The location of the mobility edge is a long-standing problem in Anderson localization. In this Letter, we show that the effective confining potential introduced in the localization landscape (LL) theory predicts the onset of delocalization in 3D tight-binding models in a large part of the energy-disorder diagram. Near the edge of the spectrum, the eigenstates are confined inside the basins of the LL-based potential. The delocalization transition corresponds to the progressive merging of these basins, resulting in the percolation of this classically allowed region throughout the system. This approach, shown to be valid both in the cases of uniform and binary disorders despite their very different phase diagrams, allows us to reinterpret the Anderson transition in the tight-binding model: the mobility edge appears to be composed of two parts, one being understood as a percolation transition.

We present a general multipole mechanism based on the lattice anapole effect leading to the excitation of high-$Q$ resonances in dielectric metasurfaces with the simplest unit cell (i.e., a unit cell with inversion symmetry containing only one nanostructure) and irradiation conditions (i.e., normal incidence). Using multipole techniques, we show analytically and numerically that these resonances are related to the conversion of bound states in the continuum (BICs) to quasi-BICs by simply changing the metasurface period. It is also shown that BICs and quasi-BICs, in turn, are realized through destructive interference (anapole effect) between multipoles of the same parity. The main advantage of such a conversion BIC to quasi-BIC compared to those proposed earlier is that it does not require distortion of symmetric properties of metasurfaces, special conditions of irradiation, or displacement of elements in composite unit cells. The results obtained give an important insight into the physics of high-$Q$ resonances in meta-optics and can simplify and expand the application of metasurfaces for tunable lasing, nonlinear generation, energy trapping manipulation, and enhanced sensing techniques.

Ferroelectric materials have potential applications in surface catalysis and photocatalysis due to the electron-hole separation induced by the built-in field. On the other hand, both the adsorption-induced charge transfer and the photogenerated free carriers influence the stability of the ferroelectric polarization. Therefore an in-depth understanding of the interaction between the charge transfer and the polarization is of the essence. In this paper, we take rutile ${\mathrm{TiO}}_2$ under applied strain as an example to study the effects of charge transfer and charge redistribution on polarization since it is known as a high-performance photocatalytic material as well as an incipient ferroelectric material. We consider charge transfer or charge redistribution induced by surface adsorption and photoexcitation and as a function of on-site Coulomb interaction. We find that the polarization generally decreases if charge transfer occurs in the ferroelectric materials, independent of the origin of the charge transfer or charge redistribution. The pseudo-Jahn-Teller theory is successfully adopted to explain the interaction between the charge transfer and macroscopic polarization. These findings give a clear physical picture of the effect of charge transfer and charge redistribution on the stability of polarization, which could contribute to the understanding of the ferroelectric catalytic and photocatalytic materials.

Based on the earlier published theory [A. Narayan, Nat. Mater. 18, 223 (2019)], a comprehensive experimental investigation of multiferroic quantum critical behavior of (Eu,Ba,Sr)${\mathrm{TiO}}_3$ polycrystalline and single-crystal samples was performed. The presence of the displacive ferroelectric quantum criticality is revealed through nonclassical ($T^2$) temperature scaling of inverse dielectric susceptibility up to 60 K. With increasing hydrostatic pressure, this ferroelectric quantum criticality is gradually suppressed. Inverse magnetic susceptibility follows classical Curie-Weiss law down to 4 K, but quantum fluctuations belonging to an antiferromagnetic phase transition ($T_{\mathrm{N}}<0.8$ K) change its scaling below 3 K to $T^{(1.7\pm 0.1)}$ and $T^{(2.1\pm 0.2)}$ for samples containing 29% and 25% of ${\mathrm{Eu}}^{2+}$ ions, respectively. Experimental indications of the coexisting ferroelectric and antiferromagnetic, i.e., multiferroic, quantum fluctuations and qualitative explanation of why they could be seen only in the immediate proximity of $T_{\mathrm{N}}$ is given.

