The dynamics of electrons and phonons in metals upon laser excitation are often described by the two-temperature model, which assumes that both subsystems are individually in thermal equilibrium. However, recent experiments show that this description is not sufficient to describe the out-of-equilibrium dynamics on ultrashort timescales. Here, assuming a thermalized electronic system, we extend and apply a parameter-free microscopic out-of-equilibrium model to describe the ultrafast laser-induced phonon and electron temperature dynamics of various metallic systems such as gold, aluminum, iron, nickel, and cobalt. We report strong deviations from the two-temperature model on the picosecond timescale for all the materials studied, even for those where the assumption of separate thermal equilibrium seemed less restrictive, like in gold. Furthermore, we demonstrate the importance of the mode dependence of the electron-phonon coupling for the relaxation process and reveal the significance of this channel in the lattice equilibration.

Orthorhombic $\mathrm{HoMn}{\mathrm{O}}_3$ is a multiferroic in which Mn antiferromagnetic order induces ferroelectricity. A second transition occurs within the multiferroic phase, in which a strong enhancement of the ferroelectric polarization occurs concomitantly to antiferromagnetic ordering of Ho $4f$ magnetic moments. Using the element selectivity of resonant x-ray diffraction, we study the magnetic order of the Mn $3d$ and Ho $4f$ moments. We explicitly show that the Mn magnetic order is affected by the Ho $4f$ magnetic ordering transition. Based on the azimuthal dependence of the $\left(0q0\right)$ and $\left(01-q0\right)$ magnetic reflections, we suggest that the Ho $4f$ order resembles an $ac$ cycloid. Using nonresonant diffraction, we show that the magnetically induced polar lattice distortion is unaffected by the Ho ordering, suggesting a mechanism through which the Ho order affects ferroelectric polarization without affecting the lattice in the same manner as the Mn order.

Motivated by the physical properties of vesignieite ${\mathrm{BaCu}}_3{\mathrm{V}}_2{\mathrm{O}}_8{\left(\mathrm{OH}\right)}_2$, we study the $J_1-J_3$ Heisenberg model on the kagome lattice, which is proposed to describe this compound for $J_1<0$ and $J_3\gg \left|J_1\right|$. The nature of the classical ground state and the possible phase transitions are investigated through analytical calculations and parallel tempering Monte Carlo simulations. For $J_1<0$ and $J_3>\frac{1+\sqrt{5}}{4}\left|J_1\right|$, the ground states are not all related by Hamiltonian symmetry. Order appears at low temperature via the order by disorder mechanism, favoring collinear configurations and leading to an emergent $q=4$ Potts parameter, that induces a finite temperature phase transition. For $J_3$ between $\frac{1}{4}\left|J_1\right|$ and $\frac{1+\sqrt{5}}{4}\left|J_1\right|$, the ground state goes through a succession of semispiral states, possibly giving rise to multiple phase transitions at low temperatures. The effects of quantum fluctuations are studied through linear spin-wave approximation and high-temperature expansions of the $S=1/2$ model.

Temperature-dependent atomic structural evolutions of liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys have been studied by in situ high-energy x-ray-diffraction (HEXRD) experiments combined with ab initio molecular-dynamics simulations. The experimental data show a reversible structural crossover at about $1050\sim 1100 (\pm 50)\mathrm{K}$ in both liquid ${\mathrm{Ag}}_{60}{\mathrm{Ga}}_{40}$ and ${\mathrm{Ag}}_{70}{\mathrm{Ga}}_{30}$ alloys. Obvious changes of the electrical resistivity, absolute thermoelectric power, and atomic diffusivity around the similar temperature range for both Ag-Ga liquids strongly support the HEXRD results. The origin of the liquid-to-liquid crossover in both Ag-Ga liquids was suggested to link with the rearrangements of Ag and Ga atoms, i.e., Ag and Ga atoms prefer to associate with themselves in the higher temperature range above 1100 K, consistent with the accelerated increase of the strong covalently bonded Ga–Ga dimers in both Ag-Ga liquids. In addition, more studies from the energy aspect are still desirable to understand the rearrangements of Ag and Ga atoms in the higher temperature range in both Ag-Ga liquids.

