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11 pages, 3191 KiB

Open AccessFeature PaperArticle

Magnetic Evolution of Carrier Doping and Spin Dynamics in Diluted Magnetic Semiconductors (Ba,Na)(Zn,Mn)₂As₂

by Guoqiang Zhao, Yipeng Cai, Kenji M. Kojima, Qi Sheng, James Beare, Graeme Luke, Xiang Li, Yi Peng, Timothy Ziman, Kan Zhao, Zheng Deng, Xiancheng Wang, Yongqing Li, Gang Su, Sadamichi Maekawa, Bo Gu, Yasutomo J. Uemura and Changqing Jin

Condens. Matter 2025, 10(2), 30; https://doi.org/10.3390/condmat10020030 - 15 May 2025

Viewed by 141

Abstract

The investigation of novel diluted magnetic semiconductors (DMSs) provides a promising platform for studying magnetism and transport characteristics, with significant implications for spintronics. DMSs based on BaZn₂As₂ are particularly noteworthy due to their high Curie temperature (T_C) [...] Read more.

The investigation of novel diluted magnetic semiconductors (DMSs) provides a promising platform for studying magnetism and transport characteristics, with significant implications for spintronics. DMSs based on BaZn₂As₂ are particularly noteworthy due to their high Curie temperature (T_C) of 260 K, diverse magnetic states, and potential for multilayer heterojunctions. This study investigates the magnetic evolution of carrier doping and spin dynamics in the asperomagnet (Ba,Na)(Zn,Mn)₂As₂, utilizing a combination of magnetization measurements, ac susceptibility, and muon spin rotation (µSR). Key findings include the following: (1) lower transition temperatures and coercive forces in (Ba,Na)(Zn,Mn)₂As₂ compared to the ferromagnet (Ba,K)(Zn,Mn)₂As₂; (2) a dynamic fluctuation peak around the transition temperature observed in both the ac susceptibility and longitudinal field (LF) µSR; and (3) the coexistence of static and dynamic states at low temperatures, exhibiting spin-glass-like characteristics. This study, to the best of our knowledge, may represent the first investigation of asperomagnetic order utilizing µSR techniques. It enhances the understanding of magnetic interactions in BaZn₂As₂-based systems and provides valuable insights into the exploration of high T_C DMSs. Full article

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

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9 pages, 1767 KiB

Open AccessArticle

Possible Superconductivity in Very Thin Magnesium Films

by Giovanni Alberto Ummarino and Alessio Zaccone

Condens. Matter 2025, 10(1), 17; https://doi.org/10.3390/condmat10010017 - 10 Mar 2025

Cited by 1 | Viewed by 1923

Abstract

It is known that noble metals such as gold, silver and copper are not superconductors; this is also true for magnesium. This is due to the weakness of the electron–phonon interaction, which makes them excellent conductors but not superconductors. As has recently been [...] Read more.

It is known that noble metals such as gold, silver and copper are not superconductors; this is also true for magnesium. This is due to the weakness of the electron–phonon interaction, which makes them excellent conductors but not superconductors. As has recently been shown for gold, silver and copper, and even for magnesium, it is possible that in very particular situations, superconductivity may occur. Quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals such as magnesium. We study this problem using a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness, L. This parameter-free theory predicts superconductivity in very thin magnesium films. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer. Full article

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

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13 pages, 17818 KiB

Open AccessArticle

Optical Mapping and On-Demand Selection of Local Hysteresis Properties in VO₂

by Melissa Alzate Banguero, Sayan Basak, Nicolas Raymond, Forrest Simmons, Pavel Salev, Ivan K. Schuller, Lionel Aigouy, Erica W. Carlson and Alexandre Zimmers

Condens. Matter 2025, 10(1), 12; https://doi.org/10.3390/condmat10010012 - 13 Feb 2025

Viewed by 999

Abstract

Quantum materials have tremendous potential for disruptive applications. However, scaling devices down has been challenging due to electronic inhomogeneities in many of these materials. Understanding and controlling these electronic patterns on a local scale has thus become crucial to further new applications. To [...] Read more.

