Condensed Matter

16 pages, 1014 KB

Open AccessArticle

On the Electronic Properties of Gold Quantum Dots at a Finite Temperature

by Rolando Saniz, Dirk Lamoen and Bart Partoens

Condens. Matter 2026, 11(3), 32; https://doi.org/10.3390/condmat11030032 - 31 Aug 2026

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Gold nanoparticles are of interest in a broad class of research fields, ranging from the basic sciences through technology to the biosciences. Not only are the environments in which they are investigated very diverse, but the temperature range is also very wide, going [...] Read more.

Gold nanoparticles are of interest in a broad class of research fields, ranging from the basic sciences through technology to the biosciences. Not only are the environments in which they are investigated very diverse, but the temperature range is also very wide, going from cryogenic temperatures to hundreds of degrees Celsius. A primary role in all the potential applications is played by the electronic properties of the nanoparticles. While these have been studied theoretically before, less attention has been given to the potential effect of temperature on them. Here, we use a particle model and model Hamiltonian to study these possible effects by means of the finite-temperature Green function formalism. We focus in particular on how much temperature would impact observations in positron annihilation and in Compton scattering experiments. For our study, we consider temperatures ranging from

- 80

°C to 1000 °C and nanoparticle diameters ranging from 1 nm to 6 nm. We find that the temperature effects are rather small and generally at the limit of experimental resolution. Still, changes near the Fermi momentum in the Compton profiles should be observable in some cases. 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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15 pages, 1817 KB

Open AccessArticle

Hidden Universal Metal in Cuprate Superconductors

by Abigail Lee and Jürgen Haase

Condens. Matter 2026, 11(3), 31; https://doi.org/10.3390/condmat11030031 - 20 Aug 2026

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Abstract

Nuclear relaxation,

1 / T_{1}

, is a very robust probe of electronic excitations in superconducting materials above and below the critical temperature of superconductivity,

T_{c}

. Here, a relaxation phenomenology of hole-doped cuprate superconductors is presented based on the majority [...] Read more.

Nuclear relaxation,

1 / T_{1}

, is a very robust probe of electronic excitations in superconducting materials above and below the critical temperature of superconductivity,

T_{c}

. Here, a relaxation phenomenology of hole-doped cuprate superconductors is presented based on the majority of the available literature data from the CuO₂ plane, without assumptions with respect to a hyperfine scenario, form factors, or particular theoretical models. Below a temperature similar to the pseudogap temperature, Heitler–Teller-type relaxation is uncovered universally; i.e., the nuclear spin relaxation above

T_{c}

is only determined by the absolute temperature,

1 / T_{1} \propto T

. All materials condense out of this metal at

T_{c}

, below which relaxation drops even faster, as expected from conventional superconductors, albeit without a Hebel–Slichter peak. It is a ’hidden metal’ in the sense that it has a vanishing uniform response and thus hardly affects the NMR shifts; it is also not seen in planar O relaxation. The hidden metal causes a temperature-independent but material-dependent planar Cu relaxation anisotropy that is strongly correlated with the size of

T_{c}

. Moreover, the rate measured with the field in the CuO₂ plane is nearly the same for all cuprates:

1 / {{}^{63}T}_{1 ⊥} T \approx 25 /

Ks, where

1 / {{}^{63}T}_{1 ‖}

is mainly responsible for the change in anisotropy. Above the hidden metal, the relaxation behavior changes and can be described by an ordinary but renormalized metal, with a reduced Cu relaxation anisotropy. The relaxation phenomenology, which should hold clues to the so-called strange metal, is also discussed in the context of the two spin components previously uncovered in the shifts, as well as the pseudogap and relation to other probes. This new phenomenology should give a better foundation for the understanding of the cuprates. Full article

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16 pages, 481 KB

Open AccessFeature PaperArticle

Phonon Softening Due to the Coupling with Charge Density Fluctuations in High-Temperature Superconducting Cuprates

by Martina Fedele, Götz Seibold and Sergio Caprara

Condens. Matter 2026, 11(3), 30; https://doi.org/10.3390/condmat11030030 - 5 Aug 2026

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Abstract

Bond-stretching phonons in hole-doped cuprates exhibit pronounced anomalies in momentum regions where charge correlations are observed, indicating a strong coupling between lattice dynamics and the charge sector. Motivated by this phenomenology, we develop a minimal theoretical framework to clarify how the bond-stretching phonon [...] Read more.