Prediction of the stable crystal structure for multinary (ternary or higher) compounds with unexplored compositions demands fast and accurate evaluation of free energies in exploring the vast configurational space. The machine-learning potential such as the neural network potential (NNP) is poised to meet this requirement but a dearth of information on the crystal structure poses a challenge in choosing training sets. Herein we propose constructing the training set from density functional theory (DFT)–based dynamical trajectories of liquid and quenched amorphous phases, which does not require any preceding information on material structures except for the chemical composition. To demonstrate suitability of the trained NNP in the crystal structure prediction, we compare NNP and DFT energies for ${\mathrm{Ba}}_2{\mathrm{AgSi}}_3$, ${\mathrm{Mg}}_2{\mathrm{SiO}}_4$, ${\mathrm{LiAlCl}}_4$, and ${\mathrm{InTe}}_2{\mathrm{O}}_5\mathrm{F}$ over experimental phases as well as low-energy crystal structures that are generated theoretically. For every material, we find strong correlations between DFT and NNP energies, ensuring that the NNPs can properly rank energies among low-energy crystalline structures. We also find that the evolutionary search using the NNPs can identify low-energy metastable phases more efficiently than the DFT-based approach. By proposing a way to developing reliable machine-learning potentials for the crystal structure prediction, this work paves the way to identifying unexplored multinary phases efficiently.

In this work we investigate defect modes localized at the ends and within the bulk of 1D metamaterial analogs of molecular aggregates. The study is undertaken in the microwave regime, where the cm scale of the metamaterial analog provides an opportunity to directly probe the modes deep within their near fields in ways not easily achieved in molecular systems. To demonstrate the power of this approach we compare our observations to predictions from a simple Su, Schrieffer, and Heeger (SSH) model and find good qualitative agreement.

The strong photon-magnon coupling between an electromagnetic cavity and two yttrium iron garnet (YIG) spheres has been investigated in the context of a strong mutual dipolar interaction between the spheres. A decrease in the coupling strength between the YIG spheres and the electromagnetic cavity is observed, along with an increase of the total magnetic losses, as the distance between the spheres is decreased. A model of inhomogeneous broadening of the ferromagnetic resonance linewidth, partly mitigated by the dipolar narrowing effect, reproduces the reduction in the coupling strength observed experimentally. These findings have important implications for the understanding of a strongly coupled photon-magnon system involving densely packed magnetic objects, such as ferromagnetic nanowire arrays, in which the total coupling strength with an electromagnetic cavity might become limited due to mutual dipolar interactions.

We investigate a paradigmatic case of topological superconductivity in a one-dimensional nanowire with $d$-orbitals and a strong interplay of spin-orbital degrees of freedom due to the competition of the orbital Rashba interaction, atomic spin-orbit coupling, and structural distortions. We demonstrate that the resulting electronic structure exhibits an orbital-dependent magnetic anisotropy which affects the topological phase diagram and the character of the Majorana bound states (MBSs). The inspection of the electronic component of the MBSs reveals that the spin-orbital polarization generally occurs along the direction of the applied Zeeman magnetic field, and transverse to the magnetic and orbital Rashba fields. The competition of symmetric and antisymmetric spin-orbit coupling remarkably leads to a misalignment of the spin and orbital moments transverse to the orbital Rashba fields, whose manifestation is essentially orbital dependent. The behavior of the spin-orbital polarization along the applied Zeeman field reflects the presence of multiple Fermi points with inequivalent orbital character in the normal state. Additionally, the spin and spatially resolved density of states leads to distinctive fingerprints of the topological phase, especially when comparing the character of the MBS with the energy excitation close to the gap edge. These findings unveil novel paths to single out hallmarks relevant for the experimental detection of MBSs.

We propose a nonperturbative numerical approach to calculate the spectrum of a many-body Hamiltonian with time and momentum resolution by exactly recreating a scattering event using the time-dependent Schrödinger equation. Akin to an actual inelastic scattering experiment, we explicitly account for the incident and scattered particles (e.g., photons, neutrons, electrons, etc.) in the Hamiltonian and obtain the spectrum by measuring the energy and momentum lost by the particle after interacting with the sample. We illustrate the method by calculating the spin excitations of a Mott-insulating Hubbard chain after a sudden quench with the aid of the time-dependent density matrix renormalization group method. Our formalism can be applied to different forms of spectroscopies, such as neutron and Compton scattering and electron-energy-loss spectroscopy, for instance.

