arXiv subjects

A. Vagov

Publications and source records attributed to A. Vagov.

28 records · Page 2Linked to original sources

arXiv2016

The BCS-BEC crossover induced by a shallow band: Pushing standard superconductivity types apart

It is well-known that the appearance of almost-empty (shallow) conduction bands in solids strongly affects their superconducting properties. In a shallow band charge carriers are depleted and have nearly zero velocities so that the crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluidity to Bose-Einstein condensation (BEC) is approached. Based on a two-band prototype system with one shallow and one deep band, we demonstrate that the fundamental phase diagram of the superconducting magnetic response changes qualitatively as compared to standard superconductors with only deep bands. The so-called intertype (IT) domain between superconductivity types I and II systematically expands in the phase diagram when passing from the BCS to BEC side: its width is inversely proportional to the squared Cooper-pair radius that shrinks several orders of magnitude through the crossover. We also show that the coupling to a stable condensate of the deep band makes the system rather robust against the otherwise strong superconducting fluctuations. Thus, the BCS-BEC crossover induced by a shallow band pushes standard superconductivity types wide apart so that the IT domain tends to dominate the phase diagram and therefore the magnetic properties of shallow-band superconductors.

cond-mat.supr-con↗

arXiv2016

Path integral description of combined Hamiltonian and non-Hamiltonian dynamics in quantum dissipative system

We present a numerical path-integral iteration scheme for the low dimensional reduced density matrix of a time-dependent quantum dissipative system. Our approach simultaneously accounts for the combined action of a microscopically modelled pure-dephasing type coupling to a continuum of harmonic oscillators representing, e.g., phonons, and further environmental interactions inducing non-Hamiltonian dynamics in the inner system represented, e.g., by Lindblad type dissipation or relaxation. Our formulation of the path-integral method allows for a numerically exact treatment of the coupling to the oscillator modes and moreover is general enough to provide a natural way to include Markovian processes that are sufficiently described by rate equations. We apply this new formalism to a model of a single semiconductor quantum dot which includes the coupling to longitudinal acoustic phonons for two cases: a) external laser excitation taking into account a phenomenological radiative decay of the excited dot state and b) a coupling of the quantum dot to a single mode of an optical cavity taking into account cavity photon losses.

quant-ph↗

arXiv2016

Shallow-band superconductors: pushing superconductivity types apart

Magnetic response is a fundamental property of superconducting materials, which helps to distinguish two superconductivity types: the ideally diamagnetic type I and type II, which can develop the mixed state with Abrikosov vortices. We demonstrate that multi-band superconductors with one shallow band, that have recently attracted much attention for their high critical temperatures and unusual properties, often stay apart of this simple classification. In a wide range of microscopic parameters such systems fall into the inter-type or transitional interval in between the standard types and display unconventional mixed state configurations.

cond-mat.supr-con↗

arXiv2016

Fast and selective phonon-assisted state preparation of a quantum dot by adiabatic undressing

We investigate theoretically the temporal behavior of a quantum dot under off-resonant optical excitation targeted at fast acoustic phonon-assisted state preparation. We demonstrate that in a preparation process driven by short laser pulses three processes can be identified: a dressing of the states during the switch on of the laser pulse, a subsequent phonon-induced relaxation and an undressing at the end of the pulse. By analyzing excitation scenarios with different pulse shapes we highlight the decisive impact of an adiabatic undressing on the final state in short pulse protocols. Furthermore, we show that in exciton-biexciton systems the laser characteristics such as the pulse detuning and the pulse length as well as the biexciton binding energy can be used to select the targeted quantum dot state.

