Condensed Matter

10 pages, 345 KiB

Open AccessFeature PaperArticle

Temperature Dependence of the Dynamical and DC Conductivity in 2D Dirac Systems: Self-Consistent Random-Phase-Approximation Approach

by Ivan Kupčić and Patrik Papac

Condens. Matter 2025, 10(1), 9; https://doi.org/10.3390/condmat10010009 (registering DOI) - 1 Feb 2025

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Abstract

We studied relaxation processes in heavily doped two-dimensional Dirac systems associated with electron scattering by acoustic and optical phonons and by static disorder. The frequency dependence of the real and imaginary parts of the relaxation function is calculated for different temperatures. The two-component [...] Read more.

We studied relaxation processes in heavily doped two-dimensional Dirac systems associated with electron scattering by acoustic and optical phonons and by static disorder. The frequency dependence of the real and imaginary parts of the relaxation function is calculated for different temperatures. The two-component low-frequency dynamical conductivity is found to be strongly dependent on temperature. At low temperatures, the imaginary part of the zero-frequency relaxation function and the DC resistivity are characterized by the scaling law

a T^{x}

with the exponent x between 2.5 and 3. Full article

(This article belongs to the Special Issue Selected Papers from the International Conference on Quantum Materials and Technologies (ICQMT2024))

11 pages, 5696 KiB

Open AccessArticle

Structure and Properties of (Fe₈₀Ga₂₀)_99.8Ce_0.2 Alloy in Cast and Hot Rolled State

by Vasily Milyutin, Irina Gervasyeva, Azambek Kalonov, Denis Shishkin, Denis Davydov and Liudmila Stashkova

Condens. Matter 2025, 10(1), 8; https://doi.org/10.3390/condmat10010008 - 30 Jan 2025

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Abstract

FeGa alloys with small additions of rare-earth elements surpass binary alloys in magnetostriction and plasticity. For this reason, they are considered promising magnetostrictive materials for various electrical engineering applications. The alloy (Fe₈₁Ga₁₉)_99.8Ce_0.2 was prepared and investigated [...] Read more.

FeGa alloys with small additions of rare-earth elements surpass binary alloys in magnetostriction and plasticity. For this reason, they are considered promising magnetostrictive materials for various electrical engineering applications. The alloy (Fe₈₁Ga₁₉)_99.8Ce_0.2 was prepared and investigated in this work. It was found that in the cast state, it has a magnetostriction of 3/2 λ about 100 ppm, saturation magnetization of 150 emu/g, tensile strength of about 300 MPa, and fracture strain of 3%. The microstructure, crystallographic texture, and behavior when heated of the alloy were investigated. Then the ingot was subjected to forging and hot rolling with a deformation degree of 90% at 1000 °C. The structure and mechanical properties of samples cut from a hot rolling sheet were studied. Their tensile strength and fracture strain increase compared to cast state up to 600 MPa and 4% correspondingly. Full article

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11 pages, 826 KiB

Open AccessArticle

Density Collective Modes of Exciton Superfluidity in Bilayer Systems

by Filippo Pascucci, Sara Conti, David Neilson, Andrea Perali and Jacques Tempere

Condens. Matter 2025, 10(1), 7; https://doi.org/10.3390/condmat10010007 - 16 Jan 2025

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Abstract

We propose a new way to establish the existence of a superfluid phase in an exciton bilayer system by exploiting the properties of its collective modes. We focus on the density collective modes and treat them within Random Phase Approximation. By comparing results [...] Read more.

We propose a new way to establish the existence of a superfluid phase in an exciton bilayer system by exploiting the properties of its collective modes. We focus on the density collective modes and treat them within Random Phase Approximation. By comparing results for the normal and superfluid states, we are able to identify unambiguous fingerprints of the exciton superfluid phase. We compare the collective modes of the exciton system and cold atom systems, and we discuss the collective modes of the exciton superfluid order parameter. Full article

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

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11 pages, 4413 KiB

Open AccessArticle

Crystal Structure Evolution of Piezoelectric Fe-Doped ZnO Film by Magnetron Co-Sputtering Technique

by Ya-Chih Cheng, Sanjaya Brahma, Sean Wu, Jow-Lay Huang and Alex C. H. Lee

Condens. Matter 2025, 10(1), 6; https://doi.org/10.3390/condmat10010006 - 15 Jan 2025

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Abstract

Zinc oxide (ZnO) exhibits piezoelectric properties due to its asymmetric structure, making it suitable for piezoelectric devices. This experiment deposited Fe-doped ZnO films on silicon substrates using a dual-target magnetron co-sputtering system. The films achieved a high c-axis orientation, and the piezoelectric coefficient [...] Read more.

