Halide perovskites have emerged in the past decade as a new important class of semiconductors for a variety of optoelectronic applications. A lot of previous studies were thus devoted to the characterization of their point defects. Positron annihilation spectroscopy is a well recognized tool for probing vacancies in materials. Recent applications of this technique to ${\mathrm{APbX}}_3$ halide perovskites are sparse and the rare theoretical predictions of positron lifetimes in these materials, published in association with experiments, do not fully agree with each other. These works suggest that vacancies on the $A$ site are not detected. In our theoretical study we focus on the role of the electron-positron correlation functional. We thoroughly revisit and compare several approximations when applied to methylammonium lead iodide (${\mathrm{MAPbI}}_3$) with or without vacancies, as well as inorganic perovskites ${\mathrm{CsPbI}}_3$ and ${\mathrm{CsPbBr}}_3$, in various phases. We show also the relationship between the size of the voids, through Voronoi volumes, and the calculated lifetimes. For the cubic phases we investigate in detail the role of polymorphism, including the distribution of vacancy formation energies and positron annihilation lifetimes. In our lifetimes calculations, apart from older and more recent semilocal approximations for the electron-positron correlation potential, we also consider the weighted density approximation, which is truly nonlocal and should better describe positron annihilation in regions with strong electronic density variations. We show that for this class of materials, and especially for cations vacancies, the influence of the chosen approximation is crucial, much stronger than in metals, alloys, and conventional semiconductors. This influence may induce one to reconsider the interpretation of experimentally determined lifetimes.

The contribution of extrinsic skew scattering to the anomalous Nernst effect (ANE) provides a potential avenue to enhance transverse thermoelectric response through spin-dependent scattering mechanism. In this study, we show the effect of ${\mathrm{Cu}}_{95}{\mathrm{Ir}}_5$ addition on the enhancement of the anomalous Nernst thermopower $\left(S_{\mathrm{ANE}}\right)$ in a CoFe thin film. A high-throughput screening based on the lock-in-thermography method for thermal responses induced by the anomalous Ettingshausen effect (AEE), the reciprocal of ANE, across a composition-spread thin film of CoFe-${\mathrm{Cu}}_{95}{\mathrm{Ir}}_5$ identified the optimal composition for ANE. Based on this screening experiment, we fabricated the specific composition ${\left(\mathrm{CoFe}\right)}_{63}{\left({\mathrm{Cu}}_{95}{\mathrm{Ir}}_5\right)}_{37}$ film and measured the electric and thermoelectric transport properties. The $S_{\mathrm{ANE}}$ value reaches 1.23 µV/K for ${\left(\mathrm{CoFe}\right)}_{63}{\left({\mathrm{Cu}}_{95}{\mathrm{Ir}}_5\right)}_{37}$ at room temperature, which is much higher than that for CoFe as well as composition-matched references ${\left(\mathrm{CoFe}\right)}_{64.85}{\left(\mathrm{Cu}\right)}_{35.15}$ and ${\left(\mathrm{CoFe}\right)}_{98.15}{\left(\mathrm{Ir}\right)}_{1.85}$, which contain the same amount of only Cu and only Ir, respectively. The scaling analysis on the anomalous Hall effect reveals that the extrinsic skew-scattering contribution is significantly greater for the ${\left(\mathrm{CoFe}\right)}_{63}{\left({\mathrm{Cu}}_{95}{\mathrm{Ir}}_5\right)}_{37}$ film compared to others at 300 K. This enhancement in the extrinsic contribution is attributable to the cooperative role of Cu and Ir; Ir acts as a dominant source of skew scattering due to large spin-orbit interaction, while Cu modifies the scattering potential symmetry and accelerates the skew scattering. These results offer an effective strategy for boosting $S_{\mathrm{ANE}}$ and provide critical insights into extrinsic mechanisms for ANE.

Noncentrosymmetric crystal structures are of significant interest due to the peculiar physical phenomena they frequently promote. Among mercury-based compounds, only a few noncentrosymmetric materials are known, with the vast majority remaining unexplored. In this work, we examine the family of noncentrosymmetric mercurides ${\mathrm{Na}}_{11}{\mathrm{Hg}}_{52}$ and $A_{11-x}{\mathrm{Hg}}_{54+x}$ ($A$ = Ca or Sr). All three materials are identified as new superconductors. The values of the critical temperatures are fairly modest, ranging from 1.7 K (${\mathrm{Ca}}_{11-x}{\mathrm{Hg}}_{54+x}$) to 3.1 K (${\mathrm{Na}}_{11}{\mathrm{Hg}}_{52}$). We postulate that the superconductivity of noncentrosymmetric ${\mathrm{Na}}_{11}{\mathrm{Hg}}_{52}$ and $A_{11-x}{\mathrm{Hg}}_{54+x}$ ($A$ = Ca or Sr) compounds is likely conventional.

