Enhancement of Rashba Spin–Orbit Interaction Due to Wave Function Engineering

We have demonstrated an enhancement of Rashba spin–orbit interaction (SOI) in In_0.53Ga_0.47As/In_0.7Ga_0.3As/In_0.53Ga_0.47As double-step structure in comparison with In_0.53Ga_0.47As normal quantum well. In the double-step structure, high electron probability density is located on the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface to enhance the interface contribution of Rashba SOI. The double-step structure is designed based on k⋅p formalism considering field and interface contributions separately. The Rashba parameter α calculated by the k⋅p formalism shows good agreement with the experimental value by analyzing weak antilocalization. The large carrier density dependence of α is due to the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface contribution as well as the energy-band bending in the In_0.7Ga_0.3As quantum well. The results of this study suggest that the precise control of interface and field contributions in Rashba SOI will make its application to semiconductor spintronics.     

The Decoherence of GaAs Quantum Dot Qubit Due to Electron–Phonon Interactions

We investigate electron charge decoherence in a GaAs single-electron semiconductor quantum dot through electron–phonon interaction. We analytically and numerically evaluate decoherence time within the Lee–Low–Pines–Huybrecht variational calculation for all coupling strengths. The dependence of decoherence time on the electron-LO-phonon coupling strength and the size of quantum dot is investigated. Our results suggest that electron–phonon interaction has very important effects on charge decoherence.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

Two-Photon Coherent Spin Flip and Polarization Rotation of Excitons in Quantum Dots

It is shown that the spin orientation and polarization of an exciton confined in a QD can be controlled via two-photon transitions, by virtually coupling the single-exciton states to the detuned ground and biexciton states. Furthermore, phonon-induced dephasing leading to loss of fidelity in the two-photon optical control schemes is studied. Exciton spin flip can be performed with errors as low as 10⁻³.     

Electron–Phonon Interaction in Doped Fullerides

We propose a theory of interaction of long wave molecular phonons with electrons in fullerides in the presence of disorder. Phonon relaxation rate and frequency renormalization are discussed. Finite electronic bandwidth reduces phonon relaxation rate at q=0. Electron–phonon coupling constants with molecular modes in fullerides are estimated. The results are in good agreement with photoemission experiments.     

The Influence of Electron–Phonon Coupling and Lattice Vibrations on the Rashba Coupling Induced Domain Wall Spin Current and Spin Torque

In the current work, a detailed calculation has been presented for spin-current and spin torque in a domain wall (DW). Specifically, we analyze both spin and momentum relaxation that are relevant to the spin-transport process. It was shown that the vibrational induced relaxations play important roles in the amount of the spin-current generated by the Rashba interaction.     

Quantum Noise of Electron–Phonon Heat Current

We analyze heat current fluctuations between electrons and phonons in a metal. In equilibrium we recover the standard result consistent with the fluctuation–dissipation theorem. Here we show that heat current noise at finite frequencies remains non-vanishing down to zero temperature. From the experimental point of view, it is a small effect and up to now elusive. We briefly discuss the impact of electron–phonon heat current fluctuations on calorimetry, particularly in the regime of single microwave-photon detection.     

Study of the Electron–Phonon Relaxation in Thin Metal Films Using Transient Thermoreflectance Technique

Electron–phonon (e–ph) relaxation in thin metal films is an important consideration in many ultra-small and ultra-fast applications. In this work, e–ph relaxation in thin gold and aluminum films has been studied using the transient thermoreflectance technique which is demonstrated sensitive enough to study the relaxation process. The optical properties of the thin metal films are different from those of bulk metal and have been measured. Based on confirmation of the measurements, the effects of metal type, film thickness, and interface on e–ph relaxation have been experimentally studied. The thermoreflectance traces of gold and aluminum films have been compared. The results show that the e–ph relaxation and the effect of electron and lattice temperatures on the thermoreflectance of gold and aluminum are quite different. The e–ph relaxation is independent of film thickness and interface.     

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.     

Electron–Phonon Interaction and Optical Spectra of Metals

Observed optical reflectivity in the infrared spectral region is compared with theoretical predictions in a strongly coupled electron–phonon system. Starting from a Fröhlich Hamiltonian, the spectral functions and their temperature dependence are derived. A full analysis including vertex corrections leads to an expression for the optical conductivity () that can be formulated in terms of the well-known optical conductivity for a quasi-isotropic system without vertex corrections. A numerical comparison between the full result and the so-called extended Drude formula, its weak coupling expansion, shows little difference over a wide range of coupling constants. Normal-state optical spectra for the high-T _c superconductors YBa₂Cu₃O₇ and La_{2 – x} Sr _x CuO₄ at optimal doping are compared with the results of model calculations. Taking the plasma frequency and from band structure calculations, the model has only one free parameter, the electron–phonon coupling constant . In both materials the overall behavior of the reflectivity can be well accounted for over a wide frequency range. Systematic differences exist only in the mid-infrared region. They become more pronounced with increasing frequency, which indicates that a detailed model for the optical response should include temperature-dependent mid-infrared bands.     

