Papers

Two-dimensional semiconductors such as monolayer MoS2 combine a useful band gap with a thickness of three atomic planes, which places the entire carrier density within a few angstroms of the surface and makes them attractive both as transistor channels and as chemical sensors. We present a self-consistent ensemble Monte Carlo (EMC) framework for monolayer MoS2 built on the ViennaEMC solver, which couples a Boltzmann transport model to Poisson's equation with degenerate free-carrier statistics. The transport model carries wavevector-dependent acoustic, polar-optical, homopolar and intervalley phonon scattering within and between the K valleys and the spin-orbit-split Q valleys, together with remote substrate phonons, screened charged impurities and surface roughness. Every intrinsic input is computed from first principles for this work, the band structure of the K and Q valleys, the phonon energies, the sound velocities and the dielectric response, and the electron-phonon couplings from density-functional perturbation theory matrix elements at 34 explicit phonon wavevectors and from Wannier interpolation with the two-dimensional long-range treatment. The model therefore contains no fitted transport parameter and no coupling taken from another calculation, and its scattering rates are checked against an independent solution of the linearised Boltzmann equation with the same couplings. The EMC velocity-field characteristic feeds a one-dimensional channel model, verified against a direct self-consistent three-dimensional EMC simulation of the gated channel, which reproduces the measured transfer characteristics of a CVD monolayer device in air and vacuum. An ambient and adsorbate extension reproduces the measured conductivity response to oxygen partial pressure and the concentration dependence of NO2 and NH3 sensing of independent monolayer devices.

We investigate deterministic coarsening dynamics in a spatially extended bistable gene toggle model with diffusive coupling. Unlike classical curvature-driven coarsening, where domain walls move continuously and annihilate gradually, the present system exhibits a qualitatively different mechanism. The domain walls remain pinned for long intervals and disappear abruptly through collective cascade events. The density of domain walls decays approximately as $\rho(t)\sim t^{-\delta}$, but the coarsening exhibits clear log-periodic oscillations superimposed on the power-law behavior. For all values of the promoter strength $\alpha$ considered, the measured exponent satisfies $\delta<0.5$, indicating a systematic deviation from the classical Allen–Cahn prediction $\delta=1/2$ for curvature-driven coarsening. We show that log-periodic oscillations are not controlled by the density of domain walls, but by the domains that disappear in each cascade. The average size of disappearing domains grows roughly linearly with cascade index, producing a constant geometric spacing of cascade times, consistent with discrete scale invariance.

With infrared Fourier-transform spectroscopy and spectroscopic ellipsometry we investigated the in-plane reflectance of the superconducting Na$_{0.27}$K$_{0.27}$Rb$_{0.27}$Fe$_{1.7}$Se$_2$ single crystals with a critical temperature $T_c\approx 34.3$ K over a broad frequency range at temperatures of 4–300 K. The normal-state response of Na$_{0.27}$K$_{0.27}$Rb$_{0.27}$Fe$_{1.7}$Se$_2$ is analyzed by a Drude-Lorentz model with one Drude component. The temperature dependences of the plasma frequency, optical conductivity, scattering rate, and dc resistivity of the Drude component in the normal state are presented. We report a Fano-shaped mode at 1430~cm$^{-1}$ indicating a coupling of the discrete mode presumably related to some electronic transition to a two-magnon continuum as well as a low-frequency broad band assigned to the acoustic magnon branch of the antiferromagnetic superstructure. These features persist in the reflectance in both the normal and superconducting states suggesting a mesoscopic coexistence of superconductivity and magnetism in Na$_{0.27}$K$_{0.27}$Rb$_{0.27}$Fe$_{1.7}$Se$_2$.

