Dynamic Manipulation of Chiral Domain Wall Spacing for Advanced Spintronic Memory and Logic Devices

Nanoscopic magnetic domain walls (DWs), via their absence or presence, enable highly interesting binary data bits. The current-controlled, high-speed, synchronous motion of sequences of chiral DWs in magnetic nanoconduits induced by current pulses makes possible high-performance spintronic memory and logic devices. The closer the spacing between neighboring DWs in an individual conduit or nanowire, the higher the data density of the device, but at the same time, the more difficult it is to read the bits. Here, we show how the DW spacing can be dynamically varied to facilitate reading for otherwise closely packed bits. In the first method, the current density is increased in portions of the conduit that, thereby, locally speeds up the DWs, decompressing them and making them easier to read. In the second method, a localized bias current is used to compress and decompress the DW spacing. Both of these methods are demonstrated experimentally and validated by micromagnetic simulations. DW compression and decompression rates as high as 88% are shown. These methods can increase the density with which DWs can be packed in future DW-based spintronic devices by more than an order of magnitude.

All-Oxide Metasurfaces Formed by Synchronized Local Ionic Gating

Ionic gating of oxide thin films has emerged as a novel way of manipulating the properties of thin films. Most studies have been carried out on single devices with a three-terminal configuration but, by exploring the electrokinetics during the ionic gating, such a configuration with initially insulating films leads to a highly non-uniform gating response of individual devices within large arrays of the devices. We show that such an issue can be circumvented by the formation of a uniform charge potential by the use of a thin conducting underlayer. This synchronized local ionic gating allows for the simultaneous manipulation of the electrical, magnetic, and/or optical properties of large arrays of devices. Designer metasurfaces formed in this way from SrCoO2.5 thin films display anomalous optical reflection of light that relies on the uniform and coherent response of all the devices. Beyond oxides, almost any material whose properties can be controlled by the addition or removal of ions via gating can form novel metasurfaces using this technique. Our findings provide insights into the electrokinetics of ionic gating and a wide range of applications using synchronized local ionic gating. This article is protected by copyright. All rights reserved.

Weyl spin-momentum locking in a chiral topological semimetal

Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking - a directional relationship between an electron's spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals.

A Cu3 BHT-Graphene van der Waals Heterostructure with Strong Interlayer Coupling for Highly Efficient Photoinduced Charge Separation

Two-dimensional van der Waals heterostructures (2D vdWhs) are of significant interest due to their intriguing physical properties critically defined by the constituent monolayers and their interlayer coupling. Synthetic access to 2D vdWhs based on chemically tunable monolayer organic 2D materials remains challenging. Herein, the fabrication of a novel organic-inorganic bilayer vdWh by combining π-conjugated 2D coordination polymer (2DCP, i.e., Cu3 BHT, BHT = benzenehexathiol) with graphene is reported. Monolayer Cu3 BHT with detectable µm2 -scale uniformity and atomic flatness is synthesized using on-water surface chemistry. A combination of diffraction and imaging techniques enables the determination of the crystal structure of monolayer Cu3 BHT with atomic precision. Leveraging the strong interlayer coupling, Cu3 BHT-graphene vdWh exhibits highly efficient photoinduced interlayer charge separation with a net electron transfer efficiency of up to 34% from Cu3 BHT to graphene, superior to those of reported bilayer 2D vdWhs and molecular-graphene vdWhs. This study unveils the potential for developing novel 2DCP-based vdWhs with intriguing physical properties.

Intrinsic supercurrent non-reciprocity coupled to the crystal structure of a van der Waals Josephson barrier

Non-reciprocal electronic transport in a spatially homogeneous system arises from the simultaneous breaking of inversion and time-reversal symmetries. Superconducting and Josephson diodes, a key ingredient for future non-dissipative quantum devices, have recently been realized. Only a few examples of a vertical superconducting diode effect have been reported and its mechanism, especially whether intrinsic or extrinsic, remains elusive. Here we demonstrate a substantial supercurrent non-reciprocity in a van der Waals vertical Josephson junction formed with a Td-WTe2 barrier and NbSe2 electrodes that clearly reflects the intrinsic crystal structure of Td-WTe2. The Josephson diode efficiency increases with the Td-WTe2 thickness up to critical thickness, and all junctions, irrespective of the barrier thickness, reveal magneto-chiral characteristics with respect to a mirror plane of Td-WTe2. Our results, together with the twist-angle-tuned magneto-chirality of a Td-WTe2 double-barrier junction, show that two-dimensional materials promise vertical Josephson diodes with high efficiency and tunability.

