The 2020 skyrmionics roadmap
Back, C., V Cros, H. Ebert, K. Everschor-Sitte, A. Fert, M. Garst, Tianping Ma, S. Mankovsky, T. L. Monchesky, M. Mostovoy, N. Nagaosa, S. S. P. Parkin, C. Pfleiderer, N. Reyren, A. Rosch, Y. Taguchi, Y. Tokura, K. von Bergmann, Jiadong Zang
Journal of Physics D 53 (36), pp 363001/1-37 (2020)
abstractThe notion of non-trivial topological winding in condensed matter systems represents a major area of present-day theoretical and experimental research. Magnetic materials offer a versatile platform that is particularly amenable for the exploration of topological spin solitons in real space such as skyrmions. First identified in non-centrosymmetric bulk materials, the rapidly growing zoology of materials systems hosting skyrmions and related topological spin solitons includes bulk compounds, surfaces, thin films, heterostructures, nano-wires and nano-dots. This underscores an exceptional potential for major breakthroughs ranging from fundamental questions to applications as driven by an interdisciplinary exchange of ideas between areas in magnetism which traditionally have been pursued rather independently. The skyrmionics Roadmap provides a review of the present state of the art and the wide range of research directions and strategies currently under way. These are, for instance, motivated by the identification of the fundamental structural properties of skyrmions and related textures, processes of nucleation and annihilation in the presence of non-trivial topological winding, an exceptionally efficient coupling to spin currents generating spin transfer torques at tiny current densities, as well as the capability to purpose-design broad-band spin dynamic and logic devices.
Realization of epitaxial NbP and TaP Weyl semimetal thin films
Bedoya-Pinto, A., A.K. Pandeya, D. Liu, H. Deniz, K. Chang, H. Tan, H. Han, J. Jena, I. Kostanovskiy, S.S.P. Parkin
ACS Nano 14 (4), pp 4405-4413 (2020)
abstractWeyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena. While WSMs were established in bulk monopnictide compounds several years ago, the growth of thin films remains a challenge. Here, we report the bottom-up synthesis of single-crystalline NbP and TaP thin films, 9 to 70 nm thick, by means of molecular beam epitaxy. The as-grown epitaxial films feature a phosphorus-rich stoichiometry, a tensile-strained unit cell, and a homogeneous surface termination, unlike their bulk crystal counterparts. These properties result in an electronic structure governed by topological surface states as directly observed using in situ momentum photoemission microscopy, along with a Fermi-level shift of -0.2 eV with respect to the intrinsic chemical potential. Although the Fermi energy of the as-grown samples is still far from the Weyl points, carrier mobilities close to 103 cm2/(V s) have been measured at room temperature in patterned Hall-bar devices. The ability to grow thin films of Weyl semimetals that can be tailored by doping or strain, is an important step toward the fabrication of functional WSM-based devices and heterostructures.
Magnetic racetrack memory: from physics to the cusp of applications within a decade
Bläsing, Robin, Asif Ali Khan, Panagiotis Filippou, Chirag Garg, Fazal Hameed, Jeronimo Castrillon, Stuart S.P Parkin
Proceedings of the IEEE 108 (8), pp 1303-1321 (2020)
abstractRacetrack memory (RTM) is a novel spintronic memory-storage technology that has the potential to overcome fundamental constraints of existing memory and storage devices. It is unique in that its core differentiating feature is the movement of data, which is composed of magnetic domain walls (DWs), by short current pulses. This enables more data to be stored per unit area compared to any other current technologies. On the one hand, RTM has the potential for mass data storage with unlimited endurance using considerably less energy than today"s technologies. On the other hand, RTM promises an ultrafast nonvolatile memory competitive with static random access memory (SRAM) but with a much smaller footprint. During the last decade, the discovery of novel physical mechanisms to operate RTM has led to a major enhancement in the efficiency with which nanoscopic, chiral DWs can be manipulated. New materials and artificially atomically engineered thin-film structures have been found to increase the speed and lower the threshold current with which the data bits can be manipulated. With these recent developments, RTM has attracted the attention of the computer architecture community that has evaluated the use of RTM at various levels in the memory stack. Recent studies advocate RTM as a promising compromise between, on the one hand, power-hungry, volatile memories and, on the other hand, slow, nonvolatile storage. By optimizing the memory subsystem, significant performance improvements can be achieved, enabling a new era of cache, graphical processing units, and high capacity memory devices. In this article, we provide an overview of the major developments of RTM technology from both the physics and computer architecture perspectives over the past decade. We identify the remaining challenges and give an outlook on its future.
Emerging materials in neuromorphic computing: Guest editorial
Burr, Geoffrey W, Abu Sebastian, Elisa Vianello, Rainer Waser, Stuart S. Parkin
APL Materials 8 (1), pp 010401/1-4 (2020)
abstractAn introduction to the APL Materials Special Issue on "Emerging Materials in Neuromorphic Computing", by the guest editors.
