Moiré superlattices formed in two-dimensional semiconductor heterobilayers provide a new realization of Hubbard model physics in which the number of electrons per effective atom can be tuned at will. We report on an exact diagonalization study of the electronic properties of half-filled narrow moiré bands in which correlation strengths are varied by changing twist angles or interaction strengths. We construct a phase diagram for the bilayer, identifying where the metal-insulator phase transition occurs, estimating the sizes of the charge gaps in the insulating phase, and commenting on the nature of the transition and the importance of subdominant interaction parameters.

The charge-neutral exciton has been predicted to carry genuine photocurrent due to the geometric Berry phase of the electronic bands, if the inversion symmetry in a crystal is broken. We detect such exciton shift current in a prototypical polar semiconductor CdS by using terahertz emission spectroscopy. A distinct peak emerges in the photocurrent spectra at the energy of the exciton resonance, which is demonstrated to result from the distinct displacements of electrons and holes in real space within the excitons to produce a finite transient charge current at subpicosecond time scale. Our findings elucidate the Berry phase physics of the charge-neutral photoexcitations and also shed light on the novel energy harvesting mechanism by exciton generation.

The nontrivial magnetic texture in real space gives rise to the intriguing phenomenon of the topological Hall effect (THE), which is relatively less explored in topological semimetals. Here, we report a large THE in the antiferromagnetic (AFM) state in single crystals of EuAgAs, an AFM Dirac semimetal. EuAgAs hosts an AFM ground state below $T_N=12$ K with a weak ferromagnetic component. The in-plane isothermal magnetization below $T_N$ exhibits a weak metamagnetic transition. We also observe chiral anomaly induced positive longitudinal magnetoconductivity, which indicates a Weyl fermion state under an applied magnetic field. The first-principles calculations reveal that EuAgAs is an AFM Dirac semimetal with a pair of Dirac cones, and therefore a Weyl semimetallic state can be realized under time-reversal symmetry breaking via an applied magnetic field. Our study establishes that EuAgAs is a system for exploiting the interplay of band topology and the topology of the magnetic texture.

The determination of the ground state of quantum many-body systems via digital quantum computers rests upon the initialization of a sufficiently educated guess. This requirement becomes more stringent the greater the system. Preparing physically motivated Ansätze on quantum hardware is therefore important to achieve a quantum advantage in the simulation of correlated electrons. In this spirit, we introduce the Gutzwiller wave function (GWF) within the context of the digital quantum simulation of the Fermi-Hubbard model. We present a quantum routine to initialize the GWF that comprises two parts. In the first, the noninteracting state associated with the $U=0$ limit of the model is prepared. In the second, the nonunitary Gutzwiller operator that selectively removes states with doubly occupied sites from the wave function is performed by adding to every lattice site an ancilla qubit, the measurement of which in the $|0\rangle$ state confirms the operator was applied. Due to its nondeterministic nature, we estimate the success rate of the algorithm in generating the GWF as a function of the lattice size and the interaction strength $U/t$. The scaling of the quantum circuit metrics and its integration in general quantum simulation algorithms are also discussed.

The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ was predicted to host the axion insulator, but the experimental evidence remains elusive. Specifically, the axion insulator is believed to carry “half-quantized” chiral currents running antiparallel on its top and bottom surfaces. However, it is challenging to measure precisely the half-quantization. Here, we propose a nonlocal surface transport device, in which the axion insulator can be distinguished from normal insulators without a precise measurement of the half-quantization. More importantly, we show that the nonlocal surface transport, as a qualitative measurement, is robust in realistic situations when the gapless side surfaces and disorder come to play. Moreover, thick electrodes can be used in the device of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ thick films, enhancing the feasibility of the surface measurements. This proposal will be insightful for the search of the axion insulator and axion in topological matter.

Monolayer materials are strongly affected by their potential fluctuations, which can be induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer ${\mathrm{WS}}_2$ on substrates with different surface topographies. We show that the ${\mathrm{WS}}_2$ monolayers on the substrates with atomically flat terraces have small effective EEA rate constants that deviate from the overall tendency and exhibit multiple exciton decay components, which cannot be accounted for by the conventional EEA model. To obtain a correct description, it is important to use a quantized EEA model. The intrinsic EEA rate constants for the flat-terrace substrates determined by this model are comparable to that of hBN-encapsulated monolayer ${\mathrm{WS}}_2$.

