Papers

Unidirectional wave propagation has emerged as a key concept in the dynamics of non-reciprocal mechanical and acoustic metamaterials. This work investigates two fundamentally distinct strategies for achieving directional wave propagation in a periodic one-dimensional mass-spring-damper lumped system: space-time modulation and spatially periodic feedback. In the first approach, the stiffness is modulated periodically in both space and time. The resulting space-time periodic system is analyzed using a Plane Wave Expansion (PWE) formulation based on the Bloch-Floquet theory to obtain the dispersion relation. The traveling modulation produces asymmetric dispersion diagrams and directional band gaps, within which elastic waves propagate preferentially in a single direction due to broken time-reversal symmetry. In the second approach, non-reciprocity is introduced through a spatially periodic feedback action. The force can depend on the displacement and/or its derivatives, such as velocity or acceleration, and is applied to the masses along the system. The lumped system with a finite number of unit cells is modeled using classical mechanics principles, yielding a state-space model of the system. The active mechanism can generate directional amplification or attenuation via the non-Hermitian skin effect (NHSE), characterized by boundary-localized modes identified by a topological invariant, the winding number. The stability of the space-time periodic system is assessed through the Lyapunov-Floquet theory. In the periodic feedback case, stability is investigated using the eigenvalues of the state matrix. These results provide design guidelines for directional wave propagation in elastic waveguides.

We summarized the room-temperature optical properties for infrared-transparent materials, defining transparency windows for two different applications: thin-film coatings (absorption coefficient $\alpha < 10 cm^{-1}$) and windows ($\alpha < 1 cm^{-1}$). The transparency requirements for thin films are substantially less stringent, enabling the use of many more optical materials for a given wavelength range. To make an easy-to-use reference, we categorized materials by chemical group, crystallinity, and typical deposition technique, and discussed practical pros and cons such as chemical and thermal stability, mechanical hardness, and cost. The datasets and plotting scripts are provided so that users can regenerate transparency-window maps for different applications. If you do optical design at infrared wavelengths, we recommend that you print out the figures and stick them on your wall.

Strong optical nonlinearities at the few-photon level are a central goal for quantum photonics, yet they remain difficult to realize in solid-state systems. In doped two-dimensional semiconductors, coupling between excitons and a degenerate Fermi sea gives rise to exciton-Fermi polarons, many-body quasiparticles whose optical response is governed by fermionic correlations. Here, using femtosecond pump-probe transient absorption microscopy, we directly image the spatially resolved nonlinear optical response of exciton-Fermi polarons in monolayer MoSe$_2$. We observe a pronounced spatial suppression of resonant absorption associated with the attractive Fermi polaron, from which we extract an optical blockade radius that is more than ten times larger than that of the neutral exciton. Microscopic analysis indicates that this extended nonlinearity arises primarily from fermion-mediated interactions between exciton-Fermi polarons. Our results establish exciton-Fermi polarons in two-dimensional semiconductors as electrically tunable, strongly interacting optical quasiparticles, and identify them as a promising platform for ultralow-power nonlinear optical devices.

The Chandler wobble (CW) – the $\sim$433-day free nutation of Earth's rotation pole – experienced an anomalous near-disappearance between 2015 and 2020, followed by a re-excitation with an approximately $180^{\circ}$ phase reversal. Using Multifractal Detrended Fluctuation Analysis (MFDFA) applied to more than six decades (1962–2024) of daily IERS EOP C04 polar motion data, this study provides the first multifractal characterisation of the CW and its recent anomaly. Global MFDFA shows that the residual polar motion components and the CW amplitude are genuine multifractal processes with strongly $q$-dependent generalised Hurst exponents and broad singularity spectra. Surrogate-data tests with shuffled and phase-randomised ensembles demonstrate that this multifractality originates from the combined action of long-range temporal correlations and heavy-tailed excitation statistics. A sliding-window analysis reveals a pronounced collapse in long-range persistence and multifractal spectral width of the geometric polar motion signal several years before and during the 2015–2020 amplitude minimum, indicating a genuine dynamical regime change rather than a simple suppression of oscillation amplitude. In contrast, the amplitude- and phase-related variables retain broad multifractal spectra and stable scaling exponents across all epochs, revealing a dynamical decoupling between the geometry of the CW and the multiscale structure of its amplitude and phase fluctuations. These findings highlight the CW amplitude as an exceptionally multifractal integrator of geophysical excitation and suggest that multifractal metrics may provide early-warning indicators of major transitions in Earth rotation dynamics.

