Physical Review B

The authors explore in detail impurity effects on arm-chair graphene nanoribbons. While limited to the framework of the Anderson hybrid model of impurity perturbations, the study reveals the possibility for Mott localization and for opening of a mobility gap under appropriate fine-tuning of the impurity energy level, its hybridization with the Dirac modes of the host material, and the impurity concentration. Special attention is dedicated to the interplay of disorder with the spin splitting of the energy spectrum of the host, caused by the Rashba spin-orbit interaction. The approach can be extended to account for further relevant effects such as electron-electron Hubbard correlations, spin-ordering effects, phonon and magnon quasiparticles, etc.

The physics of spin currents has always attracted the attention of Professor Rashba who made seminal contributions to the field. Here, the authors develop the theory of the spin and valley Hall effects in systems of interacting fermions and bosons. In high-mobility two-dimensional semiconductors, the interparticle scattering dominates over the impurity and phonon scattering and controls the transport effects. The authors demonstrate theoretically that the collisions between the quasiparticles give rise to the spin and valley Hall currents via the skew-scattering effect. They also calculate the efficiency of such a process.

To what extent can photonic or polaritonic systems model electron spin-dependent phenomena? Even in the single-particle description the main limitation comes from the time-reversal symmetry operation, which is different for electrons and polaritons. This paper analyzes the qualitative differences between the trapped polariton states in the presence of transverse-electric–transverse-magnetic splitting as compared to the trapped electron states in the presence of Rashba spin-orbit coupling. It is shown that the coupling of orbital and polarization (spin) degrees of freedom is enhanced in polariton rings, leading to the formation of a ground state with a polarization-vortex structure.

In a remarkable manifestation of the Rashba spin-orbit coupling effect, spin polarized carriers injected into a narrow two-dimensional electron system channel have ballistic trajectories and a weakly relativistic Fermi velocity. In the carrier rest frame, the intrinsic electric field in the quantum well transforms as an effective magnetic field, $H^{*}$, transverse to the trajectories. Electrons with initial spin momentum and polarization orientations along the channel precess because of $H^{*}$ with a wavelength $\lambda$. For long mean free paths $l$, spin detection probes at distances $L$ < $l$ record conductance oscillations with wavelength $\lambda$. Two unique experiments that demonstrate these oscillations are discussed.

Recently, the double spin-flip Raman scattering with simultaneous spin reversal of nonequilibrium, localized, and long-lived electrons and holes has been observed in semiconductor perovskites. The authors propose as mechanisms responsible for the double spin-flip processes (i) optical excitation of a localized exciton and its exchange interaction with localized particles, (ii) the process involving biexciton as a resonant intermediate state, and (iii) direct excitation of exciton-polaritons and their scattering by localized carriers. The conditions are discussed, under which the efficiency of double and triple spin-flip scattering can be comparable to that of the first-order process.

It is well known that electron interactions in disordered systems lead to quantum corrections to conductivity at low temperatures. This paper is devoted to the investigation of such corrections in the context of the spin Hall effect. In the Rashba model of spin Hall conductivity, an incoming electron scatters off Friedel’s oscillations in the electron density of a two-dimensional electron gas created by an impurity resulting in two interfering paths. The authors derive the quantum kinetic equation that captures these processes, which is applicable in both diffusive and ballistic transport regimes. They also discuss the frequency dependence of the spin Hall conductivity.

How can electrons tunneling through a superconductor–normal metal–superconductor junction create an anomalous Josephson effect? The authors posed this question to Dall-E 2, a new open source artificial intelligence (AI) system, and got an image result depicting this quantum process. Remarkably this AI art matches almost verbatim the actual Feynman diagram that captures the physics of this phenomenon. In this work, the authors calculate these diagrams, and analyze anomalous Josephson current in the planar double-barrier configurations of hybrid circuits. They use the paradigmatic Rashba model of spin-orbit coupling to describe the noncentrosymmetric normal region of a junction.

