We describe the dynamics of magnetic skyrmions in synthetic antiferromagnets (SAF) with a finite interlayer coupling. Due to the opposite gyrovector of the skyrmions in the two SAF layers, the coupled skyrmions reach a stationary regime with a spatial separation in a direction orthogonal to their velocity. As a consequence, and contrary to the ferromagnetic situation, a transient regime necessarily occurs with a finite acceleration, related to an inertia, and that limits its time response. The formalism developed here, based on two coupled Thiele equations, allows a quantitative description of this phenomenon. The time constant associated with the transient regime scales inversely with the antiferromagnetic coupling constant. We also show that the coupling force reaches a maximal value at a finite skyrmion separation. This sets a maximum velocity limit, beyond which the coupling force cannot stabilize the bound state.

${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ is a self-intercalated transition metal dichalcogenide that hosts tunable electronic filling and magnetism in its semimetallic band structure. Recent angle-resolved photoemission spectroscopy (ARPES) studies have unveiled a systematic shift in this semimetallic band structure relative to the chemical potential with increased Cr doping. This paper presents the temperature and magnetic field dependence of the longitudinal thermopower $S_{xx}$ for different ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$ compositions. We show that as doping increases, the sign of $S_{xx}$ changes from positive to negative at the critical doping level of $\delta \sim 0.5$. This observed doping-dependent trend in the thermopower is consistent with the evolution of the semimetallic band structure from ARPES. Importantly, an anomalous enhancement of the thermoelectric response near $T_{\mathrm{C}}$ is also observed around $\delta \sim 0.5$. Combining information from magnetometry and ARPES measurements, existence of the critical nature of the doping level ${\delta }_c\left(\sim 0.5\right)$ is unveiled in magnetic semimetal ${\mathrm{Cr}}_{1+\delta }{\mathrm{Te}}_2$, where antiferromagnetic fluctuation and near-Fermi-energy pseudogap formation play a potential vital role in enhancing thermoelectric energy conversion.

We apply a recently developed method for solving the linearized phonon Boltzmann equation to study the hydrodynamic thermal transport in dielectrics in the collective limit, i.e., when normal collisions dominate resistive ones. The method recovers Guyer and Krumhansl results for a single Debye branch and extends them to general dispersion relations and branches. Specifically, we obtain explicit microscopic expressions for the phonon distribution and for the transport coefficients in this limit. We find that the phonon distribution differs from the commonly used displaced distribution in two terms: one accounting for viscous flow and another one which allows us to solve a long-standing issue on drifting and driftless second-sound velocities. Thus, the new method allows us to generalize previous results and fill some gaps on fundamental aspects of the collective limit through a simple mathematical formalism. We compare the hydrodynamic framework with previous models and discuss its limitations.

The unconventional properties of magic angle twisted bilayer graphene (TBG), such as correlated insulator states and superconductivity, have drawn significant attention to areas where electron-electron interactions play an important role. However, the role of electron-electron interactions on the plasmon properties of TBG has rarely been studied. In this study, we focus on TBG with a twist angle near the magic angle (1.32 °), and examine the plasmon and optical absorption properties using the continuum model involving Hartree potentials that describe the electron-electron interaction. We find that the Hartree potentials decrease the energy of the intraband plasmon due to the band flattening driven by the interaction between electrons, whereas the energy of the interband plasmon is slightly increased by the Hartree potentials. Moreover, the splitting of the interband absorption peak occurs due to the effects of the Hartree potentials, which can be verified in experiments. Interestingly, the ultrahigh wave location (${\lambda }_{\mathrm{air}}/{\lambda }_p\sim 2000$) of the intraband plasmon enables TBG to be a superior platform for future terahertz optoelectronic devices.

Ultrashort x-ray pulses can be used to disentangle magnetic and structural dynamics and are accordingly utilized here to study the photoexcitation of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ (NNO), a model nickelate exhibiting structural and magnetic dynamics that conspire to induce an insulator-to-metal transition (IMT). During the course of the photoinduced IMT with above-gap excitation, we observe an ultrafast ($<180$ fs) quenching of magnetic order followed by a time delayed collapse of the insulating phase probed by x-ray absorption and THz transmission (450 fs) that correlates with the slowest optical phonon mode involved in the structural transition. A simultaneous order-disorder response at the Ni site and displacive response at the Nd site coexist in the ultrafast magnetic response. Crucially, we observe the optical phonon through its coherent coupling with Nd magnetic order, demonstrating that the magnetic and structural degrees of freedom both contribute in driving the IMT. Density functional theory calculations reveal a consistent scenario where optically driven intersite charge transfer drives a collapse of antiferromagnetic order that in turn destabilizes the charge-ordered phase resulting in an IMT. These experiments provide different modalities for the control of electronic phase transitions in quantum materials based on the ultrafast interplay between structural and magnetic orders created by femtosecond photoexcitation.