Bond-stretching phonons in hole-doped cuprates exhibit pronounced anomalies in momentum regions where charge correlations are observed, indicating a strong coupling between lattice dynamics and the charge sector. Motivated by this phenomenology, we develop a minimal theoretical framework to clarify how the bond-stretching phonon is renormalized by the proximity to charge-order instabilities and why short-ranged dynamical charge density fluctuations (CDFs) can provide a dominant contribution even in the presence of nearly static charge density wave (CDW) correlations. Starting from a correlated Fermi-liquid description with Coulomb-frustrated charge ordering and electron–phonon coupling, we formulate a two-mode random-phase-approximation treatment in which a low-energy phonon channel involved in the charge-order instability is coupled, through the electronic polarization, to the higher-energy bond-stretching branch. The resulting off-diagonal phonon self-energy transfers the low-energy charge softening to the bond-stretching phonon. We then introduce an average self-energy description to account for the coexistence of CDW and CDF components. The model shows that, although a nearly static CDW component produces a stronger local softening, a broader CDF component can dominate the effective phonon self-energy because of its larger reciprocal-space volume. The analysis identifies two key parameters controlling the bond-stretching anomaly: the characteristic energy of charge correlations and the effective electron–phonon coupling. Full article

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

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18 pages, 2297 KB

Open AccessArticle

DFT-Guided Molecular Engineering of Donor–Bridge–Acceptor Semiconductors for Organic Photovoltaics Solar Cells

by Massimo Ottonelli and Marina Alloisio

Condens. Matter 2026, 11(3), 29; https://doi.org/10.3390/condmat11030029 - 31 Jul 2026

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Abstract

Organic semiconductors offer a potential class of materials for organic photovoltaic (OPV) applications due to their tunable optoelectronic properties and low-cost processing. A methodical DFT/TD-DFT study of a library of organic donor–π–acceptor (D–π–A) compounds based on triphenylamine donors, thiophene-based π-bridges, and benzothiadiazole/malononitrile acceptors [...] Read more.

Organic semiconductors offer a potential class of materials for organic photovoltaic (OPV) applications due to their tunable optoelectronic properties and low-cost processing. A methodical DFT/TD-DFT study of a library of organic donor–π–acceptor (D–π–A) compounds based on triphenylamine donors, thiophene-based π-bridges, and benzothiadiazole/malononitrile acceptors is presented in this work, with the goal of rationalizing the structure–property relationships governing their photovoltaic behavior. CAM-B3LYP calculations were used to analyze the role of donor, bridge, and acceptor units in modulating frontier-orbital alignment, charge-transfer character, and optical absorption properties, as well as to evaluate the active-layer thickness in the estimation of the light-harvesting efficiency. The results, which are intended as internal comparative descriptors rather than predictive device efficiencies, reveal that the most pronounced bathochromic shifts and most favorable optical responses are not simply associated with the strongest donor or acceptor moieties, but rather arise from an optimal balance between frontier-orbital delocalization and charge-transfer character across the molecular framework. A preliminary assessment of photovoltaic descriptors suggests that the proposed computational workflow may provide useful guidelines for the descriptor-guided design and screening of next-generation organic photovoltaic materials. Full article

(This article belongs to the Section Physics of Materials)

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43 pages, 5187 KB

Open AccessFeature PaperArticle

Delayed Conceptual Unification in the Theory of Hole Superconductivity

by J. E. Hirsch

Condens. Matter 2026, 11(3), 28; https://doi.org/10.3390/condmat11030028 - 20 Jul 2026

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Abstract

The theory of hole superconductivity has developed over more than three decades through a sequence of steps addressing distinct physical problems. This paper identifies and documents a recurring structural pattern in that development: ideas introduced to solve one problem were only later recognized [...] Read more.

