Journal of Physics: Condensed Matter

Fundamental physical constants govern key effects in high-energy particle physics and astrophysics, including the stability of particles, nuclear reactions, formation and evolution of stars, synthesis of heavy nuclei and emergence of stable molecular structures. Here, we show that fundamental constants also set an upper bound for the frequency of phonons in condensed matter phases, or how rapidly an atom can vibrate in these phases. This bound is in agreement with ab initio simulations of atomic hydrogen and high-temperature hydride superconductors, and implies an upper limit to the superconducting transition temperature $T_\mathrm{c}$ in condensed matter. Fundamental constants set this limit to the order of 10²–10³ K. This range is consistent with our calculations of $T_\mathrm{c}$ from optimal Eliashberg functions. As a corollary, we observe that the very existence of the current research of finding $T_{\mathrm{c}}$ at and above 300 K is due to the observed values of fundamental constants. We finally discuss how fundamental constants affect the observability and operation of other effects and phenomena including phase transitions.

Chirality refers to the asymmetry of objects that cannot be superimposed on their mirror image. It is a concept that exists in various scientific fields and has profound consequences. Although these are perhaps most widely recognized within biology, chemistry, and pharmacology, recent advances in chiral phonons, topological systems, crystal enantiomorphic materials, and magneto-chiral materials have brought this topic to the forefront of condensed matter physics research. Our review discusses the symmetry requirements and the features associated with structural chirality in inorganic materials. This allows us to explore the nature of phase transitions in these systems, the coupling between order parameters, and their impact on the material's physical properties. We highlight essential contributions to the field, particularly recent progress in the study of chiral phonons, altermagnetism, magnetochirality between others. Despite the rarity of naturally occurring inorganic chiral crystals, this review also highlights a significant knowledge gap, presenting challenges and opportunities for structural chirality mostly at the fundamental level, e.g. chiral displacive phase transitions, possibilities of tuning and switching structural chirality by external means (electric, magnetic, or strain fields), whether chirality could be an independent order parameter, and whether structural chirality could be quantified, etc. Beyond simply summarizing this field of research, this review aims to inspire further research in materials science by addressing future challenges, encouraging the exploration of chirality beyond traditional boundaries, and seeking the development of innovative materials with superior or new properties.

The transition from planar to three-dimensional (3D) magnetic nanostructures represents a significant advancement in both fundamental research and practical applications, offering vast potential for next-generation technologies like ultrahigh-density storage, memory, logic, and neuromorphic computing. Despite being a relatively new field, the emergence of 3D nanomagnetism presents numerous opportunities for innovation, prompting the creation of a comprehensive roadmap by leading international researchers. This roadmap aims to facilitate collaboration and interdisciplinary dialogue to address challenges in materials science, physics, engineering, and computing. The roadmap comprises eighteen sections, roughly divided into three blocks. The first block explores the fundamentals of 3D nanomagnetism, focusing on recent trends in fabrication techniques and imaging methods crucial for understanding complex spin textures, curved surfaces, and small-scale interactions. Techniques such as two-photon lithography and focused electron beam-induced deposition enable the creation of intricate 3D architectures, while advanced imaging methods like electron holography and synchrotron x-ray tomography provide nanoscale spatial resolution for studying magnetization dynamics in three dimensions. Various 3D magnetic systems, including coupled multilayer systems, artificial spin-ice, magneto-plasmonic systems, topological spin textures, and molecular magnets are discussed. The second block introduces analytical and numerical methods for investigating 3D nanomagnetic structures and curvilinear systems, highlighting geometrically curved architectures, interconnected nanowire systems, and other complex geometries. Finite element methods are emphasized for capturing complex geometries, along with direct frequency domain solutions for addressing magnonic problems. The final block focuses on 3D magnonic crystals and networks, exploring their fundamental properties and potential applications in magnonic circuits, memory, and spintronics. Computational approaches using 3D nanomagnetic systems and complex topological textures in 3D spintronics are highlighted for their potential to enable faster and more energy-efficient computing.

