Spin-orbit Energy Research Articles

Voltage-driven spin-transfer torque in a magnetic particle

We discuss a spin-transfer torque device, where the role of the soft ferromagnetic layer is played by a magnetic particle or a magnetic molecule, in weak tunnel contact with two spin polarized leads. We investigate if the magnetization of the particle can be manipulated electronically, in the regime where the critical current for magnetization switching is negligibly weak, which could be due to the reduced particle dimensions. Using master equation simulations to evaluate the effects of spin-orbit anisotropy energy fluctuations on spin-transfer, we obtain reliable reading and writing of the magnetization state of such magnetic particle, and find that the device relies on a critical voltage rather than a critical current. The critical voltage is governed by the spin-orbit energy shifts of discrete levels in the particle. This finding opens a possibility to significantly reduce the power dissipation involved in spin-transfer torque switching, by using very small magnetic particles or molecules.

The Renner-Teller effect in HCCCN(+)(X̃(2)Π) studied by zero-kinetic energy photoelectron spectroscopy and theoretical calculations.

The spin-vibronic energy levels of the cyanoacetylene cation have been measured using the one-photon zero-kinetic energy (ZEKE) photoelectron spectroscopic method. All three degenerate vibrational modes showing vibronic coupling, i.e., Renner-Teller (RT) effect, have been observed. All the splitting spin-vibronic energy levels of the fundamental H-C≡C bending vibration (v5) have been determined. The spin-vibronic energy levels of the degenerate vibrational modes have also been calculated using a diabatic model in which the harmonic terms as well as all the second-order vibronic coupling terms are used. The theoretical predictions are in good agreement with the experimental data and are used to assign the ZEKE spectrum. It is found that the RT effects for the H-(CC)-CN bending (v7) and the C-C≡N bending (v6) vibrations are weak, whereas they are strong for the H-C≡C bending (v5) vibration. The cross-mode RT couplings between any of the two degenerate vibrations are strong. The spin-orbit resolved fundamental vibrational energy levels of the C≡N stretching (v2) and C-H stretching (v1) vibrations have also been observed. The spin-orbit energy splitting of the ground state has been determined for the first time as 43 ± 2 cm(-1), and the ionization energy of HCCCN is found to be 93 903.5 ± 2 cm(-1).

Orbital engineering of two-dimensional materials with hydrogenation: A realization of giant gap and strongly correlated topological insulators

Orbital interaction plays an important role in topological insulators. Using first-principles calculations, we demonstrate that hydrogenation can change two-dimensional (2D) trivial insulator/semimetal-like Pb, Mo, and W to a ${Z}_{2}$ topological insulator with giant gaps by filtering unwanted orbitals such as the ${p}_{z}$ orbital in Pb, and the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ in Mo and W. For PbH, the large intrinsic spin-orbit coupling (SOC) confined in the in-plane orbitals ${p}_{x,y}$ results in a bulk gap of 1.07 eV. For the case of MoH and WH, hydrogenation results in a novel $s{d}^{4}$ hybridization with a Dirac cone formed by the out-of-plane orbital ${d}_{yz/xz}$ orbitals. Furthermore, due to the electron-electron interaction and the strong SOC in the $4d$ and $5d$ elements, the bulk band gap is significantly enhanced with a value of 1.28 eV for MoH and 0.30 eV for WH. The strong correlation effect also induces several topological phase transitions in WH like the Dirac semimetal and Mott insulating phase, while MoH remains a topological insulator even with large correlation effect. We propose that these new strongly correlated 2D materials can be realized experimentally by using substrate such as the boron nitride sheet, which can exhibit various novel properties such as the Dirac semimetal phase with large spin-orbit energy splitting as well as the quantum spin Hall property.

Observation of topological transition of Fermi surface from a spindle torus to a torus in bulk Rashba spin-split BiTeCl

The recently observed large Rashba-type spin splitting in the BiTeX (X = I, Br, Cl) bulk states due to the absence of inversion asymmetry and large charge polarity enables observation of the transition in Fermi surface topology from spindle-torus to torus with varying the carrier density. These BiTeX systems with high spin-orbit energy scales offer an ideal platform for achieving practical spintronic applications and realizing non-trivial phenomena such as topological superconductivity and Majorana fermions. Here we use Shubnikov-de Haas oscillations to investigate the electronic structure of the bulk conduction band of BiTeCl single crystals with different carrier densities. We observe the topological transition of the Fermi surface (FS) from a spindle-torus to a torus. The Landau level fan diagram reveals the expected non-trivial {\pi} Berry phase for both the inner and outer FSs. Angle-dependent oscillation measurements reveal three-dimensional FS topology when the Fermi level lies in the vicinity of the Dirac point. All the observations are consistent with large Rashba spin-orbit splitting in the bulk conduction band.

