or Weyl-like crossings along special lines in momentum space create either a closed ring or line of degeneracies, rather than discrete points, has become a hot topic in topological quantum matter. Here, we review the experimentally confirmed and theoretically predicted topological nodal line semimetals, focusing in particular on the symmetry protection mechanisms of the nodal lines in various materials. Three different mechanisms: a combination of inversion and time-reversal symmetry, mirror reflection symmetry, and non-symmorphic symmetry and their robustness under the effect of spin orbit coupling are discussed. We also present a new Weyl nodal line material, the Te-square net compound KCu. Finally, we discuss potential experimental signatures for observing exotic properties of nodal line physics.
Bifunctional heterostructure assembly of NiFe LDH nanosheets on NiCoP nanowires for highly efficient and stable overall water splitting
Zhang, Haojie, Xiaopeng Li, Angelika Hähnel, Volker Naumann, Chao Lin, Sara Azimi, Stefan L. Schweizer, A. Wouter Maijenburg, Ralf B. Wehrspohn
Advanced Functional Materials 28 (14), pp 1706847/1-10 (2018)
abstract3D hierarchical heterostructure NiFe LDH@NiCoP/NF electrodes are prepared successfully on nickel foam with special interface engineering and synergistic effects. This research finds that the as-prepared NiFe LDH@NiCoP/NF electrodes have a more sophisticated inner structure and intensive interface than a simple physical mixture. The NiFe LDH@NiCoP/NF electrodes require an overpotential as low as 120 and 220 mV to deliver 10 mA cm−2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH, respectively. Tafel and electrochemical impedance spectroscopy further reveal a favorable kinetic during electrolysis. Specifically, the NiFe LDH@NiCoP/NF electrodes are simultaneously used as cathode and anode for overall water splitting, which requires a cell voltage of 1.57 V at 10 mA cm−2. Furthermore, the synergistic effect of the heterostructure improves the structural stability and promotes the generation of active phases during HER and OER, resulting in excellent stability over 100 h of continuous operation. Moreover, the strategy and interface engineering of the introduced heterostructure can also be used to prepare other bifunctional and cost-efficient electrocatalysts for various applications.
High performance Al3Sc alloy doped Al3Sc-Sb2Te chalcogenides for phase change memory application
Zhang, S., L. Wu, W. Song, X. Zhou, Z. Song, J. Shi, J. Zhang, S. Feng
Journal of Materials Chemistry C 6, pp 4177-4182 (2018)
abstractAn Al3Sc alloy doped Al3Sc-Sb2Te chalcogenide was put forward to avoid oxidation of pure Sc and enhance data retention ability. Compared with conventional Ge2Sb2Te5 and Sc-doped Sc0.2Sb2Te3 materials, the Al3Sc alloy doped Al3Sc-Sb2Te chalcogenide has much better thermal stability and data retention. X-ray diffraction and transmission electron microscopy analysis reveal that the structure of Al3Sc-Sb2Te has no obvious change compared with Sb2Te, except for the refined grain size. The Al3Sc-Sb2Te based device shows excellent reversible SET-RESET properties with a suitable operation window and a low power consumption of 1.38 x 10−11 J. In addition, a good cyclability of 1 x 106 cycles can be obtained with a large resistance ratio of about 102. Our calculations reveal that Al and Sc atoms prefer bonding with Te atoms. These stable local patterns stabilize glassy states and result in high stability.