Special Issue "Fluctuations and Highly Non-linear Phenomena in Superfluids and Superconductors II"

A special issue of Condensed Matter (ISSN 2410-3896).

Deadline for manuscript submissions: closed (31 January 2020).

Special Issue Editors

Prof. Dr. Andrea Perali
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Guest Editor
Università di Camerino, Scuola del Farmaco e Divisione di Fisica, Edificio di Fisica, Via Madonna delle Carceri 9, 62032 Camerino (MC), Italy
Interests: high-Tc superconductivity (theory and phenomenology), multiband superconductivity, quantum size effects and shape resonances in superconductors, nanoscale superconductors, superconducting heterostructures, BCS-BEC crossover, pseudogap, superconducting fluctuations, ultracold fermions: superfluidity and BCS-BEC crossover, electron–hole superfluidity
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Prof. Dr. Luca Dell'Anna
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Guest Editor
Dipartimento di Fisica e Astronomia "Galileo Galilei", Università degli Studi di Padova, Via F. Marzolo 8, I-35131 Padova, Italy
Interests: ultracold atoms; disordered systems; mesoscopic physics
Special Issues and Collections in MDPI journals
Prof. Dr. Luca Salasnich
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Guest Editor
Department of Physics and Astronomy "Galileo Galilei", University of Padova, Via Marzolo 8, 35131 Padova, Italy
Interests: Bose-Einstein condensation; ultracold atoms; quantum statistical physics
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue is dedicated to the proceedings of the conference SuperFluctuations 2019 (http://www.multisuper.org/superfluctuations-2019), which will be held in Padova, Italy, 2–4 September 2019. This international conference will highlight recent advances in diverse fields of superconductivity and superfluidity. You are invited to submit a full manuscript for consideration and possible publication in this joint Special Issue. Submissions will be rapidly reviewed and published immediately, if accepted. Invited and leading contributed papers will focus on the following topics:

  • Fluctuations and BCS-BEC crossover phenomena in multicomponent and low dimensional systems;
  • Quantum Technologies and Novel Phenomena with Bose and Fermi Mixtures;
  • Highly nonlinear phenomena: Josephson and Andreev effects, topological defects, skyrmions and solitons, vortex states;
  • Novel quantum phenomena in multicomponent / multigap superconductors and superfluids;
  • Innovative numerical methods: Machine Learning and its applications.

Prof. Dr. Andrea Perali
Prof. Dr. Luca Dell'Anna
Prof. Dr. Luca Salasnich
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Condensed Matter is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • BCS-BEC crossober
  • multicomponent superfluids
  • Bose-Fermi mixtures
  • machine learning
  • topological quantum matter
  • quantum technologies
  • vortex states

Published Papers (10 papers)

