We develop a fast and accurate machine-learned interatomic potential for the Mo-Nb-Ta-V-W quinary system and use it to study segregation and defects in the body-centered-cubic refractory high-entropy alloy MoNbTaVW. In the bulk alloy, we observe clear ordering of mainly Mo-Ta and V-W binaries at low temperatures. In damaged crystals, our simulations reveal clear segregation of vanadium, the smallest atom in the alloy, to compressed interstitial-rich regions such as radiation-induced dislocation loops. Vanadium also dominates the population of single self-interstitial atoms. In contrast, due to its larger size and low surface energy, niobium segregates to spacious regions such as the inner surfaces of voids. When annealing samples with supersaturated concentrations of defects, we find that in complete contrast to W, interstitial atoms in MoNbTaVW cluster to create only small $(\sim 1 \mathrm{nm})$ experimentally invisible dislocation loops enriched by vanadium. By comparison to W, we explain this by the reduced but three-dimensional migration of interstitials, the immobility of dislocation loops, and the increased mobility of vacancies in the high-entropy alloy, which together promote defect recombination over clustering.

We theoretically investigate the high-order harmonic generation (HHG) from ZnO crystals driven by orthogonally polarized two-color (OTC) laser fields. By solving the semiconductor Bloch equation (SBE) in two dimensions, it is found that the HHG yields show a high sensitivity to the relative phase between the two fields. By changing the laser intensity, the yields of HHG show two maxima in the high-order region. To reveal the physical origin of these phenomena, we analyze the behavior of the electron-hole pair in the real and reciprocal spaces by the recollision model, respectively. It could qualitatively interpret the results by SBE simulations. Our analysis demonstrates that the OTC laser fields could control the electron-hole dynamics and HHG processes efficiently.

The extremely large magnetoresistance (XMR) observed in many topologically nontrivial and trivial semimetals has attracted much attention in relation to its underlying physical mechanism. In this paper, by combining the band structure and Fermi surface (FS) calculations with the Hall resistivity and de Haas–van Alphen (dHvA) oscillation measurements, we studied the anisotropy of magnetoresistance (MR) of ${\mathrm{ReO}}_3$ with a simple cubic structure, an “ordinary” nonmagnetic metal considered previously. We found that ${\mathrm{ReO}}_3$ exhibits almost all the characteristics of XMR semimetals: the nearly quadratic field dependence of MR, a field-induced upturn in resistivity followed by a plateau at low temperatures, and high mobilities of charge carriers. It was found that for magnetic field H applied along the $c$ axis, the MR exhibits an unsaturated H${}^{1.75}$ dependence, which was argued to arise from the complete carrier compensation supported by the Hall resistivity measurements. For H applied along the direction of ${15}^{\circ }$ relative to the $c$ axis, an unsaturated H${}^{1.90}$ dependence of MR up to $(9.43\times {10}^3)\%$ at 10 K and 9 T was observed, which was explained by the existence of electron open orbits extending along the $k_x$ direction. Two mechanisms responsible for XMR observed usually in the semimetals occur also in the simple metal ${\mathrm{ReO}}_3$ due to its peculiar FS (two closed electron pockets and one open electron pocket), once again indicating that the details of FS geometrical configuration are a key factor for the observed XMR in materials.

Interactions between particles in quantum many-body systems play a crucial role in determining the electric, magnetic, optical, and thermal properties of the system. The recent progress in the laser-pulse technique has enabled the manipulations and measurements of physical properties on ultrafast timescales. Here we propose a method for the direct and ultrafast probing of quantum many-body interaction through coherent two-dimensional (2D) spectroscopy. Using a two-band fermionic Hubbard model for the minimum two-site lattice system, we find that the 2D spectrum of a noninteracting system contains only diagonal peaks; the interparticle interaction manifests itself in the emergence of off-diagonal peaks in the 2D spectrum before all the peaks coalesce into a single diagonal peak as the system approaches the strongly interacting limit. The evolution of the 2D spectrum as a function of the time delay between the second and third laser pulses can provide important information on the ultrafast time variation of the interaction.

We study an anisotropic two-dimensional non-Hermitian Lieb lattice, where the staggered gain and loss present in the horizontal and vertical directions, respectively. The intracell nonreciprocal coupling generates magnetic flux enclosed in the unit cell of the Lieb lattice and creates nontrivial topology. The active and dissipative topological edge states are along the horizontal and vertical directions, respectively. The two-dimensional non-Hermitian Lieb lattice also supports a passive topological corner state. At appropriate magnetic flux, the non-Hermiticity can alter the corner state from one corner to the opposite corner as the non-Hermiticity increases. The gapless phase of the Lieb lattice is characterized by different configurations of exceptional points in the Brillouin zone. The topology of the anisotropic non-Hermitian Lieb lattices can be verified in many experimental platforms including the optical waveguide lattices, photonic crystals, and electronic circuits.

