Special Issue "Quantum Crystals"

A special issue of Crystals (ISSN 2073-4352).

Deadline for manuscript submissions: closed (31 March 2018)

Special Issue Editors

Guest Editor
Prof. Dr. Claudio Cazorla

School of Materials Science and Engineering UNSW Australia
Website | E-Mail
Phone: +61 (0)9385 5918
Fax: +61 (0)9385 6565
Interests: multiferroics, oxide materials, fast-ion conductors, density functional theory, simulation methods, quantum crystals, caloric materials
Co-Guest Editor
Prof. Dr. Jordi Boronat

Polytechnic University of Catalonia, Department of Physics and Nuclear Engineering, Barcelona, Spain
Website | E-Mail
Interests: condensed matter physics, quantum liquids, Bose-Einstein condensation; quantum crystals; quantum Monte Carlo

Special Issue Information

Dear Colleagues,

At low temperatures, the kinetic energy per particle in a quantum crystal is much larger than kBT (where kB represents the Boltzmann constant and T the temperature) and the spatial fluctuations of the atoms around their equilibrium lattice positions are up to ~10% of the distance to the neighboring lattice sites. These intriguing qualities can be understood only in terms of quantum mechanical arguments.

The study of quantum crystals is very important to understand nature. Hydrogen and helium, two archetypal quantum species, are the most abundant elements in the universe and thus an exhaustive knowledge of their condensed matter phases is crucial for understanding the chemical composition and past and future evolution of planetary bodies. Quantum solids are also sought after for technological applications, including high-pressure synthesis, nuclear energy, gas storage, quantum computing, and nanoelectronics.

We invite investigators to submit research papers discussing the experimental and theoretical understanding of quantum crystals. Quantum crystals include, but are not limited to, rare-gases (e.g., He and Ne), light-weight molecular crystals (e.g., H2 and CH4), light-weight covalent, ionic and metallic solids (e.g., graphite, LiH, and Li), quantum paraelectrics (e.g., SrTiO3 and KTaO3), Wigner crystals, vortex lattices, and dipole systems.

The potential topics for this Special Issue include, but are not restricted to:

-    Experimental synthesis and characterization of quantum crystals
-    Theory and simulation 
-    Crystalline defects in quantum crystals
-    Elastic and mechanical properties
-    Quantum low-dimensional systems (i.e., thin films, one-dimensional systems, and clusters)
-    Quantum solids at high-pressure conditions
-    Gas-storage and structural properties of quantum materials
-    H-bonded ferroelectrics and quantum paraelectrics

Prof. Dr. Claudio Cazorla
Prof. Dr. Jordi Boronat
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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. Crystals is an international peer-reviewed open access monthly 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 1200 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

  • Helium
  • Hydrogen
  • Zero-point motion
  • Isotopic effects
  • Quantum simulation methods
  • Crystalline defects
  • Quantum materials

Published Papers (5 papers)

