We investigate the decay of spatial correlations of $\mathcal{PT}$-symmetric non-Hermitian one-dimensional models that host higher-order exceptional points. Beyond a certain correlation length, they develop anomalous power-law behavior that indicates strong suppression of correlations in the non-Hermitian setups as compared to the Hermitian ones. The correlation length is also reflected in the entanglement entropy where it marks a change from logarithmic growth at short distance to a constant value at large distance, characteristic of an insulator, despite the spectrum being gapless. Two different families of models are investigated, both having a similar spectrum constrained by particle-hole symmetry. The first model offers an experimentally attractive way to generate arbitrary higher-order exceptional points and represents a non-Hermitian extension of the Dirac Hamiltonian for general spin. At the critical point, it displays a decay of the correlations $\sim 1/x^2$ and $1/x^3$ irrespective of the order of the exceptional point. The second model is constructed using unidirectional hopping and displays enhanced suppression of correlations $\sim 1/x^a, a\ge 2$ with a power law that depends on the order of the exceptional point.

Experimental studies of Be-doped GaN by photoluminescence (PL) confirmed theoretical predictions that the ${\mathrm{Be}}_{\mathrm{Ga}}$ acceptor in GaN has dual nature, namely a coexistence of a deep state with a localized hole and a shallow state with a delocalized hole. Electron transitions via the deep polaronic states of this defect cause the broad yellow luminescence band with a maximum at 2.1–2.2 eV. Analysis of the PL dependence on temperature reveals two polaronic configurations (labeled Be1 and Be2) with the −/0 transition levels at 0.3–0.4 eV above the valence band. The Be1-related luminescence band transforms into the Be2-related luminescence band at a critical temperature of about 100 K. In addition, the shallow state of the ${\mathrm{Be}}_{\mathrm{Ga}}$ (Be3) is found at 0.2 eV above the valence band. The associated ultraviolet PL band with the main peak at 3.26 eV can be activated by transitions of holes from state Be2 to state Be3 over a potential barrier at $T>100$ K. The origin of another Be-related acceptor with a level at 0.113 eV above the valence band, which is responsible for the ultraviolet PL band with a maximum at 3.38 eV, remains unknown. We do not find experimental evidence for the dual nature of other acceptors, such as ${\mathrm{Mg}}_{\mathrm{Ga}}$ in GaN and ${\mathrm{Li}}_{\mathrm{Zn}}$ in ZnO.

Ultrastrong coupling between optical and material excitations is a distinct regime of electromagnetic interaction that enables a variety of physical phenomena. Traditional ways to ultrastrong light-matter coupling involve the use of some sorts of quantum emitters, such as organic dyes, quantum wells, superconducting artificial atoms, or transitions of two-dimensional electron gases. Often, reaching ultrastrong coupling requires special conditions, including high vacuum, strong magnetic fields, and low temperatures. Recent reports indicate that a high degree of light-matter coupling can be attained at ambient conditions with plasmonic meta-atoms—artificial metallic nanostructures that replace quantum emitters. Yet, the fundamental limits on the coupling strength that can be attained with this platform have not been identified. Here, using a Hamiltonian approach we theoretically analyze spectra of polaritonic states and examine the upper limits of the collective plasmon-photon coupling strength in dense assemblies of plasmonic meta-atoms. Starting off with spheres, we identify the universal upper bounds on the normalized collective coupling strength $\eta$ between ensembles of plasmonic meta-atoms and free-space photons. Next, we examine spheroidal metallic meta-atoms and show that a strongly elongated meta-atom is the optimal geometry for attaining the highest value of the collective coupling strength in the array of meta-atoms. The results could be valuable for the field of polaritonics studies, quantum technology, and modifying material properties.

