Spin relaxation in single-layer graphene with tunable mobility

Graphene is an attractive material for spintronics due to theoretical predictions of long spin lifetimes arising from low spin-orbit and hyperfine couplings. In experiments, however, spin lifetimes in single-layer graphene (SLG) measured via Hanle effects are much shorter than expected theoretically. Thus, the origin of spin relaxation in SLG is a major issue for graphene spintronics. Despite extensive theoretical and experimental work addressing this question, there is still little clarity on the microscopic origin of spin relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to tune the mobility between 2700 and 12?000 cm(2)/(V s), we successfully isolate the effect of charged impurity scattering on spin relaxation in SLG. Our results demonstrate that, while charged impurities can greatly affect mobility, the spin lifetimes are not affected by charged impurity scattering.     

Spin-Orbit Coupling in Fe-Based Superconductors

We study the spin resonance peak in recently discovered iron-based superconductors. The resonance peak observed in inelastic neutron scattering experiments agrees well with predicted results for the extended s-wave (s _±) gap symmetry. Recent neutron scattering measurements show that there is a disparity between transverse and longitudinal components of the dynamical spin susceptibility. Such breaking of the spin-rotational invariance in the spin-liquid phase can occur due to spin-orbit coupling. We study the role of the spin-orbit interaction in the multiorbital model for Fe-pnictides and show how it affects the spin resonance feature.     

Weak localization experiments on magnesium films: effects of substrate,ion implantation,microwaves, temperature,and resistivity/thickness on the phase and spin-orbit relaxation

Weak localization magnetoresistance is used to investigate spin-orbit and the phase relaxation rates. The spin-orbit rate is independent of temperature. By plotting the spin-orbit rate as a function of the inverse thickness of the films, we separate the spin-orbit relaxation into two parts: one from the scattering off the bulk imperfections, and one from the scattering against the two surfaces. The ratio between the spin-orbit and the impurity relaxation rates in the bulk was found to be close to 2 × 10^–5 for all the different samples. The influence of implanted heavy ions on the spin-orbit relaxation was also investigated. The dependence of the phase relaxation rate on resistivity, film thickness, and temperature has been studied. Theoretical results for electron-electron and electron-phonon scattering are compared to our data. We consider two novel temperature-independent phase relaxation mechanisms which may explain the residual rate we observe. The influence of a high-frequency electromagnetic field on the phase relaxation rate was investigated. For small microwave power levels the phase relaxation rate was found to increase linearly with microwave power. In the absence of a magnetic field and for samples having a dominating spin-orbit interaction (antilocalization) the resistance increases with microwave power, but turns into a decrease at high microwave power levels. For those samples having very weak spin-orbit interaction the resistance decreases continuously as we apply microwaves. This is roughly the expected behavior, but the observed change in resistance was larger than that calculated at small microwave power levels.     

Strong tuning of Rashba spin-orbit interaction in single InAs nanowires

A key concept in the emerging field of spintronics is the gate voltage or electric field control of spin precession via the effective magnetic field generated by the Rashba spin-orbit interaction. Here, we demonstrate the generation and tuning of electric field induced Rashba spin-orbit interaction in InAs nanowires where a strong electric field is created by either a double gate or a solid electrolyte surrounding gate. In particular, the electrolyte gating enables 6-fold tuning of Rashba coefficient and nearly 3 orders of magnitude tuning of spin relaxation time within only 1 V of gate bias. Such a dramatic tuning of spin-orbit interaction in nanowires may have implications in nanowire-based spintronic devices.     

Spin transport in self assembled all-metal nanowire spin valves: a study of the pure Elliott-Yafet mechanism

We report experimental study of spin transport in all metal nanowire spin valve structures. The nanowires have a diameter of 50 nm and consist of three layers--cobalt, copper, and nickel. Based on the experimental observations, we determine that the primary spin relaxation mechanism in the paramagnet layer--copper--is the Elliott-Yafet mode associated with elastic scattering caused by charged states on the surface of the nanowires. This mode is overwhelmingly dominant over all other modes, so that we are able to study the pure Elliott-Yafet mechanism in isolation. We deduce that the spin diffusion length associated with this mechanism is about 16 nm in our nanowires and is fairly temperature independent in the range 1-100 K, which is consistent with the spin relaxation being associated with elastic scattering by surface states. The corresponding spin relaxation time is about 100 femtoseconds. We also find that the spin relaxation rate is fairly independent of the electric field driving the current in the field range 0.3-3 kV/cm.     