The Kondo effect refers to the screening of a spin-1/2 impurity by a cloud of conduction electrons, then forming a many-body Fermi liquid ground state. Theoretical calculations suggest that the Kondo resonance can interact with light and should give rise to a $\pi$-phase shift of the photon signal in the case where the ground state is a Fermi liquid. This $\pi$-phase shift of light is driven from the Korringa-Shiba relation of quantum impurity Fermi-liquid ground states. We report the first observation of such a $\pi$-phase shift in a graphene double quantum dot within a circuit quantum electrodynamics architecture where the microwave photons couple to the pseudospin or charge degrees of freedom. We study the evolution of the $\pi$-phase shift as a function of temperature and bias voltage. The observed Kondo temperature $T_K\sim 550\text{mK}$ is in agreement with dc conductance measurements. All our results support the formation of a Kondo resonance located above the Fermi level of the electronic reservoirs with the occurrence of an $SU\left(4\right)$ Fermi-liquid ground state. We finally study how the Kondo-photon interaction can be tuned by inter-dot electron tunnel coupling strengths. Our findings may contribute to a better understanding of many-body physics in hybrid circuit systems, and open up applications in atomic thin materials from the light-matter interaction.

Topologically protected nanoscale spin textures, known as magnetic skyrmions, possess particlelike properties and feature emergent magnetism effects. In bulk cubic helimagnets, distinct skyrmion resonant modes are already identified using a technique such as ferromagnetic resonance in spintronics. However, direct light-matter coupling between microwave photons and skyrmion resonance modes still needs to be demonstrated. Utilizing two distinct cavity systems, we observe a direct interaction between the cavity resonant mode and two resonant skyrmion modes, the counterclockwise gyration and breathing modes, in bulk ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. For both resonant modes, we find the largest coupling strength at 57 K indicated by an enhancement of the cavity linewidth at the degeneracy point. We study the effective coupling strength as a function of temperature within the expected skyrmion phase. We attribute the maximum in effective coupling strength to the presence of a large number of skyrmions, and correspondingly to a completely stable skyrmion lattice. Our experimental findings indicate that the coupling between photons and resonant modes of magnetic skyrmions depends on the relative density of these topological particles instead of the pure spin number in the system.

We present a class of quantum dimer models on the kagome lattice with full translational invariance that feature a quantum many-body scar state of analytically known entanglement properties within their spectra. Using exact diagonalization on lattices of up to 60 sites, we show that nonscar states conform to the eigenstate thermalization hypothesis. Specifically, we show that energies are distributed according to the Gaussian ensemble expected of their respective symmetry sector, illustrate the existence of the scar from bipartite entanglement properties, and demonstrate revival phenomena in studies of fidelity dynamics.

One way to induce insulator-to-metal transitions in the spin-orbit Mott insulator ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, in contrast to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below $E_F$ coexisting with the metallic ones that we assign to nonbonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control either the charge transfer or the transition threshold.

We study the coarsening of three-dimensional antiphase domain structures via Monte Carlo simulations. Linear lattice dimensions of $N=1024$ enable us to reach a scaling regime covering about 2.5 orders of magnitude of linear scale. With short-range interactions on cubic lattices at temperatures of $0.75T_{\text{c}}$, the resulting antiphase domain structures are isotropic, which allows us to describe the real-space correlation functions by a common function scaled by a time-dependent parameter. We compare abstract Potts models and realistic models of atomic order in compounds and show that those with same ground-state degeneracy $q$ lead to equivalent antiphase domain structures, while the scaling functions for different $q$ show slight but significant deviations. Finally, we quantitatively discuss notions of real-space scale (specific interface area) and reciprocal-space scale (superstructure peak width) and thus give numerically exact values for the corresponding parameter $K$ of the Scherrer equation.

The critical phenomenon of the magnetic phase transition in the van der Waals itinerant ferromagnet ${\mathrm{Fe}}_4{\mathrm{GeTe}}_2$ has been investigated by measuring the dc magnetization. The compound undergoes a continuous paramagnetic to ferromagnetic phase transition at the Curie temperature $T_C=270.0\mathrm{K}$. A detailed analysis of magnetization isotherms measured in the vicinity of $T_C$ with magnetic field $H$ applied parallel to the $ab$ plane yields the asymptotic critical exponents $\beta =0.37, \gamma =1.20$, and $\delta =4.21$, whereas the magnetization isotherms for $H\parallel c$ yield $\beta =0.34, \gamma =1.20$, and $\delta =4.52$. Both sets of the critical exponents obey the scaling relation and scaling equation of the magnetic state predicted by the scaling theory. The renormalization group theory analysis for the critical exponents and the analysis based on the self-consistent renormalization theory of spin fluctuations manifest a quasi-two-dimensional itinerant magnetism in the ${\mathrm{Fe}}_4{\mathrm{GeTe}}_2$ single crystal where the magnetic exchange coupling $J\left(r\right)$ is the long-range type that decreases spatially slower than $r^{-4.76}$. Moreover, a small reduction in the value of $\beta$ for $H\parallel c$ attests to the observed weak effective uniaxial magnetic anisotropy of ${\mathrm{Fe}}_4{\mathrm{GeTe}}_2$.