The self-assembly of photonic nanostructures in insects involves chitin, proteins, and lipids. While synthetic photonic systems have been extensively studied, current lipid-based self-assembly systems are limited in periodicity to $68\,\text{nm}$ compared to photonic length scales ($\approx 450\,\text{nm}$) observed in biological organisms. We hypothesise that lipids facilitate how structural colour arises in vivo by acting as templates for the self-assembly of biopolymers via lipidic lyotropic liquid crystal mesophases. Here, we aim to understand and identify how structural colour is produced in insects by the co-assembly of lipids and biopolymers. We study the effect of biopolymers, pH, temperature, surface charge, and stability on lipid vesicles using dynamic light scattering, X-ray scattering, and zeta potential analysis. Using cryo-electron microscopy, we demonstrate that these vesicles interact with the biopolymers and generate periodic nanostructures with periodicities ranging from $700\,\text{nm}$ to $1.2\,\mu\text{m}$ (more than ten times larger than for purely lipidic systems) and dimensionalities ranging from 1D to 3D. Our results establish that lipid mesophases and biopolymers can induce reorganisation into ordered nanostructures, overcoming key limitations of periodicities achieved by lipid-only systems, and providing a methodology for recreating the physicochemical mechanisms underlying biophotonic structural colour.

Can a Laughlin anyon fluid become a topological superconductor? We answer this question for a $\nu=1/3$ fractional Chern insulator (FCI) using a $U(3)$ infrared parton theory. Three charge-$e/3$ constituents form the electron, while three constituent pairs form a gauge-invariant charge-$2e$ Cooper pair. The resulting superconductors share an ordinary charge-$2e$ sector but differ in their neutral color response, giving an SC$^\ast$ with the parent Laughlin $U(1)_3$ order, a chiral topological superconductor with $c_-=3/2$, and a strong-pairing molecular superconductor with $c_-=3$. The same framework organizes a nearby $\sigma_{xy}=0$ charge density wave (CDW) state. At commensurate filling, the normal state and the $c_-=3$ and $c_-=3/2$ superconducting descendants are naturally tied to a period-three density order background, while the SC$^\ast$ branch can preserve microscopic translations. Away from commensuration, a chargon metal can pair into the same $c_-=3/2$ topological superconductor. The FCI, reentrant CDW, and chiral superconductivity are unified in the $U(3)$ parton theory.

Calcium batteries (CBs) are an attractive post-Li-ion technology, offering the appeal of Ca's natural abundance and high volumetric energy density. However, practical realization of CBs remains limited by the scarcity of positive electrode (cathode) materials that support reversible Ca$^{2+}$ (de)intercalation under electrochemical conditions. To address this challenge, we screen the materials project (MP) database for novel host structures that can intercalate Ca using geometry- and chemistry-based design principles. Specifically, we employ the Voronoi polyhedral volume as a descriptor of site compatibility for hosting Ca in potential frameworks. Further, we down-select candidate structures progressively through criteria including charge neutrality, absence of non-Ca mobile cations, thermodynamic (meta)stability, average voltage, and Ca migration barriers ($E_m$) using foundational machine-learning (ML) models. Subsequently, we validate the ensemble-ML-predicted $E_m$ in a subset of the final candidates using density functional theory based nudged elastic band calculations. Overall, from an initial pool of 52,945 MP structures, our workflow identifies 37 promising Ca cathode candidates, several of which exhibit favorable combinations of thermodynamic (meta)stability, voltage, and Ca-mobility, marking them as strong candidates for synthesis and electrochemical characterization. Particularly, we identify two Ca cathode candidates with markedly low Ca$^{2+}~E_m$ (CaSc$_2$V$_2$O$_8$ and CaVSO$_4$F$_3$), and four cathode candidates with thermodynamically stable charged states (Ca$_3$(CoO$_2$)$_4$, Ca$_3$Mn$_4$(TeO$_6$)$_2$, CaVF$_5$, and CaVSO$_4$F$_3$). Beyond identifying Ca-cathodes, our work establishes geometry-based descriptors and ML-based workflows as transferable methods for high-throughput screening, enabling the rapid discovery of novel materials for battery and other applications.

