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We investigate the spatial decay and temporal localization properties of quasimodes (i.e., scattering resonances) of two-dimensional Vogel spirals, composed of deterministic, aperiodic arrays of electric dipoles. By determining the structural entropy and localization maps of Vogel spirals using the Green's matrix method, we show that three distinctive decay types of quasimodes coexist in Vogel spirals: exponential, power-law, and Gaussian. While the exponential and the power-law decays typically occur in disordered media and multifractal systems, respectively, the Gaussian decay is demonstrated to characterize, on average, the most localized quasimodes of Vogel spirals, both spatially (smallest participation ratios) and temporarily (longest lifetimes). These decay forms are demonstrated by a no-fitting analysis of the localization maps, independently corroborated by calculating the electric field in real space, which also provides a direct evidence of the algebraic spatial decay of critical quasimodes. Altogether our findings unveil a rich spectrum of both long-lived and spatially localized quasimodes that coexist in Vogel spirals and can be of direct relevance to novel optical functionalities for applications to light sources and sensing devices., 8 pages, 3 figures

In order to explore the spin accumulation, evaluating the spin galvanic and spin Hall effect, we utilize the semi-classical Boltzmann equation based on input from the relativistic Korringa-Kohn-Rostoker Green's function method, within the density functional theory. We calculate the spin accumulation including multiple contributions, especially skew-scattering (scattering-in term) and compare this to three different approximations, which include the isotropic and anisotropic relaxation time approximation. For heavy metals, with strong intrinsic spin-orbit coupling, we find that almost all the effects are captured within the anisotropic relaxation time approximation. On the other hand, in light metals the contributions from the vertex corrections (scattering-in term) are comparable to the induced effect in anisotropic relaxation time approximation. We put a particular focus on the influence of the atomic character of the substitutional impurities on the spin accumulation as well as the dependence on the impurity position. As impurities will break space inversion symmetry of the thin film, this will give rise to both symmetric and antisymmetric contributions to the spin accumulation. In general, we find the impurities at the surface generate the largest efficiency of charge-to-spin conversion in case of the spin accumulation. Comparing our results to existing experimental findings for Pt we find a good agreement.

Spin relaxation and decoherence is at the heart of spintronics and spin-based quantum information science. Currently, theoretical approaches that can accurately predict spin relaxation of general solids including necessary scattering pathways and capable for ns to ms simulation time are urgently needed. We present a first-principles real-time density-matrix approach based on Lindblad dynamics to simulate ultrafast spin dynamics for general solid-state systems. Through the complete first-principles descriptions of pump, probe and scattering processes including electron-phonon, electron-impurity and electron-electron scatterings with self-consistent electronic spin-orbit couplings, our method can directly simulate the ultrafast pump-probe measurements for coupled spin and electron dynamics over ns at any temperatures and doping levels. We first apply this method to a prototypical system GaAs and obtain excellent agreement with experiments. We find that the relative contributions of different scattering mechanisms and phonon modes differ considerably between spin and carrier relaxation processes. In sharp contrast to previous work based on model Hamiltonians, we point out that the electron-electron scattering is negligible at room temperature but becomes dominant at low temperatures for spin relaxation in n-type GaAs. We further examine ultrafast dynamics in novel spin-valleytronic materials - monolayer and bilayer WSe2 with realistic defects. We find that spin relaxation is highly sensitive to local symmetry and chemical bonds around defects. Our work provides a predictive computational platform for spin dynamics in solids, which has unprecedented potentials for designing new materials ideal for spintronics and quantum information technology., Comment: 10 pages, 8 figures

We establish the theoretical foundation of the Floquet graphene antidot lattice, whereby massless Dirac fermions are driven periodically by a circularly polarized electromagnetic field while having their motion excluded from an array of nanoholes. The properties of interest are encoded in the quasienergy spectra, which are computed nonperturbatively within the Floquet formalism. We find that a rich Floquet phase diagram emerges as the amplitude of the drive field is varied. Notably, the Dirac dispersion can be restored in real time relative to the gapped equilibrium state, which may enable the creation of an optoelectronic switch or a dynamically tunable electronic waveguide. As the amplitude is increased, the ability to shift the quasienergy gap between high-symmetry points can change which crystal momenta dominate in the scattering processes relevant to electronic transport and optical emission. Furthermore, the bands can be flattened near the $\mathrm{\ensuremath{\Gamma}}$ point, which is indicative of selective dynamical localization. Lastly, quadratic and linear dispersions emerge in orthogonal directions at the $M$ point, signaling a Floquet semi-Dirac material. Importantly, all our predictions are valid for experimentally accessible near-IR radiation, which corresponds to the above bandwidth limit for the graphene antidot lattice. Cycling between engineered Floquet electronic phases may play a key role in the development of next-generation on-chip devices for optoelectronic applications.

We study the effects of Rashba splitting on two-leg ladders with weakly screened Coulomb interactions. Past research has shown that in this class of systems the two backscattering channels with the largest amplitude favor ordering of canonically conjugated collective fields which effectively renders the system gapless. Here we show that the band-dependent Rashba spin-orbit interaction breaks this symmetry of scattering channels, leading to a new fixed point with yet unexplored instabilities. Exotic properties can be found, for instance, the mixing of the magnetism with the charge-density wave. We then investigate the physical consequences of this partial spectral gap opening. We find a striking difference in signatures of order observed in single-particle (spectral-function) and two-body (susceptibilities) experimental probes. We conclude this paper by comparing theoretical and experimental results obtained on the Au-Si(553) platform. In STM measurements, we identify the lowest-lying soliton excitation as a hallmark of the gapped sine-Gordon model in an apparently metallic system. This implies the presence of partial spectral gaps opening. Furthermore, by ARPES measurements we confirm the expected temperature dependence of the outer bands backfolding. These two findings constitute the experimental evidence of the many-body physics proposed here.

