arXiv subjects

M. D. Croitoru

Publications and source records attributed to M. D. Croitoru.

At least 19 recordsLinked to original sources

arXiv2023

Interference-induced surface superconductivity:Enhancement by tuning the Debye energy

In the usual perception, surface superconductivity is associated with the surface nucleation of a superconducting condensate above the upper critical field in type-II superconductors or with a rearrangement of phonon properties and the electron-phonon coupling near surfaces/interfaces. Recently, it has been found that there is another example when the surface superconducting temperature is increased up to 20-25% as compared to the bulk one due to constructive interference of superconducting pair states. In the present work, we demonstrate that in fact, such an interferenceinduced enhancement can be much more pronounced, up to nearly 70%. Furthermore, here it is shown that such an interference enhancement persists over a wide range of microscopic parameters.

cond-mat.supr-con↗

arXiv2020

Multiband superconductors with degenerate excitation gaps

There is a tacit assumption that multiband superconductors are essentially the same as multigap superconductors. More precisely, it is usually assumed that the number of excitation gaps in the single-particle energy spectrum of a uniform superconductor determines the number of contributing bands in the corresponding superconducting model. Here we demonstrate that contrary to this widely accepted viewpoint, the superconducting magnetic properties are sensitive to the number of contributing bands even when the corresponding excitation gaps are degenerate and cannot be distinguished. In particular, we find that the crossover between superconductivity types I and II - the intertype regime - is strongly affected by difference between characteristic lengths of multiple contributing condensates. The reason for this is that condensates with diverse characteristic lengths coexisting in one system interfere constructively or destructively, which results in multi-condensate magnetic phenomena regardless of the presence/absence of the multigap structure in the single-particle excitation spectrum.

cond-mat.supr-con↗

arXiv2018

Persistent oscillations of the order parameter and interaction quench phase diagram for a confined Bardeen-Cooper-Schrieffer Fermi gas

We present a numerical study of the interaction quench dynamics in a superfluid ultracold Fermi gas confined in a three-dimensional cigar-shaped harmonic trap. In the present paper we investigate the amplitude mode of the superfluid order parameter after interaction quenches which start deep in the BCS phase and end in the BCS-BEC crossover regime. To this end, we exploit the Bogoliubov-de Gennes formalism which takes the confinement potential explicitly into account and provides a microscopic fully coherent description of the system. We find an anharmonic nonlinear oscillation of the modulus of the superfluid order parameter, i.e., of the Higgs mode. This oscillation persists for large times with only a small amplitude modulation being visible. We connect the frequency and the mean value of this oscillation with the breaking of Cooper pairs in the superfluid phase. Additionally, we demonstrate that the occurrence of this persistent oscillation is connected to the onset of chaotic dynamics in our model. Finally, we calculate an interaction quench phase diagram of the Higgs mode for quenches on the BCS side of the BCS-BEC crossover and discuss its properties as a function of the aspect ratio of the cigar-shaped trap.

cond-mat.quant-gas↗

arXiv2018

Dynamical vanishing of the order parameter in a confined Bardeen-Cooper-Schrieffer Fermi gas after an interaction quench

We present a numerical study of the Higgs mode in an ultracold confined Fermi gas after an interaction quench and find a dynamical vanishing of the superfluid order parameter. Our calculations are done within a microscopic density-matrix approach in the Bogoliubov-de Gennes framework which takes the three-dimensional cigar-shaped confinement explicitly into account. In this framework, we study the amplitude mode of the order parameter after interaction quenches starting on the BCS side of the BEC-BCS crossover close to the transition and ending in the BCS regime. We demonstrate the emergence of a dynamically vanishing superfluid order parameter in the spatiotemporal dynamics in a three-dimensional trap. Further, we show that the signal averaged over the whole trap mirrors the spatiotemporal behavior and allows us to systematically study the effects of the system size and aspect ratio on the observed dynamics. Our analysis enables us to connect the confinement-induced modifications of the dynamics to the pairing properties of the system. Finally, we demonstrate that the signature of the Higgs mode is contained in the dynamical signal of the condensate fraction, which, therefore, might provide a new experimental access to the nonadiabatic regime of the Higgs mode.

cond-mat.quant-gas↗

arXiv2017

Quench dynamics of a confined ultracold Fermi gas: Direct visibility of the Goldstone mode in the single-particle excitations

