Thermal transport is a pivotal probe for studying low-energy, charge-neutral quasiparticles in insulating magnets. In this Letter, we report an observation of large magnetothermal conductivity and thermal Hall effect (THE) in a van der Waals antiferromagnet ${\mathrm{FeCl}}_2$. The magnetothermal conductivity reaches over $\sim 700\%$, indicating strong magnon-phonon coupling. Furthermore, we find an appreciable thermal Hall signal which changes sign concurrently with the spin-flip transition from the antiferromagnetic state to the polarized ferromagnetic state. Our theoretical calculations suggest that, in addition to the Berry curvature induced at the anticrossing points of the hybridized magnon and acoustic phonon modes of ${\mathrm{FeCl}}_2$, other mechanisms are needed to account for the magnitude of the observed THE.

The sparse version of the Sachdev-Ye-Kitaev (SYK) model reproduces essential features of the original SYK model while reducing the number of disorder parameters. In this Research Letter, we propose a further simplification of the model which we call the binary-coupling sparse SYK model. We set the nonzero couplings to be $\pm 1$, rather than being sampled from a continuous distribution such as Gaussian. Remarkably, this simplification turns out to be an improvement: The binary-coupling model exhibits strong correlations in the spectrum, which is the important feature of the original SYK model that leads to the quick onset of the random-matrix universality, more efficiently in terms of the number of nonzero terms. This model is better suited for analog or digital quantum simulations of quantum chaotic behavior and holographic metals due to its simplicity and scaling properties.

We show that time-domain analysis of high harmonic generation from solids with subcycle resolution and broad, continuous spectral coverage is now experimentally viable due to the emergence of sensitive, solid-state optical-field-sampling techniques. With this experimental opportunity in mind, we use time-dependent density functional theory to explore what insights such a time-domain, field-resolved analysis might uncover about the extreme nonlinear electron dynamics responsible for high-harmonic generation (HHG) within solids. We illustrate how simple, visual analysis of the time-domain fields provides clear insight into the interplay between intra- and interband dynamical processes underlying nonlinear light generation when spectral signatures do not. Importantly, we observe conditions where the dominant emission mechanism suddenly switches from intra- to interband over a subcycle region of time within the pulse envelope of the driving wave form. This complex field response means that phase-resolved techniques requiring a certain level of periodicity are, in general, inadequate for the study of HHG from solids. We find that field-resolved measurements having both subcycle time resolution and broad, nearly continuous spectral coverage are required for a general understanding of solid-state HHG.

We provide a guideline to generate entanglement of quantum many-body states by applying key ideas of the Higgs mechanism to systems without gauge structures. By demoting a ${\mathbb{Z}}_2$ gauge structure, the model with the global ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry is constructed on a square lattice. We show that its ground state is a symmetry-protected-topological state protected by the global ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry and translational symmetry, in contrast to the previous work where one-form symmetry is introduced for protection. Furthermore, by demoting a $\mathbb{U}\left(1\right)$ gauge structure, an intriguing gapless state with the global $\mathbb{U}\left(1\right)\times \mathbb{Z}$ is constructed. We discuss the generalization of our model to different symmetries and lattices. Plausible applications to quantum simulators, such as Rydberg atoms and trapped ions, are also discussed.

The addition of a London dispersion correction to standard Kohn-Sham density-functional theory is essential for an accurate description of noncovalent interactions. While several dispersion-corrected density functionals (DC-DFs) have shown excellent performance for hard solids at ambient conditions, their transferability to metallic systems at ambient conditions or under isotropic compression has not been systematically examined. In this study, we assess the ability of selected DC-DFs to describe the equations of state (EOSs) of selected elemental metals and intermetallic compounds up to several gigapascals of pressure. EOS-derived properties, such as the unit-cell volume, the bulk modulus, and its pressure derivative, were then evaluated with and without thermal effects and the results compared with experimental reference data. We also assess the ability of the DC-DFs to predict the phase-transition pressures for a set of intermetallic compounds. The results of this study establish that London dispersion physics, and even dispersion contributions from the core electrons, is important in the description of bulk metals.

The Hartree-Fock generalized Kadanoff-Baym ansatz (HF-GKBA) offers an approximate numerical procedure for propagating the two-time nonequilibrium Green's function (NEGF). Here, using the $GW$ self-energy, we compare the HF-GKBA to exact results for a variety of systems with long- and short-range interactions, different two-body interaction strengths, and various nonequilibrium preparations. We find excellent agreement between the HF-GKBA and exact time evolution in models when more realistic long-range exponentially decaying interactions are considered. This agreement persists for long times and for intermediate to strong interaction strengths. In large systems, HF-GKBA becomes prohibitively expensive for long-time evolutions. For this reason, we look at the use of dynamical mode decomposition (DMD) to reconstruct long-time NEGF trajectories from a sample of the initial trajectory. Using no more than 16% of the total time evolution, we reconstruct the total trajectory with high fidelity. Our results show the potential for DMD to be used in conjunction with HF-GKBA to calculate long-time trajectories in large-scale systems.

