Condensed Matter

31 pages, 5710 KB

Open AccessFeature PaperReview

Recent Progress in the Theory of Flat Bands and Their Realization

by Izumi Hase

Condens. Matter 2025, 10(4), 64; https://doi.org/10.3390/condmat10040064 - 5 Dec 2025

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Flat electronic bands, characterized by a nearly dispersionless energy spectrum, have emerged as fertile ground for exploring strong correlation effects, unconventional magnetism, and topological phases. This review paper provides an overview of the theoretical basis, material realization, and emergent phenomena associated with flat [...] Read more.

Flat electronic bands, characterized by a nearly dispersionless energy spectrum, have emerged as fertile ground for exploring strong correlation effects, unconventional magnetism, and topological phases. This review paper provides an overview of the theoretical basis, material realization, and emergent phenomena associated with flat bands. We begin by discussing the geometric and topological origins of flat bands in lattice systems, emphasizing mechanisms such as destructive interference and compact localized states. We will also explain the relationship between quantum metrics and flat bands, which are recent theoretical findings. We then survey various classes of materials—ranging from engineered lattices and Moiré structures to transition metal compounds—where flat bands have been theoretically predicted or experimentally observed. The interplay between flat-band physics and strong correlations is explored through recent developments in ferromagnetism, superconductivity, and various Hall effects. Finally, we outline open questions and potential directions for future research, including the quest for ideal flat-band systems, the role of spin–orbit coupling, and the impact of disorder. This review aims to bridge fundamental concepts with cutting-edge advances, highlighting the rich physics and material prospects of flat bands. Full article

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12 pages, 4044 KB

Open AccessArticle

Unveiling the Role of Graphene in Enhancing the Mechanical Properties of Electrodeposited Ni Composites

by Bingqian Zhang, Junhao Zhu, Zhihua Yuan and Peide Han

Condens. Matter 2025, 10(4), 63; https://doi.org/10.3390/condmat10040063 - 5 Dec 2025

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Abstract

Graphene holds significant promise as an ideal reinforcing phase. However, its tendency to irreversibly aggregate and its unclear impact on electrodeposition mechanisms have hindered the full exploitation of its advantages for enhancing material mechanical properties. In this study, we produced a graphene/Ni composite [...] Read more.

Graphene holds significant promise as an ideal reinforcing phase. However, its tendency to irreversibly aggregate and its unclear impact on electrodeposition mechanisms have hindered the full exploitation of its advantages for enhancing material mechanical properties. In this study, we produced a graphene/Ni composite reinforced with reduced graphene oxide (rGO) via a simple, scalable, and cost-effective electrodeposition approach. The incorporation of graphene not only raised the cathodic polarization potential but also enhanced the transport of ions. As a result, the presence of rGO significantly influenced the grain size, grain distribution, and the proportion of growth twins-3(111). Compared with Ni, the graphene/Ni composite exhibited improvements of 14.8% in strength and 16.8% in fracture elongation. Additionally, first-principles calculations confirmed that superior electronic conductivity and all elastic moduli along with Poisson’s ratio were found to be higher in the composite. Our findings offer fundamental insights into the role of rGO in governing the structural evolution of graphene/metal composites. Full article

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12 pages, 3746 KB

Open AccessArticle

Spectral Characterization of CeF₃-YF₃-TbF₃ Nanoparticles for Temperature Sensing in 80–320 K Temperature Range

by Svetlana Kalinichenko and Maksim Pudovkin

Condens. Matter 2025, 10(4), 62; https://doi.org/10.3390/condmat10040062 - 3 Dec 2025

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Abstract

The studied Ce_0.5Y_0.5−XTb_XF₃ (X = 0, 0.001, 0.002, 0.005, 0.01, and 0.05) nanoparticles were synthesized via the water-based co-precipitation method. All the samples demonstrated diameters in the 17–20 nm range and a hexagonal phase corresponding to [...] Read more.

