Carrier Spin Research Articles

Emergence of cubic ordered full-plane persistent spin textures in lead-free materials

The materials preserving a uniform spin configuration in the momentum space, known as persistent spin texture (PST), provide long carrier spin lifetimes through persistent spin-helix mechanism. However, most of the PSTs studied until now are attributed to the linear in $\mathbit{k}$ splitting and cease to exist locally around certain high-symmetry points of the first Brillouin zone (BZ). The PSTs with purely cubic spin splittings (PCS) have drawn attention owing to unique benefits in spin transport. Here, using relativistic first-principles calculations supplemented with $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ analysis, we report the emergence of PCS belonging to ${D}_{3h}$ point group, which is enforced by in-plane mirror and threefold rotation operations. In addition, the in-plane mirror symmetry operation sustains the PST in a larger region, i.e., full planes of BZ alongside giant spin splitting. The observed PSTs provide a route to nondephasing spin transport with larger spin Hall conductivity, thereby offering a promising platform for future spintronics devices.

Full-zone persistent spin textures with giant spin splitting in two-dimensional group IV–V compounds

Persistent spin texture (PST), a property of solid-state materials maintaining unidirectional spin polarization in the momentum k-space, offers a route to deliver the necessary long carrier spin lifetimes through the persistent spin helix (PSH) mechanism. However, most of the discovered PST locally occurred in the small part around certain high symmetry k-points or lines in the first Brillouin zone (FBZ), thus limiting the stability of the PSH state. Herein, by symmetry analysis and first-principles calculations, we report the emergence of full-zone PST (FZPST), a phenomenon displaying the PST in the whole FBZ, in the two-dimensional group IV–V (A = Si, Sn, Ge; B = Bi, Sb) compounds. Due to the existence of the in-plane mirror symmetry operation in the wave vector point group symmetry for the arbitrary in the whole FBZ, fully out-of-plane spin polarization is observed in the k-space, thus maintaining the FZPST. Importantly, we observed giant spin splitting in which the PST sustains, supporting large spin–orbit coupling parameters and small wavelengths of the PSH states. Our analysis demonstrated that the FZPST is robust for the non-degenerate bands, which can be effectively controlled by the application of an external electric field, thus offering a promising platform for future spintronic applications.

First-principles studies of the mixed-dimensional van der Waals heterostructures of graphene/MnF4

Constructing a mixed-dimensional (MD) graphene-based van der Waals heterostructure (vdWH) is a viable technique for opening the bandgap and introducing spin polarization in graphene. In this work, we discovered that the adjacent MnF4 can manipulate the carrier doping, bandgap opening, and spin polarization of graphene in the MD vdWH of graphene/MnF4 comprised of two-dimensional (2D) graphene and one-dimensional atomic wire (1D AW) MnF4. By adopting first-principles calculations, we found that graphene can achieve effective p-type doping with the carrier density up to ∼8.89 × 1013–1.03 × 1014 cm−2. With a twisted angle of θ = 10.89° and the compressed distance of dMn-Gra = 2.84 Å, the opened bandgap of graphene (Eg-Gra) achieves 35 and 57 meV for spin-up and spin-down channels due to the sublattice symmetry-breaking in graphene, and the spin splitting energy (ΔES) at the Dirac point reaches 78.7 meV as a result of the graphene–MnF4 interlayer interaction. Remarkably, Eg-Gra is increased to 64 and 79 meV for spin-up and spin-down channels, and ΔES with 202.7 meV is obtained at dMn-Gra = 2.84 Å when the width of 1D MnF4 is doubled. Meanwhile, the n-type Ohmic contact is also realized. Our work underscores the rich interplay in the graphene/MnF4 MD vdWH and provides a significant route with fundamental insights to engineer the spintronic band properties of graphene.