We present a first-principles framework to extract deformation potentials in silicon based on density-functional theory (DFT) and density-functional perturbation theory (DFPT). We compute the electronic band structures, phonon dispersion relations, and electron-phonon matrix elements to extract deformation potentials for acoustic and optical phonons for all possible processes. The matrix elements clearly show the separation between intra- and intervalley scattering in the conduction band, and quantify the strength of the scattering events in the degenerate bands of the valence band. We then use an advanced numerical Boltzmann transport equation (BTE) simulator that couples DFT electronic structures and energy/momentum-dependent scattering rates to compute the transport properties for electrons and holes. By incorporating ionized impurity scattering as well, we calculate the \ud n-type and p-type mobility versus carrier density and make comparisons to experiments, indicating excellent agreement. The fact that the method we present uses well-established theoretical tools and requires the extraction of only a limited number of matrix elements, makes it generally computationally very attractive, especially for semiconductors with a large unit cell and lower symmetry.

The tunnel magnetoresistance (TMR) is one of the most important spintronic phenomena but its reduction at finite temperature is a severe drawback for applications. Here, we reveal a crucial determinant of the drawback, that is, the $s$-$d$ exchange interaction between conduction $s$ and localized $d$ electrons at interfacial ferromagnetic layers. By calculating the temperature dependence of the TMR ratio in Fe/MgO/Fe(001), we show that the obtained TMR ratio significantly decreases with increasing temperature owing to the spin-flip scattering in the $\Delta_1$ state induced by the $s$-$d$ exchange interaction. The material dependence of the coupling constant $J_{sd}$ is also discussed on the basis of a nonempirical method., Comment: 6 pages, 5 figures, 1 table

Random scattering of photons in disordered one-dimensional solids gives rise to an exponential suppression of transmission, which is known as Anderson localization. Here, we experimentally study Anderson localization in a superconducting waveguide quantum electrodynamics system comprising eight individually tunable qubits coupled to a photonic continuum of a waveguide. Employing the qubit frequency control, we artificially introduce frequency disorder to the system and observe an exponential suppression of the transmission coefficient in the vicinity of its subradiant dark modes. The localization length decreases with the disorder strength, which we control in-situ by varying individual qubit frequencies. Employing a one-dimensional non-interacting model of coupled qubits and photons, we are able to support and complement the experimental results. The difference between our investigation and previous studies of localization in qubit arrays is the coupling via a common waveguide, allowing us to explore the localization of mediating photons in an intrinsically open system. The experiment opens the door to the study of various localization phenomena on a new platform, which offers a high degree of control and readout possibilities., 15 pages including appendix, 9 figures. Comments welcome

Publisher Copyright: © 2021 American Physical Society. The recently developed concept of time-varying metamaterials paves the way for advanced methods of electromagnetic property modulation, such as magnet-free isolators, frequency converters, and other nonreciprocal photonic components. Here, we analyze the metamaterial based on the periodically arranged temporally modulated graphene layers via a steplike graphene Fermi level. The material properties are described by the effective medium theory. The electromagnetic wave temporal scattering phenomenon caused by rapid steplike graphene Fermi level switching is considered. It is demonstrated that the frequency of scattered waves increases as the Fermi level grows rapidly and vice versa. The relation between the amplitudes of forward- and backward-scattered waves is found using the Laplace transform technique. The proposed concept allows us to efficiently generate frequency harmonics under instant switching of the graphene Fermi level at a few meV. Our investigation provides a basement for the realization of temporally modulated multilayer structures based on functional tunable materials as power-efficient frequency converters.

Non-Hermitian systems with parity-time $(\mathcal{P}\mathcal{T})$ symmetry have attracted increasing interest due to their intriguing properties and associated applications. Inspired by that, we propose here an acoustic anti-parity-time $(\mathrm{A}\text{\ensuremath{-}}\mathcal{P}\mathcal{T})$ symmetric structure with metamaterials featuring balanced positive and negative indices, together with equal gain/loss. According to the derived scattering matrix, we demonstrate its extraordinary scattering properties. For example, the spontaneous phase transition of the scattering matrix is observed, by simply modulating the frequency, gain/loss, or geometric width. In the absence of gain/loss, the $\mathrm{A}\text{\ensuremath{-}}\mathcal{P}\mathcal{T}$ symmetric structure degrades into a pair of complementary media, resulting in bidirectional total transmission. At the zero and pole of the scattering matrix, the structure behaves as an acoustic coherent perfect absorber and equivalent laser, respectively, which can perfectly absorb two-port coherent incident waves and radiate two coherent output waves with extensive and equal amplitudes. Different from that in the $\mathcal{P}\mathcal{T}$ symmetric structure, these three effects based on the $\mathrm{A}\text{\ensuremath{-}}\mathcal{P}\mathcal{T}$ symmetric structure are all achieved in the symmetric phase of the scattering matrix, resulting in the symmetric intensity distributions. Besides, the coherent perfect absorption and lasing modes of the $\mathrm{A}\text{\ensuremath{-}}\mathcal{P}\mathcal{T}$ symmetric structure are continuous and symmetric in the parameter space, which may facilitate further experimental realizations. The proposed $\mathrm{A}\text{\ensuremath{-}}\mathcal{P}\mathcal{T}$ symmetric structure provides an alternative method to demonstrate the physics of the non-Hermitian Hamiltonian, and may offer an alternative approach to design acoustic functional devices such as absorbers, sensors, and amplifiers.

We demonstrate robust ergodicity breaking in interacting many-body systems in arbitrary Euclidian dimension based on disorder-free many-body localization. Translationally invariant fine-tuned single-particle lattice Hamiltonians can host dispersionless (flat) bands only. Suitable short-range many-body interactions enforce complete suppression of particle transport due to local constraints and lead to ergodicity breaking termed many-body flat-band localization. However, heat might still flow between spatially locked charges. We demonstrate that heat transport is completely suppressed in one dimension. In higher dimensions we establish a universal bound on the filling fraction below which the heat transport is suppressed. The bound is based on the mapping to a classical percolation problem. Above the bound, the heat transport in percolation clusters is additionally affected by emerging bulk disorder and edge scattering induced by local constraints, which work in favor of arresting the heat flow and might keep the ergodicity breaking above the universal bound. We discuss explicit examples in one and two dimensions.