Quantum materials have tremendous potential for disruptive applications. However, scaling devices down has been challenging due to electronic inhomogeneities in many of these materials. Understanding and controlling these electronic patterns on a local scale has thus become crucial to further new applications. To address this issue, we have developed a new optical microscopy method that allows for the precise quasi-continuous filming of the insulator-to-metal transition in VO

_{2}

with fine temperature steps. This enables us to track metal and insulator domains over thousands of images and quantify, for the first time, the local hysteresis properties of VO

_{2}

thin films. The analysis of the maps has allowed us to quantify cycle-to-cycle reproducibility of the local transitions and reveals a positive correlation between the local insulator–metal transition temperatures T

_{c}

and the local hysteresis widths

Δ T_{c}

. These maps also enable the optical selection of regions of high or low transition temperature in combination with large or nearly absent local hysteresis. These maps pave the way to understand and use stochasticity to advantage in these materials by picking on-demand transition properties, allowing the scaling down of devices such as optical switches, infrared microbolometers and spiking neural networks. Full article

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

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18 pages, 1287 KiB

Open AccessArticle

Inhomogeneous Evolution of a Dense Ensemble of Optically Pumped Excitons to a Charge Transfer State

by Natasha Kirova and Serguei Brazovskii

Condens. Matter 2025, 10(1), 11; https://doi.org/10.3390/condmat10010011 - 9 Feb 2025

Viewed by 665

Abstract

Phase transformations induced by short optical pulses are mainstream in studies on the dynamics of cooperative electronic states. We present a semiphenomenological modeling of spatiotemporal effects expected when optical excitons are intricate with the order parameter such as in, e.g., organic compounds with [...] Read more.

Phase transformations induced by short optical pulses are mainstream in studies on the dynamics of cooperative electronic states. We present a semiphenomenological modeling of spatiotemporal effects expected when optical excitons are intricate with the order parameter such as in, e.g., organic compounds with neutral-ionic ferroelectric phase transitions. A conceptual complication appears here, where both the excitation and the ground state ordering are built from the intermolecular electronic transfer. To describe both thermodynamic and dynamic effects on the same root, we adopt, for the phase transition, a view of the excitonic insulator—a hypothetical phase of a semiconductor that appears if the exciton energy becomes negative. After the initial pumping pulse, a quasi-condensate of excitons can appear as a macroscopic quantum state that then evolves, while interacting with other degrees of freedom which are prone to an instability. The self-trapping of excitons enhances their density, which can locally surpass a critical value to trigger the phase transformation. The system is stratified in domains that evolve through dynamical phase transitions and may persist even after the initiating excitons have recombined. Full article

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

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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 926

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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14 pages, 1835 KiB

Open AccessArticle

Magnetic and Electronic Inhomogeneity in Sm_1−xEu_xB₆

by M. Victoria Ale Crivillero, Priscila F. S. Rosa, Zachary Fisk, Jens Müller, Pedro Schlottmann and Steffen Wirth

Condens. Matter 2024, 9(4), 55; https://doi.org/10.3390/condmat9040055 - 13 Dec 2024

Cited by 1 | Viewed by 1000

Abstract

While SmB₆ attracts attention as a possible topological Kondo insulator, EuB₆ is known to host magnetic polarons that give rise to large magnetoresistive effects above its ferromagnetic order transition. Here, we investigate single crystals of Sm_1−xEu_xB₆ [...] Read more.

While SmB₆ attracts attention as a possible topological Kondo insulator, EuB₆ is known to host magnetic polarons that give rise to large magnetoresistive effects above its ferromagnetic order transition. Here, we investigate single crystals of Sm_1−xEu_xB₆ by magnetic and magnetotransport measurements to explore a possible interplay of these two intriguing phenomena, with a focus on the Eu-rich substitutions. Sm_0.01Eu_0.99B₆ exhibits generally similar behavior as EuB₆. Interestingly, Sm_0.05Eu_0.95B₆ combines a global antiferromagnetic order with local polaron formation. A pronounced hysteresis is found in the magnetoresistance of Sm_0.1Eu_0.9B₆ at low temperature (

T =

1.9 K) and applied magnetic fields between 2.3 and 3.6 T. The latter is in agreement with a phenomenological model that predicts the stabilization of ferromagnetic polarons with an increasing magnetic field within materials with a global antiferromagnetic order. Full article

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

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11 pages, 1091 KiB

Open AccessArticle

The Effect of an Anisotropic Scattering Rate on the Magnetoresistance of a Metal: A Cuprate-Inspired Analysis

by Giovanni Mirarchi and Sergio Caprara

Condens. Matter 2024, 9(4), 52; https://doi.org/10.3390/condmat9040052 - 29 Nov 2024

Viewed by 826

Abstract

Inspired by the phenomenology of high-critical-temperature superconducting cuprates, we investigate the effect of an anisotropic scattering rate on the magnetoresistance of a metal, relying on Chambers’ solution to the Boltzmann equation. We find that if the scattering rate is enhanced near points of [...] Read more.

