Intriguing lattice dynamics have been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in $\mathrm{S}{\mathrm{r}}_{14}\mathrm{C}{\mathrm{u}}_{24}{\mathrm{O}}_{41}$, which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7–1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in $\mathrm{S}{\mathrm{r}}_{14}\mathrm{C}{\mathrm{u}}_{24}{\mathrm{O}}_{41}$ crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.

The study of quantum quenches in integrable systems has significantly advanced with the introduction of the quench action method, a versatile analytical approach to nonequilibrium dynamics. However, its application is limited to those cases where the overlaps between the initial state and the eigenstates of the Hamiltonian governing the time evolution are known exactly. Conversely, in this work we consider physically interesting initial states for which such overlaps are still unknown. In particular, we focus on different classes of product states in spin-$1/2$ and spin-1 integrable chains, such as tilted ferromagnets and antiferromagnets. We get around the missing overlaps by following a recent approach based on the knowledge of complete sets of quasilocal charges. This allows us to provide a closed-form analytical characterization of the effective stationary state reached at long times after the quench, through the Bethe ansatz distributions of particles and holes. We compute the asymptotic value of local correlations and check our predictions against numerical data.

The angular dependence of the transmission of sound in air through four types of two-dimensional slit arrays formed of aluminium slats is explored, both experimentally and numerically. For a simple, subwavelength periodic slit array, it is well known that Fabry-Perot-like waveguide resonances, supported by the slit cavities, coupled to diffracted evanescent waves, result in enhanced acoustic transmission at frequencies determined by the length, width, and separation of each slit cavity. We demonstrate that altering the spacing or width of some of the slits to form a compound array (i.e., an array having a basis comprised of more than one slit) results in sharp dips in the transmission spectra, which may have a strong angular dependence. These features correspond to phase resonances, which have been studied extensively in the electromagnetic case. This geometry allows for additional near-field configurations compared to the simple array, whereby the field in adjacent cavities can be out of phase. Several types of compound slit arrays are investigated; one such structure is optimized to minimize the effect of boundary-layer loss mechanisms present in each slit cavity, thereby achieving a deep, sharp transmission minimum in a broad maximum.

We present, theoretically and experimentally, the observation of the Rashba effect in photonic lattices, where the effect is brought about by an artificial gauge field, induced by the geometry of the system. In doing that, we demonstrate a particular form of coupling between pseudospin and momentum, resulting in spin-dependent shifts in the spectrum. Our system consists of two coupled, oppositely tilted waveguide arrays, where the evolution of an optical beam allows for probing the dynamics of the evolving wave packets, and the formation of spectral splitting. We show that the Rashba effect can be amplified or decreased through optical nonlinear effects, which correspond to mean-field interactions in various systems such as cold-atom lattices and exciton-polariton condensates.

The search for new ferroelectric (FE) materials holds promise for broadening our understanding of FE mechanisms and extending the range of application of FE materials. Here we investigate a class of $AB{\mathrm{O}}_3$ and $A_{2}B{B}^{\prime }{\mathrm{O}}_6$ materials that can be derived from the $X_2{\mathrm{O}}_3$ corundum structure by mixing two or three ordered cations on the $X$ site. Most such corundum derivatives have a polar structure, but it is unclear whether the polarization is reversible, which is a requirement for a FE material. In this paper, we propose a method to study the FE reversal path of materials in the corundum derivative family. We first categorize the corundum derivatives into four classes and show that only two of these allow for the possibility of FE reversal. We then calculate the energy profile and energy barrier of the FE reversal path using first-principles density functional methods with a structural constraint. Furthermore, we identify several empirical measures that can provide a rule of thumb for estimating the energy barriers. Finally, the conditions under which the magnetic ordering is compatible with ferroelectricity are determined. These results lead us to predict several potentially new FE materials.

Recent experiments access the time-resolved photoelectron signal originating from plasmon satellites in correlated materials and address their buildup and decay in real time. Motivated by these developments, we present the Kadanoff-Baym formalism for the nonequilibrium time evolution of interacting fermions and bosons. In contrast to the fermionic case, the bosons are described by second-order differential equations. Solution of the bosonic Kadanoff-Baym equations—which is the central ingredient of this work—requires substantial modification of the usual two-times electronic propagation scheme. The solution is quite general and can be applied to a number of problems, such as the interaction of electrons with quantized photons, phonons, and other bosonic excitations. Here the formalism is applied to the photoemission from a deep core hole accompanied by plasmon excitation. We compute the time-resolved photoelectron spectra and discuss the effects of intrinsic and extrinsic electron energy losses and their interference.