Microscopic manipulation of ferroelectric domains in SnSe monolayers at room temperature
Chang, Kai, Felix Küster, Brandon J. Miller, Jing-Rong Ji, Jia-Lu Zhang, Paolo Sessi, Salvador Barraz-Lopez, Stuart S. Parkin
Nano Letters 20 (9), pp 6590-6597 (2020)
abstractTwo-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and nonvolatile electronic devices based on vertical and coplanar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene and the demonstration of controlled room-temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which are difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.
Experimental formation of monolayer group-IV monochalcogenides
Chang, Kai, Stuart S. Parkin
Journal of Applied Physics 127 (22), pp 220902/1-14 (2020)
abstractMonolayer group-IV monochalcogenides (MX, M=Ge, Sn, Pb; X=S, Se, Te) are a family of novel two-dimensional (2D) materials that have atomic structures closely related to that of the staggered black phosphorus lattice. The structure of most monolayer MX materials exhibits a broken inversion symmetry and many of them exhibit ferroelectricity with a reversible in-plane electric polarization. A further consequence of the noncentrosymmetric structure is that when coupled with strong spin-orbit coupling, many MX materials are promising for the future applications in non-linear optics, photovoltaics, spintronics, and valleytronics. Nevertheless, because of the relatively large exfoliation energy, the creation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developing field of 2D material heterostructures. In this Perspective, we review recent developments in experimental routes to the creation of the monolayer MX, including molecular beam epitaxy and two-step etching methods. Other approaches that could be used to prepare the monolayer MX are also discussed, such as liquid phase exfoliation and solution-phase synthesis. A quantitative comparison between these different methods is also presented.
Evidence of higher-order topology in multilayer WTe2from Josephson coupling through anisotropic hinge states
Choi, Yong-Bin, Yingming Xie, Chui-Zhen Chen, Jinho Park, Su-Beom Song, Jiho Yoon, B. J. Kim, Takashi Taniguchi, Kenji Watanabe, Jonghwan Kim, Kin Chung Fong, Mazhar N. Ali, Kam Tuen Law, Gil-Ho Lee
Nature Materials 19 (9), pp 974-979 (2020)
abstractTd-WTe2(non-centrosymmetric and orthorhombic), a type-II Weyl semimetal, is expected to have higher-order topological phases with topologically protected, helical one-dimensional hinge states when its Weyl points are annihilated. However, the detection of these hinge states is difficult due to the semimetallic behaviour of the bulk. In this study, we have spatially resolved the hinge states by analysing the magnetic field interference of the supercurrent in Nb-WTe2-Nb proximity Josephson junctions. The Josephson current along theaaxis of the WTe2crystal, but not along thebaxis, showed a sharp enhancement at the edges of the junction, and the amount of enhanced Josephson current was comparable to the upper limits of a single one-dimensional helical channel. Our experimental observations suggest a higher-order topological phase in WTe2and its corresponding anisotropic topological hinge states, in agreement with theoretical calculations. Our work paves the way for the study of hinge states in topological transition-metal dichalcogenides and analogous phases.
Author correction: Evidence of higher-order topology in multilayer WTe2from Josephson coupling through anisotropic hinge states
Choi, Yong-Bin, Yingming Xie, Chui-Zhen Chen, Jinho Park, Su-Beom Song, Jiho Yoon, B. J. Kim, Takashi Taniguchi, Kenji Watanabe, Jonghwan Kim, Kin Chung Fong, Mazhar N. Ali, Kam Tuen Law, Gil-Ho Lee
Nature Materials 19 (9), pp 1036 (2020)
abstractIn the version of this Article originally published, the following sentence was missing from the end of the Acknowledgements: `M.N.A. acknowledges support from the Alexander von Humboldt Foundation’s Sofia Kovalevskaja Award and the BMBF MINERVA ARCHES Award.' This has now been corrected in all versions of this Article.
Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite
Cui, Bin, Yu Huan, Jifan Hu
Journal of Physics: Condensed Matter 32 (34), pp 344001/1-7 (2020)
abstractThe polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal-insulator or paramagnet-ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO3 of 40 nm could be transformed to AFM SrCoO2.5 with ordered oxygen vacancy planes either vertical (V-SrCoO2.5) or parallel (P-SrCoO2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO2.5 are along [010] and [110], respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO3 and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.
Parallel broadband femtosecond reflection spectroscopy at a soft X-ray free-electron laser
Engel, Robin Y., Piter S. Miedema, Diego Turenne, Igor Vaskivskyi, Günter Brenner, Siarhei Dziarzhytski, Marion Kuhlmann, Jan O. Schunck, Florian Döring, Andriy Styervoyedov, Stuart S.P Parkin, Christian David, Christian Schüßler-Langeheine, Hermann A. Dürr, Martin Beye
Applied Sciences 10 (19), pp 6947/1-8 (2020)
abstractX-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.
