The electric Grüneisen parameter, which characterizes the electrocaloric effect in the quantum spin system, has been introduced. We have calculated the electric Grüneisen parameter for the biaxial spin-chain system with strong coupling between the spin and the electric subsystems. It is shown that the low-temperature electric field dependence of the electric Grüneisen parameter manifests features at the critical values of the electric field and tends to constant at high temperatures. It is determined by the quantum critical points of the spin system. Such a behavior of the electric Grüneisen parameter can be observed in electrocaloric experiments with spin-chain materials.

We report unambiguously experimental evidence of a strong orbital current in Nb films with weak spin-orbit coupling via the spin-torque ferromagnetic resonance spectrum for Fe/Nb and Co/Nb bilayers. The sign change in the dampinglike torque in Co/Nb demonstrates a large spin-orbit correlation and thus great efficiency of orbital torque in Co/Nb. By studying the efficiency as a function of the thickness of Nb sublayer, we reveal a long orbital diffusion length (∼3.1 nm) of Nb. Further planar Hall resistance measurements at positive and negative applying current confirm the nonlocal orbital transport in ferromagnetic-metal/Nb heterostructures.

We report a combined experimental and theoretical study of the exchange bias (EB), structural and magnetic properties of manganese-rich ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{41.5+x}{\mathrm{Sn}}_{8.5-x}$ $(x=0, 0.75, \text{and} 1)$ Heusler alloys. All these alloys crystallize in a tetragonal structure at room temperature. Ab initio simulation using density functional theory (DFT) on off-stoichiometric ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{42.1875}{\mathrm{Sn}}_{7.8125}$ alloy (very close to the actual composition) predicts the tetragonal structure with the ferrimagnetic arrangement as the ground state, in good agreement with the experimental results. These alloys are found to exhibit a large exchange bias at low temperatures, with a maximum EB field of 7.1 kOe at 2 K for $x=1$, which is significantly larger than that reported for any Mn-Ni–based Heusler systems. Frequency dependence of spin freezing temperature obeying the scaling law confirms the presence of a cluster spin glass (CSG)–like state in these alloys. The latter is also confirmed from the memory and aging effect studies in zero-field cooling and field-cooled protocols for these alloys. The large EB is attributed to the strong exchange coupling between the CSG clusters embedded in an otherwise ferrimagnetic matrix. The exchange mechanism behind the large EB effect is discussed in the light of varying bond lengths between different magnetic pairs, and hence the magnetic coupling, due to the large tetragonal distortion. The present experimental findings are found to be in good agreement with the DFT results.

A numerical analysis on the computational capability of physical reservoir computing utilizing a spin-torque oscillator with two free layers is reported. Conventional spintronics devices usually consist of two ferromagnets, where the direction of magnetization in one layer, called the free layer, can move while that of the other, the reference layer, is fixed. Recently, however, devices with two free layers, where the reference layer is replaced by another free layer, have been developed for various practical applications. Adding another free layer drastically changes the dynamical response of the device through the couplings via the spin-transfer effect and the dipole magnetic field. A numerical simulation of the Landau-Lifshitz-Gilbert equation and a statistical analyses of the Lyapunov exponent and the synchronization index reveal the appearance of an amplitude-modulated oscillation and chaos in the oscillators with two free layers. Such complex dynamics qualitatively change the computational capability of physical reservoir computing because the computational resource is dynamics of the physical system. An evaluation of the short-term memory capacity clarifies that oscillators with two free layers have a larger capacity than those of conventional oscillators. An enhancement in capacity near the edge of echo state property, i.e., the boundary between zero and finite synchronization index, is also found.

We calculate the quantum Lyapunov exponent ${\lambda }_L$ and butterfly velocity $v_B$ in the dilute Bose gas at temperature $T$ deep in the Bose-Einstein condensation phase. The generalized Boltzmann equation approach is used for calculating out-of-time-ordered correlators, from which ${\lambda }_L$ and $v_B$ are extracted. At very low temperature where elementary excitations are phonon-like, we find ${\lambda }_L\propto T^5$ and $v_B\sim c$, the sound velocity. At relatively high temperature, we have ${\lambda }_L\propto T$ and $v_B\sim c{(T/T_{*})}^{0.23}$. We find that ${\lambda }_L$ is always comparable to the damping rate of a quasiparticle, whose energy depends suitably on $T$. The chaos diffusion constant $D_L=v_B^2/{\lambda }_L$, on the other hand, differs from the energy diffusion constant $D_E$. We find $D_E\ll D_L$ at very low temperature and $D_E\gg D_L$ otherwise.