Zinc oxide (ZnO) exhibits piezoelectric properties due to its asymmetric structure, making it suitable for piezoelectric devices. This experiment deposited Fe-doped ZnO films on silicon substrates using a dual-target magnetron co-sputtering system. The films achieved a high c-axis orientation, and the piezoelectric coefficient of the film reached its optimal value of 44.35 pC/N when doped with 0.5 at% of Fe. This value is approximately three times that of undoped ZnO films with a piezoelectric coefficient of 13.04 pC/N. The study utilized a diffractometer, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy to evaluate the crystal structure evolution of the zinc oxide films and employed X-ray photoelectron spectroscopy to assess the valence state of the Fe ions. Full article

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20 pages, 12787 KiB

Open AccessArticle

Exploring the Properties of Quantum Scars in a Toy Model

by Sudip Sinha and Subhasis Sinha

Condens. Matter 2025, 10(1), 5; https://doi.org/10.3390/condmat10010005 - 12 Jan 2025

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Abstract

We introduce the concept of ergodicity and explore its deviation caused by quantum scars in an isolated quantum system, employing a pedagogical approach based on a toy model. Quantum scars, originally identified as traces of classically unstable orbits in certain wavefunctions of chaotic [...] Read more.

We introduce the concept of ergodicity and explore its deviation caused by quantum scars in an isolated quantum system, employing a pedagogical approach based on a toy model. Quantum scars, originally identified as traces of classically unstable orbits in certain wavefunctions of chaotic systems, have recently regained interest for their role in non-ergodic dynamics, as they retain memory of their initial states. We elucidate these features of quantum scars within the same framework of this toy model. The integrable part of the model consists of two large spins, with a classical counterpart, which we combine with a random matrix to induce ergodic behavior. Scarred states can be selectively generated from the integrable spin Hamiltonian by protecting them from the ergodic states using a projector method. Deformed projectors mimic the ‘quantum leakage’ of scarred states, enabling tunable mixing with ergodic states and thereby controlling the degree of scarring. In this simple model, we investigate various properties of quantum scarring and shed light on different aspects of many-body quantum scars observed in more complex quantum systems. Notably, the underlying classicality can be revealed through the entanglement spectrum and the dynamics of ‘out-of-time-ordered correlators’. Full article

(This article belongs to the Special Issue Non-equilibrium Dynamics in Ultra-Cold Quantum Gases)

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16 pages, 326 KiB

Open AccessArticle

Quantum Kinetic Theory of the Spin Hall Effect for Disordered Graphene with Rashba Spin–Orbit Coupling

by Roberto Raimondi and Thierry Valet

Condens. Matter 2025, 10(1), 4; https://doi.org/10.3390/condmat10010004 - 6 Jan 2025

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Abstract

The spin Hall effect for the model Hamiltonian of graphene with Rashba spin–orbit coupling is analyzed by means of a recently derived quantum kinetic theory of the linear response for multi-band electron systems. The latter expresses the interband part of the density matrix [...] Read more.

The spin Hall effect for the model Hamiltonian of graphene with Rashba spin–orbit coupling is analyzed by means of a recently derived quantum kinetic theory of the linear response for multi-band electron systems. The latter expresses the interband part of the density matrix in terms of the intraband occupation numbers, which can be obtained as solutions of a Boltzmann transport equation. The analysis, which, in the case of the model here considered, can be carried out in a completely analytical way, thus provides an effective pedagogical illustration of the general theory. While our results agree with those previously obtained with alternative approaches for the same model, our comparatively simpler and more physically transparent derivation illustrates the advantages of our formalism when dealing with non trivial multi-band Hamiltonians. Full article

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

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23 pages, 6551 KiB

Open AccessFeature PaperArticle

Anomalous Polarization in One-Dimensional Aperiodic Insulators

by Anouar Moustaj, Julius Krebbekx and Cristiane Morais Smith

Condens. Matter 2025, 10(1), 3; https://doi.org/10.3390/condmat10010003 - 6 Jan 2025

Viewed by 374

Abstract

Multilevel charge pumping is a feature that was recently observed in quasiperiodic systems. In this work, we show that it is more generic and appears in different aperiodic systems. Additionally, we show that for aperiodic systems admitting arbitrarily long palindromic factors, the charge [...] Read more.

