Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.

Recent x-ray magnetic circular dichroism experimental investigations on the magnetism of Pt overlayers deposited on ferromagnetically ordered Co films demonstrated the possibility of controlling the induced magnetic moment in Pt by applying an external electric field [Phys. Rev. Lett. 120, 157203 (2018)]. Based on first-principles calculations, we investigate the electronic and magnetic properties of Pt layers in $\mathrm{Pd}\left(001\right)/\mathrm{Co}/\mathrm{Pt}$ thin film structures exposed to an external electric field. Due to the Co underlayer, the surface Pt layers have induced moments that are modified by an external electric field. The field-induced changes can be explained by the modified spin-dependent orbital hybridization that varies nonlinearly with the field strength. We calculate the x-ray absorption and the x-ray magnetic circular dichroism spectra for an applied external electric field and examine its impact on the spectra in the Pt layer around the $L_2$ and $L_3$ edges. We also determine the layer-dependent magnetocrystalline anisotropy, and we show that the anisotropy can be tuned easily in the different layers by the external electric field.

Knowledge of the magnetic ordering temperature is one of the fundamental physical properties characterizing material behavior. Magnetism is not only related to the magnitude of magnetic moments within specific magnetic ordering, but it also influences other physical properties. In the present work, we determine magnetic ordering temperatures for high-entropy alloys (HEAs), with examples of the Al- and Mo-based Cantor alloy derivatives. Nowadays, HEAs represent a promising class of materials in the sense of mechanical engineering. Regarding the paramagnetic state of the well-known and well-explored Cantor alloy, we deal with its Mo and Al derivatives, which offer higher possible ordering temperatures. We discuss the differences between $p$-type and $d$-type substitution as well as their influence on the magnetic behavior. We determine the ordering temperatures based on the ab initio calculated magnetic exchange interactions in terms of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. Based on the thorough description of magnetic pair exchange interactions, a particular composition's influence on the achieved critical temperatures is described.

The curling spin wave modes of a ferromagnetic vortex confined to a microscale disk have been directly imaged in response to a microwave field excitation using time-resolved scanning Kerr microscopy. Micromagnetic simulations have been used to explore the interaction of gyrotropic vortex core dynamics with the curling modes observed in the region of circulating in-plane magnetization. Hybridization of the fundamental gyrotropic mode with the degenerate, lowest frequency, azimuthal modes has previously been reported to lead to their splitting and counterpropagating motion, as we observe in our spectra and measured images. The curling nature of the modes can be ascribed to asymmetry in the static and dynamic magnetization across the disk thickness, but here we also present evidence that spiral spin waves emitted by the core can influence the spatial character of higher frequency curling modes for which hybridization is permitted only with gyrotropic modes of the same sense of azimuthal motion. While it is challenging to identify if such modes are truly hybridized from the mode dispersion in a confined disk, our simulations reveal that spiral spin waves from the core may act as mediators of the interaction between the core dynamics and azimuthal modes, enhancing the spiral nature of the curling mode. At higher frequency, modes with radial character only do not exhibit marked curling, but instead show evidence of interaction with spin waves generated at the edge of the disk. The measured spatiotemporal character of the observed curling modes is accurately reproduced by our simulations, which reveal the emission of propagating short-wavelength spiral spin waves from both core and edge regions of the disk. Our simulations suggest that the propagating modes are not inconsequential, but may play a role in the dynamic overlap required for hybridization of modes of the core and in-plane magnetized regions. These results are of importance to the fields of magnonics and spintronics that aim to utilize spin wave emission from highly localized, nanoscale regions of nonuniform magnetization, and their subsequent interaction with modes that may be supported nearby.

In the novel superfluid polar phase realized in liquid ${}^3\mathrm{He}$ in highly anisotropic aerogels, a quantum transition to the polar-distorted A (PdA) phase may occur at low but finite pressure $P_c\left(0\right)$. It is shown that the nontrivial quantum dynamics of the critical fluctuation of PdA order is induced by the presence of both columnar impurity scattering leading to Anderson's theorem for the polar phase and the line node of the quasiparticle gap in the state, and that, in contrast to the situation of the normal to B-phase transition in isotropic aerogels, a weakly divergent behavior of the compressibility appears in the quantum critical region close to $P_c\left(0\right)$.

The parent compounds of iron-based superconductors are magnetically ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hund's coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ series consists of magnetically ordered bad metal ($x=0$), superconducting ($x\approx 0.02$) and magnetically ordered semiconducing/insulating ($x\approx 0.5$) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use x-ray absorption spectroscopy and resonant inelastic x-ray scattering to study the valence state of Fe and spin dynamics in two ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ compounds ($x=0$ and 0.47). We find that magnetism in both compounds arises from ${\mathrm{Fe}}^{2+}$ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in ${\mathrm{NaFe}}_{0.53}{\mathrm{Cu}}_{0.47}\mathrm{As}$ arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.

