arXiv subjects

P. Kleinert

Publications and source records attributed to P. Kleinert.

14 recordsLinked to original sources

arXiv2009

Parquet theory for correlated thermodynamic Greens functions

We address the problem of finding self-consistent parquet equations for two-body correlated Greens functions that arise out of a cumulant expansion. The general theory as developed for non-relativistic electron systems reproduces the conventional two-body parquet diagrammatic technique for the vertex functions. As an application, the crossing-invariant linearized basic parquet equations are treated for the correlated two-particle Greens function.

cond-mat.stat-mech↗

arXiv2008

Excitation of spin waves on a cylindrical semiconductor heterostructure with Rashba spin-orbit interaction

Elementary excitations in a paramagnetic semiconductor quantum well confined to a cylindrical surface are theoretically studied on the basis of coupled spin-charge drift-diffusion equations. The electric-field-mediated eigenmodes are optically excited by an oscillating interference pattern, which induces a current in the outer circuit. For a cylinder with a given radius, sharp resonances are predicted to occur in the steady-state current response, which are due to weakly damped spin remagnetization waves.

cond-mat.mtrl-sci↗

arXiv2008

Two-time quantum transport and quantum diffusion

Based on the nonequilibrium Green's function technique, a unified theory is developed that covers quantum transport and quantum diffusion in bulk semiconductors on the same footing. This approach, which is applicable to transport via extended and localized states, extends previous semi-phenomenological studies and puts them on a firm microscopic basis. The approach is sufficiently general and applies not only to well-studied quantum transport problems, but also to models, in which the Hamiltonian does not commute with the dipole operator. It is shown that, even for the unified treatment of quantum transport and quantum diffusion in homogeneous systems, all quasi-momenta of the carrier distribution function are present and fulfill their specific function. Particular emphasis is put on the double-time nature of quantum kinetics. To demonstrate the existence of robust macroscopic transport effects that have a true double-time character, a phononless steady-state current is identified that appears only beyond the generalized Kadanoff-Baym ansatz.

cond-mat.stat-mech↗

arXiv2008

Electric-field driven long-lived spin excitations on a cylindrical surface with spin-orbit interaction

Based on quantum-kinetic equations, coupled spin-charge drift-diffusion equations are derived for a two-dimensional electron gas on a cylindrical surface. Besides the Rashba and Dresselhaus spin-orbit interaction, the elastic scattering on impurities, and a constant electric field are taken into account. From the solution of the drift-diffusion equations, a long-lived spin excitation is identified for spins coupled to the Rashba term on a cylinder with a given radius. The electric-field driven weakly damped spin waves are manifest in the components of the magnetization and have the potential for non-ballistic spin-device applications.

cond-mat.mtrl-sci↗

arXiv2008

Electric-field induced spin excitations in two-dimensional spin-orbit coupled systems

Rigorous coupled spin-charge drift-diffusion equations are derived from quantum-kinetic equations for the spin-density matrix that incorporate effects due to k-linear spin-orbit interaction, an in-plane electric field, and the elastic scattering on nonmagnetic impurities. The explicit analytical solution for the induced magnetization exhibits a pole structure, from which the dispersion relations of spin excitations are identified. Applications of the general approach refer to the excitation of long-lived field-induced spin waves by optically generated spin and charge patterns. This approach transfers methods known in the physics of space-charge waves to the treatment of spin eigenmodes. In addition, the amplification of an oscillating electric field by spin injection is demonstrated.

cond-mat.mtrl-sci↗

arXiv2008

Superconductivity from four Fermion complexes

Superconductivity is studied for a fermionic system with attractive four-body interaction. Applying a Green function approach, the gap equation is derived. From the solution, the transition temperature is calculated. Under the condition that the respective coupling constants are comparable, the transition temperature of four-fermion complexes is considerably larger than the corresponding BCS value.

cond-mat.supr-con↗

arXiv2007

Field-induced spin excitations in Rashba-Dresselhaus two-dimensional electron systems probed by surface acoustic waves

A 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 the field-induced eigen-modes of the spin-charge coupled system, 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. Sharp resonances in the field dependence of the in-plane and out-of-plane magnetization are identified.

