Strong Spin-orbit Coupling Research Articles

Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi3Bi5

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi3Bi5, which is isostructural to the AV3Sb5 (A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi3Bi5 from the measured electronic structures. In addition, around the Brillouin zone centre, {{mathbb{Z}}}_{2} nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Large damping-like torque and efficient current-induced magnetization reversal in Ti/Tb–Co/Cr structures

Seeking the magnetic heterostructures with large current-induced torque efficiency is currently one of the core hotspots in spintronics. In this work, we report the large and composition-dependent damping-like (DL) torque in the structure consisting of light metal Cr and Ti layers and a ferrimagnetic Tb–Co layer. The DL torque efficiency in the structures reaches a maximum (around −0.55) as the Tb content in the Tb–Co layer is in the range of 0.15–0.18. This composition-dependent behavior is different from that observed in the usual heavy-metal/ferrimagnetic structures. We also demonstrated the efficient current-induced magnetization reversal in these structures with a low threshold current density down to 8×1010 A/m2. In comparison, only very low efficiency values of −0.06 and −0.086 are obtained in the Ti/Co/Cr and Ti/Tb–Co/SiN control samples, respectively, suggesting that the large DL torque in the Ti/Tb–Co/Cr structures may originate from the orbital Hall effect in the Cr metal. By fitting the Cr layer thickness dependence of the torque efficiency with a simple orbital current diffusion model, we obtained an effective orbital Hall angle of −0.57±0.02 for the Ti/Tb0.85Co0.15/Cr samples. This work demonstrated the possibility to enhance the orbital torque effect by using the magnetic layer containing the element with strong spin–orbit coupling.

Giant spin-splitting and tunable spin-momentum locked transport in room temperature collinear antiferromagnetic semimetallic CrO monolayer

In this work, we present theoretical predictions of a two-dimensional collinear antiferromagnetic semimetal, CrO, which exhibits a giant spin-split band structure, spin-momentum locked transport properties, and a high Néel temperature. Specifically, CrO features two pairs of spin-polarized anisotropic Weyl points at the Fermi level. By manipulating the position of these Weyl points with strain, we demonstrate that four different antiferromagnetic spintronic states with zero net magnetic moments can be achieved, including semimetals with two spin-polarized transport channels, half-semimetals, semiconductors with two spin-polarized transport channels, and half-semiconductors. The strain-induced semiconducting state also preserves the ultra-high carrier mobility of Weyl points, and the bandgap can be easily tuned. These findings offer a good avenue in spintronics without net magnetic moment or strong spin–orbit coupling and could lead to the development of antiferromagnetic materials for spintronic applications.

The Role of Polaronic States on the Spin Dynamics in Solution-Processed Two-Dimensional Layered Perovskite with Different Layer Thickness.

2D lead halide perovskites (LHPs) show strong excitonic and spin-orbit coupling effects, generating a facile spin injection. Besides, they possess a polaron character due to the soft crystal lattice, which can prolong the spin lifetime, making them favorable materials for spintronic applications. Here, the spin dynamics of 2D PEA2 PbI4 (MAPbI3 )n -l thin films with different layers by temperature- and pump fluence-dependent circularly polarization-resolved transient absorption (TA) measurements is studied. These results indicate that the spin depolarization mechanism is gradually converted from the Maialle-Silva-Sham (MSS) mechanism to the polaronic states protection mechanism with the layer number increasing from <n> = 1 to 3, which is determined by the interplay between the strength of Coulomb exchange interaction and the strength of polaronic effect. While for <n> ≥ 4, the Elliot-Yafet (EY) impurities mechanism is proposed, in which the formed polaronic states with free charge carriers no longer play the protective role.

Synthesis and characterization of half-Heusler ScPtBi films via three-source magnetron co-sputtering on Nb superconductor buffer layer