Phonon spectra of detwinned ${\mathrm{SrFe}}_2{\mathrm{As}}_2$ crystals, as measured by inelastic x-ray scattering, show clear anisotropy accompanying the magnetostructural transition at 200 K. We model the mode splitting using magnetic DFT calculations, including a phenomenological reduction in force-constant anisotropy that can be attributed to magnetic fluctuations. This serves as a starting point for a general model of phonons in this material applicable to both the antiferromagnetically ordered phase and the paramagnetic phase. Using this model, the measured splitting in the magnetic phase below $T_N$, and the measured phonon linewidth, we set a lower bound on the mean magnetic fluctuation frequency above $T_N$ at 210 K.

This paper provides a review and fundamental physical interpretation for the effective densities and moduli of acoustic metamaterials. We introduce the terminology of hidden force and hidden source of volume: the effective density or modulus is negative when the hidden force or source of volume is larger than, and operates in antiphase to, respectively, the force or volume change that would be obtained in their absence. We demonstrate this ansatz for some established acoustic metamaterials with elements based on membranes, Helmholtz resonators, springs, and masses. The hidden force for membrane-based acoustic metamaterials, for instance, is the force from the membrane tension. The hidden source for a Helmholtz-resonator-based metamaterial is the extra air volume injected from the resonator cavity. We also explain the analogous concepts for pure mass-and-spring systems, in which case, hidden forces can arise from masses and springs fixed inside other masses, whereas hidden sources—more aptly termed hidden expanders of displacement in this case—can arise from light rigid trusses coupled to extra degrees of freedom for mechanical motion such as the case of coupling to masses that move at right angles to the wave-propagation direction. This overall picture provides a powerful tool for conceptual understanding and design of new acoustic metamaterials, and avoids common pitfalls involved in determining the effective parameters of such materials.

Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations $\hslash {\omega }_j$(q), phonon densities of states $g$($\hslash \omega$), and isochoric temperature-dependent vibrational heat capacities $c_v\left(T\right)$. We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities $c_v\left(T\right)$, computed within the harmonic approximation from $\hslash {\omega }_j$(q) values, increase with temperature from $0.4\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 100 K to $1.4\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 200 K and $1.9\times {10}^{-4}\mathrm{eV}/\mathrm{atom}\mathrm{K}$ at 300 K, in excellent agreement with isobaric heat capacity values $c_p\left(T\right)$ between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence ($\theta \lesssim {\theta }_c$, where ${\theta }_c$ is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to $\lesssim 10\mathrm{nm}$.

We study the real-time dynamics of a translationally invariant quantum spin chain, based on the East kinetically constrained glass model, in search for evidence of many-body localization in the absence of disorder. Numerical simulations indicate a change, controlled by a coupling parameter, from a regime of fast relaxation-corresponding to thermalization-to a regime of very slow relaxation. This slowly relaxing regime is characterized by dynamical features usually associated with nonergodicity and many-body localization (MBL): memory of initial conditions, logarithmic growth of entanglement entropy, and nonexponential decay of time correlators. We show that slow relaxation is a consequence of sensitivity to spatial fluctuations in the initial state. While numerical results and physical considerations indicate that relaxation time scales grow markedly with size, our finite size results are consistent both with an MBL transition, expected to only occur in disordered systems, and with a pronounced quasi-MBL crossover.

We generalize the Wannier interpolation of the electron-phonon matrix elements to the case of polar-optical coupling in polar semiconductors. We verify our methodological developments against experiments, by calculating the widths of the electronic bands due to electron-phonon scattering in GaAs, the prototype polar semiconductor. The calculated widths are then used to estimate the broadenings of excitons at critical points in GaAs and the electron-phonon relaxation times of hot electrons. Our findings are in good agreement with available experimental data. Finally, we demonstrate that while the Fröhlich interaction is the dominant scattering process for electrons/holes close to the valley minima, in agreement with low-field transport results, at higher energies, the intervalley scattering dominates the relaxation dynamics of hot electrons or holes. The capability of interpolating the polar-optical coupling opens new perspectives in the calculation of optical absorption and transport properties in semiconductors and thermoelectrics.

We report a polarized Raman study of the Weyl semimetal TaAs. We observe all the optical phonons, with energies and symmetries consistent with our first-principles calculations. We detect additional excitations assigned to multiple-phonon excitations. These excitations are accompanied by broad peaks separated by $140 {\mathrm{cm}}^{-1}$ that are also most likely associated with multiple-phonon excitations. We also noticed a sizable $B_1$ component for the spectral background, for which the origin remains unclear.

We consider near-field heat transfer with nonzero chemical potential for photons, as can occur between two semiconductor bodies, held at different temperatures with at least one of the bodies under external bias. We show that the dependence of radiative heat flux on chemical potential enables electronic control of both the direction and magnitude of near-field heat transfer between the two bodies. Moreover such a configuration can operate as a solid-state cooling device whose efficiency can approach the Carnot limit in the ideal case. Significant cooling can also be achieved in the presence of inherent nonidealities including Auger recombination and parasitic phonon-polariton heat transfer.