Nuclear magnetic resonance in small superconducting particles. I

A theoretical treatment of the effects of superconducting fluctuations on nuclear spin-lattice relaxation in small particles given by imánek, Imbro, and MacLaughlin is refined, taking into account the Zeeman energy and the pairbreaking effect introduced by a magnetic field dependence of impurity vertex corrections. The dimensions of the particle are supposed to be less than the coherence length and the penetration depth, but still large enough for electronic levels to have a continuous spectrum. In this paper, spin-orbit interactions are not considered, though they have a large effect on the role of the Zeeman energy. Calculations are valid in the dirty limit and at temperatures not necessarily close to the transition temperature of the bulk superconductor. The spin susceptibility and the nuclear spin-lattice relaxation time are calculated as functions of the particle size and the field. The former turns out to be in good agreement with the theoretical results by Mühlschlegel, Scalapino, and Denton, in which energy levels are considered to be discrete. It is shown that the Zeeman energy and the pair-breaking mechanism give opposite effects on the behavior of the relaxation time at low temperatures. Recent experiments have found a characteristic field dependence of the relaxation time, which is reproduced fairly well by the present calculations.     

Spin relaxation in InGaN quantum disks in GaN nanowires

The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of ～100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott-Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D'yakonov-Perel' relaxation mechanism in the presence of Rashba spin-orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.     

Generalized de Gennes-Takahashi-Tachiki proximity effect theory

We generalize the de Gennes-Takahashi-Tachiki theory of coupled dirty superconductors to include the effects of orbital diamagnetism, spin (Pauli) paramagnetism, spin-orbit scattering, and magnetic impurity scattering. Some new expressions for thin film sandwiches and superlattices are obtained.     

Magnetotransport Calculations for Two-Dimensional Electrons with Spin-Orbit Interaction

We present calculations of the magnetoconductivity in a two-dimensional electron system including the Rashba spin-orbit interaction. Essential for these calculations is an extension of the self-consistent Born approximation which takes into account the electron spin degree of freedom. The calculated magnetoconductivity exhibits, besides the beating in the Shubnikov-de Haas oscillations, a modulation related to the spin-orbit induced crossings of Landau levels, as a consequence of spin-conserving scattering between spin-orbit coupled states.     

D'yakonov–Perel' Spin Relaxation Controlled by Electron–Electron Scattering

The D'yakonov–Perel' mechanism of spin relaxation is connected with the spin splitting of the electron dispersion curve in crystals lacking a center of symmetry. In a two-dimensional noncentrosymmetric system, e.g. quantum well or heterojunction, the spin splitting is a linear function of k, at least for small values of k. We demonstrate that the spin relaxation time τ_s due to the spin splitting is controlled not only by momentum relaxation processes as widely accepted but also by electron–electron collisions which have no effect on the electron mobility. In order to calculate the time τ_s taking into account the electron–electron scattering, we have solved the two-dimensional kinetic equation for the electron spin density matrix. The result has been compared with that obtained assuming the momentum scattering to occur due to elastic scattering of electrons by ionized impurities. We have also extended the quasi-elastic approximation to describe the electron–electron collision integral for a spin-polarized three-dimensional electron gas.     

Electrically tuned spin-orbit interaction in an InAs self-assembled quantum dot

Electrical control over electron spin is a prerequisite for spintronics spin-based quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spin-orbit interaction in two-dimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spin-orbit interaction in the range 50-150 μeV while maintaining the electron occupation of a single self-assembled InAs quantum dot. We determine the spin-orbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.     

Spin-orbit coupling induced interference in quantum corrals

Lack of inversion symmetry at a metallic surface can lead to an observable spin-orbit interaction. For certain metal surfaces, such as the Au(111) surface, the experimentally observed spin-orbit coupling results in spin rotation lengths on the order of tens of nanometers, which is the typical length scale associated with quantum corral structures formed on metal surfaces. In this work, multiple scattering theory is used to calculate the local density of states (LDOS) of quantum corral structures composed of nonmagnetic adatoms in the presence of spin-orbit coupling. Contrary to previous theoretical predictions, spin-orbit coupling induced modulations are observed in the theoretical LDOS, which should be observable using scanning tunneling microscopy.     

Retardation effects in the longitudinal spin relaxation in metals. II

The memory kernel of the longitudinal dynamical impurity susceptibility is calculated as a function of frequency, magnetic field, and temperature up to third order in the exchange coupling of thes-d model. It is shown that the strong retardation effects for the spin relaxation due to freezing out of the spin-flip scattering are modified by Kondo anomalies only slightly.Supported by the Deutscher Akademischer Austauschdienst.     

Hole spin relaxation in Ge-Si core-shell nanowire qubits

Controlling decoherence is the biggest challenge in efforts to develop quantum information hardware. Single electron spins in gallium arsenide are a leading candidate among implementations of solid-state quantum bits, but their strong coupling to nuclear spins produces high decoherence rates. Group IV semiconductors, on the other hand, have relatively low nuclear spin densities, making them an attractive platform for spin quantum bits. However, device fabrication remains a challenge, particularly with respect to the control of materials and interfaces. Here, we demonstrate state preparation, pulsed gate control and charge-sensing spin readout of hole spins confined in a Ge-Si core-shell nanowire. With fast gating, we measure T(1) spin relaxation times of up to 0.6 ms in coupled quantum dots at zero magnetic field. Relaxation time increases as the magnetic field is reduced, which is consistent with a spin-orbit mechanism that is usually masked by hyperfine contributions.     

