Signature of pressure-induced topological phase transition in ZrTe5.

The layered van der Waals material ZrTe5 is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe5 nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe5 is a weak TI in ambient conditions.

Anomalies in the Dirac bands in the proximity of correlated electrons.

Dirac fermions, particles with zero rest mass, are observed in topological materials and are believed to play a key role in the exotic phenomena in fundamental science and the advancement of quantum technology. Most of the topological systems studied so far are weakly correlated systems and the study of their properties in the presence of electron correlation is an interesting emerging area of research, where the electron correlation is expected to enhance the effective mass of the particles. Here, we studied the properties of Dirac bands in a non-symmorphic layered Kondo lattice system, CeAgSb2, employing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations. In addition to the Dirac cones due to non-symmorphic symmetry, this material hosts Dirac fermions in the squarenet layer in the proximity of a strongly correlated Ce layer exhibiting Kondo behavior. Experimental results reveal crossings of the highly dispersive linear bands at the Brillouin zone boundary due to non-symmorphic symmetry. In addition, there are anisotropic Dirac cones constituted by the squarenet Sb 5p states forming a diamond-shaped nodal line. These Dirac bands are linear in a wide energy range with an unusually high slope. Interestingly, near the local Ce 4f bands, these bands exhibit a change in the slope akin to the formation of a 'kink' observed in other materials due to electron-phonon coupling. The emergence of such exotic properties in proximity to strongly correlated electronic states has significant implications in the study of complex quantum materials including unconventional superconductors.

Transfer of 2D Films: From Imperfection to Perfection.

Atomically thin 2D films and their van der Waals heterostructures have demonstrated immense potential for breakthroughs and innovations in science and technology. Integrating 2D films into electronics and optoelectronics devices and their applications in electronics and optoelectronics can lead to improve device efficiencies and tunability. Consequently, there has been steady progress in large-area 2D films for both front- and back-end technologies, with a keen interest in optimizing different growth and synthetic techniques. Parallelly, a significant amount of attention has been directed toward efficient transfer techniques of 2D films on different substrates. Current methods for synthesizing 2D films often involve high-temperature synthesis, precursors, and growth stimulants with highly chemical reactivity. This limitation hinders the widespread applications of 2D films. As a result, reports concerning transfer strategies of 2D films from bare substrates to target substrates have proliferated, showcasing varying degrees of cleanliness, surface damage, and material uniformity. This review aims to evaluate, discuss, and provide an overview of the most advanced transfer methods to date, encompassing wet, dry, and quasi-dry transfer methods. The processes, mechanisms, and pros and cons of each transfer method are critically summarized. Furthermore, we discuss the feasibility of these 2D film transfer methods, concerning their applications in devices and various technology platforms.

Large out-of-plane spin-orbit torque in topological Weyl semimetal TaIrTe4.

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin-orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin-torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ / 2e) (Ωm)-1, which is an order of magnitude higher than the reported values in other materials.

Localization and interaction of interlayer excitons in MoSe2/WSe2 heterobilayers.

Transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform to explore unique excitonic physics via the properties of the constituent TMDs and external stimuli. Interlayer excitons (IXs) can form in TMD heterobilayers as delocalized or localized states. However, the localization of IX in different types of potential traps, the emergence of biexcitons in the high-excitation regime, and the impact of potential traps on biexciton formation have remained elusive. In our work, we observe two types of potential traps in a MoSe2/WSe2 heterobilayer, which result in significantly different emission behavior of IXs at different temperatures. We identify the origin of these traps as localized defect states and the moiré potential of the TMD heterobilayer. Furthermore, with strong excitation intensity, a superlinear emission behavior indicates the emergence of interlayer biexcitons, whose formation peaks at a specific temperature. Our work elucidates the different excitation and temperature regimes required for the formation of both localized and delocalized IX and biexcitons and, thus, contributes to a better understanding and application of the rich exciton physics in TMD heterostructures.

A ferromagnetic Eu-Pt surface compound grown below hexagonal boron nitride.

One of the fundamental applications for monolayer-thick 2D materials is their use as protective layers of metal surfaces and in situ intercalated reactive materials in ambient conditions. Here we investigate the structural, electronic, and magnetic properties, as well as the chemical stability in air of a very reactive metal, Europium, after intercalation between a hexagonal boron nitride (hBN) layer and a Pt substrate. We demonstrate that Eu intercalation leads to a hBN-covered ferromagnetic EuPt2 surface alloy with divalent Eu2+ atoms at the interface. We expose the system to ambient conditions and find a partial conservation of the di-valent signal and hence the Eu-Pt interface. The use of a curved Pt substrate allows us to explore the changes in the Eu valence state and the ambient pressure protection at different substrate planes. The interfacial EuPt2 surface alloy formation remains the same, but the resistance of the protecting hBN layer to ambient conditions is reduced, likely due to a rougher surface and a more discontinuous hBN coating.

A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe5 GeTe2 /Graphene Heterostructure.