Recently "the Hat" monotile was introduced into the family of aperiodic tilings and quasicrystals boasting physical properties lying at the boundary of ordered and disordered systems. Here we study the two-dimensional wave transport, transverse localization and scaling properties of the quantum modes in a Monotile quasilattice. Our system is based on reconfigurable optical lattices for cavity-polaritons which provide flexible means to study wavepacket dynamics, strong nonlinear phenomena, and power-driven condensation in this new type of an aperiodic tiling. We confirm the existence of localized and critical states in the Monotile through direct diagonalization of the Schrödinger equation. Scaling analysis on the moments of the wavefunction distribution reveals anomalous transport regimes of super-diffusive and near sub-diffusive polariton transport associated with the fractal structure of the Monotile Hilbert space. We propose a strategy using resonantly excited polariton fluids to verify our findings.

Stiff and dense fibrin networks in chronic blood clots impede drug penetration, limiting the efficacy of thrombolytic therapies. Acoustic cavitation of microbubbles is a promising strategy to enhance drug delivery in soft tissues. However, the interaction of these bubbles with stiff fibrin networks has yet to be investigated. Here, we show that ultrasound-driven bubbles undergoing periodic oscillations can penetrate and alter dense fibrin networks. The penetrated bubbles create three-dimensional paths that enable nanobeads (matrix transport markers) to infiltrate up to 200 \textmu m deep into the mesh. Radial bubble oscillation is found to be the dominant forcing mechanism on fibrin fibers. Combining mechanical measurements with these observations reveals that the radial stress from a single bubble oscillation is far below the fracture strength of fibrin fibers. Instead, repeated sub-fracture loading from thousands of oscillations progressively accumulates damage and softens the network until it yields - the plausible mechanism for bubble penetration through the fiber mesh. We further explored this fibrin softening at a range of peak applied forces. At low force, the fibrin network initially softens, but is resistant to further damage after hundreds of cycles. At higher forces, networks continue to soften without reaching a stable state, indicating the progressive accumulation of damage. These results show that cavitation can enhance matrix transport in dense fiber mesh by softening and structurally altering fibrin networks. The underlying physics is governed by the viscoplastic mechanics of bubble-fibrin interactions. These findings establish a mechanistic framework to design comprehensive treatment strategies for fibrotic aged clots.

Anyons are usually characterized by their topological data and their fractional quantum numbers under global symmetries. In lattice systems such as fractional Chern insulators (FCI), they are also mobile quasiparticles. Their motion controls the possible ground states of the dilute anyon gas obtained by doping an FCI, including possible superconducting states. We show how tunneling probes can measure this motion. In scanning tunneling spectroscopy, weak disorder produces spatially oscillating quasiparticle-interference patterns whose branches reveal the dispersion of fractionalized constituents. In quantum twisting microscopy, planar momentum-conserving tunneling selects the total momentum of the injected electron, so the continuum thresholds of fractionalized electron spectra encode the dispersion of the constituent anyons. The resulting spectra distinguish compact electron-like excitations, bound anyon molecules, and unbound anyon continuum.

