arXiv subjects

V. N. Gladilin

Publications and source records attributed to V. N. Gladilin.

At least 19 recordsLinked to original sources

arXiv2020

Vortices in nonequilibrium photon condensates

We present a theoretical study of vortices in arrays of photon condensates. Even when interactions are negligible, as is the case in current experiments, pumping and losses can lead to a finite vortex core size. While some properties of photon condensate vortices, such as their self-acceleration and the generation of vortex pairs by a moving vortex, resemble those in interacting polariton condensates far from equilibrium, in several aspects they differ from previously studied systems: the vortex core size is determined by the balance between pumping and tunneling, the core appears oblate in the direction of its motion and new vortex pairs can spontaneously nucleate in the core region.

cond-mat.quant-gas↗

arXiv2018

Unstable and stable regimes of polariton condensation

Modulational instabilities play a key role in a wide range of nonlinear optical phenomena, leading e.g. to the formation of spatial and temporal solitons, rogue waves and chaotic dynamics. Here we experimentally demonstrate the existence of a modulational instability in condensates of cavity polaritons, arising from the strong coupling of cavity photons with quantum well excitons. For this purpose we investigate the spatiotemporal coherence properties of polariton condensates in GaAs-based microcavities under continuous-wave pumping. The chaotic behavior of the instability results in a strongly reduced spatial and temporal coherence and a significantly inhomogeneous density. Additionally we show how the instability can be tamed by introducing a periodic potential so that condensation occurs into negative mass states, leading to largely improved coherence and homogeneity. These results pave the way to the exploration of long-range order in dissipative quantum fluids of light within a controlled platform.

cond-mat.mes-hall↗

arXiv2012

Stable "antiferromagnetic" vortex lattice imprinted into a type-II superconductor

In type-II superconductors, where vortices and antivortices tend to annihilate, only a "ferromagnetic" vortex lattice, with the same orientation of vortex magnetic moments, is usually formed in a homogeneous external magnetic field. Using the time-dependent Ginzburg-Landau formalism, we demonstrate that a checkerboard vortex-antivortex lattice ("antiferromagnetic vortex lattice"), imprinted onto a superconducting film by a periodic array of underlying clockwise and counterclockwise microcoils generating spatially periodic positive and negative magnetic field pulses and then trapped by an array of artificial pinning centers, remains stable even after the imprinting magnetic field pulse is switched off.

cond-mat.supr-con↗

arXiv2012

Aharonov-Bohm oscillations in the vortex dynamics in superconducting hollow cylinders

Using time-dependent Ginzburg-Landau theory we demonstrate that the Aharonov-Bohm (AB) effect, resulting from a Berry phase shift of the (macroscopic) wavefunction, is revealed through the dynamics of topological phase defects present in that same wavefunction. We study vortices and antivortices on the surface of a hollow superconducting cylinder, moving on circular orbits as they are subjected to the force from the current flowing parallel to the cylinder axis. Due to the AB effect the orbit deflections, caused by a magnetic field component along the cylinder axis, become periodic as a function of field, leading to strong and robust resistance oscillations.

cond-mat.supr-con↗

arXiv2012

Negative-μ regime in the ac magnetic response of superconductor nanoshells

The time-dependent Ginzburg-Landau formalism is applied to analyze the vortex states and vortex dynamics in superconducting spherical nanoshells, subjected to mutually perpendicular strong dc and weak ac magnetic fields. We demonstrate that nonuniformity of the shell thickness can dramatically affect the ac magnetic response of a 3D array of superconducting nanoshells. Remarkably, this response is strongly influenced not only by the relevant geometric and material parameters and the ac-field frequency but also by the magnitude of the applied dc field: by changing this field the real part of the effective ac magnetic permeability can be tuned from positive values significantly larger than one down to negative values.

