The recent observation of superconductivity in infinite-layer ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films has attracted a lot of attention, since this compound is electronically and structurally analogous to the superconducting cuprates. Due to the challenges in the phase stabilization upon chemical doping with Sr, we synthesized artificial superlattices of ${\mathrm{LaNiO}}_3$ embedded in insulating ${\mathrm{LaGaO}}_3$, and we used layer-selective topotactic reactions to reduce the nickelate layers to ${\mathrm{LaNiO}}_2$. Hole doping is achieved via interfacial oxygen atoms and tuned via the layer thickness. We used electrical transport measurements, transmission electron microscopy, and x-ray spectroscopy together with ab initio calculations to track changes in the local nickel electronic configuration upon reduction, and we found that these changes are reversible. Our experimental and theoretical data indicate that the doped holes are trapped at the interfacial quadratic pyramidal Ni sites. Calculations for electron-doped cases predict a different behavior, with evenly distributed electrons among the layers, thus opening up interesting perspectives for interfacial doping of transition-metal oxides.

A van Hove singularity (VHS) often significantly amplifies the electronic instability of a crystalline solid, including correlation-induced phenomena such as Hund's metallicity. We perform a systematic study on the interplay between Hund's coupling and electronic structures with a VHS focusing on Hund's metallicity. We construct a simplified tight-binding model targeting cubic perovskite materials and test the effects of the VHS utilizing dynamical mean-field theory with an exact diagonalization solver. The quasiparticle weight and the low-frequency power exponent of the self-energy provide a quantitative estimation of metallicity over the phase diagram. We find the VHS to substantially enhance Hund's metallicity. The results here suggest a range of parameters through which a VHS can bring great synergy with Hund's coupling.

We apply a hydrodynamic theory to study transverse magnetosonic resonances in a viscous two-dimensional electron fluid under an in-plane Stern-Gerlach force (SGF), a perpendicular magnetic field with cyclotron frequency ${\omega }_c$, and an alternating electric field with frequency $\omega$. The SGF leads to a splitting in the dispersion curve of the transverse magnetosound wave and hinders the long-wavelength plasmonic excitation. The effective diagonal viscosity coefficient can be enhanced by about one order of magnitude due to the SGF. The variation of absorption power $Y$ with ${\omega }_c$ exhibits a viscoelastic (VE) resonance at ${\omega }_c=\omega /2$ and transverse magnetosound resonances. The SGF can raise the heights of all resonant peaks and leave the peak positions almost completely unchanged. The most substantial magnetosonic resonant peak, much weaker than the VE peak in the absence of SGF, can be tuned by the SGF to be well above the VE peak. Our results indicate that the SGF can be used to tune the VE resonance and transverse magnetosound resonances, which is relevant to the manipulation of photoresistance and photovoltaic effects in viscous electron fluids.

Based on a recent generalization of Bloch's theorem, we present a Bloch Ansatz for the Harper model with an arbitrary rational magnetic flux in various geometries, and solve the associated Ansatz equations analytically. In the case of a cylinder and a particular boundary condition, the energy spectrum of edge states has no dependence on the length of the cylinder, which allows us to construct a quasi-one-dimensional edge theory that is exact and describes two edges simultaneously. We prove that energies of bulk states, generating the so-called Hofstadter's butterfly, depend on a single geometry-dependent spectral parameter and have exactly the same functional form for the cylinder and the torus with general twisted boundary conditions, and argue that the (edge) bulk spectrum of a semi-infinite cylinder in an irrational magnetic field is (the complement of) a Cantor set. Finally, realizing that the bulk projection of the Harper Hamiltonian is a linear form over a deformed Weyl algebra, we introduce a Bethe Ansatz valid for both cylinder and torus geometries.

Rare-earth nickelates feature an insulator-to-metal transition (IMT) that can be electrically triggered. We study the dynamics of this electrically induced transition by comparing the time-dependent transport properties of two distinct members of the ${Re\mathrm{NiO}}_3$ family: ${\mathrm{NdNiO}}_3$ and ${\mathrm{SmNiO}}_3$. We report stark differences in the nucleation and growth of the metallic phase, which evolve more rapidly for ${\mathrm{NdNiO}}_3$. With the aid of simulations, we identify the amplitude of the resistivity change across the IMT as the key parameter controlling the switching speed. Our results are in accordance with recent experiments in the ${\mathrm{VO}}_x$ family, contributing to a unified vision of the field-induced IMT dynamics across different families of correlated oxides.