We present a numerical study of a confined ultracold Fermi gas showing that the Goldstone mode of the BCS gap is directly visible in the dynamics of the single-particle excitations. To this end, we investigate the low-energy dynamic response of a confined Fermi gas to a rapid change of the scattering length (i.e., an interaction quench). Based on a fully microscopic time-dependent density-matrix approach within the Bogoliubov-de Gennes formalism that includes a 3D harmonic confinement we simulate and identify the emergence of the Goldstone mode in a cigar-shaped $^6$Li gas. We show that the quench leads to a low-frequency in-phase oscillation of the single-particle occupations. Complete inversion is achieved for occupations corresponding to the lowest-lying single-particle states.

cond-mat.quant-gas↗

arXiv2014

Higgs mode in the quench dynamics of a confined ultracold Fermi gas in the BCS regime

The Higgs amplitude mode of the order parameter of an ultracold confined Fermi gas in the BCS regime after a quench of the coupling constant is analyzed theoretically. Characteristic features are a damped oscillation which at a certain transition time changes into a rather irregular dynamics. We compare the numerical solution of the full set of nonlinear equations of motion for the normal and anomalous Bogoliubov quasiparticle excitations with a linearized approximation. In doing so the transition time as well as the difference between resonant systems, i.e., systems where the Fermi energy is close to a subband minimum, and off-resonant systems can be well understood and traced back to the system and geometry parameters.

cond-mat.quant-gas↗

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↗

arXiv2012

The extended Lawrence-Doniach model: the temperature evolution of the in-plane magnetic field anisotropy

Using the quasi-classical formalism we provide the description of the temperature and field-direction dependence of the in-plane upper critical field in layered superconductors, taking into account the interlayer Josephson coupling and the paramagnetic spin splitting. We generalize the Lawrence-Doniach model for the case of high magnetic fields and show that the re-entrant superconductivity is naturally described by our formalism when neglecting the Pauli pair breaking effect. We demonstrate that in layered superconductors the in-plane anisotropy of the onset of superconductivity exhibits four different\ temperature regimes: from the Ginzburg-Landau type in the vicinity of the critical temperature $T_{c0}$ with anisotropies of coherence lengths, up to the FFLO type induced by the strong interference between the modulation vector and the orbital effect. Our results are in agreement with the experimental measurements of the field-angle dependence of the superconducting onset temperature of the organic compound (TMTSF)$_{2} $ClO$_{4}$.

cond-mat.supr-con↗

arXiv2012

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

We investigate theoretically under which conditions a stable and high-fidelity preparation of the biexciton state in a quantum dot can be realized by means of adiabatic rapid passage in the presence of acoustic phonon coupling. Our analysis is based on a numerically complete real-time path integral approach and comprises different schemes of optical driving using frequency-swept (chirped) pulses. We show that depending on the size of the biexciton binding energy, resonant two-photon excitations or two-color schemes can be favorable. It is demonstrated that the carrier-phonon interaction strongly affects the efficiency of the protocols and that a robust preparation of the biexciton is restricted to positive chirps and low temperatures. A considerable increase of the biexciton yield can be achieved realizing temperatures below 4 K.

cond-mat.mes-hall↗

arXiv2012

Atypical BCS-BEC crossover induced by quantum-size effects

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.

cond-mat.quant-gas↗

arXiv2012

In-plane magnetic field anisotropy of the FFLO state in layered superconductors

There are strong experimental evidences of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state formation in layered organic superconductors in parallel magnetic field. We study theoretically the interplay between the orbital effect and the FFLO modulation in this case and demonstrate that the in-plane critical field anisotropy drastically changes at the transition to the FFLO state. The very peculiar angular dependence of the superconducting onset temperature which is predicted may serve for unambiguous identification of the FFLO modulation. The obtained results permit us to suggest the modulated phase stabilization as the origin of the magnetic-field angle dependence of the onset of superconductivity experimentally observed in (TMTSF)$_{2}$ClO$_{4}$ organic conductors.

cond-mat.supr-con↗

arXiv2011

BCS-BEC crossover-like phenomena driven by quantum-size effects in quasi-one-dimensional fermionic condensates