The half-Heusler compounds exhibit simultaneous electrical, optical, and magnetic properties hence extensive research efforts have recently been dedicated to their development in diverse applications. Herein, we detail the fabrication and magnetic characterization of thin film ScPtBi was grown at 300 °C on Nb-buffered Si (100) single crystals (Nb/Si (100)) by using magnetron co-sputtering. X-ray diffraction examinations have unveiled the polycrystalline growth nature of the films. It is seen that the obtained ScPtBi thin film is strained by 1.4% compared to the single crystal of ScPtBi. The morphological observations, conducted by scanning electron microscopy (SEM), revealed that the Nb buffer layer has a smooth and crack-free surface. Moreover, it has been observed that the Bi islands are formed on the surface of ScPtBi film, which is confirmed by the high-resolution scanning transmission electron microscopy (HR-STEM) analyses. The presence of the weak antilocalization (WAL) effect has been observed in the samples. The Hikami-Larkin-Nagaoka (HLN) equation was used to analyze experimental data, aiming to calculate the phase coherence length (lφ) and the coefficient α. As the temperature increased, the phase coherence length decreased from 39 nm to 24 nm. The coefficient α was determined to be in the range of 500 to 1100. As a result, it has been concluded that the presence of the WAL effect might be connected to the strong spin-orbit coupling (SOC) nature of the films instead of from topologically protected surface states.

Antiferromagnetic Spin Fluctuations and Unconventional Superconductivity in Topological Superconductor Candidate YPtBi Revealed by ^{195}Pt-NMR.

We report ^{195}Pt nuclear magnetic resonance (NMR) measurements on topological superconductor candidate YPtBi, which has broken inversion symmetry and topological nontrivial band structures due to the strong spin-orbit coupling. In the normal state, we find that Knight shift K is field- and temperature independent, suggesting that the contribution from the topological bands is very small at low temperatures. However, the spin-lattice relaxation rate 1/T_{1} divided by temperature (T), 1/T_{1}T, increases with decreasing T, implying the existence of antiferromagnetic spin fluctuations. In the superconducting state, no Hebel-Slichter coherence peak is seen below T_{c} and 1/T_{1} follows T^{3} variation, indicating the unconventional superconductivity. The finite spin susceptibility at zero-temperature limit and the anomalous increase of the NMR linewidth below T_{c} point to a mixed state of spin-singlet and spin-triplet (or spin-septet) pairing.

Perspective on antiferromagnetic iridates for spintronics

Antiferromagnetic (AFM) iridates are recently discovered to be a fertile playground for exploring emergent phenomena relevant to the intriguing interplay among multiple degrees of freedom, such as strong spin–orbit coupling, electron correlation, and the crystal field effect. These phenomena offer interesting routes for probing as well as controlling AFM order in iridate, which is essential in AFM spintronics. In this perspective, we will briefly review recent studies on AFM iridates that host large potential for advancing the reading (anisotropic magnetoresistance effect, etc.) and writing (magnetic field control of AFM order, etc.) functionalities of AFM spintronics. We will also discuss promising directions for expanding the research of AFM iridate based spintronics from the perspectives of material growth, manipulation protocol, and characterization technique.

Unconventional hidden Rashba effects in two-dimensional InTe

Much attention has been paid to the hidden Rashba effect exhibiting an intriguing spin-layer locking phenomenon observed in two-dimensional materials with inversion symmetry. This effect provides a new possibility for manipulating Rashba spin polarization even in centrosymmetric materials. We propose a mechanism exhibiting an unconventional hidden Rashba effect showing a unique helical spin texture in which two Fermi circles in different bands have the same spin helicity exhibiting a complete spin separation in space. We demonstrate such an unconventional hidden Rashba effect by showing the unique electronic structures of two-dimensional InTe with inversion symmetry investigated by the first-principles calculations. It is found that spins in both the inner and outer bands with the one helicity are located in the top sublayer whereas spins with the other helicity are in the bottom sublayer indicating a complete spatial spin separation. Strong spin-orbit coupling and a band inversion among two pairs of spin-degenerate bands are the main origins for the unconventional hidden Rashba involving two pairs of spin-degenerate bands rather than one pair, which gets usually involved in the conventional (hidden) Rashba effects. This new type of the hidden Rashba effect observed in two-dimensional InTe would broaden our understanding of the underlying physics of spin polarization phenomena eventually leading to a potential application in future spintronics.