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Magnonics is a research field that has gained an increasing interest in both the fundamental and applied sciences in recent years. This field aims to explore and functionalize collective spin excitations in magnetically ordered materials for modern information technologies, sensing applications and advanced computational schemes. Spin waves, also known as magnons, carry spin angular momenta that allow for the transmission, storage and processing of information without moving charges. In integrated circuits, magnons enable on-chip data processing at ultrahigh frequencies without the Joule heating, which currently limits clock frequencies in conventional data processors to a few GHz. Recent developments in the field indicate that functional magnonic building blocks for in-memory computation, neural networks and Ising machines are within reach. At the same time, the miniaturization of magnonic circuits advances continuously as the synergy of materials science, electrical engineering and nanotechnology allows for novel on-chip excitation and detection schemes. Such circuits can already enable magnon wavelengths of 50 nm at microwave frequencies in a 5G frequency band. Research into non-charge-based technologies is urgently needed in view of the rapid growth of machine learning and artificial intelligence applications, which consume substantial energy when implemented on conventional data processing units. In its first part, the 2024 Magnonics Roadmap provides an update on the recent developments and achievements in the field of nano-magnonics while defining its future avenues and challenges. In its second part, the Roadmap addresses the rapidly growing research endeavors on hybrid structures and magnonics-enabled quantum engineering. We anticipate that these directions will continue to attract researchers to the field and, in addition to showcasing intriguing science, will enable unprecedented functionalities that enhance the efficiency of alternative information technologies and computational schemes.

Activity and autonomous motion are fundamental aspects of many living and engineering systems. Here, the scale of biological agents covers a wide range, from nanomotors, cytoskeleton, and cells, to insects, fish, birds, and people. Inspired by biological active systems, various types of autonomous synthetic nano- and micromachines have been designed, which provide the basis for multifunctional, highly responsive, intelligent active materials. A major challenge for understanding and designing active matter is their inherent non-equilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Furthermore, interactions in ensembles of active agents are often non-additive and non-reciprocal. An important aspect of biological agents is their ability to sense the environment, process this information, and adjust their motion accordingly. It is an important goal for the engineering of micro-robotic systems to achieve similar functionality. Many fundamental properties of motile active matter are by now reasonably well understood and under control. Thus, the ground is now prepared for the study of physical aspects and mechanisms of motion in complex environments, the behavior of systems with new physical features like chirality, the development of novel micromachines and microbots, the emergent collective behavior and swarming of intelligent self-propelled particles, and particular features of microbial systems. The vast complexity of phenomena and mechanisms involved in the self-organization and dynamics of motile active matter poses major challenges, which can only be addressed by a truly interdisciplinary effort involving scientists from biology, chemistry, ecology, engineering, mathematics, and physics. The 2025 motile active matter roadmap of Journal of Physics: Condensed Matter reviews the current state of the art of the field and provides guidance for further progress in this fascinating research area.

Scientific simulation codes are public property sustained by the community. Modern technology allows anyone to join scientific software projects, from anywhere, remotely via the internet. The phonopy and phono3py codes are widely used open-source phonon calculation codes. This review describes a collection of computational methods and techniques implemented in these codes and shows their implementation strategies as a whole, aiming to be useful for the community. Some of the techniques presented here are not limited to phonon calculations and may therefore be useful in other areas of condensed matter physics.

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

The industrialization has severely impacted the ecosystem because of intensive use of chemicals and gases, causing the undesired outcomes such as hazardous gases, e.g. carbon monoxide (CO), nitrous oxide (NO_x), ammonia (NH₃), hydrogen (H₂), hydrogen sulfide (H₂S) and even volatile organic compounds. These hazardous gases are not only impacting the living beings but also the entire ecosystem. Thus, it becomes essential to monitor these gases for their efficient management. There are continuous efforts to realize such sensors, which rely on the functional materials properties. The widely used such sensors use metal oxide nanomaterials. However, these are not very sensitive and operate at higher temperatures. In contrast, two-dimensional (2D) materials such as Graphene, Borophene, MXenes, and transition metal dichalcogenides (TMDs) including doping, functionalization, and heterostructures offer unique physical, chemical, and optoelectronic properties. The chemical properties with high specific surface area of 2D materials make them suitable for gas sensing applications. The present review covers the recent developments on 2D-layered material, including MoS₂, WS₂, h-BN, and Graphene, as well as their heterostructures for gas sensing applications. The review article also emphasizes their synthesis and characterization techniques, especially for 2D materials. The electronic properties of these materials are highly sensitive to any chemical changes, resulting in significant changes in their resistance. It led to the development of the highly scalable chemiresistive-based gas sensor. The sensing parameters such as sensitivity, selectivity, gas concentration, limit of detection, temperature, humidity, response, reproducibility, stability, recovery, and response time are discussed in detail to understand the gas sensing characteristics of these 2D materials. This review also includes the past developments, current status, and future scope of these 2D materials as highly efficient gas sensors. Thus, this review article may lead the researchers to design and develop highly sensitive gas sensors based on 2D materials.