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Research

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Open AccessArticle
Effect of Inter-Well Interactions on Non-Linear Beam Splitters for Matter-Wave Interferometers
Condens. Matter 2020, 5(2), 31; https://doi.org/10.3390/condmat5020031 - 21 Apr 2020
Abstract
We study the non-linear beam splitter in matter-wave interferometers using ultracold quantum gases in a double-well configuration in presence of non-local interactions inducing inter-well density-density coupling, as they can be realized, e.g., with dipolar gases. We explore this effect after considering different input [...] Read more.
We study the non-linear beam splitter in matter-wave interferometers using ultracold quantum gases in a double-well configuration in presence of non-local interactions inducing inter-well density-density coupling, as they can be realized, e.g., with dipolar gases. We explore this effect after considering different input states, in the form of either coherent, or Twin-Fock, or NOON states. We first review the non-interacting limit and the case in which only the local interaction is present, including the study of sensitivity near the self-trapping threshold. Then, we consider the two-mode model in the presence of inter-well interactions and consider the scaling of the sensitivity as a function of the non-local coupling strength. Our analysis clearly shows that non-local interactions can compensate the degradation of the sensitivity induced by local interactions, so that they may be used to restore optimal sensitivity. Full article
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Open AccessFeature PaperArticle
Transition Metal Dichalcogenides as Strategy for High Temperature Electron-Hole Superfluidity
Condens. Matter 2020, 5(1), 22; https://doi.org/10.3390/condmat5010022 - 22 Mar 2020
Abstract
Condensation of spatially indirect excitons, with the electrons and holes confined in two separate layers, has recently been observed in two different double layer heterostructures. High transition temperatures were reported in a double Transition Metal Dichalcogenide (TMD) monolayer system. We briefly review electron-hole [...] Read more.
Condensation of spatially indirect excitons, with the electrons and holes confined in two separate layers, has recently been observed in two different double layer heterostructures. High transition temperatures were reported in a double Transition Metal Dichalcogenide (TMD) monolayer system. We briefly review electron-hole double layer systems that have been proposed as candidates for this interesting phenomenon. We investigate the double TMD system WSe 2 /hBN/MoSe 2 , using a mean-field approach that includes multiband effects due to the spin-orbit coupling and self-consistent screening of the electron-hole Coulomb interaction. We demonstrate that the transition temperature observed in the double TMD monolayers, which is remarkably high relative to the other systems, is the result of (i) the large electron and hole effective masses in TMDs, (ii) the large TMD band gaps, and (iii) the presence of multiple superfluid condensates in the TMD system. The net effect is that the superfluidity is strong across a wide range of densities, which leads to high transition temperatures that extend as high as T B K T = 150 K. Full article
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Open AccessArticle
A Dual-Species Bose-Einstein Condensate with Attractive Interspecies Interactions
Condens. Matter 2020, 5(1), 21; https://doi.org/10.3390/condmat5010021 - 21 Mar 2020
Cited by 1
Abstract
We report on the production of a 41 K- 87 Rb dual-species Bose–Einstein condensate with tunable interspecies interaction and we study the mixture in the attractive regime; i.e., for negative values of the interspecies scattering length a 12 . The binary condensate is [...] Read more.
We report on the production of a 41 K- 87 Rb dual-species Bose–Einstein condensate with tunable interspecies interaction and we study the mixture in the attractive regime; i.e., for negative values of the interspecies scattering length a 12 . The binary condensate is prepared in the ground state and confined in a pure optical trap. We exploit Feshbach resonances for tuning the value of a 12 . After compensating the gravitational sag between the two species with a magnetic field gradient, we drive the mixture into the attractive regime. We let the system evolve both in free space and in an optical waveguide. In both geometries, for strong attractive interactions, we observe the formation of self-bound states, recognizable as quantum droplets. Our findings prove that robust, long-lived droplet states can be realized in attractive two-species mixtures, despite the two atomic components possibly experiencing different potentials. Full article
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Open AccessArticle
Spectral Function of a Boson Ladder in an Artificial Gauge Field
Condens. Matter 2020, 5(1), 15; https://doi.org/10.3390/condmat5010015 - 10 Mar 2020
Cited by 1
Abstract
We calculate the spectral function of a boson ladder in an artificial magnetic field by means of analytic approaches based on bosonization and Bogoliubov theory. We discuss the evolution of the spectral function at increasing effective magnetic flux, from the Meissner to the [...] Read more.
We calculate the spectral function of a boson ladder in an artificial magnetic field by means of analytic approaches based on bosonization and Bogoliubov theory. We discuss the evolution of the spectral function at increasing effective magnetic flux, from the Meissner to the Vortex phase, focussing on the effects of incommensurations in momentum space. At low flux, in the Meissner phase, the spectral function displays both a gapless branch and a gapped one, while at higher flux, in the Vortex phase, the spectral function displays two gapless branches and the spectral weight is shifted at a wavevector associated to the underlying vortex spatial structure, which can indicate a supersolid-like behavior. While the Bogoliubov theory, valid at weak interactions, predicts sharp delta-like features in the spectral function, at stronger interactions we find power-law broadening of the spectral functions due to quantum fluctuations as well as additional spectral weight at higher momenta due to backscattering and incommensuration effects. These features could be accessed in ultracold atom experiments using radio-frequency spectroscopy techniques. Full article