D''yakonov–Perel'' Spin Relaxation Controlled by Electron–Electron Scattering

The D'yakonov–Perel' mechanism of spin relaxation is connected with the spin splitting of the electron dispersion curve in crystals lacking a center of symmetry. In a two-dimensional noncentrosymmetric system, e.g. quantum well or heterojunction, the spin splitting is a linear function of k, at least for small values of k. We demonstrate that the spin relaxation time _s due to the spin splitting is controlled not only by momentum relaxation processes as widely accepted but also by electron–electron collisions which have no effect on the electron mobility. In order to calculate the time _s taking into account the electron–electron scattering, we have solved the two-dimensional kinetic equation for the electron spin density matrix. The result has been compared with that obtained assuming the momentum scattering to occur due to elastic scattering of electrons by ionized impurities. We have also extended the quasi-elastic approximation to describe the electron–electron collision integral for a spin-polarized three-dimensional electron gas.     

Electric-Field Induced Spin Excitation in Quantum Wells with Rashba–Dresselhaus Spin-Orbit Coupling

The spin-rotation symmetry in spin-orbit coupled two-dimensional electron systems gives rise to a long-lived spin excitation that is robust against short-range impurity scattering. The influence of a constant in-plane electric field on this persistent spin helix is studied. To probe field-mediated spin modes, a surface acoustic wave is exploited that provides the wave-vector for resonant excitation. The approach takes advantage of methods worked out in the field of space-charge waves. A sharp resonance in the electric field dependence of the magnetization is identified.     

On the longitudinal static and dynamic susceptibility of spin-1/2 Kondo systems

The impurity spin polarization, static susceptibility, and longitudinal impurity spin relaxation rate are calculated for thes-d model as function of temperature and magnetic field for ferromagnetic and antiferromagnetic exchange coupling. The thermodynamic functions and the dynamical susceptibility are obtained from the impurity relaxation spectrum, which is approximated by taking into account the infrared-like singularities. For antiferromagnetic coupling the zero-field susceptibility obeys a Curie-Weiss law1/χ～4.6(T+θ) for high and intermediate temperatures and it approaches the finite value1/χ～3.8θ for zero temperature. The zero-field relaxation rate is much larger than the Korringa value; it decreases with temperature and approaches the nonzero value1/T ₁～1.2θ for zero temperature. The relaxation rate decreases with increasing field. The results for the spin polarization agree well with the experimental data for the Cu:Fe alloy.     

Frequency dependence of AC susceptibility of bcc solid3He

We have measured AC susceptibility of bcc solid³ He in a two orders of magnitude with a wider range than in the previous study. For the sample of 23.0 cm³/mol, the relaxation time between the Zeeman and exchange reservoirs in the paramagnetic state is on the order of 10 ms, while the relaxation time in the ordered state decreases down to one hundredth of the value in the paramagnetic state. This small relaxation time is related to the process of spin wave relaxation in the ordered state.     

Low Frequency Susceptibility of High-Density bcc Solid 3He in the Paramagnetic and Nuclear-Ordered States

We have measured the AC magnetic susceptibility and static magnetization of high-density bcc solid ³ He through the nuclear-ordering transition. The susceptibility in the paramagnetic state strongly depends on the frequency of the measuring field. Near the transition temperature a sharp peak in the real part of the AC susceptibility and an abrupt depression in the imaginary part are observed. The transition temperature indicated by the AC susceptibility is higher than that derived from static magnetization. We analyzed the new behavior in the susceptibility in terms of the spin relaxation between the Zeeman system and the exchange system. The relaxation time in the energy flow in the two systems is in the range of milliseconds in the paramagnetic state, and decreases drastically by two orders of magnitude in the ordered state. The relaxation time in the paramagnetic state is interpreted as due to exchange narrowing, while in the ordered state is explained to be the drift time of the spin wave limited by the size of the sample grown in the pores of the sintered silver. The ordering temperature is given as a function of molar volume in the entire range of the bcc phase.     

Spin-dependent plasmonics based on interfering topological defects

Observation of spin-dependent plasmonics based on the interference of topological defects in the near-field is presented. We utilize the surface plasmons' scattering dynamics from localized vortex sources to create spinoptical devices as an ensemble of isolated nanoantennas to observe a "giant" spin-dependent plasmonic vortex and a spin-dependent plasmonic focusing lens. The spin-orbit point spread function, a spiral wavefront, is introduced, where the optical spin is a degree of freedom.