Characterizing the many-body localization (MBL) transition in strongly disordered and interacting quantum systems is an important issue in the field of condensed matter physics. We study the single particle Green's functions for a disordered interacting system in one dimension using exact diagnonalization in the infinite temperature limit and provide strong evidence that single particle excitations carry signatures of delocalization to MBL transition. In the delocalized phase, the typical values of the local density of states and the scattering rate are finite while in the MBL phase, the typical values for both the quantities become vanishingly small. The probability distribution functions of the local density of states and the scattering rate are broad log-normal distributions in the delocalized phase while the distributions become very narrow and sharply peaked close to zero in the MBL phase. We also study the eigenstate Green's function for all the many-body eigenstates and demonstrate that both, the energy resolved typical scattering rate and the typical local density of states, can track the many-body mobility edges., Comment: A few changes to the text to improve clarity. No change in results. Close to the published version

Quantum transport simulations are rapidly evolving and now encompass well-controlled tensor network techniques for many-body limits. One powerful approach combines matrix product states with extended reservoirs. In this method, continuous reservoirs are represented by explicit, discretized counterparts and a chemical potential or temperature drop is maintained by external relaxation. Currents are strongly influenced by relaxation when it is very weak or strong, resulting in a simulation analog of Kramers' turnover for solution-phase chemical reactions. At intermediate relaxation, the intrinsic conductance, that given by the Landauer or Meir-Wingreen expressions, moderates the current. We demonstrate that strong impurity scattering (i.e., a small steady-state current) reveals anomalous transport regimes within this methodology at weak-to-moderate and moderate-to-strong relaxation. The former is due to virtual transitions and the latter to unphysical broadening of the populated density of states. Thus, the turnover analog has $five$ standard transport regimes, further constraining the parameters that lead to recovery of the intrinsic conductance. In the worst case, the common strategy of choosing a relaxation strength proportional to the reservoir level spacing can prevent convergence to the continuum limit. This advocates a simulation strategy where one utilizes the current versus relaxation turnover profiles to identify simulation parameters that most efficiently reproduce the intrinsic physical behavior., 16 pages, 5 figures

The bulk-boundary correspondence in one dimension asserts that the physical quantities defined in the bulk and at the edge are connected, as well established in the argument for electric polarization. Recently, a spectral bulk-boundary correspondence (SBBC), an extended version of the conventional bulk-boundary correspondence to energy-dependent spectral functions, such as Green's functions, has been proposed in chiral symmetric systems, in which the chiral operator anticommutes with the Hamiltonian. In this study, we extend the SBBC to a system with impurity scattering and dynamical self-energies, regardless of the presence or absence of a gap in the energy spectrum. Moreover, the SBBC is observed to hold even in a system without chiral symmetry, which substantially generalizes its concept. The SBBC is demonstrated with concrete models, such as superconducting nanowires and a Su-Schrieffer-Heeger model. Its potential applications and certain remaining issues are also discussed., Comment: 16 pages, 12 figures

Recent studies have revealed the significance of electron-phonon interaction (EPI) in phonon transport at intermediate temperatures. In some metals, the EPI can even dominate over the anharmonic phonon-phonon (ph-ph) scattering, leading to an anomalous phonon transport regime in which the lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ becomes nearly temperature ($T$) independent in contrast to the usual $1/T$ dependence. However, the experimental verification of this anomalous transport regime is very challenging due to the difficulty in separating the phonon contributions from the dominating electron ones to the measured total thermal conductivity in metals. In this work, using first-principles calculations, we predict that in bilayer graphene, the phonon transport can be driven to the anomalous regime by tuning the doping level. At high doping levels close to the Van Hove singularity, the EPI can result in a fivefold reduction of ${\ensuremath{\kappa}}_{L}$ at room temperature, and ${\ensuremath{\kappa}}_{L}$ becomes $T$ independent. This anomalous behavior is found to have its origin in three aspects: (i) mirror symmetry breaking enables direct coupling between flexural phonons, the dominant carriers of ${\ensuremath{\kappa}}_{L}$, and electrons; (ii) dominance of normal processes in the anharmonic ph-ph scattering facilitates the EPI to be more prominent; (iii) dominance of these normal ph-ph processes induces the indirect effect of EPI on ${\ensuremath{\kappa}}_{L}$. This is distinct from monolayer graphene, where the mirror symmetry prohibits the direct scattering of the flexural phonons by electrons and only the indirect EPI affects ${\ensuremath{\kappa}}_{L}$. This work gives insight into the manipulation of heat conduction via externally induced EPI in two-dimensional materials in which mirror symmetry breaks and normal processes dominate the ph-ph scattering.

We present an empirical description of experimental spin-torque switching probability for the CoFeB/MgO/CoFeB type of magnetic tunnel junctions beyond macrospin limit, parametrizing measurement data for direct comparison with the corresponding macrospin asymptote expression. We show that, near 35 nm in diameter, spin-torque switching speed in these tunnel devices is faster than macrospin-limit predictions. These devices have a faster reduction of switching error rate versus spin-torque drive amplitude than macrospin. While the functional form similar to macrospin can still describe experimental data satisfactorily, the parameters no longer correspond to materials values. Instead they reflect the nonuniform nature of the switching process. Further, the parameters depend on the resistance-area product ${r}_{A}$ of the junction, with higher ${r}_{A}$ causing a steeper slope of switching error versus switching current. This ${r}_{A}$ dependence could not originate from low-bias spin-dependent tunneling. These observations suggest that, in addition to nonuniform nonlinear dynamics during switching, it is also important to consider higher-order dynamic processes, including a high-bias tunnel electron's spin-flip scattering, voltage-induced change to interface magnetism, and possibly Joule heating.

We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f, 4 pages article with 4 figures, submitted to Physical Review B Letters, concatenated with 12 pages supplementary information (having 15 figures) in a 17 pages article with concantenated bibliography

Surface lattice resonance (SLR) is the collective excitation of nanoparticle resonances arising from the hybridization between localized surface plasmons (LSPs) and propagating Rayleigh anomalies (RAs). When comparing with the corresponding LSPs, SLRs exhibit much higher quality factor. In fact, as the quality factor depends on the constituting resonances and their hybridization, how one can parametrize it in an analytic form is an important issue. We have studied the SLRs arising from 2D Au monopartite nanoparticle arrays by angle- and polarization-resolved reflectivity spectroscopy, temporal coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulation. The scattering matrix of the SLRs is formulated, revealing the importance of the spectral detuning and the interaction strengths between the LSP and the RAs in governing the quality factor. We then extend the CMT approach to study bipartite arrays where nanoparticle dimer is employed and find the coupling between two LSPs plays a major role in further boosting the quality factor. Specifically, the coupling takes part in controlling the detuning factor as well as determining whether the coupled bright or dark mode is hybridized with the RAs. The dark mode hybridization can strongly enhance the quality factor which is otherwise not possible in the monopartite counterparts., Comment: 33 pages, 10 figures

