Rashba-type Spin Splitting Research Articles

Three-dimensional bulk band dispersion in polar BiTeI with giant Rashba-type spin splitting

In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive $h\nu$-dependent soft x-ray angle resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of ${\sim}300$ meV at the conduction-band-minimum and valence-band-maximum located in the $k_z=\pi/c$ plane. It provides the first direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound.

Role ofdorbitals in the Rashba-type spin splitting for noble-metal surfaces

We investigate the Rashba-type spin splitting in the Shockley surface states on Au(111) and Ag(111) surfaces, based on first-principles calculations. By turning on and off spin-orbit interaction (SOI) partly, we show that although the surface states are mainly of p-orbital character with only small d-orbital one, d-channel SOI determines the splitting and the spin direction while p-channel SOI has minor and negative effects. The small d-orbital character of the surface states, present even without SOI, varies linearly with the crystal momentum k, resulting in the linear k dependence of the splitting, the Hallmark of the Rashba type. As a way to perturb the d-orbital character of the surface states, we discuss effects of electron and hole doping to the Au(111) surface.

Ideal Two-Dimensional Electron Systems with a Giant Rashba-Type Spin Splitting in Real Materials: Surfaces of Bismuth Tellurohalides

Spintronics is aimed at actively controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction in a two-dimensional electron system immersed in an inversion-asymmetric environment. The spin-orbit-induced spin splitting of the two-dimensional electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in two-dimensional electron systems that reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Γ-point energy lying deep inside the bulk band gap. The giant spin splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations.

Manipulating the Rashba-type spin splitting and spin texture of Pb quantum well states

Using spin- and angle-resolved photoemission spectroscopy we show that the spin splitting and the spin texture of quantum well states in thin Pb films can be manipulated through changes in the interface to the substrate. Compared with films grown on the Pb reconstructed Si(111) substrate the Bi interface reduces the spin splitting of the Pb states by a factor of 2 and the spin polarization vector is rotated by 32${}^{\ensuremath{\circ}}$ out of the sample plane. The spin splitting on an Ag reconstructed substrate is below the experimental resolution. Based on a model supported by ab initio calculations we interpret these changes as due to a modification of the charge density distribution in the quantum well states.

Giant Rashba-type spin splitting in bulk BiTeI

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.

Structural influence on the Rashba-type spin splitting in surface alloys

The Bi/Ag(111), Pb/Ag(111), and Sb/Ag(111) surface alloys exhibit a two-dimensional band structure with a strongly enhanced Rashba-type spin splitting, which is in part attributed to the structural asymmetry resulting from an outward relaxation of the alloy atoms. In order to gain further insight into the spin splitting mechanism, we have experimentally determined the outward relaxation of the alloy atoms in these surface alloys using quantitative low-energy electron diffraction. The structure plays an important role in the size of the spin splitting as it dictates the potential landscape, the symmetry as well as the orbital character. Furthermore, we discuss the band ordering of the Pb/Ag(111) surface alloy as well as the reproducible formation of Sb/Ag(111) surface alloys with unfaulted (face-centered cubic) and faulted (hexagonally close-packed) top layer stacking.

Silicon Surface with Giant Spin Splitting

We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken inducing a giant spin splitting with a Rashba energy of about 140 meV, much larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.

Unconventional spin topology in surface alloys with Rashba-type spin splitting

The spins of a pair of spin-orbit split surface states at a metal surface are usually antiparallelly aligned, in accord with the Rashba model for a two-dimensional electron gas. By first-principles calculations and two-photon photoemission experiments we provide evidence that in the surface alloy Bi/Cu(111) the spins of an unoccupied pair of surface states are parallelly aligned. This unconventional spin polarization, which is not consistent with that imposed by the Rashba model, is explained by hybridization of surface states with different orbital character and is attributed to the spin-orbit interaction. Since hybridization is a fundamental effect our findings are relevant for spin electronics in general.

Rashba Effect of the Tl-covered Ge(111) Surface

We studied electronic structure and Rashba-type spin splitting in the (3×1) and (1×1) structures on Tl/Ge(111) using angle-resolved photoelectron spectroscopy (ARPES) and first-principles calculation incorporating the spin-orbit interaction. The calculations showed that Tl behaves as a monovalent ion and surface states involved with the Tl-Ge bond are formed mainly by the Tl 6p orbital and the Ge dangling bond. The surface states show k-dependent spin splitting up to ∼200 and ∼800 meV for the (3×1) and (1×1) structures, respectively. The occupied surface states were observed by ARPES. Their spin splittings were predicted to be 150−200 meV, while these were not resolved in the measurement. The largest splitting of ∼800 meV is calculated for the unoccupied states at K− on the (1×1) structure. The wavefunction is strongly localized in the Tl overlayer, indicating that the heavy core potential of Tl is essential for the large spin splitting.

Rashba effect at the surfaces of rare-earth metals and their monoxides

We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.

Band structure ofTl/Ge(111)−(3×1): Angle-resolved photoemission and first-principles prediction of giant Rashba effect

We have investigated the atomic and electronic structure of the $\text{Tl}/\text{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ surface using angle-resolved photoelectron spectroscopy (ARPES) and first-principles total-energy-band calculation. The experimental surface bands exhibit semiconducting electronic structure and are well reproduced by the band calculation based on the honeycomb-chain channel structure with Tl adsorbed at ${H}_{3}$ site. Electron density maps of the surface-related states show the formation of the $\text{Ge}=\text{Ge}$ double-bond-like state as observed on the alkali-metal-induced $\text{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ surfaces. In our scalar-relativistic all-electron calculation, the Tl-derived surface bands show a Rashba-type spin splitting in the projected bulk band gap, which is not clearly resolved by ARPES due to a limited energy resolution. They show complicated dispersion due to the hybridization with the bulk states in the projected bulk band region. The lowest unoccupied surface band, which is an antibonding state formed between $\text{Tl}\text{ }6p$ and Ge dangling bonds and is highly localized in the topmost surface layer, shows a large spin-orbit splitting of $\ensuremath{\sim}200\text{ }\text{meV}$.

First-principles investigation of structural and electronic properties of ultrathin Bi films

Employing first-principles calculations, we perform a systematic study of the electronic properties of thin (one to six bilayers) films of the semimetal bismuth in (111) and (110) orientation. Due to the different coordination of the surface atoms in these two cases, we find a large variation of the conducting properties of the films, ranging from small-band-gap semiconducting to semimetallic and metallic. The evolution of the Bi(111) and Bi(110) surface states can be monitored as a function of the film thickness. Another interesting feature is provided by the strong spin-orbit effects in Bi and the resulting Rashba-type spin splitting of the surface states. The relaxations, band structures, Fermi surfaces, and densities of states are presented and discussed with respect to possible applications in the field of spintronics.

Spin-dependent photocurrent induced by Rashba-type spin splitting inAl0.25Ga0.75N∕GaNheterostructures

Circular and linear photogalvanic effects are experimentally demonstrated in 0001-oriented Al0.25Ga0.75N / GaN heterostructures. By changing the incident angle of the pumping light beam, it is possible to manipulate the relative strength between the circular and linear photogalvanic effects. The spin-dependent signal is evidenced by the sign change upon reversing the radiation helicity. Its consistency with the strength of the Rashba-type spin splitting as determined from the beating of Shubnikov–de Haas oscillations reveals the underlying mechanism responsible for the observed effect. The measured spin-dependent photocurrent is larger than that of the intersubband transition by two orders of magnitude at room temperature. AlGaN / GaN heterostructures therefore provide an excellent opportunity for the generation of spin polarized current to be used in spintronics.