Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111)

Observation of giant non-reciprocal charge transport from quantum Hall states in a topological insulator

Symmetry breaking in quantum materials is of great importance and can lead to non-reciprocal charge transport. Topological insulators provide a unique platform to study non-reciprocal charge transport due to their surface states, especially quantum Hall states under an external magnetic field. Here we report the observation of non-reciprocal charge transport mediated by quantum Hall states in devices composed of the intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S, which is attributed to asymmetric scattering between quantum Hall states and Dirac surface states. A giant non-reciprocal coefficient of up to 2.26 × 105 A-1 is found. Our work not only reveals the properties of non-reciprocal charge transport of quantum Hall states in topological insulators but also paves the way for future electronic devices.

Coexistence of Magnon-Induced and Rashba-Induced Unidirectional Magnetoresistance in Antiferromagnets

Unidirectional magnetoresistance (UMR) has been intensively studied in ferromagnetic systems, which is mainly induced by spin-dependent and spin-flip electron scattering. Yet, UMR in antiferromagnetic (AFM) systems has not been fully understood to date. In this work, we reported UMR in a YFeO₃/Pt heterostructure where YFeO₃ is a typical AFM insulator. Magnetic-field dependence and temperature dependence of transport measurements indicate that magnon dynamics and interfacial Rashba splitting are two individual origins for AFM UMR, which is consistent with the UMR theory in ferromagnetic systems. We further established a comprehensive theoretical model that incorporates micromagnetic simulation, density functional theory calculation, and the tight-binding model, which explain the observed AFM UMR phenomenon well. Our work sheds light on the intrinsic transport property of the AFM system and may facilitate the development of AFM spintronic devices.

Room-Temperature Gate-Tunable Nonreciprocal Charge Transport in Lattice-Matched InSb/CdTe Heterostructures

Symmetry manipulation can be used to effectively tailor the physical order in solid-state systems. With the breaking of both the inversion and time-reversal symmetries, nonreciprocal magneto-transport may arise in nonmagnetic systems to enrich spin-orbit effects. Here, the observation of unidirectional magnetoresistance (UMR) in lattice-matched InSb/CdTe films is investigated up to room temperature. Benefiting from the strong built-in electric field of 0.13 V nm^-1 in the heterojunction region, the resulting Rashba-type spin-orbit coupling and quantum confinement result in a distinct sinusoidal UMR signal with a nonreciprocal coefficient that is 1-2 orders of magnitude larger than most non-centrosymmetric materials at 298 K. Moreover, this heterostructure configuration enables highly efficient gate tuning of the rectification response, wherein the UMR amplitude is enhanced by 40%. The results of this study advocate the use of narrow-bandgap semiconductor-based hybrid systems with robust spin textures as suitable platforms for the pursuit of controllable chiral spin-orbit applications.

Role of Hidden Spin Polarization in Nonreciprocal Transport of Antiferromagnets

The discovery of hidden spin polarization (HSP) in centrosymmetric nonmagnetic crystals, i.e., spatially distributed spin polarization originated from local symmetry breaking, has promised an expanded material pool for future spintronics. However, the measurements of such exotic effects have been limited to subtle space- and momentum-resolved techniques, unfortunately, hindering their applications. Here, we theoretically predict macroscopic non-reciprocal transports induced by HSP when coupling another spatially distributed quantity, such as staggered local moments in a space-time PT-symmetric antiferromagnet. By using a four-band model Hamiltonian, we demonstrate that HSP plays a crucial role in determining the asymmetric bands with respect to opposite momenta. Such band asymmetry leads to non-reciprocal nonlinear conductivity, exemplified by tetragonal CuMnAs via first-principles calculations. We further provide the material design principles for large nonlinear conductivity, including two-dimensional nature, multiple band crossings near the Fermi level, and symmetry protected HSP. Our Letter not only reveals direct spintronic applications of HSP (such as Néel order detection), but also sheds light on finding observables of other hidden effects, such as hidden optical polarization and hidden Berry curvature.

