Since real devices necessarily contain defects, understanding wave propagation in disordered systems has proven a deep and important issue that has led to several important developments in the field of electronic transport and metal-insulator transitions, in particular Anderson localization. In this work, we consider a two-dimensional square lattice of pinned magnetic spins with nearest-neighbor interactions and we randomly replace a fixed proportion of spins with nonmagnetic defects carrying no spin. We focus on the linear spin-wave regime and address the propagation of a spin-wave excitation with initial momentum ${\mathit{k}}_0$. We compute the disorder-averaged momentum distribution obtained at time $t$ and show that the system exhibits two regimes. At low defect density, typical disorder configurations only involve a single percolating magnetic cluster interspersed with single defects essentially and the physics is driven by Anderson localization. In this case, the momentum distribution features the emergence of two known emblematic signatures of coherent transport, namely the coherent backscattering peak located at $-{\mathit{k}}_0$ and the coherent forward scattering peak located at ${\mathit{k}}_0$. At long times, the momentum distribution becomes stationary. However, when increasing the defect density, site percolation starts to set in and typical disorder configurations display more and more disconnected clusters of different sizes and shapes. At the same time, the coherent forward scattering peak starts to oscillate in time with well-defined frequencies. These oscillation frequencies represent eigenenergy differences in the regular, disorder-immune part of the Hamiltonian spectrum. This regular spectrum originates from the small-size magnetic clusters and its weight grows as the system undergoes site percolation and small clusters proliferate. Our system offers a unique spectroscopic signature of cluster formation in site percolation problems.

We investigate the effect of structural periodicity on elastic longitudinal and flexural wave propagation in parity-time ($\mathcal{PT}$) symmetric prototypical elastic metamaterial rods and beams. Through theoretical analysis and numerical demonstration, we reveal the link between the observed band structure of infinite periodic $\mathcal{PT}$ symmetric elastic metamaterials and the unique scattering properties of the corresponding finite periodic $\mathcal{PT}$ symmetric systems. In particular, we study the coherent perfect absorber and laser (CPAL). There is an observed periodic-like occurrence of the CPAL formation when varying the cell number in the finite periodic metamaterial that correlates to the band structure of the infinite periodic system. For this, we provide simple equations to estimate the periodicity and the minimum cell number. Using a piezoelectric-aided elastic metamaterial consisting of piezoelectric patches and active circuits, we propose a feasible configuration to realize tunable flexural wave CPALs in a periodic-structured $\mathcal{PT}$ symmetric metamaterial beam, and demonstrate that the CPAL can be induced in the Bragg's bandgap.

We introduce a non-Hermitian platform allowing us to obtain a three-dimensional exceptional surface in a magnon-polariton coupled system. A two-tone driving system gives rise to a complex effective coupling, leading to a pseudo-Hermitian Hamiltonian. Independent tuning of the real and imaginary components of the coupling provides additional degrees of freedom, allowing us to form a four-dimensional parameter space in which a three-dimensional exceptional surface is proposed. We show that this pseudo-Hermitian Hamiltonian has the most general form in cavity spintronics with $CPT$ symmetry. Our exceptional surface allows us to form a $\mathcal{PT}$-like circle where the $\mathcal{PT}$ and anti-$\mathcal{PT}$-symmetric exceptional points are unified. Moreover, we show that the $CPT$ symmetry in our system can be reduced to $\mathcal{PT}$ symmetry. Our proposal makes the cavity magnon polariton a rich platform for realizing intriguing phenomena of exceptional points regardless of fabrication imperfections or exact detuning of the system frequencies.

We investigate phase stability and vacancy formation in fcc Fe-Ni alloys over a broad composition-temperature range, via a density functional theory parametrized effective interaction model, which includes explicitly spin and chemical variables. On-lattice Monte Carlo simulations based on this model are used to predict the temperature evolution of the magnetochemical phase. The experimental composition-dependent Curie and chemical order-disorder transition temperatures are successfully predicted. We point out a significant effect of chemical and magnetic orders on the magnetic and chemical transitions, respectively. The resulting phase diagram shows a magnetically driven phase separation around 10–40% Ni and 570–700 K, between ferromagnetic and paramagnetic solid solutions, in agreement with experimental observations. We compute vacancy formation magnetic free energy as a function of temperature and alloy composition. We identify opposite magnetic and chemical disordering effects on vacancy formation in the alloys with 50% and 75% Ni. We find that thermal magnetic effects on vacancy formation are much larger in concentrated Fe-Ni alloys than in fcc Fe and Ni due to a stronger magnetic interaction.

