Condensed Matter

10 pages, 596 KiB

Open AccessArticle

On the Kinetic Energy Driven Superconductivity in the Two-Dimensional Hubbard Model

by Takashi Yanagisawa, Kunihiko Yamaji and Mitake Miyazaki

Condens. Matter 2021, 6(1), 12; https://doi.org/10.3390/condmat6010012 - 26 Feb 2021

Cited by 2 | Viewed by 3324

We investigate the role of kinetic energy for the stability of superconducting state in the two-dimensional Hubbard model on the basis of an optimization variational Monte Carlo method. The wave function is optimized by multiplying by correlation operators of site off-diagonal type. This [...] Read more.

We investigate the role of kinetic energy for the stability of superconducting state in the two-dimensional Hubbard model on the basis of an optimization variational Monte Carlo method. The wave function is optimized by multiplying by correlation operators of site off-diagonal type. This wave function is written in an exponential-type form given as

ψ_{λ} = exp (- λ K) ψ_{G}

for the Gutzwiller wave function

ψ_{G}

and a kinetic operator K. The kinetic correlation operator

exp (- λ K)

plays an important role in the emergence of superconductivity in large-U region of the two-dimensional Hubbard model, where U is the on-site Coulomb repulsive interaction. We show that the superconducting condensation energy mainly originates from the kinetic energy in the strongly correlated region. This may indicate a possibility of high-temperature superconductivity due to the kinetic energy effect in correlated electron systems. Full article

(This article belongs to the Special Issue Quantum Complex Matter 2020)

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22 pages, 345 KiB

Open AccessArticle

Z₂ Topological Order and Topological Protection of Majorana Fermion Qubits

by Rukhsan Ul Haq and Louis H. Kauffman

Condens. Matter 2021, 6(1), 11; https://doi.org/10.3390/condmat6010011 - 24 Feb 2021

Cited by 2 | Viewed by 4184

Abstract

The Kitaev chain model exhibits topological order that manifests as topological degeneracy, Majorana edge modes and

Z_{2}

topological invariant of the bulk spectrum. This model can be obtained from a transverse field Ising model(TFIM) using the Jordan–Wigner transformation. TFIM has neither topological [...] Read more.

The Kitaev chain model exhibits topological order that manifests as topological degeneracy, Majorana edge modes and

Z_{2}

topological invariant of the bulk spectrum. This model can be obtained from a transverse field Ising model(TFIM) using the Jordan–Wigner transformation. TFIM has neither topological degeneracy nor any edge modes. Topological degeneracy associated with topological order is central to topological quantum computation. In this paper, we explore topological protection of the ground state manifold in the case of Majorana fermion models which exhibit

Z_{2}

topological order. We show that there are at least two different ways to understand this topological protection of Majorana fermion qubits: one way is based on fermionic mode operators and the other is based on anti-commuting symmetry operators. We also show how these two different ways are related to each other. We provide a very general approach to understanding the topological protection of Majorana fermion qubits in the case of lattice Hamiltonians. We then show how in topological phases in Majorana fermion models gives rise to new braid group representations. So, we give a unifying and broad perspective of topological phases in Majorana fermion models based on anti-commuting symmetry operators and braid group representations of Majorana fermions as anyons. Full article

(This article belongs to the Section Condensed Matter Theory)

9 pages, 9415 KiB

Open AccessArticle

Fingerprint-Based Detection of Non-Local Effects in the Electronic Structure of a Simple Single Component Covalent System

by Behnam Parsaeifard, Deb Sankar De, Jonas A. Finkler and Stefan Goedecker

Condens. Matter 2021, 6(1), 9; https://doi.org/10.3390/condmat6010009 - 20 Feb 2021

Cited by 12 | Viewed by 3452

Abstract

Using fingerprints used mainly in machine learning schemes of the potential energy surface, we detect in a fully algorithmic way long range effects on local physical properties in a simple covalent system of carbon atoms. The fact that these long range effects exist [...] Read more.

