The Grüneisen ratio $\mathrm{\Gamma }$, i.e., the singular part of the ratio of thermal expansion to the specific heat, has been broadly employed to explore both finite $T$ and quantum critical points (QCPs). For a genuine quantum phase transition (QPT), thermal fluctuations are absent and thus the thermodynamic $\mathrm{\Gamma }$ cannot be employed. We propose a quantum analog to $\mathrm{\Gamma }$ that computes entanglement as a function of a tuning parameter $\lambda$ and show that QPTs take place only for systems in which the ground-state energy depends on $\lambda$ nonlinearly. Furthermore, we demonstrate the breakdown of the Hellmann-Feynman theorem in the thermodynamic limit at any QCP. We showcase our approach using the quantum one-dimensional Ising model with a transverse field and Kane's quantum computer. The slowing down of the dynamics and thus the “creation of mass” close to any QCP/QPT is also discussed.

We present a theory to realize entangled quantum spin states with fractional magnetization. The origin of magnetization reduction is partly emergent antiferromagnetism, that is, the spin liquefaction of ferromagnetism. We study a ferromagnetic bilinear coupling region of the spin-$S$ ($\geqq 1$) bilinear-biquadratic spin chain based on (i) a rigorous eigenstate correspondence between the spin-$S$ model and spin-$\frac{1}{2}$ model and (ii) a numerical exact-diagonalization calculation up to $S=3$. As a result, we obtain a fractional magnetized $M=1-1/\left(2S\right)$ phase, where ground states have quantum entanglement-reflecting corresponding spin-$\frac{1}{2}$ antiferromagnetic ground states in a ferromagnetic background. This spin-liquefaction theory of ferromagnets can be generalized to any-dimensional lattices even under a magnetic field. This fractional ferromagnetism opens another research field of quantum ferromagnets.

We investigate the effects of introducing a boost (a Zeeman field parallel to the spin quantization axis) at the proximitized helical edge of a two-dimensional (2D) quantum spin Hall insulator. Our self-consistent analysis finds that a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting phase may emerge at the edge when the boost is larger than a critical value tied to the induced pairing gap. A nontrivial consequence of retaining the 2D bulk in the model is that this boundary FFLO state supports a finite magnetization as well as finite current (flowing along the edge). This has implications for a proper treatment of the ultraviolet cutoff in analyses employing the effective one-dimensional (1D) helical edge model. Our results may be contrasted with previous studies of such 1D models, which found that the FFLO phase either does not appear for any value of the boost (in non-self-consistent calculations), or that it self-consistently appears even for infinitesimal boost, but carries no current and magnetization.

The ${\mathrm{Et}}_2{\mathrm{Me}}_2\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ salt is a Mott insulator at room temperature with layers of equivalent dimers ${\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ forming a two-dimensional triangular lattice. Although spin liquid behavior has been reported in other systems with a quasitriangular lattice, ${\mathrm{Et}}_2{\mathrm{Me}}_2\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ shows no spin liquid behavior irrespective of the expected high spin frustration due to its almost perfect equilateral triangular structure. Here, we report a high resolution structural analysis that reveals the existence of an intermediate incommensurate phase between the highly frustrated room temperature phase and the charge separated phase characterized by differently charged dimers coupled to different intradimer Pd-Pd distances. The structure of the incommensurate phase exhibits no modulation of the Pd-Pd distance but a large steric effect between the cation and the terminal sulfur atom of the $\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2$ molecules. In addition to this steric frustration, the interdimer transfer integrals coupled to the arch distortion in the dimer are also modulated, suggesting that the spin state in the incommensurate phase is due to an incommensurate modulation of the transfer integral. We demonstrate that the presence of the incommensurate phase originates from different frustrated competitive forces and discuss its magnetic and optical properties.

Dzyaloshinskii-Moriya interaction (DMI) is of particular interest as it plays a primary role in stabilizing topological chiral magnetism such as skyrmions and has been intensively studied due to its potential applications for next-generation information storage technologies. For two-dimensional (2D) magnets, although Janus structure offers a paradigm to realize strong DMI, further studies are confined by the strict fabrication and the necessity of heavy atoms. Here, using first-principles calculations, we propose a strategy beyond this limit, where Li atoms are absorbed on the surface of a 2D magnet, resulting in a strong DMI through the Rashba effect by introducing an effective electric field generated by the perpendicular electronic polarization due to the charge transfer after the absorption. Furthermore, taking ${\mathrm{MnSe}}_2$ monolayer as an example, we predict the various phase transitions of skyrmions under strain manipulations and external magnetic fields. Our findings pave a road toward obtaining topological chiral magnetism.

Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet ${\mathrm{YTiO}}_3$, with spin-spin exchange disorder introduced via La substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functional theory calculations predict minimal changes in exchange constants as a result of average structural changes due to La substitution, in agreement with the data. The absence of significant changes in the spin-wave dispersion, the considerable lifetime effect, and the reduced ordered magnetic moment previously observed in the La-substituted system are qualitatively captured by an isotropic, nearest-neighbor, three-dimensional Heisenberg ferromagnet model with random antiferromagnetic exchange. We therefore establish ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ as a model system for the study of antiferromagnetic spin-exchange disorder in a three-dimensional Heisenberg ferromagnet.

We investigate theoretically the superfluid phase transition of helium-3 under nanoscale confinement of one spatial dimension realized in recent experiments. Instead of the $3\times 3$ complex matrix order parameter found in the three-dimensional system, the quasi-two-dimensional superfluid is described by a reduced $3\times 2$ complex matrix. It features a nodal quasiparticle spectrum, regardless of the value of the order parameter. The origin of the $3\times 2$ order parameter is first illustrated via the two-particle Cooper problem, where Cooper pairs in the $p_x$ and $p_y$ orbitals are shown to have a lower bound-state energy than those in $p_z$ orbitals, hinting at their energetically favorable role at the phase transition. We then compute the Landau free energy under confinement within the mean-field approximation and show that the critical temperature for condensation of the $3\times 2$ order parameter is larger than for other competing phases. Through exact minimization of the mean-field free energy, we show that mean-field theory predicts precisely two energetically degenerate superfluid orders to emerge at the transition that are not related by symmetry: The A-phase and the planar phase. Beyond the mean-field approximation, we show that strong-coupling corrections favor the A-phase observed in experiment, whereas weak-coupling perturbative renormalization group predicts the planar phase to be stable.

Defects in frustrated antiferromagnetic spin chains are universally present in geometrically frustrated systems. We consider the defects of the one-dimensional, spin-$s$ XXZ chain with single-ion anisotropy on a periodic chain with $N$ sites that was famously studied by Haldane. For $N$ odd, the antiferromagnetic model is frustrated, and the ground state must include a soliton defect. We consider the Heisenberg interaction perturbatively and determine the corresponding perturbative solitonic ground state. Then we compute the entanglement spectrum, entanglement entropy (EE), capacity of entanglement (CE), and spin correlations in the solitonic ground state. For weak frustration, we find an algebraic violation of the area law for the EE consistent with recent results on weakly frustrated chains. Our analysis then moves beyond the weak frustration regime, and we obtain a novel extensive scaling law for the EE when strong frustration prevails, signaling large entanglement, and failure of the quasiparticle interpretation in this regime. Enhanced frustration results in less total correlations but relatively more nonlocal correlations.

Conductivity in a multiband system can be divided into intra- and interband contributions, and the latter further into symmetric and antisymmetric parts. In a flat band, intraband conductivity vanishes and the antisymmetric interband contribution, proportional to the Berry curvature, corresponds to the anomalous Hall effect. We investigate whether the symmetric interband conductivity, related to the quantum metric, can be finite in the zero frequency and flat band limit. Starting from the Kubo-Greenwood formula with a finite scattering rate $\eta$, we show that the DC conductivity is zero in a flat band when taking the clean limit $\left(\eta \to 0\right)$. If commonly used approximations involving derivatives of the Fermi distribution are used, finite conductivity appears at zero temperature $T=0$; we show however that this is an artifact due to the lack of Fermi surfaces in a (partially) flat band. We then analyze the DC conductivity using the Kubo-Streda formula, and note similar problems at $T=0$. The predictions of the Kubo-Greenwood formula (without the approximation) and the Kubo-Streda formula differ significantly at low temperatures. We illustrate the results within the Su-Schrieffer-Heger model where one expects vanishing DC conductivity in the dimerized limit as the unit cells are disconnected. We discuss the implications of our results to previous work which has proposed the possibility of finite flat band DC conductivity proportional to the quantum metric. Our results also highlight that care should be taken when applying established transport and linear response approaches in the flat band context, since many of them utilize the existence of a Fermi surface and assume scattering to be weak compared to kinetic energy.

Callaway's simplified heat flow model is often used to confirm experimental realizations of unconventional, hydrodynamic, and Poiseuille phonon transport in ultrahigh thermal conductivity $\left(\kappa \right)$ materials, due to its simplicity and low computational cost. Here, we show that the Callaway model works exceptionally well for most ultrahigh-$\kappa$ materials like diamond and boron nitride, but fails dramatically for boron arsenide (BAs) and boron antimonide (BSb). This failure is driven by the inability of the Callaway model to effectively describe the severely restricted phonon scattering in BAs and BSb, where many scattering selection rules are activated simultaneously. Our work highlights the powerful predictive capability of the Callaway model, and gives insights into the nature of phonon scattering in ultrahigh-$\kappa$ materials and the suitability of Callaway's description of heat flow through them.