We investigate the stabilization of topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phases, with a specific emphasis on the intraband FFLO phase, in a one-dimensional Fermi gas subjected to an external magnetic field. This research highlights the crucial role of the interplay between Rashba spin-orbit coupling (RSOC) and Dresselhaus spin-orbit coupling (DSOC). Employing a Fermi-Hubbard model alongside the density matrix renormalization group (DMRG) method, we examine the combined effects of RSOC and DSOC on these exotic superfluid phases, taking into account attractive fermionic interactions. Our principal finding reveals that while RSOC primarily stabilizes conventional zero-momentum pairing, DSOC performs a distinct and crucial role in selectively stabilizing the intraband FFLO phase. This stabilization is achieved by enhancing spin polarization within a single helicity band and suppressing interband coherence, thereby facilitating the formation of finite-momentum FFLO pairs within the same band and resulting in the emergence of a topologically nontrivial superfluid. This targeted control of intraband FFLO pairing paves the way for new strategies in the manipulation of superfluid phases in spin-orbit coupled systems and offers essential insights for experimental realizations in ultracold atomic gases, with implications for topological quantum computing and Majorana fermions.

There are multiple possible origins of transport anisotropies in metals and superconductors. For instance, rotational symmetry can be spontaneously broken in the normal state as a result of electronic nematic order inducing anisotropies in an otherwise $s$-wave superconducting state. Another possibility is that the dominant source of rotational symmetry breaking is the superconductor itself and its vestiges that may survive in the normal state. We here theoretically analyze the correlations of transport anisotropies in the normal and the corresponding superconducting phase for different scenarios of broken symmetry, either coming solely from the normal state, solely from the superconductor and its vestiges in the metallic regime, or simultaneously from both. We further include both zero-momentum and finite-momentum pairing; we develop a theory of vestigial order for the latter, characterized by broken rotational and translational symmetry. Our findings reveal that the relative transport anisotropies in the normal and superconducting phases sensitively depend on the scenario, including the form of vestigial order and, in some cases, the parity of the superconducting order parameter. As such, measuring the directional dependence of the critical current and resistivity can provide strong constraints on the origin of rotational symmetry breaking. We demonstrate our findings in minimal models relevant to twisted multilayer graphene, rhombohedral graphene, and twisted transition-metal dichalcogenides.

Neutron scattering experiments on the spin-triplet superconductor ${\mathrm{UTe}}_2$ have established that the dominant low-energy magnetic response is along Brillouin zone boundaries, resembling the magnetic susceptibility of narrow-gap interband excitations. We report a study of the sensitivity of these excitations to magnetic field along the crystallographic $\widehat{a}$ axis. Up to fields of ${\mu }_{0}H\hspace{0.17em}=\hspace{0.17em}13$ T, the maximal inelastic neutron spectral weight increases in energy transfer, with a pronounced increase in $d\hslash {\omega }_{\mathrm{peak}}/dH$ near ${\mu }_{0}H\hspace{0.17em}=\hspace{0.17em}7$ T. This behavior parallels the field- and temperature-dependent features of the electrical resistivity that are associated with Kondo hybridization. Our measurements suggest that $\widehat{a}$ axis fields near ${\mu }_{0}H\hspace{0.17em}=\hspace{0.17em}7$ T induce a change in the hybridization between heavy $f$ electrons and the bare conduction band.

Recently, moiré superlattices have been found on the surface of topological insulators due to the rotational misalignment of topmost layers. In this work, we study the effects of moiré superlattices on the Landé $g$-factor and hence Landau levels of topological surface states. We find that an extra Zeeman term besides the bulk one can be induced in moiré surface states, due to the spin-orbit coupling and moiré superlattice on the surface. As a result, the originally field-independent zeroth Landau level of moiré surface states could be tilted by the out-of-plane magnetic field.