Effects of strong phonon anharmonicity of a type-I clathrate ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Sn}}_{30}$ induced by the quadruple-well potential of guest atoms were investigated. Phonon transport including a coherent interbranch component was analyzed using a first-principles-based self-consistent phonon (SCP) theory that gives temperature-dependent harmonic interatomic force constants and by solving off-diagonal components of group velocity operator. Experimentally observed thermal conductivities have been reasonably reproduced by considering both lattice and electron contributions. Through the analysis with the SCP theory, we found that hardening of guest modes leads to an increase in lattice thermal conductivity at frequencies below those of framework-dominant flat modes ($<40{\mathrm{cm}}^{-1}$), which finally results in the slow decay of the total lattice thermal conductivity with increasing temperature. Detailed analyses revealed that the increase in lattice thermal conductivity at low frequency is attributed to (a) the increase in both group velocities and lifetimes of phonon modes located at frequencies below that of the flat guest modes and (b) abnormal increase in lifetimes of phonon modes located between frequencies of the flat guest and framework modes with increasing temperature.

We investigate the competition between acoustic phonon mediated superconductivity and the long-range Coulomb interaction in moiréless graphene multilayers, specifically, Bernal bilayer graphene, rhombohedral trilayer graphene, and ABCA-stacked tetralayer graphene. In these graphene multilayers, the acoustic phonons can realize, through electron-phonon coupling, both spin-singlet and spin-triplet pairings, and the intra-sublattice pairings ($s$-wave spin-singlet and $f$-wave spin-triplet) are the dominant channels. Our theory naturally explains the distinct recent experimental findings in Bernal bilayer graphene and rhombohedral trilayer graphene, and we further predict the existence of superconductivity in ABCA tetralayer graphene arising from electron-phonon interactions. In particular, we demonstrate that the acoustic phonon mediated superconductivity prevails over a wide range of doping in rhombohedral trilayer graphene and ABCA tetralayer graphene while superconductivity exists only in a narrow range of doping near the Van Hove singularity in Bernal bilayer graphene. Key features of our theory are the inclusion of realistic band structures with the appropriate Van Hove singularities and Coulomb repulsion effects (the so-called “${\mu }^{*}$ effect”) opposing the phonon-induced superconducting pairing. We also discuss how intervalley scatterings can suppress the spin-triplet spin-polarized superconductivity. Our work provides detailed prediction based on electron–acoustic phonon interaction induced graphene superconductivity, which should be investigated in future experiments.

We theoretically study the nonlinear valley polarization dynamics of excitons in atom-thin semiconductors. The presence of significant polarization slows down valley relaxation due to an effective magnetic field resulting from exciton-exciton interactions. We address temporal dynamics of valley-polarized excitons and study the steady states of the polarized exciton gas. We demonstrate bistability of the valley polarization where two steady states with low- and high-valley polarization are formed. We study the effects of fluctuations and noise in such a system. We evaluate valley-polarization autocorrelation functions and demonstrate that for a high-polarization regime the fluctuations are characterized by high amplitude and long relaxation time. We study the switching between the low- and high-valley polarized states caused by the noise in the system and demonstrate that the state with high-valley polarization is preferential in a wide range of pumping rates.

The renaissance of interest in halide perovskites, triggered by their unprecedented performance in optoelectronic applications, elicited worldwide efforts to uncover a variety of intriguing physical properties, with a particular interest in spin-orbit effects. The current work presents the first magneto-optical experimental evidence for anisotropic electron-hole interactions arising from bulk orthorhombic ${\mathrm{MAPbBr}}_3$. Magneto-photoluminescence spectra, monitored along with several different crystallographic directions, were dominated by dual exciton emission peaks, while each exhibited a highly nonlinear response to a magnetic field. Moreover, these plots depicted asymmetry from $-B_0$ to $+B_0$, with a strong dependence on the axis of observations. A theoretical model implementing anisotropy in the electron-hole interaction, Rashba effect, Landé $g$ factors, and a lesser contribution from an Overhauser effect, corroborated the experimental results. These research discoveries expand the possible applications of excitons in halide perovskites toward optoelectronic and information devices.

