arXiv subjects

J. Tempere

Publications and source records attributed to J. Tempere.

At least 37 records · Page 2Linked to original sources

arXiv2015

Soliton core filling in superfluid Fermi gases with spin-imbalance

In this paper the properties of dark solitons in superfluid Fermi gases with spin-imbalance are studied by means of a recently developed effective field theory [S. N. Klimin, J. Tempere, G. Lombardi, J. T. Devreese, Eur. Phys. J. B 88, 122 (2015)] suitable to describe the BEC-BCS crossover in ultracold gases in an extended range of temperatures as compared to the usual Ginzburg-Landau treatments. The spatial profiles for the total density and for the density of the excess-spin component, and the changes of their properties across the BEC-BCS crossover are examined in different conditions of temperature and imbalance. The presence of population imbalance is shown to strongly affect the structure of the soliton excitation by filling its core with unpaired atoms. This in turn influences the dynamical properties of the soliton since the additional particles in the core have to be dragged along thus altering the effective mass.

cond-mat.quant-gas↗

arXiv2015

Contact potential instability in the path-integral description of itinerant ferromagnetism

It has long been predicted that a two-component non-localized Fermi gas will exhibit spontaneous polarization for sufficiently strong repulsive interactions, a phenomenon which is called itinerant ferromagnetism. Recent experiments with ultracold atomic gases have reached the interaction strength for which theoretical models have predicted the occurrence of the normal-to-itinerant-ferromagnetic phase transition, but so far this transition has not been observed. The instability of the repulsive branch of the Feshbach resonance prevents the formation of the itinerant ferromagnetic state, but it is not clear whether this is the only instability impeding its experimental realization. In this article, we use the path-integral formalism with density fields in the Hubbard-Stratonovich transformation to study the stability of a homogeneous two-component Fermi gas with contact interactions. Within the saddle-point approximation we show that none of the extrema of the action are minima, meaning all extrema are unstable to small density fluctuations. This implies a more general mechanical instability of the polarized (itinerant ferromagnetic) and normal states of the system in the path-integral formalism. We find that it is important to consider the stability of the system when studying itinerant ferromagnetism. Since (mechanical) stability may be influenced by the details of the interaction potential, we suggest the use of a more realistic potential than the contact potential in future theoretical descriptions.

cond-mat.quant-gas↗

arXiv2015

Extension of the Ginzburg - Landau approach for ultracold Fermi gases below a critical temperature

In the context of superfluid Fermi gases, the Ginzburg - Landau (GL) formalism for the macroscopic wave function has been successfully extended to the whole temperature range where the superfluid state exists. After reviewing the formalism, we first investigate the temperature-dependent correction to the standard GL expansion (which is valid close to $T_{c}$). Deviations from the standard GL formalism are particularly important for the kinetic energy contribution to the GL energy functional, which in turn influences the healing length of the macroscopic wave function. We apply the formalism to variationally describe vortices in a strong-coupling Fermi gas in the BEC-BCS crossover regime, in a two-band system. The healing lengths, derived as variational parameters in the vortex wave function, are shown to exhibit hidden criticality well below $T_{c}$.

cond-mat.quant-gas↗

arXiv2015

Finite temperature effective field theory and two-band superfluidity in Fermi gases

We develop a description of fermionic superfluids in terms of an effective field theory for the pairing order parameter. Our effective field theory improves on the existing Ginzburg - Landau theory for superfluid Fermi gases in that it is not restricted to temperatures close to the critical temperature. This is achieved by taking into account long-range fluctuations to all orders. The results of the present effective field theory compare well with the results obtained in the framework of the Bogoliubov - de Gennes method. The advantage of an effective field theory over Bogoliubov - de Gennes calculations is that much less computation time is required. In the second part of the paper, we extend the effective field theory to the case of a two-band superfluid. The present theory allows us to reveal the presence of two healing lengths in the two-band superfluids, to analyze the finite-temperature vortex structure in the BEC-BCS crossover, and to obtain the ground state parameters and spectra of collective excitations. For the Leggett mode our treatment provides an interpretation of the observation of this mode in two-band superconductors.