Gallium nitride has been studied thoroughly in recent decades as the material is relevant for applications in blue-light-emitting devices and laser diodes, as well as high-power and high-frequency transistors. This material has rarely been explored by cross-sectional scanning tunneling microscopy (X-STM), in spite of the relevance of its $m$ plane, which constitutes the output facet of commercial lasers and the sidewalls of GaN nanowires, and has been intensively explored as an alternative orientation for light emitters due to its nonpolar nature. Here we successfully investigate a clean $m$ plane of wurtzite GaN and study the presence of gallium vacancies by X-STM at liquid nitrogen temperature. We observe Friedel oscillations around the defects and characterize the dependence of the oscillation radius on the applied bias. This physical phenomenon, combined with the tunneling conditions, clarifies the charge state of the vacancy and the origin of the free carriers in GaN that screen the Ga vacancies. We observe a predicted bi-stability of the N atom neighboring the Ga vacancy and perform density functional theory calculations to complement the experimental results. We obtain high-quality clean cleavages on the GaN $m$ plane, which will allow for the study of other nanostructured GaN-based materials and devices.

The quantitative measurement of charges at the nanoscale yields important insights into fundamental physical, chemical, or biological processes. In principle, charges can be probed by the sharp tip of a dynamic atomic force microscope, however, quantitative measurements are still a challenge as a large number of parameters and effects contribute to the measured signal. Here, we introduce the fundamental framework for charge force microscopy (CFM) and investigate charges located in, on, or above the surface of a dielectric substrate supported by a metal electrode. We present a comprehensive analysis of CFM signal generation and, in particular, unravel the dependency of the CFM signal on the probe oscillation amplitude, on system parameters such as the substrate dielectric constant or the tip geometry, and on the vertical and lateral position of charges. Most importantly, we untangle the influence from nearby charges when quantifying the magnitude of a central charge of interest in presence of many surrounding charges. We find that charge quantification from regular imaging bears many ambiguities, while mapping the CFM signal perpendicular to the sample surface allows to untangle many signal contributions. Thus, by accounting for measurement parameters and nonlocal influences, quantitative measurements are possible with CFM.

Maxwell lattices, where the number of degrees of freedom equals the number of constraints, are known to host topologically protected zero-frequency modes and states of self-stress, characterized by a topological index called topological polarization. In this Letter, we show that in addition to these known topological modes, with the help of a mirror symmetry, the inherent chiral symmetry of Maxwell lattices creates another topological index, the mirror-graded winding number (MGWN). This MGWN is a higher-order topological index, which gives rise to topological zero modes and states of self-stress at mirror-invariant domain walls and corners between two systems with different MGWNs. We further show that two systems with the same topological polarization can have different MGWNs, indicating that these two topological indices are fundamentally distinct.

We show how resonant (near-field) coupling affects wave transport in disordered media through ultrasonic experiments in concentrated suspensions. The samples consist of resonant emulsions in which oil droplets are suspended in a liquid gel. By varying the volume fraction of droplets $\varphi$ up to 40%, the limits of the independent scattering approximation are experimentally demonstrated as soon as $\varphi >10\%$. For these concentrated samples, the proximity of resonant scatterers induces a renormalization of the surrounding medium, leading to a reduction in scattering strength. Hence, maximum scattering is reached at an intermediate droplet concentration $(20\%<\varphi <30\%)$ where subdiffusive wave transport is observed. These results are very relevant for designing materials for the study of wave transport phenomena such as Anderson localization.

We study a Heisenberg-Dzyaloshinskĭ-Moriya Hamiltonian on AB-stacked kagome bilayers at finite temperature. In a large portion of the parameter space, we observe three qualitative changes upon cooling the system: a crossover from a Heisenberg paramagnet to an XY chiral paramagnet, a Kosterlitz-Thouless transition to a chiral nematic phase, and a fluctuation-induced first-order transition to an Ising-like phase. We characterize the properties of phases numerically using Monte Carlo finite-size analysis. To further explain the nature of the observed phase transitions, we develop an analytical coarse-graining procedure that maps the Hamiltonian onto a generalized XY model on a triangular lattice. To leading order, this effective model includes both bilinear and biquadratic interactions and is able to correctly predict the two phase transitions. Lastly, we study the Ising fluctuations at low temperatures and establish that the origin of the first-order transition stems from the quasidegenerate ring manifold in the momentum space.