Heterobilayer transition metal dichalcogenide (TMDC) moiré systems provide an ideal framework to investigate strongly correlated physics. Here we theoretically study bosonic many-body phases of excitons in moiré TMDCs. By using two moiré models and cluster mean-field theory, we reveal that, due to nonlocal Coulomb interactions, moiré excitons can feature exotic supersolid phases, i.e., superfluids of broken translational invariance, and correlated insulating states. The correlated phases exist at experimentally accessible temperatures and are tunable via the twist angle and exciton density.

We analyze the confinement of electronic surface states in a model of a topological insulator nanowire. Spin-momentum locking in the surface states reduces unwanted backscattering in the presence of nonmagnetic disorder and is known to counteract localization for certain values of magnetic flux threading the wire. We show that intentional backscattering can be induced for a range of conditions in the presence of a nanowire constriction. We propose a geometry for a nanowire that involves two constrictions and show that these regions form effective barriers that allow for the formation of a quantum dot. We analyze the zero-temperature noninteracting electronic transport through the device using the Landauer-Büttiker approach and show how externally applied magnetic flux parallel to the nanowire and electrostatic gates can be used to control the spectrum of the quantum dot and the electronic transport through the surface states of the model device.

Breakdown of Landau's hypothesis of adiabatic continuation from noninteracting to fully interacting electrons is commonly believed to bring about a violation of Luttinger's theorem. Here, we elucidate what may go wrong in the proof of Luttinger's theorem. The analysis provides a simple way to correct Luttinger's expression of the electron number in single-band models where perturbation theory breaks down through the birth of a Luttinger surface without symmetry breaking. In those cases, we find that the Fermi volume only accounts for the doping away from half-filling. In the hypothetical circumstance of a non-symmetry-breaking Mott insulator with a Luttinger surface, our analysis predicts the noteworthy existence of quasiparticles whose Fermi surface is just the Luttinger one. Therefore, those quasiparticles can be legitimately regarded as spinons, and the Mott insulator with a Luttinger surface as realization of a spin-liquid insulator.

We theoretically investigate the escape rate occurring via quantum tunneling in a system affected by tailored dissipation. Specifically, we study the environmental assisted quantum tunneling of the superconducting phase in a current-biased Josephson junction. We consider Ohmic resistors inducing dissipation both in the phase and in the charge of the quantum circuit. We find that the charge dissipation leads to an enhancement of the quantum escape rate. This effect appears already in the low Ohmic regime and also occurs in the presence of phase dissipation that favors localization. Inserting realistic circuit parameters, we address the question of its experimental observability and discuss suitable parameter spaces for the observation of the enhanced rate.

In atomically thin semiconductors based on transition metal dichalcogenides, photoexcitation can be used to generate high densities of electron-hole pairs. Due to optical nonlinearities, which originate from Pauli blocking and many-body effects of the excited carriers, the generated carrier density will deviate from a linear increase in pump fluence. In this paper, we describe nonlinear absorption properties and excited carrier dynamics using a theoretical approach that combines results from ab initio electronic-state calculations with a many-body treatment of optical excitation. We determine the validity range of a linear approximation for the excited carrier density vs pump power and identify the role and magnitude of optical nonlinearities at elevated excitation carrier densities for ${\mathrm{MoS}}_2, {\mathrm{MoSe}}_2, {\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ considering various excitation conditions. For photoexcitation at the bandgap, we find that the use of a linear absorption coefficient of the unexcited system can strongly underestimate the achievable carrier density in ${\mathrm{MoS}}_2$ due to many-body renormalizations of the two-particle density of states. The same holds for excitation of the high-energy band continuum in W-based materials.

We investigate the dynamical effects of a magnetic flux quench in the Su-Schrieffer-Heeger model in a one-dimensional ring geometry. We show that even when the system is initially in the half-filled insulating state, the flux quench induces a time-dependent current that eventually reaches a finite stationary value. Such persistent current, which exists also in the thermodynamic limit, cannot be captured by the linear response theory and is the hallmark of nonlinear dynamical effects occurring in the presence of dimerization. Moreover, we show that for a range of values of dimerization strength and initial flux, the system exhibits dynamical quantum phase transitions, even though the quench is performed within the same topological class of the model.