We consider systems characterized by the presence of a rapidly oscillating force. A general method is presented for the construction of the effective action governing the large-scale nonlinear dynamics of such systems order by order in inverse powers of the oscillation frequency $\omega$. The explicit expression for the effective Lagrangian is derived up to ${\cal O}(1/\omega^6)$ next-to-next-to-leading approximation. The general structure of the high-frequency expansion reveals a broad class of nonlinear systems whose transition curves are identical to those of the linear Mathieu equation, which enables a fully nonperturbative stability analysis in the case of strong driving and nonlinearity. The method is generalized to velocity-dependent forces and configuration space with curvature, characteristic to systems with constraints. Several applications are discussed in detail, including the dynamical magnetic trapping of electric charges.

We investigate memory effects and quantum transport in two-dimensional lattice systems within the framework of non-equilibrium Green's functions and Schwinger-Keldysh non-equilibrium quantum field theory. Starting from a 2D tight-binding Hamiltonian, we employ the Dyson expansion on the Keldysh contour and the second-order Born and self-consistent Born Approximation to derive the electronic self-energies associated with elastic and inelastic scattering mechanisms.Static disorder produces a local self-energy and a rapidly decaying memory kernel, characteristic of Markovian dynamics, whereas electron-phonon coupling generates temporally nonlocal self-energies and genuine Non-Markovian behavior. We demonstrate that these distinct memory signatures are directly reflected in the spectral function, which we propose as a diagnostic probe of non-equilibrium memory effects. Further we explore 1PI and 2PI effective actions to see their memory perspectives studying their coarse-graining behavior. Building on this theoretical framework, we further apply the conventional NEGF formalism to two paradigmatic two-dimensional models-the Hofstadter and an RKKY-coupled system to explore how different microscopic Hamiltonians influence Markovian and Non-Markovian nature. Our results provide a unified connection between scattering mechanisms, memory effects, and quantum transport in low-dimensional systems.

Entanglement and quantum correlations are central to the physics of quantum materials, yet they have remained notoriously difficult to access experimentally. Accessing these phenomena in solids requires quantum optical probes that operate at the native length and time scales of material excitations, below the diffraction limit of light. Developing the requisite tools has previously been infeasible due to the weak intensities of state-of-the-art quantum light sources and the inefficiency of light coupling in near-field light-matter interactions. In this work, we address these challenges and report the development of a quantum light scattering-type scanning near-field optical microscope (q-SNOM) that enables quantum-optical studies of solid-state systems with nanoscale spatial resolution. As a first demonstration, we visualize the self-interference of single hybrid light-matter polaritons in the prototypical van der Waals semiconductor MoS2. We also introduce a polaritonic time-of-flight metrology that exploits the temporal correlations among entangled photons to observe the quasiparticle propagation dynamics at femtosecond time scales. This work establishes a new experimental paradigm for exploring quantum effects in materials at the nanoscale.

We propose a local notion of strong-to-weak spontaneous symmetry breaking (SW-SSB), through a local one-point fidelity correlator. Compared with the previous definition in terms of a two-point fidelity correlator, our local formulation offers two key advantages: (1) it is easier to detect in large systems: for a system of size $N$ and with ${\rm poly}(N)$ amount of resources, one can detect the local fidelity order up to volume scale $O(\log(N))$; and (2) the local SW-SSB order remains well defined in the thermodynamic limit, where the density matrix itself is not well defined. We show that key features of SW-SSB, including stability under finite-depth symmetric channels and long-range conditional mutual information, persist within this local framework. Our definition is conceptually analogous to local thermalization, as exemplified by pure states obeying the eigenstate thermalization hypothesis (ETH). For critical states, the local one-point fidelity correlator defines an interesting class of defect problems. We demonstrate the applicability of the local formulation through several concrete examples, and derive the universal scaling behavior of the local fidelity correlator in a range of critical systems, including ground states of conformal field theories as well as ballistic and diffusive free-fermion metals.