Thickness-Tunable Zoology of Magnetic Spin Textures Observed in Fe5GeTe2

The family of two-dimensional (2D) van der Waals (vdW) materials provides a playground for tuning structural and magnetic interactions to create a wide variety of spin textures. Of particular interest is the ferromagnetic compound Fe5GeTe2 that we show displays a range of complex spin textures as well as complex crystal structures. Here, using a high-brailliance laboratory X-ray source, we show that the majority (1 × 1) Fe5GeTe2 (FGT5) phase exhibits a structure that was previously considered as being centrosymmetric but rather lacks inversion symmetry. In addition, FGT5 exhibits a minority phase that exhibits a long-range ordered (√3 × √3)-R30° superstructure. This superstructure is highly interesting in that it is innately 2D without any lattice periodicity perpendicular to the vdW layers, and furthermore, the superstructure is a result of ordered Te vacancies in one of the topmost layers of the FGT5 sheets rather than being a result of vertical Fe ordering as earlier suggested. We show, from direct real-space magnetic imaging, evidence for three distinct magnetic ground states in lamellae of FGT5 that are stabilized with increasing lamella thickness, namely, a multidomain state, a stripe phase, and an unusual fractal state. In the stripe phase we also observe unconventional type-I and type-II bubbles where the spin texture in the central region of the bubbles is nonuniform, unlike conventional bubbles. In addition, we find a bobber or a cocoon-like spin texture in thick (∼170 μm) FGT5 that emerges from the fractal state in the presence of a magnetic field. Among all the 2D vdW magnets we have thus demonstrated that FGT5 hosts perhaps the richest variety of magnetic phases that, thereby, make it a highly interesting platform for the subtle tuning of magnetic interactions.

Observation of the Fluctuation Spin Hall Effect in a Low-Resistivity Antiferromagnet

The spin Hall effect (SHE) can generate a pure spin current by an electric current, which is promisingly used to electrically control magnetization. To reduce the power consumption of this control, a giant spin Hall angle (SHA) in the SHE is desired in low-resistivity systems for practical applications. Here, critical spin fluctuation near the antiferromagnetic (AFM) phase transition in chromium (Cr) is proven to be an effective mechanism for creating an additional part of the SHE, named the fluctuation spin Hall effect. The SHA is significantly enhanced when the temperature approaches the Néel temperature (TN) of Cr and has a peak value of -0.36 near TN. This value is higher than the room-temperature value by 153% and leads to a low normalized power consumption among known spin-orbit torque materials. This study demonstrates the critical spin fluctuation as a prospective way to increase the SHA and enriches the AFM material candidates for spin-orbitronic devices.

Long-lived spin waves in a metallic antiferromagnet

Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo2P2, which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo2P2, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo2P2, we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

Generation of out-of-plane polarized spin current by spin swapping

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn₃Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn₃Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn₃Sn layer and whose temperature dependence follows that of this layer's antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn₃Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.

Anisotropic proximity-induced superconductivity and edge supercurrent in Kagome metal, K1-xV3Sb5

Materials with Kagome nets are of particular importance for their potential combination of strong correlation, exotic magnetism, and electronic topology. KV₃Sb₅ was discovered to be a layered topological metal with a Kagome net of vanadium. Here, we fabricated Josephson Junctions of K_1-xV₃Sb₅ and induced superconductivity over long junction lengths. Through magnetoresistance and current versus phase measurements, we observed a magnetic field sweeping direction-dependent magnetoresistance and an anisotropic interference pattern with a Fraunhofer pattern for in-plane magnetic field but a suppression of critical current for out-of-plane magnetic field. These results indicate an anisotropic internal magnetic field in K_1-xV₃Sb₅ that influences the superconducting coupling in the junction, possibly giving rise to spin-triplet superconductivity. In addition, the observation of long-lived fast oscillations shows evidence of spatially localized conducting channels arising from edge states. These observations pave the way for studying unconventional superconductivity and Josephson device based on Kagome metals with electron correlation and topology.

Chiral antiferromagnetic Josephson junctions as spin-triplet supercurrent spin valves and d.c. SQUIDs

Spin-triplet supercurrent spin valves are of practical importance for the realization of superconducting spintronic logic circuits. In ferromagnetic Josephson junctions, the magnetic-field-controlled non-collinearity between the spin-mixer and spin-rotator magnetizations switches the spin-polarized triplet supercurrents on and off. Here we report an antiferromagnetic equivalent of such spin-triplet supercurrent spin valves in chiral antiferromagnetic Josephson junctions as well as a direct-current superconducting quantum interference device. We employ the topological chiral antiferromagnet Mn3Ge, in which the Berry curvature of the band structure produces fictitious magnetic fields, and the non-collinear atomic-scale spin arrangement accommodates triplet Cooper pairing over long distances (>150 nm). We theoretically verify the observed supercurrent spin-valve behaviours under a small magnetic field of <2 mT for current-biased junctions and the direct-current superconducting quantum interference device functionality. Our calculations reproduce the observed hysteretic field interference of the Josephson critical current and link these to the magnetic-field-modulated antiferromagnetic texture that alters the Berry curvature. Our work employs band topology to control the pairing amplitude of spin-triplet Cooper pairs in a single chiral antiferromagnet.

Atomic Displacements Enabling the Observation of the Anomalous Hall Effect in a Non-Collinear Antiferromagnet

Antiferromagnets with non-collinear spin structures display various properties that make them attractive for spintronic devices. Some of the most interesting examples are an anomalous Hall effect despite negligible magnetization and a spin Hall effect with unusual spin polarization directions. However, these effects can only be observed when the sample is set predominantly into a single antiferromagnetic domain state. This can only be achieved when the compensated spin structure is perturbed and displays weak moments due to spin canting that allows for external domain control. In thin films of cubic non-collinear antiferromagnets, this imbalance is previously assumed to require tetragonal distortions induced by substrate strain. Here, it is shown that in Mn₃ SnN and Mn₃ GaN, spin canting is due to structural symmetry lowering induced by large displacements of the magnetic manganese atoms away from high-symmetry positions. These displacements remain hidden in X-ray diffraction when only probing the lattice metric and require measurement of a large set of scattering vectors to resolve the local atomic positions. In Mn₃ SnN, the induced net moments enable the observation of the anomalous Hall effect with an unusual temperature dependence, which is conjectured to result from a bulk-like temperature-dependent coherent spin rotation within the kagome plane.