Largely suppressed magneto-thermal conductivity and enhanced magneto-thermoelectric properties in PtSn4
Fu, Chenguang, Satya N. Guin, Thomas Scaffidi, Yan Sun, Rana Saha, Sarah J. Watzman, Abhay K. Srivastava, Guowei Li, Walter Schnelle, Stuart S. Parkin, Claudia Felser, Johannes Gooth
Research 2020, pp 4643507/1-8 (2020)
abstractHighly conductive topological semimetals with exotic electronic structures offer fertile ground for the investigation of the electrical and thermal transport behavior of quasiparticles. Here, we find that the layer-structured Dirac semimetal PtSn4 exhibits a largely suppressed thermal conductivity under a magnetic field. At low temperatures, a dramatic decrease in the thermal conductivity of PtSn4 by more than two orders of magnitude is obtained at 9T. Moreover, PtSn4 shows both strong longitudinal and transverse thermoelectric responses under a magnetic field. Large power factor and Nernst power factor of approximately 80-100 μW·cm−1·K−2 are obtained around 15K in various magnetic fields. As a result, the thermoelectric figure of merit zT is strongly enhanced by more than 30 times, compared to that without a magnetic field. This work provides a paradigm for the decoupling of the electron and hole transport behavior of highly conductive topological semimetals and is helpful for developing topological semimetals for thermoelectric energy conversion.
Efficient chiral-domain-wall motion driven by spin-orbit torque in metastable platinum films
Garg, Chirag, See-Hun Yang, Leslie Thompson, Teya Topuria, Amir Capua, Brian Hughes, Timothy Phung, Panagiotis Filippou, Stuart S. Parkin
Physical Review Applied 14 (3), pp 034052/1-8 (2020)
abstractThe properties and characteristics of thin-film materials strongly depend on their textures and orientations. However, the attainable film morphologies are severely limited by substrates and growth thermodynamics. Metastable films that otherwise cannot be grown by conventional growth methods may overcome these limitations, thus allowing a dramatic expansion of the spectrum of film textures and orientations. Here we present a means to grow metastable platinum layers that are deposited from platinum alloyed with bismuth surfactant material. This is distinct from conventional surfactant-aided growth of films in which surfactants are typically deposited onto the substrate before film deposition, altering the film growth mode but not the film morphology by tuning the surface energy. Surprisingly, we find that almost no bismuth is incorporated into the platinum layer, but rather the structural morphology of this layer is significantly altered. When this metastable platinum layer is applied to spin-orbit-torque technology, a huge increase in the current-driven velocity of chiral domain walls in perpendicularly magnetized wire on top of the metastable platinum layer is observed for otherwise the same current density, while the platinum resistivity is found to barely increase. Our findings show that the metastable film grown from material alloyed with surfactant is promising for the development of devices in various fields, such as spintronics, semiconductors, and quantum materials.
Interfacial control of ferromagnetism in ultrathin SrRuO3 films sandwiched between ferroelectric BaTiO3 layers
Gu, Youdi, Cheng Song, Qinghua Zhang, Fan Li, Hengxin Tan, Kun Xu, Jia Li, Muhammad S Saleem, Muhammad U Fayaz, Jingjing Peng, Fengxia Hu, Lin Gu, Wei Liu, Zhidong Zhang, Feng Pan
ACS Applied Materials and Interfaces 12 (5), pp 6707-6715 (2020)
abstractInterfaces between materials provide an intellectually rich arena for fundamental scientific discovery and device design. However, the frustration of magnetization and conductivity of perovskite oxide films under reduced dimensionality is detrimental to their device performance, preventing their active low-dimensional application. Herein, by inserting the ultrathin 4d ferromagnetic SrRuO3 layer between ferroelectric BaTiO3 layers to form a sandwich heterostructure, we observe enhanced physical properties in ultrathin SrRuO3 films, including longitudinal conductivity, Curie temperature, and saturated magnetic moment. Especially, the saturated magnetization can be enhanced to  ∼ 3.12 μB/Ru in ultrathin BaTiO3/SrRuO3/BaTiO3 trilayers, which is beyond the theoretical limit of bulk value (2 μB/Ru). This observation is attributed to the synergistic ferroelectric proximity effect (SFPE) at upper and lower BaTiO3/SrRuO3 heterointerfaces, as revealed by the high-resolution lattice structure analysis. This SFPE in dual-ferroelectric interface cooperatively induces ferroelectric-like lattice distortions in RuO6 oxygen octahedra and subsequent spin-state crossover in SrRuO3, which in turn accounts for the observed enhanced magnetization. Besides the fundamental significance of interface-induced spin-lattice coupling, our findings also provide a viable route to the electrical control of magnetic ordering, taking a step toward low-power applications in all-oxide spintronics.