Advances in synchrotron x-ray Raman scattering (XRS) techniques enable probing of pressure-induced element-specific bonding transitions in diverse oxides beyond megabar pressures. Evolution of the electronic structures under extreme pressure involves an emergence of additional oxygen $1s$ electron excitation pattern and shifts in the electronic states toward high-energy ranges in an XRS spectrum. Structural origins of such changes and proper spectral proxy for the corresponding evolution in electronic density of states remain to be established. Here, we calculated the O $K$-edge features in the XRS spectrum for diverse crystalline ${\mathrm{SiO}}_2$ polymorphs, from α-quartz (1 atm) to a pyrite-type structure (at ∼271 GPa), beyond the current experimental pressure limit (∼160 GPa). The ab initio results unravel the electronic origins of the evolution in the XRS patterns, such as the emergence of the bimodal feature with decreasing oxygen-oxygen distance $\left(d_{\mathrm{O}\text{−}\mathrm{O}}\right) <\sim 2.5 \AA$ and a subsequent merging of the doublet patterns with a further decrease in $d_{\mathrm{O}\text{−}\mathrm{O}}$ down to $\sim 2.3–2.4 \AA$. Ab initio simulations of the XRS spectrum of pyrite-type ${\mathrm{SiO}}_2$ phase at $\sim 271 \mathrm{GPa}$ revealed the significant electron dispersion in $2p^{*}$ states and the pronounced electron density between edge-sharing oxygen atoms with $d_{\mathrm{O}\text{−}\mathrm{O}}$ of ∼2.1 Å, in contrast to an earlier prediction with a negligible electron density between those oxygen atoms. The pressure-induced increase in electron interactions leads to a systematic change in the edge energy at half of the spectrum area $\left(E_M\right)$ and the edge energy at the center of mass of the spectrum $\left(E_C\right)$ of XRS patterns beyond multimegabar pressures. Particularly, the $E_M$ and $E_C$ increase linearly with decreasing oxygen proximity $\left(d_{\mathrm{O}\text{−}\mathrm{O}}\right)$ and bulk density up to 270 GPa. The $E_M$ and $E_C$ for ${\mathrm{MgSiO}}_3$ phases show larger changes with varying $d_{\mathrm{O}\text{−}\mathrm{O}}$ than those in ${\mathrm{SiO}}_2$ phases, demonstrating the effect of the electronic hybridization between Mg and O on the evolution of the XRS patterns. The strong linear correlation among $E_M$, $E_C$, and $d_{\mathrm{O}\text{−}\mathrm{O}}$ (and density) up to ∼270 GPa confirms that the $E_M$ and $E_C$ can be effective spectral proxies to quantify the densification of crystalline and noncrystalline oxides. The results with the statistical evaluations of the electronic density of states provide improved prospects for the prediction of the evolution in core-level excitation features of diverse complex, multicomponent oxides under multimegabar pressure conditions toward the deep lower mantle of Earth and other super-Earth bodies.

The urgent need for reliable simulation tools to match the extreme accuracy needed to control tailored quantum devices highlights the importance of understanding open quantum systems and their modeling. To this end, we compare here the commonly used Redfield and Lindblad master equations against numerically exact results in the case of one and two resonant qubits transversely coupled at a single point to a Drude-cut ohmic bath. All the relevant parameters are varied over a broad range, which allows us to give detailed predictions about the validity and physically meaningful applicability of the weak-coupling approaches. We characterize the accuracy of the approximate approaches by comparing the maximum difference of their system evolution superoperators with numerically exact results. After optimizing the parameters of the approximate models to minimize the difference, we also explore if and to what extent the weak-coupling equations can be applied at least as phenomenological models. Optimization may lead to an accurate reproduction of experimental data, but yet our results are important to estimate the reliability of the extracted parameter values such as the bath temperature. Our findings set general guidelines for the range of validity of the usual Born-Markov master equations and indicate that they fail to accurately describe the physics in a surprisingly broad range of parameters, in particular, at low temperatures. Since quantum-technological devices operate there, their accurate modeling calls for a careful choice of methods.