Systems of hard rods of equal length in a one-dimensional harmonic trap have been observed to exhibit peculiar non-ergodic behavior that might suggest the existence of a novel hidden conserved quantity beyond the two well known ones, i.e., the total energy and the center-of-mass energy. In this work, we investigate this possibility by systematically constraining the forms of the conserved quantities, and we rigorously rule out the existence of any extra hidden conserved quantity that is analytic in the positions and momenta of the rods involved. We do so by showing two key results: conservation during free motion demands the $U(1)$ invariance of these quantities under rotations of the position and momenta of each rod, and conservation during collisions demand an $S_N$ invariance under the permutation of the momenta of the rods as long as one of the rods have non-zero length. We then show that these conditions imply that any conserved quantity is functionally dependent on the two known conserved quantities. In addition, we show that in the special case where all rods have zero length (i.e., when they are point particles), conservation under collisions only requires invariance under a smaller $S_N$ group of permutations of the labels of the rods, which leads to a much larger set of analytic conserved quantities that we explicitly write down. In all, this rigorously clarifies the structure of conserved quantities in the hard rod problem, and motivates the application of such systematic methods to other classical systems.

The Higgs and Nambu-Goldstone modes are paradigmatic collective excitations in superconductors and superfluids. These modes are commonly analyzed within the pseudospin formulation of BCS theory, where the dynamics are obtained from the Heisenberg equations of motion for pseudospins. However, this construction becomes inconsistent when directly extended to non-Hermitian systems. In this work, we develop a consistent pseudospin framework for non-Hermitian fermionic superfluids based on the metricized formulation of non-Hermitian quantum mechanics. We apply this formalism to a driven non-Hermitian BCS-type Hamiltonian with complex pairing interaction and analyze its collective excitation spectrum. We find that, in addition to the conventional Higgs (amplitude) mode, the system hosts a novel phase mode that has no counterpart in Hermitian superfluids. Remarkably, this mode is gapped even in the absence of the Anderson-Higgs mechanism, as is the case in neutral superfluids. Furthermore, the resonance spectrum depends explicitly on the initial nongauge phase of the complex order parameter and the dynamical response remains finite at resonance, in contrast to the divergence characteristic of Hermitian systems. These resonances disappear upon the emergence of exceptional points.

The generalized Lotka-Volterra (GLV) framework, recently advanced by dynamical mean-field theory, enables the systematic analysis of large ecological networks. When combined with structured interaction topologies, previous studies have shown that the viability of species, defined by having a positive stationary abundance, depends on the number of their interacting neighbors or the "degree." While such model studies usually assume that interaction strengths follow an identical distribution across all connected pairs, real ecological communities often exhibit correlations between interaction strength and a species' degree. To capture this overlooked feature, we introduce degree-dependent interaction strengths into a generalized random LV model. We identify two distinct regimes: a hub-favored phase, where highly connected species survive preferentially, and a hub-suppressed phase, where they face higher extinction risks. We analytically derive the phase boundary where these degree-dependent strengths precisely balance the connectivity effect, leaving all species equally susceptible. Crucially, these phases induce opposing shifts in the degree-degree correlation or "assortativity" of the network of viable species: the hub-favored phase enhances disassortativity by selectively removing the interactions between low-degree species, whereas the hub-suppressed phase reduces it as the interactions involving hub species tend to disappear. Ultimately, our findings demonstrate that degree-dependent interactions are a fundamental mechanism not only for shaping species survival, but for naturally reproducing the wide range of assortativity values observed in real ecological networks.

We observe pump-induced double anticrossings whose gap sizes and center-frequency shifts depend strongly on pump power, indicating a nonlinear mechanism. The anticrossings vanish at high magnetic fields, where energy conservation suppresses three-magnon splitting, thereby identifying the underlying process as three-magnon scattering. We attribute the double anticrossings to nondegenerate three-magnon splitting, which generates two magnon populations at distinct frequencies. Each population forms a standing-wave mode and couples independently to the Kittel mode, giving rise to two effective coupling channels. These results demonstrate that nonlinear magnon interactions can dynamically generate multiple coupling channels within a single system.