Excitons in quantum dots are excellent sources of polarization-entangled photon pairs, but a quantitative understanding of their interaction with the nuclear spin bath is still missing. Here we investigate the role of hyperfine energy shifts using experimentally accessible parameters and derive an upper limit to the achievable entanglement fidelity. Our results are consistent with all available literature, indicate that spin noise is often the dominant process limiting the entanglement in InGaAs quantum dots, and suggest routes to alleviate its effect.

Due to both its scale and complexity, medium-range order is difficult to study. In this paper, we present a computational tool which identifies the shortest path of bonded atoms which connect any pair of atoms an arbitrary distance apart. We call these atomic structures chains and they are well suited for characterizing medium-range order. We apply this tool to force enhanced atomic refinement produced models of 50% zirconia tantala. These models show a systematic change in the medium-range order caused by heat treatment, which, by using this tool, we attribute to chains becoming kinkier. We also perform an in-depth analysis on specific chains to demonstrate that peaks seen in the pair distribution function are correlated to each other. Zirconia tantala is used as an example of the utility of this tool, but we can easily apply it to other materials.

Antiferromagnets have attracted growing attention owing to their intriguing characteristics and promising applications in the field of spintronics. In this work, the antiferromagnetic attribute of single crystallin ${L1}_0\mathrm{MnPd}$ films epitaxially grown on MgO(001) substrates is proven by the exchange bias observed in ${L1}_0 \mathrm{MnPd}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ bilayers. The Néel vector in ${L1}_0\mathrm{MnPd}$ films is verified to be aligned along the $c$ axis by magnetoresistance measurements in longitudinal, transverse, and polar geometries of the external magnetic field. Moreover, the anomalous Hall effect is observed to become weak with increasing MnPd film thickness, indicating the existence of uncompensated magnetic moments on the surface and the collinear spin structure of ${L1}_0\mathrm{MnPd}$ films. Finally, exhibiting an anomaly at low temperatures, the sheet resistivity consists of the square and logarithmic temperature dependencies, due to magnon-electron scattering and an analogous orbital two-channel Kondo effect, respectively. Of most importance, the uncompensated magnetic moments are found to govern magnetotransport properties of ${L1}_0\mathrm{MnPd}$ films.

We present a multimodal spectroscopic paradigm that enables independent measurement of charge and spin degrees of freedom (DOF) in strongly correlated materials. This spin-based technique probes symmetry-specific Hamiltonian parameters by analyzing how the time delay between applied pulses ($\tau$) affects the response. We demonstrate ways in which charge DOF that couple through the quadrupolar interaction (inversion symmetric) can be independently measured even in the presence of large magnetic noise (inversion asymmetric). The method quantifies both the strength of the interactions and their distribution (noise). We provide protocols to directly and independently measure the distribution of interaction strengths, even when the average value of the interaction is zero. By independently measuring distributions of different forms of disorder, this methodology can elucidate which microscopic symmetry drives a phase transition. We discuss potential applications to study complex phase transitions in strongly interacting quantum materials.

Two possible physical mechanisms of superconductivity suppression at superconductor/antiferromagnet (S/AF) interfaces, which work even for interfaces with compensated antiferromagnets, were reported. One of them suggests that the Néel order of the AF induces rapidly oscillating spin-triplet correlations in the S layer. They are called Néel triplets, and they suppress singlet superconductivity. Nonmagnetic disorder destroys this type of triplet correlations. As a result, the critical temperature of the S/AF bilayer grows with impurity strength. The second mechanism, on the contrary, suggests that nonmagnetic impurity scattering suppresses superconductivity in S/AF hybrids. The predictions were made in the framework of two different quasiclassical approaches [G. A. Bobkov et al., Phys. Rev. B 106, 144512 (2022) and E. H. Fyhn et al., Phys. Rev. Lett. 131, 076001 (2023)]. Here we suggest the unified theory of the proximity effect in thin-film S/AF structures, which incorporates both pictures as limiting cases, and we study the proximity effect at S/AF interfaces for arbitrary impurity strength, chemical potential, and the value of the Néel exchange field.