In the present work, the structural, magnetic, and thermodynamic properties of ${\mathrm{Dy}}_{1-x}{\mathrm{Er}}_x{\mathrm{Ni}}_2$ $(x=0.25,0.5,0.75)$ Laves-phase intermetallics, being continuous solid solutions in the ${\mathrm{DyNi}}_2\text{−}{\mathrm{ErNi}}_2$ system, synthesized by arc melting, are experimentally studied and analyzed in accordance with mutual substitution within the rare-earth sublattice. The existence of Laves-phase superstructure (space group $F\overline{4}3m$) in the substituted ${\mathrm{Dy}}_{1-x}{\mathrm{Er}}_x{\mathrm{Ni}}_2$ compounds was found by x-ray diffraction analysis. Low-temperature magnetic studies showed the ferromagnetic behavior of the compounds; their Curie temperatures correlate with the erbium content; namely, they decrease from 16.8 K for ${\mathrm{Dy}}_{0.75}{\mathrm{Er}}_{0.25}{\mathrm{Ni}}_2$ to 10.0 K for ${\mathrm{Dy}}_{0.25}{\mathrm{Er}}_{0.75}{\mathrm{Ni}}_2$. Heat capacity measurements performed in magnetic fields of 1 and 2 T allowed us to estimate the isothermal magnetic entropy and adiabatic temperature changes for ${\mathrm{Dy}}_{1-x}{\mathrm{Er}}_x{\mathrm{Ni}}_2$. Independently, a theoretical analysis was performed for a polycrystalline sample in the frame of a model Hamiltonian taking into account the Zeeman exchange interaction and crystal electric field anisotropy and was expanded to experimental results. Composites with optimum proportions of the individual ${\mathrm{Dy}}_{0.75}{\mathrm{Er}}_{0.25}{\mathrm{Ni}}_2$, ${\mathrm{Dy}}_{0.5}{\mathrm{Er}}_{0.5}{\mathrm{Ni}}_2$, and ${\mathrm{Dy}}_{0.25}{\mathrm{Er}}_{0.75}{\mathrm{Ni}}_2$ components were theoretically determined. The results indicate that the proposed composites are good candidates to be used as the refrigerant material in a magnetic refrigerator performing an Ericsson cycle at low temperatures. The relative cooling power RCP, refrigerant capacity RC, temperature averaged entropy change TEC, and the changes of emitted or absorbed quantity of heat ΔQ were calculated for investigated samples. Direct measurements of the adiabatic temperature change in the vicinity of the magnetic transition temperature were carried out in magnetic fields up to 14 T. The regularities found are discussed in terms of Landau's theory of second-order magnetic phase transitions.

Magnetic impurities in a superconductor induce Yu-Shiba-Rusinov (YSR) states inside the superconducting gap, whose energy depends on the strength of the coupling to the impurity and on the density of states (DOS) at the Fermi level. We consider a DOS exhibiting a logarithmic or a power-law divergence at the Fermi level due to Van Hove singularities (VHSs) and higher-order Van Hove singularities (HOVHSs), respectively. We find that the energy of the YSR states has the same functional form as in the constant DOS scenario, with the effect of the singularity being an enhancement of the effective coupling constants. In particular, the critical magnetic coupling strength at which the Shiba transition occurs is always lowered by a factor $1/\rho (\mathrm{\Delta }/E_{\mathrm{c}}$), where $\mathrm{\Delta }$ is the superconducting gap, $E_{\mathrm{c}}$ is the bandwidth, and $\rho \left(E\right)$ is the factor in the DOS which diverges at $E=0$ for a VHS or HOVHS. Further, since the critical magnetic coupling is significantly reduced, a new regime becomes accessible in which the transition point is controlled by the nonmagnetic coupling constant. Interestingly, the slope of the Shiba energy curve at the Shiba transition is independent of impurity parameters and purely reflects the band structure. Additionally, we find that our main conclusions remain valid even when the Fermi level is not precisely tuned to the Van Hove singularity, but instead lies in an energy range of the order of the superconducting gap. Our results show that tuning a superconducting material towards a VHS or HOVHS enhances the possibilities for engineering YSR states and for characterizing the superconductor itself.