We investigate the dc magnetoconductivity of two-dimensional Dirac cones and gapped nodal rings (GNRs) subjected to a perpendicular magnetic field, which quantises the electronic spectrum into Landau levels (LLs). Working in the ultraquantum limit, where only the lowest LL (LLL) is partially occupied, we employ the Kubo–Bastin formalism to compute the transport coefficients for pointlike, Gaussian, and Yukawa impurity-potentials. For the Dirac case, the longitudinal conductivity is field-independent for pointlike impurities and a monotonic function of $B$ for the Gaussian and Yukawa potentials, while the Hall conductivity vanishes identically owing to the particle-hole symmetry of the two neighbouring LLs. The GNR case is qualitatively different: its non-monotonic stretched-checkmark LL spectrum causes the effective LLL to migrate to successively lower indices as the field increases, producing a pronounced oscillatory structure in both conductivities. The longitudinal response develops resonant peaks at LLL degeneracies, while the Hall conductivity traces out a sawtooth pattern with sharp zero-crossings at these same points. These results establish distinct transport fingerprints for the two systems in the extreme quantum limit, and provide a theoretical framework for interpreting magnetotransport experiments on GNRs.

Superconductivity in doped SrTiO$_3$ occurs over a wide range of carrier densities, including those for which the Fermi energy is below the polar longitudinal optical phonon scale. In this regime, the assumptions underpinning conventional implementations of Migdal-Eliashberg theory, including frequency cutoffs at the phonon scale and a Coulomb pseudopotential $\mu^\ast$, are not valid. We solve the finite-temperature $GW$ equations with full momentum and frequency dependence, without cutoffs or $\mu^\ast$, for polar one-band models of SrTiO$_3$, using effective masses and three-phonon dielectric functions parameterized from ab initio calculations. Comparing different self-consistency levels, namely $G_0W_0$, $GW_0$, and fully self-consistent $GW$, we find that the one-shot ($G_0W_0$) kernel overestimates the pairing-onset temperature by one to two orders of magnitude. The dominant suppression comes from replacing $G_0$ by $G$, thereby incorporating the phonon renormalization factor in the electron Green function. Using the self-consistently computed interaction $W$ further lowers and narrows the pairing-onset dome. In the dilute limit, our calculations identify the pairing channel as the Fröhlich phonon interaction screened by the incipient ferroelectricity of the material, with plasmonic and electronic screening effects negligible. The numerical solution of the full equations reveals a pairing-onset scale that remains non-zero as the density tends to zero, whereas Fermi-surface projection or Fermi-energy frequency truncation removes it. This work highlights the relevance of incipient ferroelectricity, the importance of self-consistency, and the need for a full momentum- and frequency-dependent treatment in modeling superconductivity in SrTiO$_3$-like doped polar semiconductors.

We develop an alchemical thermodynamic integration scheme that couples ab initio force calculators on the fly during Monte Carlo and molecular dynamics simulations. The implementation is validated against existing hybrid-Hamiltonian approaches. The scheme yields ab initio free energies of high-pressure Fe-Ni and ambient Li-Na liquid solutions that agree with previous calculations and reproduce the experimentally observed Li-Na miscibility gap. The code has an efficiency comparable to standard ab initio molecular dynamics. These results establish this scheme as a practical alchemical-integration framework for first-principles free-energy calculations in solutions.

Van der Waals MoS2 graphene heterostructures are compelling candidates for high performance electronic and optoelectronic device applications. However, the interlayer charge transfer typically quenches the photoluminescence of monolayer MoS2, limiting the use of these heterostructures in light emitting applications. In this work, we report an anomalous photoluminescence enhancement in n-type monolayer MoS2 by integrating it with monolayer graphene via polystyrene assisted wet transfer process. Photoluminescence spectroscopy reveals a dominant trion to exciton conversion in the heterostructure. Kelvin probe force microscopy shows a 600 meV increase in the work function of MoS2 upon heterostructure formation. This work function shift, together with the higher work function of graphene, signifies electron transfer from MoS2 to graphene. Shifts in the graphene G and 2D Raman modes further corroborate the interlayer charge transfer. Hydroxyl and epoxy functionalization of graphene following heterostructure formation is evidenced by X ray photoelectron spectroscopy. DFT based Bader charge analysis quantifies the role of these functional groups in facilitating interlayer charge transfer. Collectively, our findings establish polystyrene assisted wet transfer as a practical interface engineering strategy for enhancing excitonic emission in MoS2 graphene heterostructures, thereby advancing their potential for scalable optoelectronic devices.