Electron bands in Bernal bilayer graphene flatten out in a transverse electric field, offering a promising platform for correlated electron physics. We discuss the spin/valley isospin magnetism resembling that seen in moiré bands. We predict that the isospin magnetism coexists with momentum-polarized phases occurring via a “flocking transition” in momentum space in which the electron distribution is spontaneously displaced in momentum space relative to the $K$ and $K^{\prime }$ valley centers. The momentum-polarized phases feature electronic nematicity manifest in the anisotropy of transport, and unusual observables such as persistent currents in the ground state. Momentum-polarized carriers “sample” the Berry curvature of the conduction band, resulting in a longitudinal and anomalous Hall conductivity that displays switching upon the flocking transition and other effects that do not occur in previously studied systems.

We present a scheme to use physical Majorana quasizero modes at each junction of a two-dimensional nanowire network to build a logical Majorana zero mode, the location of which is controllable through gate voltages. The wire network is a way to realize a recent proposal to imprint a Kekulé vortex pattern on a honeycomb lattice via gate voltages. We show that a specific type of junction, other than a naive Y or T junction, is needed to realize, without breaking time-reversal symmetry, an artificial “graphene” system with Majorana fermions instead of complex ones at each site. The junction we propose (i) traps exactly one physical Majorana (quasi)zero mode at each site of either a brick wall or honeycomb lattice and (ii) allows this mode to hybridize with all three neighboring sites. Using a lattice of these junctions and starting from an electronic, tight-binding model for the wires, we imprint the voltage patterns corresponding to the Kekulé vortex and observe the emergence of the logical Majorana zero mode at the vortex core. By suitable adiabatic modulations of gate voltages as a function of time, the positions of individual Kekulé vortices can be moved, and in this manner, non-Abelian braiding of the logical Majoranas bound to the vortex cores can be performed. We provide the range of parameters where these excitations could be realized experimentally.

Real-space entanglement spectrum (RSES) of a quantum Hall (QH) wave function gives a natural route to infer the nature of its edge excitations. Computation of RSES becomes expensive with increase in number of particles and number of included Landau levels (LL). RSES can be efficiently computed using Monte Carlo (MC) methods for trial states that can be written as products of determinants such as the composite fermion (CF) and parton states. This computational efficiency applies also to the RSES of lowest Landau level (LLL) projected CF and parton states, however, LLL projection to be used here requires approximations that generalize the Jain Kamilla (JK) projection. This work is a careful study of how this approximation should be done. We identify the approximation that is closest in spirit to JK projection, and perform tests of the approximations involved in the projection by comparing the MC results with the RSES obtained from computationally expensive but exact methods. We present the techniques and use them to calculate exact RSES of the exact LLL projected bosonic Jain $2/3$ state in bipartition of systems of sizes up to $N=24$ on the sphere. For the lowest few angular momentum sectors of the RSES, we present evidence to show that MC results closely match the exact spectra. We also compare other plausible projection schemes. We also calculate the exact RSES of the unprojected fermionic Jain $2/5$ state obtained from exact diagonalization of the Trugman-Kivelson Hamiltonian in the two lowest LLs on the sphere. By comparing with the RSES of the unprojected $2/5$ state from Monte Carlo methods, we show that the latter is practically exact.

We report the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) data for the noncollinear Weyl semimetal CeAlSi. The anomalous Hall conductivity (${\sigma }_{ij}^A$) was measured for two different orientations of the magnetic field ($B$), namely ${\sigma }_{yz}^A$ for $B\left|\right|a$ and ${\sigma }_{xy}^A$ for $B\left|\right|c$, where $a$ and $c$ denote the crystallographic axes. We find that ${\sigma }_{xy}^A$ and ${\sigma }_{yz}^A$ are of opposite sign and both are large below the Curie temperature ($T_{\mathrm{C}}$). In the paramagnetic phase, ${\sigma }_{xy}^A$ rises even more and goes through a maximum at $T\approx 170\mathrm{K}$, whereas the absolute value of ${\sigma }_{yz}^A$ decreases with increasing temperature. The origin of the sign difference between ${\sigma }_{xy}^A$ and ${\sigma }_{yz}^A$ was attributed to the reconstruction of the band structure under the variation of the spin orientation. Further, in a system where humps in the AHE are present and scalar spin chirality is zero, we show that the $\mathbf{k}$-space topology plays an important role to determine the transport properties at both low and high temperatures. We also observed the anomalous contribution in the Nernst conductivity (${\alpha }_{xy}^A$) measured for $B\left|\right|c$. ${\alpha }_{xy}^A/T$ turns out to be sizeable in the magnetic phase and above $T_{\mathrm{C}}$ slowly decreases with temperature. We were able to recreate the temperature dependencies of ${\sigma }_{xy}^A$ and ${\alpha }_{xy}^A/T$ in the paramagnetic phase using a single band toy model assuming a nonzero Berry curvature in the vicinity of the Weyl node. A decisive factor appears to be a small energy distance between the Fermi level and a Weyl point.