Band tilt induced nonlinear Nernst effect in topological insulators: An efficient generation of high-performance spin polarization

Topological insulators (TIs) hold promise as a platform for spintronics applications due to the fascinating spin-momentum locking (SML) of the surface states. One particular interest lies in using TIs as spin-polarized sources for spintronics structures. Here, we propose the band tilt induced nonlinear Nernst effect (NLNE) in TIs as a clean and efficient route to generate high-performance spin polarization (SP). We show that in the presence of SML and hexagonal warping effect a finite band tilt can induce an imbalance of two spin carriers and effectively give rise to spin-polarized NLNE current in TIs. In our scheme, both the spin current and charge current regime can be achieved under the thermal drive. The obtainable SP can be efficiently tuned in either a smooth or rapid way, exhibiting highly flexible tunability. In addition, near-unity SP can be generated within a wide range of tunable parameters, which is also found to be robust against temperature. Therefore, our work provides a mechanism to realize controllable room-temperature high-degree SP based on TIs, which is of essential importance for future spintronics applications.

Terahertz response of ultrafast spin polarization in semi-insulating GaAs

Due to its high sensitivity and time-resolved ability, terahertz time-domain spectroscopy is a powerful tool for investigating ultrafast carrier dynamics in semiconductors. In addition to charges, spins of ultrafast carriers provide an alternate degree of freedom to design modern electronic devices but are rarely studied by terahertz time-domain spectroscopy. Here, ultrafast spin polarization in semi-insulating GaAs is studied by optical-pump terahertz-probe experiments at room temperature. We used circularly and linearly polarized femtosecond laser pulses to inject nonequilibrium carriers in GaAs and observed that both the transmitted and reflected terahertz signals exhibited different dynamical evolutions under the excitations of linearly and circularly polarized laser pulses, which are ascribed to the generation and relaxation of spin-polarized electrons. The lifetime of the ultrafast spin polarization was obtained from our experiments, highlighting the potentialities of terahertz spectroscopy for the investigation of spin relaxation in semiconductors.

Synthesis, Crystal Structure and Magnetic Properties of a Trinuclear Copper(II) Complex Based on P-Cresol-Substituted Bis(α-Nitronyl Nitroxide) Biradical.

Trinuclear copper(II) complex [CuII3(NIT2PhO)2Cl4] was synthesized with p-cresol-substituted bis(α-nitronyl nitroxide) biradical: 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (NIT2PhOH). The crystal structure of this heterospin complex was determined using single-crystal X-ray diffraction analysis and exhibits four unusual seven-membered metallocycles formed from the coordination of oxygen atoms of the N-O groups and of bridging phenoxo (µ-PhO−) moieties with copper(II) ions. The crystal structure analysis reveals an incipient agostic interaction between a square planar copper center and a hydrogen-carbon bond from one methyl group carried on the coordinated nitronyl-nitroxide radical. The intramolecular Cu∙∙∙H-C interaction involves a six-membered metallocycle and may stabilize the copper center in square planar coordination mode. From the magnetic susceptibility measurements, the complex, which totals seven S = 1/2 spin carriers, has almost a ground state spin S = 1/2 at room temperature ascribed to strong antiferromagnetic interaction between the nitronyl nitroxide moieties and the copper(II) centers and in between the copper(II) centers through the bridging phenoxo oxygen atom.

Bipolar semiconductor in two-dimensional covalent organic frameworks

As a tunable spintronic device, the bipolar magnetic semiconductor (BMS) has been intensively studied in recent years, but the candidate materials are limited in the magnetic system. In this work, we report an alternative strategy to realize it in a nonmagnetic system called bipolar semiconductor (BS). Utilizing doping-induced ferromagnetism, the enantiomorphic flat bands in BS are used to switch the spin carriers, achieving the same function as the BMS. Based on the frontier orbital of the molecular building blocks, a universal approach is proposed to search the BS in two-dimensional (2D) covalent organic frameworks (COFs), which is confirmed by first-principles calculations in the experimentally synthesized fluoride-graphdiyne (F-GDY). Moreover, the spin-polarized channels and giant anomalous Hall effect can also be created in F-GDY through the light irradiation. Our results demonstrate a new spintronic application for 2D COFs without the intrinsic magnetism.