We performed $^{75}$As nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements on a higher-quality single-crystalline CeRh$_2$As$_2$, a heavy-fermion superconductor exhibiting multiple superconducting (SC) phases under magnetic fields along the $c$ axis. This SC multiphase is believed to originate from staggered Rashba spin-orbit coupling associated with locally broken inversion symmetry. In addition to superconductivity, CeRh$_2$As$_2$ exhibits phase I below $T_0\sim0.5$ K and an antiferromagnetic (AFM) state below $T_{N}\sim 0.25$ K in the early-stage samples. In the higher-quality sample, the AFM transition becomes more pronounced, and $T_{N}$ increases to nearly coincide with $T_{SC}$. The NQR spectra at the As(1) site imply an internal field with an incommensurate distribution, indicating a two-dimensional incommensurate modulation of the magnetic structure superimposed on a $Q=0$ A-type AFM component. Moreover, a pronounced decrease in the NQR intensity at $T_0$ well-above $T_{N}$ and an abrupt increase in the internal field at $T_{N}$ suggest the emergence of a slowly fluctuating AFM order at $T_0$ which becomes static at $T_{N}$.

The quadrature scaling of magnetoresistance has been widely adopted as a hallmark of the strange metal state. However, whether this scaling signals quantum criticality or reflects conventional transport behavior remains controversial. Here, by systematically investigating the magnetotransport properties of NiTe2 nanosheets, we demonstrate that the quadrature scaling is not a unique signature of strange metallicity. We find that the scaling holds only when the crossover field , marking the transition from quadratic to linear magnetoresistance, is sufficiently small relative to the applied field range. Through controlled simulations, we show that the scaling emerges whenever linear magnetoresistance dominates, irrespective of its origin, and fails when the linear regime is inaccessible. This conclusion is supported by observations in SrTiO3 based heterostructures, where quadrature scaling appears despite the absence of strange metal behavior. Our results establish that the quadrature scaling merely reflects the presence of linear magneto resistance, urging caution in using this scaling as a diagnostic tool for exploring the strange metal state.

Materials with negative thermal expansion (NTE) are essential for applications requiring precise control of thermal expansion. Owing to their exceptional chemical tunability, flexible architectures, and low-energy lattice vibrations, metal-organic frameworks (MOFs) represent a rich platform for exploring NTE. However, uncovering the structural motifs that govern NTE across the enormous MOF design space remains experimentally challenging, and large-scale first-principles phonon calculations are computationally prohibitive. Here, we comprehensively evaluate the factors influencing NTE in MOFs by utilizing a high-throughput workflow based on MACE-MP-MOF0, a machine learning interatomic potential fine-tuned for MOFs with near-ab initio accuracy, to construct PhononMOFdb, a database of phonons, inelastic neutron scattering spectra, bulk moduli, and heat capacities for over 12,000 MOFs. High-throughput screening of this database reveals that highly porous cubic topology frameworks with heavier, lower-valent metal nodes favor strong NTE, while linker functionalization provides a practical handle for tuning NTE magnitude and sign without compromising mechanical stability. Experimental validation via high-resolution temperature-dependent synchrotron powder X-ray diffraction on the Ce-UiO-66 MOF and its brominated variants confirms the design recipe and yields volumetric NTE coefficients surpassing current records. This work establishes a data-driven strategy for engineering NTE in MOFs, showing how machine learning-accelerated discovery and targeted experimental validation together unlock predictive materials design.

