Although quasiparticles in flat bands have zero group velocity, they can display an anomalous velocity due to the quantum geometry. We address the thermal and thermoelectric transport in flat bands in the clean limit with a small amount of broadening due to inelastic scattering. We derive general Kubo formulas for flat bands in the dc limit up to linear order in the broadening and extract expressions for the thermal conductivity, the Seebeck and Nernst coefficients. Under the condition of zero particle flow, the Seebeck coefficient for flat Chern bands is topological up to second-order corrections in the broadening. We identify thermal and thermoelectric transport signatures of two generic flat Chern bands and also of the generalized flattened Lieb model, which describes a family of three equally spaced flat Chern bands where the middle one is topologically trivial. Both families of Hamiltonians saturate the lower bound of the trace of the quantum metric, permitting the derivation of general transport results with minimal assumptions. Finally, we address the saturation of the quantum metric lower bound for a generic family of Hamiltonians with an arbitrary number of flat Chern bands corresponding to SU(2) coherent states. We find that only the extremal bands in this class of Hamiltonians saturate the bound, provided that the momentum dependence of their Hamiltonians is described by a meromorphic function.

We study the effects of imperfect nesting in the density wave (DW) state on various electronic properties within a simple two-dimensional tight-binding model. The discussed model reflects the main features of quasi-1D metals where the DW emerges. We show that a DW with imperfect nesting leads to unusual singularities in the quasiparticle density of states and to a power-law renormalization of the superconducting critical temperature. Our results are derived at arbitrary large antinesting and may help to understand the phase diagram of the wide class of density wave superconductors. We also compute the conductivity tensor in a wide temperature range, including the DW transition, and obtain a satisfactory agreement with the experimental data on rare-earth trichalcogenides and many other DW materials.

We present a numerical approach to calculate inelastic light scattering spectra from gold nanocrystals, based on the finite element method. This approach is validated by comparison with previous analytic calculations for spherically symmetric scatterers. Superellipsoid nanocrystals are considered in order to smoothly vary the shape from octahedra to cubes via spheres, while preserving cubic symmetry. Spectra are calculated and discussed taking into account the irreducible representation of the involved vibration modes. A strong increase in the inelastically scattered light intensity is observed for small variations of the shape around the sphere. This increase is related to variations of the electric field inside the nanocrystals, which are very small for small nanospheres but increase quickly for nonspherical nanocrystals. This strong dependence with shape must be taken into account when interpreting experimental spectra acquired from inhomogeneous ensembles of nanocrystals whose shape dispersion are usually neglected. The overall changes in the spectra when varying the shape of the nanocrystals provide additional insight into previously published results. Preliminary calculations for chiral shapes further show a significant difference between spectra obtained with right or left circularly polarized light.

Starting from a double quantum dot realization of a minimal Kitaev chain, we demonstrate that non-Hermiticity stabilizes the so-called poor man's Majorana zero modes in a region of parameter space that is much broader than in the Hermitian regime. In particular, we consider the simplest non-Hermitian mechanism which naturally appears due to coupling to normal reservoirs and is commonly present in all transport experiments. Specifically, such couplings induce exceptional points which connect stable and highly tunable zero energy real lines that are well separated from the quasicontinuum. Such zero-energy lines reflect spectral degeneracies protected by topology and represent the non-Hermitian generalization of the Hermitian poor man's Majorana modes occurring at single points in parameter space. Our findings pave the way for realizing robust non-Hermitian effects by combining unconventional superconductors and non-Hermitian topology.