The studied Ce_0.5Y_0.5−XTb_XF₃ (X = 0, 0.001, 0.002, 0.005, 0.01, and 0.05) nanoparticles were synthesized via the water-based co-precipitation method. All the samples demonstrated diameters in the 17–20 nm range and a hexagonal phase corresponding to the phase of CeF₃. Under 266 nm excitation (4f–5d absorption band of Ce³⁺), the luminescence spectrum shape was notably dependent on temperature. The integrated luminescence intensity ratio (LIR) of Ce³⁺ and Tb³⁺ (⁵D₄–⁷F₃) peaks was chosen as a temperature-dependent parameter. It was shown that the LIR functions linearly decay. The rate of decay decreases with the increase in Tb³⁺ concentration. This was explained by the fact that in the case of low Tb³⁺ concentrations, the spectral temperature dependence is mostly based on effective thermal quenching of Ce³⁺ luminescence. At higher Tb³⁺ concentrations, there is a higher probability of Ce³⁺ to Tb³⁺ energy transfer. Here, the efficiency of the temperature dependence of this process is lower, and the rate of LIR decay is lower as well. Full article

(This article belongs to the Section Spectroscopy and Imaging in Condensed Matter)

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13 pages, 705 KB

Open AccessFeature PaperArticle

Magnetic-Field Oscillations of the Critical Temperature in Ultraclean, Two-Dimensional Type-I Superconductors

by Aiying Zhao, Richard A. Klemm and Qiang Gu

Condens. Matter 2025, 10(4), 61; https://doi.org/10.3390/condmat10040061 - 29 Nov 2025

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Abstract

We investigate the influence of Landau Levels (LLs) and Zeeman energy, induced by an applied magnetic field

B

, on the critical temperature

T_{c}

for two-dimensional (2D) ultraclean superconductors using a fully quantum mechanical approach within the Bardeen–Cooper–Schrieffer (BCS) theory. In contrast [...] Read more.

We investigate the influence of Landau Levels (LLs) and Zeeman energy, induced by an applied magnetic field

B

, on the critical temperature

T_{c}

for two-dimensional (2D) ultraclean superconductors using a fully quantum mechanical approach within the Bardeen–Cooper–Schrieffer (BCS) theory. In contrast to standard BCS theory, it allows for Cooper pair formation between electrons with opposite spins and momenta along the

B

direction, both on the same or on neighboring LLs. Our quantum mechanical treatment of LLs reveals that the critical temperature

T_{c}

for electrons paired on the same LL exhibits oscillations around the BCS critical temperature at low magnetic fields. The Zeeman energy leads to a decrease in

T_{c} (B)

with increasing

B

for electrons paired both on the same and on neighboring LLs. Notably, as the g-factor increases,

T_{c} (B)

decreases faster as the magnetic field increases for a larger g-factor than for a smaller one. Full article

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Graphical abstract

17 pages, 1336 KB

Open AccessArticle

Transitions from Coplanar Double-Q to Noncoplanar Triple-Q States Induced by High-Harmonic Wave-Vector Interaction

by Satoru Hayami

Condens. Matter 2025, 10(4), 60; https://doi.org/10.3390/condmat10040060 - 28 Nov 2025

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Abstract

We theoretically investigate topological transitions between coplanar and noncoplanar magnetic states in centrosymmetric itinerant magnets on a square lattice. A canonical effective spin model incorporating bilinear and biquadratic exchange interactions at finite wave vectors is analyzed to elucidate the emergence of multiple-Q [...] Read more.

We theoretically investigate topological transitions between coplanar and noncoplanar magnetic states in centrosymmetric itinerant magnets on a square lattice. A canonical effective spin model incorporating bilinear and biquadratic exchange interactions at finite wave vectors is analyzed to elucidate the emergence of multiple-Q magnetic orders. By taking into account high-harmonic wave-vector interactions, we demonstrate that a coplanar double-Q spin texture continuously evolves into a noncoplanar triple-Q state carrying a finite scalar spin chirality. The stability of these multiple-Q states is examined using simulated annealing as a function of the relative strengths of the high-harmonic coupling, the biquadratic interaction, and the external magnetic field. The resulting phase diagrams reveal a competition between double-Q and triple-Q states, where the noncoplanar triple-Q phase is stabilized through the cooperative effect of the high-harmonic and biquadratic interactions. Real-space spin textures, spin structure factors, and scalar spin chirality distributions are analyzed to characterize the distinct magnetic phases and the topological transitions connecting them. These findings provide a microscopic framework for understanding the emergence of noncoplanar magnetic textures driven by the interplay between two- and four-spin interactions in centrosymmetric itinerant magnets. Full article

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9 pages, 664 KB

Open AccessOpinion

Conservation in High-Field Quantum Transport

by Mukunda P. Das and Frederick Green

Condens. Matter 2025, 10(4), 59; https://doi.org/10.3390/condmat10040059 - 27 Nov 2025

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Abstract

This article provides an overview of the role of microscopic conservation in charge transport at small scales and at driving fields beyond the linear-response limit. As a practical example, we recall the measurement and theory of interband coupling effects in a quantum point [...] Read more.