Magnetron-sputtering inert-gas condensation (MS-IGC) provides a scalable, environmentally friendly vapour-phase synthesis approach for preparing customised nanoparticles (NPs) with bespoke properties via fine-tuning several deposition parameters. However, this high-level control comes with a caveat: the synthesis mechanisms are affected by deposition parameters in complicated ways, often yielding unpredictable outputs. This report details the working mechanism of a typical MS-IGC system, achieving in situ synthesis, size screening, and directional deposition of nanoclusters through the synergistic operation of the three vacuum chambers (condensation, screening, and deposition). The study systematically explores the regulation laws of multiple key process parameters (e.g., inert-gas flows, aggregation length, etc.) on the formation, size distribution, and deposition behaviour of nanoclusters, to rationalise their chosen values toward optimised output. To this end, multiple linear regression analysis was performed to isolate the effect of each deposition parameter and thus quantify its effect on the NP size and size distribution. Our results indicate that parameters that may prolong the nascent NPs' residence inside the condensation chamber (most prominently, the exit nozzle diameter) can positively affect the final NP size. This study expands the understanding of NP formation, enabling improved experimental control and process optimisation.

Disorder fundamentally reshapes how crystalline systems respond to external forces, yet it remains unclear whether disorder drives interacting lattices toward glassy states or instead fragments them into domains separated by mobile interfaces. Here, we investigate vortex motion in superconducting island arrays, where disorder is introduced in a controlled manner by tuning the magnetic field away from commensurate vortex fillings. By driving vortices with an applied current, we observe a two-step depinning transition at incommensurate fillings. Comparison with molecular vortex model simulations shows that this intermediate regime is consistent with domain wall motion in a polycrystalline vortex lattice. While two-step depinning has been explored theoretically in driven periodic systems, direct experimental evidence linking this behavior to interface-dominated vortex motion has been lacking. Our results demonstrate that disordered, interacting vortex systems with strong periodic pinning can favor interface physics over homogeneous glassy dynamics.

In recent years, interest in single-atom catalysts supported on carbon-based structures has grown considerably due to their high catalytic activity and efficient uses of metal atoms. However, the design and characterization of these materials through first-principles calculations are computationally expensive, limiting the exploration of a large number of possible configurations. Here, we developed a framework based on graph neural networks (GNNs) to predict the adsorption energies of transition metals on graphene quantum dots (GQDs). The model was trained using data obtained from density functional theory calculations and achieved an $R^2$ of 0.906 with an MAE of 0.101 eV, while reducing computational cost by roughly six orders of magnitude relative to DFT. This methodology provides an efficient tool for the accelerated screening and rational design of new catalysts based on carbon nanostructures.

The continuous demand for improved batteries motivates the discovery and advancement of materials with improved transport. Ionic diffusivity is the relevant material property where its measurement depends on accurate assessment of the active material length-scale, generally from the mass-specific surface area. In this perspective, we argue for renewed focus on diffusivity comparisons. A procedural review of 303 recent open-access publications about battery material development revealed two aspects: (1) 49% of publications support structure-property transport claims using diffusivity values and (2) of those reporting diffusivity values, 15% clearly stated that the length scale was measured after grinding-alone or stated that grinding was not used at all. Diffusivity assessment rationally requires length scale measurement after grinding (grind-measure), rather than the reverse. A range of measurement methods are compared, including SEM, BET, and SAXS as well as the resulting apparent diffusivities. Common errors and pitfalls of each of these approaches are described. As an example, datasets are presented for TiNb2O7 (TNO1) and Ti2Nb10O29 (TNO2) made from sol-gel (SOL) and solid state (SS) techniques using rigorous quantitative measurements to separately compare material diffusivities and galvanostatic performance. Here, the SOL samples had shorter length-scales and lower diffusion coefficients. Galvanostatic cell measurements, however, revealed that the shorter length-scales more than compensated for the lower diffusivities with better overall high-rate capacity retention. This example shows how cell level metrics often differ from underlying diffusivities. We argue that materials development needs renewed focus on property measurements like diffusivity where best-practices are important to derive meaningful insights towards structure-property relationships.