Atomically thin two-dimensional (2D) semiconductors, particularly transition metal dichalcogenides, have emerged as exciting quantum materials with versatile applications in optoelectronics. The stacking configuration between van der Waals (vdW) layers significantly influences their optical, electrical, and thermal properties. This work demonstrates stacking-dependent Raman intensity and frequency in chemical vapor deposition-grown stacked bilayer $\mathrm{W}{\mathrm{S}}_2$. The variation in intensity is explained based on the difference in optical phonon lifetimes and full width at half maxima of Raman peaks for two different stackings (phases). Considering these differences as the fingerprints for stacking configurations, the mirror twin boundaries (MTBs) and different phases are addressed in a single bilayer $\mathrm{W}{\mathrm{S}}_2$. We propose a model based on spontaneous symmetry breaking and atomic reconstruction across the MTBs that explains the existence of two distinct phases (2H and 3R). Further, substrate-supported thermal conductivities of these phases are evaluated across the MTBs using optothermal Raman scattering. These findings suggest thermal conductivity has different values, even in a single bilayer $\mathrm{W}{\mathrm{S}}_2$ with different phases. This is attributed to the variation in phonon scattering rate in different phases. The first-principles calculations predict a smaller group velocity of ZA phonons and hence, a higher scattering rate leading to lower thermal conductivity in the 2H phase and vice versa for the 3R phase. Our findings could be helpful for thermoelectric, directional heat transport, and nanoheat spreaders in 2D vdW semiconductors by engineering bilayer 2D materials with different phases and manipulating their thermal conductivities.

Using phenomenological Landau free energy, we systematically analyze the influence of so-called umklapp contributions on the phase diagram of a solid hosting a commensurate charge density wave phase. The umklapp terms may change the transition into the ordered state from continuous (second order) to discontinuous (first order). Additional elements, such as critical and tricritical points, emerge on the phase diagram under suitable conditions. The proposed mechanism is generalized for the case of a nearly commensurate charge density wave, as well as spin-density wave orders. Several charge-density-wave-hosting alloys are used as experimentally available examples illustrating the formalism.

Skyrmions are stable topological textures that have garnered substantial attention within the ferroelectric community for their exotic functional properties. While previous studies have questioned the feasibility of ${\left[001\right]}_{\text{pc}}$ skyrmion tubes in rhombohedral ${\mathrm{BaTiO}}_3$ due to the high-energy cost of ${180}^{\circ }$domain walls, we demonstrate here their stabilization with topological charges of $\mathcal{Q}=\pm 1$ from density functional theory and second-principles calculations. By enabling extensive vortex and antivortex polarization configurations, the expected prohibitive energetic barriers are overcome while preserving the topological nature of the structures. Notably, we extend these findings to demonstrate the appearance of skyrmion and antiskyrmion tubes in other related materials, highlighting their broader relevance. Furthermore, our computational experiments indicate that these structures can be directly stabilized and reversibly switched by applied electric fields, establishing a straightforward route for their practical realization and functional control in nanoelectronic devices.

Emergent ${\mathrm{Ising}}_h^2$ integrability is anticipated in a quantum Ising ladder composed of two weakly coupled critical transverse field Ising chains. The system is remarkable for including eight types of massive relativistic particles, whose scattering matrix and mass spectrum are characterized by the ${\mathcal{D}}_8^{\left(1\right)}$ Lie algebra. In this article, by computing the spin dynamical structure factors using analytical form-factor approach, we clearly identify dispersive single-particle excitations of (anti) soliton and breathers as well as their multiparticle continua in the spectra, which is further confirmed by numerical simulations. We show that the selection rule inherent in the parity and topological charge of the theory causes a significant result: charge-parity-odd particles, termed dark particles, cannot be directly excited from the ground state through any local or quasilocal operations. This, in turn, suggests the long lifetime of the lightest dark particle.