Interaction Effects in a 1D Flat Band at a Topological Crystalline Step Edge

Step edges of topological crystalline insulators can be viewed as predecessors of higher-order topology, as they embody one-dimensional edge channels embedded in an effective three-dimensional electronic vacuum emanating from the topological crystalline insulator. Using scanning tunneling microscopy and spectroscopy, we investigate the behavior of such edge channels in Pb_1-xSn_xSe under doping. Once the energy position of the step edge is brought close to the Fermi level, we observe the opening of a correlation gap. The experimental results are rationalized in terms of interaction effects which are enhanced since the electronic density is collapsed to a one-dimensional channel. This constitutes a unique system to study how topology and many-body electronic effects intertwine, which we model theoretically through a Hartree-Fock analysis.

Ultralow Energy Domain Wall Device for Spin-Based Neuromorphic Computing

Neuromorphic computing (NC) is gaining wide acceptance as a potential technology to achieve low-power intelligent devices. To realize NC, researchers investigate various types of synthetic neurons and synaptic devices, such as memristors and spintronic devices. In comparison, spintronics-based neurons and synapses have potentially higher endurance. However, for realizing low-power devices, domain wall (DW) devices that show DW motion at low energies─typically below pJ/bit─are favored. Here, we demonstrate DW motion at current densities as low as 10⁶ A/m² by engineering the β-W spin-orbit coupling (SOC) material. With our design, we achieve ultralow pinning fields and current density reduction by a factor of 10⁴. The energy required to move the DW by a distance of about 18.6 μm is 0.4 fJ, which translates into the energy consumption of 27 aJ/bit for a bit-length of 1 μm. With a meander DW device configuration, we have established a controlled DW motion for synapse applications and have shown the direction to make ultralow energy spin-based neuromorphic elements.

Strain-driven formation of epitaxial nanostructures in brownmillerite strontium cobaltite thin films

Nanostructured materials can display unique physical properties and are of particular interest for their new functionalities. Epitaxial growth is a promising approach for the controlled synthesis of nanostructures with desired structures and crystallinity. SrCoOx is a particularly intriguing material owing to a topotactic phase transition between an antiferromagnetic insulating brownmillerite SrCoO2.5 (BM-SCO) phase and a ferromagnetic metallic perovskite SrCoO3-δ (P-SCO) phase depending on the oxygen concentration. Here, we present the formation and control of epitaxial BM-SCO nanostructures by substrate-induced anisotropic strain. Perovskite substrates with a (110)-orientation and which allow for compressive strain result in the creation of BM-SCO nanobars, while (111)-oriented substrates give rise to the formation of BM-SCO nanoislands. We have found that substrate-induced anisotropic strain coupled with the orientation of crystalline domains determines the shape and facet of the nanostructures, while their size can be tuned by the degree of strain. Moreover, the nanostructures can be transformed between antiferromagnetic BM-SCO and ferromagnetic P-SCO via ionic liquid gating. Thus, this study provides insights into the design of epitaxial nanostructures whose structure and physical properties can be readily controlled.

Reversal of Anomalous Hall Effect and Octahedral Tilting in SrRuO3 Thin Films via Hydrogen Spillover

The perovskite SrRuO₃ (SRO) is a strongly correlated oxide whose physical and structural properties are strongly intertwined. Notably, SRO is an itinerant ferromagnet that exhibits a large anomalous Hall effect (AHE) whose sign can be readily modified. Here, a hydrogen spillover method is used to tailor the properties of SRO thin films via hydrogen incorporation. It is found that the magnetization and Curie temperature of the films are strongly reduced and, at the same time, the structure evolves from an orthorhombic to a tetragonal phase as the hydrogen content is increased up to ≈0.9 H per SRO formula unit. The structural phase transition is shown, via in situ crystal truncation rod measurements, to be related to tilting of the RuO₆ octahedral units. The significant changes observed in magnetization are shown, via density functional theory (DFT), to be a consequence of shifts in the Fermi level. The reported findings provide new insights into the physical properties of SRO via tailoring its lattice symmetry and emergent physical phenomena via the hydrogen spillover technique.

All-electrical reading and writing of spin chirality

Spintronics promises potential data encoding and computing technologies. Spin chirality plays a very important role in the properties of many topological and noncollinear magnetic materials. Here, we propose the all-electrical detection and manipulation of spin chirality in insulating chiral antiferromagnets. We demonstrate that the spin chirality in insulating epitaxial films of TbMnO₃ can be read electrically via the spin Seebeck effect and can be switched by electric fields via the multiferroic coupling of the spin chirality to the ferroelectric polarization. Moreover, multivalued states of the spin chirality can be realized by the combined application of electric and magnetic fields. Our results are a path toward next-generation, low-energy consumption memory and logic devices that rely on spin chirality.