Inspired by the phenomenology of high-critical-temperature superconducting cuprates, we investigate the effect of an anisotropic scattering rate on the magnetoresistance of a metal, relying on Chambers’ solution to the Boltzmann equation. We find that if the scattering rate is enhanced near points of the Fermi surface with a locally higher density of states, an extended regime is found where the magnetoresistance varies linearly with the magnetic field. We then apply our results to fit the experimental magnetoresistance of La_1.6−xNd_0.4Sr_xCuO₄ and speculate about the possible source of anisotropic scattering. Full article

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

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16 pages, 509 KiB

Open AccessFeature PaperArticle

Single-Defect-Induced Peculiarities in Inverse Faraday-Based Switching of Superconducting Current-Carrying States near a Critical Temperature

by Mihail D. Croitoru and Alexander I. Buzdin

Condens. Matter 2024, 9(4), 48; https://doi.org/10.3390/condmat9040048 - 12 Nov 2024

Viewed by 960

Abstract

The Inverse Faraday Effect (IFE) is a phenomenon that enables non-thermal magnetization in various types of materials through the interaction with circularly polarized light. This study investigates the impact of single defects on the ability of circularly polarized radiation to switch between distinct [...] Read more.

The Inverse Faraday Effect (IFE) is a phenomenon that enables non-thermal magnetization in various types of materials through the interaction with circularly polarized light. This study investigates the impact of single defects on the ability of circularly polarized radiation to switch between distinct superconducting current states, when the magnetic flux through a superconducting ring equals half the quantum flux,

Φ_{0} / 2

. Using both analytical methods within the standard Ginzburg–Landau theory and numerical simulations based on the stochastic time-dependent Ginzburg–Landau approach, we demonstrate that while circularly polarized light can effectively switch between current-carrying superconducting states, the presence of a single defect significantly affects this switching mechanism. We establish critical temperature conditions above which the switching effect completely disappears, offering insights into the limitations imposed by a single defect on the dynamics of light-induced IFE-based magnetization in superconductors. Full article

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

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13 pages, 1122 KiB

Open AccessFeature PaperArticle

Simple Model for T_c and Pairing Symmetry Changes in Sr₂RuO₄ Under (100) Uniaxial Strain

by Macauley Curtis, Martin Gradhand and James F. Annett

Condens. Matter 2024, 9(4), 44; https://doi.org/10.3390/condmat9040044 - 1 Nov 2024

Viewed by 1189

Abstract

Uniaxial strain in the (100) direction has the effect of increasing the superconducting

T_{c}

in Sr₂RuO₄ from

1.5

K to over 3 K. The enhanced

T_{c}

corresponds to a Lifshitz transition in the Fermi surface topology of this [...] Read more.

Uniaxial strain in the (100) direction has the effect of increasing the superconducting

T_{c}

in Sr₂RuO₄ from

1.5

K to over 3 K. The enhanced

T_{c}

corresponds to a Lifshitz transition in the Fermi surface topology of this unconventional superconductor. We model this using a simple two-dimensional one-band model for the

γ

sheet of the Fermi surface. This reproduces the experimental

T_{c}

results well if we assume a

d_{x^{2} - y^{2}}

singlet pairing state. On the other hand, the triplet state

p_{x} + i p_{y}

does not show any distinct peaks in

T_{c}

associated with the Lifshitz transition. A mixed symmetry state pairing of the form

d + i g

can both describe the

T_{c}

changes and show a distinct transition temperature for time-reversal symmetry breaking (TRSB). Full article

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

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10 pages, 2113 KiB

Open AccessFeature PaperArticle

Kondo Versus Fano in Superconducting Artificial High-Tc Heterostructures

by Gaetano Campi, Gennady Logvenov, Sergio Caprara, Antonio Valletta and Antonio Bianconi

Condens. Matter 2024, 9(4), 43; https://doi.org/10.3390/condmat9040043 - 31 Oct 2024

Cited by 1 | Viewed by 1256

Abstract

Recently, the quest for high-Tc superconductors has evolved from the trial-and-error methodology to the growth of nanostructured artificial high-Tc superlattices (AHTSs) with tailor-made superconducting functional properties by quantum design. Here, we report the growth by molecular beam epitaxy (MBE) of a superlattice of [...] Read more.