Room-temperature spin-orbit torque switching in a manganite-based heterostructure

The oxide ferromagnet ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) possesses low saturation magnetization $({M}_{s})$, a low magnetic damping constant $(\ensuremath{\alpha})$, and high carrier spin polarization, which promise superior functionalities in spintronics devices. For applications, these devices are to be integrated into electronic circuits, which require the control of the magnetization of LSMO by electrical means. However, a reliable electrical switching method remains largely unexplored. Here, we study the current-induced spin-orbit torque (SOT) in an all-oxide $\mathrm{SrIr}{\mathrm{O}}_{3}/\mathrm{LSMO}$ bilayer. We found the magnetization of a 15 nm LSMO can be switched by electrical current, as confirmed from both the electrical transport measurement and the magnetic optical measurement. By taking advantage of the strain-mediated magnetic anisotropy, the magnetic easy axis of LSMO was controlled to be either perpendicular or parallel to the charge current direction such that a type-$y$ or a type-$x$ SOT switching was achieved, respectively. The dampinglike SOT efficiency is characterized to be 0.15 at room temperature according to the harmonic Hall voltage analysis. We also studied the temperature dependences of the SOT effective field and current-induced switching. Our demonstration of the electrical switching of LSMO magnetization at room temperature may stimulate SOT studies in a wide variety of all-oxide systems.

Spin splitting in low-symmetry quantum wells beyond Rashba and Dresselhaus terms

Spin-orbit interaction in semiconductor structures with broken space inversion symmetry leads to spin splitting of electron and hole states even in the absence of magnetic field. We discover that, beyond the Rashba and Dresselhaus contributions, there is an additional type of the zero-field spin splitting which is caused by the interplay of the cubic shape of crystal unit cell and macroscopic structure asymmetry. In quantum wells grown along low-symmetry crystallographic axes, this type of spin-orbit interaction couples the out-of-plane component of carrier's spin with the in-plane momentum whereas the coupling strength is controlled by structure inversion asymmetry. We carry out numerical calculations and develop an analytical theory, which demonstrate that this interaction dominates $\mathbit{k}$-linear spin splitting of heavy-hole subbands in a wide range of parameters.

Atomic and mesoscopic structure of Dy-based surface alloys on noble metals

Surface alloys are a highly flexible class of low dimensional materials with the opportunity to tune and control the spin and charge carrier functionalities on the nanoscale. Here, we focus on the atomic and mesoscopic structural details of three distinct binary rare-earth-noble metals (NM) surface alloys by employing scanning tunneling microscopy and low energy electron diffraction. Using Dysprosium as the guest element on fcc(111) NM substrates, we identify the formation of non-commensurate surface alloy superstructures, which lead to homogeneous moiré patterns for DyCu2/Cu(111) and DyAu2/Au(111), while an inhomogeneous one is found for DyAg2/Ag(111). The local structure was analyzed for these samples and the observed differences are discussed in the light of the lattice mismatches of the alloy layer with respect to the underlying substrate. For the particularly intriguing case of a DyAg2 surface alloy, the surface alloy layer does not show a uniform long-range periodic structure, but consists of local hexagonal tiles separated by extended domain walls, which occur likely to relieve the in-plane strain within the DyAg2 surface alloy layer. Our findings clearly demonstrate that surface alloying is an intriguing tool to tailor the local atomic structure as well as the mesoscopic moiré structures of metallic heterostructures.

More than a couple of applications benefit from spin-heat coupling

Spin-heat coupling, the interaction between the spin and heat of energy carriers, can be used to tweak materials’ properties in data storage, spintronics, metrology development

High-quality two-dimensional electron gas in undoped InSb quantum wells

We report on transport experiments through high-mobility gate-tunable undoped InSb quantum wells (QWs). Due to the elimination of any Si modulation doping, the gate-defined two-dimensional electron gases in the QWs display a significantly increased mobility of $260\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ at a rather low density of $2.4\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}2}$. Using magnetotransport experiments, we characterize spin-orbit interactions by measuring weak antilocalization. Furthermore, by measuring Shubnikov--de Haas oscillations in tilted magnetic fields, we find that the $g$ factor agrees with $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory calculations at low magnetic fields but grows with spin polarization and carrier density at high magnetic fields. Additionally, signatures of Ising quantum Hall ferromagnetism are found at filling factor $\ensuremath{\nu}=2$ for tilt angles where the Landau level energy equals the Zeeman energy. Despite the high mobility, the undoped InSb QWs exhibit no fractional quantum Hall effect up to magnetic fields of 25 T.