Ferromagnetic ${\mathrm{Co}}_2\mathrm{MnGa}$ has recently attracted significant attention due to effects related to the nontrivial topology of its band structure. However, a systematic study of canonical magnetogalvanic transport effects is missing. Focusing on high quality thin films, here we systematically measure anisotropic magnetoresistance (AMR) and its thermoelectric counterpart anisotropic magnetothermopower (AMTP). We model the AMR data by free energy minimization within the Stoner-Wohlfarth formalism and conclude that both crystalline and noncrystalline components of this magnetotransport phenomenon are present in ${\mathrm{Co}}_2\mathrm{MnGa}$. The AMTP is, in comparison to the AMR, large in relative terms, since the Seebeck coefficient ${\mathrm{\Sigma }}_0$ is small, which is discussed in the context of the Mott rule and of phonon drag. A further analysis of AMTP components is presented.

Here, the impact of small Fermi energy on the selected parameters of the superconducting state in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{BiO}}_3$ (BKBO) is studied at $x\in (0.3,0.4,0.5)$. This is done by employing the adiabatic and nonadiabatic Eliashberg equations in the context of the available experimental data. It is found that the retardation, strong-coupling, and the nonadiabatic effects notably influence the superconducting phase in BKBO. In particular, the electron-electron interaction, approximated here by the Coulomb pseudopotential, is argued to be reduced by the nonadiabatic effects that supplement retardation and allow for the phonon-mediated superconductivity. These findings are reinforced by further analysis of the Coulomb pseudopotential cutoff and the isotope effect. The former confirms that signatures of the nonadiabatic effects are visible even when high-order pseudopotential terms are considered, whereas the latter analysis shows a reduction of the isotope coefficient with respect to the canonical BCS level, as caused by the interplay of the retardation and strong-coupling effects. In summary, obtained results confirm recent theoretical and experimental studies which suggest a phonon-mediated mechanism of superconductivity in BKBO. However, they also point out that this phase cannot be properly described by the conventional theory due to the existence of somewhat unusual effects.

Superconductors without inversion symmetry in their crystal structure are known to exhibit unconventional properties. Recently, based on the measured temperature dependence of the magnetic field penetration depth, superconductivity in noncentrosymmetric $\mathrm{Th}\mathrm{Co}{\mathrm{C}}_2$ was proposed to be a nodal $d$ wave and mediated by the spin fluctuations. Moreover, a non-BCS behavior of the temperature dependence of the electronic specific heat and the magnetic upper critical field were reported. In this work, the electronic structure, phonons, and electron-phonon coupling are studied in $\mathrm{Th}\mathrm{Co}{\mathrm{C}}_2$ on the basis of ab initio computations. The effect of the spin-orbit coupling on the electronic structure and electron-phonon interaction is analyzed, and a large splitting of the electronic band structure is found. The calculated electron-phonon coupling constant $\lambda =0.59$ remains in decent agreement with the experimental estimates, suggesting that the electron-phonon interaction is strong enough to explain superconductivity with $T_c\simeq 2.5$ K. Nevertheless, we show that the conventional isotropic Eliashberg formalism is unable to describe the thermodynamic properties of the superconducting state, as calculated temperature dependence of the electronic specific heat and magnetic penetration depth deviate from experiments, which is likely driven by the strong spin-orbit coupling and inversion symmetry breaking. In addition, to shed more light on the pairing mechanism, we propose to measure the carbon isotope effect, as our calculations based on the electron-phonon coupling predict the observation of the isotope effect with an exponent $\alpha \simeq 0.15$.

We investigate the induced superconducting pair correlations in junctions between a conventional spin-singlet $s$-wave superconductor and a disordered normal metal. Decomposing the pair amplitude based on its symmetries in the time domain, we demonstrate that the odd-time, or equivalently odd-frequency, spin-singlet $p$-wave correlations are both significant in size and entirely robust against random nonmagnetic disorder. We find that these odd-frequency correlations can even be generated by disorder. Our results show that anisotropic odd-frequency pairing represent an important fraction of the proximity-induced correlations in disordered superconducting hybrid structures.

Spin-nutation resonance has been well explored in one-sublattice ferromagnets. Here, we investigate the spin nutation in two-sublattice antiferromagnets as well as, for comparison, ferrimagnets with inter- and intrasublattice nutation coupling. In particular, we derive the susceptibility of the two-sublattice magnetic system in response to an applied external magnetic field. To this end, the antiferromagnetic and ferrimagnetic (sub-THz) precession and THz nutation resonance frequencies are calculated. Our results show that the precession resonance frequencies and effective damping decrease with intrasublattic nutation coupling, while they increase with intersublattice nutation in an antiferromagnet. However, we find that the THz nutation resonance frequencies decrease with both the intra- and intersublattice nutation couplings. For ferrimagnets, conversely, we calculate two nutation modes with distinct frequencies, unlike antiferromagnets. The exchangelike precession resonance frequency of ferrimagnets decreases with intrasublattice nutation coupling and increases with intersublattice nutation coupling, as antiferromagnets, but the ferromagneticlike precession frequency of ferrimagnets is practically invariant to the intra- and intersublattice nutation couplings.