Metasurfaces are becoming important tools for the control of thermal radiation. Understanding their functional possibilities on computational grounds requires evaluating the response of the biperiodic layered system for many degrees of freedom, including several radiation directions and polarizations, while varying lattice spacing, thicknesses, and/or materials of homogeneous layers, over a range of frequencies. The diverse set of cases that need to be considered in simulations prompts for efficient numerical tools to handle them. To respond to this need, we present a method for computing the thermal radiation from metasurfaces that combines the directional Kirchhoff's law with efficient $T$-matrix-based calculations. We show that such a method can accurately reproduce experimental data from a metasurface made of platinum square plates. Additionally, we predict highly circularly polarized emissivity from a chiral metasurface. When comparing CPU-times, the method outperforms other approaches such as rigorous coupled wave analysis already at the modest number of 61 cases per frequency.

In condensed matter physics, experimental control over material properties reflects a deep understanding of the underlying physics. In recent years, meaningful progress has been made towards a description of the physics of correlated electron systems, but examples of control of these systems remain rare. In this work, we confirm a phase diagram theoretically proposed for organic Mott insulators. We use $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_{2}X$ ($X$ = Br, Cl) [BEDT-TTF = bis(ethylenedithio)tetrathiafuvalene] materials as experimental realization of the proposed model and demonstrate the ability to tune them both ways across a phase border between a Mott insulator with a uniformly distributed charge and a charge-ordered state through the application of uniaxial strain. We induce charge order at 33 K in the quantum dipole liquid material $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Br}$ through the application of tensile strain of 0.4% along the $c$ axis. We suppress charge order down to 10 K in $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$ by applying a tensile strain of 1.6% along the $b$ axis. We use Raman scattering spectroscopy to probe the charge state through analysis of charge-sensitive molecular vibrations and a low-frequency mode of collective dipole fluctuations close to the phase border.

We report a single-crystal neutron diffraction study of the crystal and magnetic structures of ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$ in its insulating phases, performed on a well-characterized sample that exhibits room-temperature low-field colossal magnetoresistance (CMR). Previous studies have reported conflicting results regarding the crystal structure and the presence or absence of charge-orbital ordering, likely due to differences in sample form and measurement techniques. By using a single crystal in which CMR is directly observed, we clarify these inconsistencies and establish a unified structural picture. This study establishes that the insulating phases correspond to a charge-orbital-ordered state in ${\mathrm{NdBaMn}}_2{\mathrm{O}}_6$. We find that both the paramagnetic and antiferromagnetic insulating phases adopt a $2\sqrt{2}{a}_p\times \sqrt{2}{a}_p\times 2c_p$ unit cell with space group $P{2}_{1}am$ and exhibit $(3x^2-r^2)/(3y^2-r^2)$-type charge and orbital ordering. The antiferromagnetic phase shows an A-type magnetic structure with inequivalent Mn moments arising from this ordering. These results demonstrate that the insulating phases correspond to a charge-orbital-ordered state and suggest that the observed CMR originates from the competition between charge-orbital ordering and magnetic structures. In addition, the polar crystal symmetry implies the presence of spontaneous polarization, mainly arising from oxygen displacements.

The pseudomode method for open quantum systems, also known as the mesoscopic leads approach, consists in replacing a structured environment by a set of auxiliary “pseudomodes” subject to local damping that approximate the environment's spectral density. Determining what parameters and geometry to use for the auxiliary modes, however, is nontrivial and involves many subtleties. In this paper we revisit this problem of pseudomode design and investigate some of these subtleties. In particular, we examine the scenario in which pseudomodes couple to each other, resulting in an effective spectral density that is no longer a sum of Lorentzians. We show that nondiagonalizability of the pseudomodes' effective single-particle non-Hermitian Hamiltonian can lead to terms in the effective spectral density which cannot be obtained by diagonalizable non-Hermitian Hamiltonians. We also present a method for constructing the pseudomode parameters to exactly match a fit to a spectral density, and in doing so illuminate the enormous freedom in this process. The case of many uncoupled pseudomodes evenly distributed in energy is explored, and we show how, contrary to conventional assumption, the effective spectral density does not necessarily converge in the limit of an infinite number of pseudomodes distributed this way. Finally, we discuss how the notion of effective spectral densities can also emerge in the context of scattering theory for noninteracting systems.