Recent cold atom experiments have realized one-dimensional anyons and enabled the tuning of 1D~statistics between bosons and fermions. Here, we analyze the symmetries, integrability, and resulting degeneracies of the underlying anyon-Hubbard model of finite length. Our results reveal a switching between symmetry classes AI, BDI, and CI in dependence on system size, particle number, and boundary conditions, and show that two anyons with periodic boundaries are integrable, while two anyons with open boundary conditions are not. We include a comprehensive analysis of all model limits, especially of interacting bosons and pseudofermions and resolve spectral signatures. We additionally reveal an exactly solvable doublon state that hides in the continuum of scattering states and the exact solution of the nullspace of two noninteracting anyons. The uncovered symmetries shape the fundamental properties of the one-dimensional anyons at hand, and the predicted states are accessible in state-of-the-art experiments.

We prove a trade-off theorem for order and disorder parameters in one-dimensional quantum spin systems with quenched disorder. For a disordered ensemble with exact Ising symmetry and average translation symmetry, any gapped ensemble must have one and only one of the following: an $O(1)$ order parameter or an $O(1)$ disorder parameter with even parity, both of the Edwards-Anderson type. The result extends to nearly gapped ensembles that accommodate Griffiths-type rare-region effects. These results offer a powerful and rigorous framework to understand the disorder effects beyond perturbative approaches. As applications, we (1) establish the existence of string order parameters for SPT phases; (2) derive a Lieb-Schultz-Mattis-type constraint for disordered ensembles, which requires a nearly gapped ensemble to spontaneously break the symmetry; and (3) discuss similar trade-off relations for disordered fermion chains, leading to an improved understanding of certain "intrinsically disordered" topological phases.

We review the notion of Yang-Baxter integrability for spin chains that have Hilbert spaces with constraints, such as a Rydberg blockade. We focus on anyonic chains, whose constraints arise from the fusion rules of the fusion categories on which they are based. We discuss the emergence of Temperley-Lieb algebras and present a new result on which types of anyonic chains exhibit them. We then give an overview of known results for integrable anyonic chains and extend them to several fusion categories up to rank $7$. Using a modification of the boost operator formalism, we find several new integrable anyonic chains and discuss some of their properties. These include spin-$\frac32$ models for $\mathfrak{su}(2)_k$ fusion categories, anyonic chains based on the Tambara-Yamagami fusion categories TY$(\mathbb{Z}_n)$, and product fusion categories Fib$\times$Fib and Fib$\times$Ising. We review recent results for spin chains based on the Haagerup-Izumi fusion category HI$(\mathbb{Z}_3)$, and present preliminary numerics for a HI$(\mathbb{Z}_5)$ model.

We introduce a framework for emulating graphs and, through them, curved spaces of arbitrary dimension, using arrays of superconducting wires. The array consists of two stacked layers of wires, horizontal and vertical, such that wires are parallel within each layer and perpendicular between layers. By discretizing a space into a graph, assigning a superconducting wire with a rigid phase to each vertex, and coupling pairs of wires through Josephson junctions according to the graph edges, arbitrary geometries and topologies can be engineered in a controlled setting. The superconducting phases then realize scalar field theories on the emergent geometry. We establish experimentally realistic conditions for implementing these architectures and develop a dictionary relating measurable circuit observables to quantities in the emulated field theory. As an application, we develop the implementation of hyperbolic (Anti-de Sitter) spaces of constant negative curvature and use them as an experimentally accessible platform to explore holographic duality in arbitrary dimensions. We investigate the effects of disorder in the Josephson couplings, which translate into metric variations in the bulk-boundary correspondence, and analyze their impact on boundary scaling exponents both analytically and numerically, finding that holographic duality remains robust even in the presence of strong disorder. Beyond holography, the framework opens a broad range of architectural possibilities, including the exploration of physics on highly nontrivial graphs and toy models of dynamical spacetimes.