Quasiperiodic systems show a universal gap structure due to quasiperiodicity, which is analogous to gap openings at the Brillouin zone boundary in periodic systems. The integrated density of states (IDoS) below those energy gaps is characterized by a few integers, which is known as the “gap labeling theorem” (GLT) for quasiperiodic systems. In this study, focusing on multilayer thin film systems such as twisted bilayer graphene and stacked transition-metal dichalcogenides, we extend the GLT for multilayer systems of arbitrary dimensions and number of layers, using an approach based on the algebra called “a noncommutative torus.” We find that the energy gaps and the associated IDoS are generally characterized by ${}_{DN}{C}_D$ integer labels in $N$ layer systems in the $D$ dimensions, when the effect of the interlayer coupling can be approximated by a quasiperiodic intralayer coupling for each layer. We demonstrate that the generalized GLT holds for quasiperiodic 1D tight-binding models by numerical simulations.

Two-dimensional Hubbard lattices with two or three holes are investigated as a function of $U$ in the large-$U$ limit. In the so-called Nagaoka limit (one-hole system at infinite $U$), it is known that the Hubbard model exhibits a ferromagnetic ground state. Here, by means of exact full configuration interaction quantum Monte Carlo simulations applied to periodic lattices up to 24 sites, we compute spin-spin correlation functions as a function of increasing $U$. The correlation functions clearly demonstrate the onset of ferromagnetic domains, centered on individual holes. The overall total spin of the wave functions remains the lowest possible (0 or $\frac{1}{2}$, depending on the number of holes). The ferromagnetic domains appear at interaction strengths comparable to the critical interaction strengths of the Nagaoka transition in finite systems with strictly one hole. The existence of such ferromagnetic domains is the signature of Nagaoka physics in Hubbard systems with a small (but greater than 1) number of holes.

We investigate the ground state of the spin $S=\frac{1}{2}$ Heisenberg antiferromagnet on the shuriken lattice, also in the presence of an external magnetic field. To this end, we employ two-dimensional tensor network techniques based on infinite projected entangled pair and simplex states considering states with different sizes of the unit cells. We show that a valence bond crystal with resonances over length six loops emerges as the ground state (at any given finite bond dimension) yielding the lowest reported estimate of the ground state energy $E_0/J=-0.4410\pm 0.0001$ for this model, estimated in the thermodynamic limit. We also study the model in the presence of an external magnetic field and find the emergence of 0, $\frac{1}{3}$, and $\frac{2}{3}$ magnetization plateaus. The $\frac{1}{3}$ and $\frac{2}{3}$ plateau states respect translation and point group symmetries and feature loop-four plaquette resonances.

Recent mean-field calculations suggest that the superconducting state of twisted bilayer graphene exhibits either a nematic order or a spontaneous breakdown of the time-reversal symmetry. The two-dimensional character of the material and the large critical temperature relative to the Fermi energy dictate that the material should have significant fluctuations. We study the effects of these fluctuations using Monte Carlo simulations. We show that in a model proposed earlier for twisted bilayer graphene there is a fluctuation-induced phase with quadrupling fermionic order for all considered parameters. This four-electron condensate, instead of superconductivity, shows a spontaneous breaking of time-reversal symmetry. Our results suggest that twisted bilayer graphene is an especially promising platform to study different types of condensates, beyond the pair-condensate paradigm.

We explore the dielectric properties of the molecular quantum-spin-liquid candidate $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$ as a function of frequency, pressure, and temperature. The transition from the incoherent semiconducting to the Mott-insulating state at the quantum Widom line yields a drop of the low-frequency dielectric constant to small positive values ${\varepsilon }_1\approx 10$. The characteristic relaxor-type peak around $T=50$ K moves to lower temperatures when pressurized. An additional feature appears and quickly grows as the percolative first-order transition is approached with rising pressure. Above 4 kbar, it dominates the dielectric response and rapidly shifts to $T\to 0$. Overall, the pressure-dependent dielectric response is remarkably similar to the less-correlated sister compound $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$. At the Mott transition, we identify a reentrant insulating behavior at $T^{★}=11$ K which might indicate a low-entropy ground state — possibly with a spin gap.