The theory of hole superconductivity has developed over more than three decades through a sequence of steps addressing distinct physical problems. This paper identifies and documents a recurring structural pattern in that development: ideas introduced to solve one problem were only later recognized as being required by independent physical constraints. By tracing a series of such delayed conceptual unifications, spanning pairing mechanism, charge expulsion, electrodynamics, spin structure, rotation, relativity, thermodynamics and momentum conservation, we highlight that the framework evolves by constraint tightening rather than by ad hoc embellishment. While this does not establish the correctness of the theory, it provides evidence that it is responding to real physical requirements uncovered progressively, in contrast to theories that accommodate discrepancies or new constraints primarily through auxiliary assumptions and ultimately fail. Full article

(This article belongs to the Special Issue 10th Anniversary of Condensed Matter: Recent Advances in Condensed Matter Physics)

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17 pages, 3305 KB

Open AccessArticle

The Interplay of Thermal Melting and Pump-Driven Melting of Charge Order: A Two-Temperature Study of the Holstein Model

by Debraj Bose, Sankha Subhra Bakshi and Pinaki Majumdar

Condens. Matter 2026, 11(3), 27; https://doi.org/10.3390/condmat11030027 - 17 Jul 2026

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Abstract

Charge order driven by electron–phonon coupling is well understood at equilibrium but pump-probe experiments raise a new question: how does this order melt and recover after strong photoexcitation? A pump pulse promotes carriers across the charge-order gap and creates a nonequilibrium high-energy electronic [...] Read more.

Charge order driven by electron–phonon coupling is well understood at equilibrium but pump-probe experiments raise a new question: how does this order melt and recover after strong photoexcitation? A pump pulse promotes carriers across the charge-order gap and creates a nonequilibrium high-energy electronic population. In a closed system, the subsequent dynamics are constrained by energy conservation. In an ‘open system’—where the system is coupled to a thermal bath at some temperature

T_{bath}

—there are new fluctuation and dissipation processes at play. One can attempt a computational scheme that incorporates coupling of electrons to a laser pump, the coupling of system phonons to a thermal bath, and the Holstein interaction that couples electrons and phonons. We attempt an approximation where the pump-induced electronic excitations are modeled by a slowly time-varying ‘electron temperature’,

T_{el} (t)

, indicative of a quasi-equilibrium electronic state. We solve the problem for different combinations of

T_{el}

and

T_{bath}

, probing the order parameter dynamics, the static properties and excitations in the long-time ‘quasi-steady state’, and establish a ‘phase diagram’ in terms of bath temperature and electron temperature. Full article

(This article belongs to the Section Condensed Matter Theory)

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17 pages, 13395 KB

Open AccessArticle

Enhancing X-Ray and Gamma-Ray Detector Calibration via AI-Driven Digital Twins: Predicting Extracorporeal Photon Emission from OpenDose Specific Absorbed Fraction Datasets Using Uncertainty-Aware Transformer Ensembles

by Muhammed Emin Bedir

Condens. Matter 2026, 11(3), 26; https://doi.org/10.3390/condmat11030026 - 15 Jul 2026

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Abstract

Patient-specific dosimetry and quantitative external counting for in vivo monitoring of radiopharmaceutical therapies require detector calibration coefficients (k) that are currently obtained through computationally intensive Monte Carlo (MC) simulations. We present a digital-twin framework that learns the mapping from organ-level Specific Absorbed Fractions [...] Read more.

Patient-specific dosimetry and quantitative external counting for in vivo monitoring of radiopharmaceutical therapies require detector calibration coefficients (k) that are currently obtained through computationally intensive Monte Carlo (MC) simulations. We present a digital-twin framework that learns the mapping from organ-level Specific Absorbed Fractions (SAFs) to clinical k values for X-ray and gamma-ray detectors, trained on 4.47 million SAF entries from the OpenDose collaboration covering the ICRP-110 adult male (AM) and adult female (AF) reference phantoms, 91 photon energies (5–10,000 keV), and 336 source organs. An uncertainty-aware ensemble combining Histogram Gradient Boosting, ExtraTrees, Random Forest, Quantile-HGBT, and a Feature-Tokenizer Transformer (414 K parameters) was stacked via ridge regression and calibrated using Conformalized Quantile Regression. The ensemble achieved a test mean absolute error of 0.220 in log10-SAF space (R² = 0.921) with an empirical 95% prediction interval coverage of 95.0%. Validation against 10,487 independent published S-values (EMDOSE) yielded a Pearson correlation of 0.990 (log space). The framework reduces k-coefficient computation time from hours of MC simulation to milliseconds, supporting real-time detector calibration for theranostic workflows. Full article

