Intrinsic Spin Research Articles

Composition and stacking dependent topology in bilayers from the graphene family

We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveils a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of nontrivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.

Efficient full spin\u2013orbit torque switching in a single layer of a perpendicularly magnetized single-crystalline ferromagnet

Spin–orbit torque (SOT), which is induced by an in-plane electric current via large spin-orbit coupling, enables an innovative method of manipulating the magnetization of ferromagnets by means of current injection. In conventional SOT bilayer systems, the magnetization switching efficiency strongly depends on the interface quality and the strength of the intrinsic spin Hall Effect. Here, we demonstrate highly efficient full SOT switching achieved by applying a current in a single layer of perpendicularly magnetized ferromagnetic semiconductor GaMnAs with an extremely small current density of ∼3.4 × 105 A cm−2, which is two orders of magnitude smaller than that needed in typical metal bilayer systems. This low required current density is attributed to the intrinsic bulk inversion asymmetry of GaMnAs as well as its high-quality single crystallinity and large spin polarization. Our findings will contribute to advancements in the electrical control of magnetism and its practical application in semiconductor devices.

Computation of intrinsic spin Hall conductivities from first principles using maximally localized Wannier functions

We present a method to compute the intrinsic spin Hall conductivity from first principles using an interpolation scheme based on maximally localized Wannier functions. After obtaining the relevant matrix elements among the ab initio Bloch states calculated on a coarse $k$-point mesh, we Fourier transform them to find the corresponding matrix elements between Wannier states. We then perform an inverse Fourier transform to interpolate the velocity and spin-current matrix elements onto a dense $k$-point mesh and use them to evaluate the spin Hall conductivity as a Brillouin-zone integral. This strategy has a much lower computational cost than a direct ab initio calculation without sacrificing the accuracy. We demonstrate that the spin Hall conductivities of platinum and doped gallium arsenide, computed with our interpolation scheme as a function of the Fermi energy, are in good agreement with those obtained in previous first-principles studies. We also discuss certain approximations that can be performed, in the spirit of the tight-binding method, to simplify the calculation of the velocity and spin-current matrix elements in the Wannier representation.

Near-unity spin Hall ratio in Ni x Cu1-x alloys.

We report a large spin Hall effect in the 3d transition metal alloy Ni x Cu1-x for x ∈ {0.3, 0.75}, detected via the ferromagnetic resonance of a permalloy (Py = Ni80Fe20) film deposited in a bilayer with the alloy. A thickness series at x = 0.6, for which the alloy is paramagnetic at room temperature, allows us to determine the spin Hall ratio θ SH ≈ 1, spin diffusion length λs, spin mixing conductance G ⇅, and damping due to spin memory loss αSML. We compare our results with similar experiments on Py/Pt bilayers measured using the same method. Ab initio band structure calculations with disorder and spin-orbit coupling suggest an intrinsic spin Hall effect in Ni x Cu1-x alloys, although the experiments here cannot distinguish between extrinsic and intrinsic mechanisms.

The energy-momentum tensor of spin-1 hadrons: formalism

We provide the complete decomposition of the local gauge-invariant energy-momentum tensor for spin-1 hadrons, including non-conserved terms for the individual parton flavors and antisymmetric contributions originating from intrinsic spin. We state sum rules for the gravitational form factors appearing in this decomposition and provide relations for the mass decomposition, work balance, total and orbital angular momentum, mass radius, and inertia tensor. Generalizing earlier work, we derive relations between the total and orbital angular momentum and the Mellin moments of twist-2 and 3 generalized parton distributions, accessible in hard exclusive processes with spin-1 targets. Throughout the work, we comment on the unique features in these relations originating from the spin-1 nature of the hadron, being absent in the lower spin cases.