Decoupling multiphase superconductivity from normal state ordering in CeRh2As2

${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ is a multiphase superconductor with ${T}_{\text{c}}=0.26$ K. The two superconducting (SC) phases, SC1 and SC2, observed for a magnetic field $H$ parallel to the $c$ axis of the tetragonal unit cell, have been interpreted as even- and odd-parity SC states, separated by a phase boundary at ${\ensuremath{\mu}}_{0}{H}^{*}=4$ T. Such parity switching is possible due to a strong Rashba spin-orbit coupling at the Ce sites located in locally noncentrosymmetric environments of the globally centrosymmetric lattice. The existence of another ordered state (phase I) below a temperature ${T}_{0}\ensuremath{\approx}0.4$ K suggests an alternative interpretation of the ${H}^{*}$ transition: It separates a mixed SC$+$I (SC1) and a pure SC (SC2) state. Here, we present a detailed study of higher-quality single crystals of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$, showing much sharper signatures at ${T}_{\text{c}}=0.31$ K and ${T}_{0}=0.48$ K. We refine the $T\ensuremath{-}H$ phase diagram of ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ and demonstrate that ${T}_{0}(H)$ and ${T}_{\text{c}}(H)$ lines meet at ${\ensuremath{\mu}}_{0}H\ensuremath{\approx}6$ T, well above ${H}^{*}$, implying no influence of phase I on the SC phase switching. A basic analysis with the Ginzburg-Landau theory indicates a weak competition between the two orders.

Fast and Tunable Phosphorescence from Organic Ionic Crystals.

Crystalline diphosphonium iodides [MeR2P-spacer-R2Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (λem values from 550 to 880 nm) and intensities (Φem reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-π interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 μs at 297 K. Radiative rate constants kr as high as 2.8 × 105 s-1 deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-π charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design of photofunctional and responsive molecular materials.

Intrinsic orbital and spin Hall effect in bismuth semimetal

We investigate the intrinsic orbital Hall conductivity (OHC) and spin Hall conductivity (SHC) in a bismuth semimetal, by employing an $sp$-orbital tight-binding model. We report a notable difference between the anisotropy of OHC and SHC whose orbital and spin polarizations lie within the basal plane. We do not observe a substantial correlation between the orbital and spin Berry curvatures with spin-orbit coupling (SOC) at the Fermi energy, disproving the correlation between OHC and SHC in a strong SOC regime. We argue the huge SHC in a Bi semimetal is attributed to its gigantic SOC which strongly affects the hybridization of the $p$ orbitals, despite its relatively small magnitude of OHC. We hope the distinct anisotropy of OHC and SHC provides a feasible means to differentiate between the two effects in experiments.

Evidence of charge transfer in three-dimensional topological insulator/antiferromagnetic Mott insulator Bi1Sb1Te1.5Se1.5/α−RuCl3 heterostructures

We report magnetotransport measurements on ${\mathrm{Bi}}_{1}{\mathrm{Sb}}_{1}{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}/{\mathrm{RuCl}}_{3}$ heterostructure nanodevices. ${\mathrm{Bi}}_{1}{\mathrm{Sb}}_{1}{\mathrm{Te}}_{1.5}{\mathrm{Se}}_{1.5}$ (BSTS) is a strong three-dimensional topological insulator (3D-TI) that hosts conducting topological surface states (TSS) enclosing an insulating bulk. $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ (namely, ${\mathrm{RuCl}}_{3}$) is an antiferromagnet that is predicted to behave as a Kitaev-like quantum spin liquid carrying chargeless Majorana excitations. Temperature ($T$)-dependent resistivity measurements on BSTS show the interplay between parallel bulk and surface transport channels. For $T&lt;150$ K, surface state transport dominates over bulk transport. Multichannel weak antilocalization (WAL) is observed as a sharp cusp in the low-field magnetoconductivity, indicating strong spin-orbit coupling. The presence of top and bottom topological surface states (TSS) including a pair of electrically coupled Rashba surface states (RSS) is indicated. The nonlinear Hall effect, explained by a two-band model, further supports this interpretation. We observed that ${\mathrm{RuCl}}_{3}$ has an electrostatic effect on the TSS of BSTS, that is, it changes bandbending at the interface as a result of charge transfer. In this regard, the change in low-$T$ logarithmic resistivity upturn in the presence of out-of-plane magnetic field is analyzed employing the Lu-Shen model, supporting the presence of gapless surface states (TSS and RSS) with a $\ensuremath{\pi}$ Berry phase.

