Strong Intradot Coulomb Interaction Research Articles

Tunability of the Andreev reflection induced by the Majorana zero mode

The Andreev reflection induced by Majorana zero mode (MZM) is theoretically investigated, by considering the existence of two coupling manners between the MZM and normal metallic lead, i.e., the direct coupling and the indirect coupling via a quantum dot. We find that in the noninteracting or weak-Coulomb cases, the low-bias conductance can be modulated by the interplay among the structural parameters. As a result, the conductance peak at the zero-bias limit has an opportunity to change into antiresonance in the case of appropriate structural parameters. When strong intradot Coulomb interaction is taken into account, the zero-bias conductance tends to show up as one peak with its width tightly related to the level, Coulomb strength, and the Zeeman splitting of the quantum dot. We believe that this work is helpful for ascertaining the signature of MZM in the Andreev reflection spectrum.

Thermoelectric unipolar spin battery in a suspended carbon nanotube

A quantum dot formed in a suspended carbon nanotube exposed to an external magnetic field is predicted to act as a thermoelectric unipolar spin battery which generates pure spin current. The built-in spin flip mechanism is a consequence of the spin-vibration interaction resulting from the interplay between the intrinsic spin–orbit coupling and the vibrational modes of the suspended carbon nanotube. On the other hand, utilizing thermoelectric effect, the temperature difference between the electron and the thermal bath to which the vibrational modes are coupled provides the driving force. We find that both magnitude and direction of the generated pure spin current are dependent on the strength of spin-vibration interaction, the sublevel configuration in dot, the temperatures of electron and thermal bath, and the tunneling rate between the dot and the pole. Moreover, in the linear response regime, the kinetic coefficient is non-monotonic in the temperature T and it reaches its maximum when is about one phonon energy. The existence of a strong intradot Coulomb interaction is irrelevant for our spin battery, provided that high-order cotunneling processes are suppressed.

Out-of-phase Andreev transports in a double-quantum-dot Cooper-pair splitter

We investigate the Andreev transports through a double-quantum-dot Cooper-pair splitter with an additional direct coupling between the dots. It is found that when finite magnetic flux is applied through this structure, its induced phase adjusts the local and crossed Andreev reflections in different manners. Moreover, in the presence of appropriate magnetic flux, the crossed Andreev reflection can be efficiently enhanced, accompanied by the suppression of the local Andreev reflection. Such a result exists in both the linear and nonlinear transport regions, even in the case of strong intradot Coulomb interaction. Therefore, such a work provides a promising scheme to enhance the splitting of the Cooper pair.

A high-efficiency double quantum dot heat engine

High-efficiency heat engine requires a large output power at the cost of less input heat energy as possible. Here we propose a heat engine composed of serially connected two quantum dots sandwiched between two metallic electrodes. The efficiency of the heat engine can approach the maximum allowable Carnot efficiency ηC. We also find that the strong intradot Coulomb interaction can induce additional work regions for the heat engine, whereas the interdot Coulomb interaction always suppresses the efficiency. Our results presented here indicate a way to fabricate high-efficiency quantum-dot thermoelectric devices.

Thermally driven unipolar and bipolar spin diode based on double quantum dots

Using the master equation approach, we theoretically propose a unipolar spin diode consisting of two serially coupled quantum dots connected to two normal metal electrodes, in which the two electrodes have different temperatures, and the two dots have different Zeeman splittings. The current through this system can be from zero to nearly 100% spin-polarized by tuning the difference of the electrode temperatures or the gate voltages applied to the quantum dots. This particular transport property is attributed to the interplay of the strong intradot Coulomb interactions, the asymmetrical energy level distributions in the two dots induced by the different Zeeman splittings and different Fermi distributions in the two electrodes caused by different temperatures. Also, a bipolar spin diode can be designed using the two unipolar spin diodes. These configurations may provide the essential elements for the design of logic circuits and even magnetic heads.

The role of Coulomb interaction in thermoelectric effects of an Aharonov–Bohminterferometer

We investigate the thermoelectric effects of an Aharonov–Bohm (AB) interferometer with aquantum dot (QD) embedded in each of its arms, where the intra-dot Coulomb interactionbetween electrons in each QD is taken into account. Using Green’s function methods andthe equation of motion (EOM) technique, we find that the Seebeck coefficient and Lorenznumber can be strongly enhanced when the chemical potential sweeps the molecularstates associated with the Fano line-shapes in the transmission spectra, due toquantum interference effects between the bonding and antibonding molecular states.It is found that enhancement of the thermoelectric effects occurs between thetwo groups of conductance peaks in the presence of strong intra-dot Coulombinteraction—the reason being that a transmission node is developed in the Coulombblockade regime. In this case, the maximum value of the Lorenz number approaches10π2kB2/(3e2). Itsthermoelectric conversion efficiency in the absence of phonon thermal conductance, described by the figureof merit ZT, approaches 2 at room temperature. Therefore, it may be used as a high-efficiencysolid-state thermoelectric conversion device under certain circumstances.