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe5 GeTe2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe5 GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe5 GeTe2 /graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5 GeTe2 along with the presence of negative spin polarization at the Fe5 GeTe2 /graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

Microwave Synthesized 2D Gold and Its 2D-2D Hybrids.

Xenes, i.e., monoelemental two-dimensional atomic sheets, are promising for sensitive and ultrafast sensor applications owing to exceptional carrier mobility; however, most of them oxidize below 500 °C and therefore cannot be employed for high-temperature applications. 2D gold, an oxidation-resistant plasmonic Xene, is extremely promising. 2D gold was experimentally realized by both atomic layer deposition and chemical synthesis using sodium citrate. However, it is imperative to develop a new facile single-step method to synthesize 2D gold. Here, liquid-phase synthesis of 2D gold is demonstrated by microwave exposure to auric chloride dispersed in dimethylformamide. Microscopies (AFM and high-resolution TEM), spectroscopies (Raman, UV-vis, and X-ray photoelectron), and X-ray diffraction establish the formation of a hexagonal crystallographic phase for 2D gold. 2D-2D hybrids of 2D gold have also been synthesized and investigated for electronic/optoelectronic behaviors and SERS-based molecular sensing. DFT band structure calculation for 2D gold and its hybrids corroborates the experimental findings.

Valley-exchange coupling probed by angle-resolved photoluminescence.

The optical properties of monolayer transition metal dichalcogenides are dominated by tightly-bound excitons. They form at distinct valleys in reciprocal space, and can interact via the valley-exchange coupling, modifying their dispersion considerably. Here, we predict that angle-resolved photoluminescence can be used to probe the changes of the excitonic dispersion. The exchange-coupling leads to a unique angle dependence of the emission intensity for both circularly and linearly-polarised light. We show that these emission characteristics can be strongly tuned by an external magnetic field due to the valley-specific Zeeman-shift. We propose that angle-dependent photoluminescence measurements involving both circular and linear optical polarisation as well as magnetic fields should act as strong verification of the role of valley-exchange coupling on excitonic dispersion and its signatures in optical spectra.

Robust Spin Interconnect with Isotropic Spin Dynamics in Chemical Vapor Deposited Graphene Layers and Boundaries.

The utilization of large-area graphene grown by chemical vapor deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of robust spin interconnects. Here, we report universal spin transport and dynamic properties in specially devised single layer, bilayer, and trilayer graphene channels and their layer boundaries and folds that are usually present in CVD graphene samples. We observe uniform spin lifetime with isotropic spin relaxation for spins with different orientations in graphene layers and their boundaries at room temperature. In all of the inhomogeneous graphene channels, the spin lifetime anisotropy ratios for spins polarized out-of-plane and in-plane are measured to be close to unity. Our analysis shows the importance of both Elliott-Yafet and D'yakonov-Perel' mechanisms with an increasing role of the latter mechanism in multilayer channels. These results of universal and isotropic spin transport on large-area inhomogeneous CVD graphene with multilayer patches and their boundaries and folds at room temperature prove its outstanding spin interconnect functionality, which is beneficial for the development of scalable spintronic circuits.

Unconventional Charge-Spin Conversion in Weyl-Semimetal WTe2.

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, a charge-current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba-Edelstein effects, the measurements indicate an unconventional charge-to-spin conversion in WTe2 , which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin-orbit interaction with a novel spin-texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all-electrical van der Waals spintronic circuits and for low-power and high-performance nonvolatile spintronic technologies.

Gate-tunable spin-galvanic effect in graphene-topological insulator van der Waals heterostructures at room temperature.

Unique electronic spin textures in topological states of matter are promising for emerging spin-orbit driven memory and logic technologies. However, there are several challenges related to the enhancement of their performance, electrical gate-tunability, interference from trivial bulk states, and heterostructure interfaces. We address these challenges by integrating two-dimensional graphene with a three-dimensional topological insulator (TI) in van der Waals heterostructures to take advantage of their remarkable spintronic properties and engineer proximity-induced spin-charge conversion phenomena. In these heterostructures, we experimentally demonstrate a gate-tunable spin-galvanic effect (SGE) at room temperature, allowing for efficient conversion of a non-equilibrium spin polarization into a transverse charge current. Systematic measurements of SGE in various device geometries via a spin switch, spin precession, and magnetization rotation experiments establish the robustness of spin-charge conversion in the Gr-TI heterostructures. Importantly, using a gate voltage, we reveal a strong electric field tunability of both amplitude and sign of the spin-galvanic signal. These findings provide an efficient route for realizing all-electrical and gate-tunable spin-orbit technology using TIs and graphene in heterostructures, which can enhance the performance and reduce power dissipation in spintronic circuits.

Electrical Control of Hybrid Monolayer Tungsten Disulfide-Plasmonic Nanoantenna Light-Matter States at Cryogenic and Room Temperatures.