Single-layer Bismuth Monobromide (SL-Bi₄Br₄) is a recently experimentally confirmed room temperature quantum spin hall insulator with a relatively large bulk band gap. In this paper, we investigate the electronic properties of SL-Bi₄Br₄ and single-layer bismuth monobromide nanoribbon (SL-Bi₄Br₄ NR) introducing different vacancy defects near the nanoribbon edges. With maximally localized wannier function (MLWF) constructed Hamiltonian we show that SL-Bi₄Br₄ NR edge states are protected by bulk topology and robust against disorder. In conjunction with MLWF and non-equilibrium Green's function, we also show that in devices made from SL-Bi₄Br₄, transmission through the topologically protected edge states do not suffer from degradation when the device is sufficiently wide. Increasing channel length and defect concentration affect only the bulk states transmission leaving edge states transmission perfectly quantized. This resilience against disorder signifies SL-Bi₄Br₄'s promising candidacy for next-generation electronic & spintronics devices application.

A review of the electronic and optical properties of Kekulé and other short wavelength modulations textures on graphene is presented. Starting from the experimental realization of such textures, the review discusses the electronic and optical properties in terms of several theoretical models like the tight-binding Hamiltonian and effective low energy models based on the Dirac equation. Other surveyed subjects are, strain effects, valley engineering, Kekulé bilayers, zitterbewegung, Kekulé interfaces, valley birefringence and the skew valley scattering. Specific signatures in the optical and electronic conductivities of Kekule textures are next discussed using several approaches like linear response theory, the random phase approximation, and Floquet theory. Plasmons are also presented by considering the dielectric function. Finally, a discussion is presented on how Kekulé textures are related with highly correlated phases, including its importance in magic angle twisted bilayer graphene superconductivity and related quantum phases.

Two-dimensional (2D) rare-earth-based square lattice (SL) quantum magnets provide a pathway to achieve distinctive ground states characterized by unusual excitations. We investigate the magnetic, heat capacity, structural, and electronic properties of a magnetic system Bi₂ErO₄Cl. This compound features a structurally ideal 2D SL composed of Er³⁺ rare-earth magnetic ions. The single-phase polycrystalline sample was synthesized using hydrothermal, followed by a vacuum-sealed tube technique. The analysis of heat capacity and magnetic data indicates that the Er³⁺ ion adopts a $\textit{J}_{\textrm{eff}} = \frac{1}{2}$ state at low temperatures. Fitting the Curie–Weiss (CW) law to the low-temperature magnetic susceptibility data reveals a CW temperature of approximately −2.1 K, suggesting antiferromagnetic (AFM) interactions between the Er³⁺ moments. Our first-principles calculations validate a 2D spin model relevant to the titled Er compound. The presence of AFM interaction between the Er³⁺ ions is further confirmed using total energy calculations (DFT+U), aligning with the experimental results. The heat capacity measurements reveal the presence of magnetic long-range order below T_N = 0.47 K. The magnetic heat capacity data follows T^1.8 power law dependence below T_N.

The industrialization has severely impacted the ecosystem because of intensive use of chemicals and gases, causing the undesired outcomes such as hazardous gases, e.g. carbon monoxide (CO), nitrous oxide (NO_x), ammonia (NH₃), hydrogen (H₂), hydrogen sulfide (H₂S) and even volatile organic compounds. These hazardous gases are not only impacting the living beings but also the entire ecosystem. Thus, it becomes essential to monitor these gases for their efficient management. There are continuous efforts to realize such sensors, which rely on the functional materials properties. The widely used such sensors use metal oxide nanomaterials. However, these are not very sensitive and operate at higher temperatures. In contrast, two-dimensional (2D) materials such as Graphene, Borophene, MXenes, and transition metal dichalcogenides (TMDs) including doping, functionalization, and heterostructures offer unique physical, chemical, and optoelectronic properties. The chemical properties with high specific surface area of 2D materials make them suitable for gas sensing applications. The present review covers the recent developments on 2D-layered material, including MoS₂, WS₂, h-BN, and Graphene, as well as their heterostructures for gas sensing applications. The review article also emphasizes their synthesis and characterization techniques, especially for 2D materials. The electronic properties of these materials are highly sensitive to any chemical changes, resulting in significant changes in their resistance. It led to the development of the highly scalable chemiresistive-based gas sensor. The sensing parameters such as sensitivity, selectivity, gas concentration, limit of detection, temperature, humidity, response, reproducibility, stability, recovery, and response time are discussed in detail to understand the gas sensing characteristics of these 2D materials. This review also includes the past developments, current status, and future scope of these 2D materials as highly efficient gas sensors. Thus, this review article may lead the researchers to design and develop highly sensitive gas sensors based on 2D materials.