Bilinear Magnetoresistance in HgTe Topological Insulator: Opposite Signs at Opposite Surfaces Demonstrated by Gate Control

Spin-orbit effects appearing in topological insulators (TI) and at Rashba interfaces are currently revolutionizing how we can manipulate spins and have led to several newly discovered effects, from spin-charge interconversion and spin-orbit torques to novel magnetoresistance phenomena. In particular, a puzzling magnetoresistance has been evidenced as bilinear in electric and magnetic fields. Here, we report the observation of bilinear magnetoresistance (BMR) in strained HgTe, a prototypical TI. We show that both the amplitude and sign of this BMR can be tuned by controlling with an electric gate the relative proportions of the opposite contributions of opposite surfaces. At magnetic fields of 1 T, the magnetoresistance is of the order of 1% and has a larger figure of merit than previously measured TIs. We propose a theoretical model giving a quantitative account of our experimental data. This phenomenon, unique to TI, offers novel opportunities to tune their electrical response for spintronics.

Large magnetoelectric resistance in the topological Dirac semimetal α-Sn

The spin-momentum locking of surface states in topological materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magnetoelectric resistance (BMER) effect offers a new approach for information reading and field sensing applications, but the effects demonstrated so far are too weak or for low temperatures. This article reports the first observation of BMER effects in topological Dirac semimetals; the BMER responses were measured at room temperature and were substantially stronger than those reported previously. The experiments used topological Dirac semimetal α-Sn thin films grown on silicon substrates. The films showed BMER responses that are 10⁶ times larger than previously measured at room temperature and are also larger than those previously obtained at low temperatures. These results represent a major advance toward realistic BMER applications. Significantly, the data also yield the first characterization of three-dimensional Fermi-level spin texture of topological surface states in α-Sn.

Giant bipolar unidirectional photomagnetoresistance

Positive magnetoresistance (PMR) and negative magnetoresistance (NMR) describe two opposite responses of resistance induced by a magnetic field. Materials with giant PMR are usually distinct from those with giant NMR due to different physical natures. Here, we report the unusual photomagnetoresistance in the van der Waals heterojunctions of WSe₂/quasi-two-dimensional electron gas, showing the coexistence of giant PMR and giant NMR. The PMR and NMR reach 1,007.5% at -9 T and -93.5% at 2.2 T in a single device, respectively. The magnetoresistance spans over two orders of magnitude on inversion of field direction, implying a giant unidirectional magnetoresistance (UMR). By adjusting the thickness of the WSe₂ layer, we achieve the maxima of PMR and NMR, which are 4,900,000% and -99.8%, respectively. The unique magnetooptical transport shows the unity of giant UMR, PMR, and NMR, referred to as giant bipolar unidirectional photomagnetoresistance. These features originate from strong out-of-plane spin splitting, magnetic field-enhanced recombination of photocarriers, and the Zeeman effect through our experimental and theoretical investigations. This work offers directions for high-performance light-tunable spintronic devices.NMR).

Circular Photogalvanic Effect in Oxide Two-Dimensional Electron Gases

Two-dimensional electron gases (2DEGs) at the LaAlO_{3}/SrTiO_{3} interface have attracted wide interest, and some exotic phenomena are observed, including 2D superconductivity, 2D magnetism, and diverse effects associated with Rashba spin-orbit coupling. Despite the intensive investigations, however, there are still hidden aspects that remain unexplored. For the first time, here we report on the circular photogalvanic effect (CPGE) for the oxide 2DEG. Spin polarized electrons are selectively excited by circular polarized light from the in-gap states of SrTiO_{3} to 2DEG and are converted into electric current via the mechanism of spin-momentum locking arising from Rashba spin-orbit coupling. Moreover, the CPGE can be effectively modified by the density and distribution of oxygen vacancies. This Letter presents an effective approach to generate and manipulate the spin polarized current, paving the way toward oxide spintronics.

Gate-tuneable and chirality-dependent charge-to-spin conversion in tellurium nanowires

Chiral materials are an ideal playground for exploring the relation between symmetry, relativistic effects and electronic transport. For instance, chiral organic molecules have been intensively studied to electrically generate spin-polarized currents in the last decade, but their poor electronic conductivity limits their potential for applications. Conversely, chiral inorganic materials such as tellurium have excellent electrical conductivity, but their potential for enabling the electrical control of spin polarization in devices remains unclear. Here, we demonstrate the all-electrical generation, manipulation and detection of spin polarization in chiral single-crystalline tellurium nanowires. By recording a large (up to 7%) and chirality-dependent unidirectional magnetoresistance, we show that the orientation of the electrically generated spin polarization is determined by the nanowire handedness and uniquely follows the current direction, while its magnitude can be manipulated by an electrostatic gate. Our results pave the way for the development of magnet-free chirality-based spintronic devices.

Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi

FeSi is a nonmagnetic narrow-gap insulator, exhibiting peculiar charge and spin dynamics beyond a simple band structure picture. Those unusual features have been attracting renewed attention from topological aspects. Although the surface conduction was demonstrated according to size-dependent resistivity in bulk crystals, its topological characteristics and consequent electromagnetic responses remain elusive. Here, we demonstrate an inherent surface ferromagnetic-metal state of FeSi thin films and its strong spin-orbit coupling (SOC) properties through multiple characterizations of two-dimensional conductance, magnetization, and spintronic functionality. Terminated covalent bonding orbitals constitute the polar surface state with momentum-dependent spin textures due to Rashba-type spin splitting, as corroborated by unidirectional magnetoresistance measurements and first-principles calculations. As a consequence of the spin-momentum locking, nonequilibrium spin accumulation causes magnetization switching. These surface properties are closely related to the Zak phase of the bulk band topology. Our findings propose another route to explore noble metal–free materials for SOC-based spin manipulation.

Nonreciprocal Transport in a Rashba Ferromagnet, Delafossite PdCoO2

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO₂, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 ± 0.3 eV Å and argue that our results qualify delafossites as a new family of oxides for nanospintronics and spin-orbitronics, beyond perovskite materials.

Spin-Charge Interconversion in KTaO3 2D Electron Gases

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO₃ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO₃ is reported. 2DEGs are generated by the simple deposition of Al metal onto KTaO₃ single crystals, characterized by angle-resolved photoemission spectroscopy and magnetotransport, and shown to display the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. Their spin-charge interconversion efficiency is then compared with that of STO-based interfaces, related to the 2DEG electronic structure, and perspectives are given for the implementation of KTaO₃ 2DEGs into spin-orbitronic devices is compared.

Synthetic Rashba spin-orbit system using a silicon metal-oxide semiconductor

The spin-orbit interaction (SOI), mainly manifesting itself in heavy elements and compound materials, has been attracting much attention as a means of manipulating and/or converting a spin degree of freedom. Here, we show that a Si metal-oxide- semiconductor (MOS) heterostructure possesses Rashba-type SOI, although Si is a light element and has lattice inversion symmetry resulting in inherently negligible SOI in bulk form. When a strong gate electric field is applied to the Si MOS, we observe spin lifetime anisotropy of propagating spins in the Si through the formation of an emergent effective magnetic field due to the SOI. Furthermore, the Rashba parameter α in the system increases linearly up to 9.8 × 10^-16 eV m for a gate electric field of 0.5 V nm^-1; that is, it is gate tuneable and the spin splitting of 0.6 μeV is relatively large. Our finding establishes a family of spin-orbit systems.

Nonreciprocal charge transport up to room temperature in bulk Rashba semiconductor α-GeTe

Nonmagnetic Rashba systems with broken inversion symmetry are expected to exhibit nonreciprocal charge transport, a new paradigm of unidirectional magnetoresistance in the absence of ferromagnetic layer. So far, most work on nonreciprocal transport has been solely limited to cryogenic temperatures, which is a major obstacle for exploiting the room-temperature two-terminal devices based on such a nonreciprocal response. Here, we report a nonreciprocal charge transport behavior up to room temperature in semiconductor α-GeTe with coexisting the surface and bulk Rashba states. The combination of the band structure measurements and theoretical calculations strongly suggest that the nonreciprocal response is ascribed to the giant bulk Rashba spin splitting rather than the surface Rashba states. Remarkably, we find that the magnitude of the nonreciprocal response shows an unexpected non-monotonical dependence on temperature. The extended theoretical model based on the second-order spin-orbit coupled magnetotransport enables us to establish the correlation between the nonlinear magnetoresistance and the spin textures in the Rashba system. Our findings offer significant fundamental insight into the physics underlying the nonreciprocity and may pave a route for future rectification devices.