cond-mat.supr-con↗

arXiv2011

Vortex core deformation and stepper motor behavior in a superconducting ratchet

We investigated experimentally the frequency dependence of a superconducting vortex ratchet effect by means of electrical transport measurements and modeled it theoretically using the time dependent Ginzburg-Landau formalism. We demonstrate that the high frequency vortex behavior can be described as a discrete motion of a particle in a periodic potential, i.e. the so called stepper motor behavior. Strikingly, in the more conventional low frequency response a transition takes place from an Abrikosov vortex rectifier to a phase slip line rectifier. This transition is characterized by a strong increase in the rectified voltage and the appearance of a pronounced hysteretic behavior.

cond-mat.supr-con↗

arXiv2010

Exciton spin dynamics in spherical CdS quantum dots

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.

cond-mat.mtrl-sci↗

arXiv2009

Co-existence of the Meissner and vortex-state on a superconducting spherical shell

We show that on superconducting spherical nanoshells, the co-existence of the Meissner state with a variety of vortex patterns drives the phase transition to higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force pushing the nucleating vortices and antivortices towards the poles, overcoming local pinning centers, preventing vortex-antivortex recombination and leading to the appearance of a Meissner belt around the sphere's equator. In sufficiently small and thin spherical shells paramagnetic vortex states can be stable, enabling spatial separation of freely moving shells with different radii and vorticity in an inhomogeneous external magnetic field.

cond-mat.supr-con↗

arXiv2007

Evidence for nonmonotonic magnetic field penetration in a type-I superconductor

Polarized neutron reflectometry (PNR) provides evidence that nonlocal electrodynamics governs the magnetic field penetration in an extreme low-k superconductor. The sample is an indium film with a large elastic mean free path (11 mkm) deposited on a silicon oxide wafer. It is shown that PNR can resolve the difference between the reflected neutron spin asymmetries predicted by the local and nonlocal theories of superconductivity. The experimental data support the nonlocal theory, which predicts a nonmonotonic decay of the magnetic field.

cond-mat.supr-con↗

arXiv2007

Two-phonon assisted exciton spin relaxation due to exchange interaction in spherical quantum dots

Spin relaxation in undoped quasi-spherical CdS quantum dots at zero magnetic fields is investigated using time- and polarization-resolved transient absorption measurements. Unlike in previous studies of these systems, the measured signals were corrected for spin-insensitive contributions to the exciton bleaching dynamics, {allowing us to determine} the pure spin-related exciton dynamics. To explain the observed room-temperature spin-relaxation time of several nanoseconds, we propose a novel mechanism based on intralevel exciton transitions with the emission of one LO phonon, the absorption of another LO phonon, and an electron spin flip, which is driven by the electron-hole exchange interaction. The transition rates, calculated in the present work for different sizes of quantum dots and temperatures, are in fair agreement with our experimental results.

cond-mat.mes-hall↗

arXiv2007

Vortices on a superconducting nanoshell: phase diagram and dynamics

In superconductors, the search for special vortex states such as giant vortices focuses on laterally confined or nanopatterned thin superconducting films, disks, rings, or polygons. We examine the possibility to realize giant vortex states and states with non-uniform vorticity on a superconducting spherical nanoshell, due to the interplay of the topology and the applied magnetic field. We derive the phase diagram and identify where, as a function of the applied magnetic field, the shell thickness and the shell radius, these different vortex phases occur. Moreover, the curved geometry allows these states (or a vortex lattice) to coexist with a Meissner state, on the same curved film. We have examined the dynamics of the decay of giant vortices or states with non-uniform vorticity into a vortex lattice, when the magnetic field is adapted so that a phase boundary is crossed.

cond-mat.supr-con↗

arXiv2007

Critical temperature and specific heat for Cooper pairing on a spherical surface

Based on an exact solution of the Bardeen-Cooper-Schrieffer type Hamiltonian on a spherical surface, we calculate the specific heat for the electron system with pair correlations on a sphere. We find that it is possible to extract from the specific heat a temperature above which many-body states with broken Cooper pairs get populated. Therefore, we define this temperature as the characteristic temperature signalling the onset of a BCS-type pair-correlated state for electrons on a spherical surface. Such spherical electron systems are realized in multielectron bubbles in liquid helium, for which the above-mentioned characteristic temperature is found to be of the order of 10-100 mK. Both the specific heat and the critical temperature show a pronounced (4-6%) odd-even parity effect that persists even for numbers of electrons as large as 10$^6$.