Quantum simulators of lattice gauge theories involve dynamics of typically short-ranged interacting particles and dynamical fields. Elimination of the latter via Gauss law leads to infinite range interactions as exemplified by the Schwinger model in a staggered formalism. This motivates the study of long-range interactions, not necessarily diminishing with the distance. Here we consider localization properties of a spin chain with interaction strength growing linearly along the chain as for the Schwinger model. We generalize the problem to models with different interaction ranges. Using exact diagonalization we find the participation ratio of all eigenstates, which allows us to quantify the localization volume in Hilbert space. Surprisingly, the localization volume changes nonmonotonically with the interaction range. Our study is relevant for quantum simulators of lattice gauge theories implemented in state-of-the-art cold atom/ion devices, and it could help to reveal hidden features in disorder-free confinement phenomena in long-range interacting systems.

The confluence of unitary dynamics and nonunitary measurements gives rise to intriguing and relevant phenomena, generally referred to as measurement-induced phase transitions. These transitions have been observed in quantum systems composed of trapped ions and superconducting quantum devices. However, their experimental realization demands substantial resources, primarily owing to the classical tracking of measurement outcomes, known as postselection of trajectories. In this work, we first describe the statistical properties of an interacting transmon array which is repeatedly measured by standard and feedback measurements. We predict the behavior of relevant quantities in the area-law phase using a combination of the replica method and non-Hermitian perturbation theory. We show numerically that a transmon array, modeled by an attractive Bose-Hubbard model, in which local measurements of the number of bosons are probabilistically interleaved, exhibits a phase transition in the entanglement entropy properties of the ensemble of trajectories in the steady state for both measurement types. Furthermore, in the numerical simulations, we observe that by using deterministic feedback operations after the local number measurements, the distribution of the number of bosons measured at a single site carries relevant information about the statistics of individual trajectories.

Recent progress in the study of many-body localization in strongly disordered interacting spin chains has emphasized the importance of distinguishing finite time prethermal behavior from long time and large volume asymptotics. We reexamine a reported nonergodic extended (NEE) regime in the interacting quasiperiodic Ganeshan-Pixley-Das Sarma model from this perspective, and propose that this regime is a prethermal feature. Indeed, we argue that the NEE regime may be identified through a change in the functional form of spin-spin autocorrelation functions, demonstrating that the NEE regime is distinguishable within intermediate-time dynamics. This is in contrast with existing conjectures relating the NEE regime to the presence of an asymptotic mobility edge in the single-particle spectrum. Thus, we propose a mechanism for the formation of an NEE regime which does not rely on asymptotic properties of the spin chain. Namely, we propose that the NEE regime emerges due to regularly spaced deep wells in the disorder potential. The highly detuned sites suppress spin transport across the system, effectively cutting the chain and producing a separation of timescales between the spreading of different operators. To support this proposal, we show that the NEE phenomenology also occurs in random models with deep wells but with no mobility edges, and does not occur in quasiperiodic models with mobility edges but with no deep wells. Our results support the broad conclusion that there is not a sharp distinction between the dynamics of quasiperiodically and randomly disordered systems in the prethermal regime. More specifically, we find that generic interacting quasiperiodic models do not have stable intermediate dynamical phases arising from their single-particle mobility edges, and that NEE phenomenology in such models is transient.

Brillouin light scattering and microstrip line ferromagnetic resonance were used to study perpendicular magnetic anisotropy (PMA), magnetic damping, and interfacial Dzyaloshinskii-Moriya interaction (iDMI) in $X$/Fe($t_{\mathrm{Fe}}$)/$Y$ thin films with varying Fe thickness $(1.4\mathrm{nm}\le t_{\mathrm{Fe}}\le 10\mathrm{nm})$, where $X$ stands for Pt, Ir, or Cu and $Y$ stands for Ir, Cu, or MgO. A particular interest is given to their temperature dependence. To accurately assess the contribution of each interface with Fe to the effective interface PMA constant, we combined direct measurements of the effective magnetization thickness dependence with the spin waves frequency mismatch. Notably, Ir/Fe and Fe/Ir, as well as Cu/Fe and Fe/Cu interfaces exhibited distinct behaviors: Fe/Ir significantly contributed to PMA and spin pumping induced damping. The damping constant showed a nonlinear dependence on the Fe layer inverse effective thickness, which was linked to a strong interfacial two-magnon scattering, particularly evident in Cu/Fe/Ir and Cu/Fe/Cu structures. Additionally, iDMI measurements revealed an opposite chirality of Pt/Fe and Ir/Fe interfaces. We also investigated how these magnetic interfacial properties vary with temperature in the Pt/Fe/Cu and Cu/Fe/Cu systems. We found that the interface PMA constant increased linearly with temperature, while the iDMI constant decreased significantly, by approximately 60%. This suggests that iDMI is highly sensitive to thermal disorder. Post-temperature measurements indicated that the temperature induces interdiffusion and impacts the magnetic properties through specific effects that are challenging to disentangle in the absence of in situ magnetization measurements.