Recent works have predicted materials featuring bands with a large spin-splitting distinct from ferromagnetic and relativistically spin-orbit-coupled systems. Materials displaying this property are known as altermagnets and feature a spin-polarized band structure reminiscent of a $d$-wave superconducting order parameter. We here consider the contact between an altermagnet and a superconductor and determine how the altermagnetism affects the fundamental process of Andreev reflection. We show that the resulting charge conductance depends strongly on the interfacial orientation of the altermagnet relative to the superconductor, displaying features similar to normal metals or ferromagnets. The zero-bias conductance peaks present at the interface in the $d$-wave case are robust toward the presence of an altermagnetic interaction. Moreover, the spin conductance also strongly depends on the orientation of the altermagnet relative the interface. These results show how the anisotropic altermagnetic state can be probed by conductance spectroscopy and how it offers voltage control over charge and spin currents that are modulated due to superconductivity.

Recent advances in laser technology now enable engineering the electronic structure of matter through strong light-matter interactions. However, the effective physicochemical properties of these laser-dressed nonequilibrium materials are not well understood. Here we develop a general theory that enables modeling and interpreting the linear optical absorption of solids that are dressed by light of arbitrary strength and photon energy. The theory applies to any crystalline solid, including dielectrics, semiconductors, semimetals, and quantum materials. In the theory, the dressing of Bloch electrons by the driving laser is treated exactly using Floquet theory. The effective optical properties of this laser-dressed material are probed through a weak laser whose effects are captured to first order in perturbation theory. Remarkably, in this nonequilibrium system the time- and space-periodic Floquet-Bloch modes play the role of the pristine eigenstates of matter as the optical absorption is seen to emerge from transitions among them. We implement the theoretical framework into a code (FloqticS: Floquet optics in solids), which is made available through GitHub and can be interfaced with first-principle based electronic-structure computational packages. To isolate the emergent phenomenology, we performed computations in a model solid with a cosine-shaped lattice potential driven by strong nonresonant light. The computations recover the dynamical Franz-Keldysh effect and identify dramatic changes in the optical absorption upon increasing the amplitude of the driving laser. The Floquet replicas open absorption sidebands separated by integer multiples of the drive photon energy. The hybridization of the Floquet-Bloch modes, create intense low-frequency absorption and stimulated emissions, and dips in the absorption spectrum. We assign these emerging effects as purely-optical tell-tale signatures of the Floquet-Bloch modes. These advances can be used to model, control, and characterize the response properties of laser-dressed materials.

The material realization of the intrinsic chiral topological superconductivity (TSC) is still a big challenge in modern condensed-matter physics. In this paper, we propose a general scheme to make chiral TSCs from nontopological superconductivities (SCs) through the newly developed “twistronics” technique. Suppose we have a $D_n$-symmetric monolayer carrying nontopological SC with pairing angular momentum $L=n/2$. Here we propose that by stacking two such monolayers with the largest twist angle, the interlay Josephson coupling can drive chiral TSC with the same $L$ in the system. An argument based on the universal Ginzburg-Landau theory is provided to understand this proposal. One known example which fits our proposal is the $d+id$ chiral TSC in the ${45}^{\circ }$-twisted bilayer cuprates. Here, we demonstrate the application of our proposal to a new example, i.e., the $f+if$ chiral TSC obtained by twisting two properly doped honeycomb-Hubbard-model monolayers by the angle ${30}^{\circ }$. This example is related to the newly synthesized ${30}^{\circ }$-twisted bilayer graphene.

We study theoretically the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states appearing locally in a superconducting thin film with a small circular magnetic cluster. By solving the Eilenberger equation in two dimensions, we calculate the pair potential, pairing correlations, free-energy density, and quasiparticle density of states for various cluster sizes and exchange potentials. Increasing the exchange potential and cluster size leads to a higher number of nodes in the pair potential. Although the free-energy density beneath the ferromagnet locally exceeds the normal-state value, the FFLO states are stabilized by the superconducting condensate away from the magnetic cluster. Analyzing the pairing-correlation functions, we show that the spatial variation of the spin-singlet $s$-wave pair potential generates $p$-wave Cooper pairs. These odd-frequency Cooper pairs play a dominant role in governing the inhomogeneous subgap spectra observed in the local density of states. Furthermore, we propose an experimental method for the detection of local FFLO states by analyzing the quasiparticle density of states.