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↗

arXiv2005

Cooper pairing and superconductivity on a spherical surface

Electrons in a multielectron bubble in helium form a spherical, two-dimensional system coupled to the ripplons at the bubble surface. The electron-ripplon coupling, known to lead to polaronic effects, is shown to give rise also to Cooper pairing. A Bardeen-Cooper-Schrieffer (BCS) Hamiltonian arises from the analysis of the electron-ripplon interaction in the bubble, and values of the coupling strength are obtained for different bubble configurations. The BCS Hamiltonian on the sphere is analysed using the Richardson method. We find that although the typical ripplon energies are smaller than the splitting between electronic levels, a redistribution of the electron density over the electronic levels is energetically favourable as pairing correlations can be enhanced. The density of states of the system with pairing correlations is derived. No gap is present, but the density of states reveals a strong step-like increase at the pair-breaking energy. This feature of the density of states should enable the unambiguous detection of the proposed state with pairing correlations in the bubble, through either capacitance spectroscopy or tunneling experiments, and allow to map out the phase diagram of the electronic system in the bubble.

cond-mat.supr-con↗

arXiv2004

Magnetic susceptibility of ultra-small superconductor grains

For assemblies of superconductor nanograins, the magnetic response is analyzed as a function of both temperature and magnetic field. In order to describe the interaction energy of electron pairs for a huge number of many-particle states, involved in calculations, we develop a simple approximation, based on the Richardson solution for the reduced BCS Hamiltonian and applicable over a wide range of the grain sizes and interaction strengths at arbitrary distributions of single-electron energy levels in a grain. Our study is focused upon ultra-small grains, where both the mean value of the nearest-neighbor spacing of single-electron energy levels in a grain and variations of this spacing from grain to grain significantly exceed the superconducting gap in bulk samples of the same material. For these ultra-small superconductor grains, the overall profiles of the magnetic susceptibility as a function of magnetic field and temperature are demonstrated to be qualitatively different from those for normal grains. We show that the analyzed signatures of pairing correlations are sufficiently stable with respect to variations of the average value of the grain size and its dispersion over an assembly of nanograins. The presence of these signatures does not depend on a particular choice of statistics, obeyed by single-electron energy levels in grains.

cond-mat.supr-con↗

arXiv2003

Optical properties of polaronic excitons in stacked quantum dots

We present a theoretical investigation of the optical properties of polaronic excitons in stacked self-assembled quantum dots, which is based on the non-adiabatic approach. A parallelepiped-shaped quantum dot is considered as a model for a self-assembled quantum dot in a stack. The exciton-phonon interaction is taken into account for all phonon modes specific for these quantum dots (bulk-like, half-space and interface phonons). We show that the coupling between stacked quantum dots can lead to a strong enhancement of the optical absorption in the spectral ranges characteristic for phonon satellites.

cond-mat.mes-hall↗

arXiv2003

Optical spectra of quantum dots: effects of non-adiabaticity

It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton-phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck-Condon progression, prescribed by the adiabatic approach.

cond-mat.mes-hall↗

arXiv2003

Quantum Transport in a Nanosize Silicon-on-Insulator Metal-Oxide-Semiconductor

An approach is developed for the determination of the current flowing through a nanosize silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFET). The quantum mechanical features of the electron transport are extracted from the numerical solution of the quantum Liouville equation in the Wigner function representation. 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 a channel length. From the Wigner function distributions, the coexistence of the diffusive and the ballistic transport naturally emerges. It is shown that the scattering mechanisms tend to reduce the ballistic component of the transport. The ballistic component increases with decreasing the channel length.

cond-mat.mes-hall↗