The orbital Hall effect generates a current of orbital angular momentum perpendicular to a charge current. Experiments suggest that this orbital current decays on a long length scale that is of the order of the spin-flip diffusion length or longer. We examine this suggestion using first-principles quantum mechanical scattering calculations to study the decay of orbital currents injected from an orbitally polarized lead into thermally disordered bulk systems of selected transition metals. We find that the decay occurs over only a few atomic layers. On this length scale the orbital current may be converted into a spin current if the spin Hall angle is sufficiently large, as for Pt. In Cu, Cr, and V with small spin Hall angles, the conversion into a spin current is negligible in the bulk and significant conversion only occurs at interfaces.

We report temperature and magnetic field dependences of the magnetization and ac susceptibility of ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$, aiming to construct a magnetic phase diagram for $H\parallel$ [001]. Its spin Hamiltonian can be described as an Ising model for a metamagnet. The superlattice structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4·{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ facilitates the reduction of interlayer antiferromagnetic interaction. As predicted by the model, there is a tricritical point in the phase diagram when the ratio of the intralayer to interlayer interaction is less than $-3/5$. The tricritical point is determined to be (12.4 K, 660 Oe) by the imaginary part of ac susceptibility due to the dissipation of domain walls of the mixed phase. The effective tricritical exponents, ${\beta }_2^{\mathrm{eff}}\sim 1.10$, ${\delta }_2^{\mathrm{eff}}\sim 1.65$, have been obtained and differ from the mean-field exponents. When the logarithmic correction factor ${|ln|t\left|\right|}^{0.5}$ is included, the data collapse with the mean-field power law. These findings were tested against the tricritical scaling hypothesis. The deviation from the Landau theoretical values results from the logarithmic correction, similar to another layered metamagnet ${\mathrm{FeCl}}_2$. As a member of the intrinsic magnetic topological insulator family, ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ features a tricritical point in its magnetic phase diagram, providing a solid foundation for future research on topological transitions and tricriticality.

The electronic structure and magnetic properties of a newly predicted stable ternary compound ${\mathrm{La}}_{18}{\mathrm{Co}}_{28}{\mathrm{Pb}}_3$ are studied using electronic structure analysis. The ground state of this compound is ferromagnetic, with three positions of nonequivalent magnetic Co atoms. A strong dependence of magnetic properties on volume shows that this system is situated near the point of magnetic instability. The coexistence of high- and low-spin ferromagnetic states as a function of volume near equilibrium was discovered. A corresponding spin tunneling splitting was estimated. The stability of the theoretically predicted magnetic ground state was tested by varying the Hubbard parameter. The thermal spin fluctuations were added to estimate the paramagnetic moment and a Curie temperature. The necessity of experimental verification of the obtained results is emphasized.

In our study, we conduct magnetization and heat capacity measurements to investigate field-induced magnetic phase transitions within the newly synthesized compound ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$, a spin-1 dimer system arranged on a triangular lattice. From our first-principles simulations, we determine that the spin system in ${\mathrm{K}}_2{\mathrm{Ni}}_2{\left({\mathrm{SeO}}_3\right)}_3$ can be represented as a two-dimensional triangular-lattice spin-1 dimer model, including an intra-dimer exchange of $J_1=0.32\mathrm{meV}$, an inter-dimer exchange of $J_2=0.79\mathrm{meV}$, and an easy-axis anisotropy of $D=0.14\mathrm{meV}$. The presence of easy-axis magnetic anisotropy explains the distinct magnetic phase diagrams observed under $c$-axis directional and in-plane magnetic fields. Notably, our investigation unveils a two-step phase transition with the magnetic field aligned with the $c$ direction. Our findings yield valuable insights into the magnetic phase transitions inherent to geometrically frustrated magnetic systems featuring dimer structures.