We investigate via exact diagonalization of finite clusters the electronic structure and magnetism of ${\mathrm{M}}_2{\mathrm{O}}_9$ dimers in the mixed-valence hexagonal perovskites ${\mathrm{A}}_3{\mathrm{B}}^{\prime }{\mathrm{M}}_2{\mathrm{O}}_9$ for various different fillings of $4d$ and $5d$ transition-metal M ions. We find that the magnetic moments of such dimers are determined by a subtle interplay of spin-orbit coupling, Hund's coupling, and Coulomb repulsion, as well as the electron filling of the M ions. Most importantly, the magnetic moments are anisotropic and temperature dependent. This behavior is a result of spin-orbit coupling, magnetic field effects, and the existence of several nearly degenerate electronic configurations whose proximity allows the occupation of excited states already at room temperature. This analysis is consistent with experimental susceptibility measurements for a variety of dimer-based materials. Furthermore, we perform a survey of ${\mathrm{A}}_3{\mathrm{B}}^{\prime }{\mathrm{M}}_2{\mathrm{O}}_9$ materials and propose ground-state phase diagrams for the experimentally relevant M fillings of $d^{4.5}$, $d^{3.5}$, and $d^{2.5}$. Finally, our results show that the usually applied Curie-Weiss law with a constant magnetic moment cannot be used in these spin-orbit-coupled materials.

Photoreflectance (PR) spectrum of bulk $2H\text{−}{\mathrm{MoS}}_2$ at energies around its direct band gap is shown to have at least three distinct spectral features over a certain temperature range, although only two are seen in absorption and reflectance spectra. These observations are compared with results from many-body perturbation theory band structure calculations that include exciton formation through the Bethe-Salpeter equation. Apart from the ground state $A(n=1)$ exciton transition around 1.92 eV at 25 K, the next spectral feature around 1.96 eV, whose origin has been much debated, is identified here with a transition $I$ with strong interlayer contributions. Its greater sensitivity to electric fields is established through electroreflectance measurements. The third one around 1.99 eV, which is prominent only in PR, is shown to arise from the first excited state $A(n=2)$ exciton transition. We discuss the previously unexplained observation as to why the $A(n=2)$ feature dominates the PR spectrum at high temperatures. The $A$ exciton is shown to have a quasi-two-dimensional (2D) nature with binding energy of $77\pm 3$ meV under a 2D hydrogenic exciton model.

We discuss the phase-dependent nonlocal thermoelectric effect in a topological Josephson junction in contact with a normal-metal probe. We show that, due to the helical nature of topological edge states, nonlocal thermoelectricity is generated by a purely Andreev interferometric mechanism. This response can be tuned by imposing a Josephson phase difference, through the application of a dissipationless current between the two superconductors, even without the need of applying an external magnetic field. We discuss in detail the origin of this effect and we also provide a realistic estimation of the nonlocal Seebeck coefficient which turns out to be of the order of a few $\mu \text{V}/\text{K}$ at temperatures of a few kelvin.

We analyze the interplay of antiferromagnetism and pairing in the two-dimensional Hubbard model with a moderate repulsive interaction. Coupled charge, magnetic, and pairing fluctuations above the energy scale of spontaneous symmetry breaking are treated by a functional renormalization group flow, while the formation of gaps and order below that scale is treated in mean-field theory. The full frequency dependences of the interaction vertices and gap functions are taken into account. We compute the magnetic and pairing gap functions as a function of doping $p$ and compare with results from a static approximation. In spite of the strong frequency dependences of the effective interactions and of the pairing gap, important physical results from previous static functional renormalization group calculations are confirmed. In particular, there is a sizable doping regime with robust pairing coexisting with Néel or incommensurate antiferromagnetism. The critical temperature for magnetic order is interpreted as the pseudogap crossover temperature. Computing the Kosterlitz-Thouless temperature from the superfluid phase stiffness, we obtain a superconducting dome in the $(p,T)$ phase diagram centered around 15% hole doping.