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Open AccessArticle
Dispersion and Damping of Phononic Excitations in Fermi Superfluid Gases in 2D
Condens. Matter 2020, 5(1), 13; https://doi.org/10.3390/condmat5010013 - 25 Feb 2020
Abstract
We calculate the sound velocity and the damping rate of the collective excitations of a 2D fermionic superfluid in a non-perturbative manner. Specifically, we focus on the Anderson–Bogoliubov excitations in the BEC-BCS crossover regime, as these modes have a sound-like dispersion at low [...] Read more.
We calculate the sound velocity and the damping rate of the collective excitations of a 2D fermionic superfluid in a non-perturbative manner. Specifically, we focus on the Anderson–Bogoliubov excitations in the BEC-BCS crossover regime, as these modes have a sound-like dispersion at low momenta. The calculation is performed within the path-integral formalism and the Gaussian pair fluctuation approximation. From the action functional, we obtain the propagator of the collective excitations and determine their dispersion relation by locating the poles of this propagator. We find that there is only one kind of collective excitation, which is stable at T = 0 and has a sound velocity of v F / 2 for all binding energies, i.e., throughout the BEC-BCS crossover. As the temperature is raised, the sound velocity decreases and the damping rate shows a non-monotonous behavior: after an initial increase, close to the critical temperature T C the damping rate decreases again. In general, higher binding energies provide higher damping rates. Finally, we calculate the response functions and propose that they can be used as another way to determine the sound velocity. Full article
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Open AccessArticle
BCS-BEC Crossover and Pairing Fluctuations in a Two Band Superfluid/Superconductor: A T Matrix Approach
Condens. Matter 2020, 5(1), 10; https://doi.org/10.3390/condmat5010010 - 21 Jan 2020
Cited by 1
Abstract
We investigate pairing fluctuation effects in a two band fermionic system, where a shallow band in the Bardeen–Cooper–Schrieffer–Bose–Einstein condensation (BCS-BEC) crossover regime is coupled with a weakly interacting deep band. Within a diagrammatic T matrix approach, we report how thermodynamic quantities such as [...] Read more.
We investigate pairing fluctuation effects in a two band fermionic system, where a shallow band in the Bardeen–Cooper–Schrieffer–Bose–Einstein condensation (BCS-BEC) crossover regime is coupled with a weakly interacting deep band. Within a diagrammatic T matrix approach, we report how thermodynamic quantities such as the critical temperature, chemical potential, and momentum distributions undergo the crossover from the BCS to BEC regime by tuning the intraband coupling in the shallow band. We also generalize the definition of Tan’s contact to a two band system and report the two contacts for different pair-exchange couplings. The present results are compared with those obtained by the simpler Nozières–Schmitt–Rink approximation. We confirm a pronounced enhancement of the critical temperature due to the multiband configuration, as well as to the pair-exchange coupling. Full article
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Open AccessArticle
Quantum-Granularity Effect in the Formation of Supermixed Solitons in Ring Lattices
Condens. Matter 2020, 5(1), 2; https://doi.org/10.3390/condmat5010002 - 08 Jan 2020
Cited by 4
Abstract
We investigate a notable class of states peculiar to a bosonic binary mixture featuring repulsive intraspecies and attractive interspecies couplings. We evidence that, for small values of the hopping amplitudes, one can access particular regimes marked by the fact that the interwell boson [...] Read more.
We investigate a notable class of states peculiar to a bosonic binary mixture featuring repulsive intraspecies and attractive interspecies couplings. We evidence that, for small values of the hopping amplitudes, one can access particular regimes marked by the fact that the interwell boson transfer occurs in a jerky fashion. This property is shown to be responsible for the emergence of a staircase-like structure in the phase diagram of a mixture confined in a ring trimer and to resemble the mechanism of the superfluid-Mott insulator transition strongly. Under certain conditions, in fact, we show that it is possible to interpret the interspecies attraction as an effective chemical potential and the supermixed soliton as an effective particle reservoir. Our investigation is developed both within a fully quantum approach based on the analysis of several quantum indicators and by means of a simple analytical approximation scheme capable of capturing the essential features of this ultraquantum effect. Full article
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Open AccessArticle
Thermal and Quantum Fluctuation Effects in Quasiperiodic Systems in External Potentials
Condens. Matter 2019, 4(4), 93; https://doi.org/10.3390/condmat4040093 - 16 Dec 2019
Abstract
We analyze the many-body phases of an ensemble of particles interacting via a Lifshitz–Petrich–Gaussian pair potential in a harmonic confinement. We focus on specific parameter regimes where we expect decagonal quasiperiodic cluster arrangements. Performing classical Monte Carlo as well as path integral quantum [...] Read more.
We analyze the many-body phases of an ensemble of particles interacting via a Lifshitz–Petrich–Gaussian pair potential in a harmonic confinement. We focus on specific parameter regimes where we expect decagonal quasiperiodic cluster arrangements. Performing classical Monte Carlo as well as path integral quantum Monte Carlo methods, we numerically simulate systems of a few thousand particles including thermal and quantum fluctuations. Our findings indicate that the competition between the intrinsic length scale of the harmonic oscillator and the wavelengths associated to the minima of the pair potential generically lead to a destruction of the quasicrystalline pattern. Extensions of this work are also discussed. Full article
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Review