Pump-probe reflectivity reveals coherent acoustic oscillations at 33 and 8.2 GHz in a Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ thin film. The acoustic response was analyzed using a modified logistic-function model, suggesting a temporal redistribution of coherent acoustic energy consistent with anharmonic decay of the higher-frequency mode into the lower-frequency mode.

The Dicke model provides a fundamental description of collective light-matter interactions and has long served as a testbed for exploring a wide range of physical phenomena in quantum optics and condensed matter physics. In this work, we develop a variational framework for investigating the finite-size Dicke model on both fully qubit-based (digital) and hybrid qubit boson based (digital-analogue) quantum computing platforms. We show that the resulting model reproduces the characteristic critical behavior of the Dicke model in the appropriate large-spin limit while remaining suitable for implementation on both classical emulators of quantum computers and actual trapped ion quantum computers, albeit in the case of latter somewhat limited by noise. Finally, we introduce a complementary hybrid qubit-bosonic variational ansatz that directly exploits the bosonic degree of freedom to reduce quantum resources and discuss its potential implementation on hybrid quantum hardware. Our results establish a scalable, symmetry-aware framework for variational quantum simulations of collective light-matter systems and provide a pathway toward efficient simulations of more general spin-boson models on near-term quantum devices.

We investigate the dissipative dynamics of a generalized Lipkin-Meshkov-Glick (LMG) model coupled to a thermal environment. In this generalized model, in addition to the conventional quadratic interaction, one considers quartic interactions between spin-$1/2$'s coupled all-to-all and evolving in presence of a transverse field. Employing the usual linear Lindblad master equation with thermally-balanced jump processes, we derive magnetisation evolution equations, and demonstrate that the corresponding stationary solution reproduces the canonical equilibrium phase diagram of the model. We then extend our analysis to a nonlinear Lindblad equation that incorporates imperfect quantum-jump processes in terms of jump-retention parameters. Here, remarkably, the system relaxes to a genuine nonequilibrium stationary state whose properties differ qualitatively from those obtained in the linear case. The jump-retention parameters provide tunable knobs that shift the phase boundaries and even modify the nature of the phase transitions with respect to the linear case. Our exact results establish a direct connection between dissipative relaxation dynamics and stationary-state behavior, while identifying controlled quantum-jump retention as a mechanism for engineering nonequilibrium phases in long-range interacting open quantum systems.

We study the generation of state $k$-designs from time evolution under a fixed Hamiltonian. Specifically, we consider the ensemble $\mathcal{E}=\left\{e^{-iHt}|\psi_0\rangle | \ t\sim \mathrm{Unif}[0,T],\, |\psi_0\rangle\sim \mathcal{E}'\right\}$, where the initial states are sampled from an ensemble $\mathcal{E}'$. For Hamiltonians drawn from the Gaussian unitary ensemble, we derive a simple relation between the frame potential of the evolved ensemble $\mathcal{E}$ and that of the initial ensemble $\mathcal{E}'$ in the large evolution time limit. This relation shows that $\mathcal{E}$ forms an exact state $k$-design in the thermodynamic limit as long as $\mathcal{E}'$ forms a state 1-design. Remarkably, we further show, both analytically and numerically, that time evolution under a simple nonintegrable mixed-field Ising Hamiltonian can generate approximate state $k$-designs with high precision, starting from product states in an appropriately chosen Pauli basis. We also analyze the finite-$T$ correction and find it scales as $O(1/T)$. To reduce the evolution time, we propose an $M$-step quench protocol that suppresses this correction to $O(1/T^M)$, which is also verified numerically. We then extend our analysis to unitary ensembles, deriving an analogous recursion relation for the unitary frame potential. Our results elucidate the mechanisms underlying recent proposals for generating unitary $k$-designs through sequential quantum quenches in a unified manner.