(This article belongs to the Special Issue Advances in X-Ray and Gamma Ray Detectors and Applications)

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9 pages, 266 KB

Open AccessReview

Wakefield Acceleration in Gamma-Ray Bursts

by Jahanvi Jahanvi, Alessandro Armando Vigliano and Francesco Longo

Condens. Matter 2026, 11(3), 25; https://doi.org/10.3390/condmat11030025 - 3 Jul 2026

Viewed by 557

Abstract

Gamma-ray bursts (GRBs) represent the most powerful explosions in the Universe, releasing extreme fluxes of non-thermal radiation across the electromagnetic spectrum. A central enigma in GRB physics remains the mechanism responsible for accelerating electrons, positrons, and hadrons to the required ultra-relativistic energies. Conventional [...] Read more.

Gamma-ray bursts (GRBs) represent the most powerful explosions in the Universe, releasing extreme fluxes of non-thermal radiation across the electromagnetic spectrum. A central enigma in GRB physics remains the mechanism responsible for accelerating electrons, positrons, and hadrons to the required ultra-relativistic energies. Conventional theories primarily invoke diffusive shock acceleration (DSA), magnetic reconnection, and relativistic turbulence. This short review first examines these canonical acceleration methods, then discusses the principles and successes of plasma wakefield acceleration as a powerful future technique for ground-based applications. Finally, we critically analyze the feasibility of applying this mechanism to the cosmological environment of GRBs, exploring why the terrestrial success of wakefield acceleration has not yet been definitively confirmed “on the sky”. 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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13 pages, 2484 KB

Open AccessArticle

Piezoelectric Double Layer Pressure Sensors: An Analytical Study and Multiphysics Simulation

by Moirangthem Shamjit Singh, Pradip Kumar Kalita and Maibam Sanju Meetei

Condens. Matter 2026, 11(3), 24; https://doi.org/10.3390/condmat11030024 - 30 Jun 2026

Viewed by 548

Abstract

This study presents both the analytical modeling and simulation of a cantilever pressure sensor with double-layer piezoelectric materials, specifically ZnO and PVDF, which have negative and positive voltage coefficients, in order to investigate the performance of the sensor and to validate the analytical [...] Read more.

This study presents both the analytical modeling and simulation of a cantilever pressure sensor with double-layer piezoelectric materials, specifically ZnO and PVDF, which have negative and positive voltage coefficients, in order to investigate the performance of the sensor and to validate the analytical model with simulation. A detailed three-dimensional sensor model was developed in FEM, comprising gold (Au) as electrodes, silicon dioxide (SiO₂) as an insulating layer, and silicon (Si) as a substrate. Simulations performed across a pressure range of 0–10 kPa revealed a linear output voltage response, and the average margin of error (MoE) between the calculated and simulated values is approximately 11.8%. The observed net potential difference exhibited a negative polarity, primarily due to the dominant effect of ZnO, which has negative piezoelectric voltage coefficients. Comparison of analytical and simulated results shows close agreement, with slope values of −1.16 mV/kPa and −1.03 mV/kPa, respectively, validating the FEM model’s accuracy. From the analytical model, it is observed that the sensitivity of the sensor varies with piezoelectric voltage coefficients, stress produced on the piezoelectric surface and the thickness of the piezoelectric material. Various simulation results show that the output voltage increases as the thickness of ZnO and PVDF decreases. Full article

(This article belongs to the Section Physics of Materials)

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Condens. Matter, Volume 11, Issue 3 (September 2026) – 9 articles

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