Local and global energy barriers for chiral domain walls in synthetic antiferromagnet-ferromagnet lateral junctions

Of great promise are synthetic antiferromagnet-based racetrack devices in which chiral composite domain walls can be efficiently moved by current. However, overcoming the trade-off between energy efficiency and thermal stability remains a major challenge. Here we show that chiral domain walls in a synthetic antiferromagnet-ferromagnet lateral junction are highly stable against large magnetic fields, while the domain walls can be efficiently moved across the junction by current. Our approach takes advantage of field-induced global energy barriers in the unique energy landscape of the junction that are added to the local energy barrier. We demonstrate that thermal fluctuations are equivalent to the magnetic field effect, thereby, surprisingly, increasing the energy barrier and further stabilizing the domain wall in the junction at higher temperatures, which is in sharp contrast to ferromagnets or synthetic antiferromagnets. We find that the threshold current density can be further decreased by tilting the junction without affecting the high domain wall stability. Furthermore, we demonstrate that chiral domain walls can be robustly confined within a ferromagnet region sandwiched on both sides by synthetic antiferromagnets and yet can be readily injected into the synthetic antiferromagnet regions by current. Our findings break the aforementioned trade-off, thereby allowing for versatile domain-wall-based memory, and logic, and beyond.

Three-dimensional racetrack memory devices designed from freestanding magnetic heterostructures

The fabrication of three-dimensional nanostructures is key to the development of next-generation nanoelectronic devices with a low device footprint. Magnetic racetrack memory encodes data in a series of magnetic domain walls that are moved by current pulses along magnetic nanowires. To date, most studies have focused on two-dimensional racetracks. Here we introduce a lift-off and transfer method to fabricate three-dimensional racetracks from freestanding magnetic heterostructures grown on a water-soluble sacrificial release layer. First, we create two-dimensional racetracks from freestanding films transferred onto sapphire substrates and show that they have nearly identical characteristics compared with the films before transfer. Second, we design three-dimensional racetracks by covering protrusions patterned on a sapphire wafer with freestanding magnetic heterostructures. We demonstrate current-induced domain-wall motion for synthetic antiferromagnetic three-dimensional racetracks with protrusions of up to 900 nm in height. Freestanding magnetic layers, as demonstrated here, may enable future spintronic devices with high packing density and low energy consumption.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS2 field-effect transistors

Efficient doping for modulating electrical properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is essential for meeting the versatile requirements for future electronic and optoelectronic devices. Because doping of semiconductors, including TMDCs, typically involves generation of charged dopants that hinder charge transport, tackling Coulomb scattering induced by the externally introduced dopants remains a key challenge in achieving ultrahigh mobility 2D semiconductor systems. In this study, we demonstrated remote charge transfer doping by simply inserting a hexagonal boron nitride layer between MoS₂ and solution-deposited n-type dopants, benzyl viologen. A quantitative analysis of temperature-dependent charge transport in remotely doped devices supports an effective suppression of the dopant-induced scattering relative to the conventional direct doping method. Our mechanistic investigation of the remote doping method promotes the charge transfer strategy as a promising method for material-level tailoring of electrical and optoelectronic devices based on TMDCs.

Skyrmion Echo in a System of Interacting Skyrmions

We consider helical rotation of skyrmions confined in the potentials formed by nanodisks. Based on numerical and analytical calculations we propose the skyrmion echo phenomenon. The physical mechanism of the skyrmion echo formation is also proposed. Because of the distortion of the lattice, impurities, or pinning effect, confined skyrmions experience slightly different local fields, which leads to dephasing of the initial signal. The interaction between skyrmions also can contribute to the dephasing process. However, switching the magnetization direction in the nanodiscs (e.g., by spin transfer torque) also switches the helical rotation of the skyrmions from clockwise to anticlockwise (or vice versa), and this restores the initial signal (which is the essence of skyrmion echo).

Zero-field polarity-reversible Josephson supercurrent diodes enabled by a proximity-magnetized Pt barrier

Simultaneous breaking of inversion and time-reversal symmetries in a conductor yields a non-reciprocal electronic transport^1-3, known as the diode or rectification effect, that is, low (ideally zero) conductance in one direction and high (ideally infinite) conductance in the other. So far, most of the diode effects observed in non-centrosymmetric polar/superconducting conductors^4-7 and Josephson junctions^8-10 require external magnetic fields to break the time-reversal symmetry. Here we report zero-field polarity-switchable Josephson supercurrent diodes, in which a proximity-magnetized Pt layer by ferrimagnetic insulating Y₃Fe₅O₁₂ serves as the Rashba(-type) Josephson barrier. The zero-field diode efficiency of our proximity-engineered device reaches up to ±35% at 2 K, with a clear square-root dependence on temperature. Measuring in-plane field-strength/angle dependences and comparing with Cu-inserted control junctions, we demonstrate that exchange spin-splitting^11-13 and Rashba(-type) spin-orbit coupling^13-15 at the Pt/Y₃Fe₅O₁₂ interface are key for the zero-field giant rectification efficiency. Our achievement advances the development of field-free absolute Josephson diodes.