In our previous publication [Ozaki and Ogata, Phys. Rev. Res. 3, 013058 (2021)], the quantization of the orbital-Zeeman (OZ) cross term in the magnetic susceptibility, or the cross term of spin Zeeman and orbital effect, was shown for the Kane-Mele model using the expansion around the Dirac points. In the present study, we accurately evaluate the orbital, spin-Zeeman, and OZ cross term of the Kane-Mele model using a recently developed formulation. This method uses the explicit real-space Bloch wave functions and enables us to accurately evaluate each contribution taking into account an infinite number of high-energy bands and core electron contribution at arbitrary chemical potential. We find that the effect beyond the previous formulations does not change the quantization of the OZ cross term. We also study the OZ cross term in the metalic case, and the sum rule of the OZ cross term with respect to the chemical potential is found. The possibility of experimental detection of the quantization is also discussed.

We have performed a time-resolved and phase-sensitive investigation of three-magnon scattering of ferromagnetic resonance (FMR) over several orders of magnitude in excitation power. We observe a regime that hosts transient oscillations of the FMR magnon population, despite higher-order magnon interactions at large powers. Also at high powers, the scattering generates ${180}^{\circ }$ phase shifts of the FMR magnons. These phase shifts correspond to reversals in the three-magnon scattering direction, between splitting and confluence. These scattering reversals are most directly observed after removing the microwave excitation, generating coherent oscillations of the FMR magnon population much larger than its steady-state value during the excitation. Our model is in strong agreement with these findings. These findings reveal the transient behavior of this three-magnon scattering process, and the nontrivial interplay between three-magnon scattering and the magnons' phases.

A quantum spin liquid is a novel ground state that can support long-range entanglement between magnetic moments, resulting in exotic spin excitations involving fractionalized $S=\frac{1}{2}$ spinons. Here, we measure the excitations in single crystals of the spin liquid candidate Zn-barlowite using resonant inelastic x-ray scattering. By analyzing the incident polarization and temperature dependences, we deduce a clear magnetic scattering contribution forming a broad continuum that surprisingly extends up to $\sim 200$ meV ($\sim 14J$, where $J$ is the magnetic exchange). The excitation spectrum indicates that significant contributions may arise from multiple pairs of spinons and/or antispinons at high energies. Furthermore, the observation of similar scattering in the hexagonal barlowite parent material above its ordering temperature suggests the presence of similar high-energy excitations, irrespective of the low-energy physics.

Establishing a comprehensive model of the rich spin dynamics in ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$ has been a challenge over the last decade. Inelastic neutron scattering experiments revealed that its elementary triplons are nondegenerate, showing the existence of significant anisotropic spin couplings. Evidence for triplon decay into two triplons has been found, but two prominent downturns in the dispersions eluded an explanation. Level repulsion due to hybridization of single triplons with the continuum of two-triplon scattering states has been proposed as an explanation. We show that this concept may explain the weak downturn at higher energies, but fails for the most pronounced downturn at lower energy. In turn, we provide evidence that this downturn is the signature of a two-triplon bound state of essentially singlet character pointing to a triplon-triplon interaction as the second crucial ingredient of the spin dynamics in this exemplary system.

In this paper, we study the structural, scattering, and wave localization properties of multifractal arrays of electric point dipoles generated from multiplicative random fields with different spectra of fractal dimensions. Specifically, using the rigorous Green's matrix method, we investigate the scattering resonances and wave localization behavior of systems with $N={10}^4$ dipoles and demonstrate an enhanced localization behavior in highly inhomogeneous multifractal structures compared to homogeneous fractals, or monofractals. We show distinctive spectral properties, such as the absence of level repulsion in the strong multiple scattering regime and power-law statistics of level spacings, which indicate a clear localization transition enhanced in nonhomogeneous multifractals. Our findings unveil the importance of multifractal structural correlations in the multiple scattering regime of electric dipole arrays and provide an efficient model for the design of multiscale nanophotonic systems with enhanced light-matter coupling and localization phenomena beyond what is possible with traditional fractal systems.