cond-mat.mes-hall↗

arXiv2013

Critical superconductors

Bogomolnyi critical point, originally introduced in string theories, is fundamental to superconductivity. At the critical temperature T_c it marks the sharp border between ideally diamagnetic bulk type-I superconductors and type-II ones that can host vortices, while itself it harbors infinitely degenerate distributions of magnetic flux in the superconducting state. Here we show that below T_c the physics of the Bogomolnyi critical point projects onto a wider range of microscopic parameters, even extremely wide for multiband superconductors. In this critical range, the degeneracy of the superconducting state at Tc is lifted into a sequence of novel topological equilibria and the customary understanding of superconducting phenomena does not suffice. As a radical departure from traditional views, we introduce the paradigm of critical superconductors, discuss their distinct magnetic properties, advocate their subdivision in terms of possible intermediate states, and demonstrate direct relevance of this paradigm to superconducting materials of prime interest today.

cond-mat.supr-con↗

arXiv2013

Competition between loss channels in quantum-dot cavity systems: unconventional consequences

We demonstrate that in quantum-dot cavity systems, the interplay between acoustic phonons and photon losses introduces novel features and characteristic dependencies in the system dynamics. In particular, the combined action of both loss mechanisms strongly affects the transition from the weak to the strong coupling regime as well as the shape of Mollow-type spectra in untypical ways. For weak coupling, where the spectra degenerate to a single line, we predict that their widths decrease with rising temperature.

cond-mat.mes-hall↗

arXiv2013

Ginzburg-Landau theory for multiband superconductors: microscopic derivation

A procedure to derive the Ginzburg-Landau (GL) theory from the multiband BCS Hamiltonian is developed in a general case with an arbitrary number of bands and arbitrary interaction matrix. It combines the standard Gor'kov truncation and a subsequent reconstruction in order to match accuracies of the obtained terms. This reconstruction recovers the phenomenological GL theory as obtained from the Landau model of phase transitions but offers explicit microscopic expressions for the relevant parameters. Detailed calculations are presented for a three-band system treated as a prototype multiband superconductor. It is demonstrated that the symmetry in the coupling matrix may lead to the chiral ground state with the phase frustration, typical for systems with broken time-reversal symmetry.

cond-mat.supr-con↗

arXiv2009

Giant Variations of Cooper-Pair Size in Nanoscale Superconductors

The Cooper-pair size (i.e., the BCS coherence length) in low-dimensional superconductors is dramatically modified by quantum-size effects. In particular, for nanowires made of conventional superconducting materials, we show that the coherence length exhibits size-dependent drops by two-three orders of magnitude and reaches values found in high-$T_c$ superconductors. This phenomenon is surprisingly similar to the well-known BCS-BEC crossover but with one important exception: it is driven by the transverse quantization of the electron spectrum rather than by the strength of the fermion-fermion interaction. Similar results can be expected for other systems with the same structure of the single-particle spectrum, e.g, for superfluid Fermi gases confined in a quantum-wire or quantum-well geometry.

cond-mat.supr-con↗

arXiv2009

Asymptotic boundary layer method for unstable trajectories: Semiclassics for individual scar wavefunctions

We extend the asymptotic boundary layer (ABL) method, originally developed for stable resonator modes, to the description of individual wavefunctions localized around unstable periodic orbits. The formalism applies to the description of scar states in fully or partially chaotic quantum systems, and also allows for the presence of smooth and sharp potentials, as well as magnetic fields. We argue that the separatrix wave function provides the largest contribution to the scars on a single wave function. This agrees with earlier results on the wave-function asymptotics and on the quantization condition of the scar states. Predictions of the ABL formalism are compared with the exact numerical solution for a strip resonator with a parabolic confinement potential and a magnetic field.

quant-ph↗

arXiv2007

Generalized Galitskii approach for the vertex function of a Fermi gas with resonant interaction

We present a generalized Galitskii approach for the Bethe-Salpeter equation for the two-particle vertex function of a Fermi system with resonant interaction by accounting for the resonant state in the scattering potential and utilizing the universal form of the resonant scattering amplitude. The procedure can be carried out both for the normal as well as for the condensate state. In both cases, the vertex function in the vicinity of the resonance is shown to formally coincide with that obtained for a weakly attractive Fermi gas. Thus we justify the popular calculational framework in which results for the weakly attractive Fermi gas are formally extrapolated into the domain of strong coupling, and further to the repulsive side of the resonance, where molecular states are formed.

cond-mat.other↗