A review of the electronic and optical properties of Kekulé and other short wavelength modulations textures on graphene is presented. Starting from the experimental realization of such textures, the review discusses the electronic and optical properties in terms of several theoretical models like the tight-binding Hamiltonian and effective low energy models based on the Dirac equation. Other surveyed subjects are, strain effects, valley engineering, Kekulé bilayers, zitterbewegung, Kekulé interfaces, valley birefringence and the skew valley scattering. Specific signatures in the optical and electronic conductivities of Kekule textures are next discussed using several approaches like linear response theory, the random phase approximation, and Floquet theory. Plasmons are also presented by considering the dielectric function. Finally, a discussion is presented on how Kekulé textures are related with highly correlated phases, including its importance in magic angle twisted bilayer graphene superconductivity and related quantum phases.

This elementary review article is aimed to the beginning graduate students interested to know basic aspects of Kitaev model. We begin with a very lucid introduction of Kitaev model and present its exact solution, Hilbert space structure, fractionalization, spin–spin correlation function and topological degeneracy in an elementary way. We then discuss the recent proposal of realizing Kitaev interaction in certain materials. Finally we present some recent experiments done on these materials, mainly magnetization, susceptibility, specific heat and thermal Hall effect to elucidate the recent status of material realization of coveted Kitaev spin-liquid phase. We end with a brief discussion on other theoretical works on Kitaev model from different many-body aspects.

Lighter isotopes typically diffuse faster than heavier isotopes; however, the case is not necessarily true for H. Predicting the kinetics of H isotope transport and reactions in substances remains a fundamental challenge in material and condensed matter physics. The peculiar experimentally observed isotope effect on H diffusivities in face-centred cubic (fcc) metals has long been an unresolved problem. Using an ab initio path-integral approach to explore the quantum mechanical nature of both electrons and nuclei, this study successfully predicts H isotope diffusivities in fcc Pd over a wide temperature range. The temperature dependence of the diffusivities follows an unusual 'reversed-S' shape on Arrhenius plots. This irregular behaviour, arising from the competition between different nuclear quantum effects (NQEs) with different temperature dependencies, reveals the mechanism of anomalous crossovers between normal and reversed isotope effects. The results illustrate that this phenomenon is common in other fcc metals (e.g. Cu and Ag), where H atoms prefer to occupy octahedral (O) sites. Conversely, in Al, where H atoms prefer to occupy tetrahedral (T) sites, the dependence of H diffusivities on temperature exhibits a familiar 'C' shape. A lattice expansion of approximately 1%–2% causes the stable position of H atoms dissolved in Pd to shift from the O to T sites, and H diffusion in expanded Pd is no longer suppressed by NQEs, as observed in Al. This finding has important implications for interpreting kinetic processes involving the crossover from classical to quantum behaviour of H atoms moving between different interstitial sites. Path-integral simulation results describing the approximate quantum dynamics of the Pd–H system, using a machine-learning-based interatomic potential with accuracy similar to the density functional theory calculations, are presented. This computational approach paves the way for elucidating the quantum behaviour of H isotopes in various materials.

This pedagogic review aims to give a gentle introduction to an exactly solvable model, the Hatsugai–Kohmoto (HK) model, which has infinite-ranged interaction but conserves the center of mass. Although this model is invented in 1992, intensive studies on its properties ranging from unconventional superconductivity, topological ordered states to non-Fermi liquid behaviors are made since 2020. We focus on its emergent non-Fermi liquid behavior and provide discussion on its thermodynamics, single-particle and two-particle correlation functions. Perturbation around solvable limit has also been explored with the help of perturbation theory, renormalization group and exact diagonalization calculation. We hope the present review will be helpful for graduate students or researchers interested in HK-like models or more generic strongly correlated electron systems.