Topological excitations or defects such as solitons are ubiquitous throughout physics, supporting numerous interesting phenomena like zero-energy modes with exotic statistics and fractionalized charges. In this paper, we study such objects through the lens of symmetry-resolved entanglement entropy. Specifically, we compute the charge-resolved entanglement entropy for a single interval in the low-lying states of the Su-Schrieffer-Heeger model in the presence of topological defects. Using a combination of exact and asymptotic analytical techniques, backed up by numerical analysis, we find that, compared to the unresolved counterpart and to the pure system, a richer structure of entanglement emerges. This includes a redistribution between its configurational and fluctuational parts due to the presence of the defect and an interesting interplay with entanglement equipartition. In particular, in a subsystem that excludes the defect, equipartition is restricted to charge sectors of the same parity, while full equipartition is restored if the subsystem includes the defect, as long as the associated zero mode remains unoccupied. Additionally, by exciting zero modes in the presence of multiple defects, we observe a significant enhancement of entanglement in certain charge sectors, due to charge splitting on the defects. The two different scenarios featuring the breakdown of entanglement equipartition are underlied by a joint mechanism, which we unveil by relating them to degeneracies in the spectrum of the entanglement Hamiltonian. In addition, equipartition is shown to stem from an equidistant entanglement spectrum.

Deviation from perfect conical dispersion in Dirac materials, such as the presence of mass or tilting, enhances the control and directionality of electronic transport. To identify these signatures, we analyze the thermal derivative spectra of optical reflectivity in doped massive tilted Dirac systems. The density of states and chemical potential are determined as preliminary steps to calculate the optical conductivity tensor at finite temperature using thermal convolution. Changes in reflection caused by temperature variations enable the clear identification of critical frequencies in the optical response. By measuring these spectral features in the thermoderivative spectrum, energy gaps and band structure tilting can be determined. A comparison is presented between the spectra of various low-energy Dirac Hamiltonians. Our findings suggest that thermal difference spectroscopy holds promise as a valuable technique for probing interband transitions of two-dimensional Dirac fermions.

We show that the regime of strong light-matter coupling with remarkable magnetic properties can be implemented in systems based on monolayers of chromium triiodide (${\mathrm{CrI}}_3$). This two-dimensional material combines the presence of strongly bound excitonic complexes with ferromagnetic ordering below the Curie temperature. Using microscopic first-principles calculations, we reveal a rich spectrum of optical transitions corresponding to both Wannier- and Frenkel-type excitons, including those containing electrons with an effective negative mass. We show that excitons of different polarizations efficiently hybridize with a photonic mode of a planar microcavity. Due to the peculiar selection rules, polariton modes become well resolved in circular polarizations. The strong optical oscillator strength of excitons and cavity confinement leads to large values of the Rabi splitting, reaching 35 meV for a single monolayer and giant Zeeman splitting between polariton modes of up to 20 meV. This makes ${\mathrm{CrI}}_3$ an excellent platform for magnetopolaritonic applications.

A conceptual problem of the electric-field-mediated polarization correlations during a stochastic formation of polarization domain structure in ferroelectrics is addressed by using an exactly solvable stochastic model of polarization development in a uniaxial ferroelectric [Phys. Rev. B 107, 144109 (2023)]. A full set of time-dependent two-point correlation coefficients between all random variables is derived analytically, evaluated numerically, and presented graphically in three dimensions. They are particularly required for the analysis of nonlinear phenomena involving spatial dispersion like optical second harmonic generation and scattering.

Much of the impact of thermal activation on the low-temperature dynamics of the thermoelastic martensitic transformation (MT) remains unresolved. This study introduces the concept of viscosity of superelasticity, referred to herein as “viscosuperelasticity,” to comprehensively interpret the slow dynamics of stress-induced isothermal MT at low temperatures. The thermal activation dynamics underlying this concept allow for deriving the time-temperature-transformation (TTT) diagrams at any holding stress from the temperature- and strain-rate-dependent superelastic stress-strain curves. Experimental results for isothermal forward and reverse MTs comprehensively agree with the derived TTT diagrams, disclosing the nonreciprocity in the dynamics between the forward and reverse MT paths. One of the representative manifestations of viscosuperelasticity is the difference in the cooling/heating-rate dependence of the forward/reverse MT starting and finishing temperatures ($T_{\mathrm{Ms}}, T_{\mathrm{Mf}}, T_{\mathrm{As}}$, and $T_{\mathrm{Af}}$). A significant decrease in $T_{\mathrm{Ms}}$ and $T_{\mathrm{Mf}}$ with increasing cooling rate explains kinetic arrest, and a gradual increase in $T_{\mathrm{As}}$ and $T_{\mathrm{Af}}$ with increasing heating rate explains the difficulty in detecting the isothermal evolution of the reverse MT. The TTT diagrams also enable the impact of thermal activation upon dynamic cooling/heating to be evaluated quantitatively, explaining the nonergodic thermal-history dependence of anelastic transformation strain, which has been believed to be an essential signature of strain glass. The concept of viscosuperelasticity offers a comprehensive interpretation of nonreciprocal isothermal dynamics, kinetic arrest, and nonergodicity in the anelastic strain as manifestations of thermal activation of MTs.