Using fingerprints used mainly in machine learning schemes of the potential energy surface, we detect in a fully algorithmic way long range effects on local physical properties in a simple covalent system of carbon atoms. The fact that these long range effects exist for many configurations implies that atomistic simulation methods, such as force fields or modern machine learning schemes, that are based on locality assumptions, are limited in accuracy. We show that the basic driving mechanism for the long range effects is charge transfer. If the charge transfer is known, locality can be recovered for certain quantities such as the band structure energy. Full article

(This article belongs to the Section Physics of Materials)

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7 pages, 1538 KiB

Open AccessCommentary

Kittel’s Brief Comments Endorsing an Alternative View on Barium Titanate

by Akira Kojima

Condens. Matter 2021, 6(1), 10; https://doi.org/10.3390/condmat6010010 - 20 Feb 2021

Viewed by 3265

Abstract

Charles Kittel has written a masterpiece book, “Introduction to Solid State Physics” (ISSP). He mentions in the chapter on ferroelectrics in detail that barium titanate is the typical displacive-type ferroelectric compound where the Ti⁴⁺ displacement develops a dipole moment, which has made [...] Read more.

Charles Kittel has written a masterpiece book, “Introduction to Solid State Physics” (ISSP). He mentions in the chapter on ferroelectrics in detail that barium titanate is the typical displacive-type ferroelectric compound where the Ti⁴⁺ displacement develops a dipole moment, which has made a deep impression in our mind. The author’s group, however, has arrived at an alternative viewpoint on the unit cell structure of barium titanate based on their exhaustive experimental studies. Accordingly, the author sent his relevant papers in 2006 and 2007 to Kittel. He endorsed the results frankly with reminiscence. He mentioned revising the ferroelectric chapter of ISSP according the author’s suggestions. It appears to be admissible to publish details now after Kittel has passed away. A long time misunderstanding of the phase transition in barium titanate is due to the text book knowledge of ISSP. Full article

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9 pages, 599 KiB

Open AccessArticle

Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band Superfluid/Superconductor

by Hiroyuki Tajima, Pierbiagio Pieri and Andrea Perali

Condens. Matter 2021, 6(1), 8; https://doi.org/10.3390/condmat6010008 - 7 Feb 2021

Cited by 1 | Viewed by 3277

Abstract

We investigate single-particle excitation properties in the normal state of a two-band superconductor or superfluid throughout the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein-condensation (BEC) crossover, within the many-body T-matrix approximation for multichannel pairing fluctuations. We address the single-particle density of states and the spectral [...] Read more.

We investigate single-particle excitation properties in the normal state of a two-band superconductor or superfluid throughout the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein-condensation (BEC) crossover, within the many-body T-matrix approximation for multichannel pairing fluctuations. We address the single-particle density of states and the spectral functions consisting of two contributions associated with a weakly interacting deep band and a strongly interacting shallow band, relevant for iron-based multiband superconductors and multicomponent fermionic superfluids. We show how the pseudogap state in the shallow band is hidden by the deep band contribution throughout the two-band BCS-BEC crossover. Our results could explain the missing pseudogap in recent scanning tunneling microscopy experiments in FeSe superconductors. Full article

(This article belongs to the Special Issue Fluctuations and Highly Non-linear Phenomena in Superfluids and Superconductors III)

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14 pages, 1801 KiB

Open AccessArticle

Plausible Physical Mechanisms for Unusual Volatile/Non- Volatile Resistive Switching in HfO₂-Based Stacks

by Cynthia P. Quinteros, Jordi Antoja-Lleonart and Beatriz Noheda

Condens. Matter 2021, 6(1), 7; https://doi.org/10.3390/condmat6010007 - 2 Feb 2021

Viewed by 7034

Abstract

Memristive devices made of silicon compatible simple oxides are of great interest for storage and logic devices in future adaptable electronics and non-digital computing applications. A series of highly desirable properties observed in an atomic-layer-deposited hafnia-based stack, triggered our interest to investigate their [...] Read more.

