Session N52: Spins in Topological and Spin-Orbit Materials I

Robust Topological Spintronics with Spin-Valley-Momentum Locking

Two-dimensional topological insulators offer a tantalizing prospect that their topological edge states are spin-momentum locked, supporting the quantum spin Hall (QSH) effect. However, such QSH states are usually fragile or limited to cryogenic temperatures, which can be easily destroyed by magnetic impurities [1]. A similar situation arises in quantum valley Hall (QVH) insulators, where the resulting valley-momentum locking unlocks easily with short-range impurities and the conductance quantization disappears [2]. To overcome these challenges, we propose a planar junction formed by the QSH and QVH insulators, where the QVH and QSH edge states can simultaneously emerge along their interface, giving a novel quantum spin-valley Hall kink (QSVHK) states [3]. Unlike the single QSH (QVH) states, such QSVHK states are robust against different disturbances and imperfections, showing ballistic spin-valley-momentum locking transport even at room temperature [3]. Based on first-principles results and our fabricated samples, we show how the QSVHK states can be realized by gate control, alloy engineering, or surface functionalization in bismuthene [3]. We further reveal how the interplay between the QSH (QVH) and quantum anomalous Hall states can generate multiple Hall effects through the electric or magnetic control [4-6], where their tunable band topology can be optically probed [7]. Such multiple Hall effects are highly tunable and feasibly realized, paving an important step towards topological spintronics and valleytronics.

 

[1] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).

[2] J. Li et al., Nat. Nanotechnol. 11, 1060 (2016).

[3] T. Zhou et al., Phys, Rev. Lett. 127, 116402 (2021).

[4] T. Zhou et al., npj Quant. Mater. 3, 39 (2018).

[5] T. Zhou et al., Phys, Rev. B 94, 235449 (2016).

[6] T. Zhou et al., Nano Lett. 15, 5149 (2015).

[7] G. Xu, T. Zhou et al., Phys. Rev. Lett. 125, 157402 (2020).   

First Principles study of transport properties 1T’- WTe2monolayer

2D transition metal dichalcogenides have drawn special attention to due to their spintronic and valleytronic properties over the past decade. Intriguing physical properties emerge from 1T’-WTe₂monolayer, is a topological insulator and exhibits quantized spin hall conductance up to a record-high phase transition temperature of 100 K. 1T’-WTe₂ sheets exhibit a band gap below 100K, but are metallic above. We investigate charge and spin transport in 1T’-WTe₂ monolayer in semimetal and semiconductor phases at finite temperatures. We applied our recently-developed first-principles density matrix dynamics (FPDM) with ab initio electron-phonon and electron-impurity scattering and self-consistent spin-orbit coupling [1].  We find the spin lifetime increases with reducing temperature in both phases. Predicted mobility in the semiconducting phase agrees reasonably with experimental measurements. Spin and carrier lifetimes are linearly proportional, according to the Elliot-Yafet (EY) mechanism. Under an external electric field, spin relaxation shows a distinct character that deviates from both the EY and D'yakonov-Perel' mechanisms. Our FPDM method provides insights into spin relaxation in 2D quantum materials beyond the traditional mechanistic regimes.

[1] J. Xu et al, Phys. Rev. B, 2021.   

Edge spin transport in disordered two-dimensional topological insulators

The spin conductance of two-dimensional topological insulators (2D TIs) is not expected to be quantized in the presence of perturbations that break the spin-rotational symmetry. However, the deviation from the pristine-limit quantization has yet to be studied in detail. In this paper we define, for the first time, the spin current operator for the helical edge modes of a 2D TI. Using the developed formalism, we consider the effects of disorder terms that break spin-rotational symmetry or give rise to edge-to-edge coupling. We then utilize a tight-binding model of topological monolayer WTe₂ and scattering matrix formalism to numerically study spin transport in a four-terminal 2D TI device. In particular, we calculate the spin conductances and characteristic spin decay length in the presence of scalar and magnetic disorder. In addition, we study the effects of inter-edge scattering in a quantum point contact geometry. We find that the spin Hall conductance is surprisingly robust to symmetry-breaking perturbations as long as inter-edge scattering is weak.   

3D C-paired Spin-Valley Locking materials in collinear antiferromagnetic order

C-paired spin-valley locking(SVL) refers that the spin and valley degree of freedoms are locked by the real space crystal symmetries. Materials with C-paired SVL can generate piezomagnetism(PZM) and transverse spin current. In this letter we first propose the general theory of C-paired SVL and conclude 47 magnetic point groups compatible with collinear antiferromagnetic(AFM) order that one C-paired SVL material must belong to. Depending on how the spin orientations of locked valleys coupled, we classified C-paired SVL into three types: collinear, non-collinear, and collinear + non-collinear. From these 3D collinear AFM materials fabricated in experiments, we predict a series of materials realizing C-paired SVL in the assistance of our package that classifies all momenta in every specific magnetic space group. Among them, CoF₂ as the representative material of PZM shows large PZM and spin Hall angle by first-principles calculations. Our study and package build the foundation for identifying and classifying the C-paired SVL phenomenon in 3D magnetic materials and provide numerous materials that own desired controllable magnetic and electronic properties.   