Heterodimensional superlattice with in-plane anomalous Hall effect

Superlattices-a periodic stacking of two-dimensional layers of two or more materials-provide a versatile scheme for engineering materials with tailored properties1,2. Here we report an intrinsic heterodimensional superlattice consisting of alternating layers of two-dimensional vanadium disulfide (VS2) and a one-dimensional vanadium sulfide (VS) chain array, deposited directly by chemical vapour deposition. This unique superlattice features an unconventional 1T stacking with a monoclinic unit cell of VS2/VS layers identified by scanning transmission electron microscopy. An unexpected Hall effect, persisting up to 380 kelvin, is observed when the magnetic field is in-plane, a condition under which the Hall effect usually vanishes. The observation of this effect is supported by theoretical calculations, and can be attributed to an unconventional anomalous Hall effect owing to an out-of-plane Berry curvature induced by an in-plane magnetic field, which is related to the one-dimensional VS chain. Our work expands the conventional understanding of superlattices and will stimulate the synthesis of more extraordinary superstructures.

Obstructed Surface States as the Descriptor for Predicting Catalytic Active Sites in Inorganic Crystalline Materials

The discovery of new catalysts that are efficient and sustainable is a major research endeavor for many industrial chemical processes. This requires an understanding and determination of the catalytic origins, which remains a challenge. Here, a novel method to identify the position of active sites based on searching for crystalline symmetry-protected obstructed atomic insulators (OAIs) that have metallic surface states is described. The obstructed Wannier charge centers (OWCCs) in OAIs are pinned by symmetries at some empty Wyckoff positions so that surfaces that accommodate these sites are guaranteed to have metallic obstructed surface states (OSSs). It is proposed and confirmed that the OSSs are the catalytic activity origins for crystalline materials. The theory on 2H-MoTe₂ , 1T'-MoTe₂ , and NiPS₃ bulk single crystals is verified, whose active sites are consistent with the calculations. Most importantly, several high-efficiency catalysts are successfully identified just by considering the number of OWCCs and the symmetry. Using the real-space-invariant theory applied to a database of 34 013 topologically trivial insulators, 1788 unique OAIs are identified, of which 465 are potential high-performance catalysts. The new methodology will facilitate and accelerate the discovery of new catalysts for a wide range of heterogeneous redox reactions.

Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn₃Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn₃Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

Giant Spin Hall Effect and Spin-Orbit Torques in 5d Transition Metal-Aluminum Alloys from Extrinsic Scattering

The generation of spin currents from charge currents via the spin Hall effect (SHE) is of fundamental and technological interest. Here, some of the largest SHEs yet observed via extrinsic scattering are found in a large class of binary compounds formed from a 5d element and aluminum, with a giant spin Hall angle (SHA) of ≈1 in the compound Os₂₂ Al₇₈ . A critical composition of the 5d element is found at which there is a structural phase boundary between poorly and highly textured crystalline material, where the SHA exhibits its largest value. Furthermore, a systematic increase is found in the spin Hall conductivity (SHC) and SHA at this critical composition as the atomic number of the 5d element is systematically increased. This clearly shows that the SHE and SHC are derived from extrinsic scattering mechanisms related to the potential mismatch between the 5d element and Al. These studies show the importance of extrinsic mechanisms derived from potential mismatch as a route to obtaining large spin Hall angles with high technological impact. Indeed, it is demonstrated that a state-of-the-art racetrack device has a several-fold increased current-induced domain wall efficiency using these materials as compared to prior-art materials.

All topological bands of all nonmagnetic stoichiometric materials

Topological quantum chemistry and symmetry-based indicators have facilitated large-scale searches for materials with topological properties at the Fermi energy (E_F). We report the implementation of a publicly accessible catalog of stable and fragile topology in all of the bands both at and away from E_F in the 96,196 processable entries in the Inorganic Crystal Structure Database. Our calculations, which represent the completion of the symmetry-indicated band topology of known nonmagnetic materials, have enabled the discovery of repeat-topological and supertopological materials, including rhombohedral bismuth and Bi₂Mg₃. We find that 52.65% of all materials are topological at E_F, roughly two-thirds of bands across all materials exhibit symmetry-indicated stable topology, and 87.99% of all materials contain at least one stable or fragile topological band.

Atomic Scale Control of Spin Current Transmission at Interfaces

Ferromagnet/heavy metal bilayers represent a central building block for spintronic devices where the magnetization of the ferromagnet can be controlled by spin currents generated in the heavy metal. The efficiency of spin current generation is paramount. Equally important is the efficient transfer of this spin current across the ferromagnet/heavy metal interface. Here, we show theoretically and experimentally that for Ta as heavy metal the interface only partially transmits the spin current while this effect is absent when Pt is used as heavy metal. This is due to magnetic moment reduction at the interface caused by 3d-5d hybridization effects. We show that this effect can be avoided by atomically thin interlayers. On the basis of our theoretical model we conclude that this is a general effect and occurs for all 5d metals with less than half-filled 5d shell.

Fermi surface chirality induced in a TaSe2 monosheet formed by a Ta/Bi2Se3 interface reaction

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintronic functionalities. In the present study we address both subjects and investigate a van der Waals-type heterostructure consisting of the topological insulator Bi₂Se₃ and a single Se-Ta-Se triple-layer (TL) of H-type TaSe₂ grown by a method which exploits an interface reaction between the adsorbed metal and selenium. We then show, using surface x-ray diffraction, that the symmetry of the TaSe₂-like TL is reduced from D_3h to C_3v resulting from a vertical atomic shift of the tantalum atom. Spin- and momentum-resolved photoemission indicates that, owing to the symmetry lowering, the states at the Fermi surface acquire an in-plane spin component forming a surface contour with a helical Rashba-like spin texture, which is coupled to the Dirac cone of the substrate. Our approach provides a route to realize chiral two-dimensional electron systems via interface engineering in van der Waals epitaxy that do not exist in the corresponding bulk materials.