Recently, the quest for high-Tc superconductors has evolved from the trial-and-error methodology to the growth of nanostructured artificial high-Tc superlattices (AHTSs) with tailor-made superconducting functional properties by quantum design. Here, we report the growth by molecular beam epitaxy (MBE) of a superlattice of Mott insulator metal interfaces (MIMIs) made of nanoscale superconducting layers of quantum confined-space charge in the Mott insulator La₂CuO₄ (LCO), with thickness L intercalated by normal metal La_1.55Sr_0.45CuO₄ (LSCO) with period d. The critical temperature shows the superconducting dome with Tc as a function of the geometrical parameter L/d showing the maximum at the magic ratio L/d = 2/3 where the Fano–Feshbach resonance enhances the superconducting critical temperature. The normal state transport data of the samples at the top of the superconducting dome exhibit Planckian T-linear resistivity. For L/d > 2/3 and L/d < 2/3, the heterostructures show a resistance following Kondo universal scaling predicted by the numerical renormalization group theory for MIMI nanoscale heterostructures. We show that the Kondo temperature, T_K, and the Kondo scattering amplitude, R_0K, vanish at L/d = 2/3, while T_K and R_0K increase at both sides of the superconducting dome, indicating that the T-linear resistance regime competes with the Kondo proximity effect in the normal phase of MIMIs. Full article

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

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12 pages, 846 KiB

Open AccessFeature PaperArticle

Undamped Higgs Modes in Strongly Interacting Superconductors

by José Lorenzana and Götz Seibold

Condens. Matter 2024, 9(4), 38; https://doi.org/10.3390/condmat9040038 - 30 Sep 2024

Viewed by 1089

Abstract

In superconductors, gauge

U (1)

symmetry is spontaneously broken. According to Goldstone’s theorem, this breaking of a continuous symmetry establishes the existence of the Bogoliubov phase mode while the gauge-invariant response also includes the amplitude fluctuations of the order parameter. The [...] Read more.

In superconductors, gauge

U (1)

symmetry is spontaneously broken. According to Goldstone’s theorem, this breaking of a continuous symmetry establishes the existence of the Bogoliubov phase mode while the gauge-invariant response also includes the amplitude fluctuations of the order parameter. The latter, which are also termed ‘Higgs’ modes in analogy with the standard model, appear at the energy of the spectral gap

2 Δ

, when the superconducting ground state is evaluated within the weak-coupling BCS theory, and, therefore, are damped. Previously, we have shown that, within the time-dependent Gutzwiller approximation (TDGA), Higgs modes appear inside the gap with a finite binding energy relative to the quasiparticle continuum. Here, we show that the binding energy of the Higgs mode becomes exponentially small in the weak-coupling limit converging to the BCS solution. On the other hand, well-defined undamped amplitude modes exist in strongly coupled superconductors when the interaction energy becomes of the order of the bandwidth. Full article

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

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9 pages, 1098 KiB

Open AccessFeature PaperArticle

Room-Temperature Superconductivity in 1D

by Carlo A. Trugenberger

Condens. Matter 2024, 9(3), 34; https://doi.org/10.3390/condmat9030034 - 8 Sep 2024

Cited by 2 | Viewed by 1994

Abstract

We review the theoretical model underpinning the recently reported room-temperature, ambient-pressure superconductivity along line defects on the surface of highly oriented pyrolytic graphite. The main ingredients for this 1D room-temperature superconductivity are pairing by effective strain gauge fields, the formation of an effective [...] Read more.

