Condensed Matter

8 pages, 2185 KiB

Open AccessArticle

Thermal and Quantum Fluctuation Effects in Quasiperiodic Systems in External Potentials

by Fabio Cinti and Tommaso Macrì

Condens. Matter 2019, 4(4), 93; https://doi.org/10.3390/condmat4040093 - 16 Dec 2019

Cited by 3 | Viewed by 2432

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

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

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12 pages, 858 KiB

Open AccessArticle

Generation of DC, AC, and Second-Harmonic Spin Currents by Electromagnetic Fields in an Inversion-Asymmetric Antiferromagnet

by Tatsuhiko N. Ikeda

Condens. Matter 2019, 4(4), 92; https://doi.org/10.3390/condmat4040092 - 11 Dec 2019

Cited by 4 | Viewed by 2145

Abstract

Manipulating spin currents in magnetic insulators is a key technology in spintronics. We theoretically study a simple inversion-asymmetric model of quantum antiferromagnets, where both the exchange interaction and the magnetic field are staggered. We calculate spin currents generated by external electric and magnetic [...] Read more.

Manipulating spin currents in magnetic insulators is a key technology in spintronics. We theoretically study a simple inversion-asymmetric model of quantum antiferromagnets, where both the exchange interaction and the magnetic field are staggered. We calculate spin currents generated by external electric and magnetic fields by using a quantum master equation. We show that an ac electric field with amplitude

E_{0}

leads, through exchange-interaction modulation, to the dc and second-order harmonic spin currents proportional to

E_{0}^{2}

. We also show that dc and ac staggered magnetic fields

B_{0}

generate the dc and ac spin currents proportional to

B_{0}

, respectively. We elucidate the mechanism by an exactly solvable model, and thereby propose the ways of spin current manipulation by electromagnetic fields. Full article

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16 pages, 4314 KiB

Open AccessArticle

Multi-Band Superconductivity and the Steep Band/Flat Band Scenario

by Annette Bussmann-Holder, Hugo Keller, Arndt Simon and Antonio Bianconi

Condens. Matter 2019, 4(4), 91; https://doi.org/10.3390/condmat4040091 - 27 Nov 2019

Cited by 14 | Viewed by 3122

Abstract

The basic features of multi-band superconductivity and its implications are derived. In particular, it is shown that enhancements of the superconducting transition temperature take place due to interband interactions. In addition, isotope effects differ substantially from the typical BCS scheme as soon as [...] Read more.

The basic features of multi-band superconductivity and its implications are derived. In particular, it is shown that enhancements of the superconducting transition temperature take place due to interband interactions. In addition, isotope effects differ substantially from the typical BCS scheme as soon as polaronic coupling effects are present. Special cases of the model are polaronic coupling in one band as realized e.g., in cuprates, coexistence of a flat band and a steep band like in MgB₂, crossovers between extreme cases. The advantages of the multiband approach as compared to the single band BCS model are elucidated and its rather frequent realization in actual systems discussed. Full article

(This article belongs to the Special Issue From cuprates to Room Temperature Superconductors)

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13 pages, 2191 KiB

Open AccessArticle

Novel Schemes for Compact FELs in the THz Region

by Andrea Doria, Gian Piero Gallerano and Emilio Giovenale

Condens. Matter 2019, 4(4), 90; https://doi.org/10.3390/condmat4040090 - 19 Nov 2019

Cited by 5 | Viewed by 2560

Abstract

The rapid advance of terahertz technologies in terms of radiation generators, systems, and scientific or industrial applications has put a particular focus on compact sources with challenging performances in terms of generated power (peak and/or average), radiation time structure, and frequency band tunability. [...] Read more.

The rapid advance of terahertz technologies in terms of radiation generators, systems, and scientific or industrial applications has put a particular focus on compact sources with challenging performances in terms of generated power (peak and/or average), radiation time structure, and frequency band tunability. Free electron laser (FEL)-based sources are probably the best candidates to express such a versatility; there are a number of schemes that have been investigated over the years to generate coherent radiation from free electrons in the mm-wave and terahertz regions of the spectrum, covering a wide frequency range from approximately 100 GHz to 10 THz. This paper proposes novel schemes for exploring the limits in the performance of radio frequency-driven free-electron devices in terms of ultrashort pulse duration, wide bandwidth operation, and energy recovery for near continuous wave (CW) operation. The aim of the present work is to demonstrate the feasibility of an FEL achieving performance comparable to a conventional photoconductive THz source, which is commonly used for time-domain spectroscopy (TDS), in terms of bandwidth and pulse duration. We will also demonstrate that a THz FEL could be very powerful and flexible in terms of tailoring its spectral features. Full article

(This article belongs to the Special Issue THz: Research Frontiers for New Sources, Imaging and Other Advanced Technologies)

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17 pages, 869 KiB

Open AccessArticle

A Quantum Model for the Dynamics of Cold Dark Matter

by Tim Zimmermann, Massimo Pietroni, Javier Madroñero, Luca Amendola and Sandro Wimberger

Condens. Matter 2019, 4(4), 89; https://doi.org/10.3390/condmat4040089 - 13 Nov 2019

Cited by 3 | Viewed by 3402

Abstract

A model for cold dark matter is given by the solution of a coupled Schrödinger–Poisson equation system. We present a numerical scheme for integrating these equations, discussing the problems arising from their nonlinear and nonlocal character. After introducing and testing our numerical approach, [...] Read more.