This article provides an overview of the role of microscopic conservation in charge transport at small scales and at driving fields beyond the linear-response limit. As a practical example, we recall the measurement and theory of interband coupling effects in a quantum point contact driven far from equilibrium. Full article

(This article belongs to the Special Issue New Advances in Condensed Matter Physics, 2nd Edition)

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19 pages, 930 KB

Open AccessReview

de Gennes–Suzuki–Kubo Quantum Ising Mean-Field Dynamics: Applications to Quantum Hysteresis, Heat Engines, and Annealing

by Soumyaditya Das, Soumyajyoti Biswas, Muktish Acharyya and Bikas K. Chakrabarti

Condens. Matter 2025, 10(4), 58; https://doi.org/10.3390/condmat10040058 - 20 Nov 2025

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Abstract

We briefly review the early development of the mean-field dynamics for cooperatively interacting quantum many-body systems, mapped to pseudo-spin (Ising-like) systems. We start with (Anderson, 1958) pseudo-spin mapping the BCS (1957) Hamiltonian of superconductivity, reducing it to a mean-field Hamiltonian of the XY [...] Read more.

We briefly review the early development of the mean-field dynamics for cooperatively interacting quantum many-body systems, mapped to pseudo-spin (Ising-like) systems. We start with (Anderson, 1958) pseudo-spin mapping the BCS (1957) Hamiltonian of superconductivity, reducing it to a mean-field Hamiltonian of the XY (or effectively Ising) model in a transverse field. Then, we obtain the mean-field estimate for the equilibrium gap in the ground-state energy at different temperatures (gap disappearing at the transition temperature), which fits Landau’s (1949) phenomenological theory of superfluidity. We then present in detail a general dynamical extension (for non-equilibrium cases) of the mean-field theory of quantum Ising systems (in a transverse field), following de Gennes’ (1963) decomposition of the mean field into the orthogonal classical cooperative (longitudinal) component and the quantum (transverse) component, with each of the component following Suzuki–Kubo (1968) mean-field dynamics. Next, we discuss its applications to quantum hysteresis in Ising magnets (in the presence of an oscillating transverse field), to quantum heat engines (employing the transverse Ising model as a working fluid), and to the quantum annealing of the Sherrington–Kirkpatrick (1975) spin glass by tuning down (to zero) the transverse field, which provides us with a very fast computational algorithm, leading to ground-state energy values converging to the best-known analytic estimate for the model. Finally, we summarize the main results obtained and draw conclusions about the effectiveness of the de Gennes–Suzuki–Kubo mean-field equations for the study of various dynamical aspects of quantum condensed matter systems. Full article

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14 pages, 7491 KB

Open AccessArticle

Impact of Overdeposition on Magnetic Behavior in Ferromagnetic Nanowire Arrays

by Oleksandr Pastukh

Condens. Matter 2025, 10(4), 57; https://doi.org/10.3390/condmat10040057 - 12 Nov 2025

Viewed by 1038

Abstract

Owing to their dimensions and high aspect ratio, magnetic nanowires possess distinctive physical and chemical properties and are of great importance in building nanoelectronics devices. Nanowires are traditionally produced by electrochemical deposition methods using alumina or polycarbonate membranes, and their parameters (porosity, size, [...] Read more.

Owing to their dimensions and high aspect ratio, magnetic nanowires possess distinctive physical and chemical properties and are of great importance in building nanoelectronics devices. Nanowires are traditionally produced by electrochemical deposition methods using alumina or polycarbonate membranes, and their parameters (porosity, size, and arrangement of pores) strongly influence the magnetic properties of nanowires. However, very often, the effect that cannot be neglected during synthesis is overdeposition. The influence of overdeposition on the magnetic properties of nanowires is often overlooked, but it can strongly alter the magnetic behavior of the system. In this study, we use micromagnetic simulations to investigate how different levels of overdeposition affect the hysteretic behavior of nanowires and their magnetization switching mechanism. It was shown that the formation of hemispherical caps on the ends of the nanowires may alter the out-of-plane magnetic anisotropy of the nanowires and strongly influence the squareness of the hysteresis loop. The demagnetizing field distribution for nanowires with overdeposition was also investigated, showing a strong influence of its spatial distribution change on the reversal mechanism and interaction between nanowires. The obtained results were compared to existing experimental observations, showing good agreement with the magnetic behavior of the system. Performed research can be of great interest to experimental groups, as it highlights the importance of controlling overdeposition during nanowire synthesis and its potential influence on magnetic performance. Full article