The construction of machine-learned interatomic potentials (MLIPs) is often limited by the cost of generating large density-functional-theory (DFT) training datasets. For systematically generated structure pools such as ASSYST, a central practical question is how many configurations must be labeled to achieve reliable accuracy. Here we assess geometry-based, label-free subset selection for training linear Atomic Cluster Expansion (ACE) potentials. Using statistical leverage scores and CUR-type sampling, we compare leverage-guided selection against random, energy-based, and force-based baselines under controlled iterative protocols. Elemental Al provides the primary benchmark, with Cu and Al-Cu alloys used for transfer validation. Leverage-guided subsets recover plateau-level energy and force accuracy using substantially smaller labeled fractions (approximately 30-40%) than random sampling, corresponding to an effective 2-3x reduction in DFT labeling for the systems studied. In alloy tests, defect energetics remain comparable across strategies once sufficient chemical diversity is included, while leverage selection maintains competitive accuracy at reduced training size. These results demonstrate that descriptor-space-guided, label-free subsampling can significantly reduce DFT workload for linear ACE models trained on ASSYST structure pools without degrading defect-level fidelity.

We investigate the propagation of classical plane waves and surface plasmon-polariton modes within the framework of pseudo-electrodynamics (PED) supplemented by a non-local Chern-Simons (CS) topological term. Starting from the dimensionally reduced action, we derive the decoupled second-order wave equations for the electromagnetic field, and for the gauge-potential. In vacuum, we show that the topological CS parameter works as a dynamical mass generator, modifying the strict transversality of plane waves, and introducing a distinct energy gap in the dispersion relation. Considering a planar conducting medium that satisfies the Ohm law, the topological mass induces a novel hybridization mechanism between the Transverse Electric (TE), and Transverse Magnetic (TM) modes, a feature entirely absent in the conventional planar plasmonics. We analyze the asymptotic limits of this system, obtaining the exact real solutions in the lossless reactive Drude regime, and deriving the complex refractive index through a quasi-local approximation in the dissipative regime.

The physics of Wigner solids is characterized by an interplay of elasticity and long-range electrostatics, endowing crystal defects with properties distinct from those in charge-neutral crystals. Recent experiments observing lattice melting and Wigner-crystal-adjacent phases in two dimensions necessitate an examination of how defects affect the long-wavelength properties of such solids. Here, we use duality techniques to construct a comprehensive framework for studying the contribution of vacancies and interstitials, dislocations, and disclinations to the effective action of two-dimensional charged crystals. This allows for a systematic investigation of the interaction energies for the different combinations of defect pairs. We further study melting transitions due to defect proliferation, assessing and justifying some of the assumptions present in the literature. In the metallic Wigner crystal phase, characterized by a finite ground state density of vacancies, we find phonons with a dispersion relation that varies in an unusual way with the vacancy density. Consequently, we discuss how thermodynamic properties of such a vacancy Fermi liquid can be probed in measurements of the melting temperature and speed of sound. The field theory developed here can serve as a starting point in the study of anomalous Hall crystals and charge density waves with defects.

Iron-chromium-aluminium (FeCrAl) alloys are leading candidates for accident-tolerant fuel cladding in light-water reactors, where their superior high-temperature oxidation resistance promises to extend coping times during loss-of-coolant accidents. The in-reactor lifetime of cladding is ultimately governed by radiation-induced microstructural evolution of which point defect transport is the dominant mechanism, however, this remains poorly understood. Here, we use a species-resolved kinetic Monte Carlo (KMC) model for vacancy diffusion in FeCrAl, parameterised by linear surrogate models trained on a database of migration barriers generated through the Hop-Decorate workflow. By sampling compositions spanning the Fe-rich to Cr-rich range of the Fe-Cr-Al system, we map how the local chemical environment controls vacancy hopping and hence macroscopic diffusivity. We find that increasing the Cr content in the alloy progressively decreases global diffusivity of vacancies even though activation energies stay relatively constant. This implies that the higher the Fe content in the alloy, the faster vacancies diffuse, thereby increasing annihilation events with fast-moving interstitials, potentially reducing irradiation induced defects and increasing radiation tolerance. Conversely, we find that Cr-rich alloy compositions stand out with a markedly elevated activation energy and significantly slower diffusion, orders of magnitude lower at accident-relevant temperatures. This indicates suppressed vacancy mobility in Cr-rich $\alpha'$ phases which are known to form under irradiation.

