Fluctuations and Highly Non-linear Phenomena in Superfluids and Superconductors VII

A special issue of Condensed Matter (ISSN 2410-3896). This special issue belongs to the section "Superconductivity".

Deadline for manuscript submissions: closed (31 January 2023) | Viewed by 9391

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Scuola del Farmaco e Divisione di Fisica, Edificio di Fisica, Università di Camerino, Via Madonna delle Carceri 9, 62032 Camerino, 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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Guest Editor
Department of Physics, Columbia University, New York, NY 10027, USA
Interests: atomic, molecular, and optical physics; condensed matter physics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

This Special Issue is dedicated to the proceedings of the conference SuperFluctuations 2022, which will be held in Padova and online on 6–8 July 2022 (conference website link). This international conference will provide an excellent forum for the presentation and discussion of recent developments in diverse fields of superconductivity and superfluidity. You are invited to submit a full manuscript for consideration and possible publication in this 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: multicomponent and low-dimensional systems, spin-orbit coupling, and FFLO states.
  • Novel quantum phenomena with Bose and Fermi mixtures.
  • Highly nonlinear phenomena: Josephson and Andreev effects, topological defects, and vortex states.
  • Electron-hole superfluidity: theory, numerical simulations, and experimental realizations.
  • Innovative numerical methods: machine learning, its synergies with QMC, and applications.
  • Quantum technologies and quantum devices/sensors based on novel superconducting/superfluid systems.

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

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Published Papers (7 papers)