We discuss the nonzero frequency noise of heat current with the explicit example of energy carried by thermal photons in a circuit. Instead of the standard circuit modeling that gives a convenient way of predicting time-averaged heat current, we describe a setup composed of two resistors forming the heat baths by collections of bosonic oscillators. In terms of average heat transport this model leads to identical results with the conventional one, but besides this, it yields a convenient way of dealing with noise as well. The nonzero-frequency heat current noise does not vanish in equilibrium even at zero temperature, a result that is known for, e.g., electron tunneling. We present a modulation method that can convert the difficult-to-measure high-frequency quantum noise down to zero frequency.

Most metallic and ceramic materials are comprised of space-filling collections of crystalline grains separated by grain boundaries. While this grain structure has been studied for more than a century, there are few rigorous results regarding its global properties available in the literature. We present a rigorous result for three-dimensional grain structures that relates the integral of the Gaussian curvature over the grain boundaries to the numbers of grains and quadruple junctions. The result is numerically verified for a grain structure consisting of periodic truncated octahedra.

We study a class of off-diagonal quasiperiodic hopping models described by a one-dimensional Su-Schrieffer-Heeger chain with quasiperiodic modulations. With regard to these models, we unveil a general dual-mapping relation in parameter space of the dimerization strength $\lambda$ and the quasiperiodic modulation strength $V$, regardless of specific details of quasiperiodic modulations. Moreover, we demonstrated semianalytically and numerically that under the specific quasiperiodic modulation, quantum criticality, namely the criticality of wave functions, can emerge and persist in a wide parameter space. These unusual results provide a distinctive paradigm compared with the diagonal quasiperiodic systems and perspectives for future investigations on quasiperiodic disorder.

We add quantum fluctuations to a classical period-doubling Hamiltonian time crystal, replacing the $N$ classical interacting angular momenta with quantum spins of size $l$. The full permutation symmetry of the Hamiltonian allows a mapping to a bosonic model and the application of exact diagonalization for a quite large system size. In the thermodynamic limit $N\to \infty$ the model is described by a system of Gross-Pitaevskii equations whose classical-chaos properties closely mirror the finite-$N$ quantum chaos. For $N\to \infty$, and $l$ finite, Rabi oscillations mark the absence of persistent period doubling, which is recovered for $l\to \infty$ with Rabi-oscillation frequency tending exponentially to 0. For the chosen initial conditions, we can represent this model in terms of Pauli matrices and apply the discrete truncated Wigner approximation. For finite $l$ this approximation reproduces no Rabi oscillations but correctly predicts the absence of period doubling. Our results show the instability of time-translation symmetry breaking in this classical system even to the smallest quantum fluctuations, because of tunneling effects.

The microscopic conditions for the emergence of nonreciprocal magnon excitations in noncentrosymmetric magnets are theoretically investigated. We show that asymmetric magnon excitations appear when a bond magnetic toroidal dipole becomes active, which is defined as a parallel component between the Dzyaloshinskii-Moriya vector and the averaged spin moments at the ends of the bonds. Depending on the magnetic structures accompanying the bond magnetic toroidal dipoles, the higher-rank magnetic toroidal multipoles can also be activated in a magnetic cluster, which describes various angle dependences of asymmetric magnon excitations. We demonstrate a variety of asymmetric magnon excitations for two magnetic systems in the noncentrosymmetric lattice structures: a one-dimensional breathing chain under a uniform crystalline electric field and a three-dimensional layered breathing kagome structure. We show that a bottom shift of magnon bands occurs by the cluster magnetic toroidal dipole and a valley splitting occurs by the cluster magnetic toroidal octupole under magnetic orderings. Our result provides a way to identify the important Dzyaloshinskii-Moriya interaction for nonreciprocal magnon excitations at the microscopic level beyond the symmetry argument, which will be useful not only to understand the origin of asymmetric band modulations observed in experiments but also to give an insight with which to explore further spintronics materials.

Two-dimensional lattices of dipolar-coupled thin film ferromagnetic nanodisks give rise to emergent superferromagnetic (SFM) order when the spacing between dots becomes sufficiently small. In this paper, we define micron-sized arrays of permalloy nanodisks arranged on a hexagonal lattice. The arrays were shaped as hexagons, squares, and rectangles to investigate finite-size effects in the SFM domain structure for such arrays. The resulting domain patterns were examined using x-ray magnetic circular dichroism photoemission electron microscopy. At room temperature, we find these SFM metamaterials to be below their blocking temperature. Distinct differences were found in the magnetic switching characteristics of horizontally and vertically oriented rectangular arrays. The results are corroborated by micromagnetic simulations.