Extrinsic-intrinsic crossover of the spin Hall effect induced by alloying

We report the observation of the crossover between the extrinsic and intrinsic spin Hall effect induced by alloying. We found that the spin Hall angle, the ratio of the spin Hall conductivity to the electric conductivity, changes drastically by tuning the composition of Au-Cu alloy. The spin Hall angle changes the sign only in a limited range of the Cu concentration due to the extrinsic skew scattering, while the intrinsic contribution becomes dominant with increasing the Cu concentration. This observation provides essential information for fundamental understanding of spin-orbit physics.

Evaluation of spin diffusion length and spin Hall angle of the antiferromagnetic Weyl semimetal Mn3Sn

Antiferromagnetic Weyl semimetal Mn$_3$Sn has shown to generate strong intrinsic anomalous Hall effect (AHE) at room temperature, due to large momentum-space Berry curvature from the time-reversal symmetry breaking electronic bands of the Kagome planes. This prompts us to investigate intrinsic spin Hall effect, a transverse phenomenon with identical origin as the intrinsic AHE. We report inverse spin Hall effect experiments in nanocrystalline Mn$_3$Sn nanowires at room temperature using spin absorption method which enables us to quantitatively derive both the spin diffusion length and the spin Hall angle in the same device. We observed clear absorption of the spin current in the Mn$_3$Sn nanowires when kept in contact with the spin transport channel of a lateral spin-valve device. We estimate spin diffusion length $\lambda_{s(Mn_3Sn)}$ $\sim$0.75 $\pm$0.67 nm from the comparison of spin signal of an identical reference lateral spin valve without Mn$_3$Sn nanowire. From inverse spin Hall measurements, we evaluate spin Hall angle $\theta_{SH}$ $\sim$5.3 $\pm$ 2.4 $\%$ and spin Hall conductivity $\sigma_{SH}$ $\sim$46.9 $\pm$ 3.4 ($\hbar/e$) ($\Omega$ cm)$^{-1}$. The estimated spin Hall conductivity agrees with both in sign and magnitude to the theoretically predicted intrinsic $\sigma_{SH}^{int}$ $\sim$36-96 ($\hbar/e$) ($\Omega$ cm)$^{-1}$. We also observed anomalous Hall effect at room temperature in nano-Hall bars prepared at the same time as the spin Hall devices. Large anomalous Hall conductivity along with adequate spin Hall conductivity makes Mn$_3$Sn a promising material for ultrafast and ultrahigh-density spintronics devices.

New insights into the physical processes that underpin cell division and the emergence of different cellular and multicellular structures

Despite decades of focused research, a detailed understanding of the fundamental physical processes that underpin biological systems (structures and processes) remains an open challenge. Within the present paper we report on biomimetic studies, which offer new insights into the process of cell division and the emergence of different cellular and multicellular structures.Experimental studies specifically investigated the impact of including different concentrations of charged bio-molecules (cytokinin and gibberellic acid) on the growth of BaCO3−SiO2 based structures. Results highlighted the role of charge density on the emergence of long-range order, underpinned by a negentropic process. This included the growth of synthetic cell-like structures, with the intrinsic capacity to divide and change morphology at cellular and multicellular scales.Detailed study of dividing structures supports a hypothesis that cell division is dependent on the establishment of a charge-induced macroscopic quantum potential and cell-scale quantum coherence, which allows a description in terms of a macroscopic Schrödinger-like equation, based on a constant different from the Planck constant. Whilst the system does not reflect full correspondence with standard quantum mechanics, many of the phenomena that we typically associate with such a system are recovered.In addition to phenomena normally associated with the Schrödinger equation, we also unexpectedly report on the emergence of intrinsic spin as a macroscopic quantum phenomena, whose origins we account for within a four-dimensional fractal space-time and a macroscopic Pauli equation, which represents the non-relativistic limit of the Dirac equation.