In heat conduction through a homogenous nanomaterial, various phonons may exhibit diverse temperatures even at the same location at a steady state, known as the local phonon nonequilibrium phenomenon. Different phonons are often considered to behave independently, and the phonons with longer mean free paths (MFPs) have smaller temperature gradients. That is, the temperature gradient exhibits the following order: ballistic phonons semiballistic phonons \ensuremath{\approx} lattice (average) temperature gradient diffusive phonons, where ballistic phonons have MFPs much larger than the characteristic length, semiballistic phonons have MFPs like the characteristic length, and diffusive phonons have MFPs much smaller than the characteristic length. However, in this paper, we reveal that the effect of phonon-phonon coupling leads nonequilibrium phonon temperature gradients to the following trend: diffusive phonon temperature gradients will decrease to the lattice temperature, and temperature gradients of some semiballistic phonons even surpass that of diffusive phonons. The diffusive phonon temperature is merged onto the lattice temperature since they have large scattering rates and can be equilibrated quickly to the lattice temperature after traveling for a short distance away from the boundaries into the nanomaterial. The semiballistic phonons have large scattering rates but not large enough to bring them down to the lattice temperature. To obtain a further understanding of the nonequilibrium phonon temperatures, we have also derived a simple analytical model which can accurately predict the temperature profiles of all individual phonons given their MFPs. Using this model, we find that, near the boundary, phonon temperatures decay with position exponentially (instead of linearly), with a rate inversely proportional to their MFPs. Our findings offer insight for the understanding and prediction of phonon nonequilibrium temperatures within nanodevices.

Surface impedance of type-II superconductors is determined by their local optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$. Standard BCS-like theoretical descriptions of $\ensuremath{\sigma}(\ensuremath{\omega})$, due to Mattis and Bardeen [Phys. Rev. 111, 412 (1958)] or Zimmermann et al. [Physica C 183, 99 (1991)], do not take pair-breaking processes into account. Therefore they do not provide a quantitative explanation of the microwave response, and in particular, they cannot predict the magnitude of the coherence peak. Here, based on the recently developed concept of Dynes superconductors, which does take also the pair-breaking scattering into account, we provide a simple but complete description of the microwave response of type-II superconductors, concentrating on the unexpected properties of clean superconductors which are often used as cavity materials.

I present a formalism to calculate the configuration-averaged lattice thermal conductivity for substitutional random alloys. The method is based on multiple-scattering approach which can capture the effect of disorder-induced configuration fluctuations on single-particle and two-particle phonon Green functions in random alloys. The randomness of the system is dealt within the augmented space theorem. This is combined with a generalized Feynman diagrammatic technique to extract various useful results in the form of mathematical expressions. I show the structure of all possible scattering diagrams up to the fourth order and subsequently illustrate how to obtain Dyson's equation from a resummation of the diagrammatic series. I also study how disorder scattering affects two-particle Green functions associated with thermal response. It was shown explicitly how the disorder scattering renormalizes both the phonon propagators as well as the heat currents. I derive the relation between these renormalized heat currents and the self-energy of the propagators. I have also studied a different class of scattering diagrams which are not related to the self-energy but rather to the vertex corrections. The configuration-averaging scheme is straightforward to apply to other relevant quantities such as joint density of states, thermal diffusivity, etc., for random alloys. The developed formalism is applied to a realistic ${\mathrm{Au}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ binary alloy. The effect of disorder-induced corrections (as compared to the simple virtual crystal approximation) turns out to be appreciable in this alloy. The configuration-averaged lattice thermal conductivity for ${\mathrm{Au}}_{50}{\mathrm{Fe}}_{50}$ shows a quadratic behavior in low-temperature regime ($T\ensuremath{\le}30$ K), which increases smoothly to a $T$-independent saturated value at high $T$. Simulated thermal diffusivity $D(\ensuremath{\nu})$ helps to numerically estimate the mobility $\mathrm{edge}\phantom{\rule{0.28em}{0ex}}({\ensuremath{\nu}}_{c})$ which in turn evaluates the fraction of localized states. $D(\ensuremath{\nu})$ is found to decrease smoothly (almost linearly) in the high-$\ensuremath{\nu}$ range, which when fitted to ${({\ensuremath{\nu}}_{c}\ensuremath{-}\ensuremath{\nu})}^{\ensuremath{\alpha}}$ gives the critical $\mathrm{exponent}\phantom{\rule{0.28em}{0ex}}(\ensuremath{\alpha})$ to be 1.018 for ${\mathrm{Au}}_{50}{\mathrm{Fe}}_{50}$ alloy. This agrees fairly well with the scaling and other theories of Andersen localization.

We have studied magnetotransport of a degenerate two-dimensional electron gas in a Hall sample in the Knudsen regime, when the mean free paths of electrons with respect to their collisions with each other and with impurities are much larger than the width of the sample. In contrast to the usually considered symmetric sample, whose both its edges reflect electrons diffusely, we considered an asymmetric sample, one edge of which reflects them diffusely, while the other specularly. It is shown that in such structure in low magnetic fields the Hall coefficient is parametrically large in comparison with its standard value. Also the situation is discussed when all types of scattering can be neglected except for scattering at the edges of the sample.

We modeled, both analytically and numerically, the magnitude and phase of spin waves propagating in thin magnetic films and scattered from mesoscale chiral magnonic resonators. Our calculations reveal a remarkably strong chiral scattering of propagating spin waves from magnon dark modes hosted by the resonator, exceeding in strength the scattering from its quasiuniform mode. We formulate conditions for the waveguide-resonator system to be used as an efficient spin-wave diode and as a phase shifter. Both these applications are found to be feasible when using the available ferromagnetic materials for the resonators.