We perform redundant x-ray diffraction versus temperature experiments in bulk transparent KTN. We find a violation of the standard perovskite cubic-to-tetragonal symmetry breaking at the Curie point in the form of an orthorhombic cell distortion. The lattice distortion spans coherently macroscopic volumes of the sample and is characterized by a negative-to-zero thermal volume expansion. Dielectric measurements and calorimetry indicate that the anomalous behavior is not associated to a distinct thermodynamic phase. The comparison of linear and nonlinear optical scattering experiments with structural data suggests that the lattice distortion is a consequence of strain relaxation to a 3D superlattice of polarization vortices.

We demonstrate that two toric code layers on the square lattice coupled by an Ising interaction display two distinct phases with intrinsic topological order. The second-order quantum phase transition between the weakly coupled ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ and the strongly coupled ${\mathbb{Z}}_2$ topological order can be described by the condensation of bosonic quasiparticles from both sides and belongs to the three-dimensional ${\mathrm{Ising}}^{*}$ universality class. This can be shown by an exact duality transformation to the transverse-field Ising model on the square lattice, which builds on the existence of an extensive number of local ${\mathbb{Z}}_2$ conserved parities. These conserved quantities correspond to the product of two adjacent star operators on different layers. Notably, we show that the low-energy effective model derived about the limit of large Ising coupling is given by an effective single-layer toric code in terms of the conserved quantities of the Ising toric code bilayer. The two topological phases are further characterized by the topological entanglement entropy which serves as a nonlocal order parameter.

Enabling the light control of complex systems on ultrashort timescales gives rise to rich physics with promising applications. Although crucial, the quantitative determination of both the longitudinal and the shear photoinduced strains still remains challenging. Here, by scrutinizing asymmetric Bragg peaks pairs $(\pm h01)$ in ${\mathrm{BiFeO}}_3$ using picosecond time-resolved x-ray diffraction experiments, we simultaneously determine the longitudinal and shear strains. Importantly, we reveal a difference in the dynamical response of the longitudinal strain with respect to the shear one due to an interplay of quasilongitudinal and quasitransverse acoustic modes, well reproduced by our model. Finally, we show that the relative amplitude of those strains can be explained only if both thermal and nonthermal processes contribute to the acoustic phonon photogeneration process.

Chiral magnets possess topological line excitations where the magnetization within each cross section forms a skyrmion texture. We study analytically and numerically the low-energy, nonlinear dynamics of such a skyrmion string in a field-polarized cubic chiral magnet, and we demonstrate that it supports solitary waves. These waves are in general nonreciprocal, i.e., their properties depend on the sign of their velocity $v$, but this nonreciprocity diminishes with decreasing $\left|v\right|$. An effective field-theoretical description of the solitary waves is derived that is valid in the limit $v\to 0$ and gives access to their profiles and their existence regime. Our analytical results are quantitatively confirmed with micromagnetic simulations for parameters appropriate for the chiral magnet FeGe. Similarities with solitary waves found in vortex filaments of fluids are pointed out.

We demonstrate the rise and fall of multiple pseudogaps in the Bardeen-Cooper-Schrieffer to Bose-Einstein condensation crossover in two-band fermionic systems having different pairing strengths in the deep band and in the shallow band. The striking features of this phenomenon are an unusual many-body screening of the pseudogap state and the importance of pair-exchange couplings, which induces multiple pseudogap formation in the two bands. The multiband configuration suppresses pairing fluctuations and the pseudogap opening in the strongly interacting shallow band at small pair-exchange couplings by screening effects, with a possible connection to the pseudogap phenomenology in iron-based superconductors. On the other hand, the multiple pseudogap mechanism is accompanied by the emergence of binary preformed Cooper pairs originating from the interplay between intraband and pair-exchange couplings.

This work reports micromagnetic simulations used to study magnetization reversal in kagome artificial spin ice, taking into account the actual edge imperfections presented by the magnetic nanoislands as a source of disorder. An important advantage of the micromagnetic approach is to discretize a magnetic element in nanometric domains, allowing access to the sample magnetic moments distribution in detail. The limit case of zero disorder is accessed considering a system composed of perfect magnetic islands. The main result is the prediction of a critical angle between the applied magnetic field and the direction of one of the sublattices, above which Dirac strings emerge. For lower values, the reversal occurs through a bidimensional fashion, being smooth or abrupt depending on whether or not roughness is present, respectively. Finally, our simulations open the question of how the contour imperfections really influence the magnetic response of artificial spin ice.