Specific heat and thermal expansion properties are investigated in two non-magnetic rare-earth cage compounds, LaB₆ and LaPt₄Ge₁₂, which represent extremes in guest-to-cage mass ratio. Using simplified phonons dispersions for the two lowest branches, a theoretical framework is proposed for the low temperature thermodynamic analysis of cage compounds. Within the quasi-harmonic approximation, the Gru ̈neisen rule is found to break down even at low temperatures. However, under the influence of the flattened branches, it should be approximatively restored at intermediates temperatures. The model accurately describes LaB₆ specific heat below 50 K. In the LaPt₄Ge₁₂ case, the description is rapidly inadequate with increasing the temperature, which points to the interference of additional low frequency phonon branches. Subsequently, thermal expansion measurements are used to investigate the Grüneisen rule in these two compounds. As predicted, there appears to be distincts Grüneisen regimes at low temperature. This study will help distinguish between phonon and magnetic contributions to the thermal expansion in the RB₆ and RPt4Ge₁₂ series.

NaNiO$_2$ is a Ni$^{3+}$-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn--Teller distortion below $T^\mathrm{JT}_\mathrm{onset}\approx480$\,K, which disappears in a broad first-order transition on heating to $T^\mathrm{JT}_\mathrm{end}\approx500$\,K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction [ACS Inorganic Chemistry 61.10 (2022): 4312-4321] and found to exhibit an increasing $T^\mathrm{JT}_\mathrm{onset}$ with pressure up to $\sim$5\,GPa. In this work, powdered NaNiO$_2$ was studied \textit{via} variable-pressure synchrotron x-ray diffraction up to pressures of $\sim$67\,GPa at 294\,K and 403\,K. Suppression of the collinear Jahn--Teller ordering is observed \textit{via} the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at $\sim$18\,GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn--Teller transition temperature, $T^\mathrm{JT}_\mathrm{onset}$, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn--Teller distortion in NaNiO$_2$ is dome-like.

The electron-electron interaction (EEI), weak localization and Kondo effect are known to correct low-temperature (low-T) resistivity in metals and semimetals. However, the impact of EEI on the anomalous Hall effect (AHE) by EEI remains a subject of debate. In this study, we investigate the EEI corrections to both the low-T longitudinal and anomalous Hall resistivities in van der Waals ferromagnetic Fe3GaTe2 single crystals with a high Curie temperature. Our findings reveal that the longitudinal resistivity is well-described by the EEI theory developed by Altshuler et al., while the anomalous Hall (AH) resistivity deviates from this theory. We found that the AH resistivity follows a T temperature dependence, and its relative rate of change is 2.6 times that of the longitudinal resistivity. These results demonstrate that EEI significantly influences the low-T AH resistivity under intrinsic mechanism in Fe3GaTe2. This observation challenges the conventional understanding that EEI does not contribute to the AHE in systems with mirror symmetry, as suggested by skew scattering and side jump models. This work opens avenues for further exploration of EEI effect in disordered magnetic materials.

We show with hybrid density functional theory calculations that chalcogen donors other than oxygen (i.e., S_N, Se_N, and Te_N) give rise to deep donor states in aluminum nitride. These donors trap a localized electron in their neutral charge state, leading to deep (+/0) donor levels that are 0.45 eV or more from the conduction band edge. As such, this behavior is distinct from the DX behavior leads to deep (+/−) levels which affects other donors such as O_N and Si_Al. We highlight how these results hint at the formation of small electron polarons in AlN, which are found to be unstable in the bulk, but metastable when bound to donor dopants like Si_Al and the chalocogens, with activation energies on the order of 0.2-0.3 eV. These results indicate that S, Se, and Te are not shallow donor dopants in aluminum nitride and identify origins of the experimentally observed ∼200-300 meV activation energies for dopant activation in donor-doped samples.

We performed inelastic neutron scattering experiments on single crystal samples of a linear magnetoelectric material Mn₃Ta₂O₈, which exhibits a collinear antiferromagnetic order, 
to reveal the spin dynamics. 
Numerous modes observed in the neutron spectra were reasonably reproduced by linear spin-wave theory on the basis of the spin Hamiltonian including eight Heisenberg interactions and an easy-plane type single-ion anisotropy. The presence of strong frustration was found in the identified spin Hamiltonian.