Dynamics of a Ferromagnetic Particle Levitated over a Superconductor

Under conditions where the angular momentum of a ferromagnetic particle is dominated by intrinsic spin, applied torque is predicted to cause gyroscopic precession of the particle. If the particle is sufficiently isolated from the environment, a measurement of spin precession can potentially yield sensitivity to torque beyond the standard quantum limit. Levitation of a micron-scale ferromagnetic particle above a superconductor is a possible method of near frictionless suspension enabling observation of ferromagnetic particle precession and ultrasensitive torque measurements. We experimentally investigate the dynamics of a micron-scale ferromagnetic particle levitated above a superconducting niobium surface. We find that the levitating particles are trapped in potential minima associated with residual magnetic flux pinned by the superconductor and, using an optical technique, characterize the quasiperiodic motion of the particles in these traps.

Giant intrinsic spin Hall effect in W3Ta and other A15 superconductors.

The spin Hall effect (SHE) is the conversion of charge current to spin current, and nonmagnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here, we predict and explain the large intrinsic SHE in β-W and the A15 family of superconductors: W3Ta, Ta3Sb, and Cr3Ir having spin Hall conductivities (SHCs) of -2250, -1400, and 1210 , respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: High symmetry combined with heavy atoms gives rise to multiple Dirac-like crossings in the electronic structure; without sufficient symmetry protection, these crossings gap due to spin-orbit coupling; and gapped crossings create large SBC.

Gravitational wave beacons

We explore spinning, precessing, unequal mass binary black holes to display the long term orbital angular momentum, $\vec{L}$, flip dynamics. We study two prototypical cases of binaries with mass ratios $q=1/7$ and $q=1/15$ and a misaligned spin of the large black hole (with an intrinsic spin magnitude of $S_2/m_2^2=0.85$). We conduct full numerical simulations, for nearly 14 and 18 orbits respectively, to evolve the binary down to merger and display a full $L$-flip cycle. The pattern of radiation of such systems is particularly interesting, displaying strong polarization-dependent variation of amplitudes at precessional frequencies, leading to distinctive observational consequences for ground, space, and pulsar timing based gravitational wave detectors. These waveform features are strongly directional dependent and measurements of gravitational waves polarizations can be exploited to disentangle the binary's parameters in various astrophysical scenarios.

Single and double hole quantum dots in strained Ge/SiGe quantum wells

Even as today’s most prominent spin-based qubit technologies are maturing in terms of capability and sophistication, there is growing interest in exploring alternate material platforms that may provide advantages, such as enhanced qubit control, longer coherence times, and improved extensibility. Recent advances in heterostructure material growth have opened new possibilities for employing hole spins in semiconductors for qubit applications. Undoped, strained Ge/SiGe quantum wells are promising candidate hosts for hole spin-based qubits due to their low disorder, large intrinsic spin–orbit coupling strength, and absence of valley states. Here, we use a simple one-layer gated device structure to demonstrate both a single quantum dot as well as coupling between two adjacent quantum dots. The hole effective mass in these undoped structures, m* ∼ 0.08 m0, is significantly lower than for electrons in Si/SiGe, pointing to the possibility of enhanced tunnel couplings in quantum dots and favorable qubit–qubit interactions in an industry-compatible semiconductor platform.

Tunability of Bandgap and Magnetism in K and Pb Codoped BiFeO3 Nanoparticles for Multiferroic Applications: The Role of Structural Transition and Fe Deficiency