We develop a controlled diagrammatic framework for periodic Anderson models,and apply it to heterostrained magic-angle twisted bilayer graphene (MATBG) at charge neutrality using the topological heavy-fermion formulation. Building on arXiv:2604.14278, we organize self-energy insertions and perform a Dyson resummation to any order in the small parameter $s^2$ – the fraction of the moiré Brillouin zone with nontrivial quantum geometry. For strained MATBG, the expansion remains controlled down to arbitrarily low temperatures as long as the strain induced energy scale is not too small. In the flat-chiral limit, an emergent approximate $\rm{U}(1)$ symmetry forbids the leading scattering channel and leaves the Mott bands sharp at order $s^2$. This is in stark contrast to the unstrained case, where the linewidth is of order $N_f s^2 U$ with $U$ the on-site $f$-$f$ Hubbard interaction and $N_f$ the number of $f$ states per site. Away from the chiral limit, the linewidth is non-zero at order $s^2$ but more than an order of magnitude smaller than in the unstrained case. The strain-induced energy scale also imprints itself directly on the spectrum: as an electron-phonon-like kink in the dispersion, and as an additional flat “trion” band – a single-particle excitation bound to a local $f$ particle-hole pair. We use the framework to predict the Quantum Twisting Microscope spectrum at one-loop order for both strained and unstrained MATBG, and compare with recent experiments.

We introduce FunKit, a Mathematica package for the derivation and tracing of functional equations from arbitrary master equations. FunKit provides an expression vocabulary and a set of rules that allow for derivations in any given field theory and master equation. It also allows users to add extensions for more specific equation systems. Therefore, it can be used in a wide range of situations, for example Dyson–Schwinger or functional RG equations, flowing reparametrisations, nPI equations, (modified) STIs and WTIs, functional Polchinski and Wegner flows, functional master equations with sources, and many others. Besides interfacing with the \FORM language to trace large tensor expressions efficiently, FunKit also provides facilities to export arbitrary Mathematica expressions to C++, Julia or Fortran code, including the results of derivations, which can then be evaluated numerically. Both the tracing and code generation can also be used independently and in combination with other packages.

We investigate $O(N)$ boundary conformal field theories (BCFTs) with boundary interactions in $d=4-\epsilon$ and $d=3-\epsilon$ employing the analytic bootstrap. By deriving universal constraints on conformal data, we show that infinitely many operator expansions can be expressed in terms of a finite set of inputs. Complementing the analytic bootstrap with a perturbative renormalization-group analysis, we identify totally new boundary fixed points in $d=4-\epsilon$, including non-unitary ones, generated by a boundary cubic coupling, and compute their conformal data to leading order. Moreover, we leverage our solution in $d=3-\epsilon$ to extract, for the first time, the boundary conformal data for the tricritical $O(N)$ model. Altogether, our approach provides a unified prescription for BCFTs with interacting boundaries and streamlines the determination of bulk and boundary operator expansions.

We investigate variational learning of quantum many-body ground states directly in measurement space using autoregressive neural networks. In particular, we represent quantum states via probability distributions of outcomes over a symmetric informationally complete positive operator-valued measure (SIC-POVM). The probability distribution is encoded in the parameters of an autoregressive neural-network-based on gated recurrent units (GRUs). Ground states are obtained by gradient descent that updates the probability distribution to minimize the energy with respect to a given Hamiltonian, while enforcing positivity constraints that ensure that the distribution of measurement outcomes correspond to a physical quantum state. We analyze the role of constraint enforcement (hierarchy of positivity conditions), variety of neural network architectures (multiple layers, dilation, and modifications of input data) in determining the success of this approach. We benchmark our approach on one-dimensional transverse-field Ising model and the Heisenberg model, along with gapping fields, for system sizes up to L=128, illustrating its efficacy across a wide variety of models.

Finite-size effects limit the accuracy with which conformal data can be extracted from lattice simulations of critical systems. While action improvement suppresses some corrections to scaling, it does not address operator-dependent effects arising from imperfect lattice representations of continuum conformal fields. In this work, we propose a data-driven method for improving lattice operators themselves, constructing estimators with enhanced overlap with the corresponding primary operators of the continuum conformal field theory. We identify improved lattice representations of leading spin and energy operators in three two-dimensional critical systems: the Ising model, the q = 3 Potts model, and the dilute q = 3 Potts model. In all cases, the resulting operators exhibit reduced corrections to scaling and yield more accurate estimates of scaling dimensions compared to conventional lattice choices. The code and analysis workflows used to produce these results are made available in an accompanying GitHub repository.