Multilevel charge pumping is a feature that was recently observed in quasiperiodic systems. In this work, we show that it is more generic and appears in different aperiodic systems. Additionally, we show that for aperiodic systems admitting arbitrarily long palindromic factors, the charge pumping protocol connects two topologically distinct insulating phases. This confirms the existence of topological phases in aperiodic systems whenever their finite-size realizations admit inversion symmetry. These phases are characterized by an anomalous edge response resulting from the bulk–boundary correspondence. We show that these signatures are all present in various chains, each representing a different class of structural aperiodicity: the Fibonacci quasicrystal, the Tribonacci quasicrystal, and the Thue–Morse chain. More specifically, we calculate three quantities: the Berry phase of the periodic approximation of the finite-size systems, the polarization response to an infinitesimal static and constant electric field in systems with open boundary conditions, and the degeneracy of the entanglement spectrum. We find that all of them provide signatures of a topologically nontrivial phase. Full article

(This article belongs to the Section Condensed Matter Theory)

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8 pages, 4625 KiB

Open AccessArticle

Topological Superconductivity of the Unconventional Type, S = 1, S^z = 0, in a Layer of Adatoms

by Edine Silva and Mucio A. Continentino

Condens. Matter 2025, 10(1), 2; https://doi.org/10.3390/condmat10010002 - 5 Jan 2025

Viewed by 479

Abstract

In this paper, we study the appearance of topological p-wave superconductivity of the type

S = 1

,

S^{z} = 0

in a layer of adatoms. This unconventional superconductivity arises due to an anti-symmetric hybridization between the orbitals of the adatoms [...] Read more.

In this paper, we study the appearance of topological p-wave superconductivity of the type

S = 1

,

S^{z} = 0

in a layer of adatoms. This unconventional superconductivity arises due to an anti-symmetric hybridization between the orbitals of the adatoms and those of the atoms in the superconducting BCS substrate. This two-dimensional system is topologically non-trivial only in the absence of a magnetic field and belongs to class DIII of the Altland–Zirnbauer classification. We obtain the Pfaffian that characterizes the topological phases of the system and its phase diagram. We discuss the differences between the two-dimensional case and a chain with the same type of superconductivity. Full article

(This article belongs to the Special Issue Superstripes Physics, 3rd Edition)

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11 pages, 4730 KiB

Open AccessArticle

Terahertz Time-Domain Spectroscopy of Substituted Gadolinium Gallium Garnet

by Ke Wang, Junchao Xu, Hironaru Murakami, Hiroyasu Yamahara, Munetoshi Seki, Hitoshi Tabata and Masayoshi Tonouchi

Condens. Matter 2025, 10(1), 1; https://doi.org/10.3390/condmat10010001 - 25 Dec 2024

Viewed by 472

Abstract

Temperature dependence of the lowest frequency transverse optical phonon (TO1) in a single crystal Substituted Gadolinium Gallium Garnet (SGGG, (001)) was studied using terahertz time-domain spectroscopy at temperatures between 80 K and 500 K. The complex dielectric constants were calculated from the optical [...] Read more.

Temperature dependence of the lowest frequency transverse optical phonon (TO1) in a single crystal Substituted Gadolinium Gallium Garnet (SGGG, (001)) was studied using terahertz time-domain spectroscopy at temperatures between 80 K and 500 K. The complex dielectric constants were calculated from the optical constants fitting with the Lorentz oscillator model. The results show that the TO1 phonon of SGGG is at 2.5 THz at room temperature, the frequency of the TO1 phonon slightly decreases, and the dumping factor clearly increases with increasing temperature. Additionally, our results demonstrate that even a small substitution can induce a phonon shift, leading to higher absorption and causing a slight degradation in thermal stability. Our work is expected to support the development of magneto-optical and spintronic devices. Full article

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Condens. Matter, Volume 10, Issue 1 (March 2025) – 9 articles

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