The two-dimensional infinite projected entangled pair state tensor network is evolved in imaginary time with the full update (FU) algorithm to simulate the Shastry-Sutherland model in a magnetic field at finite temperature directly in the thermodynamic limit. We focus on the phase transition into the $m=\frac{1}{2}$ magnetization plateau, which was observed in experiments on ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$. For the largest simulated bond dimension, the early evolution in the high-temperature regime is simulated with the simple update (SU) scheme and then, as the correlation length increases, continued with the FU scheme towards the critical regime. We apply a small symmetry-breaking bias field and then extrapolate towards zero bias using a simple scaling theory in the bias field. The combined SU $+$ FU scheme provides an accurate estimate of the critical temperature, even though the results could not be fully converged in the bond dimension in the vicinity of the transition. The critical temperature estimate is improved with a generalized scaling theory that combines two divergent length scales: One due to the bias, and the other due to the finite bond dimension. The obtained results are consistent with the transition being in the universality class of the two-dimensional classical Ising model. The estimated critical temperature is 3.5(2) K, which is well above the temperature 2.1 K used in the experiments.

In this paper, starting from a quantum master equation, we discuss the interaction between two negatively charged nitrogen-vacancy color centers in diamond via exciton-polaritons propagating in a two-dimensional transition metal dichalcogenide layer in close proximity to a diamond crystal. We focus on the optical 1.945 eV transition and model the nitrogen-vacancy color centers as two-level (artificial) atoms. We find that the interaction parameters and the energy-level renormalization constants are extremely sensitive to the distance of the nitrogen-vacancy centers to the transition-metal dichalcogenide layer. Analytical expressions are obtained for the spectrum of the exciton-polaritons and for the damping constants entering the Lindblad equation. The conditions for occurrence of exciton mediated superradiance are discussed.

We study disorder effects in a two-dimensional system with chiral symmetry and find that disorder can induce a quadrupole topological insulating phase (a higher-order topological phase with quadrupole moments) from a topologically trivial phase. Their topological properties manifest in a topological invariant defined based on effective boundary Hamiltonians, the quadrupole moment, and zero-energy corner modes. We find gapped and gapless topological phases and a Griffiths regime. In the gapless topological phase, all the states are localized, while in the Griffiths regime, the states at zero energy become multifractal. We further apply the self-consistent Born approximation to show that the induced topological phase arises from disorder renormalized masses. We finally introduce a practical experimental scheme with topoelectrical circuits where the predicted topological phenomena can be observed by impedance measurements. Our work opens the door to studying higher-order topological Anderson insulators and their localization properties.

We investigate heat circulators where a phase coherent region is contacted by three leads that are either normal- or superconducting. A magnetic field, and potentially the superconducting phases, allow to control the preferential direction of the heat flow between the three-different temperature-biased contacts. The main goal of this study is to analyze the requirements for heat circulation in nonideal devices, in particular focusing on sample-to-sample variations. Quite generally, we find that the circulation performance of the devices is good as long as only a few transport channels are involved. We compare the performance of circulators with normal conducting contacts to those with superconducting contacts and find that the circulation coefficients are essentially unchanged.

Photoemission driven by ultrafast optical fields enables spatiotemporal control of electron motion with extremely high precision. Here, we present a quantum model for ultrafast photoelectron emission from a dc-biased surface induced by laser pulses of arbitrary duration, ranging from subcycle to continuous wave, by solving the time-dependent Schrödinger equation exactly. The single formulation is valid from photon-driven electron emission in low intensity optical fields to field-driven emission in high intensity optical fields. We find the emitted charge per pulse oscillatorily increases with pulse repetition rate, due to varying coherent interaction of neighboring laser pulses. For a well-separated single pulse, our results recover the experimentally observed vanishing carrier-envelope phase sensitivity in the optical-field regime. We also find that applying a large dc field to the photoemitter is able to greatly enhance the photoemission current and in the meantime substantially shorten the current pulse.

Measuring local temperatures of open systems out of equilibrium is emerging as a novel approach to study the local thermodynamic properties of nanosystems. An operational protocol has been proposed to determine the local temperature by coupling a probe to the system and then minimizing the perturbation to a certain local observable of the probed system. In this paper, we first show that such a local temperature is unique for a single quantum impurity and the given local observable. We then extend this protocol to open systems consisting of multiple quantum impurities by proposing a local minimal perturbation condition (LMPC). The influence of quantum resonances on the local temperature is elucidated by both analytic and numerical results. In particular, we demonstrate that quantum resonances may give rise to strong oscillations of the local temperature along a multi-impurity chain under a thermal bias.