We have developed a technique combining the accuracy of quantum Monte Carlo in describing the electron correlation with the efficiency of a machine learning potential (MLP). We use kernel regression in combination with the smooth overlap of atomic position (SOAP) features, implemented here in a very efficient way. The key ingredients are as follows: (i) a sparsification technique, based on farthest point sampling, ensuring generality and transferability of our MLPs, and (ii) the so-called $\mathrm{\Delta }$–learning, allowing a small training data set, a fundamental property for highly accurate but computationally demanding calculations, such as the ones based on quantum Monte Carlo. As an application we present a benchmark study of the liquid-liquid transition of high-pressure hydrogen and show the quality of our MLP, by emphasizing the importance of high accuracy for this very debated subject, where experiments are difficult in the laboratory, and theory is still far from being conclusive.

The electronic stopping power of protons traveling along the channeling and off-channeling trajectories in platinum (Pt) is reported based on time-dependent density functional theory calculations for electrons combined with Ehrenfest molecular dynamics simulations for ions in real time and real space. We provided an intuitive description of the electronic stopping power for a wide range of ion energies, and revealed the excitation mechanism for the inner $4f$ electrons of Pt. The comparison of calculation results with experimental data showed that conduction electrons are sufficient to describe the electronic stopping power of Pt accurately in the low-energy range. The excitation of $4f$ electrons contributes substantially to the electronic stopping power of Pt as the proton velocity exceeds 1.0 a.u. Our calculated stopping power is in quantitative agreement with the experimental data up to the stopping maximum, and showed that the stopping power obtained from the off-channeling geometry is greatly improved in comparison with the channeling results. Finally, the velocity dependence of the mean steady-state charge of protons was quantified, and the results showed that the instantaneous charge state of protons exhibits periodic oscillations in the low-energy region, while the periodic oscillations disappears in the high-energy region due to more ionization of protons.

The magnetic characteristics of polycrystalline exchange-biased antiferromagnet/ferromagnet bilayers are determined by a complex interplay of parameters, describing structural and magnetic properties of the material system, including, in particular, the grain volume distribution of the antiferromagnet. An ideal characterization of such systems would be a nondestructive determination of the relevant parameters for each individual grain. This is in most cases not feasible, since typical characterization methods average over larger areas. Here, we show that it is, however, possible to determine averaged microscopic parameters from averaged macroscopic magnetic quantities measured by vectorial Kerr magnetometry in comparison to an elaborate model. In particular, we estimate the magnetically effective antiferromagnetic grain size distribution, being essential for the interface exchange coupling to the ferromagnetic layer. We found that the distribution of magnetically active grain sizes differs from the structural one, indicating that the antiferromagnetic order, relevant for the exchange bias, extends only over a part of the grains' structural volumes.

The propagation of incoherent elastic energy in a three-dimensional solid due to the scattering by many randomly placed and oriented, pinned dislocation segments is considered in a continuum mechanics framework. The scattering mechanism is that of an elastic string of length $L$ that re-radiates as a response to an incoming wave. The scatterers are thus not static but have their own dynamics. A Bethe-Salpeter (BS) equation is established and a Ward-Takahashi identity (WTI) is demonstrated. The BS equation is written as a spectral problem that, using the WTI, is solved in the diffusive limit. To leading order, a diffusion behavior indeed results and an explicit formula for the diffusion coefficient is obtained. It can be evaluated in an independent scattering approximation in the absence of intrinsic damping. It depends not only on the bare longitudinal and transverse wave velocities but also on the renormalized velocities as well as attenuation coefficients of the coherent waves. The influence of the length scale given by $L$, and of the resonant behavior for frequencies near the resonance frequency of the strings can be explicitly identified. A Kubo representation for the diffusion constant can be identified. Previous generic results, obtained with an energy transfer formalism, are recovered when the number of dislocations per unit volume is small. This includes the equipartition of diffusive energy density which, however, does not hold in general. The formalism bears a number of similarities with the behavior of electromagnetic waves in a medium with a random distribution of dielectric scatterers; the elastic interaction, however, is momentum dependent.