A review of the electronic and optical properties of Kekulé and other short wavelength modulations textures on graphene is presented. Starting from the experimental realization of such textures, the review discusses the electronic and optical properties in terms of several theoretical models like the tight-binding Hamiltonian and effective low energy models based on the Dirac equation. Other surveyed subjects are, strain effects, valley engineering, Kekulé bilayers, zitterbewegung, Kekulé interfaces, valley birefringence and the skew valley scattering. Specific signatures in the optical and electronic conductivities of Kekule textures are next discussed using several approaches like linear response theory, the random phase approximation, and Floquet theory. Plasmons are also presented by considering the dielectric function. Finally, a discussion is presented on how Kekulé textures are related with highly correlated phases, including its importance in magic angle twisted bilayer graphene superconductivity and related quantum phases.

NaNiO$_2$ is a Ni$^{3+}$-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn--Teller distortion below $T^\mathrm{JT}_\mathrm{onset}\approx480$\,K, which disappears in a broad first-order transition on heating to $T^\mathrm{JT}_\mathrm{end}\approx500$\,K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction [ACS Inorganic Chemistry 61.10 (2022): 4312-4321] and found to exhibit an increasing $T^\mathrm{JT}_\mathrm{onset}$ with pressure up to $\sim$5\,GPa. In this work, powdered NaNiO$_2$ was studied \textit{via} variable-pressure synchrotron x-ray diffraction up to pressures of $\sim$67\,GPa at 294\,K and 403\,K. Suppression of the collinear Jahn--Teller ordering is observed \textit{via} the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at $\sim$18\,GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn--Teller transition temperature, $T^\mathrm{JT}_\mathrm{onset}$, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn--Teller distortion in NaNiO$_2$ is dome-like.

We show with hybrid density functional theory calculations that chalcogen donors other than oxygen (i.e., S_N, Se_N, and Te_N) give rise to deep donor states in aluminum nitride. These donors trap a localized electron in their neutral charge state, leading to deep (+/0) donor levels that are 0.45 eV or more from the conduction band edge. As such, this behavior is distinct from the DX behavior leads to deep (+/−) levels which affects other donors such as O_N and Si_Al. We highlight how these results hint at the formation of small electron polarons in AlN, which are found to be unstable in the bulk, but metastable when bound to donor dopants like Si_Al and the chalocogens, with activation energies on the order of 0.2-0.3 eV. These results indicate that S, Se, and Te are not shallow donor dopants in aluminum nitride and identify origins of the experimentally observed ∼200-300 meV activation energies for dopant activation in donor-doped samples.

When compressed, certain lattices undergo phase transitions that may allow nuclei to gain significant kinetic energy. To explore the dynamics of this phenomenon, we develop a methodology to study Coulomb coupled N-body systems constrained to a sphere, as in the Thomson problem. We initialize N total Boron nuclei as point particles on the surface of the sphere, allowing them to equilibrate via Coulomb scattering with a viscous damping term. To simulate a phase transition, we remove $N_\mathrm{rm}$ particles, forcing the system to rearrange into a new equilibrium. With this model, we consider the Thomson problem as a dynamical system, providing a framework to explore how non-zero temperature affects structural imperfections in Thomson minima. We develop a scaling relation for the average peak kinetic energy attained by a single particle as a function of N and $N_\mathrm{rm}$. For certain values of N, we find an order of magnitude energy gain when increasing $N_\mathrm{rm}$ from 1 to 6. The model may help to design a lattice that maximizes the energy output.

The Mpemba effect has initially been noticed in macroscopic systems—namely that hot water can freeze faster than cold water—but recently its extension to open quantum systems has attracted significant attention. This phenomenon can be explained in the context of nonequilibrium thermodynamics of Markovian systems, relying on the amplitudes of different decay modes of the system dynamics. Here, we study the Mpemba effect in a single-level quantum dot coupled to a thermal bath, highlighting the role of the sign and magnitude of the electron–electron interaction in the occurrence of the Mpemba effect. We gain physical insights into the decay modes from a dissipative symmetry of this system called fermionic duality. Based on this analysis of the relaxation to equilibrium of the dot, we derive criteria for the occurrence of the Mpemba effect using two thermodynamically relevant measures of the distance to equilibrium, the nonequilibrium free energy and the dot energy. We furthermore compare this effect to a possible exponential speedup of the relaxation. Finally, we propose experimentally relevant schemes for the state preparation and explore different ways of observing the Mpemba effect in quantum dots in experiments.