We study, both theoretically and experimentally, the features on the current-voltage characteristic of a highly transparent Josephson junction caused by transition of the superconducting leads to the normal state. These features appear due to the suppression of the Andreev excess current. We show that by tracing the dependence of the voltage, at which the transition occurs, on the bath temperature and by analyzing the suppression of the excess current by the bias voltage one can recover the temperature dependence of the heat flow out of the junction. We verify theory predictions by fabricating two highly transparent superconductor-graphene-superconductor (SGS) Josephson junctions with suspended and nonsuspended graphene as a nonsuperconducting section between Al leads. Applying the above mentioned technique we show that the cooling power of the suspended junction depends on the bath temperature as $\propto T_{\mathrm{bath}}^{3.1}$ close to the superconducting critical temperature.

Two-dimensional ferromagnets have recently drawn extensive attention, and here we study the electronic structure and magnetic properties of the bulk and monolayer of CrSBr using first-principles calculations and Monte Carlo simulations. Our results show that bulk CrSBr is a magnetic semiconductor and has the easy magnetization $b$ axis, hard $c$ axis, and intermediate $a$ axis. Thus the experimental triaxial magnetic anisotropy (MA) is well reproduced here, and it is identified to be the joint effects of spin-orbit coupling and magnetic dipole-dipole interaction. We find that bulk CrSBr has a strong ferromagnetic (FM) intralayer coupling but a marginal interlayer one. We also study the CrSBr monolayer in detail and find that the intralayer FM exchange persists and the shape anisotropy has a more pronounced contribution to the MA. Using the parameters of the FM exchange and the triaxial MA, our Monte Carlo simulations show that the CrSBr monolayer has Curie temperature $T_{\mathrm{C}}=175$ K. Moreover, we find that a uniaxial tensile (compressive) strain along the $a$ $\left(b\right)$ axis would further increase the $T_{\mathrm{C}}$.

Magnetic skyrmions are topologically distinct swirls of magnetic moments which display particlelike behavior, including the ability to undergo thermally driven diffusion. In this paper we study the thermally activated motion of arrays of skyrmions using temperature dependent micromagnetic simulations where the skyrmions form spontaneously. In particular, we study the interaction of skyrmions with grain boundaries, which are a typical feature of sputtered ultrathin films used in experimental devices. We find the interactions lead to two distinct regimes. For longer lag times the grains lead to a reduction in the diffusion coefficient, which is strongest for grain sizes similar to the skyrmion diameter. At shorter lag times the presence of grains enhances the effective diffusion coefficient due to the gyrotropic motion of the skyrmions induced by their interactions with grain boundaries. For grain sizes significantly larger than the skyrmion diameter clustering of the skyrmions occurs in grains with lower magnetic anisotropy.

Experimental detection of spin dynamics of the quantum spin liquid phase, which occurs when the quantum fluctuations lead to fractionalized spin states, remains a challenge. In this paper, we illustrate spin-liquid signatures in the geometrically frustrated layered kagome system $\mathrm{YBa}{\mathrm{Co}}_{4-x}{\mathrm{Fe}}_x{\mathrm{O}}_{7+\delta }$ ($x=0$ and $x=0.6$) over a wide temperature range using micro-Raman spectroscopy. The thermal response of the gapped broad band over a high temperature range possibly marks a spin liquid phase in this system. We report the appearance of sharp spectral features in the high temperature regime with unusual thermal behavior for $\mathrm{YBa}{\mathrm{Co}}_{4-x}{\mathrm{Fe}}_x{\mathrm{O}}_{7+\delta }$ ($x=0.6$). Besides, atypical oscillation of a phonon mode frequency with temperature suggests an unconventional electron-lattice coupling in the doped compound via a strong correlation between their lattice vibration and magnetic spin dynamics.