Based on micromagnetic simulations and experimental observations of the magnetization and lattice dynamics after the direct optical excitation of the magnetic insulator Bi : YIG or indirect excitation via an optically opaque Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy and magneto-elastic coupling. The strain and temperature of the lattice are quantified via modeling ultrafast x-ray diffraction data. Measurements of the time-resolved magneto-optical Kerr effect agree well with the magnetization dynamics simulated according to the excitation via two mechanisms: the magneto-elastic coupling to the experimentally verified strain dynamics and the ultrafast temperature-induced transient change in the magnetic anisotropy. The numerical modeling proves that, for direct excitation, both mechanisms drive the fundamental mode with opposite phase. The relative ratio of standing spin wave amplitudes of higher-order modes indicates that both mechanisms are substantially active.

Pure and gold-doped amorphous silicon membranes were irradiated with swift heavy ions (75 MeV Ag or 1.1 GeV Au ions) and studied using small-angle x-ray scattering. The samples that were irradiated with 1.1 GeV Au ions produced a scattering pattern consistent with core-shell-type ion tracks of 2.0 $\pm$ 0.1 nm (core) and 7.0 $\pm$ 0.3 nm (total) radius irrespective of gold doping, consistent with radii previously observed [9]. The density in the core is nearly 4% different from that of the surrounding material. The entire track (core $+$ shell) is slightly less dense than the surrounding material, yielding an expansion or hammering constant $A$ of $0.036\pm 0.003$ ${\mathrm{nm}}^2$ per ion track, consistent with the macroscopic “hammering” deformation. No tracks were found in samples irradiated with 75 MeV Ag ions, and no signature specific to the gold impurity doping could be observed.

In a semiclassical model, magnetic fields can originate from the orbital magnetic moments of electrons. Here, using the Biot-Savart law, we theoretically predict the effective magnetic fields induced by chiral phonons in the point-charge model. Taking tellurium as an example, it is found that by applying a temperature gradient, the average effective magnetic field per mode for a specific location within a unit cell can reach the order of 0.01 T at room temperature. We prove that the effective magnetic field induced by chiral phonons has the ability to manipulate the Curie temperature and spontaneous magnetization in a ferromagnetic material or even in a general material.

${\mathrm{U}}_3{\mathrm{Cu}}_4{\mathrm{Ge}}_4$ is a uniaxial ferromagnet that displays a first-order magnetization process (FOMP) at 25 T for field applied along the hard $b$ axis. Here, we report on ultrasound and magnetostriction measurements of ${\mathrm{U}}_3{\mathrm{Cu}}_4{\mathrm{Ge}}_4$ in static and pulsed magnetic fields up to 40 T. The FOMP causes stepwise anomalies in the magnetoelastic properties. ${\mathrm{U}}_3{\mathrm{Cu}}_4{\mathrm{Ge}}_4$ elongates along the $a$ and $c$ axes and shrinks along the $b$ axis, leading to an almost zero volume effect. The sound velocities of the longitudinal and transverse acoustic waves decrease sharply at the FOMP, whereas the sound attenuation shows pronounced peaks. An analysis of the ultrasound data using a mean-field theory suggests the existence of quadrupolar interactions and crystal-electric-field effects. The $5f$ electronic states are between itinerant and localized, typical of uranium-based intermetallic compounds.

We determine and analyze collective normal modes of a finite disk-shaped two-dimensional vortex crystal formed in a compressible bosonic superfluid in an artificial magnetic field. Using the microscopic Gross-Pitaevskii theory in the lowest Landau level approximation, we generate vortex crystal ground states and solve the Bogoliubov–de Gennes equations for small amplitude collective oscillations. We find chiral surface waves that propagate at frequencies larger than those of the bulk Tkachenko modes. Furthermore, we study low frequency bulk excitations and identify a Ruderman mode, which we find is well described by a previously developed low-energy effective field theory.