We have measured crystalline electric field (CEF) excitations of ${\mathrm{Nd}}^{3+}$ ions in the two-channel Kondo lattice candidates $\mathrm{Nd}{\mathit{T}}_2{\mathrm{Zn}}_{20}$ $(T = \mathrm{Co}, \mathrm{Rh}, \text{and} \mathrm{Ir})$ by means of inelastic neutron scattering (INS). In the INS measurements at 5 K, dispersionless excitations were observed at 3.8 and 7.2 meV for $T=\mathrm{Co}$, 3.1 and 5.8 meV for $T=\mathrm{Rh}$, and 3.0 and 5.3 meV for $T=\mathrm{Ir}$. Analyses of the temperature dependence of the INS spectra confirm that the CEF ground states are the ${\mathrm{\Gamma }}_6$ doublet, that is a requisite for manifestation of the magnetic two-channel Kondo effect. For $T=\mathrm{Co}$, a shoulder was observed at 7.7 meV close to the CEF excitation peak centered at 7.2 meV. The shoulder is attributed to a bound state of the CEF and low-lying optical phonon excitations.

In this work we propose a two-dimensional topological photonic system of Si rods, whose unit cell is composed of six artificial atoms located on the sites of a triangular lattice. By perturbing the original lattice, by alternately increasing (type A) and shrinking (type B) the radius of the rods, we find edge modes that are highly localized at the interface between the type A and type B photonic crystals. The proposed structure is robust against complex defects and allows transmission of the propagating electromagnetic waves without any reflection. The interface edge modes present a typical behavior corresponding to that of a topological insulator. More specifically, the edge mode presents a pseudospin topological behavior. Our numerical results show the robustness, originating from the topological protection, of this mode against defects, disorder, and reflection. Also, the localization at the interface state permits the confinement of the light, where the interface behaves as a waveguide for the propagation of electromagnetic waves. The geometric perturbation considered here can easily be implemented using nanolithography.

We study the steady-state dc transport characteristics of a system comprised of an interacting quantum dot, modeled as an Anderson impurity, coupled to two metallic noninteracting leads with Rashba spin-orbit coupling (SOC), using an interpolative perturbative approach (IPA). The single-particle spectra, current, and differential conductance are obtained in weak- and strong-coupling regimes over a wide range of SOC and bias values. Extensive benchmarking of the IPA validates the method in the linear as well as nonlinear response regime. The universal, zero-bias ($V_{sd}=0$) peak with a width proportional to the Kondo scale ($T_K$) and two nonuniversal finite-bias peaks around $V_{sd}=\pm U$ in the zero-temperature differential conductance show a clear separation with increasing $U$ or increasing SOC. In the strong-coupling regime, increasing temperature induces melting of the zero-bias peak, leading to a crossover from a three-peak conductance to a two-peak conductance. Recent experiments find the emergence of a two-peak structure by increasing SOC at a fixed temperature. Our results appear to provide a qualitative explanation of these observations as a SOC tuned crossover from weak/intermediate to strong coupling, and a simultaneous crossover from low-$T/T_K$ to high-$T/T_K$ ratio. We also reproduce the experimentally observed temperature dependence of the zero-bias conductance.

The correspondence between $1/2$ modes in the entanglement spectrum (ES) and boundary states is a powerful quantum-informative window into gapped topological phases, where the bulk gap is a key to theoretically approaching the correspondence. In this paper, we investigate the correspondence in gapless topological phases. We choose higher-order Weyl semimetals (HOWSMs) with higher-order Weyl nodes in the bulk and hinge arcs and Fermi arcs on the boundary. We find that the aim of quantum-informatively identifying boundary states ultimately drives us to make full use of the eigenquantities of the entanglement Hamiltonian: the ES as well as Schmidt vectors [entanglement wave functions (EWFs)]. While both hinge arcs and Fermi arcs contribute to $1/2$ modes, the EWFs corresponding to hinge arcs and Fermi arcs are respectively localized on the virtual hinges and surfaces of symmetry-allowed entanglement partitions. We conclude that the full entanglement data, i.e., ES and EWFs, should be employed to identify boundary states and symmetry protection. While HOWSMs are prototypical noninteracting examples, our work on the necessity and effectiveness of EWFs sheds light on both theoretical and numerical explorations of the entanglement signature in general gapless topological phases.

The cavity axion polariton represents a new frontier in the field of topological matter and light-matter interactions. To realize strong axion-photon coupling and low damping of the axion mode, which are two key factors for quantum information applications, we studied the short-wavelength axion modes and the axion-axion interaction theoretically. Our results reveal an unconventional coupling between photon and standing wave axion modes that is in the strong-coupling regime as the number of antinodes is large. When two axion modes are coupled with cavity photons, cavity-mediated axion-axion coupling gives rise to the axion dark mode. Our results suggest that short-wavelength axion dark modes may be potential candidates for quantum manipulation and storage.