In mixed states of quantum systems, symmetries come in two types: strong and weak. Furthermore, it has been argued that in quantum many-body systems, strong symmetries can be "spontaneously broken" down to weak symmetries. An issue is that as previously formulated, such "strong-to-weak symmetry breaking" appears to be a fairly non-local effect. In this paper, we show how to understand and diagnose strong symmetries and strong-to-weak symmetry breaking in an explicitly local way. Our main technical tool is a rigorous definition of strong symmetry in the limit of infinite volume, which generalizes the conventional finite-volume definitions, and for which we give several equivalent formulations, including one involving the concept of "local charge coherence". Finally, we introduce von Neumann systems, which in infinite-volume symmetries are intermediate between strong and weak symmetries. We derive a Lieb-Schultz-Mattis type anomaly constraint for von Neumann symmetries (and therefore, in particular, strong symmetries) in quantum spin chains.

We construct a continuous-time, positively divisible non-Markovian process with memory of the initial state that satisfies the differential Chapman–Kolmogorov equation. In the stationary state, the correlation function exhibits exponential decay, a behavior typically regarded as characteristic of Markovian dynamics. Nevertheless, the memory is preserved throughout the evolution of the process, manifesting itself in observable statistical quantities. We further demonstrate that mutual information serves as a reliable measure of the non-Markovian character of the process. As an application, we study a random walk driven by the constructed process and show that the memory effect breaks ergodicity and modifies transport properties such as the diffusion coefficient.

The antiferromagnet on the ruby lattice is expected to host a range of exotic emergent phenomena, yet its material realization has remained elusive. Here we show that the layered transition metal fluorides CsBaFe$_3$F$_{12}$ and CsBaCr$_3$F$_{12}$ with Fe$^{3+}$ and Cr$^{3+}$ ions realize only slightly distorted ruby lattice geometries with spin moments $S=5/2$ and $S=3/2$, respectively. Their microscopic Hamiltonians, calculated with DFT energy mapping, are dominated by short-ranged antiferromagnetic interactions within the ruby layers. Classical Monte Carlo simulations reveal strong frustration in both compounds, with local Néel correlations on the hexagonal plaquettes and distinct long-range ordering tendencies governed by weaker triangular links. For CsBaFe$_3$F$_{12}$, the calculated thermodynamic behaviour is consistent with the experimentally reported magnetic ordering scale. For CsBaCr$_3$F$_{12}$, classical Monte Carlo and Luttinger-Tisza analysis reveal competing low-energy ordering wave vectors, strong finite-size sensitivity, and a tendency toward incommensurate order. Overall, our results establish these fluorides as experimentally accessible ruby-lattice antiferromagnets and provide quantitative predictions for future neutron-scattering studies.

Two-dimensional van der Waals materials exhibit a variety of correlated electron phases, and optical driving offers a promising route toward manipulating them. For example, cavity-enhanced, continuous-wave (CW) Raman excitation has been suggested as a way to coherently and superradiantly populate phonons or charge density waves via material excitons. A steady-state phonon population may be sustained with sufficiently strong electron-phonon coupling to drive novel collective response. We describe an apparatus built to meet the requirements of such an experimental program: Namely, an ultrahigh-vacuum system housing a length-tunable confocal Fabry-Pérot cavity with an intracavity sample, both cryogenically cooled and stabilized against vibrations. A four-axis nanopositioner aligns the sample and supports electrical leads for sample carrier density modulation and transport measurements. Transmission through the multimode cavity enables in situ sample imaging for alignment; the sample is a transition metal dichalcogenide in this work. Operating near the confocal geometry concentrates the optical field into a localized supermode that substantially enhances light-matter coupling. This enhancement is preserved despite the millimeter-scale cavity length, which provides room for sample alignment and exchange.