cond-mat.quant-gas↗

arXiv2015

Optical properties of inhomogeneous metallic hydrogen plasmas

We investigate the optical properties of hydrogen as it undergoes a transition from the insulating molecular to the metallic atomic phase, when heated by a pulsed laser at megabar pressures in a diamond anvil cell. Most current experiments attempt to observe this transition by detecting a change in the optical reflectance and/or transmittance. Theoretical models for this change are based on the dielectric function calculated for bulk, homogeneous slabs of material. Experimentally, one expects a hydrogen plasma density that varies on a length scale not substantially smaller than the wave length of the probing light. We show that taking this inhomogeneity into account can lead to significant corrections in the reflectance and transmittance. We present a technique to calculate the optical properties of systems with a smoothly varying density of charge carriers, determine the optical response for metallic hydrogen in the diamond anvil cell experiment and contrast this with the standard results. Analyzing recent experimental results we obtain $σ^{Drude}_{DC}=(2.1 \pm 1.3) \times 10^3$ ($Ω$ cm)$^{-1}$ for the conductivity of metallic hydrogen at 170 GPa and 1250 K.

cond-mat.mtrl-sci↗

arXiv2014

Phase correlations and quasicondensate in a two-dimensional ultracold Fermi gas

The interplay between dimensionality, coherence and interaction in superfluid Fermi gases is analyzed by the phase correlation function of the field of fermionic pairs. We calculate this phase correlation function for a two-dimensional superfluid Fermi gas with $s$-wave interactions within the Gaussian pair fluctuation formalism. The spatial behavior of the correlation function is shown to exhibit a rapid (exponential) decay at short distances and a characteristic algebraic decay at large distances, with an exponent matching that expected from Berezinskii-Kosterlitz-Thouless theory of 2D Bose superfluids. We conclude that the Gaussian pair fluctuation approximation is able to capture the physics of quasi long-range order in two-dimensional Fermi gases.

cond-mat.quant-gas↗

arXiv2014

Finite temperature effective field theory for dark solitons in superfluid Fermi gases

We use a finite temperature effective field theory recently developed for superfluid Fermi gases to investigate the properties of dark solitons in these superfluids. Our approach provides an analytic solution for the dip in the order parameter and the phase profile accross the soliton, which can be compared with results obtained in the framework of the Bogoliubov - de Gennes equations. We present results in the whole range of the BCS-BEC crossover, for arbitrary temperatures, and taking into account Gaussian fluctuations about the saddle point. The obtained analytic solutions yield an exact energy-momentum relation for a dark soliton showing that the soliton in a Fermi gas behaves like a classical particle even at nonzero temperatures. The spatial profile of the pair field and for the parameters of state for the soliton are analytically studied. In the strong-coupling regime and/or for sufficiently high temperatures, the obtained analytic solutions match well the numeric results obtained using the Bogoliubov - de Gennes equations.

cond-mat.quant-gas↗

arXiv2014

Interface superconductivity in LaAlO$_{3}$-SrTiO$_{3}$ heterostructures

The interface superconductivity in LaAlO$_{3}$-SrTiO$_{3}$ heterostructures reveals a non-monotonic behavior of the critical temperature as a function of the two-dimensional density of charge carriers. We develop a theoretical description of interface superconductivity in strongly polar heterostructures, based on the dielectric function formalism. The density dependence of the critical temperature is calculated accounting for all phonon branches including different types of optical (interface and half-space) and acoustic phonons. The LO- and acoustic-phonon-mediated electron-electron interaction is shown to be the dominating mechanism governing the superconducting phase transition in the heterostructure.

cond-mat.supr-con↗

arXiv2014

Bipolarons and multi-polarons consisting of impurity atoms in a Bose-Einstein condensate

The variational Feynman formalism for the polaron, extended to an all-coupling treatment of bipolarons, is applied for two impurity atoms in a Bose-Einstein condensate. This shows that if the polaronic coupling strength is large enough the impurities will form a bound state (the bipolaron). As a function of the mutual repulsion between the impurities two types of bipolaron are distinguished: a tightly bound bipolaron at weak repulsion and a dumbbell bipolaron at strong repulsion. Apart from the binding energy, also the evolution of the bipolaron radius and its effective mass are examined as a function of the strength of the repulsive interaction between the impurities and of the polaronic cupling strength. We then apply the strong-coupling formalism to multiple impuritiy atoms in a condensate which leads to the prediction of multi-polaron formation in the strong coupling regime. The results of the two formalisms are compared for two impurities in a condensate which results in a general qualitative agreement and a quantitative agreement at strong coupling. Typically the system of impurity atoms in a Bose-Einstein condensate is expected to exhibit the polaronic weak coupling regime. However, the polaronic coupling strength is in principle tunable with a Feshbach resonance.

cond-mat.quant-gas↗

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

Polaronic properties of an impurity in a Bose-Einstein condensate in reduced dimensions

