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

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Cover Story (view full-size image) A dark matter model of recent interest in cosmology is Fuzzy Dark Matter. Here, dark matter is [...] Read more.
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Open AccessArticle
Thermal and Quantum Fluctuation Effects in Quasiperiodic Systems in External Potentials
Condens. Matter 2019, 4(4), 93; https://doi.org/10.3390/condmat4040093 - 16 Dec 2019
Viewed by 571
Abstract
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
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Open AccessArticle
Generation of DC, AC, and Second-Harmonic Spin Currents by Electromagnetic Fields in an Inversion-Asymmetric Antiferromagnet
Condens. Matter 2019, 4(4), 92; https://doi.org/10.3390/condmat4040092 - 11 Dec 2019
Viewed by 699
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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Open AccessArticle
Multi-Band Superconductivity and the Steep Band/Flat Band Scenario
Condens. Matter 2019, 4(4), 91; https://doi.org/10.3390/condmat4040091 - 27 Nov 2019
Cited by 3 | Viewed by 649
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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Open AccessArticle
Novel Schemes for Compact FELs in the THz Region
Condens. Matter 2019, 4(4), 90; https://doi.org/10.3390/condmat4040090 - 19 Nov 2019
Viewed by 657
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
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Open AccessArticle
A Quantum Model for the Dynamics of Cold Dark Matter
Condens. Matter 2019, 4(4), 89; https://doi.org/10.3390/condmat4040089 - 13 Nov 2019
Cited by 1 | Viewed by 1028
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) Printed Edition available
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Open AccessEditorial
Nanoscience and Nanotechnology, Proceedings of the INFN-LNF 2018 Conference
Condens. Matter 2019, 4(4), 88; https://doi.org/10.3390/condmat4040088 - 13 Nov 2019
Viewed by 490
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
Open AccessReview
Probing Phase Separation and Local Lattice Distortions in Cuprates by Raman Spectroscopy
Condens. Matter 2019, 4(4), 87; https://doi.org/10.3390/condmat4040087 - 01 Nov 2019
Viewed by 530
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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Open AccessArticle
Mössbauer Spectroscopy of Triphylite (LiFePO4) at Low Temperatures
Condens. Matter 2019, 4(4), 86; https://doi.org/10.3390/condmat4040086 - 18 Oct 2019
Viewed by 711
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
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Open AccessArticle
Quantum State of the Fermionic Carriers in a Transport Channel Connecting Particle Reservoirs
Condens. Matter 2019, 4(4), 85; https://doi.org/10.3390/condmat4040085 - 15 Oct 2019
Cited by 2 | Viewed by 552
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) Printed Edition available
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Open AccessReview
Dzyaloshinskii–Moriya Coupling in 3d Insulators
Condens. Matter 2019, 4(4), 84; https://doi.org/10.3390/condmat4040084 - 12 Oct 2019
Cited by 2 | Viewed by 841
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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