The energy landscape of helium-nitrogen mixtures is explored by ab initio evolutionary searches, which predicted several stable helium-nitrogen compounds in the pressure range from 25 to 100 GPa. Among these high-pressure compounds, the monoclinic structure of ${\mathrm{HeN}}_{22}$ consists of neutral He atoms, partially ionic dimers ${\mathrm{N}}_2^{\delta -}$, and lanternlike cages ${\mathrm{N}}_{20}^{\delta +}$, showing a certain amount of charge transfer within the nitrogen framework. The monoclinic (${\mathrm{HeN}}_{20}{)}^{\delta +}{\mathrm{N}}_2^{\delta -}$ may be quenchable to ambient pressure with the estimated energy density of 10.44 kJ/g, which is $\sim 2.4$ times larger than that of trinitrotoluene, indicating a very promising high-energy-density material.

We characterized the temperature dependence of the complex dielectric susceptibility spectrum of a perovskite $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{Br}}_3$ single crystal by using heterodyne interference spectroscopy. Owing to a time-domain measurement that is not influenced by incoherent lattice relaxations, we were able to detect a low-energy indirect-gap band. We observed two peaks in the energy region below the exciton transition at the $R$ point in the imaginary part of the spectrum. The temperature dependences of the energies of these peaks were different from the dependence of the energy of the direct-transition exciton. The trends of both the splitting width and peak intensities can be explained by assuming a Rashba spin-split band.

Half-metallic ferromagnetic Heusler compounds represent an important class of materials for spintronic applications. We experimentally observe the dispersion of electronic bands with spin resolution in three representative Heusler compounds (${\mathrm{Co}}_2\mathrm{MnGa}, {\mathrm{Co}}_2\mathrm{MnSi}$, and ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$). Bulk-sensitive measurements at very low photon energy are complemented by spin-integrated hard x-ray angular resolved photoemission spectroscopy. The dispersion of majority and minority electrons allows us to link exchange splitting and band filling to the number of valence electrons $N_v$. Photoexcitation at $h\nu =6.05\mathrm{eV}$ gives access to the spin-polarization texture $\mathit{P}\left(E_B,k_x,k_y\right)$ of the bulk bands covering a $\left(k_x,k_y\right)$ range of 60% of the Brillouin zone. We find that ${\mathrm{Co}}_2\mathrm{MnSi}$ and ${\mathrm{Co}}_2{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ exhibit minority band gaps of 0.5 and 0.35 eV, i.e., decreasing band gap with increasing $N_{\mathrm{V}}$ in contrast to the rigid-band expectation. For ${\mathrm{Co}}_2\mathrm{MnGa}$, the minority valence state maximum lies at approx. 0.2 eV above $E_{\mathrm{F}}$.

The topologically nontrivial band structure of A15 compounds has drawn a significant amount of attention owing to the possible realization of topological superconductivity. The superconducting ground state of the A15 alloys ${\mathrm{Ti}}_{3}X$ ($X=\mathrm{Ir},\mathrm{Sb}$), by muon spectroscopy, has been investigated. Zero-field muon spin relaxation measurements have shown that time-reversal symmetry is preserved in these materials. Furthermore, specific-heat and transverse-field muon rotation measurements rule out any possibility of a nodal or anisotropic superconducting gap but reveal a conventional isotropic $s$-wave gap structure. This supports the idea that these A15 superconductors are time-reversal-symmetry-preserved topological superconducting candidate compounds.

One challenge of studying the many-body localization transition is defining the length scale that diverges upon the transition to the ergodic phase. In this manuscript we explore the localization properties of a ring with onsite disorder subject to an imaginary magnetic flux. We connect the imaginary flux which delocalizes single-particle orbitals of an Anderson-localized ring with the localization length of an open chain. We thus identify the delocalizing imaginary flux per site with an inverse localization length characterizing the transport properties of the open chain. We put this intuition to use by exploring the phase diagram of a disordered interacting chain, and we find that the inverse imaginary flux per bond provides an accessible description of the transition and its diverging localization length.

We study the exchange between electron and magnon spins at the interface of an antiferromagnet and a heavy metal at finite temperatures. The underlying physical mechanism is based on spin torque associated with the creation/annihilation of thermal magnons with right-hand and left-hand polarization. The creation/annihilation process depends strongly on the relative orientation between the polarization of the electron and the magnon spins. For a sufficiently strong spin-transfer torque (STT), the conversion process becomes nonlinear, generating a nonzero net spin pumping current in the antiferromagnet that can be detected in the neighboring metal layer. Applying an external magnetic field renders possible the manipulation of the STT driving thermal spin pumping. Our theoretical results are experimentally feasible and are of a direct relevance to antiferromagnet-based spintronic devices.