Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime (original) (raw)
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Cotunneling through two-level quantum dots weakly coupled to ferromagnetic leads
Europhysics Letters (EPL), 2006
The spin-polarized transport through two-level quantum dots weakly coupled to ferromagnetic leads is considered theoretically in the Coulomb blockade regime. It is assumed that the dot is doubly occupied, so that the current flows due to cotunneling through singlet and triplet states of the dot. It is shown that transport characteristics strongly depend on the ground state of quantum dot. If the ground state is a singlet, differential conductance (G) displays a broad minimum at low bias voltage, while tunnel magnetoresistance (TMR) is given by the Julliere value. If triplet is the ground state of the system, there is a maximum in differential conductance at zero bias when the leads are magnetized in antiparallel. The maximum is accompanied by a minimum in TMR. The different behavior of G and TMR may thus help to determine the ground state of the dot and the energy difference between the singlet and triplet states.
Physical Review B, 2008
Non-equilibrium transport through a quantum dot with one spin-split single-particle level is studied in the cotunneling regime at low temperatures. The Coulomb diamond can be subdivided into parts differing in at least one of two respects: what kind of tunneling processes (i) determine the single-particle occupations and (ii) mainly contribute to the current. No finite systematic perturbation expansion of the occupations and the current can be found that is valid within the entire Coulomb diamond. We therefore construct a non-systematic solution, which is physically correct and perturbative in the whole cotunneling regime, while smoothly crossing-over between the different regions. With this solution the impact of an intrinsic spin-flip relaxation on the transport is investigated. We focus on peaks in the differential conductance that mark the onset of cotunneling-mediated sequential transport. It is shown that these peaks are maximally pronounced at a relaxation roughly as fast as sequential tunneling. The approach as well as the presented results can be generalized to quantum dots with few levels.
IEEE Transactions on Magnetics, 2008
A model is proposed to study the spin dependent transport of a quantum dot coupled to two noncollinear ferromagnetic leads configuration, in the sequential tunneling regime. The angular deviation () and lead polarization () dependence of the tunneling current and tunneling magnetoresistance (TMR) are investigated for both singly occupied and doubly occupied dot, in the presence of spin flip effect. The current and TMR are found to be more sensitive to and the spin flip rate in the singly-occupied bias regime.
Quantitative study of spin-flip cotunneling transport in a quantum dot
Physical Review B, 2012
We report detailed transport measurements in a quantum dot in a spin-flip co-tunneling regime, and a quantitative comparison of the data to microscopic theory. The quantum dot is fabricated by lateral gating of a GaAs/AlGaAs heterostructure, and the conductance is measured in the presence of an in-plane Zeeman field. We focus on the ratio of the nonlinear conductance values at bias voltages exceeding the Zeeman threshold, a regime that permits a spin flip on the dot, to those below the Zeeman threshold, when the spin flip on the dot is energetically forbidden. The data obtained in three different odd-occupation dot states show good quantitative agreement with the theory with no adjustable parameters. We also compare the theoretical results to the predictions of a phenomenological form used previously for the analysis of non-linear co-tunneling conductance, specifically the determination of the heterostructure g-factor, and find good agreement between the two.
Transport through a double-quantum-dot system with noncollinearly polarized leads
Physical Review B, 2008
We investigate linear and nonlinear transport in a double quantum dot system weakly coupled to spinpolarized leads. In the linear regime, the conductance as well as the nonequilibrium spin accumulation are evaluated in analytic form. The conductance as a function of the gate voltage exhibits four peaks of different heights with mirror symmetry with respect to the charge neutrality point. As the polarization angle is varied, due to exchange effects, the position and shape of the peaks change in a characteristic way, which preserves the electron-hole symmetry of the problem. In the nonlinear regime, various spin-blockade effects are observed. Moreover, negative differential conductance features occur for noncollinear magnetizations of the leads. In the considered sequential tunneling limit, the tunneling magnetoresistance ͑TMR͒ is always positive with a characteristic gate voltage dependence for noncollinear magnetization. If a magnetic field is added to the system, the TMR can become negative.
Transport through a double quantum dot in the sequential tunneling and cotunneling regimes
Physical Review B, 2004
We study transport through a double quantum dot, both in the sequential tunneling and cotunneling regimes. Using a master equation approach, we find that, in the sequential tunneling regime, the differential conductance G as a function of the bias voltage ∆µ has a number of satellite peaks with respect to the main peak of the Coulomb blockade diamond. The position of these peaks is related to the interdot tunnel splitting and the singlet-triplet splitting. We find satellite peaks with both positive and negative values of differential conductance for realistic parameter regimes. Relating our theory to a microscopic (Hund-Mulliken) model for the double dot, we find a temperature regime for which the Hubbard ratio (=tunnel coupling over on-site Coulomb repulsion) can be extracted from G(∆µ) in the cotunneling regime. In addition, we consider a combined effect of cotunneling and sequential tunneling, which leads to new peaks (dips) in G(∆µ) inside the Coulomb blockade diamond below some temperature scales, which we specify.
Spin Blockades in Linear and Nonlinear Transport through Quantum Dots
Physical Review Letters, 1995
The transport properties of a quantum dot that is weakly coupled to leads are investigated by using the exact quantum states of a finite number of interacting electrons. It is shown that in addition to the Coulomb blockade, spin selection rules strongly influence the low temperature transport, and lead to experimentally observable effects. Transition probabilities between states that correspond to successive electron numbers vanish if the total spins differ by | ∆S |> 1/2. In non-linear transport, this can lead to negative differential conductances. The linear conductance peaks are suppressed if transitions between successive ground states are forbidden.
Physical Review B, 2008
We study the effects of spin-flip scatterings on the time-dependent transport properties through a magnetic quantum dot attached to normal and ferromagnetic leads. The transient spin-dynamics as well as the steady-state tunneling magnetoresistance (TMR) of the system are investigated. The absence of a definite spin quantization axis requires the time-propagation of two-component spinors. We present numerical results in which the electrodes are treated both as one-dimensional tightbinding wires and in the wide-band limit approximation. In the latter case we derive a transparent analytic formula for the spin-resolved current, and transient oscillations damped over different timescales are identified. We also find a novel regime for the TMR inversion. For any given strength of the spin-flip coupling the TMR becomes negative provided the ferromagnetic polarization is larger than some critical value. Finally we show how the full knowledge of the transient response allows for enhancing the spin-current by properly tuning the period of a pulsed bias.
Zero-bias anomaly in cotunneling transport through quantum-dot spin valves
Physical Review B, 2005
We predict a new zero-bias anomaly in the differential conductance through a quantum dot coupled to two ferromagnetic leads with antiparallel magnetization. The anomaly differs in origin and properties from other anomalies in transport through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade valleys with an unpaired dot electron. It is a consequence of the interplay of single-and double-barrier cotunneling processes and their effect on the spin accumulation in the dot. The anomaly becomes significantly modified when a magnetic field is applied.
Spin Blockade in Non-linear Transport through Quantum Dots
Europhysics Letters (EPL), 1994
The influence of excited levels on the nonlinear transport properties of a quantum dot weakly coupled to ideal leads is studied using a master-equation approach. A quantum mechanical model for interacting electrons is used to determine the spectrum of the dot. The current-voltage characteristic shows Coulomb blockade and additional finestructure that is related to the excited states of the correlated electrons. Asymmetric coupling to the leads causes asymmetric conductance peaks. It is demonstrated that spin selection rules can lead to regions of negative differential conductance.