Quantum confinement is known to influence fermionic condensates, resulting in quantum-size oscillations of superfluid/superconducting properties. Here we show that the impact of quantum-size effects is even more dramatic. Under realistic conditions, a significant phase-space reconfiguration induced by quantum-size effects opens a quasi-molecule channel in the fermionic pairing so that the condensed pairs exhibit features typical of a molecular state. As an illustration we consider a quasi-one-dimensional fermionic condensate, as realized, e.g., in cigar-shaped atomic Fermi gases or superconducting quantum wires. In this case the transverse quantization of the particle motion favors pairing through a coherent superposition of quantum channels that are formed due to the grouping of single-particle levels into a series of well distinguished subbands. Whenever the bottom of a subband approaches the Fermi level, the longitudinal spatial distribution of fermions in a condensed pair becomes strongly localized within the corresponding quantum channel. The fermionic pairs in this channel resemble molecules with bosonic character.

cond-mat.quant-gas↗

arXiv2010

Metallic nanograins: spatially nonuniform pairing induced by quantum confinement

It is well-known that the formation of discrete electron levels strongly influences the pairing in metallic nanograins. Here we focus on another effect of quantum confinement in superconducting grains that was not studied previously, i.e., spatially nonuniform pairing. This effect is very significant when single-electron levels form bunches and/or a kind of shell structure: in highly symmetric grains the order parameter can exhibit variations with position by an order of magnitude. Nonuniform pairing is closely related to a quantum-confinement induced modification of the pairing-interaction matrix elements and size-dependent pinning of the chemical potential to groups of degenerate or nearly degenerate levels. For illustration we consider spherical metallic nanograins. We show that the relevant matrix elements are as a rule enhanced in the presence of quantum confinement, which favors spatial variations of the order parameter, compensating the corresponding energy cost. The size-dependent pinning of the chemical potential further increases the spatial variation of the pair condensate. The role of nonuniform pairing is smaller in less symmetric confining geometries and/or in the presence of disorder. However, it always remains of importance when the energy spacing between discrete electron levels $δ$ is approaching the scale of the bulk gap $Δ_B$, i.e., $δ> 0.1$-$0.2\,Δ_B$.

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

Superconducting nanowires: quantum confinement and spatially dependent Hartree-Fock potential

It is well-known that in bulk, the solution of the Bogoliubov-de Gennes equations is the same whether or not the Hartree-Fock term is included. Here the Hartree-Fock potential is position independent and, so, gives the same contribution to both the single-electron energies and the Fermi level (the chemical potential). Thus, the single-electron energies measured from the Fermi level (they control the solution) stay the same. It is not the case for nanostructured superconductors, where quantum confinement breaks the translational symmetry and results in a position dependent Hartree-Fock potential. In this case its contribution to the single-electron energies depends on the relevant quantum numbers. We numerically solved the Bogoliubov-de Gennes equations with the Hartree-Fock term for a clean superconducting nanocylinder and found a shift of the curve representing the thickness-dependent oscillations of the critical superconducting temperature to larger diameters.

cond-mat.supr-con↗

arXiv2009

Size-dependent enhancement of superconductivity in nanowires

A shape-dependent superconducting resonance can be expected when an energy level associated with the transverse motion in a wire passes through the Fermi surface. We show that the recently observed width-dependent increase of $T_c$ in ${\rm Al}$ and ${\rm Sn}$ nanowires is a consequence of this shape resonance effect.

cond-mat.supr-con↗

arXiv2008

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 D < 10-15 nm, 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., D < 5 nm.

cond-mat.supr-con↗

arXiv2006

Influence of the Characteristics of the STM-tip on the Electroluminescence Spectra

We analyze the influence of the characteristics of the STM-tip (applied voltage, tip radius) on the electroluminescence spectra from an STM-tip-induced quantum dot taking into account the many-body effects. We find that positions of electroluminescence peaks, attributed to the electron-hole recombination in the quantum dot, are very sensitive to the shape and size of the confinement potential as determined by the tip radius and the applied voltage. A critical value of the tip radius is found, at which the luminescence peak positions as a function of the tip radius manifest a transition from decreasing behavior for smaller radii to increasing behavior for larger radii. We find that this critical value of the tip radius is related to the confinement in the lateral and normal direction.

cond-mat.mes-hall↗