Valley-Polarized Spin Splittings in Magnetized Pt Dichalcogenides

Using first-principles simulations, we show that pristine and Janus Pt dichalcogenides thin films (such as SePtTe) when deposited on the magnetic substrate α-MnSe develop giant proximity-induced magnetism and spin-valley polarization. Based on the analysis of symmetries of the system, we demonstrate that these effects are the result of an interplay of charge redistribution, broken inversion and time-reversal symmetries together with spin-orbit interactions. These mechanisms concur to the formation of unequal K and K' valleys in the conduction bands characterized by spin-splittings of the order of hundreds of meV. Remarkably, these properties are independent of the layer thickness and are present in both few-layers (semiconducting) and multilayered (metallic) heterostructures. Our results provide a platform for exploring novel spin-valley physics in low-dimensional materials, with promising applications in spintronic systems displaying spin-orbit torques that are resilient to disorder and temperature effects.   

Tunable spin-charge conversion in topological Dirac semimetals

We theoretically demonstrate that topological Dirac semimetals (TDSMs) can provide a platform for realizing both electrically and magnetically tunable spin-charge conversion. With time-reversal symmetry, the spin component along the rotation axis (z-axis) is approximately conserved, which leads to an anisotropic spin Hall effect—the spin Hall current relies on the relative orientation between the external electric field and the z-axis. The application of a magnetic field, on the other hand, breaks time-reversal symmetry, driving the TDSM into a Weyl semimetal phase and thus, partially converting the spin current to a charge Hall current. Using the Kubo formulas, we numerically evaluate the spin and charge Hall conductivities based on a low-energy TDSM Hamiltonian together with the Zeeman coupling. Besides the conventional spin Hall conductivity element σ_xy^z, we find that unconventional components, such as σ_xy^x and σ_xy^y, also exist and vary as the magnetic field rotates. The charge Hall conductivity also exhibits appreciable tunability that is distinct from the spin Hall conductivities. We show that such tunability originates from the interplay of symmetry and band topology of the TDSMs.   

Spin Hall conductivity of polycrystalline TaAs thin films grown by molecular beam epitaxy

We report the synthesis and characterization of polycrystalline TaAs thin films grown by molecular beam epitaxy. We used transmission electron microscopy, energy-dispersive X-ray spectroscopy and reflection high energy electron diffraction to confirm the polycrystallinity and composition of our films. Furthermore, we combined them with a soft ferromagnet (NiFe) and used spin torque ferromagnetic resonance (ST-FMR) to study the charge to spin conversion in the heterostructure. We performed electrical transport measurements and experimentally determined the spin Hall conductivity of polycrystalline TaAs in the thin film regime. Finally, we compare our results with the charge to spin conversion produced by the spin Hall effect in a NiFe/Ta control sample.   

Room Temperature Spin Transport in Cd3As2

As the physical limits of CMOS loom closer, alternative state variable paradigms become increasingly important. Devices utilizing the electron spin as a state variable are especially promising due to their intrinsic non-volatility, speed, and versatility. Fully incorporating spintronic devices into next-generation computing systems requires optimized architectures and materials capable of efficiently harnessing the electron spin. One particularly promising class of materials are topological Dirac semimetals (TDS), exemplified by Cd₃As₂. TDS materials have high mobilities, 3D Dirac cones, and can exist in multiple quantum phases. We demonstrate the function of Cd₃As₂ as a channel for the flow of spin currents by incorporating it with hybrid graphene/MgO tunnel barriers as a non-local spin valve, the basic unit of spintronic devices for logic operations. We show that the spin valves operate at least up to room temperature.[1] We quantify the spintronic transport in the devices by measuring the spin Hall effect/inverse spin Hall effect, observing spin Hall angles up to θ_SH = 1.5 and spin diffusion lengths of 10-40 µm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd₃As₂, are enabling steps toward realizing practical spintronic-based computing systems.   

