Special Issue "Proceedings of the conference SuperFluctuations 2017"

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

Deadline for manuscript submissions: closed (31 January 2018)

Special Issue Editors

Guest Editor
Prof. Andrea Perali

Università di Camerino, Scuola del Farmaco e Divisione di Fisica, Edificio di Fisica, Via Madonna delle Carceri 9, 62032 Camerino (MC), Italy
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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
Guest Editor
Prof. Dr. Luca Dell'Anna

Dipartimento di Fisica e Astronomia "Galileo Galilei", Università degli Studi di Padova, Via F. Marzolo 8, I-35131 Padova, Italy
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Interests: ultracold atoms; disordered systems; mesoscopic physics
Guest Editor
Prof. Dr. Luca Salasnich

Department of Physics and Astronomy "Galileo Galilei", University of Padova, Via Marzolo 8, 35131 Padova, Italy
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Phone: +39 049 827 7132
Interests: Bose-Einstein condensation; Bose-Einstein condensation; Ultracold Atoms; Quantum Statistical Physics; Quantum Statistical Physics

Special Issue Information

Dear Colleagues,

This Special Issue is dedicated to SuperFluctuations 2017, which will be held in San Benedetto del Tronto, Italy, 6–8 September 2017 (http://www.multisuper.org/superfluctuations-2017).

Among the main topics of the Conference is the study of fluctuations and BCS-BEC crossover phenomena in low dimensional systems. Very recently, the BCS-BEC crossover has been realized in quasi two-dimensional (2D) ultracold gases made of alkali-metal atoms. Contrary to the 3D case, mean-field theories are completely unreliable for the study of strongly-interacting superfluid fermions in two dimensions because of the huge increase of quantum fluctuations. The 2D BCS-BEC crossover is also interesting for high-TC superconductivity where the phase diagram of cuprate superconductors can be interpreted in terms of a BCS-BEC crossover as doping is varied. The critical temperature TC has a wide fluctuation region with pseudo-gap effects not yet fully understood.

Another hot topic of the Conference is multiband and multigap superconductors, which have demonstrated the potential for realizing novel coherent quantum phenomena and to control system parameters to enhance the superconducting critical temperature and the pairing energy gaps. Nanostructuring of bulk superconductors in the form of nanofilms, nanostripes, and nanoclusters, or introducing controlled non homogeneities has shown to be able to induce multi-gap and multiband superconductivity and superconducting shape resonances. Very recently, it has been suggested that iron-based superconductors have composite superconductivity, consisting of strong-coupling BEC in the electron band and weak-coupling BCS-like superconductivity in the hole band. Indeed, the intermediate crossover regime between BCS and BEC superconductivity is now considered as one of the most promising regimes in the search for high-TC superconductivity.

Main Topics:

  • -Fluctuations and BCS-BEC crossover phenomena in low dimensional systems;
  • -Hybrid systems, superconductivity at the interfaces, and coexistence of phases;
  • -Highly nonlinear phenomena: Josephson and Andreev effects, topological defects, skyrmions and solitons, vortex states;
  • -Novel phenomena in multicomponent/multigap superconductors and superfluids.

Prof. Dr. Andrea Perali
Guest Editor

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Published Papers (5 papers)