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17 pages, 1277 KB

Open AccessPerspective

Nanoscale Lattice Heterostructure in High-Tc Superconductors

by Annette Bussmann-Holder, Jürgen Haase, Hugo Keller, Reinhard K. Kremer, Sergei I. Mukhin, Alexey P. Menushenkov, Andrei Ivanov, Alexey Kuznetsov, Victor Velasco, Steven D. Conradson, Gaetano Campi and Antonio Bianconi

Condens. Matter 2025, 10(4), 56; https://doi.org/10.3390/condmat10040056 - 30 Oct 2025

Cited by 1 | Viewed by 2647

Abstract

Low-temperature superconductivity has been known since 1957 to be described by BCS theory for effective single-band metals controlled by the density of states at the Fermi level, very far from band edges, the electron–phonon coupling constant l, and the energy of the boson [...] Read more.

Low-temperature superconductivity has been known since 1957 to be described by BCS theory for effective single-band metals controlled by the density of states at the Fermi level, very far from band edges, the electron–phonon coupling constant l, and the energy of the boson in the pairing interaction w₀, but BCS has failed to predict high-temperature superconductivity in different materials above about 23 K. High-temperature superconductivity above 35 K, since 1986, has been a matter of materials science, where manipulating the lattice complexity of high-temperature superconducting ceramic oxides (HTSCs) has driven materials scientists to grow new HTSC quantum materials up to 138 K in HgBa₂Ca₂Cu₃O₈ (Hg1223) at ambient pressure and near room temperature in pressurized hydrides. This perspective covers the major results of materials scientists over the last 39 years in terms of investigating the role of lattice inhomogeneity detected in these new quantum complex materials. We highlight the nanoscale heterogeneity in these complex materials and elucidate their special role played in the physics of HTSCs. Especially, it is highlighted that the geometry of lattice and charge complex heterogeneity at the nanoscale is essential and intrinsic in the mechanism of rising quantum coherence at high temperatures. Full article

(This article belongs to the Special Issue Superstripes Physics, 4th Edition)

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16 pages, 13109 KB

Open AccessArticle

Photonic Glasses in Ferrofluid Thin Films

by Alberto Tufaile and Adriana Pedrosa Biscaia Tufaile

Condens. Matter 2025, 10(4), 55; https://doi.org/10.3390/condmat10040055 - 27 Oct 2025

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Abstract

This study investigates the dynamic magneto-optical properties of ferrofluid thin films, focusing on how magnetic fields induce light–matter interactions using a device known as Ferrocell. Our findings reveal that incident light interacts with self-assembled, anisotropic nanoparticle structures, transforming the ferrofluid into a highly [...] Read more.

This study investigates the dynamic magneto-optical properties of ferrofluid thin films, focusing on how magnetic fields induce light–matter interactions using a device known as Ferrocell. Our findings reveal that incident light interacts with self-assembled, anisotropic nanoparticle structures, transforming the ferrofluid into a highly responsive optical medium. Monochromatic laser experiments confirmed the direct correlation between laser color and diffracted light color offering direct insights into particle orientation and aggregate morphology. We observed significant chromatic shifts, especially in regions under strong perpendicular magnetic fields, which provide compelling evidence of structural colors. This phenomenon stems from wavelength-selective interference and diffraction, reminiscent of photonic crystal behavior, yet characterized by short-range order, classifying the material as a photonic glass. Full article

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6 pages, 282 KB

Open AccessEditorial

Breakthroughs in Interdisciplinary Research with High-Energy Accelerators by Guido Barbiellini

by Antonio Bianconi

Condens. Matter 2025, 10(4), 54; https://doi.org/10.3390/condmat10040054 - 24 Oct 2025

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Abstract

The National Laboratories in Frascati (LNF INFN) were conceived and created by a group of collaborators of Enrico Fermi, including Edoardo Amaldi, Gilberto Bernardini, and Enrico Persico, after World War II, with the goal of hosting a 1 GeV electron synchrotron for nuclear [...] Read more.