We study by numerical simulation the magnetic order in ensembles of randomly packed magnetic spherical particles which, induced by their uniaxial anisotropy in the strong coupling limit, behave as Ising dipoles. We explore the role of the frozen disorder in the positions of the particles assuming a common fixed direction for the easy axes of all spheres. We look at two types of spatially disordered configurations. In the first one we consider isotropic positional distributions which can be obtained from the liquid state of the hard sphere fluid. We derive the phase diagram in the $T-\mathrm{\Phi }$ plane, where $T$ is the temperature and $\mathrm{\Phi }$ is the volume fraction. This diagram exhibits long-range ferromagnetic order at low $T$ for volume fractions above the threshold ${\mathrm{\Phi }}_c=0.157$ predicted by mean-field calculations. For $\mathrm{\Phi }\lesssim {\mathrm{\Phi }}_c$ a spin-glass phase forms with the same marginal behavior found for other strongly disordered dipolar systems. The second type of spatial configurations we study are anisotropic distributions that can be obtained by freezing a dipolar hard sphere liquid in its polarized state at low temperature. This structural anisotropy enhances the ferromagnetic order present in isotropic hard sphere configurations.

Convincing signatures of Majorana zero modes (MZMs) are one necessary requirement for achieving fault-tolerant quantum computations based on topological superconductivity (TSC). In addition to improving fabrication techniques, exploring the stoichiometric TSC platform is another route to suppressing the influences of trivial in-gap modes on the signatures of MZMs. Stoichiometric transition metal dichalcogenides (TMD) with topological surface states (TSSs) are promising but the field range of inducing magnetic vortices to harbor MZMs is limited by the small perpendicular upper critical field. Here, we propose that the line defects of chalcogen vacancies (CVs) embedded in TMDs are the stoichiometric TSC candidates for realizing stable MZMs without needing the TSSs under a wide range of in-plane magnetic fields. Detailed analysis and calculations on the 1H-$\mathrm{Mo}{X}_2$, 1H-$\mathrm{W}{X}_2$, and 1T-$\mathrm{Pt}{X}_2$ ($X=S$, Se, or Te) monolayers with CV line defects indicate the antisymmetric spin-orbit coupling effect, known as the origin of odd-parity pairing, is ensured by noncentrosymmetric point group symmetry. First-principles TSC phase diagrams are constructed to facilitate experimental detection of convincing signatures for the MZMs located at both ends of the line defect. Our findings enrich the stoichiometric TSC candidates and will promote device fabrication to manipulating and storing quantum information based on the device-friendly TMDs.

Thermal and electrical conductivity are the two most critical material properties in the design of miniaturized modern devices and the cooling of integrated circuits. In principle, thermal and electrical energy transport are two independent physical processes because they are associated with different energy carriers, i.e., phonons that carry heat and electric charges that carry current. However, it is still unknown how the two kinds of energy would spread in a material, such as graphene, with both high thermal conductivity and high electric conductivity at the same time in response to an external excitation. In this paper, we show that the thermal energy and electric potential energy of a graphene nanosheet exhibit quite different decoupled transport behaviors subject to an external flexural or planar excitation by using large-scale ab initio nonequilibrium molecular dynamics simulations up to thousands of atoms. It is found that the thermal energy has a higher transport velocity than the electric potential energy in response to a flexural excitation, while the electric potential energy has a higher transport velocity in response to a planar excitation. The dependence of transport behavior on the excitation strength is investigated. We find that, induced by the different responses to the excitation strength, the thermal energy and electric potential energy possess similar variations subject to the flexural excitations and opposite variations subject to the planar excitations. Anomalous diffusion of the thermal and electric potential energy in this nonequilibrium excitation process is also studied to understand the decoupled carrier mobilities. Furthermore, the cross-correlation function between the thermal energy and the electric potential energy is calculated to numerically demonstrate the decoupled variation. After an initial sharp drop, the cross-correlation function exhibits an exponential decay subject to both flexural and planar excitations. Our findings provide insight into the complex transport behavior of thermal and electric potential energy in crystalline solids and a promising method for designing adjustable thermal and thermoelectric devices.