BiFeO3 is the most well-known single-phase material for multiferroic and photovoltaic applications because of its large ferroelectric polarization and high operating temperature. However, direct magnetoelectric coupling between ferroelectricity and magnetic order in bulk BiFeO3 is exceedingly weak, resulting from its intrinsic cycloidal spin structure. Fortunately, nanostructured BiFeO3 could exhibit multiferroism due to superficial uncompensated spins, thereby altering the magnetic order of the bulk phase. Here, the optical property and magnetism of Bi1–x(KPb)xFeO3 (0 ≤ x ≤ 0.1) nanoparticles are studied. X-ray powder diffraction and Raman spectroscopy reveal the apparent increase of cubic crystal phase (Pm-3m) and the emergence of Fe deficiency with the increase of K and Pb codoping content in BiFeO3 nanoparticles. Fe deficiency and its change in Bi1–x(KPb)xFeO3 nanoparticles are further confirmed by EDX and XPS spectra, besides the presence of Bi3+, Fe3+, and Pb4+ ions. The optical bandgap is narrowed gradually in the samples with the increase of K and Pb codoping content, and the corresponding phenomenological model is proposed. Magnetic hysteresis and magnetization versus temperature curves indicate the coexistence of antiferromagnetic and ferromagnetic orderings in Bi1–x(KPb)xFeO3 nanoparticles. The saturation magnetization first decreases and then increases with K and Pb codoping concentration increasing. A correlation among the structural transition, Fe deficiency, and magnetic property has been established in detail, which provides a possibility to further understand the cycloidal spin magnetic structure of BiFeO3 and improve its magnetic property for multiferroic applications.

Geometry and electronic structure of monolayer, bilayer, and multilayer Janus WSSe

Newly synthesized Janus transition-metal dichalcogenides MXY ($M=\mathrm{Mo}$, W; $X\ensuremath{\ne}Y=\mathrm{S}$, Se, Te) possess intrinsic Rashba spin splitting and out-of-plane dipole moment due to the breaking of mirror symmetry. Taking WSSe as an example, we present a first-principles investigation of the structural stability and electronic properties of mono-, bi-, and multilayer MXY. Results show that S atoms contribute more than Se atoms in the valence-band maximum at the $\mathrm{\ensuremath{\Gamma}}$ point, which can be greatly affected by interlayer interactions. The high-symmetry AA\ensuremath{'} stacking is still the most stable pattern, but there are various orders of chalcogen atomic layers in each stacking. The most preferred order of two adjacent layers is S-Se-Se-S, followed by Se-S-Se-S. The Se-S-Se-S--ordered WSSe bilayer is found to have significant layer splitting due to the net dipole moment, which has great potential for solar cells. Layer-dependent Rashba splittings exist in asymmetry-ordered WSSe bilayers, that can be tuned by changing the interlayer distance, originating from the regulation of interlayer electrostatic interaction. However, there is not layer splitting in a symmetrically stacked WSSe bilayer and opposite Rashba splitting appears in the two layers at a sufficiently large interlayer distance. The electronic structures and spin splittings can be easily modulated by controlling the chalcogen atomic-layer order, so that we can obtain the desired properties from mono-, bi-, and multilayer MXY.

Randomness-induced spin-liquid-like phase in the spin- 12 J1−J2 triangular Heisenberg model

We study the effects of bond randomness in the spin-$1/2$ $J_1-J_2$ triangular Heisenberg model using exact diagonalization and density matrix renormalization group. With increasing bond randomness, we identify a randomness induced spin-liquid-like phase without any magnetic order, dimer order, spin glass order, or valence-bond glass order. The finite-size scaling of gaps suggests the gapless nature of both spin triplet and singlet excitations, which is further supported by the broad continuum of dynamical spin structure factor. By studying the bipartite entanglement spectrum of the system on cylinder geometry, we identify the features of the low-lying entanglement spectrum in the spin-liquid-like phase, which may distinguish this randomness induced spin-liquid-like phase and the intrinsic spin liquid phase in the clean $J_1 - J_2$ triangular Heisenberg model. We further discuss the nature of this spin-liquid-like phase and the indication of our results for understanding spin-liquid-like materials with triangular-lattice structure.