The mixed-valent iridate ${\mathrm{Ba}}_3{\mathrm{InIr}}_2{\mathrm{O}}_9$ has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits ${\mathrm{Ir}}^{4.5+}$ ions in face-sharing ${\mathrm{IrO}}_6$ octahedra forming ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimers with three $t_{2g}$ holes per dimer. Our results establish ${\mathrm{Ba}}_3{\mathrm{InIr}}_2{\mathrm{O}}_9$ as a cluster Mott insulator. Strong intradimer hopping delocalizes the three $t_{2g}$ holes in quasimolecular dimer states while interdimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of ${\mathrm{Ba}}_3{\mathrm{InIr}}_2{\mathrm{O}}_9$ emerges from spin-orbit entangled quasimolecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of $5d$ compounds. Using single-crystal x-ray diffraction we find the monoclinic space group $C2/c$ already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasimolecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of ${\mathrm{Ba}}_3{\mathrm{InIr}}_2{\mathrm{O}}_9$ to a hopping-induced phase transition based on the condensation of a quasimolecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding $j=\frac{1}{2}$ orbital and the third hole in a bonding $j=\frac{3}{2}$ orbital.

We study the electrical transport of a two-dimensional non-Fermi-liquid with disorder, and we determine the first quantum correction to the semiclassical dc conductivity due to quantum interference. We consider a system with $N$ flavors of fermions coupled to SU($N$) critical matrix bosons. Motivated by the SYK model, we employ the bilocal field formalism and derive a set of finite-temperature saddle-point equations governing the fermionic and bosonic self-energies in the large-$N$ limit. Interestingly, disorder smearing induces a marginal Fermi-liquid (MFL) self-energy for the fermions. We next consider fluctuations around the saddle points and derive a MFL-Finkel'stein nonlinear sigma model. We find that the Altshuler-Aronov quantum conductance correction gives linear-$T$ resistivity that can dominate over the Drude result at low temperatures. The strong temperature dependence of the quantum correction arises due to rapid relaxation of the mediating quantum-critical bosons. We verify that our calculations explicitly satisfy the Ward identity at the semiclassical and quantum levels. Our results establish that quantum interference persists in two-particle hydrodynamic modes, even when quasiparticles are subject to strong (Planckian) dissipation.

Identifying quantum critical points (QCPs) and their associated fluctuations may hold the key to unraveling the unusual electronic phenomena observed in cuprate superconductors. Recently, signatures of quantum fluctuations associated with charge order (CO) have been inferred from the anomalous enhancement of CO excitations that accompany the reduction of the CO order parameter in the superconducting state. To gain more insight into the interplay between CO and superconductivity, here we investigate the doping dependence of this phenomenon throughout the Bi-2212 cuprate phase diagram using resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge. As doping increases, the CO wave vector decreases, saturating near a commensurate value of 0.25 reciprocal lattice unit beyond a characteristic doping $p_c$, where the correlation length becomes shorter than the apparent periodicity ($4a_0$). Such behavior is indicative of the fluctuating nature of the CO; the proliferation of CO excitations in the superconducting state also appears strongest at $p_c$, consistent with expected behavior at a CO QCP. Intriguingly, $p_c$ appears to be near optimal doping, where the superconducting transition temperature $T_c$ is maximal.

We provide a negative sign free formulation of the auxiliary field quantum Monte Carlo algorithm for generalized Kitaev models with higher symmetries. Our formulation is based on the Abrikosov fermion representation of the spin-1/2 degree of freedom and the phase pinning approach [T. Sato and F. F. Assaad, Phys. Rev. B 104, L081106 (2021)]. Enhancing the number of fermion flavors or orbitals from 1 to $N$ allows one to generalize the inherent $Z_2$ global symmetry to $Z_2\times \mathrm{S}\mathrm{U}{\left(N\right)}_o$. Using this general approach, we study the $Z_2\times \mathrm{S}\mathrm{U}{\left(2\right)}_o$ Kitaev-Heisenberg model reflecting the competition between the isotropic Heisenberg exchange and Kitaev-type bond-directional exchange interactions. We show that the symmetry enhancement provides a path to escape frustration and that the spin liquid phases in the original $Z_2$-symmetric model are not present in this model. Nevertheless, the ground-state phase diagram is extremely rich and has points with higher global and local continuous symmetries as well as deconfined quantum critical points.