It is difficult to completely eliminate disorder during the fabrication of graphene-based nanodevices. From a simulation perspective, it is straightforward to determine the electronic transport properties of disordered devices if complete information about the disorder and the Hamiltonian describing it is available. However, to do the reverse and determine information about the nature of the disorder purely from transport measurements is a far more difficult task. In this work, we apply a recently developed inversion technique to identify important structural information about edge-disordered zigzag graphene nanoribbons. The inversion tool decodes the electronic transmission spectrum to obtain the overall level of edge vacancies in this type of device. We also consider the role of spin-polarised states at the ribbon edges and demonstrate that, in addition to edge roughness, the inversion procedure can also be used to detect the presence of magnetism in such nanoribbons. We finally show that if the transmission for both spin orientations is available, for example by using ferromagnetic contacts in a transport measurement, then additional structural information about the relative concentration of defects on each edge can be derived.

We use low-temperature scanning tunneling microscopy (LT-STM) to characterize the early stages of silver fluorination. On Ag(100), we observe only one adsorbate species, which shows a bias-dependent STM topography. Notably, at negative bias voltages, $V_\textrm{B} \lt 0$, the apparent shape can be described as a round protrusion surrounded by a moat-like depression (sombrero). As the voltage increases, the apparent shape changes, eventually evolving into a round depression. From the STM images, we determine the adsorption site to be the hollow position. On Ag(110) we find adsorbates with three distinct STM topographies. One type exhibits the same shape change with $V_\textrm{B}$ as observed on Ag(100), that is, from a sombrero shape to a round depression as the voltage changes from negative to positive values; the other two types are observed as round depressions regardless of $V_\textrm{B}$. From the STM images, we find the three adsorbates to be sitting on the short-bridge, hollow and top position on the Ag(110) surface, with a relative abundance of 60%, 35% and 5%.

Due to the time reversal symmetry, the linear anomalous Hall effect (AHE) usually vanishes in $\textrm{MoS}_{2}$ monolayer. In contrast, the nonlinear AHE plays an essential role in such system when the uniaxial strain breaks the $C_{3v}$ symmetry and eventually results in the nonzero Berry curvature dipole (BCD). We find that not only the magnitude of the AHE but also the nonlinear Hall angle can be tuned by the strain. Especially the nonlinear Hall angle exhibits a deep relationship which is analogy to the birefraction phenomenon in optics. It actually results from the pseudotensor nature of the BCD moment. Besides the ordinary positive and negative crystals in optics, there are two more birefraction-like cases corresponding to an imaginary refraction index ratio in monolayer $\textrm{MoS}_{2}$. Our findings shed lights on the strain controlled electronic devices based on the two-dimensional materials with BCD.

Purely electrical control of the Dzyaloshinskii–Moriya interaction (DMI) without any external magnetic field is explored in a magnetic Weyl semimetal (WSM). The underlying mechanism for the DMI in the WSM is the recently identified asymmetrical indirect spin–spin interaction compatible with the inversion symmetry of the structure. While the necessary imbalance in the fermion population of opposite chirality is normally achieved with non-orthogonal external electric and magnetic fields (i.e. the axial anomaly), it is found that the intrinsic axial magnetic field characteristic to an inhomogeneous magnetic texture can play the role of the magnetic field. When applied to the magnetic domain walls as specific examples, our theoretical analysis clearly illustrates that the resulting DMI is pinned by and can in turn significantly affect the wall textures. As the appearance and strength of the DMI can be solely controlled by the applied electric field, this mechanism enables electrical modulation of magnetic domains including their excitation in the WSMs. Numerical calculations highlight significant advantages of the WSM over the conventional magnetic materials in spintronic applications such as the racetrack memory.

Diborides AB₂ crystallizing in the hexagonal C32 structure exhibit a wide range of magnetic and electronic properties depending on the choice of the element A and the precise values of the lattice constants a and c. ErB₂ represents a typical rare-earth diboride, exhibiting easy-plane ferromagnetic order below 14 K. We report a study of the evolution of the electrical transport properties of ErB₂ when tuning the lattice constants under pressures up to 5.6 GPa. Using Bridgman-type pressure cells with polycrystalline diamond anvils and steatite as the solid pressure medium, quasi-hydrostatic conditions are provided. We find that magnetic order is stabilized under pressure and discuss the influence of uniaxial components by comparing measurements on polycrystalline and single-crystalline samples.