Shot Noise in a Spin-Diode Geometry

We apply the master equation technique to calculate shot noise in a system composed of single level quantum dot attached to a normal metal lead and to a ferromagnetic lead (NM-QD-FM). It is known that this system operates as a spin-diode, giving unpolarized currents for forward bias and polarized current for reverse bias. This effect is observed when only one electron can tunnel at a time through the dot, due to the strong intradot Coulomb interaction. We find that the shot noise also presents a signature of this spin-diode effect, with a super-Poissonian shot noise for forward and a sub-Poissonian shot noise for reverse bias voltages. The shot noise thus can provide further experimental evidence of the spin-rectification in the NM-QD-FM geometry.

Two-electron bound states in a continuum in quantum dots

A bound state in a continuum (BIC) might appear in open quantum dots for the variation in the dot’s shape. By means of the equations of motion of the Green’s functions, we investigate the effect of strong intradot Coulomb interactions on that phenomenon within the framework of the impurity Anderson model. The equation that the imaginary part of the poles of the Green’s function equals zero yields the condition for BICs. As a result, we show that the Coulomb interactions replicate the single-electron BICs into two-electron ones.

Double Rashba Quantum Dots Ring as a Spin Filter

We theoretically propose a double quantum dots (QDs) ring to filter the electron spin that works due to the Rashba spin–orbit interaction (RSOI) existing inside the QDs, the spin-dependent inter-dot tunneling coupling and the magnetic flux penetrating through the ring. By varying the RSOI-induced phase factor, the magnetic flux and the strength of the spin-dependent inter-dot tunneling coupling, which arises from a constant magnetic field applied on the tunneling junction between the QDs, a 100% spin-polarized conductance can be obtained. We show that both the spin orientations and the magnitude of it can be controlled by adjusting the above-mentioned parameters. The spin filtering effect is robust even in the presence of strong intra-dot Coulomb interactions and arbitrary dot-lead coupling configurations.

Shell effects in nonlinear magnetotransport through small quantum dots

Nonlinear transport through a quantum dot attached to two metallic contacts is studied in the limit of a weak and a strong intradot Coulomb interaction. The nonequilibrium self-consistent mean-field equations for the energies and spectral weights of one-electron transitions are formulated in the strong-interaction regime. The self-consistent solutions result in a decrease of bias voltage threshold for nonzero current, contrary to the weak case. With an increase of the bias voltage window, the solutions lead to a significant deviation from the Gibbs statistics: the populations of states involved in tunneling are equalizing even at low temperatures. For a symmetric dot-contact coupling we obtain simple analytical relations between the heights of the current steps and degeneracies of a spectrum of a circular dot in a perpendicular magnetic field, in both regimes.

Spin-polarized current through a lateral double quantum dot with spin–orbit interaction

Abstract We study the spin-polarized current through a vertical double quantum dot scheme. Both the Rashba spin–orbit (RSO) interaction inside one of the quantum dots and the strong intradot Coulomb interactions on the two dots are taken into account by using the second-quantized form of the Hamiltonian. Due to the existence of the RSO interaction, spin-up and spin-down electrons couple to the external leads with different strengths, and then a spin polarized current can be driven out of the middle lead by controlling a set of structure parameters and the external bias voltage. Moreover, by properly adjusting the dot levels and the external bias voltages, a pure spin current with no accompanying charge current can be generated in the weak coupling regime. We show that the difference between the intradot Coulomb interactions strongly influences the spin-polarized currents flowing through the middle lead and is undesirable in the generation of the net spin current. Based on the RSO interaction, the structure we propose can efficiently polarize the electron spin without the usage of any magnetic field or ferromagnetic material. This device can be used as a spin-battery and is realizable using the present available technologies.

Current–voltage characteristics in strongly correlated double quantum dots

We have studied the current–voltage properties of a double quantum dot (DQD) connected by leads in arrangements that vary from series to symmetrical parallel configurations, in the presence of strong intradot Coulomb interaction. The influences of the connecting configurations and the difference between dot levels on the magnitude and symmetry of the total current are examined. We find that the connecting configurations of the dots can determine the number of the current paths and in turn determine the magnitude of the current, while the coupling strengths between the dots and the leads together with the difference of dot levels determine the current–voltage symmetry. The negative differential conductance observed in serial DQD can be explained in terms of the reduction of the current paths.