We propose a method to calculate finite-temperature properties of a quantum many-body system for a microcanonical ensemble by introducing a pure quantum state named here an energy-filtered random-phase state, which is also a potentially promising application of near-term quantum computers. In our formalism, a microcanonical ensemble is specified by two parameters, i.e., the energy of the system and its associated energy window. Accordingly, the density of states is expressed as a sum of Gaussians centered at the target energy with its spread corresponding to the width of the energy window. We then show that the thermodynamic quantities such as entropy and temperature are calculated by evaluating the trace of the time-evolution operator and the trace of the time-evolution operator multiplied by the Hamiltonian of the system. We also describe how these traces can be evaluated using random diagonal-unitary circuits appropriate to quantum computation. The pure quantum state representing our microcanonical ensemble is related to a state of the form introduced by Wall and Neuhauser for the filter diagonalization method [M. R. Wall and D. Neuhauser, J. Chem. Phys. 102, 8011 (1995)], and therefore we refer to it as an energy-filtered random-phase state. The energy-filtered random-phase state is essentially a Fourier transform of a time-evolved state whose initial state is prepared as a random-phase state, and the cut-off time in the time-integral for the Fourier transform sets the inverse of the width of the energy window. The proposed method is demonstrated numerically by calculating thermodynamic quantities for the one-dimensional spin-1/2 Heisenberg model on small clusters up to 28 qubits, showing that the method is most efficient for the target energy around which the dense distribution of energy eigenstates is found.

We study the Atiyah-Hirzebruch spectral sequence (AHSS) for equivariant $K$ theory in the context of band theory. Various concepts in band theory, such as irreps at high-symmetry points, compatibility relations, topological gapless points, and singularities, fit naturally into the AHSS. As an application of the AHSS, we get the complete list of topological invariants for 230 space groups without time-reversal or particle-hole invariance. We find that many torsion topological invariants appear even for symmorphic space groups.

Lifshitz transition is a kind of topological phase transition in which the Fermi surface is reconstructed. It can occur in the two-dimensional (2D) tilted Dirac materials when the energy bands change between the type-I phase ($0<t<1$) and the type-II phase ($t>1$) through the type-III phase ($t=1$), where different tilts are parametrized by the values of $t$. In order to characterize the Lifshitz transition therein, we theoretically investigate the longitudinal optical conductivities (LOCs) in type-I, type-II, and type-III Dirac materials within linear response theory. In the undoped case, the LOCs are constants either independent of the tilt parameter in both type-I and type-III phases or determined by the tilt parameter in the type-II phase. In the doped case, the LOCs are anisotropic and possess two critical frequencies determined by $\omega ={\omega }_1\left(t\right)$ and $\omega ={\omega }_2\left(t\right)$, which are also confirmed by the joint density of state. The tilt parameter and chemical potential can be extracted from optical experiments by measuring the positions of these two critical boundaries and their separation $\mathrm{\Delta }\omega \left(t\right)={\omega }_2\left(t\right)-{\omega }_1\left(t\right)$. With increasing the tilting, the separation becomes larger in the type-I phase whereas smaller in the type-II phase. The LOCs in the regime of large photon energy are exactly the same as that in the undoped case. The type of 2D tilted Dirac bands can be determined by the asymptotic background values, critical boundaries, and their separation in the LOCs. These can therefore be taken as signatures of Lifshitz transition therein. The results of this work are expected to be qualitatively valid for a large number of 2D tilted Dirac materials, such as 8-Pmmn borophene monolayer, $\alpha -{\mathrm{SnS}}_2, {\mathrm{TaCoTe}}_2, {\mathrm{TaIrTe}}_4$, and $1T^{\prime }$ transition metal dichalcogenides, due to the underlying intrinsic similarities of 2D tilted Dirac bands.

We explore the physics of a spin-1/2 Heisenberg chain with Kondo interaction, $J_k$, to a two-dimensional electron gas. At weak $J_k$ the problem maps onto a Heisenberg chain locally coupled to a dissipative Ohmic bath. At the decoupled fixed point, the dissipation is a marginally relevant perturbation and drives long-range antiferromagnetic order along the chain. In the dynamical spin structure factor we observe a quadratic low-energy dispersion akin to Landau-damped Goldstone modes. At large $J_k$ Kondo screening dominates, and the spin correlations of the chain inherit the power law of the host metal, akin to a paramagnetic heavy Fermi liquid. In both phases we observe heavy bands near the Fermi energy in the composite-fermion spectral function. Our results, obtained from auxiliary-field quantum Monte Carlo simulations, provide a negative-sign-free realization of a quantum transition between an antiferromagnetic metal and a heavy-fermion metal. We discuss the relevance of our results in the context of scanning tunneling spectroscopy experiments of magnetic adatom chains on metallic surfaces.