Current limitations of spintronics devices based on bulk topological materials stimulate the search for new materials and structures with interesting spin properties. Here the authors report a chiral spin texture around the Fermi level related to structural symmetry breaking in a TaSe₂ layer grown on a Bi2Se3 surface.

Observation of fractional spin textures in a Heusler material

Recently a zoology of non-collinear chiral spin textures has been discovered, most of which, such as skyrmions and antiskyrmions, have integer topological charges. Here we report the experimental real-space observation of the formation and stability of fractional antiskyrmions and fractional elliptical skyrmions in a Heusler material. These fractional objects appear, over a wide range of temperature and magnetic field, at the edges of a sample, whose interior is occupied by an array of nano-objects with integer topological charges, in agreement with our simulations. We explore the evolution of these objects in the presence of magnetic fields and show their interconversion to objects with integer topological charges. This means the topological charge can be varied continuously. These fractional spin textures are not just another type of skyrmion, but are essentially a new state of matter that emerges and lives only at the boundary of a magnetic system. The coexistence of both integer and fractionally charged spin textures in the same material makes the Heusler family of compounds unique for the manipulation of the real-space topology of spin textures and thus an exciting platform for spintronic and magnonic applications.

Skyrmions and anti-skyrmions are magnetic textures that have garnered much interest due to their stability. Here, Jena et al demonstrate the existence of fractional spin textures at the edges of Heusler alloy sample, which can have continuous variable topological charges.

Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO_3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO_2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Enhanced thin film analysis via High Resolution RBS using the NEC CARBS system

Advancements in thin film deposition techniques can now produce films of only a few monolayers in thicknesses, with multiple applications emerging in the nano technology field. This maturing manufacturing technique is driving the need for diagnostics tools able to accurately measure depth profiles. To meet this need, the Compact Automated Rutherford Back-Scattering (CARBS) system is under development at National Electrostatics Corp. (NEC) for nanometer thin film analysis using High-resolution RBS (HRBS) within a 4 x 4meter footprint. We present the recent development of the system and demonstrate a study of HRBS applied to a 30nm CoAl alloy film. We compare the performance of the CARBS system with the conventional NEC HRBS end station and discuss the advantages of HRBS over SIMS method.

Ultrafast Sub-100 fs All-Optical Modulation and Efficient Third-Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films

Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10^-4 % that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump-probe measurements with sub-10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all-optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion, and all-optical processing.

Magnetic Skyrmions in a Thickness Tunable 2D Ferromagnet from a Defect Driven Dzyaloshinskii-Moriya Interaction

There is considerable interest in van der Waals (vdW) materials as potential hosts for chiral skyrmionic spin textures. Of particular interest is the ferromagnetic, metallic compound Fe₃ GeTe₂ (FGT), which has a comparatively high Curie temperature (150-220 K). Several recent studies have reported the observation of chiral Néel skyrmions in this compound, which is inconsistent with its presumed centrosymmetric structure. Here the observation of Néel type skyrmions in single crystals of FGT via Lorentz transmission electron microscopy (LTEM) is reported. It is shown from detailed X-ray diffraction structure analysis that FGT lacks an inversion symmetry as a result of an asymmetric distribution of Fe vacancies. This vacancy-induced breaking of the inversion symmetry of this compound is a surprising and novel observation and is a prerequisite for a Dzyaloshinskii-Moriya vector exchange interaction which accounts for the chiral Néel skyrmion phase. This phenomenon is likely to be common to many 2D vdW materials and suggests a path to the preparation of many such acentric compounds. Furthermore, it is found that the skyrmion size in FGT is strongly dependent on its thickness: the skyrmion size increases from ≈100 to ≈750 nm as the thickness of the lamella is increased from ≈90 nm to ≈2 µm. This extreme size tunability is a feature common to many low symmetry ferro- and ferri-magnetic compounds.

Heusler-based synthetic antiferrimagnets

Antiferromagnet spintronic devices eliminate or mitigate long-range dipolar fields, thereby promising ultrafast operation. For spin transport electronics, one of the most successful strategies is the creation of metallic synthetic antiferromagnets, which, to date, have largely been formed from transition metals and their alloys. Here, we show that synthetic antiferrimagnetic sandwiches can be formed using exchange coupling spacer layers composed of atomically ordered RuAl layers and ultrathin, perpendicularly magnetized, tetragonal ferrimagnetic Heusler layers. Chemically ordered RuAl layers can both be grown on top of a Heusler layer and allow for the growth of ordered Heusler layers deposited on top of it that are as thin as one unit cell. The RuAl spacer layer gives rise to a thickness-dependent oscillatory interlayer coupling with an oscillation period of ~1.1 nm. The observation of ultrathin ordered synthetic antiferrimagnets substantially expands the family of synthetic antiferromagnets and magnetic compounds for spintronic technologies.