Motion signatures of nonstationary electromagnetic bodies are imprinted in their scattering spectrum. While the Doppler frequency shift holds information about the velocity of its center of mass, internal degrees of freedom in a nonrigid body, such as rotation and vibration, introduce nontrivial spectral distortions, termed micro-Doppler signatures. Contemporary analytic characterization of such signatures typically neglects subwavelength electromagnetic coupling, which can dominate the scattering signatures of motion. To address this overlooked scattering regime, a theory of moving coupled dipoles is used to model a moving nonrigid body. The method is verified experimentally in the microwave regime, demonstrating remote sensing of subwavelength information. The method can be useful for analyzing and characterizing effects that frequently emerge in radar science, healthcare monitoring, optical manipulation of particles, and many other applications, where remote sensing and classification of motion are important.

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20--100 bilayers (BL) was measured at $T$= 9 K in magnetic fields up to $B=4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, oriented in-plane parallel and perpendicular to the electric dc current $I$. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between $B$ and $I$ within the surface plane was found. Only for $B\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}I$ an increase of MC was detected that is, to first approximation, $\ensuremath{\propto}{B}^{2}$. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, $d$, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with $\ensuremath{\alpha}=\ensuremath{-}0.3$ and a coupling strength that decays $\ensuremath{\propto}1/d$ with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for $dg60$ BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

The dynamic structure factor $S(Q,E)$ of liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ has been obtained by inelastic x-ray scattering, where $Q$ and $E$ are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped $Q$ dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on $Q$ like an optical mode with decreasing $Q$ to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|{\mathrm{\ensuremath{\Delta}}}_{k}|$). Here, we report the measurement of a strain-induced gap modulation ($\ensuremath{\partial}|{\mathrm{\ensuremath{\Delta}}}_{k}|$) in uniaxially strained ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ utilizing angle-resolved photoemission spectroscopy and in situ strain tuning. We found that the uniaxial strain drives ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ into a nematic superconducting state which breaks the fourfold rotational symmetry of the superconducting pairing. The superconducting gap increases on the ${d}_{yz}$ electron and hole pockets while it decreases on the ${d}_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The ${d}_{xz}$ and ${d}_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intraorbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\ensuremath{\partial}|{\mathrm{\ensuremath{\Delta}}}_{k}|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.

We present x-ray spectral evidence for the evolution of valence- and spin-specific tetragonal distortions in single-layer cobaltates. Measurements of Co ${L}_{3}$-edge resonant inelastic x-ray scattering reveal the ${t}_{2g}$ electronic structure of Co in hole-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ ($x=0.5$, 0.7, and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the ${t}_{2g}$ states of high-spin ${\mathrm{Co}}^{2+}$ decreases, whereas that of low-spin ${\mathrm{Co}}^{3+}$ increases and the population fraction of high-spin ${\mathrm{Co}}^{3+}$ increases. The results enable us to clarify the origin of the change of magnetic anisotropy and in-plane resistivity in the mixed-valence cobaltates caused by the interplay of spin-orbit coupling and tetragonal distortion.

We propose an interpolation scheme to describe pair correlations in crystals with many-body interactions that requires only information on relative displacements for the nearest neighbours and in the long range. Using crystalline Ni as a test case, the scheme is shown to deliver the functional form for the radial distribution function at least as well as molecular dynamics simulations. The results provide a fast route for verification of interatomic potentials and study of many-body interactions using a combination of x-ray scattering and x-ray absorption spectroscopy.

We present a quantitative theory of the gate-voltage tuned superconductor-to-insulator transition (SIT) observed experimentally in the 2D electron system created in the (111) interface between crystalline ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ and ${\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}$. Considering two fundamental opposing effects of Cooper-pair fluctuations---the critical conductivity enhancement, known as paraconductivity, and its suppression associated with the loss of unpaired electrons due to Cooper-pairs formation---we generalize the standard thermal fluctuations theory to include interaction between fluctuations within the self-consistent field approximation and quantum tunneling between mesoscopic superconducting puddles within a phenomenological approach. Relying on the quantitative agreement found between our theory and a large body of experimental sheet-resistance data, we conclude that spin-orbit scatterings, via significant enhancement of the interaction between fluctuations, strongly enhance the sheet resistance peak at high fields and reveal anomalous metallic behavior at low fields, due to mixing of relatively heavy electron bands with a light electron band near a Lifshitz point. The large enhancement of the interaction between fluctuations at high fields, where the sheet resistance is strongly amplified, is shown to result in localization of Cooper-pair fluctuations within mesoscopic puddles.

We have investigated the effects of spatial-inversion and time-reversal symmetry breaking on the acoustic phonon branches in chiral MnSi using high-resolution inelastic x-ray scattering and first-principles calculations. We find a momentum-transfer-dependent ( $q$ -dependent) splitting between transverse phonon bands having angular momentum parallel or antiparallel to $q$ . This is understood by a phenomenological theory using a gyrotropic tensor. We observed no significant impact from the time-reversal symmetry breaking induced by a magnetic field (energy shifts 0.3 meV). This suggests the effect of time-reversal symmetry breaking is small or is restricted to a very-low-energy regime in this material, possibly due to small spin-orbit interaction.

Electrically induced formation of valley-polarized free-carrier domains is theoretically investigated in two-dimensional (2D) honeycomb lattice systems exhibiting topological valley transport and the valley Hall effect. The results show that under a strong electric field $E$ applied along a nanostrip, the domains can be formed across the strip when this transverse dimension is comparable to the intervalley diffusion length ${L}_{iv}$. Further, these domains are found to be characterized by two distinct length scales dependent on $E$ and ${L}_{iv}$, i.e., the extended diffusion length ${L}_{\mathrm{ext}}$ ($\ensuremath{\propto}{L}_{iv}E$) and the compressed diffusion length ${L}_{\mathrm{com}}$ ($\ensuremath{\propto}{L}_{iv}/E$). The former determines the extension of the domain plateaus, whereas the latter specifies the width of the domain wall. Within each of the domains (excluding a narrow region of the domain wall; ${L}_{\mathrm{ext}}\ensuremath{\gg}{L}_{\mathrm{com}}$), the charge carriers are fully polarized belonging to only one of the valleys, providing a pure bulk valley current inside the considered domain region. The domain properties including the polarization amplitude and domain wall position are analyzed as a function of the applied electric field and intervalley scattering rates at the edges. The position of the domain wall, which determines the relative extension of the plateaus, can be controlled by a transverse current flowing between the Hall-type contacts. The current-voltage characteristic demonstrates a superlinear behavior in the range of electric fields corresponding to the valley-polarization domain formation. This feature is a distinctive signature of the anomalous transport arising from Bloch-band Berry curvature, which enhances the longitudinal conductance and diminishes the channel resistance. By reversing the applied electric field, the valley polarization and localization of the valley-polarized currents can be abruptly switchable. We suggest that the studied scheme of electrically induced domains of valley polarization in the 2D honeycomb lattice systems can be used in novel applications with all-electrical control and manipulation of the valley degree of freedom.