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Research

18 pages, 482 KiB  
Article
Charge-Density Waves vs. Superconductivity: Some Results and Future Perspectives
by Giulia Venditti and Sergio Caprara
Condens. Matter 2023, 8(3), 54; https://doi.org/10.3390/condmat8030054 - 27 Jun 2023
Cited by 2 | Viewed by 1564
Abstract
Increasing experimental evidence suggests the occurrence of filamentary superconductivity in different (quasi) two-dimensional physical systems. In this piece of work, we discuss the proposal that under certain circumstances, this occurrence may be related to the competition with a phase characterized by charge ordering [...] Read more.
Increasing experimental evidence suggests the occurrence of filamentary superconductivity in different (quasi) two-dimensional physical systems. In this piece of work, we discuss the proposal that under certain circumstances, this occurrence may be related to the competition with a phase characterized by charge ordering in the form of charge-density waves. We provide a brief summary of experimental evidence supporting our argument in two paradigmatic classes of materials, namely transition metal dichalcogenides and cuprates superconductors. We present a simple Ginzburg–Landau two-order-parameters model as a starting point to address the study of such competition. We finally discuss the outcomes of a more sophisticated model, already presented in the literature and encoding the presence of impurities, and how it can be further improved in order to really address the interplay between charge-density waves and superconductivity and the possible occurrence of filamentary superconductivity at the domain walls between different charge-ordered regions. Full article
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12 pages, 760 KiB  
Article
Quadriexciton Binding Energy in Electron–Hole Bilayers
by Cesare Malosso, Gaetano Senatore and Stefania De Palo
Condens. Matter 2023, 8(2), 44; https://doi.org/10.3390/condmat8020044 - 10 May 2023
Viewed by 1117
Abstract
Excitonic condensation and superfluidity have recently received a renewed attention, due to the fabrication of bilayer systems in which electrons and holes are spatially separated and form stable pairs known as indirect excitons. Dichalcogenides- and graphene-based bilayers are nowadays built and investigated, giving [...] Read more.
Excitonic condensation and superfluidity have recently received a renewed attention, due to the fabrication of bilayer systems in which electrons and holes are spatially separated and form stable pairs known as indirect excitons. Dichalcogenides- and graphene-based bilayers are nowadays built and investigated, giving access to systems with (i) only spin degeneracy and (ii) spin and valley degeneracy. Simulation studies performed in the last decades at T=0 for simple, model electron–hole bilayers, as function of the interlayer distance and in-layer carrier density, have revealed in case (i) the formation of biexcitons in a tiny region of the parameter space and in case (ii) the formation of stable compounds made of four electrons and four holes (quadriexcitons) in a sizable region of the parameter space. Of some interest is the relation of the properties of isolated biexcitons (quadriexcitons) and those of their finite-density counterpart. In fact, the isolated biexciton has been repeatedly studied in the last years with simulations and other techniques. No simulations, instead, are available to our knowledge for the isolated quadriexciton, for which we present here results of the first quantum Monte Carlo (QMC) study. Stability with respect to the dissociation into biexcitons and the pair correlations while varying the interlayer distance d are discussed. Full article
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12 pages, 1433 KiB  
Article
Low-Lying Collective Excitations of Superconductors and Charged Superfluids
by Serghei Klimin, Jacques Tempere and Hadrien Kurkjian
Condens. Matter 2023, 8(2), 42; https://doi.org/10.3390/condmat8020042 - 03 May 2023
Viewed by 1287
Abstract
We investigate theoretically the momentum-dependent frequency and damping of low-lying collective excitations of superconductors and charged superfluids in the BCS–BEC crossover regime. The study is based on the Gaussian pair-and-density fluctuation method for the propagator of Gaussian fluctuations of the pair and density [...] Read more.
We investigate theoretically the momentum-dependent frequency and damping of low-lying collective excitations of superconductors and charged superfluids in the BCS–BEC crossover regime. The study is based on the Gaussian pair-and-density fluctuation method for the propagator of Gaussian fluctuations of the pair and density fields. Eigenfrequencies and damping rates are determined in a mutually consistent nonperturbative way as complex poles of the fluctuation propagator. Particular attention is paid to new features with respect to preceding theoretical studies, which were devoted to collective excitations of superconductors in the far BCS regime. We find that at a sufficiently strong coupling, new branches of collective excitations appear, which manifest different behavior as functions of the momentum and the temperature. Full article
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8 pages, 310 KiB  
Article
FL* Interpretation of a Dichotomy in the Spin Susceptibility of the Cuprates
by Pieralberto Marchetti
Condens. Matter 2023, 8(2), 30; https://doi.org/10.3390/condmat8020030 - 23 Mar 2023
Cited by 1 | Viewed by 1062
Abstract
We propose that some dichotomic Fermi liquid versus non-Fermi liquid behaviours of physical quantities in hole-doped cuprates can be explained in terms of the FL* fractionalized Fermi liquid concept, introduced some years ago, even beyond the region of underdoping. The particle excitations of [...] Read more.
We propose that some dichotomic Fermi liquid versus non-Fermi liquid behaviours of physical quantities in hole-doped cuprates can be explained in terms of the FL* fractionalized Fermi liquid concept, introduced some years ago, even beyond the region of underdoping. The particle excitations of this FL* system are the holon carrying charge, the spinon carrying spin 1/2, gauge fluctuations coupling them and the hole as a spinon–holon bound state or resonance due to gauge binding. In our proposal, physical responses have a Fermi-liquid-type behaviour if they are dominated by the hole resonance, whereas a non-Fermi liquid behaviour appears if they are dominated by spinon–spinon (and possibly also holon–holon) gauge interactions. The specific case of spin susceptibility in the so-called "strange metal phase" is discussed. The uniform susceptibility turns out to be hole-dominated, the spin-lattice relaxation rate in the Cu sites is spinon-dominated. Full article
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8 pages, 518 KiB  
Article
Effect of Transverse Confinement on a Quasi-One-Dimensional Dipolar Bose Gas
by Stefania De Palo, Edmond Orignac, Roberta Citro and Luca Salasnich
Condens. Matter 2023, 8(1), 26; https://doi.org/10.3390/condmat8010026 - 05 Mar 2023
Cited by 1 | Viewed by 1008
Abstract
We study a gas of bosonic dipolar atoms in the presence of a transverse harmonic trapping potential by using an improved variational Bethe ansatz, which includes the transverse width of the atomic cloud as a variational parameter. Our calculations show that the system [...] Read more.
We study a gas of bosonic dipolar atoms in the presence of a transverse harmonic trapping potential by using an improved variational Bethe ansatz, which includes the transverse width of the atomic cloud as a variational parameter. Our calculations show that the system behavior evolves from quasi-one dimensional to a strictly one-dimensional one by changing the atom–atom interaction, or the axial density, or the frequency of the transverse confinement. Quite remarkably, in the droplet phase induced by the attractive dipolar interaction the system becomes sub-one dimensional when the transverse width is smaller than the characteristic length of the transverse harmonic confinement. Full article
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7 pages, 326 KiB  
Article
Second-Neighbor Hopping Effects in the Two-Dimensional Attractive Hubbard Model
by Rodrigo Alves Fontenele, Nathan Vasconcelos, Natanael Carvalho Costa, Thereza Paiva and Raimundo Rocha dos Santos
Condens. Matter 2023, 8(1), 11; https://doi.org/10.3390/condmat8010011 - 17 Jan 2023
Viewed by 1427
Abstract
The emergence of superconductivity (SC) in lattice models, such as the attractive Hubbard one, has renewed interest since the realization of cold-atom experiments. However, reducing the temperature in these experiments is a bottleneck; therefore, investigating how to increase the energy scale for SC [...] Read more.
The emergence of superconductivity (SC) in lattice models, such as the attractive Hubbard one, has renewed interest since the realization of cold-atom experiments. However, reducing the temperature in these experiments is a bottleneck; therefore, investigating how to increase the energy scale for SC is crucial to cold atoms. In view of this, we examine the effects of next-nearest-neighbor hoppings (t) on the pairing properties of the attractive Hubbard model in a square lattice. To this end, we analyze the model through unbiased Quantum Monte Carlo simulations for fixed density n=0.87, and perform finite-size scaling analysis to the thermodynamic limit. As our main result, we notice that the existence of further hopping channels leads to an enhancement of the pairing correlations, which, in turn, increases the ground-state order parameter Δ. Finally, at finite temperatures, for t/t0, this enhancement of pairing correlations leads to an increase in the critical temperature Tc. That is, the fine-tuning of second-neighbor hoppings increases the energy scales for SC, and may be a route by which cold-atom experiments can achieve such a phase and to help us further understand the nature of this phenomenon. Full article
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12 pages, 328 KiB  
Article
Rabi Coupled Fermions in the BCS–BEC Crossover
by Luca Dell’Anna, Federico De Bettin and Luca Salasnich
Condens. Matter 2022, 7(4), 59; https://doi.org/10.3390/condmat7040059 - 22 Oct 2022
Viewed by 1230
Abstract
We investigate the three-dimensional BCS–BEC crossover in the presence of a Rabi coupling, which strongly affects several properties of the system, such as the chemical potential, the pairing gap and the superfluid density. We determine the critical interaction strength, below which the system [...] Read more.
We investigate the three-dimensional BCS–BEC crossover in the presence of a Rabi coupling, which strongly affects several properties of the system, such as the chemical potential, the pairing gap and the superfluid density. We determine the critical interaction strength, below which the system is normal also at zero temperature. Finally, we calculate the effect of the Rabi coupling on the critical temperature of the superfluid-to-normal phase transition by using different theoretical schemes. Full article
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