Ferroelectric materials can be used for the development of multiple device concepts combining non-volatility, small dimensions, low-power actuation, and electrical tunability. Such development demands efficient and precise design of simulation tools describing the polarization texture. However, most existing ferroelectric solvers are CPU-based and rely on simplified electrostatic treatments and reduced-dimensional representations of the polarization field. These approximations limit their ability to capture finite-size and boundary effects and restrict the range of domain structures and domain walls that can be realistically simulated. Here, we present a fully GPU (graphics processing units)-accelerated and scalable numerical solver, named PETASPIN_microelectrics, for computing the full polarization vector field of ferroelectric systems using the Ginzburg-Landau formalism. Our solver incorporates an optimized and validated calculation of the full electrostatic field and enables the parallel execution of multiple simulations. We systematically validated the solver with several benchmark problems, including phase transitions in BaTiO3 and ferroelectric domain wall profiles. Our simulations reproduce temperature-driven hysteretic phase transitions in BaTiO3. We also reproduce hysteresis loops and demonstrate stabilization of a three-dimensional hybrid skyrmion in a PbTiO3/SrTiO3 bilayer system. Our results show quantitative agreement with predictions from an analytical theory and prior experimental studies. The proposed solver provides an efficient, accurate platform for large-scale simulations of ferroelectric materials including stabilization of topological textures supporting predictive modeling for next-generation of ferroelectric device design.

Experimental synthesis of theoretically predicted materials with controlled elemental coordination environments can lead to realization of useful properties, such as facile ion transport or ferroelectric switching. Among such materials are new ternary nitrides in the Mg-W-N composition space, where several new stable and metastable compounds have been predicted and synthesized recently in bulk and film forms. Here, we report for the first time on the bulk synthesis of Mg$_3$WN$_4$ in a wurtzite-derived crystal structure via a solid state metathesis reaction. $In$ $situ$ synchrotron powder X-ray diffraction shows how the ion exchange proceeds from Li$_6$WN$_4$ + 3 MgCl$_2$ precursors to Mg$_3$WN$_4$ + 6 LiCl products, with the reaction starting slowly near 380 $^\circ$C and completing by 600 $^\circ$C, including the presence of a competing disordered rocksalt-derived phase (Mg,W)N above 440 $^\circ$C. The follow up $ex$ $situ$ powder synthesis at 400 $^\circ$C for 0.5 hour with 10% excess MgCl$_2$ reveals the cation-ordered nature of the wurtzite-derived Mg$_3$WN$_4$ structure with polar symmetry confirmed by second harmonic generation measurements. Optical absorption spectra, chemical composition analysis, and electron microscopy imaging suggests that bulk wurtzite Mg$_3$WN$_4$ is prone to defect formation. Overall, this study shows that selective $ex$ $situ$ synthesis of the phase pure ternary nitrides, informed by in situ measurements, is possible by carefully controlling the thermal budget of the reaction, and paves a way towards property characterization of wurtzite Mg$_3$WN$_4$.

Liquid water is fundamentally important, and its accurate computer simulation has been the driving force for myriad methodological developments. Ab initio molecular dynamics with forces obtained from density functional theory (DFT) is now a standard tool widely used by researchers. However, we reveal that previous studies of liquid water using the same widely-used density functional (revPBE-D3) exhibit significant discrepancies with one another, varying by over 20% in the diffusion coefficient and 10% in the density, raising fundamental questions about reproducibility. By combining modern long-range machine-learning interatomic potentials that enable robust statistical sampling with carefully converged DFT training data, we resolve these discrepancies, achieving consensus across six diverse community codes. Our predictions differ markedly from previous literature: we show that most previous results overestimate the density and underestimate the diffusion coefficient of revPBE-D3 water due to basis set incompleteness and pseudopotential inconsistencies, coupled with limitations in statistical sampling (in some cases). These benchmark values provide a reliable reference for validating current and future implementations of DFT-based ab initio molecular dynamics. Reaching agreement establishes confidence and credibility and serves as a prerequisite for the systematic assessment of new density functionals and numerical approximations.