In bulk and low-dimensional extended systems, the screening of excitations by the electron cloud is a key feature governing spectroscopic properties. Widely used computational approaches, especially in the framework of many-body perturbation theory, such as the $GW$ approximation and the resulting approximate Bethe-Salpeter equation, are explicitly formulated in terms of the screened Coulomb interaction. In the present work, we explore the effect of screening in absorption and electron energy loss spectroscopy, concentrating on the effect of local distortions on the screening and elucidating the resulting changes in the various spectra. Using the layered bulk oxide ${\mathrm{V}}_2{\mathrm{O}}_5$ as prototype material, we show in which way local distortions affect the screening, and in which way changes in the screening impact electron energy loss and absorption spectra including excitons. We highlight cancellations that make many-body effects in the spectra very robust with respect to structural modifications, while the band structure undergoes significant changes and the nature of the excitations may also be affected. This yields insight concerning the structure-properties relations that are crucial for the use of ${\mathrm{V}}_2{\mathrm{O}}_5$ as energy storage material, and more generally, that may be used to optimize the analysis and the calculation of electronic spectra in complex materials.

The Kitaev quantum spin liquidand massively quantum entangled states, are so scarce in nature that searching for new candidate systems remains a great challenge. A honeycomb heterostructure could be a promising route to realize and utilize such an exotic quantum phase by providing additional controllability of Hamiltonian and device compatibility, respectively. Here, we provide epitaxial honeycomb oxide thin film ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$, a candidate of Kitaev quantum spin liquid proposed recently. We found a spin glass and antiferromagnetic ground states depending on Na stoichiometry, signifying not only the importance of Na vacancy control but also strong frustration in ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$. Despite its classical ground state, the field-dependent magnetic susceptibility shows remarkable scaling collapse with a single critical exponent, which can be interpreted as evidence of quantum criticality. Its electronic ground state and derived spin Hamiltonian from optical spectroscopy are consistent with the predicted Kitaev model. Our work provides a unique route to the realization and utilization of Kitaev quantum spin liquid.

Recent quasi-two-dimensional (quasi-2D) systems with judicious exploitation of the atomic monolayer or few-layer architecture exhibit unprecedented physical properties that challenge the conventional wisdom on condensed matter physics. Here we show that the infinite layer ${\mathrm{SrCuO}}_2$ (SCO), a topical cuprate Mott insulator in bulk form, can manifest an unexpected metallic state in the quasi-2D limit when SCO is grown on ${\mathrm{TiO}}_2$-terminated ${\mathrm{SrTiO}}_3$ (STO) substrates. The sheet resistance does not conform to Landau's Fermi liquid paradigm. Hard x-ray core-level photoemission spectra demonstrate a definitive Fermi level that resembles the hole doped metal. Soft x-ray absorption spectroscopy also reveals features analogous to those of a hole doped Mott insulator. Based on these results, we conclude that the hole doping does not occur at the interfaces between SCO and STO; instead, it comes from the transient layers between the chain-type and the planar-type structures within the SCO slab. The present work reveals a metallic state in the infinite layer SCO and invites further examination to elucidate the spatial extent of this state.

In this paper, first, we present a general formulation to investigate the ground-state and elementary excitations of an excitonic insulator (EI) in real materials. In addition, we discuss the out-of-equilibrium state induced (albeit transiently) by high-intensity light illumination of a conventional two-dimensional (2D) insulator. We then present various band structure models which allow us to study the transition from a conventional insulator to an EI in 2D materials as a function of the dielectric constant, the conventional-insulator gap (and chemical potential), the bandwidths of the conduction and valence bands, and the Bravais lattice unit-cell size. One of the goals of this investigation is to determine which range of these experimentally determined parameters to consider in order to find the best candidate materials to realize the excitonic insulator. The numerical solution to the EI gap equation for various band structures shows a significant and interesting momentum dependence of the EI gap function $\mathrm{\Delta }\left(\vec{k}\right)$ and of the zero-temperature electron and hole momentum distribution across the Brillouin zone. Last, we discuss the fact that these features can be detected by tunneling microscopy.

The Hubbard model on an anisotropic triangular lattice in two dimensions, a fundamental model for frustrated electron physics, displays a wide variety of phases and phase transitions. This work investigates the model using the ladder dual fermion approximation which captures local correlations nonperturbatively but approximates nonlocal correlations. We find metallic, one-dimensional antiferromagnetic, noncollinear antiferromagnetic, square-lattice antiferromagnetic, and spiral phases but no evidence of collinear antiferromagnetic order in different parts of the phase diagram. Analyzing the spin susceptibility in detail, we see both regions of agreement and of discrepancy with previous work. The case of ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ is discussed in detail.