Mikhail D. Croitoru

Oscillations of the superconducting temperature induced by quantum well states in thin metallic films : Numerical solution of the Bogoliubov-de Gennes equations

Quantum confinement of electrons in a high-quality metallic nanofilm results in the formation of quantumwell states. The band of single-electron states is split into a series of subbands moving in energy with changing film thickness. When the bottom of such a subband passes through the Fermi surface, a shape resonance appears leading to oscillations of the critical temperature Tc as a function of film thickness. We present a quantitative description of recent experimental data on the film-thickness dependence of Tc in Pb nanofilms with atomic-scale uniformity in thickness Science 306, 1915 2004 through a numerical solution of the Bogoliubov‐de Gennes equations. DOI: 10.1103/PhysRevB.75.014519

Shape resonances in the superconducting order parameter of ultrathin nanowires

We study the shape resonance effect associated with the confined transverse superconducting modes of a cylindrical nanowire in the clean limit. Results of numerical investigations of the Bogoliubov-de Gennes equations show significant deviations of the energy gap parameter from its bulk value with a profound effect on the transition temperature. The most striking is that the size of the resonances is found to be by about order of magnitude larger than in ultrathin metallic films with the same width.

Magnetic-field induced quantum-size cascades in superconducting nanowires

In high-quality nanowires, quantum confinement of the transverse electron motion splits the band of single-electron states in a series of subbands. This changes in a qualitative way the scenario of the magnetic-field induced superconductor-to-normal transition. We numerically solve the Bogoliubov-de Gennes equations for a clean metallic cylindrical nanowire at zero temperature in a parallel magnetic field and find that for diameters

Atypical BCS-BEC crossover induced by quantum-size effects

D\ensuremath{\lesssim}10\text{--}15\text{ }\text{nm}

Quantum transport in a nanosize double-gate metal-oxide-semiconductor field-effect transistor

, this transition occurs as a cascade of subsequent jumps in the order parameter (this is opposed to the smooth second-order phase transition in the mesoscopic regime). Each jump is associated with the depairing of electrons in one of the single-electron subbands. As a set of subbands contribute to the order parameter, the depairing process occurs as a cascade of jumps. We find pronounced quantum-size oscillations of the critical magnetic field with giant resonant enhancements. In addition to these orbital effects, the paramagnetic breakdown of Cooper pairing also contributes but only for smaller diameters, i.e.,

Ultrafast terahertz-field-induced dynamics of superconducting bulk and quasi-1D samples

D\ensuremath{\lesssim}5\text{ }\text{nm}

Biexciton state preparation in a quantum dot via adiabatic rapid passage: Comparison between two control protocols and impact of phonon-induced dephasing

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Dependence of superconducting properties on the size and shape of a nanoscale superconductor: from nanowire to film

Quantum-size oscillations of the basic physical characteristics of a confined fermionic condensate are a well-known phenomenon. Its conventional understanding is based on the single-particle physics, whereby the oscillations follow the size-dependent changes in the single-particle density of states. Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which demonstrates an important many-body aspect of the quantum-size effects, overlooked previously. The many-body physics is revealed in the atypical crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the Bose-Einstein condensate (BEC) induced by the size quantization of the particle motion. Quantized perpendicular spectrum results in the formation of single-particle subbands (shells) so that the aggregate fermionic condensate becomes a coherent mixture of subband condensates. Each time when the lower edge of a subband crosses the chemical potential, the BCS-BEC crossover is approached in this subband, and the aggregate condensate contains both the BCS and BEC-like components.

Phonon limited superconducting correlations in metallic nanograins.

A model for the nanosize double-gate silicon-on-insulator metal-oxide-semiconductor field-effect transistors is developed, which enables both physical accuracy and flexibility. The quantum Liouville equation in the Wigner function representation has been used to deal with the quantum transport problem. Accounting for electron scattering due to ionized impurities, acoustic phonons, and surface roughness at the Si∕SiO2 interface, device characteristics are obtained as a function of structure parameters. From the analysis of the Wigner function, the coexistence of diffusive and ballistic transport naturally emerges. The obtained I-V characteristics show that the double-gate device is a potential structure for ultimate complementary metal-oxide-semiconductor scaling.

Competition between pure dephasing and photon losses in the dynamics of a dot-cavity system

Within a density-matrix formalism based on the Bardeen–Cooper–Schrieffer (BCS) model and the Bogoliubov–de Gennes equations we provide a description of the dynamics of the non-equilibrium superconducting pairing induced by a terahertz (THz) laser pulse in bulk and quasi-one-dimensional (1D) samples of conventional (BCS-type) superconductors. A cross-over from an adiabatic to a non-adiabatic regime takes place for short and intense THz pulses. In the non-adiabatic regime, the order parameter performs a damped oscillation. We discuss how the parameters of the THz pulse influence the amplitude and the mean value of the oscillation in bulk samples. It is demonstrated that for high intensities the non-adiabatic regime can be reached even for pulses longer than the oscillation period. For the 1D samples we find that the oscillation may attenuate with a different power law. This is analysed by comparing the THz-induced dynamics with the dynamics induced by a sudden switching of the pairing strength, which exhibits essentially the same behaviour. The numerical calculations show that the exponent of the power law depends critically on the density of states in the Debye window and therefore changes in an oscillatory way with the confinement strength. Irregularities in the decay of the oscillation are predicted when the 1D quantum wire is cut short to an elongated zero-dimensional quantum dot structure.