Antennas transmit information wirelessly from one location to another via electromagnetic waves. Miniaturizing them, however, is challenging since the radiation efficiencies of all traditional antennas, based on the principles of classical electromagnetics, plummet when their dimensions are shrunk to tiny fractions of the radiated wavelength. Lately, a new generation of antennas whose operations are underpinned by non-classical principles have been demonstrated and they can overcome this limitation. This enables embedded applications that were hitherto inaccessible. In addition, beam steering, which normally requires a large phased array (multiple antenna elements each much larger than the wavelength), can now be accomplished with a single element much smaller than the wavelength. Some of these antennas also have stealth attributes for secure and covert communication, which makes this new genre a disruptive new technology.

The motion of liquids and soft matter is over-damped and `slow' when viscosity dominates. In this (low Reynolds-number) limit and when the system is isothermal, the equations of motion may be generated via Onsager's variational principle, which neglects inertia. This variational approach is immensely powerful, being used to obtain equations of motion for colloidal fluids, droplets on surfaces and much more. However, inertia can play a role, manifesting as vibrations and under-damped motion. Here we show how to extend this variational framework so that it remains valid for when damping/dissipation and inertia are both equally important.

We carefully investigate the recombination dynamics of localized defect emission in monolayer WSe$_2$ on SiO$_2$/Si substrate using time-resolved photoluminescence over the temperature range of 4 K to 120 K. We observe two long-lived optical lifetimes, one few nanosecond and one hundreds of nanoseconds. PL decay profile of these long lived states is fit well by a power-law, and based on laser fluence studies, the likely origin of the power-law is due to a distribution of life-times rather than many body interactions. Temperature-dependence of these rates shows that thermal detrapping governs these long-lived channels and we obtained a value of 60 meV for the characteristics energy in a Bose-Einstein model. We performed spin-resolved density functional calculations for selenium vacancies, the most likely source of native defects, to elucidate on spin- and momentum-forbidden pathways which are the likely origin of these long lifetimes. Such detailed understanding of long-lived defect states is crucial for quantum and optoelectronic applications.

Manganese telluride ($\text{MnTe}$) has recently emerged as a prototypical $g$-wave altermagnet, providing an ideal platform to investigate the non-equilibrium excitations of altermagnetic order. Here, we report simultaneous spatial mapping of the local equilibrium orientation of the Néel vector, $\varphi_L(\mathbf{r})$, alongside the amplitude, $\Delta\varphi(\mathbf{r},t)$, and frequency, $\Omega(\mathbf{r})$, of photoexcited spin waves. Based on these measurements, we place a remarkably low upper bound of $\approx 60~\mu\text{eV}$ (0.7 K) on the spin-wave gap arising from intrinsic anisotropy. This exceptionally weak hexagonal anisotropy ($K_6$) renders the altermagnetic order highly susceptible to optical tuning, allowing coherent spin waves to be driven by a photoinduced enhancement of $K_6$. Above a threshold pump fluence, our spatial maps reveal that this photomodulation manifests as a six-fold symmetric sawtooth dependence of $\Delta\varphi$ on $\varphi_L$ and a cycloid-like modulation of $\Omega$. Ultimately, the near-isotropy of the Néel vector in $\text{MnTe}$ enables optical and mechanical control over the orientation of spin-splitting in the electronic band structure, offering new pathways for altermagnetic spintronics.