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Research

Open AccessArticle The Inverse-Square Interaction Phase Diagram: Unitarity in the Bosonic Ground State
Crystals 2018, 8(6), 246; https://doi.org/10.3390/cryst8060246
Received: 22 April 2018 / Revised: 4 June 2018 / Accepted: 4 June 2018 / Published: 8 June 2018
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Abstract
Ground-state properties of bosons interacting via inverse square potential (three dimensional Calogero-Sutherland model) are analyzed. A number of quantities scale with the density and can be naturally expressed in units of the Fermi energy and Fermi momentum multiplied by a dimensionless constant (Bertsch
[...] Read more.
Ground-state properties of bosons interacting via inverse square potential (three dimensional Calogero-Sutherland model) are analyzed. A number of quantities scale with the density and can be naturally expressed in units of the Fermi energy and Fermi momentum multiplied by a dimensionless constant (Bertsch parameter). Two analytical approaches are developed: the Bogoliubov theory for weak and the harmonic approximation (HA) for strong interactions. Diffusion Monte Carlo method is used to obtain the ground-state properties in a non-perturbative manner. We report the dependence of the Bertsch parameter on the interaction strength and construct a Padé approximant which fits the numerical data and reproduces correctly the asymptotic limits of weak and strong interactions. We find good agreement with beyond-mean field theory for the energy and the condensate fraction. The pair distribution function and the static structure factor are reported for a number of characteristic interactions. We demonstrate that the system experiences a gas-solid phase transition as a function of the dimensionless interaction strength. A peculiarity of the system is that by changing the density it is not possible to induce the phase transition. We show that the low-lying excitation spectrum contains plasmons in both phases, in agreement with the Bogoliubov and HA theories. Finally, we argue that this model can be interpreted as a realization of the unitary limit of a Bose system with the advantage that the system stays in the genuine ground state contrarily to the metastable state realized in experiments with short-range Bose gases. Full article
(This article belongs to the Special Issue Quantum Crystals)
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Open AccessArticle On the Phase Diagrams of 4He Adsorbed on Graphene and Graphite from Quantum Simulation Methods
Crystals 2018, 8(5), 202; https://doi.org/10.3390/cryst8050202
Received: 1 April 2018 / Revised: 28 April 2018 / Accepted: 29 April 2018 / Published: 4 May 2018
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Abstract
The ground-state phase diagrams of 4 He adsorbed on graphene and graphite are calculated using quantum simulation methods. In this work, a systematic investigation of the approximations used in such simulations is carried out. Particular focus is placed on the helium–helium (He–He) and
[...] Read more.
The ground-state phase diagrams of 4 He adsorbed on graphene and graphite are calculated using quantum simulation methods. In this work, a systematic investigation of the approximations used in such simulations is carried out. Particular focus is placed on the helium–helium (He–He) and helium–carbon (He–C) interactions, as well as their modern approximations. On careful consideration of other approximations and convergence, the simulations are otherwise (numerically) exact. The He–He interaction as approximated by a sum of pairwise potentials is quantitatively assessed. A similar analysis is made for the He–C interaction, but more thoroughly and with a focus on surface corrugation. The importance of many-body effects is discussed. Altogether, the results provide “reference data” for the considered systems. Using comparisons with experiments and first-principle calculations, conclusions are drawn regarding the quantitative accuracy of these modern approximations to these interactions. Full article
(This article belongs to the Special Issue Quantum Crystals)
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Open AccessArticle Ferroelectric Relaxor Quantum Crystals
Crystals 2018, 8(4), 180; https://doi.org/10.3390/cryst8040180
Received: 27 February 2018 / Revised: 17 April 2018 / Accepted: 18 April 2018 / Published: 21 April 2018
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Abstract
A discussion is given of ferroelectrics (FEs) that have their Curie temperatures Tc very near absolute zero. These have differences in their dynamics in comparison with higher-temperature systems, since domain wall motion occurs via quantum mechanical tunneling and not by thermally activated
[...] Read more.
A discussion is given of ferroelectrics (FEs) that have their Curie temperatures Tc very near absolute zero. These have differences in their dynamics in comparison with higher-temperature systems, since domain wall motion occurs via quantum mechanical tunneling and not by thermally activated diffusion. Emphasis in the present paper is on FEs that have relaxor characteristics. In such systems, the temperature at which the isothermal electric susceptibility ε(T,f) peaks is a strong function of frequency, and it decreases with decreasing frequency. This is due to glassy viscosity and is symbolic of non-equilibrium dynamics, usually described by a Vogel-Fulcher equation. It permits an extra dimension with which to examine the transitions. The second half of this paper reviews domain wall instabilities and asks about their presence in QCP ferroelectrics, which has not yet been reported and may be unobservable due to the absence of thermal diffusion of walls near T = 0; in this respect, we note that diffusion does exist in ferroelectric relaxors, even at T = 0, by virtue of their glassy, viscous dynamics. Full article
(This article belongs to the Special Issue Quantum Crystals)
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Open AccessArticle Exciton Spectroscopy of Spatially Separated Electrons and Holes in the Dielectric Quantum Dots
Crystals 2018, 8(4), 148; https://doi.org/10.3390/cryst8040148
Received: 26 February 2018 / Revised: 23 March 2018 / Accepted: 25 March 2018 / Published: 27 March 2018
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Abstract
It is shown that in the potential energy of an exciton of spatially separated electrons and holes (hole moves in the amount of quantum dots (QDs), and the electron is localized on a spherical surface section (QD—dielectric matrix)) taking into account centrifugal energy
[...] Read more.
It is shown that in the potential energy of an exciton of spatially separated electrons and holes (hole moves in the amount of quantum dots (QDs), and the electron is localized on a spherical surface section (QD—dielectric matrix)) taking into account centrifugal energy gives rise band of the quasi-stationary surface exciton states that with the increase of the radius of QD becomes stationary state. The mechanisms of formation of the spectra of interband and intraband absorption (emission) of light in nanosystems containing aluminum oxide QDs, placed in the matrix of vacuum oil, are presented. It is shown that the electron transitions in the area of the surface exciton states cause significant absorption in the visible and near infrared wavelengths, and cause the experimentally observed significant blurring of the absorption edge. Full article
(This article belongs to the Special Issue Quantum Crystals)
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Open AccessArticle Dislocation Structure and Mobility in Hcp Rare-Gas Solids: Quantum versus Classical
Crystals 2018, 8(2), 64; https://doi.org/10.3390/cryst8020064
Received: 13 December 2017 / Revised: 17 January 2018 / Accepted: 20 January 2018 / Published: 29 January 2018
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Abstract
We study the structural and mobility properties of edge dislocations in rare-gas crystals with the hexagonal close-packed (hcp) structure by using classical simulation techniques. Our results are discussed in the light of recent experimental and theoretical studies on hcp 4He, an archetypal
[...] Read more.
We study the structural and mobility properties of edge dislocations in rare-gas crystals with the hexagonal close-packed (hcp) structure by using classical simulation techniques. Our results are discussed in the light of recent experimental and theoretical studies on hcp 4 He, an archetypal quantum crystal. According to our simulations classical hcp rare-gas crystals present a strong tendency towards dislocation dissociation into Shockley partials in the basal plane, similarly to what is observed in solid helium. This is due to the presence of a low-energy metastable stacking fault, of the order of 0.1 mJ/m 2 , that can get further reduced by quantum nuclear effects. We compute the minimum shear stress that induces glide of dislocations within the hcp basal plane at zero temperature, namely, the Peierls stress, and find a characteristic value of the order of 1 MPa. This threshold value is similar to the Peierls stress reported for metallic hcp solids (Zr and Cd) but orders of magnitude larger than the one estimated for solid helium. We find, however, that in contrast to classical hcp metals but in analogy to solid helium, glide of edge dislocations can be thermally activated at very low temperatures, T∼10 K, in the absence of any applied shear stress. Full article
(This article belongs to the Special Issue Quantum Crystals)
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