Anomalous excitations of atomically crafted quantum magnets

High-energy resolution spectroscopic studies of quantum magnets proved extremely valuable in accessing magnetodynamics quantities, such as energy barriers, magnetic interactions, and lifetime of excited states. Here, we investigate a previously unexplored flavor of low-energy spin excitations for quantum spins coupled to an electron bath. In sharp contrast to the usual tunneling signature of two steps symmetrically centered around the Fermi level, we find a single step in the conductance. Combining time-dependent and many-body perturbation theories, magnetic field-dependent tunneling spectra are explained as the result of an interplay between weak magnetic anisotropy energy, magnetic interactions, and Stoner-like electron-hole excitations that are strongly dependent on the magnetic states of the nanostructures. The results are rationalized in terms of a noncollinear magnetic ground state and the dominance of ferro- and antiferromagnetic interactions. The atomically crafted nanomagnets offer an appealing model for the exploration of electrically pumped spin systems.

Josephson diode effect from Cooper pair momentum in a topological semimetal

Cooper pairs in non-centrosymmetric superconductors can acquire finite centre-of-mass momentum in the presence of an external magnetic field. Recent theory predicts that such finite-momentum pairing can lead to an asymmetric critical current, where a dissipationless supercurrent can flow along one direction but not in the opposite one. Here we report the discovery of a giant Josephson diode effect in Josephson junctions formed from a type-II Dirac semimetal, NiTe₂. A distinguishing feature is that the asymmetry in the critical current depends sensitively on the magnitude and direction of an applied magnetic field and achieves its maximum value when the magnetic field is perpendicular to the current and is of the order of just 10 mT. Moreover, the asymmetry changes sign several times with an increasing field. These characteristic features are accounted for by a model based on finite-momentum Cooper pairing that largely originates from the Zeeman shift of spin-helical topological surface states. The finite pairing momentum is further established, and its value determined, from the evolution of the interference pattern under an in-plane magnetic field. The observed giant magnitude of the asymmetry in critical current and the clear exposition of its underlying mechanism paves the way to build novel superconducting computing devices using the Josephson diode effect.

Long range and highly tunable interaction between local spins coupled to a superconducting condensate

Interfacing magnetism with superconducting condensates is rapidly emerging as a viable route for the development of innovative quantum technologies. In this context, the development of rational design strategies to controllably tune the interaction between magnetic moments is crucial. Here we address this problem demonstrating the possibility of tuning the interaction between local spins coupled through a superconducting condensate with atomic scale precision. By using Cr atoms coupled to superconducting Nb, we use atomic manipulation techniques to precisely control the relative distance between local spins along distinct crystallographic directions while simultaneously sensing their coupling by scanning tunneling spectroscopy. Our results reveal the existence of highly anisotropic interactions, lasting up to very long distances, demonstrating the possibility of crossing a quantum phase transition by acting on the direction and interatomic distance between spins. The high tunability provides novel opportunities for the realization of topological superconductivity and the rational design of magneto-superconducting interfaces.

Molecular Dopant-Dependent Charge Transport in Surface-Charge-Transfer-Doped Tungsten Diselenide Field Effect Transistors

The controllability of carrier density and major carrier type of transition metal dichalcogenides(TMDCs) is critical for electronic and optoelectronic device applications. To utilize doping in TMDC devices, it is important to understand the role of dopants in charge transport properties of TMDCs. Here, the effects of molecular doping on the charge transport properties of tungsten diselenide (WSe₂ ) are investigated using three p-type molecular dopants, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄ -TCNQ), tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue), and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd-COCF₃ )₃ ). The temperature-dependent transport measurements show that the dopant counterions on WSe₂ surface can induce Coulomb scattering in WSe₂ channel and the degree of scattering is significantly dependent on the dopant. Furthermore, the quantitative analysis revealed that the amount of charge transfer between WSe₂ and dopants is related to not only doping density, but also the contribution of each dopant ion toward Coulomb scattering. The first-principles density functional theory calculations show that the amount of charge transfer is mainly determined by intrinsic properties of the dopant molecules such as relative frontier orbital positions and their spin configurations. The authors' systematic investigation of the charge transport of doped TMDCs will be directly relevant for pursuing molecular routes for efficient and controllable doping in TMDC nanoelectronic devices.

Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer

[Figure: see text].

Role of Two-Dimensional Ising Superconductivity in the Nonequilibrium Quasiparticle Spin-to-Charge Conversion Efficiency

Nonequilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana Fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here nonlocal magnon devices to examine how such 2D Ising superconductivity affects the conversion efficiency of magnon spin to quasiparticle charge in superconducting flakes of 2H-NbSe₂ transferred onto ferrimagnetic insulating Y₃Fe₅O₁₂. Comparison with a reference device based on a conventionally paired superconductor shows that the Y₃Fe₅O₁₂-induced in-plane (IP) exchange spin-splitting in the NbSe₂ flake is hindered by its inherent out-of-plane (OOP) spin-orbit field, which, in turn, limits the transition-state enhancement of the spin-to-charge conversion efficiency. Our out-of-equilibrium study highlights the significance of symmetry matching between underlying Cooper pairs and exchange-induced spin-splitting for the giant transition-state spin-to-charge conversion and may have implications toward proximity-engineered spin-polarized triplet pairing via tuning the relative strength of IP exchange and OOP spin-orbit fields in ferromagnetic insulator/2D Ising SC bilayers.