Recent experiments have shown that nearly metallic nanotubes remain insulating throughout magnetic field sweeps, in which the magnetic flux through the nanotube is expected to pass through a critical point where the single-particle band gap closes in one valley. Here, the authors investigate correlated insulating states of zigzag carbon nanotubes, which emerge from the interplay of electron-electron interactions, magnetic flux, strain, and spin-orbit coupling. They find that the gap for charged excitations generically remains open near the critical flux value, and predict a novel mirror symmetry breaking phase that may arise in the experimentally relevant regime of spin-orbit coupling.

Back in 1958 E.I. Rashba predicted the so-called resonant (long-range exchange) interaction between an electron and a hole in Wannier-Mott excitons. This interaction underlies exciton fine structure and its spin dynamics. Here, the author develops a model of the coherent exciton spin-valley dynamics in two-dimensional transition metal dichalcogenides caused by the interplay of the wave-vector dependent exchange interaction ($LT$ splitting) and wave-vector independent anisotropic splitting induced by elastic strain. The author uncoversdifferent regimes of coherent spin dynamics of two-dimensional excitons and outlines the possibilities to disentangle the strain-induced from the $LT$ splitting.

Most studies to date have found, by means of various approximate or phenomenological methods, that the Fano factor or the shot noise in a double barrier structure are insensitive to dephasing caused by inelastic scattering. In this paper, the authors explore the status of this statement by deriving a general Landauer-Büttiker-type formula that expresses the current noise and the Fano factor in a one-dimensional conductor through inelastic scattering amplitudes.

Electrically induced space separation of free carriers from different valleys in bulk multivalley semiconductors in restricted geometry was first reported by E. I. Rashba in 1965. This Rashba valley-dependent size effect envisioned the use of a valley index in future electronics. It remains relevant at present for the rapidly developing field of valleytronics. The authors show that electrically induced valley-polarized domains can arise in two-dimensional multivalley semiconductor nanosystems with anisotropic valleys ($n$-Si, $n$-AlAs, etc.). The formation of such domains leads to space separation of carriers from different valleys, generates valley currents, and induces electric (magnetic) fields.

Structural-inversion-asymmetry effects on charge carriers can be described by a spin-orbit (Rashba) effective magnetic field. The direction of this field is fixed by the carrier momentum and axis of asymmetry - properties that form the basis of the spin transistor. The authors have used a sensitive terahertz magnetophotoresponse method to probe electrons in an asymmetric quantum well of InAs in the integer quantum-Hall-effect regime. Remarkable multiple-line spin-resonance spectra are attributed to oppositely directed Rashba fields of electrons in counterpropagating quasi-one-dimensional channels on opposite sample edges. Similar effects might lead to ultrafast electric-field control of spins for any spin-polarized topological edge channels.

In 1964, Solomon Pekar and Emmanuel Rashba observed that nonuniform magnetization couples the electron spin to its orbital motion. It turns out that, for an electron in a Néel antiferromagnet, a smooth texture of the staggered magnetization also produces spin-orbit coupling. For certain locations of electron band extrema in the Brillouin zone, this spin-orbit coupling gives rise to skyrmion-electron bound states. Such bound states turn the skyrmion into a charged particle in the presence of dopant carriers.

In two-dimensional hole systems, the Rashba spin-orbit interaction leads to a spin dependent momentum. A perpendicular magnetic field spatially separates holes with different spins, creating a mass spectrometer for spin. Spin-resolved magnetic focussing has been used to measure spin polarization from the amplitude of magnetic focussing peaks. In this work, the authors show that the $k^3$ form of the Rashba spin-orbit interaction term for semiconductor holes changes the scattering rate, which has an exponential effect on the focussing peak amplitude. This result further demonstrates the impact of the Rashba spin-orbit interaction in the field of semiconductor hole spin physics.

Liquid He-4 is free from magnetic defects, making it an ideal substrate for electrons with long-lived spin states. When electrons are localized in quantum dots and a strong magnetic field is applied along the surface, such states may serve as qubit states. A nonuniform magnetic field from a current-carrying wire submerged into helium enables switchable spin-orbit coupling. Bringing the frequency of orbital intradot vibrations close to the Larmor frequency allows for obtaining a strong Rashba electric dipole spin resonance and efficient interdot spin coupling. The authors show that the associated spin relaxation can be made sufficiently slow, permitting high-fidelity gate operations.

