Coherent tunnelling through two quantum dots with Coulomb interaction (original) (raw)
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Coulomb interaction and transport in tunnel junctions and quantum dots
Physica B: Condensed Matter, 1993
In the first part of the paper the AC conductance of a quasi-one-dimensional tunnel junction involving a potential barrier is calculated in linear response. Its frequency dependence is used to define a dynamical capacitance. The influence of phase breaking electron-phonon interactions is investigated. It is argued that Coulomb interaction is of minor importance at higher frequencies and that dynamic and static capacitances are the same. The argument provides a high-frequency limit for turnstile operation. In the second part, the quantum mechanical properties of few interacting electrons in quantum dots are considered. Including the spin degree of freedom, the spectral properties of up to four interacting electrons confined within a quasi-one-dimensional system of finite length with Coulomb interactions are investigated by numerical diagonalization. The limitations of the description in terms of a capacitance are discussed. For sufficiently low density the electrons become localized, forming a Wigner molecule. The energetically lowest excitations are identified as vibrational and tunneling modes, both being collective modes involving all the electrons.
Spin-polarized transport through weakly coupled double quantum dots in the Coulomb-blockade regime
Physical Review B, 2007
We analyze cotunneling transport through two quantum dots in series weakly coupled to external ferromagnetic leads. In the Coulomb blockade regime the electric current flows due to third-order tunneling, while the second-order single-barrier processes have indirect impact on the current by changing the occupation probabilities of the double dot system. We predict a zero-bias maximum in the differential conductance, whose magnitude is conditioned by the value of the inter-dot Coulomb interaction. This maximum is present in both magnetic configurations of the system and results from asymmetry in cotunneling through different virtual states. Furthermore, we show that tunnel magnetoresistance exhibits a distinctively different behavior depending on temperature, being rather independent of the value of inter-dot correlation. Moreover, we find negative TMR in some range of the bias voltage.
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.
Coherent versus Sequential Electron Tunneling in Quantum Dots
Physical Review Letters, 2003
Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak-heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels. PACS numbers: 73.21.La, 03.65.Yz Recent experimental studies of electronic transport through nearly isolated quantum dots [1, 2] assess the importance of quantum coherence and the nature of dephasing mechanisms in finite interacting electronic systems. Of particular interest is the Coulomb blockade regime, where the thermal energy k B T is much smaller than the charging energy E C necessary to add an electron to the quantum dot. In this regime the conductance depends primarily on the quantum properties of the dot, such as its resonance levels and the corresponding line widths due to the coupling between the dot and leads. Electrons are allowed to tunnel through the quantum dot whenever the charging energy is compensated by an external potential and the dot energy levels are in resonance with the chemical potential at the leads (small bias limit). The tunneling condition can be attained, for instance, by a tunable gate voltage V g . In a typical experiment V g is varied to obtain the conductance spectrum, a sequence of sharp (Coulomb blockade) peaks.
Charge transport through quantum dots via time-varying tunnel coupling
Physical Review B, 2001
We describe a mechanism for charge pumping through tunnel-coupled quantum dots in the regime of strong Coulomb blockade. The quantum state of an additional electron within the structure is steered by changing the tunneling couplings between neighboring dots. Appropriate tailoring of the interdot tunneling rates allows one to design the instantaneous eigenvalues of the system Hamiltonian. A combination of adiabatic following and Landau-Zener tunneling results in the transfer of charge from one dot to the neighboring one. Coupling to electron reservoirs via weak tunnel barriers then allows one to implement an electron pump.
Transport properties of quantum dots
Annalen der Physik, 2010
Linear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current-voltage characteristics show Coulomb blockade. The excited states lead to additional finestructure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a 'spin blockade' which decreases the current when certain excited states become involved in the transport. In two-dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many-electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments. electron tunneling (SET) transistors . In SET-devices, a controlled transfer of electrons -one by one -can be achieved [5] by applying . This could also open the way to a new current standard based on counting the electrons that pass the device per unit of time .
Negative differential conductance induced by electronic correlation in a double quantum dot molecule
Physical Review B, 2008
Spin-charge states of correlated electrons in a one-dimensional quantum dot attached to interacting leads are studied in the non-linear transport regime. With non-symmetric tunnel barriers, regions of negative differential conductance induced by spin-charge separation are found. They are due to a correlation-induced trapping of higher-spin states without magnetic field, and associated with a strong increase in the fluctuations of the electron spin.
Electronic Correlations in Transport through Coupled Quantum Dots
Physical Review Letters, 1999
The conductance through two quantum dots in series is studied using general qualitative arguments and quantitative slave-boson mean-field theory. It is demonstrated that measurements of the conductance can explore the phase diagram of the two-impurity Anderson model. Competition between the Kondo effect and the interdot magnetic exchange leads to a two-plateau structure in the conductance as a function to the gate voltage and a two or three peak structure in the conductance versus interdot tunneling. [S0031-9007(99)09017-1]
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.