Memristive devices made of silicon compatible simple oxides are of great interest for storage and logic devices in future adaptable electronics and non-digital computing applications. A series of highly desirable properties observed in an atomic-layer-deposited hafnia-based stack, triggered our interest to investigate their suitability for technological implementations. In this paper, we report our attempts to reproduce the observed behaviour within the framework of a proposed underlying mechanism. The inability of achieving the electrical response of the original batch indicates that a key aspect in those devices has remained undetected. By comparing newly made devices with the original ones, we gather some clues on the plausible alternative mechanisms that could give rise to comparable electrical behaviours. Full article

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Graphical abstract

23 pages, 535 KiB

Open AccessArticle

Scaling of Phase Diagram and Critical Point Parameters in Liquid-Vapour Phase Transition of Metallic Fluids

by S.V.G. Menon

Condens. Matter 2021, 6(1), 6; https://doi.org/10.3390/condmat6010006 - 28 Jan 2021

Cited by 3 | Viewed by 3573

Abstract

The first objective of this paper is to investigate the scaling behavior of liquid-vapor phase transition in FCC and BCCmetals starting from the zero-temperature four-parameter formula for cohesive energy. The effective potentials between the atoms in the solid are determined while using lattice [...] Read more.

The first objective of this paper is to investigate the scaling behavior of liquid-vapor phase transition in FCC and BCCmetals starting from the zero-temperature four-parameter formula for cohesive energy. The effective potentials between the atoms in the solid are determined while using lattice inversion techniques as a function of scaling variables in the four-parameter formula. These potentials are split into repulsive and attractive parts, as per the Weeks–Chandler–Anderson prescription, and used in the coupling-parameter expansion for solving the Ornstein–Zernike equation that was supplemented with an accurate closure. Thermodynamic quantities obtained via the correlation functions are used in order to obtain critical point parameters and liquid-vapor phase diagrams. Their dependence on the scaling variables in the cohesive energy formula are also determined. An equally important second objective of the paper is to revisit coupling parameter expansion for solving the Ornstein–Zernike equation. The Newton–Armijo non-linear solver and Krylov-space based linear solvers are employed in this regard. These methods generate a robust algorithm that can be used to span the entire fluid region, except very low temperatures. The accuracy of the method is established by comparing the phase diagrams with those that were obtained via computer simulation. The avoidance of the ’no-solution-region’ of the Ornstein-Zernike equation in coupling-parameter expansion is also discussed. Details of the method and complete algorithm provided here would make this technique more accessible to researchers investigating the thermodynamic properties of one component fluids. Full article

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3 pages, 151 KiB

Open AccessEditorial

Acknowledgment to Reviewers of Condensed Matter in 2020

by Condensed Matter Editorial Office

Condens. Matter 2021, 6(1), 5; https://doi.org/10.3390/condmat6010005 - 25 Jan 2021

Viewed by 2259

Abstract

Peer review is the driving force of journal development, and reviewers are gatekeepers who ensure that Condensed Matter maintains its standards for the high quality of its published papers [...] Full article

9 pages, 539 KiB

Open AccessArticle

Moving Pearl Vortices in Thin-Film Superconductors

by Vladimir Kogan and Norio Nakagawa

Condens. Matter 2021, 6(1), 4; https://doi.org/10.3390/condmat6010004 - 24 Jan 2021

Cited by 4 | Viewed by 3450

Abstract

The magnetic field

h_{z}

of a moving Pearl vortex in a superconducting thin-film in

(x, y)

plane is studied with the help of the time-dependent London equation. It is found that for a vortex at the origin moving in [...] Read more.