A New Look at Spin Transport: Spinning a Negative into a Strength

Approaching the finite lifetimes of spin carriers as an advantage to modulate their lifetimes and transmit information

Semiclassical response of disordered conductors: Extrinsic carrier velocity and spin and field-corrected collision integral

The semiclassical equations of motion are widely used to describe carrier transport in conducting materials. Nevertheless, the substantial challenge of incorporating disorder systematically into the semiclassical model persists, leading to quantitative inaccuracies and occasionally erroneous predictions for the expectation values of physical observables. In the present work we provide a general prescription for reformulating the semiclassical equations of motion for carriers in disordered conductors by taking the quantum mechanical density matrix as the starting point. We focus on external electric fields, without magnetic fields, and spin-independent disorder. The density matrix approach allows averaging over impurity configurations, and the trace of the velocity operator with the disorder-averaged density matrix can be reinterpreted as the semiclassical velocity weighted by the Boltzmann distribution function. Through this rationale the well-known intrinsic group and anomalous velocities are trivially recovered, while we demonstrate the existence of an extrinsic interband velocity, namely a disorder correction to the semiclassical velocity of Bloch electrons, mediated by the interband matrix elements of the Berry connection. A similar correction is present in the non-equilibrium expectation value of the spin operator, contributing to spin-orbit torques. To obtain agreement with diagrammatic approaches the scattering term in the Boltzmann equation is corrected to first order in the electric field, and the Boltzmann equation is solved up to sub-leading order in the disorder potential. Our prescription ensures all vertex corrections present in diagrammatic treatments are taken into account, and to illustrate this we discuss model cases in topological insulators, including the anomalous Hall effect as well as spin-orbit torques.

High-Throughput Computational Screening for Bipolar Magnetic Semiconductors.

Searching ferromagnetic semiconductor materials with electrically controllable spin polarization is a long-term challenge for spintronics. Bipolar magnetic semiconductors (BMS), with valence and conduction band edges fully spin polarized in different spin directions, show great promise in this aspect because the carrier spin polarization direction can be easily tuned by voltage gate. Here, we propose a standard high-throughput computational screening scheme for searching BMS materials. The application of this scheme to the Materials Project database gives 11 intrinsic BMS materials (1 experimental and 10 theoretical) from nearly ~40000 structures. Among them, a room-temperature BMS Li2V3TeO8 (mp-771246) is discovered with a Curie temperature of 478 K. Moreover, the BMS feature can be maintained well when cutting the bulk Li2V3TeO8 into (001) nanofilms for realistic applications. This work provides a feasible solution for discovering novel intrinsic BMS materials from various crystal structure databases, paving the way for realizing electric-field controlled spintronics devices.

Nonmonotonic quantum phase gathering in curved spintronic circuits

Spin carriers propagating along quantum circuits gather quantum spin phases depending on the circuit's size, shape, and spin-orbit coupling (SOC) strength. These phases typically grow monotonically with the SOC strength, as found in Rashba quantum wires and rings. In this work we show that the spin-phase gathering can be engineered by geometric means, viz. by the geometric curvature of the circuits, to be non-monotonic. We demonstrate this peculiar property by using one-dimensional polygonal models where flat segments alternate with highly curved vertices. The complex interplay between dynamic and geometric spin-phase components -- triggered by a series of emergent spin degeneracy points -- leads to bounded, global spin phases. Moreover, we show that the particulars of the spin-phase gathering have observable consequences in the Aharonov-Casher conductance of Rashba loops, a connection that passed unnoticed in previous works.