A new highly anisotropic Rh-based Heusler compound for magnetic recording
He, Yangkun, Gerhard H. Fecher, Chenguang Fu, Yu Pan, Kaustuv Manna, Johannes Kroder, Ajay Jha, Xiao Wang, Zhiwei Hu, Stefan Agrestini, Javier Herrero-Martin, Manuel Valvidares, Yurii Skourski, Walter Schnelle, Plamen Stamenov, Horst Borrmann, Liu Hao Tjeng, Rudolf Schäfer, Stuart S. Parkin, John Micha Coey, Claudia Felser
Advanced Materials PREPRINT, pp 2004331/1-8 (2020)
abstractThe development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh2CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m−3 is combined with a saturation magnetization of μ0Ms = 0.52 T at 2 K (2.2 MJ m−3 and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighboring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its Tc of 450 K, together with a thermal conductivity of 20 W m−1 K−1, make Rh2CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.
Elliptical Bloch skyrmion chiral twins in an antiskyrmion system
Jena, Jagannath, Börge Göbel, Tianping Ma, Vivek Kumar, Rana Saha, Ingrid Mertig, Claudia Felser, Stuart S.P Parkin
Nature Communications 11, pp 1115/1-9 (2020)
abstractSkyrmions and antiskyrmions are distinct topological chiral spin textures that have been observed in various material systems depending on the symmetry of the crystal structure. Here we show, using Lorentz transmission electron microscopy, that arrays of skyrmions can be stabilized in a tetragonal inverse Heusler with D2d symmetry whose Dzyaloshinskii-Moriya interaction (DMI) otherwise supports antiskyrmions. These skyrmions can be distinguished from those previously found in several B20 systems which have only one chirality and are circular in shape. We find Bloch-type elliptical skyrmions with opposite chiralities whose major axis is oriented along two specific crystal directions: [010] and [100]. These structures are metastable over a wide temperature range and we show that they are stabilized by long-range dipole-dipole interactions. The possibility of forming two distinct chiral spin textures with opposite topological charges of ±1 in one material makes the family of D2d materials exceptional.
Observation of magnetic antiskyrmions in the low magnetization ferrimagnet Mn2Rh0.95Ir0.05Sn
Jena, Jagannath, Rolf Stinshoff, Rana Saha, Abhay K. Srivastava, Tianping Ma, Hakan Deniz, Peter Werner, Claudia Felser, Stuart S. Parkin
Nano Letters 20 (1), pp 59-65 (2020)
abstractRecently, magnetic antiskyrmions were discovered in Mn1.4Pt0.9Pd0.1Sn, an inverse tetragonal Heusler compound that is nominally a ferrimagnet, but which can only be formed with substantial Mn vacancies. The vacancies reduce considerably the compensation of the moments between the two expected antiferromagnetically coupled Mn sub-lattices so that the overall magnetization is very high and the compound is almost a "ferromagne". Here, we report the observation of antiskyrmions in a second inverse tetragonal Heusler compound, Mn2Rh0.95Ir0.05Sn, which can be formed stoichiometrically without any Mn vacancies and which thus exhibits a much smaller magnetization. Individual and lattices of antiskyrmions can be stabilized over a wide range of temperature from near room temperature to 100 K, the base temperature of the Lorentz transmission electron microscope used to image them. In low magnetic fields helical spin textures are found which evolve into antiskyrmion structures in the presence of small magnetic fields. A weaker Dzyaloshinskii-Moriya interaction (DMI), that stabilizes the antiskyrmions, is expected for the 4d element Rh as compared to the 5d element Pt, so that the observation of antiskyrmions in Mn2Rh0.95Ir0.05Sn establishes the intrinsic stability of antiskyrmions in these Heusler compounds. Moreover, the finding of antiskyrmions with substantially lower magnetization promises, via chemical tuning, even zero moment antiskyrmions with important technological import.
VO2 electro-optic memory and oscillator for neuromorphic computing
Jeong, Junho, Youngho Jung, Zhongnan Qu, Bin Cui, Ankita Khanda, Ankita Sharma, Stuart S. Parkin, Joyce K. S Poon
CLEO: Science and Innovations 2020, pp STh3R.2 /1-2 (2020)
abstractWe demonstrate optical memory and light-triggered electrical oscillations in a VO2 electro optic micro-wire device for potential applications in neuromorphic computing architectures.
Signatures of sixfold degenerate exotic fermions in a superconducting Metal PdSb2
Kumar, Nitesh, Mengyu Yao, Jayit Nayak, Maia G. Vergniory, Joern Bannies, Zhijun Wang, Niels B. M Schröter, Vladimir N Strocov, Lukas Müchler, Wujun Shi, Emile D. L Rienks, J. L. Manes, Chandra Shekhar, Stuart S. Parkin, Joerg Fink, Gerhard H. Fecher, Yan Sun, B. Andrei Bernevig, Claudia Felser
Advanced Materials 32 (11), pp 1906046/1-6 (2020)
abstractMultifold degenerate points in the electronic structure of metals lead to exotic behaviors. These range from twofold and fourfold degenerate Weyl and Dirac points, respectively, to sixfold and eightfold degenerate points that are predicted to give rise, under modest magnetic fields or strain, to topological semimetallic behaviors. The present study shows that the nonsymmorphic compound PdSb2 hosts six-component fermions or sextuplets. Using angle-resolved photoemission spectroscopy, crossing points formed by three twofold degenerate parabolic bands are directly observed at the corner of the Brillouin zone. The group theory analysis proves that under weak spin-orbit interaction, a band inversion occurs.