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Research

Open AccessArticle Suppression of Quantum-Mechanical Collapse in Bosonic Gases with Intrinsic Repulsion: A Brief Review
Condens. Matter 2018, 3(2), 15; https://doi.org/10.3390/condmat3020015
Received: 12 March 2018 / Revised: 16 April 2018 / Accepted: 18 April 2018 / Published: 23 April 2018
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Abstract
It is known that attractive potential ~1/r2 gives rise to the critical quantum collapse in the framework of the three-dimensional (3D) linear Schrödinger equation. This article summarizes theoretical analysis, chiefly published in several original papers, which demonstrates suppression
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It is known that attractive potential ~ 1 / r 2 gives rise to the critical quantum collapse in the framework of the three-dimensional (3D) linear Schrödinger equation. This article summarizes theoretical analysis, chiefly published in several original papers, which demonstrates suppression of the collapse caused by this potential, and the creation of the otherwise missing ground state in a 3D gas of bosonic dipoles pulled by the same potential to the central charge, with repulsive contact interactions between them, represented by the cubic term in the respective Gross–Pitaevskii equation (GPE). In two dimensions (2D), quintic self-repulsion is necessary for the suppression of the collapse; alternatively, this may be provided by the effective quartic repulsion produced by the Lee–Huang–Yang correction to the GPE. 3D states carrying angular momentum are constructed in the model with the symmetry reduced from spherical to cylindrical by an external polarizing field. Interplay of the collapse suppression and miscibility–immiscibility transition is considered in a binary condensate. The consideration of the 3D setting in the form of the many-body quantum system, with the help of the Monte Carlo method, demonstrates that, although the quantum collapse cannot be fully suppressed, the self-trapped states predicted by the GPE exist in the many-body setting as metastable modes protected against the collapse by a tall potential barrier. Full article
(This article belongs to the Special Issue Proceedings of the conference SuperFluctuations 2017)
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Open AccessArticle Effective Control of Chemical Potentials by Rabi Coupling with RF-Fields in Ultracold Mixtures
Condens. Matter 2018, 3(2), 14; https://doi.org/10.3390/condmat3020014
Received: 19 February 2018 / Revised: 5 April 2018 / Accepted: 12 April 2018 / Published: 17 April 2018
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Abstract
We show that a linear term coupling the atoms of an ultracold binary mixture provides a simple method to induce an effective and tunable population imbalance between them. This term is easily realized by Rabi coupling between different hyperfine levels of the same
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We show that a linear term coupling the atoms of an ultracold binary mixture provides a simple method to induce an effective and tunable population imbalance between them. This term is easily realized by Rabi coupling between different hyperfine levels of the same atomic species. The resulting effective imbalance holds for one-particle states dressed by the Rabi coupling and obtained by diagonalizing the mixing matrix of the Rabi term. This way of controlling the chemical potentials applies to both bosonic and fermionic atoms and it also allows for spatially- and temporally-dependent imbalances. As a first application, we show that, in the case of two attractive fermionic hyperfine levels with equal chemical potentials coupled by the Rabi pulse, the same superfluid properties of an imbalanced binary mixture are recovered. We finally discuss the properties of m-species mixtures in the presence of SU(m)-invariant interactions. Full article
(This article belongs to the Special Issue Proceedings of the conference SuperFluctuations 2017)
Open AccessArticle Fermionic Properties of Two Interacting Bosons in a Two-Dimensional Harmonic Trap
Condens. Matter 2018, 3(1), 9; https://doi.org/10.3390/condmat3010009
Received: 30 January 2018 / Revised: 9 March 2018 / Accepted: 12 March 2018 / Published: 15 March 2018
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Abstract
The system of two interacting bosons in a two-dimensional harmonic trap is compared with the system consisting of two noninteracting fermions in the same potential. In particular, we discuss how the properties of the ground state of the system, e.g., the different contributions
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The system of two interacting bosons in a two-dimensional harmonic trap is compared with the system consisting of two noninteracting fermions in the same potential. In particular, we discuss how the properties of the ground state of the system, e.g., the different contributions to the total energy, change as we vary both the strength and range of the atom–atom interaction. In particular, we focus on the short-range and strong interacting limit of the two-boson system and compare it to the noninteracting two-fermion system by properly symmetrizing the corresponding degenerate ground state wave functions. In that limit, we show that the density profile of the two-boson system has a tendency similar to the system of two noninteracting fermions. Similarly, the correlations induced when the interaction strength is increased result in a similar pair correlation function for both systems. Full article
(This article belongs to the Special Issue Proceedings of the conference SuperFluctuations 2017)
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Open AccessArticle The BKT Universality Class in the Presence of Correlated Disorder
Condens. Matter 2018, 3(1), 8; https://doi.org/10.3390/condmat3010008
Received: 30 January 2018 / Revised: 5 March 2018 / Accepted: 6 March 2018 / Published: 9 March 2018
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Abstract
The correct detection of the Berezinskii-Kosterlitz-Thouless (BKT) transition in quasi-two-dimensional superconductors still remains a controversial issue. Its main signatures, indeed, are often at odds with the theoretical expectations. In a recent work (Maccari, I.; Benfatto, L.; Castellani, C. Phys. Rev. B 2017, 96,
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The correct detection of the Berezinskii-Kosterlitz-Thouless (BKT) transition in quasi-two-dimensional superconductors still remains a controversial issue. Its main signatures, indeed, are often at odds with the theoretical expectations. In a recent work (Maccari, I.; Benfatto, L.; Castellani, C. Phys. Rev. B 2017, 96, 060508), we have shown that the presence of spatially correlated disorder plays a key role in this sense because it is the reason underlying the experimentally-observed smearing of the universal superfluid-density jump. In the present paper we closely investigate the effects of correlated disorder on the BKT transition, specifically addressing the issue of whether or not it changes the BKT universality class. Full article
(This article belongs to the Special Issue Proceedings of the conference SuperFluctuations 2017)
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Open AccessArticle Two-Time Correlation Functions in Dissipative and Interacting Bose–Hubbard Chains
Condens. Matter 2018, 3(1), 2; https://doi.org/10.3390/condmat3010002
Received: 6 December 2017 / Revised: 29 December 2017 / Accepted: 31 December 2017 / Published: 3 January 2018
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Abstract
A method is presented for the systematic derivation of a hierarchy of coupled equations for the computation of two-time correlation functions of operators for open many-body quantum systems. We show how these systems of equations can be closed in mean-field and beyond approximations.
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A method is presented for the systematic derivation of a hierarchy of coupled equations for the computation of two-time correlation functions of operators for open many-body quantum systems. We show how these systems of equations can be closed in mean-field and beyond approximations. Results for the specific example of the spectral weight functions are discussed. Our method allows one to access the full temporal evolution, not just the stationary solution, of non-equilibrium open quantum problems described by a Markovian master equation. Full article
(This article belongs to the Special Issue Proceedings of the conference SuperFluctuations 2017)
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