The spin and orbital motion are coupled because the electron spin interacts with a relativistic magnetic field, generated by moving charges that the electron sees in its rest frame. While the celebrated Rashba spin-orbit coupling (SOC) has been studied for decades, the authors reveal here, using novel symmetry, that even simple trends regarding its strength are not understood. Unlike the common understanding that stronger Rashba SOC should enhance many phenomena, from magnetoresistance to exotic spin-triplet superconductivity, the authors propose and experimentally confirm a more nuanced picture in which strong Rashba SOC in junctions with single ferromagnets can be either a friend or a foe.

The spontaneous electric polarization in bulk crystals like wurtzite defies a naive definition using classical electromagnetism, and its measurable signatures have remained unclear. It was also bulk wurtzite for which Emmanuel Rashba discovered in 1959 what is called today the Rashba effect. The authors argue that the Rashba effect is the long-sought hallmark of a spontaneous electric polarization in polar crystals like wurtzite. Beyond that, they develop a comprehensive theory of multipolar order in crystals. Treating electric and magnetic order on equal footing, they identify five categories of polarized matter that are characterized by distinct observable signatures.

The authors study the continuum limit of two-dimensional chiral magnets in which the Dzyaloshinskii-Moriya interaction (DMI) is due to the interplay between a smooth magnetic texture and spin-orbit coupling. They investigate corrections to the DMI free energy, in a gradient expansion, and also in the limits of small Rashba spin-orbit coupling or small exchange coupling, revealing non-Lifshitz invariants in higher-order terms. These contributions are significant for filling near the conduction band bottom at strong Rashba spin-orbit coupling and can stabilize nontrivial spin structures.

The authors investigate the proximity-induced spin-orbit coupling in twisted graphene/topological insulator (Bi${}_2$Se${}_3$ and Bi${}_2$Te${}_3$) heterostructures from first principles. Fitting the band structures around the graphene Dirac cone to an established spin-orbit Hamiltonian yields the twist angle dependencies of the spin-orbit couplings. Amongst other findings, an interesting form of the Rashba spin-orbit coupling arises for such structures. It entails a large radial component of the in-plane spin structure around the Dirac cone, which can be used to realize collinear charge-to-spin conversion.

Surface alloys between the heavy metals Bi, Pb, or Sb and noble metals, such as Ag(111), are known for their giant Rashba splitting. Although thallium (Tl) should result in isostructural surface alloys, its structural and electronic properties remained elusive. The authors present here a detailed work on the structural and electronic properties of Tl films epitaxially grown on Ag(111) surfaces. By combining experimental LEED, AES, STM/STS, and IPE with $ab$ $initio$ band structure and charge distribution calculations, the $s$,$p_z$-derived surface state of the TlAg${}_2$ surface alloy is identified, with a weak, but finite, Rashba splitting.

The authors theoretically demonstrate two simple magneto-optical measurements which can be used to confirm the existence of Rashba terms in two-dimensional semiconductors even in samples of suboptimal quality, where the transition linewidths are comparable to, or larger than, the Landau-level splitting. The first suggested technique is magnetic circular dichroism (MCD), where a giant MCD signal should be observed at the band edge in the presence of Rashba terms. The second one is the comparison of the diamagnetic shift, measured in absorption, with that in photoluminescence which results from the dispersion minimum of the Rashba exciton.

In 1957, Rashba and Davidov published an article on optical absorption in a molecular crystal, considering the interaction of the created exciton with phonons. The problem reduces to calculating the spectral density for the exciton. Here, the authors describe several approximations designed to obtain the behavior of the low-energy tail of the spectral density, where conventional perturbation methods do not work. The analysis includes both the thermal population and quantum motion of the phonons, and an application is made to a one-dimensional continuum model with acoustic phonons.

Quantum machines have been a fascination for years. This paper examines a specific quantum machine - a spin driven by two harmonically related light beams. When the spin adopts a specific Floquet state, it receives energy from one beam and transfers it to the other, acting as an energy frequency converter. The authors study the power distribution of this 2-beam and spin system, uncovering an intriguing transition in distribution width, tuned by the drive amplitude.