Electronic bands in chirally stacked $n$ layer carbon-based honeycomb heterostructures, encompassing rhombohedral or ABC $(n\ge 3)$, Bernal or AB bilayer $(n=2)$, and monolayer $(n=1)$ graphene, possess fourfold valley and spin degeneracy. Such systems with $n\ge 2$, when subject to external perpendicular electric displacement fields, feature a fully degenerate metal at high doping, a spin polarized but valley degenerate half-metal at moderate doping, and a nondegenerate quarter metal at low doping. Due to the fully polarized nature of the quasiparticles in the quarter metal, realized around one particular valley otherwise chosen spontaneously, it can sustain a single local superconducting ground state, representing a pair density wave that is chiral and odd parity in nature. From a leading order renormalization group analysis, here we show that repulsive density-density interaction among such polarized fermionic excitations can foster the pair density wave phase at low temperatures. Connections with experimentally observed superconducting states in the close vicinity of the quarter metal in some members of such graphene heterostructure families are discussed, and possible routes to realize such a paired state in optical honeycomb lattices are highlighted.

We show that the large-$n$ limit of the $O\left(n\right)$ quantum rotor model defined on a general graph has the same critical behavior as the corresponding quantum spherical model and that the critical exponents depend solely on the spectral dimension $d_s$ of the graph. To this end, we employ a classical to quantum mapping and use known results for the large-$n$ limit of the classical $O\left(n\right)$ model on graphs. Away from the critical point, we discuss the interplay between the Laplacian and the Adjacency matrix in the whole parameter plane of the quantum Hamiltonian. These results allow us to paint the full picture of the $O\left(n\right)$ quantum rotor model on graphs in the large-$n$ limit.

The magnetic behavior of the Fe sublattice in the pseudobinary Laves phase materials $\left(\mathrm{Hf},\mathrm{Mo}\right)\mathrm{F}{\mathrm{e}}_2$ was thoroughly investigated in the paramagnetic regime as well as in the magnetically ordered state. The analysis of the paramagnetic phase has led to the determination of the Curie-Weiss temperature and effective paramagnetic moment whereas the spontaneous magnetic moment was obtained at temperatures ranging from 4 K up to the Curie point. The number of magnetic charge carriers was calculated both above and below the ferromagnetic ordering temperature in order to evaluate the degree of itinerancy (degree of delocalization) of Fe magnetism in the $\mathrm{H}{\mathrm{f}}_{1-x}\mathrm{M}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_2$ series of intermetallic compounds. It is shown that the Curie and Curie-Weiss temperatures decrease strongly upon increasing Mo concentration. Of particular interest is the thermal dependence of the reciprocal magnetic susceptibility in the intermediate composition range ($0.05\le x\le 0.15$): an unusual transition between two distinct Curie-Weiss behaviors is observed at a critical temperature $T^{*}$. The experimental data were analyzed and discussed within the framework of the Kuz'min's model, the self-consistent renormalization and Takahashi's theory of spin fluctuations. $T_0$ and $T_{\mathrm{A}}$ parameters characterizing the widths of the spin fluctuation spectrum in energy and wave-vector space, respectively, have been estimated. The transverse spin fluctuations are predominant for the parent intermetallic $\mathrm{HfF}{\mathrm{e}}_2$, in contrast, the longitudinal spin fluctuations gradually grow for the richest Mo compositions. Furthermore, Mo for Hf substitution induces a weakening of ferromagnetic exchange interactions: a change from long-range ferromagnetism (for $\mathrm{HfF}{\mathrm{e}}_2$) towards systems with competing exchange interactions (for Mo-rich side) is observed. In this family of compounds, the generalized Rhodes-Wohlfarth ratio points out an itinerant magnetism. The $3d$ Fe magnetism evolves from nearly localized character to strongly delocalized one as the Mo fraction is increased.

Motivated by recent experimental studies, we investigate the spectrum of the nearest-neighbor triangular XXZ model within the $1/S$ expansion, in the limit in which the exchange couplings present a strong easy-axis anisotropy $J_{xy}/J_{zz}\ll 1$. We show that, in the limit in which $1/S\to 0$ and $J_{xy}\to 0$ at fixed $V=J_{zz}/\left(S{J}_{xy}\right)$, the triangular spin model can be reduced to an effective boson model with quartic interactions on the honeycomb lattice. This effective model interpolates between a spin wave ($V\to 0$) and a strong-coupling limit ($V\to \infty$) and encodes in a simple framework the regimes discussed by Kleine et al. [Z. Phys. B: Condens. Matter 86, 405 (1992); Z. Phys. B: Condens. Matter, 87, 103 (1992)]. For zero field, the classical ground state of the model presents an accidental degeneracy, which can be traced to a simple symmetry of the classical energy. The model thus offers a transparent realization of a theory with quantum order-by-disorder and a pseudo-Goldstone mode. We analyze the spectrum at zero magnetic field by calculating the self-energy at one-loop order. In the calculation, we introduce a self-consistent renormalization of the energy scale and of the pseudo-Goldstone energy gap; the latter renormalization is essential to remove infrared divergences in the on-shell corrections to the energy dispersion. Finally, we discuss qualitatively the structure of the one-loop corrections in comparison with the spectrum observed experimentally in ${\mathrm{K}}_2\mathrm{Co}{\left({\mathrm{SeO}}_3\right)}_2$.