We theoretically study the scattering behavior of spin waves (SWs) at the interface of an antiferromagnetically coupled (AFMC) heterojunction. It is shown that the SWs passing through the interface are evanescent and the incident waves are all reflected back, demonstrating a magnetization-dependent magnon blocking effect in this structure. We also analytically derive the expressions for the decay length of the evanescent waves (EWs). The theoretical result indicates that with the increase of the spin-wave (SW) frequency, the decay length decreases and the EWs are more concentrated at the interface, showing a magnonic skin effect (MSE) which is similar to the skin effect of electromagnetic waves. Furthermore, a positive magnonic Goos-H\"anchen shift (MGHS) of the reflected waves is also predicted. It can be understood by an effective reflection interface shift induced by the nonzero decay length of the EWs. The results of micromagnetic simulations are consistent well with all the theoretical findings. Based on the above findings, we also propose a magnon valve without spacers, which shows 100% on-off ratio for magnons. Our work provides insights into SW transmissions in the system of AFMC heterostructures and will serve as a promising tool for future magnonic devices.

We measure the Shubnikov--de Haas effect in thin-film ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ grown on an $({\mathrm{LaAlO}}_{3}{)}_{0.29}\text{\ensuremath{-}}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_{3}{)}_{0.71}$ substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, and that the quasiparticle lifetime is consistent with the ${T}_{\mathrm{c}}$ of comparably clean, single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. Unlike single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is $1.3\ifmmode\pm\else\textpm\fi{}0.1$ times longer than the quantum lifetime. This may suggest that extended (rather than point) defects could be the dominant source of quasiparticle scattering in these films. To test this hypothesis, we perform cross-sectional scanning transmission electron microscopy and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ through the fabrication of devices---such as planar tunnel junctions and superconducting quantum interference devices.

Detailed analysis reveals that an incorporation of stabilized defect complexes within metallic thin films, though a highly disordering and nonequilibrium process, gives rise to superconductivity, Fermi-liquid (FL) transport, and a universal correlation among them. This remarkable manifestation of correlated macroscopic quantum effects is attributed to a phonon-mediated electron-electron, e-e, scattering channel which encompasses both Koshino-Taylor and Bergmann's pseudo-Umklapp processes. This channel---denoted below as pseudo-Umklapp e-e scattering channel---is distinctly different from traditional ones in that disorder leads to a breakdown of lattice momentum conservation (significantly enlarging available phase space), to a spectral weight transfer towards lower frequencies (modifying electron-phonon coupling constant $\ensuremath{\lambda}$), and to a relaxation of kinematic constraints (all phonic polarization modes become available for mediation). On modeling the distorted structure in terms of Hosemann's paracrystal and using standard quantum many-body techniques, we demonstrate the role of distortion and softening in establishing this pseudo-Umklapp channel and, consequently, the surge of superconductivity, the FL transport, and the correlation of their parameters. This unifying approach allows us to derive analytical expressions for ${T}_{c}({\ensuremath{\rho}}_{\ensuremath{\circ}})$ (hallmark of superconductivity), the coefficient $A({\ensuremath{\rho}}_{\ensuremath{\circ}})$ (hallmark of FL transport), and the universal kinematic scaling relation $\mathrm{ln}(\frac{{T}_{c}}{\ensuremath{\theta}})\ensuremath{\propto}{A}^{\frac{\ensuremath{-}1}{2}}$: All are in satisfactory agreement with experiments ($\ensuremath{\theta}$ is an energy scale; residual resistivity ${\ensuremath{\rho}}_{\ensuremath{\circ}}$ measures the extent of disorder).

We consider a problem of superconductivity coexistence with the spin-density-wave order in disordered multiband metals. It is assumed that random variations of the disorder potential on short length scales render the interactions between electrons to become spatially correlated. As a consequence, both superconducting and magnetic order parameters become spatially inhomogeneous and are described by the universal phenomenological quantities, whereas all the microscopic details are encoded in the correlation function of the coupling strength fluctuations. We consider a minimal model with two nested two-dimensional Fermi surfaces and disorder potentials which include both intra- and interband scattering. The model is analyzed using the quasiclassical approach to show that short-scale pairing-potential disorder leads to a broadening of the coexistence region.

The auxiliary and itinerant coherent potential approximations (ACPA and ICPA) have been proposed as effective methods for treating both mass and force-constant disorder in lattice vibration. In this work we make a direct comparison of ACPA and ICPA to identify the differences between these two methods for simulating vibrational properties of disordered materials and devices. We investigate the major approximations in the disorder self-energies of these two methods by using the diagrammatic method. The single-site ACPA neglects the crossing diagrams describing nonlocal correlations, while ICPA utilizes only the single-fluctuation states in the self-consistency and presents extra errors due to a change in the terms with nearest-neighbor fluctuation. To further demonstrate the important differences between the ACPA and ICPA in the phonon spectral and transport properties, we extend ICPA in combination with the Keldysh nonequilibrium Green's function technique to simulate quantum transport and introduce a Green's-function-based spectral unfolding technique to use with molecular ACPA and the supercell methods for comparison. By studying the disordered one- and three-dimensional alloys, we find that ACPA and ICPA agree very well in the weak scattering regime due to weak force-constant disorder or low phonon frequency. However, for the strong scattering of force-constant disorder, it is found that the two methods present important deviations in the linewidth (and height) of disorder-averaged phonon spectra. Compared to the ACPA and exact results, the problematic treatment of the nearest-neighbor fluctuation in ICPA can present unphysical scattering and induce large errors in the phonon transport, presenting important limitations of ICPA for transport simulation.