Photophysical electronic structure of double-perovskites A2GdTaO6 (A = Ba and Sr)

X-ray photoemission spectroscopy (XPS) measurements of double perovskite oxides, Ba2GdTaO6 and Sr2GdTaO6 are performed in the energy window of 0–1300 eV. Density functional theory calculations are initiated with the Vienna ab initio Simulation Package to understand the electronic structure of the systems. The calculated DOS has been compared with the experimental valence band XPS spectra. It has been observed that the Ta-5d and O-2p states are hybridized in the valence band. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds, which has been used to explain the electrical conduction mechanism of the systems. The calculated ratio of the spin-orbit interaction energy for Ba 3d and 4d states matches well with the observed experimental results.

Spin-orbit interaction in InSb nanowires

We use magnetoconductance measurements in dual-gated InSb nanowire devices together with a theoretical analysis of weak antilocalization to accurately extract spin-orbit strength. In particular, we show that magnetoconductance in our three-dimensional wires is very different compared to wires in two-dimensional electron gases. We obtain a large Rashba spin-orbit strength of $0.5 -1\,\text{eV\r{A}}$ corresponding to a spin-orbit energy of $0.25-1\,\text{meV}$. These values underline the potential of InSb nanowires in the study of Majorana fermions in hybrid semiconductor-superconductor devices.

Proton and neutron correlations in10B

We investigate positive-parity states of $^{10}$B with the calculation of antisymmetrized molecular dynamics focusing on $pn$ pair correlations. We discuss effects of the spin-orbit interaction on energy spectra and $pn$ correlations of the $J^\pi T=1^+_10$, $=3^+_10$, and $0^+_11$ states. The $1^+_10$ state has almost no energy gain of the spin-orbit interaction, whereas the $3^+_10$ state gains the spin-orbit interaction energy largely to come down to the ground state. We interpret a part of the two-body spin-orbit interaction in the adopted effective interactions as a contribution of the genuine $NNN$ force, and find it to be essential for the level ordering of the $3^+_10$ and $1^+_10$ states in $^{10}$B. We also apply a $2\alpha+pn$ model to discuss effects of the spin-orbit interaction on $T=0$ and $T=1$ $pn$ pairs around the 2$\alpha$ core. In the spin-aligned $J^\pi T=3^+0$ state, the spin-orbit interaction affects the $(ST)=(10)$ pair attractively and keeps the pair close to the core, whereas, in the $1^+0$ state, it gives a minor effect to the $(ST)=(10)$ pair. In the $0^+1$ state, the $(ST)=(01)$ pair is somewhat dissociated by the spin-orbit interaction.

Photoreflectance analysis of annealed vanadium-doped GaAs thin films grown by metalorganic vapor phase epitaxy

Abstract In this study, we investigate the optical properties of annealed vanadium-doped GaAs films grown on GaAs substrates by metalorganic vapor phase epitaxy. The temperature dependence of the photoreflectance (PR) of as-grown GaAs:V films has been studied. We used the fit with Third-Derivative Functional Form model to evaluate the physical parameters. The temperature dependence of band gap and spin–orbit energies can be described by the Bose–Einstein statistical expression. The PR spectra of the samples are measured after thermal annealing in order to check any improvement in the optical quality of the material. The PR signal amplitude of GaAs:V samples decreased after thermal annealing. Degradation of the PR signal for annealing temperature at about 850 °C is observed revealing a poor quality of the layer surface states and an important density of the recombination centers. The lock-in phase analysis of PR spectra allows to determine the time constant for GaAs:V sample before and after thermal annealing.

The Renner-Teller effect in HCCCl(+)(X̃(2)Π) studied by zero-kinetic energy photoelectron spectroscopy and ab initio calculations.

The spin-vibronic energy levels of the chloroacetylene cation up to 4000 cm(-1) above the ground state have been measured using the one-photon zero-kinetic energy photoelectron spectroscopic method. The spin-vibronic energy levels have also been calculated using a diabatic model, in which the potential energy surfaces are expressed by expansions of internal coordinates, and the Hamiltonian matrix equation is solved using a variational method with harmonic basis functions. The calculated spin-vibronic energy levels are in good agreement with the experimental data. The Renner-Teller (RT) parameters describing the vibronic coupling for the H-C≡C bending mode (ε4), Cl-C≡C bending mode (ε5), the cross-mode vibronic coupling (ε45) of the two bending vibrations, and their vibrational frequencies (ω4 and ω5) have also been determined using an effective Hamiltonian matrix treatment. In comparison with the spin-orbit interaction, the RT effect in the H-C≡C bending (ε4) mode is strong, while the RT effect in the Cl-C≡C bending mode is weak. There is a strong cross-mode vibronic coupling of the two bending vibrations, which may be due to a vibronic resonance between the two bending vibrations. The spin-orbit energy splitting of the ground state has been determined for the first time and is found to be 209 ± 2 cm(-1).