Sliding ferroelectricity is highly attractive for its low-energy barriers and fatigue resistance. As the origin of these exotic properties, its unconventional switching dynamics remains poorly understood: How does an electric field drive a perpendicular sliding? Taking $h\text{−}\mathrm{BN}$ bilayer as a model system, its switching dynamics is studied using ab initio calculations. The off-diagonal Born effective charge leads to the perpendicular relationship between the electric field and ionic movements. Interestingly, the rules of intrinsic coercive field are distinct between $h\text{−}\mathrm{BN}$ bilayer and conventional ferroelectrics. For $h\text{−}\mathrm{BN}$ bilayer, any perturbation breaking the in-plane symmetry plays a key role to assist the avalanchelike switching dynamics. Moreover, the exotic large off-diagonal Born effective charge near the $P=0$ intermediate state results in a wriggling motion of domain walls in $h\text{−}\mathrm{BN}$ bilayer. Our results reveal the key factors in the ferroelectric switching of sliding ferroelectrics at room temperature.

Artificial magnetic domains are induced into a soft-magnetic in-plane ${\mathrm{Co}}_{60}{\mathrm{Fe}}_{20}{\mathrm{B}}_{19}{\mathrm{Si}}_1$ (CoFeBSi) alloy by interlayer coupling to a perpendicular-to-plane FePt underlayer. We present images documenting the thickness dependence of the size of magnetic domains inside the CoFeBSi ferromagnetic layer. The magnetic domain walls in FePt induce a continuous progression of domain walls into the CoFeBSi at the vicinity of the interface to FePt. With femtosecond laser pulses, the recurrent domain pattern in CoFeBSi has been investigated after reversing the magnetization and we observe a reorganization into the thickness-dependent multidomain state inside the CoFeBSi with strong domain-wall pinning at the interface.

Effective control of spin-wave (SW) dynamics is among the current topical goals of research in magnonics. Electric field control of SW dynamics in insulating magnetic materials by creating the Aharonov-Casher (AC) topological phase without energy dissipation due to joule heating is a highly preferred way. The AC phase is purely a quantum phenomenon and has no classical interpretation. It manifests as the electric-field-induced shift in the SW's phase, group velocity direction, and SW's attenuation. Within a linear approximation, the magnonic AC effect can be considered by adding a Dzyaloshinskii-Moriya-like interaction between neighboring spins, which is proportional to the magnitude and sign of the applied electric field. We study the topological AC effect on magnetization dynamics in two-sublattice easy-axis insulating antiferromagnets. The analytical calculation indicates that a static electric field is an effective tool for selectively and successfully manipulating right-handed and left-handed polarized SWs, their amplitude, and propagation length. Our theory also reveals the electric field effect on anomalous magnon dispersion characteristics—a superluminal-like propagation of magnons at nanoscale distances. We also make numerical evaluations for antiferromagnetic dielectric to illustrate the theoretical predictions. The AC topological effect gives an effective method for the chirality-selective manipulation of magnon dynamics and opens promising research directions and practical applications of antiferromagnets in magnonics.

Knight shifts along the $b$ and $c$ axes $(K_b \text{and} K_c)$ at two crystallographically distinct Te sites were measured down to 70 mK using ${}^{125}\mathrm{Te}$ nuclear magnetic resonance on an ultraclean ${\mathrm{UTe}}_2$ single crystal with a superconducting (SC) transition temperature $T_c=2.1$ K. This was carried out to determine the $\mathit{d}$-vector components, which are the order parameter in the spin-triplet pairing. Although the decrease in $K_b$ and $K_c$ is comparable to the theoretical estimation of the SC diamagnetic shielding effect, it is confirmed, by taking the difference between two Knight shifts at the distinct Te sites, that the spin susceptibility along the $b$ and $c$ axes decreases in the SC state. Taking into account the large decrease in $K_a$ in the SC state, we conclude that the $\mathit{d}$ vector has components along all three crystal axes.