We theoretically investigate the instability toward a skyrmion crystal (SkX) in frustrated triangular magnets. By performing simulated annealing and Monte Carlo simulations, we show that a frustrated spin model with easy-plane single-ion anisotropy exhibits a SkX in an in-plane external magnetic field. The emergent SkX is a counterpart of that induced by easy-axis anisotropy in an out-of-plane magnetic field, both of which show the topological Hall effect. We also find that a variety of multiple-$Q$ states distinct from the SkX are stabilized in the presence of easy-plane anisotropy. Our results provide a possibility to realize a SkX in easy-plane frustrated magnets by applying an in-plane magnetic field to in-plane cycloidal spiral magnets.

Using a two-dimensional square lattice Heisenberg model with a Rashba-type Dzyaloshinskii-Moriya interaction, we demonstrate that chiral spin fluctuations can give rise to a thermal Hall effect in the absence of any static spin texture or momentum space topology. It is shown by means of Monte Carlo and stochastic spin dynamics simulations that the thermal Hall response is finite at elevated temperature outside of the linear spin wave regime and consistent with the presence of thermal fluctuation-induced nontrivial topology. Our result suggests that the high-fluctuation phases outside of the conventional regime of magnonics may yet be a promising area of exploration for spin-based electronics.

We report the structural and magnetic ground state properties of the monoclinic compound barium iridium oxide ${\mathrm{Ba}}_4{\mathrm{Ir}}_3{\mathrm{O}}_{10}$ using a combination of resonant x-ray scattering, magnetometry, and thermodynamic techniques. Magnetic susceptibility exhibits a pronounced antiferromagnetic transition at $T_{\text{N}}\approx 25$ K, a weaker anomaly at $T_{\text{S}}\approx 142$ K, and strong magnetic anisotropy at all temperatures. Resonant elastic x-ray scattering experiments reveal a second order structural phase transition at $T_{\text{S}}$ and a magnetic transition at $T_{\text{N}}$. Both structural and magnetic superlattice peaks are observed at $L$ = half integer values. The magnetization anomaly at $T_{\text{S}}$ implies the presence of magnetoelastic coupling, which conceivably facilitates the symmetry lowering. Mean field critical scattering is observed above $T_{\text{S}}$. The magnetic structure of the antiferromagnetic ground state is discussed based on the measured magnetic superlattice peak intensity. Our study not only presents essential information for understanding the intertwined structural and magnetic properties in ${\mathrm{Ba}}_4{\mathrm{Ir}}_3{\mathrm{O}}_{10}$ but also highlights the necessary ingredients for exploring novel ground states with octahedra trimers.

We propose a picture for the magnetic properties of superconductor/ferromagnet (S/F) heterostructures based on Nb and permalloy $(\mathrm{Py}:{\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80})$. By measuring the magnetic moment as a function of the temperature in S/F/S trilayers for different thicknesses of the middle F layer, we give evidence of the presence of a magnetic stray field of the F layer. For values of F-layer thickness below a threshold, we establish a correlation between the magnetic measurements of the S/F/S trilayers and the anomalous magnetic dependence of the critical current in S/Insulator/thin superconducting film/F/S (SIsFS) Josephson junctions (JJs). These complementary investigations provide a self-consistent method to fully characterize S/F heterostructures and possibly demonstrate effects arising from the mutual interactions between ferromagnetism and superconductivity. A shift in the Fraunhofer critical current oscillations has been observed in the opposite direction to the one commonly observed in JJs with F barriers, as it has been recently predicted by inverse and electromagnetic proximity theories. This inverse memory effect is relevant for the design of these heterostructures as memory cells and spintronic devices.