Spectroscopic evidence for an additional symmetry breaking in the nematic state of FeSe superconductor
Li, Cong, Xianxin Wu, Le Wang, Defa Liu, Yongqing Cai, Yang Wang, Qiang Gao, Chunyao Song, Jianwei Huang, Chenxiao Dong, Jing Liu, Ping Ai, Hailan Luo, ChaoHui Yin, Guodong Liu, Yuan Huang, Qingyan Wang, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Zuyan Xu, Youguo Shi, Jiangping Hu, Tao Xiang, Lin Zhao, X. J. Zhou
Physical Review X 10 (3), pp 031033/1-10 (2020)
abstractThe iron-based superconductor FeSe has attracted much recent attention because of its simple crystal structure, distinct electronic structure, and rich physics exhibited by itself and its derivatives. Determination of its intrinsic electronic structure is crucial to understanding its physical properties and superconductivity mechanism. Both theoretical and experimental studies so far have provided a picture that FeSe consists of one holelike Fermi surface around the Brillouin zone center in its nematic state. Here we report direct observation of two holelike Fermi surface sheets around the Brillouin zone center, and the splitting of the associated bands, in the nematic state of FeSe by taking high-resolution laser-based angle-resolved photoemission measurements. These results indicate that, in addition to nematic order and spin-orbit coupling, there is an additional order in FeSe that breaks either inversion or time-reversal symmetries. The new Fermi surface topology asks for reexamination of the existing theoretical and experimental understanding of FeSe and stimulates further efforts to identify the origin of the hidden order in its nematic state.
Hexagonal rare-earth manganites and ferrites: a review of improper ferroelectricity, magnetoelectric coupling, and unusual domain walls
Li, Menglei, Hengxin Tan, Wenhui Duan
Physical Chemistry Chemical Physics 22 (26), pp 14415-14432 (2020)
abstractHexagonal rare-earth manganites and ferrites are well-known improper ferroelectrics with low-temperature antiferromagnetism/weak ferromagnetism. In recent decades, new multi-functional device concepts and applications have provoked the exploration for multiferroics which simultaneously possess ferroelectric and magnetic orders. As a promising platform for multiferroicity, hexagonal manganites and ferrites are attracting great research interest among the fundamental scientific and technological communities. Moreover, the novel type of vortex-like ferroelectric domain walls are locked to the antiphase structural domain walls, providing an extra degree of freedom to tune the magnetoelectric coupling and other properties such as conductance. Here, we summarize the main experimental achievements and up-to-date theoretical understanding of the ferroelectric, magnetic, and magnetoelectric properties, as well as the intriguing domain patterns in hexagonal rare-earth manganites and ferrites. Recent work on non-stoichiometric compounds will also be briefly introduced.
Titanium as a substrate for three-dimensional hybrid electrodes for vanadium redox flow battery applications
Lu, Xubin, Fan Li, Matthias Steimecke, Muhammad Tariq, Mark Hartmann, Michael Bron
ChemElectroChem 7 (3), pp 737-744 (2020)
abstractTitanium, either in the form of a Ti foil or in form of a Ti mesh, was used as a novel substrate to grow nitrogen-doped carbon nanotubes (NCNTs) through chemical vapor deposition at moderate temperatures over electrodeposited iron particles. The thus-prepared high-surface-area electrodes were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical performance towards the V(IV)/V(V) redox couple was investigated by cyclic voltammetry. The parameters for iron particle electrodeposition were adjusted towards high and uniform substrate coverage. Nanotube growth from acetonitrile at moderate temperatures (600 °C) led to N-containing CNTs with a high amount of graphitic nitrogen. NCNTs grown over Ti substrates provide promising performances towards the V(IV)/V(V) as well as the V(III)/V(IV) redox pair. In general, the results of this study show that Ti might be a suitable electrocatalyst substrate for various applications in electrochemical energy conversion.
Plasma-etched functionalized graphene as a metal-free electrode catalyst in solid acid fuel cells
Lu, Xubin, Xin Yang, Muhammad Tariq, Fan Li, Matthias Steimecke, Jia Li, Arom Varga, Michael Bron, Bernd Abel
Journal of Materials Chemistry A 8 (5), pp 2445-2452 (2020)
abstractThe Oxygen and nitrogen plasma treatment were applied to produce graphene with abundant edges, oxygen functional groups, and nitrogen doping. The plasma-etched graphene was then used as a metal-free electrocatalyst in a solid acid fuel cell. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the graphene layers. Alternating-current impedance spectroscopy of a hybrid electrode containing the plasma-etched graphene and cesium dihydrogen phosphate as proton-conducting solid acid illustrated its remarkable catalytic activity under cathodic conditions. Thus, both O2 and N2 plasma treatment activated the material. While O2 plasma was a more effective activator than N2 plasma, it also resulted in higher degradation rates. A combination of density functional theory calculations and experimental results indicated that zigzag carbon was the most active site for the oxygen reduction reaction on both O2 and N2 plasma-etched graphene. Furthermore, the armchair carbons adjacent to the surface oxygen groups and doped heteroatoms were also important active sites for O2 and N2 plasma-etched graphene, respectively. The results of this study will guide future endeavors in the development of non-precious metal catalysts for use as fuel cell cathodes.