Intrinsic spin Hall conductivity of the semimetals MoTe2 and WTe2

We report a comprehensive study on the intrinsic spin Hall conductivity (SHC) of semimetals MoTe2 and WTe2 by ab initio calculation. Large SHC and desirable spin Hall angles have been discovered, due to the strong spin orbit coupling effect and low charge conductivity in semimetals. Diverse anisotropic SHC values, attributed to the unusual reduced-symmetry crystalline structure, have been revealed. We report an effective method on SHC optimization by electron doping, and exhibit the mechanism of SHC variation respect to the energy shifting by the spin Berry curvature. Our work provides insights into the realization of strong spin Hall effects in 2D systems.

Giant spin Hall effect in two-dimensional monochalcogenides

One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials.

Dependence of spin pumping in W/CoFeB heterostructures on the structural phase of tungsten

We report the systematic dependence of spin pumping in tungsten (W)/CoFeB heterostructures on the structural phase of W, which is intricately related to argon gas pressure (${p}_{\mathrm{Ar}}$) maintained during the sputter deposition. We found that with increasing ${p}_{\mathrm{Ar}}$ the structural phase of W changes from mixed ($\ensuremath{\alpha}+\ensuremath{\beta}$) phase to pure $\ensuremath{\beta}$ phase. The $\ensuremath{\beta}$-W is stabilized in films for the high thickness of 40 nm which is desirable for spin devices. Using ferromagnetic resonance measurement of W(${p}_{\mathrm{Ar}}$)/CoFeB heterostructures, it is shown that enhancement of magnetic damping (${\ensuremath{\alpha}}_{\mathrm{eff}}$) from spin pumping is more in $\ensuremath{\beta}$-W compared to ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W. The effective spin mixing conductance (${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) is estimated for different phases of W from the linear evolution of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ with the inverse thickness of the CoFeB layer. For $\ensuremath{\beta}$-W, the ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ is found to be larger than that of ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W and it is attributed to different interface structure. Thus, effective tuning of spin pumping efficiency can be achieved using different W crystal phases. We also studied the dependence of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ on $\ensuremath{\beta}$-W film thickness to calculate the value of spin-diffusion length (${\ensuremath{\lambda}}_{\mathrm{SD}}$) and intrinsic spin mixing conductance (${g}_{\ensuremath{\beta}\text{\ensuremath{-}}W}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) using both the ballistic and diffusive spin transport models.

The observation of inherent spin Seebeck effect in Rh/YIG hybrid structure

Rhodium (Rh) is a 4d metal possessing a large spin orbit coupling strength and spin-Hall conductivity with a very small magnetic susceptibility, implying an insignificant magnetic proximity effect (MPE). We report here the observation of longitudinal spin Seebeck effect (LSSE) using Rh as a normal metal. A Rh film was sputtered on nanometer thick YIG films of highly crystalline nature and extremely low magnetic damping to obtain Rh/YIG hybrid structure. A clear thermal voltage Vth (SSE voltage) was obtained when a temperature gradient was applied on the Rh/YIG hybrid. The Rh film showed a very weak anomalous Hall resistance and the magneto-resistive testing clearly ruled out the magnetization of the Rh films via MPE. The anisotropic magnetoresistance (AMR) revealed a clear spin hall magnetoresistance (SMR) signal in Rh film implying a purely intrinsic spin current generation, free from any parasitic magnetic effects. The work can open a new window in the study of pure and uncontaminated spin current, generated in ferromagnetic insulators, using Rh as spin current detector.

Strain engineering of the intrinsic spin Hall conductivity in a SrTiO3 quantum well

The intrinsic spin Hall conductivity of a two-dimensional gas confined to SrTiO$_3$, such as occurs at an LaAlO$_3$/SrTiO$_3$ interface, is calculated from the Kubo formula. The effect of strain in the [001] (normal to the quantum well direction) and the [111] direction is incorporated into a full tight-binding Hamiltonian. We show that the spin-charge conversion ratio can be significantly altered through strain and gate voltage by tuning the chemical potential. Strain direction is also a significant factor in the spin Hall response as this direction affects the alignment of the conduction bands.