Long-range supercurrents through a chiral non-collinear antiferromagnet in lateral Josephson junctions

The proximity-coupling of a chiral non-collinear antiferromagnet (AFM)^1-5 with a singlet superconductor allows spin-unpolarized singlet Cooper pairs to be converted into spin-polarized triplet pairs^6-8, thereby enabling non-dissipative, long-range spin correlations^9-14. The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase¹⁵ in AFMs with non-collinear atomic-scale spin arrangements^1-5. Here we report long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn₃Ge (refs. ^3-5). The Josephson supercurrents in this chiral AFM decay by approximately one to two orders of magnitude slower than would be expected for singlet pair correlations^9-14 and their response to an external magnetic field reflects a clear spatial quantum interference. Given the long-range supercurrents present in both single- and mixed-phase Mn₃Ge, but absent in a collinear AFM IrMn¹⁶, our results pave a way for the topological generation of spin-polarized triplet pairs^6-8 via Berry phase engineering¹⁵ of the chiral AFMs.

Ionitronic manipulation of current-induced domain wall motion in synthetic antiferromagnets

The current induced motion of domain walls forms the basis of several advanced spintronic technologies. The most efficient domain wall motion is found in synthetic antiferromagnetic (SAF) structures that are composed of an upper and a lower ferromagnetic layer coupled antiferromagnetically via a thin ruthenium layer. The antiferromagnetic coupling gives rise to a giant exchange torque with which current moves domain walls at maximum velocities when the magnetic moments of the two layers are matched. Here we show that the velocity of domain walls in SAF nanowires can be reversibly tuned by several hundred m/s in a non-volatile manner by ionic liquid gating. Ionic liquid gating results in reversible changes in oxidation of the upper magnetic layer in the SAF over a wide gate-voltage window. This changes the delicate balance in the magnetic properties of the SAF and, thereby, results in large changes in the exchange coupling torque and the current-induced domain wall velocity. Furthermore, we demonstrate an example of an ionitronic-based spintronic switch as a component of a potential logic technology towards energy-efficient, all electrical, memory-in-logic.

Synthetic anti-ferromagnets, where two ferromagnetic layers are coupled anti-ferromagnetically via a spacer, are known for their very large current-induced domain wall velocities. Here, Guan et al show that the velocity of the domain walls in synthetic anti-ferromagnetic nanowires can be tuned over a wide range due to reversible oxidization via ionic liquid gating.

Nanoscale Noncollinear Spin Textures in Thin Films of a D2d Heusler Compound

Magnetic nano-objects, namely antiskyrmions and Bloch skyrmions, have been found to coexist in single-crystalline lamellae formed from bulk crystals of inverse tetragonal Heusler compounds with D_2d symmetry. Here evidence is shown for magnetic nano-objects in epitaxial thin films of Mn₂ RhSn formed by magnetron sputtering. These nano-objects exhibit a wide range of sizes with stability with respect to magnetic field and temperature that is similar to single-crystalline lamellae. However, the nano-objects do not form well-defined arrays, nor is any evidence found for helical spin textures. This is speculated to likely be a consequence of the poorer homogeneity of chemical ordering in the thin films. However, evidence is found for elliptically distorted nano-objects along perpendicular crystallographic directions within the epitaxial films, which is consistent with elliptical Bloch skyrmions observed in single-crystalline lamellae. Thus, these measurements provide strong evidence for the formation of noncollinear spin textures in thin films of Mn₂ RhSn. Using these films, it is shown that individual nano-objects can be deleted using a local magnetic field from a magnetic tip and collections of nano-objects can be similarly written. These observations suggest a path toward the use of these objects in thin films with D_2d symmetry as magnetic memory elements.

Vortex-Oriented Ferroelectric Domains in SnTe/PbTe Monolayer Lateral Heterostructures

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

Origin of the quasi-quantized Hall effect in ZrTe5

The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe5 samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe5 electronic structure and its Dirac-type semi-metallic character.

Large Fermi-Energy Shift and Suppression of Trivial Surface States in NbP Weyl Semimetal Thin Films

Weyl semimetals, a class of 3D topological materials, exhibit a unique electronic structure featuring linear band crossings and disjoint surface states (Fermi-arcs). While first demonstrations of topologically driven phenomena have been realized in bulk crystals, efficient routes to control the electronic structure have remained largely unexplored. Here, a dramatic modification of the electronic structure in epitaxially grown NbP Weyl semimetal thin films is reported, using in situ surface engineering and chemical doping strategies that do not alter the NbP lattice structure and symmetry, retaining its topological nature. Through the preparation of a dangling-bond-free, P-terminated surface which manifests in a surface reconstruction, all the well-known trivial surface states of NbP are fully suppressed, resulting in a purely topological Fermi-arc dispersion. In addition, a substantial Fermi-energy shift from -0.2 to 0.3 eV across the Weyl points is achieved by surface chemical doping, unlocking access to the hitherto unexplored n-type region of the Weyl spectrum. These findings constitute a milestone toward surface-state and Fermi-level engineering of topological bands in Weyl semimetals, and, while there are still challenges in minimizing doping-driven disorder and grain boundary density in the films, they do represent a major advance to realize device heterostructures based on Weyl physics.