Tunable magnetic antiskyrmion size and helical period from nanometers to micrometers in a D2d Heusler compound
Ma, Tianping, Ankit K. Sharma, Rana Saha, Abhay K. Srivastava, Peter Werner, Praveen Vir, Vivek Kumar, Claudia Felser, Stuart S. Parkin
Advanced Materials 32 (28), pp 2002043/1-7 (2020)
abstractSkyrmions and antiskyrmions are magnetic nano-objects with distinct chiral, noncollinear spin textures that are found in various magnetic systems with crystal symmetries that give rise to specific Dzyaloshinskii-Moriya exchange vectors. These magnetic nano-objects are associated with closely related helical spin textures that can form in the same material. The skyrmion size and the period of the helix are generally considered as being determined, in large part, by the ratio of the magnitude of the Heisenberg to that of the Dzyaloshinskii-Moriya exchange interaction. In this work, it is shown by real-space magnetic imaging that the helix period λ and the size of the antiskyrmion daSk in the D2d compound Mn1.4PtSn can be systematically tuned by more than an order of magnitude from ≈ 100 nm to more than 1.1 μm by varying the thickness of the lamella in which they are observed. The chiral spin texture is verified to be preserved even up to micrometer-thick layers. This extreme size tunability is shown to arise from long-range magnetodipolar interactions, which typically play a much less important role for B20 skyrmions. This tunability in size makes antiskyrmions very attractive for technological applications.
Structure and magnetism of EuS on Bi2Se3(0001)
Meyerheim, Holger L., Arthur Ernst, Katayoon Mohseni, Andrey Polyakov, V Maznichenko Igor, Pawel A. Buczek, Alessandro Coati, Stuart S. Parkin
Physica Status Solidi B PREPRINT, (2020)
abstractThe rocksalt-type ferromagnetic (FM) insulator EuS (bulkTC = 17 K) grown on Bi2Se3with well-matched (111) plane of the film and (0001) plane of the substrate is studied. The system may feature magnetic proximity effect breaking the time-reversal symmetry and opening a bandgap in the metallic topologically protected surface state of Bi2Se3. The experimental X-ray diffraction studies are combined with ab initio calculations to resolve contradictory results concerning the enhancement of the TC up to 300 K and the degree of induced magnetization in the system. It is concluded that previous studies relied on idealized and unconfirmed structure models. Herein, it is shown by surface X-ray diffraction (SXRD) with ab initio calculations that a two double layer-thick EuS film grows with a sharp interface and without chemical intermixing in a single domain state in an FCC-type stacking on the Bi2Se3(0001) surface in which the topmost layer is metallic, thereby lifting polarity. A large pz-orbital-derived top-layer sulfur magnetic moment of 0.6 μB is found, whereas for europium, μEu = 6.9 μB throughout the film is found. No magnetization within the first Bi2Se3quintuple layer is found. The calculation of the exchange parameters Jij indicates a complex FM and antiferromagnetic ordering between europium and sulfur with a maximum Neel temperature of 226 K.
Doping-induced spin Hall ratio enhancement in A15-phase, Ta-doped β-W thin films
Minhas, Mohsin Z., Avanindra Pandeya, Bharat Grover, Alessandro Fumarola, Ilya Kostanovskiy, Binoy K. Hazra, Wolfgang Hoppe, Georg Woltersdorf, Amilcar Bedoya-Pinto, Stuart S. Parkin, Mazhar N. Ali
JPhys Materials 3 (4), pp 044001/1-7 (2020)
abstractAs spintronic devices become more and more prevalent, the desire to find Pt-free materials with large spin Hall effects is increasing. Previously it was shown that β-W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that β-W/Ta alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase β-W films doped with Ta (W4−xTax where x= 0.34 ± 0.06) deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was  ∼ 9% larger than pure β-W films. We also found that the SHR"s in devices with Co(2)Fe(6)B(2)were nearly twice as large as the SHR's in devices with Co4Fe4B2. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W3Ta alloy should be feasible, opening the door to commercially viable, Pt-free, spintronic devices.
Spin-transport in superconductors
Ohnishi, K., S. Komori, G. Yang, K. -R. Jeon, L. A. B. O Olthof, X. Montiel, M. G. Blamire, J. W. A. Robinson
Applied Physics Letters 116 (13), pp 130501/1-5 (2020)
abstractSpin-transport in superconductors is a subject of fundamental and technical importance with the potential for applications in superconducting-based cryogenic memory and logic. Research in this area is rapidly intensifying with recent discoveries establishing the field of superconducting spintronics. In this perspective, we provide an overview of the experimental state-of-the-art with a particular focus on local and nonlocal spin-transport in superconductors and propose device schemes to demonstrate the viability of superconducting spin-based devices.