We investigate the piezomagnetic effect in pyrochlore-type oxides ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$, which exhibit a noncoplanar magnetic structure called the all-in/all-out antiferromagnetic order at low temperatures. The all-in/all-out magnetic order can be viewed as a ferroic order of the $xyz$-type magnetic octupoles. In both oxide materials, we observe a linear increase in magnetization with applied stress. We then estimate the powder-averaged piezomagnetic tensor at the lowest measured temperature 10 K to be $Q=6.36\times {10}^{-6}$ ${\mu }_{\text{B}}/\mathrm{Ir}/\mathrm{MPa}$ for ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and $Q=3.83\times {10}^{-7}$ ${\mu }_{\text{B}}/\mathrm{Os}/\mathrm{MPa}$ for ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. We discuss the microscopic mechanism of the piezomagnetic effect based on the stress-induced modification of the g-tensor anisotropy and Dzyaloshinskii-Moriya interactions. This work supports further development of piezomagnetic materials using magnetic multipoles.

Uncovering the rich and intricate characteristics of three-dimensional (3D) magnetic textures is essential for functional materials such as magnetic multilayers, where the delicate balance of various magnetic interactions leads to complex 3D spin arrangements. Among these textures, skyrmionic cocoons—tubular 3D magnetic structures characterized by a closed magnetization surface wrapping around a core—have emerged as particularly intriguing. Stabilized by competing magnetic interactions, these textures reside within a fraction of the thickness of the magnetic material and exhibit a typical lateral size of approximately 100 nm. Here, we present a vector tomographic reconstruction of the 3D magnetization in aperiodic Pt/Co/Al chiral multilayers, where skyrmionic cocoons have been recently reported. Using soft x-ray holography with extended reference by autocorrelation linear differential operator (HERALDO), we acquire tomographic projections of the magnetic configuration with a constant out-of-plane field and reconstruct the full 3D magnetization vector field with a spatial resolution of approximately 30 nm, as determined by Fourier shell correlation. This resolution allows us to observe critical features of the cocoons, such as their vertical misalignment. Our findings provide direct three-dimensional insight into the internal structure, vertical extent, and chirality of skyrmionic cocoons, enabled by magnetic-field-compatible vector tomography and qualitatively supported by micromagnetic simulations.

The term “two-dimensional coherent spectroscopy” (2DCS) usually refers to experimental setups where a coherently generated electric field in a sample is recorded over many runs as a function of two time variables: the delay $\tau$ between two consequent excitation pulses and the time $t$ over which the signal is emitted. Even if its implementation in the femtosecond time domain for investigation of vibrational molecular states was developed more than two decades ago, its experimental development in the terahertz domain with application to interacting electronic systems is still in its infancy. The present work aims at providing a theoretical framework for the description and interpretation of 2DCS by using the same many-body language based on a perturbative diagrammatic expansion that has been largely developed and applied in the literature to linear spectroscopy. By focusing on the case of centrosymmetric systems, we show how the problem of the theoretical interpretation of the 2D maps can be recast in the solution of two different but complementary problems. The first one is the evaluation of a third-order response function to the gauge field, whose derivation in the velocity gauge leads to a semianalytical expression for the computation of the 2D maps in the case of simple band dispersions and a Gaussianlike pulse envelope. This has the twofold advantage of reducing considerably the computational complexity and to guide the assignment of the spectral features to microscopic processes, as we demonstrate explicitly for a toy model of electrons in a semiconductinglike band structure. The second one is a careful description of multiwave propagation effects inside the material, that for bulk systems can completely screen the intrinsic properties of the nonlinear response, as we show for the case of soft superconducting Josephson plasmons. Our results provide a theoretical foundation prone to further extension to several interacting systems and offer a flexible method for modeling realistically the nonlinear responses across arbitrary spectral widths of the driving fields.