Intrinsic damping of collective spin modes in a two-dimensional fermi liquid with spin-orbit coupling.

A Fermi liquid with spin-orbit coupling (SOC) is expected to support a new set of collective modes: oscillations of magnetization in the absence of the magnetic field. We show that these modes are damped by the electron-electron interaction even in the limit of an infinitely long wavelength (q=0). The linewidth of the collective mode is on the order of Δ¯2/E(F), where Δ¯ is a characteristic spin-orbit energy splitting and E(F) is the Fermi energy. Such damping is in stark contrast to known damping mechanisms of both charge and spin collective modes in the absence of SOC, all of which disappear at q=0, and arises because none of the components of total spin is conserved in the presence of SOC.

Spectroscopy and heats of formation of CXI (X = Br, Cl, F) iodocarbenes: quantum chemical characterisation of the X̃(1 A′), ã(3 A″) and Ã(1 A″) states

The equilibrium energies of the iodocarbenes CXI (X = Br, Cl, F) in their , and states and their atomisation and dissociation energies in the complete basis limit were determined by extrapolating valence correlated (R/U)CCSD(T) and Davidson corrected multi-reference configuration interaction (MRCI) energies calculated with the aug-cc-pVxZ (x = T,Q,5) basis sets and the ECP28MDF pseudopotential of iodine plus corrections for core and core–valence correlation, scalar relativity, spin–orbit coupling and zero-point energies. Spin–orbit energies were computed in a large basis of configurations chosen so as to accurately describe dissociation to the 3P and 2P states of C and of the halogens X and I, respectively. The computed singlet–triplet splittings are 13.6, 14.4 and 27.3 kcal mol−1 for X = Br, Cl and F, respectively. The enthalpies of formation at 0 K are predicted to be 97.4, 82.6 and 38.1 kcal mol−1 with estimated errors of ±1.0 kcal mol−1. The excitation energies (T00) in CBrI and CClI are calculated to be 41.1 and 41.7 kcal mol−1, respectively. The Renner–Teller intersections in both molecules are predicted to be substantially higher than the dissociation barriers on the surfaces. By contrast, in CFI the state is found to be unbound with respect to dissociation.

Temperature-dependent Coulomb excitations in silicene

The temperature-dependent Coulomb screening and excitation spectrum of electrons in silicene are studied by the tight-binding model and the random-phase approximation. With the spin–orbit interaction, monolayer silicene is a narrow-gap semiconductor. At finite temperatures, the interplay between the intraband and interband transitions could lead to an undamped plasmon mode at low frequencies. The plasmon mode only exists in a limited region of temperature and momentum, corresponding to the constrained gap transition. Beyond that region, another damped plasmon mode dominates the excitation spectrum. The drastic change in the plasmon behavior might be observed experimentally, which could allow for the identification of the spin–orbit energy gap.

Mixing of parity of a nucleon pair at the nuclear surface due to the spin-orbit potential inF18

We investigate the structure of $^{18}\mathrm{F}$ with the microscopic wave function based on the three-body $^{16}\mathrm{O}+p+n$ model. In the calculation of the generator coordinate method of the three-body model, $T=0$ energy spectra of ${J}^{\ensuremath{\pi}}={1}^{+},\phantom{\rule{0.16em}{0ex}}{3}^{+}$, and ${5}^{+}$ states and $T=1$ spectra of ${J}^{\ensuremath{\pi}}={0}^{+},\phantom{\rule{0.16em}{0ex}}{2}^{+}$ states in $^{18}\mathrm{F}$ are described reasonably. Based on the dinucleon picture, the effect of the spin-orbit force on the $T=0$ and $T=1 pn$ pairs around the $^{16}\mathrm{O}$ core is discussed. The $T=1$ pair in the ${J}^{\ensuremath{\pi}}={0}^{+}$ state gains the spin-orbit potential energy involving the odd-parity mixing in the pair. The spin-orbit potential energy gain with the parity mixing is not so efficient for the $T=0$ pair in the ${J}^{\ensuremath{\pi}}={1}^{+}$ state. The parity mixing in the pair is regarded as the internal symmetry breaking of the pair in the spin-orbit potential at the nuclear surface.