The application of optical lattices allows a tuning of the geometry of Bose-Einstein condensates to effectively reduced dimensions. In the context of solid state physics the consideration of the low-dimensional Fröhlich polaron results in an extension of the polaronic strong coupling regime. With this motivation we apply the Jensen-Feynman variational principle to calculate the ground state properties of the polaron consisting of an impurity in a Bose-Einstein condensate in reduced dimensions. Also the response of this system to Bragg scattering is calculated. We show that reducing the dimension leads to a larger amplitude of the polaronic features and is expected to facilitate the experimental observation of polaronic properties. In optical lattices not only Feshbach resonances but also confinement-induced resonances can be used to tune the polaronic coupling strength. This opens up the possibility to experimentally reveal the intermediate and strong polaronic coupling regimes and resolve outstanding theoretical questions regarding polaron theory.

cond-mat.quant-gas↗

arXiv2012

Pseudogap and preformed pairs in the imbalanced Fermi gas in two dimensions

The physics of the pseudogap state is intimately linked with the pairing mechanism that gives rise to superfluidity in quantum gases and to superconductivity in high-Tc cuprates, and therefore, both in quantum gases and superconductors, the pseudogap state and preformed pairs have been under intensive experimental scrutiny. Here, we develop a path integral treatment that provides a divergence-free description of the paired state in two-dimensional Fermi gases. Within this formalism, we derive the pseudogap temperature and the pair fluctuation spectral function, and compare these results with the recent experimental measument of the pairing in the two-dimensional Fermi gas. The removal of the infrared divergence in the number equations is shown both numerically and analytically, through a study of the long-wavelength and low-energy limit of the pair fluctuation density. Besides the pseudogap temperature, also the pair formation temperature and the critical temperature for superfluidity are derived. The latter corresponds to the Berezinski-Kosterlitz-Thouless (BKT) temperature. The pseudogap temperature, which coincides with the pair formation temperature in mean field, is found to be suppressed with respect to the pair formation temperature by fluctuations. This suppression is strongest for large binding energies of the pairs. Finally, we investigate how the pair formation temperature, the pseudogap temperature and the BKT temperature behave as a function of both binding energy and imbalance between the pairing partners in the Fermi gas. This allows to set up phase diagrams for the two-dimensional Fermi gas, in which the superfluid phase, the phase-fluctuating quasicondensate, and the normal state can be identified.

cond-mat.quant-gas↗

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↗

arXiv2012

Microscopic mechanisms for the Fermi-liquid behavior of Nb-doped strontium titanate

The relaxation rate in Nb-doped strontium titanate involving different scattering channels is investigated theoretically. It is demonstrated that the total relaxation rate in SrTiO_{3} is provided mainly by two mechanisms. The Baber electron-electron scattering with participation of both Coulomb and phonon-mediated electron-electron interactions provides the T^{2}-dependence of the relaxation rate. The scattering on the potential landscape caused by impurities is responsible for the residual relaxation rate at low temperatures. A good agreement with experiment is achieved accounting for all phonon branches in strontium titanate, both the optical and acoustic phonons. It is shown that the effective electron-electron interaction can be attractive in strontium titanate, and provides superconductivity at low temperatures and Fermi-liquid response in a wide range of temperatures. Thus our microscopic model supports the notion that superconductivity and Fermi-liquid properties of n-type SrTiO_{3} have a common origin.

cond-mat.str-el↗

arXiv2011

Pair excitations and parameters of state of imbalanced Fermi gases at finite temperatures

The spectra of low-lying pair excitations for an imbalanced two-component superfluid Fermi gas are analytically derived within the path-integral formalism taking into account Gaussian fluctuations about the saddle point. The spectra are obtained for nonzero temperatures, both with and without imbalance, and for arbitrary interaction strength. On the basis of the pair excitation spectrum, we have calculated the thermodynamic parameters of state of cold fermions and the first and second sound velocities. The parameters of pair excitations show a remarkable agreement with the Monte Carlo data and with experiment.

cond-mat.quant-gas↗

arXiv2011

Many-polaron description of impurities in a Bose-Einstein condensate in the weak coupling regime

The weak coupling many-polaron formalism is applied to the case of the polaronic system consisting of impurities in a Bose-Einstein condensate. This allows to investigate the groundstate properties and the response of the system to Bragg spectroscopy. This theory is then applied to the system of spin-polarized fermionic lithium-6 impurities in a sodium condensate. The Bragg spectrum reveals a peak which corresponds to the emission of Bogoliubov excitations. Both ground state properties and the response spectrum show that the polaronic effect vanishes at large densities. We also look at two possibilities to define the polaronic effective mass and observe that this results in a different quantitative behavior if multiple impurities are involved.

cond-mat.quant-gas↗

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↗