A model for cold dark matter is given by the solution of a coupled Schrödinger–Poisson equation system. We present a numerical scheme for integrating these equations, discussing the problems arising from their nonlinear and nonlocal character. After introducing and testing our numerical approach, we illustrate key features of the system by numerical examples in

1 + 1

dimensions. In particular, we study the properties of asymptotic states to which the numerical solutions converge for artificial initial conditions. Full article

(This article belongs to the Special Issue Many Body Quantum Chaos)

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Graphical abstract

5 pages, 193 KiB

Open AccessEditorial

Nanoscience and Nanotechnology, Proceedings of the INFN-LNF 2018 Conference

by Stefano Bellucci

Condens. Matter 2019, 4(4), 88; https://doi.org/10.3390/condmat4040088 - 13 Nov 2019

Viewed by 1903

Abstract

The NEXT Nanotechnology group at INFN-Laboratori Nazionali di Frascati (LNF) has organized, since the year 2000, a yearly series of international meetings in the area of nanotechnology. The 2018 conference has been devoted to recent developments in nanoscience and their manifold technological applications. [...] Read more.

The NEXT Nanotechnology group at INFN-Laboratori Nazionali di Frascati (LNF) has organized, since the year 2000, a yearly series of international meetings in the area of nanotechnology. The 2018 conference has been devoted to recent developments in nanoscience and their manifold technological applications. These consisted of a number of tutorial/keynote lectures, as well as research talks presenting frontier nanoscience research developments and innovative nanotechnologies in the areas of biology, medicine, aerospace, optoelectronics, energy, materials and characterizations, low-dimensional nanostructures and devices. Selected, original papers based on the 2018 conference talks and related discussions have been published, after a careful refereeing process, in the MDPI journal Condensed Matter, and are currently included in the present dedicated issue. Full article

(This article belongs to the Special Issue Nanoscience and Nanotechnology, Proceedings of the INFN-LNF 2018 Conference)

23 pages, 5058 KiB

Open AccessReview

Probing Phase Separation and Local Lattice Distortions in Cuprates by Raman Spectroscopy

by Efthymios Liarokapis

Condens. Matter 2019, 4(4), 87; https://doi.org/10.3390/condmat4040087 - 01 Nov 2019

Cited by 4 | Viewed by 2736

Abstract

It is generally accepted that high temperature superconductors emerge when extra carriers are introduced in the parent state, which looks like a Mott insulator. Competition of the order parameters drives the system into a poorly defined pseudogap state before acquiring the normal Fermi [...] Read more.

It is generally accepted that high temperature superconductors emerge when extra carriers are introduced in the parent state, which looks like a Mott insulator. Competition of the order parameters drives the system into a poorly defined pseudogap state before acquiring the normal Fermi liquid behavior with further doping. Within the low doping level, the system has the tendency for mesoscopic phase separation, which seems to be a general characteristic in all high Tc compounds, but also in the materials of colossal magnetoresistance or the relaxor ferroelectrics. In all these systems, metastable phases can be created by tuning physical variables, such as doping or pressure, and the competing order parameters can drive the compound to various states. Structural instabilities are expected at critical points and Raman spectroscopy is ideal for detecting them, since it is a very sensitive technique for detecting small lattice modifications and instabilities. In this article, phase separation and lattice distortions are examined on the most characteristic family of high temperature superconductors, the cuprates. The effect of doping or atomic substitutions on cuprates is examined concerning the induced phase separation and hydrostatic pressure for activating small local lattice distortions at the edge of lattice instability. Full article

(This article belongs to the Special Issue From cuprates to Room Temperature Superconductors)

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15 pages, 3138 KiB

Open AccessArticle

Mössbauer Spectroscopy of Triphylite (LiFePO₄) at Low Temperatures

by Tomáš Kmječ, Jaroslav Kohout, Milan Dopita, Miroslav Veverka and Jan Kuriplach

Condens. Matter 2019, 4(4), 86; https://doi.org/10.3390/condmat4040086 - 18 Oct 2019

Cited by 7 | Viewed by 3284

Abstract

Low temperature magnetic ordering in the LiFePO

_{4}

compound is investigated experimentally using Mössbauer spectroscopy and theoretically via first principles calculations. The evaluation of experiment carried out on a powder sample is compatible with an antiferromagnetic order of Fe ion magnetic moments. When [...] Read more.