cond-mat.mtrl-sci↗

arXiv2007

Spin polarization in biased Rashba-Dresselhaus two-dimensional electron systems

Based on spin-charge coupled drift-diffusion equations, which are derived from kinetic equations for the spin-density matrix in a rigorous manner, the electric-field-induced nonequilibrium spin polarization is treated for a two-dimensional electron gas with both Rashba and Dresselhaus spin-orbit coupling. Most emphasis is put on the consideration of the field-mediated spin dynamics for a model with equal Rashba and Dresselhaus coupling constants, in which the spin relaxation is strongly suppressed. Weakly damped electric-field-induced spin excitations are identified, which remind of space-charge waves in crystals.

cond-mat.other↗

arXiv2007

Oscillation of spin polarization in a two-dimensional hole gas under a perpendicular magnetic field

Spin-charge coupling is studied for a strongly confined two-dimensional hole gas subject to a perpendicular magnetic field. The study is based on spin-charge coupled drift-diffusion equations derived from quantum-kinetic equations in an exact manner. The spin-orbit interaction induces an extra out-of-plane spin polarization. This contribution exhibits a persistent oscillatory pattern in the strong-coupling regime.

cond-mat.mes-hall↗

arXiv2007

Coupled spin-charge drift-diffusion equations for the Rashba model

Coupled spin-charge drift-diffusion equations are derived for a biased two-dimensional electron gas with weak Rashba spin-orbit interaction. The basic equations formally agree with recent results obtained for spin-orbit coupled small polarons. It is shown that effects of an in-plane electric field on a homogeneous spin system can completely be described by an associated in-plane magnetic field. Exploiting this analogy, we predict among other things the electric-field equivalent of the Hanle effect.

cond-mat.mes-hall↗

arXiv2007

Coupled spin-charge drift-diffusion approach for a two-dimensional electron gas with Rashba spin-orbit coupling

Based on kinetic equations for the density matrix, drift-diffusion equations are derived for a two-dimensional electron gas with Rashba spin-orbit coupling. Universal results are obtained for the weak coupling case. Most interesting is the observation that with increasing spin-orbit coupling strengths there is a sharp transition between spin diffusion and ballistic spin transport. For strong spin-orbit coupling, when the elastic scattering time is much larger than the spin relaxation time, undamped spin-coherent waves are identified. The existence of these long-lived spin-coherent states is confirmed by exact analytical results obtained from microscopic kinetic equations valid in the ballistic regime.

cond-mat.stat-mech↗

arXiv2007

Spin-Hall effect and spin-coherent excitations in a strongly confined two-dimensional hole gas

Based on a rigorous quantum-kinetic approach, spin-charge coupled drift-diffusion equations are derived for a strongly confined two-dimensional hole gas. An electric field leads to a coupling between the spin and charge degrees of freedom. For weak spin-orbit interaction, this coupling gives rise to the intrinsic spin-Hall effect. There exists a threshold value of the spin-orbit coupling constant that separates spin diffusion from ballistic spin transport. In the latter regime, undamped spin-coherent oscillations are observed. This result is confirmed by an exact microscopic approach valid in the ballistic regime.

cond-mat.stat-mech↗

arXiv2006

Theory of spin-Hall transport of heavy holes in semiconductor quantum wells

Based on a proper definition of the spin current, we investigate the spin-Hall effect of heavy holes in narrow quantum wells in the presence of Rashba spin-orbit coupling by using a spin-density matrix approach. In contrast to previous results obtained on the basis of the conventional definition of the spin current, we arrive at the conclusion that an electric-field-induced steady-state spin-Hall current does not exist in both, pure and disordered infinite samples. Only an ac field can induce a spin-Hall effect in such systems.

cond-mat.dis-nn↗

arXiv2005

Theory of electric-field-induced spin accumulation and spin current in the two-dimensional Rashba model

Based on the spin-density-matrix approach, both the electric-field-induced spin accumulation and the spin current are systematically studied for the two-dimensional Rashba model. Eigenmodes of spin excitations give rise to resonances in the frequency domain. Utilizing a general and physically well-founded definition of the spin current, we obtain results that differ remarkably from previous findings. It is shown that there is a close relationship between the spin accumulation and the spin current, which is due to the prescription of a quasi-chemical potential and which does not result from a conservation law. Physical ambiguities are removed that plagued former approaches with respect to a spin-Hall current that is independent of the electric field. For the clean Rashba model, the intrinsic spin-Hall conductivity exhibits a logarithmic divergency in the low-frequency regime.

cond-mat.mes-hall↗