Quantum Dot in the Pseudo-gap Sate with Spin–Orbit Interaction

We study the linear conductance in quantum dot with spin–orbit interaction coupled to Fermi liquid leads with a power-low density of states. The conductance at zero temperature is calculated as a function of the power exponent from the density of state ρ(ω)∼|ω−E _F | ^r at the Fermi energy E _F and the different energy rates. The phase shift of the conduction electrons is also r-dependent. The model can be used in the study of the quantum phase transition.     

Effect of Charge State in Nearby Quantum Dots on Quantum Hall Systems

Transport properties have been measured on two-dimensional electron system (2DES) with a nearby InAs self-assembled quantum dot layer, in order to clarify the anomalous behavior of magnetoresistance, ρ _xx , and Hall resistance, ρ _xy at $\nu=\frac{1}{3}$ , which was an unsolved problem in our previous paper (Takehana et al.: J. Phys. Soc. Jpn. 75, 114713, 2006). The significant suppression of both ρ _xx and ρ _xy was found to be reduced in the vicinity of $\nu=\frac{1}{3}$ , which indicates that the fractional quantum Hall effect prevent the spin-flip process between localized electrons and that extended 2DES, according to the discussion of the previous paper.     

Very Large Rashba Spin–Orbit Splitting in HgTe Quantum Wells

A large Rashba spin splitting has been observed in the first conduction subband of n-type modulation doped HgTe quantum wells with an inverted band structure via an investigation of Shubnikov–de Haas oscillations as a function of gate voltage. Self-consistent Hartree calculations of the band structure based on an 8 × 8 k p model quantitatively describe the experimental results. It has been shown that the heavy-hole nature of the H1 conduction subband greatly influences the spatial distribution of electrons in the quantum well and also enhances the Rashba spin splitting at large electron densities. These are unique features of type III heterostructures in the inverted band regime. The k ³ dispersion predicted by an analytical model is a good approximation of the self-consistent Hartree calculations for small values of the in-plane wave vector k . This is in contrast to the commonly used k dispersion for the conduction subband in type I heterojunctions.     

Infinite Spin Diffusion Length of Any Spin Polarization Along Direction Perpendicular to Effective Magnetic Field from Dresselhaus and Rashba Spin–Orbit Couplings with Identical Strengths in (001) GaAs Quantum Wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek et al. (Nature 458:610, 2009) provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin–orbit couplings with identical strengths in (001) GaAs quantum wells, predicted by Cheng et al. (Phys. Rev. B 75:205328, 2007).     

Pseudo-gap and Vertex Correction of Electron–Phonon Interaction in High Transition-Temperature Superconductors

The strong-coupling Eliashberg theory plus vertex correction is used to calculate the maps of transition temperature (T _c) in parameter-space characterizing superconductivity. Based on these T _c maps, complex crossover behaviors are found when electron?Cphonon interaction increases from weak-coupling region to strong-coupling region. The doping-dependent T _c of cuprate superconductors and most importantly the emergence of pseudo-gap region can be explained as the effects of vertex correction.     

On the Electron–Electron Interaction in Superconductors

By using an attractive potential that varies in reverse proportion with respect to the third power of the distance between the two particles, we resolved the Schrodinger equation for an electron pair. We applied these statements to superconductors and obtained a good agreement with experimental data and BCS theory.     

Emergence of Pairing Interaction in the Hubbard Model in the Strong Coupling Limit

We implement the rotationally-invariant formulation of the two-dimensional Hubbard model, with nearest-neighbors hopping t, which allows for the analytic study of the system in the low-energy limit. Both U(1) and SU(2) gauge transformations are used to factorize the charge and spin contribution to the original electron operator in terms of the corresponding gauge fields. The Hubbard Coulomb energy U-term is then expressed in terms of quantum phase variables conjugate to the local charge and variable spin quantization axis, providing a useful representation of strongly correlated systems. It is shown that these gauge fields play a similar role as phonons in the BCS theory: they act as the “glue” for fermion pairing. By tracing out gauge degrees of freedom the form of paired states is established and the strength of the pairing potential is determined. It is found that the attractive pairing potential in the effective low-energy fermionic action is non-zero in a rather narrow range of U/t.     

Subnanosecond Single Electron Source in the Time-Domain

We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy. The average single electron emission process is recorded with a 0.5 ns time resolution using a real-time fast acquisition card. Single electron signals are compared to simulation based on scattering theory approach adapted for finite excitation energies.