M. Crisan

Thermoelectric transport properties of a T-shaped double quantum dot system in the Coulomb blockade regime

We investigate the thermoelectric properties of a T-shaped double quantum dot system described by a generalized Anderson Hamiltonian. The system’s electrical conduction (G) and the fundamental thermoelectric parameters such as the Seebeck coefficient (S) and the thermal conductivity (κ), along with the system’s thermoelectric figure of merit (ZT) are numerically estimated based on a Green’s function formalism that includes contributions up to the Hartree-Fock level. Our results account for finite on-site Coulomb interaction terms in both component quantum dots and discuss various ways leading to an enhanced thermoelectric figure of merit for the system. We demonstrate that the presence of Fano resonances in the Coulomb blockade regime is responsible for a strong violation of the Wiedemann-Franz law and a considerable enhancement of the system’s figure of merit (ZT).

Kondo effect in spin-orbit mesoscopic interferometers

We consider a flux-threaded Aharonov-Bohm ring with an embedded quantum dot coupled to two normal leads. The local Rashba spin-orbit interaction acting on the dot electrons leads to a spin-dependent phase factor in addition to the Aharonov-Bohm phase caused by the external flux. Using the numerical renormalizationgroup method, we find a splitting of the Kondo resonance at the Fermi level which can be compensated by an external magnetic field. To fully understand the nature of this compensation effect, we perform a scaling analysis and derive an expression for the effective magnetic field. The analysis is based on a tight-binding model which leads to an effective Anderson model with a spin-dependent density of states for the transformed lead states. We find that the effective field originates from the combined effect of Rashba interaction and magnetic flux and that it contains important corrections due to electron-electron interactions. We show that the compensating field is an oscillatory function of both the spin-orbit and the Aharonov-Bohm phases. Moreover, the effective field never vanishes due to the particle-hole symmetry breaking independently of the gate voltage.

ELECTRON-FLUCTUATION INTERACTION IN A NON-FERMI-LIQUID SUPERCONDUCTOR

We studied the influence of the amplitude fluctuations of a non-Fermi superconductor on the energy spectrum of the 2D Anderson non-Fermi system. The classical fluctuations give a temperature dependence in the pseudogap induced in the fermionic excitations.

Transport and current noise characteristics of a T-shape double-quantum-dot system

We consider the transport and the noise characteristics for the case of a T-shape double-quantum-dot system using the equation of motion method. Our theoretical results, obtained in an approximation equivalent to the Hartree-Fock approximation, account for non-zero on-site Coulomb interaction in both the detector and side dots. The existence of a non-zero Coulomb interaction implies an additional two resonances in the detectors dot density of states and thereafter affects the electronic transport properties of the system. The systems conductance presents two Fano dips as a function of the energy of the localized electronic level in the side dot. The Fano dips in the systems conductance can be observed for both strong (fast detector) and weak coupling (slow detector) between the detector dot and the external electrodes. Due to stronger electronic correlations, the noise characteristics in the case of a slow detector are much higher. This setup may be of interest for the practical realization of qubit states in quantum dot systems.

Electronic Green's functions in a T-shaped multi-quantum dot system

Abstract We developed a set of equations to calculate the electronic Greens functions in a T-shaped multi-quantum dot system using the equation of motion method. We model the system using a generalized Anderson Hamiltonian which accounts for finite intradot on-site Coulomb interaction in all component dots as well as for the interdot electron tunneling between adjacent quantum dots. Our results are obtained within and beyond the Hartree–Fock approximation and provide a path to evaluate all the electronic correlations in the multi-quantum dot system in the Coulomb blockade regime. Both approximations provide information on the physical effects related to the finite intradot on-site Coulomb interaction. As a particular example for our generalized results, we considered the simplest T-shaped system consisting of two dots and proved that our approximation introduces important corrections in the detector and side dots Greens functions, and implicitly in the evaluation of the systems transport properties. The multi-quantum dot T-shaped setup may be of interest for the practical realization of qubit states in quantum dot systems.

Localized magnetic states in Rashba dots

Instituto de F´isica Interdisciplinar y Sistemas Complejos (CSIC-UIB), E-07122 Palma de Mallorca, Spain(Dated: February 16, 2009)We study the formation of local moments in quantum dots arising in quasi-one dimensional elec-tron wires due to localized spin-orbit (Rashba) interaction. Using an Anderson-like model to describethe occurrence of the magnetic moments in these Rashba dots, we calculate the local magnetizationwithin the mean-field approximation. We find that the magnetization becomes a nontrivial func-tion of the Rashba coupling strength. We discuss both the equilibrium and nonequilibrium cases.Interestingly, we obtain a magnetic phase which is stable at large bias due to the Rashba interaction.

Critical temperature of a van Hove superconductor with spin–charge separation

Abstract We studied a spin–charge separation superconductor with a van Hove density of states. The critical temperature and the influence of the Coulomb interaction have been calculated in the mean field model. We also calculated the Ginzburg–Landau coefficients as a function of temperature.

The Gap-to- Tc Ratio of a van Hove Superconductor

We give an analytical expression for the gap-to-Tc ratio (R) of a superconductor with a van Hove singularity in the density of states. Our calculation yields R in very good agreement with the results obtained numerically by S. Ratanaburi et al. [J. Supercond. 9, 485 (1996)].

Superconductivity and critical coupling in non-Fermi liquids

The critical temperature of a superconductor with a non-Fermi liquid ground state has been calculated. The density of states has two different forms and is energy dependent. The critical values of the coupling factor λ were calculated. The constant density of states results are obtained as particular cases. These results are an extension of the work done by Grosu et al. [Phys. Rev. B56, 8298 (1997)].

NMR parameters in gapped graphene systems

We calculate the nuclear spin-lattice relaxation time and the Knight shift for the case of gapped graphene systems. Our calculations consider both the massive and massless gap scenarios. Both the spin-lattice relaxation time and the Knight shift depend on temperature, chemical potential, and the value of the electronic energy gap. In particular, at the Dirac point, the electronic energy gap has stronger effects on the system nuclear magnetic resonance parameters in the case of the massless gap scenario. Differently, at large values of the chemical potential, both gap scenarios behave in a similar way and the gapped graphene system approaches a Fermi gas from the nuclear magnetic resonance parameters point of view. Our results are important for nuclear magnetic resonance measurements that target the 13C active nuclei in graphene samples.