Lattice thermal conductivities of zincblende- and wurtzite-type compounds with 33 combinations of elements are calculated with the single-mode relaxation-time approximation and a full solution of the linearized phonon Boltzmann equation from first-principles anharmonic lattice dynamics calculations. In nine zincblende-type compounds, distributions of phonon linewidths (inverse phonon lifetimes) are discussed in detail. The phonon linewidths vary nonsmoothly with respect to wave vector, which is explained from the imaginary parts of the self-energies. It is presented that detailed combination of phonon-phonon interaction strength and three-phonon selection rule is critically important to determine phonon lifetime for these compounds. This indicates difficulty to predict phonon lifetime quantitatively without anharmonic force constants. However, it is shown that joint density of states weighted by phonon numbers, which is calculated only from harmonic force constants, can be potentially used for a screening of the lattice thermal conductivity of materials.

We study how polaronic states form as a function of time due to strong electron-phonon coupling, starting from a hot electron distribution which is representative of a photoinduced metallic state immediately after laser excitation. For this purpose we provide the exact solution of the single-electron Holstein model within nonequilibrium dynamical mean-field theory. In particular, this allows us to reveal key features of the transient metallic state in the numerically most challenging regime, the adiabatic regime, in which phonon frequencies are smaller than the electronic bandwidth: Initial coherent phonon oscillations are strongly damped, leaving the system in a mixture of excited polaron states and metastable delocalized states. We compute the time-resolved photoemission spectrum, which makes it possible to disentangle two contributions. The existence of long-lived delocalized states suggests ways to externally control transient properties of photodoped metals.

We study the real-time dynamics of a highly excited charge carrier coupled to quantum phonons via a Holstein-type electron-phonon coupling. This is a prototypical example for the nonequilibrium dynamics in an interacting many-body system where excess energy is transferred from electronic to phononic degrees of freedom. We use diagonalization in a limited functional space (LFS) to study the nonequilibrium dynamics on a finite one-dimensional chain. This method agrees with exact diagonalization and the time-evolving block-decimation method, in both the relaxation regime and the long-time stationary state, and among these three methods it is the most efficient and versatile one for this problem. We perform a comprehensive analysis of the time evolution by calculating the electron, phonon and electron-phonon coupling energies, and the electronic momentum distribution function. The numerical results are compared to analytical solutions for short times, for a small hopping amplitude and for a weak electron-phonon coupling. In the latter case, the relaxation dynamics obtained from the Boltzmann equation agrees very well with the LFS data. We also study the time dependence of the eigenstates of the single-site reduced density matrix, which defines the so-called optimal phonon modes. We discuss their structure in nonequilibrium and the distribution of their weights. Our analysis shows that the structure of optimal phonon modes contains very useful information for the interpretation of the numerical data.

The efficiencies and coefficients of performance of three-terminal devices, comprising two electronic terminals and a thermal one (e.g., a boson bath), are discussed. In particular, two procedures are analyzed. (a) One of the electronic terminals is cooled by investing thermal power (from the thermal bath) and electric power (from voltage applied across the electronic junction); (b) the invested thermal power from the boson bath is exploited to cool one electronic terminal and to produce electric power. Rather surprisingly, the coefficient of performance of (b) can be enhanced as compared to that of (a).

A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers.

Twisted metamaterials, or arrays of identical planar metasurfaces stacked with a sequential rotation, have been recently introduced to realize broadband circular dichroism. Here we develop a generalized Floquet analysis to obtain the exact modal solutions for eigenwaves supported by these structures. The dispersion relation and wave propagation in twisted metamaterials are discussed in detail. Our analysis shows how the modal dispersion in these metamaterials becomes inherently different from the one of conventional periodic structures and how the eigenmodes support specific circular polarization properties based on a lattice effect, even when achiral inclusions are considered. These wave properties are ideal to realize optical devices that manipulate the polarization state of light over broad bandwidths. By analyzing the physical nature of these modes, including complex modes, we also extend the application of twisted metamaterials to realize passband and stop-band nanophotonic structures with strong polarization manipulation properties.

We present a microscopic theory for ultrafast control of solids with high-intensity terahertz frequency optical pulses. When resonant with selected infrared-active vibrations, these pulses transiently modify the crystal structure and lead to new collective electronic properties. The theory predicts the dynamical path taken by the crystal lattice using first-principles calculations of the energy surface and classical equations of motion, as well as symmetry considerations. Two classes of dynamics are identified. In the perturbative regime, displacements along the normal mode coordinate of symmetry-preserving Raman active modes can be achieved by cubic anharmonicities. This explains the light-induced insulator-to-metal transition reported experimentally in manganites. We predict a regime in which ultrafast instabilities that break crystal symmetry can be induced. This nonperturbative effect involves a quartic anharmonic coupling and occurs above a critical threshold, below which the nonlinear dynamics of the driven mode displays softening and dynamical stabilization.