Topological semimetals and metals have been extensively studied in nonmagnetic materials, while those in magnetic materials are still quite rare. Here, based on first-principles calculations, we reveal that a family of hexagonal compounds $X{\mathrm{TiO}}_2$ ($X$ = Li, Na, K, Rb) with ferromagnetic ground states host excellent topological electronic structures. In their low-energy band structures, they show two Weyl nodal loops and two Weyl nodal surfaces, where each nodal loop and nodal surface is contributed by the electron states in the same spin channel, namely, spin-polarized nodal loops and nodal surfaces. In particular, the nodal loops in these materials show type-II band dispersions and are robust against spin-orbit coupling because of the protection of the mirror symmetry. These nodal loops are featured with spin-polarized drumhead surface states. We build an effective Hamiltonian, which can well describe the type-II nodal loops in the system. Besides the type-II nodal loops, these materials also possess two spin-polarized nodal surfaces, protected by specific nonsymmorphic symmetry. The findings in this paper can serve as good material platforms to study the fundamental physics of type-II nodal loop and nodal surface fermions in a time-reversal-breaking system.

Larmor precession of the quasiparticle spin about a transverse magnetic field leads to the oscillations in the spontaneous photon echo signal due to the shuffling of the optical coherence between optically accessible (bright) and inaccessible (dark) states. Here we report on a new nonoscillating photon echo regime observed in the presence of nonequal dephasing rates of bright and dark states. This regime enables the observation of the long-lived dark optical coherence. As a simple mechanical analogy, we suggest a charged particle moving in the magnetic field through the medium with anisotropic viscous friction. We demonstrate the dark coherence retrieval in the spontaneous photon echo from excitons in the InGaAs/GaAs quantum well.

We use signal enhancement in a quantum resonator for the characterization of a thin layer of vanadium hydride using neutron reflectometry and demonstrate that pressure-concentration isotherms and expansion coefficients can be extracted from the measurement of totally externally reflected neutrons only. Moreover, a consistent data analysis of the attenuation cross section allows us to detect and quantify off-specular and small angle scattering. As our experiments are effective direct beam measurements, combined with resonant signal enhancement, counting times become considerably reduced. This allows us to overcome the challenges resulting from the comparatively low brilliance of neutron beams for grazing incidence scattering experiments. Further, we discuss the potential of resonant enhancement to increase any scattering, which is of particular interest for grazing incidence small angle neutron scattering and spectroscopy.

Carrier mobility in graphene on a GaAs substrate and its change due to the adsorption of molecular magnets, ${\mathrm{Mn}}_{12}$ and $[{\mathrm{Mn}}_3{]}_2$, on the surface of graphene, is calculated from first principles. Phonon limited mobility is also calculated for comparison. For ${\mathrm{Mn}}_{12}$ adsorption on graphene, the mobility is compared for different organic ligands of ${\mathrm{Mn}}_{12}$ $(\text{−}\mathrm{H}, \text{−}{\mathrm{CH}}_3, \mathrm{and} \text{−}\mathrm{CH}{\mathrm{Cl}}_2)$, while for $[{\mathrm{Mn}}_3{]}_2$ dimers, the mobility is calculated for different linkers that yield the ferromagnetic and anti-ferromagnetic configurations of $[{\mathrm{Mn}}_3{]}_2$, as well as for different orientations of the molecule. Significant changes in carrier mobility due to the adsorption of the molecules and due to differences in linkers suggest mobility measurement as a possible sensitive probe of magnetic molecules.

Twisted bilayer graphene (TBG) with interlayer twist angles near the magic angle ≈1.08° hosts flat bands and exhibits correlated states including Mott-like insulators, superconductivity, and magnetism. A linear-in-temperature normal state resistivity in TBG has been attributed to an exotic Planckian dissipation mechanism but can be equally well explained in terms of conventional electron-phonon scattering. To address this issue, we perform combined temperature-dependent transport measurements of both the longitudinal and Hall resistivities in near-magic-angle TBG. While the observed longitudinal resistivity follows linear temperature T dependence consistent with previous reports, the Hall resistance shows an anomalous T dependence with the cotangent of the Hall angle $\cot {\mathrm{\Theta }}_H\propto T^2$. Boltzmann theory for quasiparticle transport predicts that both the resistivity and $\cot {\mathrm{\Theta }}_H$ should have the same T dependence, contradicting the observed behavior. This failure of quasiparticle-based theories is reminiscent of other correlated strange metals such as cuprates.