Observation of Rashba zero-field spin splitting in a strained germanium 2D hole gas

We report the observation, through Shubnikov-de Haas oscillations in the magnetoresistance, of spin splitting caused by the Rashba spin-orbit interaction in a strained Ge quantum well epitaxially grown on a standard Si(001) substrate. The Shubnikov-de Haas oscillations display a beating pattern due to the spin split Landau levels. The spin-orbit parameter and Rashba spin-splitting energy are found to be 1.0 × 10−28 eVm3 and 1.4 meV, respectively. This energy is comparable to 2D electron gases in III-V semiconductors, but substantially larger than in Si, and illustrates the suitability of Ge for modulated hole spin transport devices.

Three-leaf quantum interference clovers in a trigonal single-molecule magnet.

We report on a single-molecule magnet where the spatial arrangement of three manganese ions and their spin-orbit coupling tensor orientations result in threefold angular modulations of the magnetization tunneling rates and quantum interference patterns that mimic the form of a three-leaf clover. Although expected in all quantum tunneling of magnetization resonances for a trigonal molecular symmetry, the threefold modulation only appears at resonances for which a longitudinal magnetic field is applied (i.e., resonance numbers |k|>0). A sixfold transverse field modulation observed at resonance k = 0 manifests as a direct consequence of a threefold corrugation of the spin-orbit coupling energy landscape, creating an effective longitudinal field which varies the resonance condition in the presence of a transverse field. The observations allow for an association between the trigonal distortion of the local spin-orbit interactions and the spatial disposition of the constituent ions, a finding that can be extrapolated to other systems where spin-orbit coupling plays a significant role.

Different temperature scaling of strain-induced magneto-crystalline anisotropy and Gilbert damping in Co2FeAl film epitaxied on GaAs

The temperature dependence of the Gilbert damping and magnetic anisotropy are investigated in L21 Co2FeAl films epitaxially grown on GaAs (001) substrate by the time resolved magneto-optical Kerr effect. We found that the in-plane biaxial anisotropy increases by more than 90% with the temperature decreasing from 300 K to 80 K, which is mainly due to the strong variation of the magneto-elastic coefficients. In contrast, the intrinsic Gilbert damping rises only about 10%, which is mainly attributed to the reduction of the electron phonon scattering rate, independent of the strain-induced spin-orbit coupling energy.

Persistent non-metallic behavior in Sr2IrO4 and Sr3Ir2O7 at high pressures

Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ∼0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.

Spin-orbit-coupled quantum wires and Majorana fermions on zigzag edges of monolayer transition-metal dichalcogenides

Majorana fermions, quantum particles with non-Abelian exchange statistics, are not only of fundamental importance, but also building blocks for fault-tolerant quantum computation. Although certain experimental breakthroughs for observing Majorana fermions have been made recently, their conclusive dection is still challenging due to the lack of proper material properties of the underlined experimental systems. Here we propose a new platform for Majorana fermions based on edge states of certain non-topological two-dimensional semiconductors with strong spin-orbit coupling, such as monolayer group-VI transition metal dichalcogenides (TMD). Using first-principles calculations and tight-binding modeling, we show that zigzag edges of monolayer TMD can host well isolated single edge band with strong spin-orbit coupling energy. Combining with proximity induced s-wave superconductivity and in-plane magnetic fields, the zigzag edge supports robust topological Majorana bound states at the edge ends, although the two-dimensional bulk itself is non-topological. Our findings points to a controllable and integrable platform for searching and manipulating Majorana fermions.

Al3+ ACCEPTORS AND THE OPTICAL PROPERTIES OF Bi12SiO20:Cr+Al

Absorption measurement is taken in the visible spectral region (650 - 850 nm). The dopants Cr3+ and Al3+ ions occupy the tetrahedral sites in the crystal lattice of doped sillenite. The electron transitions in Cr3+ are presented in the case of Bi12SiO20:Cr and Bi12SiO20:Cr+Al. The total cross-section of combined chromium-aluminium absorption structure and the spin-orbit coupling energy of Cr3+ are calculated.

Spin–orbit effects on the structural, homotop, and magnetic configurations of small pure and Fe-doped Pt clusters

We report ab initio calculations of the atomic and electronic structures, and related magnetic properties of platinum clusters of seven atoms both pure and substitutionally doped with an iron impurity. A relativistic Hamiltonian including spin–orbit coupling effects is self-consistently solved in the noncollinear framework within the density functional theory as implemented in the VASP code. We show that spin–orbit coupling is crucial for determining the energetic order of structural isomers of small platinum clusters, but not for the low-lying iron-doped platinum clusters. We analyze the influence of Fe doping on the overall improvement of the magnetic efficiency of the nanoparticle, i.e., large total moment and high stability against remagnetization. Results are reported for the spin–orbit energy, the magnetic anisotropy energy corresponding to the remagnetization barrier, and the orbital contribution to the total magnetic moment, which can be experimentally measured in free-standing clusters by means of X-ray magnetic circular dichroism spectroscopy.