Order-by-disorder, whereby fluctuations lift an accidental classical ground state degeneracy to stabilize a subset of ordered states, is a recurrent and prominent theme in the field of frustrated magnetism where magnetic moments, or spins, are subject to competing spin-spin interactions. Thus far, such a phenomenon has been discussed in systems where the quantum ground state is not a “classical” product state. In such a case, both thermal and quantum fluctuations act to lift the accidental classical degeneracy, begging the question of whether one mechanism of order-by-disorder is possible without the other. In this paper, we present results exposing an uncharted route to order-by-disorder, one without quantum zero-point fluctuations, in the ferromagnetic pyrochlore Heisenberg system with the Dzyaloshinskii-Moriya (DM) interaction as the leading perturbation. We prove that any collinear ferromagnetic state is an exact eigenstate even in the presence of the anisotropic DM interaction, while thermal fluctuations give rise to a preference in the magnetization direction. Using linear spin wave theory, we find that the anisotropy appears at lowest order as a subleading term in the low-temperature expansion of the free energy, proportional to $T^{7/2}$. Our results thus show that the phenomenon of thermal order-by-disorder can in principle occur even in the absence of quantum zero-point fluctuations driving quantum order-by-disorder—this being so in particular when the accidentally degenerate ground state of the classical model turns out to be an exact eigenstate of the quantum version of the model. However, and in addition, we find that when the DM interaction is large, the fully polarized ferromagnetic ground state becomes unstable for a spin-$\frac{1}{2}$ system within the framework of nonlinear spin wave theory, a result that is presumably closely related to the recent report of a quantum spin liquid in this spin-$\frac{1}{2}$ model at $D/J\approx 2$ in Ref. [Lozano-Gómez et al., Proc. Natl. Acad. Sci. USA 121, e2403487121 (2024)].

Structural and magnetic properties of a two-dimensional spin-$\frac{5}{2}$ triangular lattice antiferromagnet ${\mathrm{NH}}_4\mathrm{Fe}{\left({\mathrm{PO}}_3\mathrm{F}\right)}_2$ are explored via x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and ${}^{31}\mathrm{P}$ nuclear magnetic resonance experiments on a polycrystalline sample. The compound portrays distorted triangular units of the ${\mathrm{Fe}}^{3+}$ ions with anisotropic bond lengths. The magnetic susceptibility shows a broad maximum around $T_{\chi }^{\max }\simeq 12\mathrm{K}$, mimicking the short-range antiferromagnetic order of a low-dimensional spin system. It shows the onset of a magnetic ordering at $T_{\mathrm{N}}\simeq 5.7\mathrm{K}$, setting the frustration ratio of $f=\frac{|{\theta }_{\mathrm{CW}}|}{T_{\mathrm{N}}}\simeq 5.7$. Such a value of $f$ reflects moderate magnetic frustration in the compound. The $dM/dH$ versus $H$ plots of the low-temperature magnetic isotherms exhibit a sharp peak at $H_{\mathrm{SF}}\simeq 1.45\mathrm{T}$, indicating a field-induced spin-flop transition and a small magnetic anisotropy. Despite a tiny anisotropy, the magnetic susceptibility and NMR shift could be modeled assuming the spin-$5/2$ isotropic triangular lattice model and the average value of the exchange coupling is estimated to be $J/k_{\mathrm{B}}\simeq 1.7\mathrm{K}$. This value of the exchange coupling is reproduced well from the saturation field of the pulse field data. The rectangular shape of the ${}^{31}\mathrm{P}$ NMR spectra below $T_{\mathrm{N}}$ unfolds that the ordering is a commensurate antiferromagnetic type. Three distinct phase regimes are clearly discerned in the $H\text{−}T$ phase diagram, redolent of a frustrated magnet with in-plane (XY-type) anisotropy.