Directional ionic transport across the oxide interface enables low-temperature epitaxy of rutile TiO2
Park, Yunkyu, Hyeji Sim, Minguk Jo, Gi-Yeop Kim, Daseob Yoon, Hyeon Han, Younghak Kim, Kyung Song, Donghwa Lee, Si-Young Choi, Junwoo Son
Nature Communications 11, pp 1401/1-10 (2020)
abstractHeterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO2/VO2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO2 epitaxial films at the lowest temperature ( < = 150°C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO2−δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.
Double photoemission from Ag and Pd surfaces: Energy relations
Schumann, Frank O., Yurii Aliaev, Ilja Kostanovskiy, Jürgen Kirschner
Physical Review B 101 (11), pp 115104/1-9 (2020)
abstractWe have investigated the electron pair emission due to single-photon absorption from Ag(100) and Pd(100) surfaces. We are interested in the energy spectra of pairs in particular near the energy cutoff. The sum energy spectra of Ag display a distinctive photon energy dependence. We also observe some fine structure. Near the high-energy cutoff the coincidence rate is too low to determine the energy position of the cutoff. Nevertheless we observe a finite signal if two 5sp electrons near the Fermi level are emitted. For Pd(100) we find sum energy spectra without fine structure and the cutoff region is approached linearly. Within the experimental accuracy the minimum energy to liberate two electrons is twice the work function.
Handedness-dependent quasiparticle interference in the two enantiomers of the topological chiral semimetal PdGa
Sessi, Paolo, Feng-Ren Fan, Felix Küster, Kaustuv Manna, Niels B. M Schroter, Jing-Rong Ji, Samuel Stolz, Jonas A. Krieger, Ding Pei, Timur K. Kim, Pavel Dudin, Cephise Cacho, Rolan Widmer, Horst Borrmann, Wujun Shi, Kai Chang, Yan Sun, Claudia Felser, Stuart S. Parkin
Nature Communications 11 (1), pp 3507/1-7 (2020)
abstractIt has recently been proposed that combining chirality with topological band theory results in a totally new class of fermions. Understanding how these unconventional quasiparticles propagate and interact remains largely unexplored so far. Here, we use scanning tunneling microscopy to visualize the electronic properties of the prototypical chiral topological semimetal PdGa. We reveal chiral quantum interference patterns of opposite spiraling directions for the two PdGa enantiomers, a direct manifestation of the change of sign of their Chern number. Additionally, we demonstrate that PdGa remains topologically non-trivial over a large energy range, experimentally detecting Fermi arcs in an energy window of more than 1.6eV that is symmetrically centered around the Fermi level. These results are a consequence of the deep connection between chirality in real and reciprocal space in this class of materials, and, thereby, establish PdGa as an ideal topological chiral semimetal. Direct visualization of chiral effects in topological chiral semimetals remains elusive. Here, Sessi et al. demonstrate that quasiparticle scattering at impurities in the two enantiomers of PdGa gives rise to handedness dependent quantum interference patterns.
Extended Nernst-Planck equation incorporating partial dehydration effect
Wang, Zhong, Zhiyang Yuan, Feng Liu
Chinese Physics Letters 37 (9), pp 094701/1-6 (2020)
abstractNovel ionic transporting phenomena emerge as nanostructures approach the molecular scale. At the sub-2nm scale, widely used continuum equations, such as the Nernst-Planck equation, break down. Here, we extend the Nernst-Planck equation by adding a partial dehydration effect. Our model agrees with the reported ion fluxes through graphene oxide laminates with sub-2nm interlayer spacing, outperforming previous models. We also predict that the selectivity sequences of alkali metal ions depend on the geometries of the nanostructures. Our model opens a new avenue for the investigation of the underlying mechanisms in nanofluidics at the sub-2nm scale.
Electric-field switching of magnetic topological charge in type-I multiferroics
Xu, Changsong, Peng Chen, Hengxin Tan, Yurong Yang, Hongjun Xiang, L. Bellaiche
Physical Review Letters 125 (3), pp 037203/1-6 (2020)
abstractApplying electric field to control magnetic properties is a very efficient way for spintronics devices. However, the control of magnetic characteristics by electric fields is not straightforward, due to the time-reversal symmetry of magnetism versus spatial inversion symmetry of electricity. Such fundamental difticulty makes it challenging to modify the topology of magnetic skyrmionic states with electric field. Here, we propose a novel mechanism that realizes the electric-field (E) switching of magnetic topological charge (Q) in a controllable and reversible fashion, through the mediation of electric polarization (P) and Dzyaloshinskii-Moriya interaction (D). Such a mechanism is coined here EPDQ. Its validity is demonstrated in a multiferroic VOI2 monolayer, which is predicted to host magnetic bimerons. The change in magnetic anisotropy is found to play a crucial role in realizing the EPDQ process and its microscopic origin is discussed. Our study thus provides a new approach toward the highly desired electricfield control of magnetism.