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Open AccessReview
Singular Mean-Field States: A Brief Review of Recent Results
Condens. Matter 2020, 5(1), 20; https://doi.org/10.3390/condmat5010020 - 20 Mar 2020
Cited by 3
Abstract
This article provides a focused review of recent findings which demonstrate, in some cases quite counter-intuitively, the existence of bound states with a singularity of the density pattern at the center; the states are physically meaningful because their total norm converges. One model [...] Read more.
This article provides a focused review of recent findings which demonstrate, in some cases quite counter-intuitively, the existence of bound states with a singularity of the density pattern at the center; the states are physically meaningful because their total norm converges. One model of this type is based on the 2D Gross–Pitaevskii equation (GPE), which combines the attractive potential r 2 and the quartic self-repulsive nonlinearity, induced by the Lee–Huang–Yang effect (quantum fluctuations around the mean-field state). The GPE demonstrates suppression of the 2D quantum collapse, driven by the attractive potential, and emergence of a stable ground state (GS), whose density features an integrable singularity r 4 / 3 at r 0 . Modes with embedded angular momentum exist too, but they are unstable. A counter-intuitive peculiarity of the model is that the GS exists even if the sign of the potential is reversed from attraction to repulsion, provided that its strength is small enough. This peculiarity finds a relevant explanation. The other model outlined in the review includes 1D, 2D, and 3D GPEs, with the septimal (seventh-order), quintic, and cubic self-repulsive terms, respectively. These equations give rise to stable singular solitons, which represent the GS for each dimension D, with the density singularity r 2 / ( 4 D ) . Such states may be considered the results of screening a “bare” delta-functional attractive potential by the respective nonlinearities. Full article
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Open AccessReview
Fractional Statistics of Charge Carriers in the One- and Two-Dimensional t-J Model: A Hint for the Cuprates?
Condens. Matter 2020, 5(1), 12; https://doi.org/10.3390/condmat5010012 - 21 Feb 2020
Abstract
We show that we can interpret the exact solution of the one-dimensional t-J model in the limit of small J in terms of charge carriers with both exchange (braid) and exclusion (Haldane) statistics with parameter 1/2. We discuss an implementation of the same [...] Read more.
We show that we can interpret the exact solution of the one-dimensional t-J model in the limit of small J in terms of charge carriers with both exchange (braid) and exclusion (Haldane) statistics with parameter 1/2. We discuss an implementation of the same statistics in the two-dimensional t-J model, emphasizing similarities and differences with respect to one dimension. In both cases, the exclusion statistics is a consequence of the no-double occupation constraint. We argue that the application of this formalism to hole-doped high Tc cuprates and the derived composite nature of the hole give a hint to grasp many unusual properties of these materials. Full article
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