Hybrid light-matter states-polaritons-have attracted considerable scientific interest recently, motivated by their potential for development of nonlinear and quantum optical schemes. To realize such states, monolayer transition metal dichalcogenides (TMDCs) have been widely employed as excitonic materials. In addition to neutral excitons, TMDCs host charged excitons, which enables active tuning of hybrid light-matter states by electrical means. Although several reports demonstrated charged exciton-polaritons in various systems, the full-range interaction control attainable at room temperature has not been realized. Here, we demonstrate electrically tunable charged exciton-plasmon polaritons in a hybrid tungsten disulfide (WS2) monolayer-plasmonic nanoantenna system. We show that electrical gating of monolayer WS2 allows tuning the oscillator strengths of neutral and charged excitons not only at cryogenic but also at room temperature, both at vacuum and atmospheric pressure. Such electrical control enables a full-range tunable switching from strong neutral exciton-plasmon coupling to strong charged exciton-plasmon coupling. Our experimental findings allow discussing beneficial and limiting factors of charged exciton-plasmon polaritons, as well as offer routes toward realization of charged polaritonic devices at ambient conditions.

Tailoring emergent spin phenomena in Dirac material heterostructures.

Dirac materials such as graphene and topological insulators (TIs) are known to have unique electronic and spintronic properties. We combine graphene with TIs in van der Waals heterostructures to demonstrate the emergence of a strong proximity-induced spin-orbit coupling in graphene. By performing spin transport and precession measurements supported by ab initio simulations, we discover a strong tunability and suppression of the spin signal and spin lifetime due to the hybridization of graphene and TI electronic bands. The enhanced spin-orbit coupling strength is estimated to be nearly an order of magnitude higher than in pristine graphene. These findings in graphene-TI heterostructures could open interesting opportunities for exploring exotic physical phenomena and new device functionalities governed by topological proximity effects.

Hall sensors batch-fabricated on all-CVD h-BN/graphene/h-BN heterostructures.

The two-dimensional (2D) material graphene is highly promising for Hall sensors due to its potential of having high charge carrier mobility and low carrier concentration at room temperature. Here, we report the scalable batch-fabrication of magnetic Hall sensors on graphene encapsulated in hexagonal boron nitride (h-BN) using commercially available large area CVD grown materials. The all-CVD grown h-BN/graphene/h-BN van der Waals heterostructures were prepared by layer transfer technique and Hall sensors were batch-fabricated with 1D edge metal contacts. The current-related Hall sensitivities up to 97 V/AT are measured at room temperature. The Hall sensors showed robust performance over the wafer scale with stable characteristics over six months in ambient environment. This work opens avenues for further development of growth and fabrication technologies of all-CVD 2D material heterostructures and allows further improvements in Hall sensor performance for practical applications.

Electrical gate control of spin current in van der Waals heterostructures at room temperature.

Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin-orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS2). Here we combine graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature. By performing non-local spin valve and Hanle measurements, we unambiguously prove the gate tunability of the spin current and spin lifetime in graphene/MoS2 vdWhs at 300 K. This unprecedented control over the spin parameters by orders of magnitude stems from the gate tuning of the Schottky barrier at the MoS2/graphene interface and MoS2 channel conductivity leading to spin dephasing in high-SOC material. Our findings demonstrate an all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures, which can be key building blocks in future device architectures.

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide.

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS2 (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5-2% has been observed, corresponding to spin polarization of 5-10% in the measured temperature range of 300-75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance.

Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures.

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.

Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators.

Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking of massless Dirac Fermions in their surface states. This helical spin polarization in three-dimensional (3D) TIs has been observed using photoemission spectroscopy up to room temperatures. Recently, spin polarized surface currents in 3D TIs were detected electrically by potentiometric measurements using ferromagnetic detector contacts. However, these electric measurements are so far limited to cryogenic temperatures. Here we report the room temperature electrical detection of the spin polarization on the surface of Bi2Se3 by employing spin sensitive ferromagnetic tunnel contacts. The current-induced spin polarization on the Bi2Se3 surface is probed by measuring the magnetoresistance while switching the magnetization direction of the ferromagnetic detector. A spin resistance of up to 70 mΩ is measured at room temperature, which increases linearly with current bias, reverses sign with current direction, and decreases with higher TI thickness. The magnitude of the spin signal, its sign, and control experiments, using different measurement geometries and interface conditions, rule out other known physical effects. These findings provide further information about the electrical detection of current-induced spin polarizations in 3D TIs at ambient temperatures and could lead to innovative spin-based technologies.

Long distance spin communication in chemical vapour deposited graphene.

Graphene is an ideal medium for long-distance spin communication in future spintronic technologies. So far, the prospect is limited by the smaller sizes of exfoliated graphene flakes and lower spin transport properties of large-area chemical vapour-deposited (CVD) graphene. Here we demonstrate a high spintronic performance in CVD graphene on SiO2/Si substrate at room temperature. We show pure spin transport and precession over long channel lengths extending up to 16 µm with a spin lifetime of 1.2 ns and a spin diffusion length ∼6 µm at room temperature. These spin parameters are up to six times higher than previous reports and highest at room temperature for any form of pristine graphene on industrial standard SiO2/Si substrates. Our detailed investigation reinforces the observed performance in CVD graphene over wafer scale and opens up new prospects for the development of lateral spin-based memory and logic applications.