Nodal-line semimetals (NLSMs) harbor a variety of novel physical properties owing to the particularities of the band degeneracies that characterize the spectrum of these materials. In symmetry-enforced NLSMs, band degeneracies, being imposed by symmetries, are robust to arbitrarily strong perturbations that preserve the symmetries. We investigate the effects of electron-electron interactions on a recently proposed vacancy-engineered NLSM known as holey graphene. Using mean-field calculations and quantum Monte Carlo simulation, we show that the Hubbard model on the depleted holey-graphene lattice at half filling exhibits a transition from a NLSM to an insulating antiferromagnetic phase for an arbitrarily weak repulsive interaction $U$. In contrast to the semimetal-insulator transition in the pristine honeycomb lattice, which occurs at a finite critical value of $U$, in the depleted lattice, the transition at $U=0$ is associated with a van Hove singularity arising from the crossing of accidental nodal lines and those enforced by symmetry. We also employ linear spin wave theory (LSWT) to the effective Heisenberg model in the strong-coupling limit and obtain the global antiferromagnetic order parameter $m_{\mathrm{AFM}}\approx 0.146$. The order parameters from both QMC and LSWT agree quantitatively. Our findings indicate that vacancy engineering offers an effective way to tailor the magnetic properties of quantum materials.

The question of thermalization of a closed quantum system is of central interest in nonequilibrium quantum many-body physics. Here we present one such study analyzing the dynamics of a closed coupled Majorana SYK system. We have a large-$q$ SYK model prepared initially at equilibrium quenched by introducing a random hopping term, thus leading to nonequilibrium dynamics. We find that the final stationary state reaches thermal equilibrium with respect to the Green's functions and energy. Accordingly, the final state is characterized by calculating its final temperature and the thermalization rate. We provide a detailed review of analytical methods and derive the required Kadanoff-Baym equations, which are then solved using the algorithm developed in this work. Our results display rich thermalization dynamics in a closed quantum system in the thermodynamic limit.

The interplay between nontrivial topology and strong electron interaction can generate a variety of exotic quantum matter. Here we theoretically propose that monolayer transition metal trihalides—${\mathrm{WBr}}_3$, ${\mathrm{WCl}}_3$, ${\mathrm{WF}}_3$ (with 2% tensile strain) and ${\mathrm{MoF}}_3$ (with 2% tensile strain)—host isolated, nearly flat valence band near the Fermi level with a higher Chern number. The nontrivial topology of these flat Chern bands arises from an effective $s{d}^2$ hybridization of transition metal atoms, which transform the apparent atomic $d$ orbitals on a hexagonal lattice into $(\tilde{s},{\tilde{p}}_{+},{\tilde{p}}_{-})$ states on an emergent triangular lattice. Interestingly, the quantum geometry of flat Chern bands in these materials is comparable with that observed in Moiré systems exhibiting fractional Chern insulator states. Furthermore, we investigate the Hofstadter butterfly spectrum of these flat Chern bands, revealing additional topological features. These natural materials hosting flat Chern bands with higher Chern numbers, if realized experimentally, could offer new playgrounds to explore exotic correlated phenomena.

Estimating the local two-particle vertex functions, which are crucial for capturing the spatial fluctuation of the effective field beyond the single-site dynamical mean field theory (DMFT), is still challenging. In our previous work [R. Mizuno et al., J. Phys. Soc. Jpn. 91, 034002 (2022)], we developed a computationally efficient method for estimating the local full vertex in DMFT, where we can obtain the local two-particle full vertex from the one-particle self-energy. In this study, we further enhance our method by refining its formulation to be more faithful to the diagrammatic structure of the full vertex. With this improvement, we can qualitatively reproduce the characteristic frequency structures of the full vertex obtained by the numerically exact methods. In particular, the improved version of the simplified full vertex captures a sharp value change in the cross structure.

We present theoretical studies on models for experimental determination of spin correlations and detection of quantum entanglement of spatially separated electrons of Cooper pairs. These models are based on both electric current cross-correlations and simpler dc current measurements. By proper optimization of the entanglement witness method, we demonstrate that entanglement detection is possible with any magnetic detectors regardless of their efficiency. This allows us to break the relatively high previously established limit of detector efficiency (spin polarization), $p>1/\sqrt{3}\approx 58\%$.