The National Laboratories in Frascati (LNF INFN) were conceived and created by a group of collaborators of Enrico Fermi, including Edoardo Amaldi, Gilberto Bernardini, and Enrico Persico, after World War II, with the goal of hosting a 1 GeV electron synchrotron for nuclear physics [...] Full article

(This article belongs to the Special Issue The Universe Observed With Particle Detectors: Celebrating the Scientific Legacy of Prof. Guido Barbiellini Amidei)

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17 pages, 3749 KB

Open AccessArticle

Exploring Low Energy Excitations in the d⁵ Iridate Double Perovskites La₂BIrO₆ (B = Zn, Mg)

by Abhisek Bandyopadhyay, Dheeraj Kumar Pandey, Carlo Meneghini, Anna Efimenko, Marco Moretti Sala and Sugata Ray

Condens. Matter 2025, 10(4), 53; https://doi.org/10.3390/condmat10040053 - 6 Oct 2025

Viewed by 1806

Abstract

We experimentally investigate the structural, magnetic, transport, and electronic properties of two d⁵ iridate double perovskite materials La₂BIrO₆ (B = Mg, Zn). Notably, despite similar crystallographic structure, the two compounds show distinctly different magnetic behaviors. The M [...] Read more.

We experimentally investigate the structural, magnetic, transport, and electronic properties of two d⁵ iridate double perovskite materials La₂BIrO₆ (B = Mg, Zn). Notably, despite similar crystallographic structure, the two compounds show distinctly different magnetic behaviors. The M = Mg compound shows an antiferromagnetic-like linear field-dependent isothermal magnetization below its transition temperature, whereas the M = Zn counterpart displays a clear hysteresis loop followed by a noticeable coercive field, indicative of ferromagnetic components arising from a non-collinear Ir spin arrangement. The local structure studies authenticate perceptible M/Ir antisite disorder in both systems, which complicates the magnetic exchange interaction scenario by introducing Ir-O-Ir superexchange pathways in addition to the nominal Ir-O-B-O-Ir super-superexchange interactions expected for an ideally ordered structure. While spin–orbit coupling (SOC) plays a crucial role in establishing insulating behavior for both these compounds, the rotational and tilting distortions of the IrO₆ (and MO₆) octahedral units further lift the ideal cubic symmetry. Finally, by measuring the Ir-L₃ edge resonant inelastic X-ray scattering (RIXS) spectra for both the compounds, giving evidence of spin–orbit-derived low-energy inter-J-state (intra t_2g) transitions (below ~1 eV), the charge transfer (O 2p → Ir 5d), and the crystal field (Ir t_2g → e_g) excitations, we put forward a qualitative argument for the interplay among effective SOC, non-cubic crystal field, and intersite hopping in these two compounds. Full article

(This article belongs to the Section Quantum Materials)

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12 pages, 8239 KB

Open AccessArticle

Impact of Molecular π-Bridge Modifications on Triphenylamine-Based Donor Materials for Organic Photovoltaic Solar Cells

by Duvalier Madrid-Úsuga, Omar J. Suárez and Alfonso Portacio

Condens. Matter 2025, 10(4), 52; https://doi.org/10.3390/condmat10040052 - 25 Sep 2025

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Abstract

This study presents a computational investigation into the design of triphenylamine-based donor chromophores incorporating 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit. Three molecular architectures (System-1 to System-3) were developed by introducing distinct thiophene-derived

π

-bridges to modulate their electronic and optical characteristics for potential application [...] Read more.

This study presents a computational investigation into the design of triphenylamine-based donor chromophores incorporating 2-(1,1-dicyanomethylene)rhodanine as the acceptor unit. Three molecular architectures (System-1 to System-3) were developed by introducing distinct thiophene-derived

π

-bridges to modulate their electronic and optical characteristics for potential application in bulk heterojunction organic solar cells (OSCs). Geometrical optimizations were performed at the B3LYP/6-31+G(d,p) level, while excited-state and absorption properties were evaluated using TD-DFT with the CAM-B3LYP functional. Frontier orbital analysis revealed efficient charge transfer from donor to acceptor moieties, with System-3 showing the narrowest HOMO–LUMO gap (1.96 eV) and the lowest excitation energy (2.968 eV). Charge transport properties, estimated from reorganization energies, indicated that System-2 exhibited the most favorable balance for ambipolar transport, featuring the lowest electron reorganization energy (0.317 eV) and competitive hole mobility. Photovoltaic parameters calculated with PC₆₁BM as acceptor predicted superior

V_{o c}

,

J_{s c}

, and fill factor values for System-2, resulting in the highest theoretical power conversion efficiency (10.95%). These findings suggest that

π

-bridge engineering in triphenylamine-based systems can significantly enhance optoelectronic performance, offering promising donor materials for next-generation OSC devices. Full article

(This article belongs to the Section Condensed Matter Theory)

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Condens. Matter, Volume 10, Issue 4 (December 2025) – 13 articles

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