We review the theoretical model underpinning the recently reported room-temperature, ambient-pressure superconductivity along line defects on the surface of highly oriented pyrolytic graphite. The main ingredients for this 1D room-temperature superconductivity are pairing by effective strain gauge fields, the formation of an effective Josephson junction array in its Bose metal state on the surface and the suppression of phase slips by dimensional embedding in an extremely well-conducting 3D bulk structure. Full article

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

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Review

Jump to: Research

14 pages, 991 KiB

Open AccessReview

Emergent Magnetic Monopoles in Quantum Matter

by Maria Cristina Diamantini

Condens. Matter 2025, 10(2), 20; https://doi.org/10.3390/condmat10020020 - 1 Apr 2025

Viewed by 373

Abstract

Magnetic monopoles, though elusive as elementary particles, emerge as quantum excitations in granular quantum materials. Under certain conditions, they can undergo Bose condensation, leading to the formation of a novel state of matter known as the superinsulator. In this state, charge carriers, Cooper [...] Read more.

Magnetic monopoles, though elusive as elementary particles, emerge as quantum excitations in granular quantum materials. Under certain conditions, they can undergo Bose condensation, leading to the formation of a novel state of matter known as the superinsulator. In this state, charge carriers, Cooper pairs and anti-Cooper pairs, are bound together by an electric flux string, forming neutral electric pions. This confinement mechanism results in an infinite resistance that persists even at finite temperatures. Superinsulators behave, thus, as dual superconductors. Full article

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

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15 pages, 1834 KiB

Open AccessReview

Multiband Superconductivity, Polarons, Jahn-Teller Polarons, Heterogeneity, and High-Temperature Superconductivity

by Annette Bussmann-Holder and Hugo Keller

Condens. Matter 2024, 9(4), 56; https://doi.org/10.3390/condmat9040056 - 19 Dec 2024

Viewed by 1003

Abstract

Early on, oxides were ruled out from superconductivity, since they are typically large-band-gap insulators. Nevertheless, a rather small number of them were found to be superconducting, with transition temperatures up to 14 K and a remarkably low carrier density. This was the starting [...] Read more.

Early on, oxides were ruled out from superconductivity, since they are typically large-band-gap insulators. Nevertheless, a rather small number of them were found to be superconducting, with transition temperatures up to 14 K and a remarkably low carrier density. This was the starting point of K. Alex Müller (KAM) becoming interested in superconductivity in oxides. Step by step, he advanced the research on oxides and finally discovered, together with J. Georg Bednorz, high-temperature superconductivity (HTSC) in the perovskite-type compound Ba-La-Cu-O. Even though he was inspired by specific and clear ideas in his search, he added new impact in the understanding of HTSC for many years after receipt of the Nobel prize for this discovery. Full article

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

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10 pages, 546 KiB

Open AccessFeature PaperReview

Josephson Critical Currents and Related Effects in Ultracold Atomic Superfluid Sytems

by Verdiana Piselli, Leonardo Pisani and Giancarlo Calvanese Strinati

Condens. Matter 2024, 9(4), 41; https://doi.org/10.3390/condmat9040041 - 30 Oct 2024

Viewed by 1047

Abstract

The Josephson and Proximity effects play a pivotal role in the design of superconducting devices for the implementation of quantum technology, ranging from the standard

A l

based to the more exotic twisted high-

T_{c}

junctions. Josephson critical currents have been recently [...] Read more.

The Josephson and Proximity effects play a pivotal role in the design of superconducting devices for the implementation of quantum technology, ranging from the standard

A l

based to the more exotic twisted high-

T_{c}

junctions. Josephson critical currents have been recently investigated also in ultracold atomic systems where a potential barrier acts as a weak link. The unifying feature of the above systems, apart from being superconducting/superfluid, is the presence of spatial inhomogeneity, a feature that has to be properly taken into account in any theoretical approach employed to investigate them. In this work, we review the novel (dubbed LPDA for Local Phase Density Approximation) approach based on a coarse graining of the Bogoliubov–de Gennes (BdG) equations. Non-local and local forms of this coarse graining were utilized when investigating Proximity and Josephson effects. Moreover, the LPDA approach was further developed to include pairing fluctuations at the level of the non-self-consistent t-matrix approximation. The resulting approach, dubbed mLPDA (

m o d i f i e d

LPDA), can be used whenever inhomegeneity and fluctuations effects simultaneously play an important role. Full article

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

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