We measured the complex conductivity, $\ensuremath{\sigma}$, of $\mathrm{Fe}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ $(x=0\ensuremath{-}0.5)$ films in the superconducting state which show a drastic increase of the superconducting transition temperature ${T}_{\text{c}}$ when the nematic order disappears. Since the magnetic penetration depth $\ensuremath{\lambda}\phantom{\rule{4pt}{0ex}}(g400$ nm) of Fe(Se,Te) is longer than the typical thickness of the film $(\ensuremath{\sim}100 \mathrm{nm})$, we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of $\ensuremath{\sigma}$. Films with a nematic order showed a qualitatively different temperature dependence in penetration depth and quasiparticle scattering time when compared with those without nematic order, suggesting that nematic order influences the superconducting gap structure. Conversely, the proportionality between superfluid density ${n}_{\text{s}}$ $(\ensuremath{\propto}{\ensuremath{\lambda}}^{\ensuremath{-}2})$ and ${T}_{\text{c}}$ was observed irrespective of the presence or absence of nematic order. This result indicates that the amount of superfluid has a stronger impact on the ${T}_{\text{c}}$ of Fe(Se,Te) than the presence or absence of nematic order. Combining these results with band dispersions calculated using density functional theory, we propose that the change of the Fermi surface associated with nematicity is the primary factor influencing the change of ${T}_{\text{c}}$ and the superconducting gap structure in Fe(Se,Te).

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic ``dark matter'' and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system.

We investigate the influence of general forms of disorder on the robustness of superconductivity in multiband materials. Specifically, we consider a general two-band system where the bands arise from an orbital degree of freedom of the electrons. Within the Born approximation, we show that the interplay of the spin-orbital structure of the normal-state Hamiltonian, disorder scattering, and superconducting pairing potentials can lead to significant deviations from the expected robustness of the superconductivity. This can be conveniently formulated in terms of the so-called ``superconducting fitness.'' In particular, we verify a key role for unconventional $s$-wave states, permitted by the spin-orbital structure and which may pair electrons that are not time-reversed partners. To exemplify the role of Fermi-surface topology and spin-orbital texture, we apply our formalism to the candidate topological superconductor ${\mathrm{Cu}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, for which only a single band crosses the Fermi energy, as well as models of the iron pnictides, which possess multiple Fermi pockets.

The measurement of Friedel oscillations (FOs) is conventionally used to recover the energy dispersion of electronic structure. Besides the energy dispersion, the modern electronic structure also embodies other key ingredients such as the geometrical and topological properties; it is one promising direction to explore the potential of FOs for the relevant measurement. Here, we present a comprehensive study of FOs in substrate-supported graphene under off-resonant circularly polarized light, in which a valley-contrasting feature and topological phase transition occur due to the combined breaking of inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$) symmetries. Depending on the position of the Fermi level, FOs may be contributed by electronic backscattering in one single valley or two valleys. In the single-valley regime, the oscillation periods of FOs can be used to determine the topological phase boundary of electronic structure, while the amplitudes of FOs distinguish trivial insulators and topological insulators in a quantitative way. In the two-valley regime, the unequal Fermi surfaces lead to a beating pattern (robust two-wave-front dislocations) of FOs contributed by intravalley (intervalley) scattering. This study implies the great potential of FOs in characterizing topological and geometrical properties of the electronic structure of two-dimensional materials.

Skew scattering---an asymmetric scattering of electrons by impurities---is one of the major mechanisms causing anomalous/spin Hall effects. Although many microscopic mechanisms for skew scattering are known, the Hall angle of anomalous Hall effect by these mechanisms is often small, typically $\ensuremath{\theta}=0.{1}^{\ensuremath{\circ}}\ensuremath{-}{1}^{\ensuremath{\circ}}$. In this paper, we study the skew scattering by three-spin clusters focusing on the strong Kondo-coupling regime. Using a $T$-matrix formalism, we calculate the scattering probability for arbitrary strength of Kondo coupling, going beyond perturbation theory in previous studies. From a systematic analysis of the scattering probability for one-, two-, and three-spin clusters, we show that three spins are necessary for the skew scattering in the absence of spin-orbit interaction. The skew scattering by the three-spin cluster produces a skew angle on the order of $0.1\ensuremath{\pi}\phantom{\rule{0.28em}{0ex}}\mathrm{rad}\phantom{\rule{0.28em}{0ex}}(\ensuremath{\sim}{18}^{\ensuremath{\circ}})$ when the electron-spin coupling is comparable to the bandwidth. We also study the relationship between the anomalous/spin Hall effects and the spin chiralities and argue that the anomalous-(spin) Hall skew angle is approximately proportional to the scalar (net vector) spin chirality even for the strong-coupling cases. This mechanism is potentially relevant to anomalous/spin Hall effects in noncentrosymmetric and frustrated magnets.

KCa${}_{2}$Fe${}_{4}$As4F${}_{2}$ and its sister compounds provide a new platform to study superconducting mechanism of iron-based superconductors. Using scanning tunneling microscopy/spectroscopy, the authors observe here a full gap feature with a gap value close to 4.6 meV in tunneling spectra measured in KCa${}_{2}$Fe${}_{4}$As4F${}_{2}$. Quasiparticle interference patterns only show the intraband scattering of the holelike $\ensuremath{\alpha}$ pocket. The derived Fermi energy of this band is only about 24 meV, which indicates a deviation from the basic requirement of the weak-coupling BCS theory.

The spin Hall effect in a heavy metal thin film allows to probe the magnetic properties of an adjacent magnetic insulator via magnetotransport measurements. Here, we investigate the magnetoresistive response of yttrium iron garnet/platinum heterostructures from room temperature to beyond the Curie temperature $T_\mathrm{C, YIG} \approx 560\,\mathrm{K}$ of the ferrimagnetic insulator. We find that the amplitude of the (local) spin Hall magnetoresistance decreases monotonically from $300\,\mathrm{K}$ towards $T_\mathrm{C}$, mimicking the evolution of the saturation magnetization of yttrium iron garnet. Interestingly, the spin Hall magnetoresistance vanishes around $500\,\mathrm{K}$, well below $T_\mathrm{C}$, which we attribute to the formation of a parasitic interface layer by interdiffusion. Around room temperature the non-local magnon-mediated magnetoresistance exhibits a power law scaling $T^{\alpha}$ with $\alpha = 3/2$, as already reported. The exponent decreases gradually to $\alpha \sim 1/2$ at around $420\,\mathrm{K}$, before the non-local magnetoresistance vanishes rapidly at a similar temperature as the spin Hall magnetoresistance. We attribute the reduced $\alpha$ at high temperatures to the increasing thermal magnon population which leads to enhanced scattering of the non-equilibrium magnon population and a reduced magnon diffusion length. Finally, we find a magnetic field independent offset voltage in the non-local signal for $T > 470\,\mathrm{K}$ which we associate with electronic leakage currents through the normally insulating yttrium iron garnet film. Indeed, this non-local offset voltage is thermally activated with an energy close to the band gap., Comment: 7 pages, 4 figures