Giant, unconventional anomalous Hall effect in the metallic frustrated magnet candidate, KV3Sb5
Yang, Shuo-Ying, Yaojia Wang, Brenden R. Ortiz, Defa Liu, Jacob Gayles, Elena Derunova, Rafael Gonzalez-Hernandez, Libor Smejkal, Yulin Chen, Stuart S. Parkin, Stephen D. Wilson, Eric S. Toberer, Tyrel McQueen, Mazhar N. Ali
Science Advances 6 (31), pp eabb6003/1-8 (2020)
abstractThe anomalous Hall effect (AHE) is one of the most fundamental phenomena in physics. In the highly conductive regime, ferromagnetic metals have been the focus of past research. Here, we report a giant extrinsic AHE in KV3Sb5, an exfoliable, highly conductive semimetal with Dirac quasiparticles and a vanadium Kagome net. Even without report of long range magnetic order, the anomalous Hall conductivity reaches 15,507 Ω−1 cm−1 with an anomalous Hall ratio of ≈ 1.8%; an order of magnitude larger than Fe. Defying theoretical expectations, KV3Sb5 shows enhanced skew scattering that scales quadratically, not linearly, with the longitudinal conductivity, possibly arising from the combination of highly conductive Dirac quasiparticles with a frustrated magnetic sublattice. This allows the possibility of reaching an anomalous Hall angle of 90° in metals. This observation raises fundamental questions about AHEs and opens new frontiers for AHE and spin Hall effect exploration, particularly in metallic frustrated magnets.
Anomalous thickness-dependent electrical conductivity in van der Waals layered transition metal halide, Nb3Cl8
Yoon, Jiho, Edouard Lesne, Kornelia Sklarek, John Sheckelton, Chris Pasco, Stuart S. Parkin, Tyrel M. McQueen, Mazhar N. Ali
Journal of Physics: Condensed Matter 32 (30), pp 304004/1-7 (2020)
abstractUnderstanding the electronic transport properties of layered, van der Waals transition metal halides (TMHs) and chalcogenides is a highly active research topic today. Of particular interest is the evolution of those properties with changing thickness as the 2D limit is approached. Here, we present the electrical conductivity of exfoliated single crystals of the TMH, cluster magnet, Nb3Cl8, over a wide range of thicknesses both with and without hexagonal boron nitride (hBN) encapsulation. The conductivity is found to increase by more than three orders of magnitude when the thickness is decreased from 280 μm to 5 nm, at 300 K. At low temperatures and below  ∼ 50 nm, the conductance becomes thickness independent, implying surface conduction is dominating. Temperature dependent conductivity measurements indicate Nb3Cl8 is an insulator, however, the effective activation energy decreases from a bulk value of 310 meV to 140 meV by 5 nm. X-ray photoelectron spectroscopy (XPS) shows mild surface oxidation in devices without hBN capping, however, no significant difference in transport is observed when compared to the capped devices, implying the thickness dependent transport behavior is intrinsic to the material. A conduction mechanism comprised of a higher conductivity surface channel in parallel with a lower conductivity interlayer channel is discussed.
Atomic layer deposition of cobalt phosphide for efficient water splitting
Zhang, Haojie, Dirk J. Hagen, Xiaopeng Li, Andreas Graff, Frank Heyroth, Bodo Fuhrmann, Ilya Kostanovskiy, Stefan L. Schweizer, Francesco Caddeo, A. Wouter Maijenburg, Stuart S. Parkin, Ralf B. Wehrspohn
Angewandte Chemie International Edition 59 (39), pp 17172-17176 (2020)
abstractTransition-metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co-P films were deposited by using PH(3)plasma as the phosphorus source and an extra H(2)plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self-limited layer-by-layer growth, and the deposited Co-P films were highly pure and smooth. The Co-P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co-P films prepared by the traditional post-phosphorization method. Moreover, the deposition of ultrathin Co-P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three-dimensional (3D) architectures.
Ionic liquid gate-induced modifications of step edges at SrCoO2.5 surfaces
Zhuang, Yuechen, Bin Cui, Hao Yang, Fang Gao, Stuart. S. Parkin
ACS Nano 14 (7), pp 8562-8569 (2020)
abstractIntense electric fields developed during gating at the interface between an ionic liquid and an oxide layer have been shown to lead to significant structural and electronic phase transitions in the entire oxide layer. An archetypical example is the reversible transformation between the brownmillerite SrCoO2.5 and the perovskite SrCoO3 engendered by ionic liquid gating. Here we show using in situ atomic force microscopy studies with photothermal excitation detection, that allows for high quality measurements in the viscous environment of the ionic liquid that the edges of atomically smooth terraces at the surface of SrCoO2.5 films are significantly modified by ionic liquid gating but that the terraces themselves remain smooth. The edges develop ridges that we show, using complementary X-ray photoemission spectroscopy studies, result from the adsorption of hydroxyl groups. Our findings exhibit a way of electrically controlled surface modifications in emergent ionitronic applications.