Cluster magnetic octupole induced out-of-plane spin polarization in antiperovskite antiferromagnet

Out-of-plane spin polarization σz has attracted increasing interests of researchers recently, due to its potential in high-density and low-power spintronic devices. Noncollinear antiferromagnet (AFM), which has unique 120° triangular spin configuration, has been discovered to possess σz. However, the physical origin of σz in noncollinear AFM is still not clear, and the external magnetic field-free switching of perpendicular magnetic layer using the corresponding σz has not been reported yet. Here, we use the cluster magnetic octupole in antiperovskite AFM Mn3SnN to demonstrate the generation of σz. σz is induced by the precession of carrier spins when currents flow through the cluster magnetic octupole, which also relies on the direction of the cluster magnetic octupole in conjunction with the applied current. With the aid of σz, current induced spin-orbit torque (SOT) switching of adjacent perpendicular ferromagnet is realized without external magnetic field. Our findings present a new perspective to the generation of out-of-plane spin polarizations via noncollinear AFM spin structure, and provide a potential path to realize ultrafast high-density applications.

Modeling of the photocurrent induced by inverse spin Hall effect under local circularly polarized photoexcitation

The inverse spin Hall effect (ISHE) has been recently demonstrated through the photoinduced inverse spin Hall effect (PISHE), where a focused laser normal to the device plane generates photocurrent that depends on the helicity of the photoexcitation. Here, we have employed a finite element method to rigorously simulate the helicity-dependent photocurrent (HDPC) under local helical photoexcitation, taking into account the complications of minority carriers, metal contact junctions, and spin relaxation. We found that the ISHE-induced electromotive force is inversely proportional to the doping level of the material, caused by the diffusion and drift current balance. Furthermore, the HDPC near the metal contact can either increase or decrease, because of the competing mechanisms of fast carrier recombination and spin relaxation at the contact. These simulation results provide insightful visualization of charge, spin, electric field, and current density distributions and deeper understanding of photoinduced ISHE.

Metallic core [Ni[formula omitted]Cr[formula omitted]] as an example of centered heterometallic rings displaying quantum effects

The molecular complex containing the bimetallic core Ni6Cr representing the centered ring geometry as well as its frustrated analogue are numerically simulated using the phenomenological and DFT approach. Within the anisotropic Heisenberg model, the magnetic properties observed are quantitatively explained. In the isotropic limit and the spin frustration switched on via antiferromagnetic interactions between the local s=1 spin carriers, the sequence of the ground states determined by the total spin S is obtained in agreement with the fundamental Lieb–Mattis theorem. It is also demonstrated that the temperature dependence of the susceptibility product χT with characteristic minimum and maximum is not only a signature of competition between the ferro- and antiferro-magnetic interaction but also a symptom of the spin frustration phenomenon.

Lead-Dominated Hyperfine Interaction Impacting the Carrier Spin Dynamics in Halide Perovskites.

The outstanding optical quality of lead halide perovskites inspires studies of their potential for the optical control of carrier spins as pursued in other materials. Entering largely uncharted territory, time-resolved pump-probe Kerr rotation is used to explore the coherent spin dynamics of electrons and holes in bulk formamidinium caesium lead iodine bromide (FA0.9 Cs0.1 PbI2.8 Br0.2 ) and to determine key parameters characterizing interactions of their spins, such as the g-factors and relaxation times. The demonstrated long spin dynamics and narrow g-factor distribution prove the perovskites as promising competitors for conventional semiconductors in spintronics. The dynamic nuclear polarization via spin-oriented holes is realized and the identification of the lead (207 Pb) isotope in optically detected nuclear magnetic resonance proves that the hole-nuclei interaction is dominated by the lead ions. A detailed theoretical analysis accounting for the specifics of the lead halide perovskite materials allows the evaluation of the underlying hyperfine interaction constants, both for electrons and holes. Recombination and spin dynamics evidence that at low temperatures, photogenerated electrons and holes are localized at different regions of the perovskite crystal, resulting in their long lifetimes up to 44μs. The findings form the base for the tailored development of spin-optoelectronic applications for the large family of lead halide perovskites and theirnanostructures.