The quantum geometry of Bloch states fundamentally affects a wide range of physical phenomena. The quantum Hall effect, for example, is governed by the Chern number, and flat-band superconductivity by the distance between the Bloch states: the quantum metric. While understanding quantum geometry phenomena in the context of fermions is well established, less is known about the role of quantum geometry in bosonic systems where particles can undergo Bose-Einstein condensation (BEC). In conventional single-band or continuum systems, excitations of a weakly interacting BEC are determined by the condensate density and the interparticle interaction energy. In contrast to this, we discover here fundamental connections between the properties of a weakly interacting BEC and the underlying quantum geometry of a multiband lattice system. We show that, in the flat-band limit, the defining physical quantities of BEC, namely, the speed of sound and the quantum depletion, are dictated solely by the quantum geometry. We find that the speed of sound becomes proportional to the quantum metric of the condensed state. Furthermore, the quantum distance between the Bloch functions forces the quantum depletion and the quantum fluctuations of the density-density correlation to obtain finite values for infinitesimally small interactions. This is in striking contrast to dispersive bands where these quantities vanish with the interaction strength. Additionally, we show how in the flat-band limit the supercurrent is carried by the quantum fluctuations and is determined by the Berry connections of the Bloch states. Our results reveal how nontrivial quantum geometry allows reaching strong quantum correlation regime of condensed bosons even with weak interactions. This is highly relevant, for example, for polariton and photon BECs where interparticle interactions are inherently small. Our predictions can be experimentally tested with flat-band lattices already implemented in ultracold gases and various photonic platforms.

The classic magnetic induction effect is usually considered in electric circuits or conductor coils. In this paper, we propose quantum induction effects induced by the Berry curvature in homogenous solids. Two different types of quantum inductions are identified in the higher order of a magnetic field. They are closely related to the magnetic-field-induced anomalous velocity proportional to the Berry curvature so that both quantum inductions become significantly enhanced near the Weyl points. Further, we propose an ac-magnetic-field measurement to detect quantum induction effects and distinguish them from classical induction.

In this work, we propose and study a one-dimensional model of the effects of optical phonon/magnon coupling. The indirect interactions between magnetic ions are conducted by the sandwiched nonmagnetic ions. The vibrations of nonmagnetic ions invoke effective magnetic fields to spins and hence couple with the magnon. The quantum solutions of the model show energy gaps in the spin-wave dispersion induced by the magnon-phonon coupling. Classical simulations reveal the existence of hybrid magnon-phonon quasiparticles and the coherent phonon (magnon) induced by magnon (phonon).

Transition-metal dichalcogenides (TMDCs) hosting type-II Dirac fermions have attracted a great deal of research interest owing to their rich application capabilities. Here, we have systematically investigated the bulk and surface electronic structures of type-II Dirac semimetal $\mathrm{Ni}{\mathrm{Te}}_2$ by means of angle-resolved photoelectron spectroscopy (ARPES) combined with first-principles calculations. As a result, a bulk Dirac point located 150 meV below the Fermi energy is directly observed by using bulk-sensitive soft x-ray ARPES. Moreover, the measured Fermi surfaces of $\mathrm{Ni}{\mathrm{Te}}_2$ are found to be strongly anisotropic. The first-principles calculations, which match remarkably well with the experimental results, show that the energy position of the Dirac point crucially depends on a small variation of the structural parameters. Our work establishes $\mathrm{Ni}{\mathrm{Te}}_2$ as an ideal platform for further investigating the anisotropic magnetotransport associated with type-II Dirac fermions in TMDCs and paves the way for prospective applications.

We apply high-resolution angle-resolved photoemission spectroscopy to explore the layered transition-metal dichalcogenide $1T-{\mathrm{VSe}}_2$ with various photon energies to investigate the three-dimensional (3D) charge-density wave (CDW) transition. Here, we provide quite comprehensive evidence of 3D Fermi surface nesting in $1T-{\mathrm{VSe}}_2$. The observed Fermi surface (FS) largely overlaps with the in-plane nesting vector reported by the electron-diffraction results. Comparing the CDW gaps below and above $T_c$, we find that the gaps open along the ellipsoidal FS around the $\overline{M}$ point, in agreement with the in-plane nesting vector. With varying the photon energy, a warping of FS and the nesting condition along $k_z$ are observed. We further observe the gap variation along the $k_z$ direction consistent with the out-of-plane nesting conditions. Although it was believed to be a layered two-dimensional material, the satisfaction of the 3D nesting condition of the FS and the variation of gap distribution suggest a 3D character of CDW in $1T-{\mathrm{VSe}}_2$. Meanwhile, a larger value of $2\mathrm{\Delta }/k_B{T}_c$ = 14.76 suggests CDW in $1T-{\mathrm{VSe}}_2$ beyond the weak-coupling region, the nesting condition and the gap correspondence implies a combined effort in the formation of CDW in this material.