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arXiv:1003.0069 · Physical Review B

Spin current through an ESR quantum dot: A real-time study

Lei Wang, Hua Wei Jiang, Jun Zhuang, Xi Dai, Xin Xie

Published Feb 26, 2010• 0 citations
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Abstract

The spin transport in a strongly interacting spin-pump nanodevice is studied using the time-dependent variational-matrix-product-state approach. The precession magnetic field generates a dissipationless spin current through the quantum dot. We compute the real-time spin current away from the equilibrium condition. Both transient and stationary states are reached in the simulation. The essentially exact results are compared with those from the Hartree-Fock approximation (HFA). It is found that correlation effect on the physical quantities at quasisteady state are captured well by the HFA for small interaction strength. However the HFA misses many features in the real-time dynamics. Results reported here may shed light on the understanding of the ultrafast processes as well as the interplay of the nonequilibrium and strongly correlated effect in the transport properties.

DOI
10.1103/physrevb.81.075323

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The spin transport in a strongly interacting spin-pump nanodevice is studied using the time-dependent variational-matrix-product-state approach. The precession magnetic field generates a dissipationless spin current through the quantum dot. We compute the real-time spin current away from the equilibrium condition. Both transient and stationary states are reached in the simulation. The essentially exact results are compared with those from the Hartree-Fock approximation (HFA). It is found that correlation effect on the physical quantities at quasisteady state are captured well by the HFA for small interaction strength. However the HFA misses many features in the real-time dynamics. Results reported here may shed light on the understanding of the ultrafast processes as well as the interplay of the nonequilibrium and strongly correlated effect in the transport properties.
The finite-temperature transport properties of the spinless interacting fermion model coupled to non-interacting leads are investigated. Employing the unrestricted time-dependent Hartree–Fock (HF) approximation, the transmission probability and the nonlinear I – V characteristics are calculated, and compared with available analytical results and with numerical data obtained from a Hubbard–Stratonovich decoupling of the interaction. In the weak interaction regime, the HF approximation reproduces the gross features of the exact I – V characteristics but fails to account for subtle properties like the particular power law for the reflected current in the interacting resonant level model.
We calculate steady-state properties of a strongly correlated quantum dot under voltage bias by means of nonequilibrium cluster perturbation theory and the nonequilibrium variational cluster approach, respectively. Results for the steady-state current are benchmarked against data from accurate matrix product state based time evolution. We show that for low to medium interaction strength, nonequilibrium cluster perturbation theory already yields good results, while for higher interaction strength the self-consistent feedback of the nonequilibrium variational cluster approach significantly enhances the accuracy. We report the current-voltage characteristics for different interaction strengths. Furthermore we investigate the nonequilibrium local density of states of the quantum dot and illustrate that within the variational approach a linear splitting and broadening of the Kondo resonance is predicted which depends on interaction strength. Calculations with applied gate voltage, away from particle-hole symmetry, reveal that the maximum current is reached at the crossover from the Kondo regime to the doubly occupied or empty quantum dot. Obtained stability diagrams compare very well to recent experimental data [A. V. Kretinin et al., Phys. Rev. B 84, 245316 (2011)].
We present a stochastic thermodynamics analysis of an electron spin resonance pumped quantum dot device in the Coulomb-blocked regime, where a pure spin current is generated without an accompanying net charge current. Based on a generalized quantum master equation beyond secular approximation, quantum coherences are accounted for in terms of an average spin in the Floquet basis. Elegantly, this average spin undergoes a precession about an effective magnetic field, which originates from the nonsecular treatment and energy renormalization. It is shown that the interaction between average spin and effective magnetic field may have the dominant roles to play in both energy transport and irreversible entropy production. In the stationary limit, the energy and entropy balance relations are also established based on the theory of counting statistics.
A general theoretical formulation for the effect of a strong on-site Coulomb interaction on the time-dependent electron transport through a quantum dot under the influence of arbitrary time-varying bias voltages and/or external fields is presented, based on slave bosons and the Keldysh nonequilibrium Green's function (GF) techniques. To avoid the difficulties of computing double-time GFs, we generalize the propagation scheme recently developed by Croy and Saalmann to combine the auxiliary-mode expansion with the celebrated Lacroix's decoupling approximation in dealing with the second-order correlated GFs and then establish a closed set of coupled equations of motion, called second-order quantum rate equations (SOQREs), for an exact description of transient dynamics of electron correlated tunneling. We verify that the stationary solution of our SOQREs is able to correctly describe the Kondo effect on a qualitative level. Moreover, a comparison with other methods, such as the second-order von Neumann approach and Hubbard-I approximation, is performed. As illustrations, we investigate the transient current behaviors in response to a step voltage pulse and a harmonic driving voltage, and linear admittance as well, in the cotunneling regime.
We exploit the pumped spin-current and current noise spectra under equilibrium conditions in a single quantum dot connected to two normal leads as an electrical scheme for detection of the electron spin resonance (ESR) and decoherence. We propose spin-resolved quantum rate equations with correlation functions in Laplace space for the analytical derivation of the zero-frequency auto- and cross-shot noise spectra of charge and spin current. Our results show that in the strong Coulomb blockade regime, ESR-induced spin flip generates a finite spin current and quantum partition noises in the absence of net charge transport. Moreover, spin shot noise is closely related to the magnetic Rabi frequency and decoherence and would be a sensitive tool to measure them.
We introduce a real time version of the functional renormalization group, which allows us to study correlation effects on nonequilibrium transport through quantum dots. Our method is equally capable to address (i) the relaxation out of a nonequilibrium initial state into a (potentially) steady state driven by a bias voltage and (ii) the dynamics governed by an explicitly time-dependent Hamiltonian. All time regimes from transient to asymptotic can be tackled; the only approximation is the consistent truncation of the flow equations at a given order. As an application we investigate the relaxation dynamics of the interacting resonant level model, which describes a fermionic quantum dot dominated by charge fluctuations. Moreover, we study incoherence and relaxation phenomena within the ohmic spin-boson model by mapping the latter to the interacting resonant level model.