We theoretically study the nonequilibrium thermoelectric transport properties of a strongly-correlated molecule (or quantum dot) embedded in a tunnel junction. Assuming that the coupling of the molecule to the contacts is asymmetric, we determine the nonlinear current driven by the voltage and temperature gradients by using the perturbation theory. However, the subsystem consisting of the molecule strongly coupled to one of the contacts is solved by using the numerical renormalization group method, which allows for accurate description of Kondo correlations. We study the temperature gradient and voltage dependence of the nonlinear and differential Seebeck coefficients for various initial configurations of the system. In particular, we show that in the Coulomb blockade regime with singly occupied molecule, both thermopowers exhibit sign changes due to the Kondo correlations at nonequilibrium conditions. Moreover, we determine the nonlinear heat current and thermoelectric efficiency, demonstrating that the system can work as a heat engine with considerable efficiency, depending on the transport regime.

We pressure tune the hydrogen bond in Fe-O–H⋅⋅⋅O-P structural segments of mixed-valence barbosalite $\left({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_2^{3+}\right){\left(\mathrm{P}{\mathrm{O}}_4\right)}_2{\left(\mathrm{OH}\right)}_2$. Infrared spectroscopy evidences changes in softening of O–H stretch modes and excessive profile broadening onset below 10 GPa. Single-crystal x-ray diffraction shows pseudosymmetrization of the original monoclinic unit cell concurs with these changes in the O–H vibrational mode. These are considered compelling indicators of proton delocalization onset below 10 GPa as hydrogen bonds are strengthened under pressure. Subsequently in the range $10-30$ GPa, Fe Mössbauer spectroscopy discerns ${\mathrm{Fe}}^{2+}\iff {\mathrm{Fe}}^{3+}$ valence fluctuations at proximate cations of the hydrogen bonds. When the original crystal potential at an ${\mathrm{Fe}}^{2+}$ site is perturbed by proton delocalization at a ligand, electron exchange is induced along ${\mathrm{Fe}}^{2+}\to L\to {\mathrm{Fe}}^{3+}$ pathways [ligand $L=\mathrm{O}$ or ${\left(\mathrm{OH}\right)}^{-}$ of shared octahedral faces]. Thus, $\left({\mathrm{Fe}}^{2+}{\mathrm{Fe}}_2^{3+}\right){\left(\mathrm{P}{\mathrm{O}}_4\right)}_2{\left(\mathrm{OH}\right)}_2$ under pressure exemplifies the interplay between proton (THz) and electron (MHz) dynamics on two disparate timescales in the same condensed phase.

We identify a three-dimensional skyrmion side-face state in chiral magnets that consists of a thin layer of modulated surface spirals and an array of phase-locked skyrmion screws. Such chiral spin structures lead to a characteristic X-shaped magnetic diffraction pattern in resonant elastic x-ray scattering, reminiscent of Photo 51 of the DNA double-helix diffraction. By measuring both thin plates and bulk ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ crystals in the field-in-plane geometry, we unambiguously identify the modulated skyrmion strings by retrieving their chirality and helix angle. The breaking of the translational symmetry along the side faces suppresses the bulk-favored conical state, providing a stabilization mechanism for the skyrmion lattice phase that has been overlooked so far.

Exploring hidden effects that have been overlooked given the nominal global crystal symmetry but are indeed visible in solid-state materials has been a fascinating subject of research recently. Here, we introduce a hidden Zeeman-type spin polarization (HZSP) in nonmagnetic bulk crystals with sublattice structures. In the momentum space of these crystals, the doubly degenerate bands formed in a certain plane can exhibit a uniform spin configuration with opposite spin orientations perpendicular to this plane, whereas such degenerate states are spatially separated in a pair of real-space sectors. Interestingly, we find that HZSP can manifest itself in both centrosymmetric and noncentrosymmetric materials. We further demonstrate the important role of nonsymmorphic twofold screw-rotational symmetry played in the formation of HZSP. Moreover, two representative material examples, i.e., centrosymmetric $\mathrm{W}{\mathrm{Se}}_2$ and noncentrosymmetric $\mathrm{Ba}{\mathrm{Bi}}_4{\mathrm{O}}_7$, are identified to show HZSP via first-principles calculations. Our finding thus not only opens different perspectives for hidden spin polarization research but also significantly broadens the range of materials towards spintronics applications.