We develop a generic all-inductive procedure to measure the band structure of spin waves in a magnetic thin stripe. In contrast to existing techniques, our method works even if several spin wave branches coexist in the investigated frequency interval, provided that the branches possess sufficiently different group velocities. We first measure the microwave scattering matrix of a network composed of distant antennas inductively coupled to the spin wave bath of the magnetic film. After a mathematical transformation to the time-domain to get the transmission impulse response, the different spin wave branches are viewed as wavepackets that reach successively the receiving antenna after different travel times. In analogy with time-of flight spectroscopy, the wavepackets are then separated by time-gating. The time-gated responses are used to recalculate the contribution of each spin wave branch to the frequency domain scattering matrix. The dispersion relation of each branch stems from the absolute phase of the time-gated transmission parameter. The spin wave wavevector can be determined unambiguously if the results for several propagation distances are combined, so as to get the dispersion relations., Comment: Submitted to the PRB. This work was funded through the FETOPEN-01-2016-2017-FET-Open research and innovation actions (CHIRON project: Grant Agreement No. 801055) and was partly supported by the French RENATECH network. This work is also supported by a public grant overseen by ANR as part of the "Investissements d'avenir" program (Labex NanoSaclay, reference: ANR-10-LABX-0035)

Resonant inelastic x-ray scattering (RIXS) is a powerful probe of elementary excitations in solids. It is now widely applied to study magnetic excitations. However, its complex cross section means that RIXS has been more difficult to interpret than inelastic neutron scattering (INS). Here we report $\ensuremath{\sim}37$ meV resolution RIXS measurements of the magnetic excitations in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$, the antiferromagnetic parent of one system of high-temperature superconductors. At high energies ($\ensuremath{\sim}2$ eV), the RIXS spectra show angular-dependent $dd$ orbital excitations in agreement with previous RIXS studies but show new structure. They are interpreted with single-site multiplet calculations. At low energies ($\ensuremath{\lesssim}0.3$ eV), we model the wave-vector-dependent single magnon RIXS intensity as the product of the calculated single-ion spin-flip RIXS cross section and the dynamical structure factor $S(\mathbf{Q},\ensuremath{\omega})$ of the spin-wave excitations. When $S(\mathbf{Q},\ensuremath{\omega})$ is extracted from our data, the wave-vector-dependence of the single-magnon pole intensity shows a similar variation to that observed by INS. Our results confirm that suitably corrected RIXS data can yield the genuine wave-vector and energy dependence of $S(\mathbf{Q},\ensuremath{\omega})$ for a cuprate antiferromagnet. In addition to spin waves, our data show structured multimagnon excitations with dispersing peaks in the intensity at energies higher than the single-magnon excitations.

Twisted bilayer graphene (TBG) with interlayer twist angles near the magic angle \ensuremath{\approx}1.08\ifmmode^\circ\else\textdegree\fi{} hosts flat bands and exhibits correlated states including Mott-like insulators, superconductivity, and magnetism. A linear-in-temperature normal state resistivity in TBG has been attributed to an exotic Planckian dissipation mechanism but can be equally well explained in terms of conventional electron-phonon scattering. To address this issue, we perform combined temperature-dependent transport measurements of both the longitudinal and Hall resistivities in near-magic-angle TBG. While the observed longitudinal resistivity follows linear temperature T dependence consistent with previous reports, the Hall resistance shows an anomalous T dependence with the cotangent of the Hall angle $\mathrm{cot}\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Theta}}}_{H}\phantom{\rule{0.28em}{0ex}}\ensuremath{\propto}\phantom{\rule{0.28em}{0ex}}{T}^{2}$. Boltzmann theory for quasiparticle transport predicts that both the resistivity and $\mathrm{cot}\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Theta}}}_{H}$ should have the same T dependence, contradicting the observed behavior. This failure of quasiparticle-based theories is reminiscent of other correlated strange metals such as cuprates.

Solid-state superconducting heterostructures can provide a possible source of entangled electrons for the purpose of quantum-information processing, and shot noise can indicate when this occurs. Shot noise cross-correlations in a ballistic NSNSN (normal-superconductor-normal-superconductor-normal) is normally negative for electron currents, but becomes positive for entangled electrons that result from the breakup of Cooper pairs. A one-to-one correspondence is found between the energies of the quasibound states and regions of positivity in the cross-correlated shot noise. The quasibound states of the NSNSN system are associated with poles of the NSNSN scattering matrix. We find that regions of positive cross-correlated shot noise distributions, and the associated emission of entangled electrons, exist over a wide range of system sizes and in the presence of multiple quasibound states. We also find that, at the quasibound-state energies, the Andreev approximation is not adequate to describe the key physical processes in the NSNSN device.

We studied the electronic structure of the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ insulator within the density-functional theory using the generalized gradient approximation while taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ undergoes a pressure-induced structural and magnetic phase transition at ${P}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}4$ GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by the formation of ${\mathrm{Ir}}_{2}$ dimers on the zigzag chains, with an Ir-Ir distance of $\ensuremath{\sim}2.66$ \AA{}, even shorter than that of metallic Ir. The strong dimerization stabilizes the bonding molecular-orbital state, leads to the collapse of the magnetism, and opens the energy gap with a concomitant electronic phase transition from a Mott insulator to band insulator. The resonant inelastic x-ray scattering spectra (RIXS) at the Ir ${L}_{3}$ edge were investigated theoretically from first principles. The calculated results are in good agreement with the experimental data. We show that the drastic reconstruction of the RIXS spectral peak at 0.7 eV associated with the structural $Fddd\ensuremath{\rightarrow}C2/c$ phase transition at ${P}_{c}$ can be related to the disappearing of the Coulomb correlations in the high-pressure $C2/c$ phase.