Ultrafast phonon diffuse scattering as a tool for observing chiral phonons in monolayer hexagonal lattices

At the 2D limit, hexagonal systems such as monolayer transition metal dichalcogenides (TMDs) and graphene exhibit unique coupled spin and momentum-valley physics (valley pseudospin) owing to broken spatial inversion symmetry and strong spin-orbit coupling. Circularly polarized light provides the means for pseudospin-selective excitation of excitons (or electrons and holes) and can yield momentum-valley polarized populations of carriers that are the subject of proposed valleytronic applications. The chirality of these excited carriers has important consequences for the available relaxation/scattering pathways, which must conserve (pseudo)angular momentum as well as energy. One available relaxation channel that satisfies these constraints is coupling to chiral phonons. Here, we show that chiral carrier-phonon coupling following valley-polarized photoexcitation is expected to lead to a strongly valley-polarized chiral phonon distribution when this relaxation mechanism is dominant. This momentum valley phonon polarization is directly measurable using ultrafast phonon diffuse scattering techniques. Using ab initio calculations, we show how the dynamic phonon occupations and valley anisotropy determined by nonequilibrium observations can provide a new window on the physical processes that drive carrier valley depolarization in monolayer TMDs.

Low hydrogen coverage [formula omitted] topological phase transition in plumbene

Two-dimensional group IV monolayers with low-buckled honeycomb structures are reported as Z2 topological insulators. However, despite the strong spin–orbit coupling, the 2D lead monolayer (known as plumbene) exhibits a trivial insulator electronic state. Here we have studied the possible trivial to Z2 topological phase transition in plumbene monolayer with low-density H decoration using the density-functional theory calculations. The adsorption of the H atom on the plumbene monolayer is energetically favorable. We examined different configurations for the adsorption of one and two H atoms on the plumbene surface. The H decoration of the surface causes the trivial to Z2 topological phase transition in the plumbene monolayer. The topological phase transition depends on the density and order of H atoms on the plumbene surface.

Hydrogenic spin-valley states of the bromine donor in 2H-MoTe2

In semiconductors, the identification of doping atomic elements allowing to encode a qubit within spin states is of intense interest for quantum technologies. In transition metal dichalcogenides semiconductors, the strong spin-orbit coupling produces locked spin-valley states with expected long coherence time. Here we study the substitutional Bromine BrTe dopant in 2H-MoTe2. Electron spin resonance measurements show that this dopant carries a spin with long-lived nanoseconds coherence time. Using scanning tunneling spectroscopy, we find that the hydrogenic wavefunctions associated with the dopant levels have characteristics spatial modulations that result from their hybridization to the Q-valleys of the conduction band. From a Fourier analysis of the conductance maps, we find that the amplitude and phase of the Fourier components change with energy according to the different irreducible representations of the impurity-site point-group symmetry. These results demonstrate that a dopant can inherit the locked spin-valley properties of the semiconductor and so exhibit long spin-coherence time.

Quantized Spin Pumping in Topological Ferromagnetic-Superconducting Nanowires.

Semiconducting nanowires with strong spin-orbit coupling in the presence of induced superconductivity and ferromagnetism can support Majorana zero modes. We study the pumping due to the precession of the magnetization in single-subband nanowires and show that spin pumping is robustly quantized when the hybrid nanowire is in the topologically nontrivial phase, whereas charge pumping is not quantized. Moreover, there exists one-to-one correspondence between the quantized conductance, entropy change and spin pumping in long topologically nontrivial nanowires but these observables are uncorrelated in the case of accidental zero-energy Andreev bound states in the trivial phase. Thus, we conclude that observation of correlated and quantized peaks in the conductance, entropy change and spin pumping would provide strong evidence of Majorana zero modes, and we elaborate how topological Majorana zero modes can be distinguished from quasi-Majorana modes potentially created by a smooth tunnel barrier at the lead-nanowire interface. Finally, we discuss peculiar interference effects affecting the spin pumping in short nanowires at very low energies.

Exchange Interaction‐Driven Surface State Hybridization in Bi2Se3Topological Insulator

AbstractTopological insulator has gapless surface state, which results from bulk band inversion due to strong spin–orbit coupling. This nontrivial topology robust to nonmagnetic disorders and defects is considered one of the biggest obstacles for modulations of the surface state. In this work, the suppression of surface properties in Bi2Se3of various thicknesses grown on antiferromagnetic NiO, which has a strong exchange interaction is investigated. Under perpendicular magnetic fields, a drastic decrease in mobilityµand in the number of phase coherent channelsαin 5 QL Bi2Se3on NiO are observed. In addition, the THz transmission study shows that an increase in the surface penetration depthξcan accelerate the hybridization of surface states, which is also verified using the optical pump THz probe. This rapid collapse of surface states indicates the unique role of antiferromagnetic materials in band overlap, suggesting that the topological surface nature can be modulated by forming an antiferromagnet‐topological insulator heterostructure.