Momentum-dependent quasiparticle lifetime in ferromagnetic EuCd­2As2arising from spin-selective scattering

The development of long-range electronically ordered phases impacts the properties of quasiparticles in a material. In a ferromagnetic (FM) metal, the electronic bands are spin split due to an effective field caused by the magnetic order. The onset of FM order often results in a sharp decrease of resistivity because of a loss of spin-disorder scattering. Here, we report on a more direct observation of the effects of the coupling between electrons and ferromagnetic order via ARPES measurements in the newly discovered ferromagnetic metal EuCd₂As₂. Our study reveals a significant increase of the quasiparticle lifetime below the ferromagnetic transition temperature, yet only in selected bands and energy ranges. In particular, the majority band is observed to be much sharper than the minority band, which leads to spin selective transport behavior useful in spintronic applications. Using analytical theory, we show that the phenomenon can be naturally explained by spin-selective scattering and a competition between magnon and disorder scattering. Our theory can account for the momentum and energy dependence enhanced quasiparticle lifetime over a wide range of temperatures.    

Spin-orbit coupling and spin-momentum locking in proustite

Nonmagnetic materials with unidirectional spin-momentum locking, or persistent spin texture (PST), have been shown to exhibit electronic spin transport properties conducive to spintronics applications. Most reported PSTs in bulk materials are observed near high symmetry time-reversal invariant momenta (TRIMs) and are protected by the present symmetries - these PSTs are known as symmetry-protected PSTs (Tao and Tsymbal 2018). In this talk, we will show the presence of a bulk PST in Ag₃AsS₃ which is not strictly symmetry-protected and discuss the characteristics of the spin-orbit coupling (SOC) which allow PSTs to exist near low symmetry TRIMs. We will show how chemical substitutions influence the band dispersion and SOC and discuss the implications for the material’s PST. Our work provides improved understanding of PSTs and how they may be designed in complex dielectrics without requiring particular crystallographic symmetries.

Tao, L. L., and Evgeny Y. Tsymbal. 2018. “Persistent Spin Texture Enforced by Symmetry.” Nature Communications 9 (1): 2763.   

Discovery Principles and Materials for Symmetry-Protected Persistent Spin Textures with Long Spin Lifetimes

Persistent spin textures (PST) in solid-state materials arise from a unidirectional spin-orbit field in momentum space and offer a route to deliver the necessary long carrier spin lifetimes utilized in future quantum microelectronic devices. Nonetheless, few bulk materials host PST owing to crystal symmetry and chemical requirements with even fewer experimentally demonstrated examples. This scarcity makes both PST materials discovery and performance assessment challenging. Here we demonstrate that bulk persistent spin textures exist in the family of layered A₃B₂O₇ oxides and that a persistent spin helix (PSH) occurs over a large region of the Brillouin zone—a feature essential to experimental realization. By solving the spin diffusion equations in the strong coupling limit and using the PST performance criteria we formulate herein, we find that the spin lifetime of the PSH is ~63ns in Sr₃Hf₂O₇—substantially larger than that predicted (~10 ns) and demonstrated experimentally (~600 ps) in GaAs/AlGaAs quantum wells. Last, we use group theory analysis and electronic structure calculations to formulate general discovery principles to identify PST in more materials.    

Intrinsic Spin-Charge Conversion in Excitonic Pseudospin Superfluid

Spin-charge conversion by inverse spin Hall effect or inverse Rashba-Edelstein effect is prevalent in spintronics but dissipative. We propose a dissipationless spin-charge conversion mechanism by an excitonic pseudospin superfluid in an electron-hole double layer system. Magnetic exchange fields lift singlet-triplet degeneracy of interlayer exciton levels. Condensation of the singlet-triplet hybridized excitons breaks both a U(1) gauge symmetry and a pseudospin rotational symmetry around the fields. By a quantum-dot model, we derive spin-charge coupled Josephson equations for an excitonic superflow in the double layer system.   

Spin-orbit unrelated spin splitting in real antiferromagnets

Our recent studies (Phys. Rev. B 102, 014422 (2020); Phys. Rev. Materials 5, 014409​ (2021)) point out an unconventional momentum-dependent spin splitting (SS) in certain antiferromagnets, present even without spin-orbit coupling (SOC) or inversion symmetry breaking. We developed the theory of magnetic and space symmetry conditions enabling different spin splitting types (SST's). Using such "Design Principles" to guide DFT material search points to specific (but not all) antiferromagnetic compounds  manifesting such spin splitting. First principles calculations provide specific predictions of spin splitting vs. momentum and spin texture, awaiting experimental testing. The idea of turning on or off the spin splitting via enabling or removing such symmetry conditions (e.g via specific atomic distortions) has been examined for the prototype case of NiO than in its undistorted AFM state lacks SS. This reveals the significant role of the nonmagnetic ligands in mediating indirect magnetic interactions (such as superexchange). The proposed alternative mechanism to spin splitting opens possibilities for discovery of novel light-atom antiferromagnets that promote effects previously tested for SOC systems such as spin-current generation and current-driven spin excitations.