Low temperature magnetic ordering in the LiFePO

_{4}

compound is investigated experimentally using Mössbauer spectroscopy and theoretically via first principles calculations. The evaluation of experiment carried out on a powder sample is compatible with an antiferromagnetic order of Fe ion magnetic moments. When an external magnetic field is applied, Fe magnetic moments start to deviate slightly from the [010] easy magnetization direction. These findings are confirmed by means of first principles calculations, which also suggest the magnitude of single ion magnetic anisotropy and orbital and spin-dipolar contributions to the magnetic hyperfine field, which is eventually in a good agreement with the experiment. Diffraction and magnetic measurements complement the study. Full article

(This article belongs to the Special Issue Rechargeable Batteries Studied Using Advanced Spectroscopic and Computational Techniques)

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10 pages, 367 KiB

Open AccessArticle

Quantum State of the Fermionic Carriers in a Transport Channel Connecting Particle Reservoirs

by Andrey R. Kolovsky and Dmitrii N. Maksimov

Condens. Matter 2019, 4(4), 85; https://doi.org/10.3390/condmat4040085 - 15 Oct 2019

Cited by 5 | Viewed by 1891

Abstract

We analyze the quantum state of fermionic carriers in a transport channel attached to a particle reservoir. The analysis is done from first principles by considering microscopic models of the reservoir and transport channel. In the case of infinite effective temperature of the [...] Read more.

We analyze the quantum state of fermionic carriers in a transport channel attached to a particle reservoir. The analysis is done from first principles by considering microscopic models of the reservoir and transport channel. In the case of infinite effective temperature of the reservoir we demonstrate a full agreement between the results of straightforward numerical simulations of the system dynamics and the solution of the master equation on the single-particle density matrix of the carriers in the channel. This allows us to predict the quantum state of carriers in the case where the transport channel connects two reservoirs with different chemical potentials. Full article

(This article belongs to the Special Issue Many Body Quantum Chaos)

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46 pages, 730 KiB

Open AccessReview

Dzyaloshinskii–Moriya Coupling in 3d Insulators

by Alexander Moskvin

Condens. Matter 2019, 4(4), 84; https://doi.org/10.3390/condmat4040084 - 12 Oct 2019

Cited by 22 | Viewed by 4334

Abstract

We present an overview of the microscopic theory of the Dzyaloshinskii–Moriya (DM) coupling in strongly correlated 3d compounds. Most attention in the paper centers around the derivation of the Dzyaloshinskii vector, its value, orientation, and sense (sign) under different types of the (super)exchange [...] Read more.

We present an overview of the microscopic theory of the Dzyaloshinskii–Moriya (DM) coupling in strongly correlated 3d compounds. Most attention in the paper centers around the derivation of the Dzyaloshinskii vector, its value, orientation, and sense (sign) under different types of the (super)exchange interaction and crystal field. We consider both the Moriya mechanism of the antisymmetric interaction and novel contributions, in particular, that of spin–orbital coupling on the intermediate ligand ions. We have predicted a novel magnetic phenomenon, weak ferrimagnetism in mixed weak ferromagnets with competing signs of Dzyaloshinskii vectors. We revisit a problem of the DM coupling for a single bond in cuprates specifying the local spin–orbital contributions to the Dzyaloshinskii vector focusing on the oxygen term. We predict a novel puzzling effect of the on-site staggered spin polarization to be a result of the on-site spin–orbital coupling and the cation-ligand spin density transfer. The intermediate ligand nuclear magnetic resonance (NMR) measurements are shown to be an effective tool to inspect the effects of the DM coupling in an external magnetic field. We predict the effect of a strong oxygen-weak antiferromagnetism in edge-shared CuO

_{2}

chains due to uncompensated oxygen Dzyaloshinskii vectors. We revisit the effects of symmetric spin anisotropy directly induced by the DM coupling. A critical analysis will be given of different approaches to exchange-relativistic coupling based on the cluster and the DFT (density functional theory) based calculations. Theoretical results are applied to different classes of 3d compounds from conventional weak ferromagnets (

α

-Fe

_{2}

O

_{3}

, FeBO

_{3}

, FeF

_{3}

, RFeO

_{3}

, RCrO

_{3}

, …) to unconventional systems such as weak ferrimagnets (e.g., RFe

_{1 - x}

Cr

_{x}

O

_{3}

), helimagnets (e.g., CsCuCl

_{3}

), and parent cuprates (La

_{2}

CuO

_{4}

, …). Full article

(This article belongs to the Special Issue From cuprates to Room Temperature Superconductors)

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Condens. Matter, Volume 4, Issue 4 (December 2019) – 10 articles

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