Ab Initio Nonadiabatic Molecular Dynamics of Ultrafast Demagnetization in Iron.

Ultrafast demagnetization in magnetic metals is the key to spintronics devices. Taking iron as a prototypical system, we investigate the demagnetization mechanism by simulating the charge and spin dynamics using nonadiabatic molecular dynamics in the presence of explicit spin-orbit coupling (SOC). Strong SOC drives ultrafast spin-flip of electrons and holes, which trigger demagnetization and remagnetization, respectively. Their confrontation decreases the demagnetization ratio and finishes the demagnetization within 167 fs, agreeing with the experimental time scale. The joint spin-flip of electrons and holes correlated with the electron-phonon coupling-induced fast electron-hole recombination further decreases the maximum demagnetization ratio, below 5% of experimental value. Although the Elliott-Yafet electron-phonon scattering model can rationalize the ultrafast spin-flip process, it fails to reproduce the experimental maximum demagnetization ratio. The study suggests the key role of SOC on spin dynamics and emphasizes the interplay between SOC and electron-phonon interactions on the ultrafast demagnetization.

Electronic structure and topological phase transition in multi-orbital triangular lattice with rotation and mirror symmetry breaking

Triangular lattice is one of the most generic structures in the two-dimensional (2D) limit with numerous exotic properties, such as magnetic frustration and quantum spin liquid. Here, we investigate the electronic properties of a multi-orbital (p x , p y , p z ) triangular lattice while focusing on the role of rotation and mirror symmetry breaking in regulating the topological phase diagram. The tight-binding modeling reveals that rotation symmetry () breaking splits the Dirac point and preserves the nodal loop. A further inclusion of broken horizontal mirror symmetry () destroys the nodal loop. Consequently, with spin–orbit coupling (SOC), the (p x , p y , p z )-orbital triangular lattice is topologically non-trivial with only or symmetry breaking. With both symmetries breaking, it becomes trivial, except when the SOC is large enough to initiate band inversion. For real materials, we propose that the recently grown AgSe/AgTe monolayer in a binary honeycomb structure is an ideal system to realize the multi-orbital triangular lattice. The first-principles calculations demonstrate that free-standing AgSe and AgTe are Z 2 non-trivial. On the Ag(111) substrate, AgSe becomes trivial because of the breaking of both and symmetries, whereas AgTe remains non-trivial because of the strong SOC of Te. Our work not only provides physical insights into the experimentally synthesized 2D binary honeycomb structures but also highlights the manipulation of electronic and topological properties through symmetry/orbital physics, which are valuable for designing new quantum materials and devices.

Chiral Hybrid Perovskites (R-/S-CLPEA)4Bi2I10 with Enhanced Chirality and Spin–Orbit Coupling Splitting for Strong Nonlinear Optical Circular Dichroism and Spin Selectivity Effects

Chiral hybrid halide perovskites have attracted considerable attention in optoelectronics and spintronics owing to their unique optical, electric, and spin–orbit coupling (SOC) properties. In this article, with focus on the design scheme to increase the nonlinear optical (NLO) circular dichroism (CD) and spin selectivity effects by assembling chlorine-modified chiral R-/S-CLPEA [CLPEA = 1-(4-chlorophenyl)ethylamine] into bismuth-based perovskites, a pair of chiral hybrid perovskites (R-/S-CLPEA)4Bi2I10 were synthesized and characterized experimentally. A large NLO CD effect and strong spin-dependent charge transport were confirmed by the experimental measurements. The unique chirality-induced spin textures were illustrated in the SOC band structures from first-principles simulations. The combined theoretical and experimental results demonstrate that it is the synergic effect of large chirality of CLPEA and strong SOC effect of the Bi2I10 dimer that endows these perovskites with unique chiral spin textures, strong NLO CD effect, and remarkable spin-dependent charge transport properties, which are of importance for the functional design of chiral perovskites in nonlinear optics and spintronics.