The magnetic field

h_{z}

of a moving Pearl vortex in a superconducting thin-film in

(x, y)

plane is studied with the help of the time-dependent London equation. It is found that for a vortex at the origin moving in

+ x

direction,

h_{z} (x, y)

is suppressed in front of the vortex,

x > 0

, and enhanced behind (

x < 0

). The distribution asymmetry is proportional to the velocity and to the conductivity of normal quasiparticles. The vortex self-energy and the interaction of two moving vortices are evaluated. Full article

(This article belongs to the Section Superconductivity)

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13 pages, 904 KiB

Open AccessArticle

On Modeling of Synthesis Process of Boron Carbide Based Nanocomposites

by Levan Chkhartishvili, Levan Antashvili, Lasha Dalakishvili, Roin Chedia, Otar Tsagareishvili and Archil Mikeladze

Condens. Matter 2021, 6(1), 3; https://doi.org/10.3390/condmat6010003 - 18 Jan 2021

Cited by 6 | Viewed by 2862

Abstract

Nanocomposites based on boron carbide B₄C are hard materials with wide field of applications in modern technologies. A system of first-order ordinary differential equations that simulates the process of chemical synthesis of nanopowders of B₄C–TiB₂ compositions containing titanium [...] Read more.

Nanocomposites based on boron carbide B₄C are hard materials with wide field of applications in modern technologies. A system of first-order ordinary differential equations that simulates the process of chemical synthesis of nanopowders of B₄C–TiB₂ compositions containing titanium diboride (TiB₂) as an additional phase is suggested and resolved numerically for a typical ratio of reaction constants. Reagents and products concentrations are found as time-functions. In this way, the optimal route of production technology of boron carbide-based nanomaterials can be identified. Full article

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25 pages, 3689 KiB

Open AccessFeature PaperReview

From SrTiO₃ to Cuprates and Back to SrTiO₃: A Way Along Alex Müller’s Scientific Career

by Annette Bussmann-Holder and Hugo Keller

Condens. Matter 2021, 6(1), 2; https://doi.org/10.3390/condmat6010002 - 31 Dec 2020

Cited by 6 | Viewed by 5377

Abstract

K.A. Müller took a long route in science leaving many traces and imprints, which have been and are still today initiations for further research activities. We “walk” along this outstanding path but are certainly not able to provide a complete picture of it, [...] Read more.

K.A. Müller took a long route in science leaving many traces and imprints, which have been and are still today initiations for further research activities. We “walk” along this outstanding path but are certainly not able to provide a complete picture of it, since the way was not always straight, often marked by unintended detours, which had novel impact on the international research society. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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8 pages, 1987 KiB

Open AccessCommunication

Common Phase and Structure Misidentifications in High-Resolution TEM Characterization of Perovskite Materials

by Yu-Hao Deng

Condens. Matter 2021, 6(1), 1; https://doi.org/10.3390/condmat6010001 - 29 Dec 2020

Cited by 21 | Viewed by 15516

Abstract

High-resolution TEM (HRTEM) is a powerful tool for structure characterization. However, methylammonium lead iodide (MAPbI₃) perovskite is highly sensitive to electron beams and easily decomposes into lead iodide (PbI₂). Misidentifications, such as PbI₂ being incorrectly labeled as perovskite, [...] Read more.

High-resolution TEM (HRTEM) is a powerful tool for structure characterization. However, methylammonium lead iodide (MAPbI₃) perovskite is highly sensitive to electron beams and easily decomposes into lead iodide (PbI₂). Misidentifications, such as PbI₂ being incorrectly labeled as perovskite, are widely present in HRTEM characterization and would negatively affect the development of perovskite research field. Here misidentifications in MAPbI₃ perovskite are summarized, classified, and corrected based on low-dose imaging and electron diffraction (ED) simulations. Corresponding crystallographic parameters of intrinsic tetragonal MAPbI₃ and the confusable hexagonal PbI₂ are presented unambiguously. Finally, the method of proper phase identification and some strategies to control the radiation damage in HRTEM are provided. This warning paves the way to avoid future misinterpretations in HRTEM characterization of perovskite and other electron beam-sensitive materials. Full article

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Condens. Matter, Volume 6, Issue 1 (March 2021) – 12 articles

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