We investigate the appearance of mobility edges in a one-dimensional non-Hermitian tight-binding model with alternating hopping constants and slowly varying quasiperiodic on-site potentials. Due to the presence of a slowly varying exponent, the parity-time $\left(\mathcal{P}\mathcal{T}\right)$ symmetry of this model is broken and its spectra are complex. It is found that the spectrum of this model can be divided into three different types of patterns depending on the magnitude of the quasiperiodic potential. As the amplitude of the potential increases from small to large, the initially well-defined mobility edges become blurred gradually and then eventually disappear for large-enough potential. This behavior of mobility edges is also confirmed by a detailed study of the winding number of complex spectra of this non-Hermitian model.

In this theoretical study, we explore Förster resonant energy transfer of a single exciton within a two-dimensional array of self-assembled quantum dots arranged randomly on a circular mesa. Employing the stochastic simulation method, we solve the equation of motion for the density matrix, considering a specified decay rate. Our analysis quantifies diffusion through the mean-square displacement from the initially excited quantum dot, revealing distinct temporal stages: ballistic, normal diffusion, and saturation. Furthermore, we observe power-law localization of the exciton. Complementing our numerical investigations, we develop approximate analytical expressions that closely align with the numerical findings.

We begin a systematic investigation of quench dynamics in higher-dimensional lattice systems considering the case of noninteracting fermions with conserved particle number. We prepare the system in a translational-invariant nonequilibrium initial state, the simplest example being a classical configuration with fermions at fixed positions on the lattice, and let it evolve in time. We characterize the system's dynamics by measuring the entanglement between a finite connected region and its complement. We observe the transmutation of entanglement entropy into thermodynamic entropy and investigate how this process depends on the shape and orientation of the region with respect to the underlying lattice. Interestingly, we find that irregular regions display a distinctive multislope entanglement growth, while the dependence on the orientation angle is generically fairly weak. This is particularly true for regions with a large (discrete) rotational symmetry group. The main tool of our analysis is the celebrated quasiparticle picture of Calabrese and Cardy, which we generalize to describe the case at hand. Specifically, we show that for generic initial configurations (even when restricting to classical ones) one has to allow for the production of multiplets involving $n>2$ quasiparticles and carrying nondiagonal correlations. We obtain quantitatively accurate predictions, tested against exact numerics, and propose an efficient Monte Carlo based scheme to evaluate them for arbitrary connected regions of generic higher-dimensional lattices.

The square ice is a canonical example of a Coulomb phase in two dimensions: Its ground state is extensively degenerate and satisfies a local constraint on the spin arrangement (the so-called ice rule). In this paper, we use a loop flip algorithm to explore the properties of this ground state that we analyze not in terms of a spin texture, but rather in terms of a spatial distribution of ice-rule satisfying vertices. More specifically, we determine for various lattice sizes the average vertex populations characterizing the ice manifold, the pairwise vertex correlations, and the size distribution of vertex clusters. Comparing these results to those obtained from random, constraint-free vertex tilings, the square ice manifold is found to resemble an almost ideal vertex gas, and the cluster size distribution of ice-rule satisfying vertices is well approximated by percolation theory. Remarkably, this description remains reasonably accurate when monopoles are present in a dilute amount, allowing a direct comparison with experiments. Revising former experimental results on two artificial square ice systems, we illustrate the interest of our approach to spot the presence of a Coulomb phase from a vertex analysis.

A pyrochlore charge ice consisting of two magnetic cation types gives rise to a Heisenberg magnet with an interesting type of long-range correlated bond disorder. If the antiferromagnetic exchange among cations of the same type dominates over the interspecies exchange, closed antiferromagnetic chains following the fully packed loops of like cations emerge as nonlocal effective low-temperature degrees of freedom. Monte Carlo simulations reveal a fluctuation driven first-order phase transition from an algebraically correlated spin liquid to a long-range ordered spin nematic, which mean-field theory captures with very high accuracy. The global $O\left(3\right)$ Heisenberg symmetry is suddenly reduced to a $Z_2$ Ising symmetry as the Néel vectors of the strongly antiferromagnetically correlated loops align with each other to optimize the entropy of their thermal fluctuations. Interestingly, the nematic transition is found to be sensitive to the size statistics of the cation loops and thus provides a direct thermodynamic probe of otherwise elusive structural properties. In turn this sensitivity offers a structural route to engineering the nematic phase stability.