The parallels between the models of the weakly interacting Bose gas and elastic pseudogauge fields coupled to the massive Dirac fermions in two spatial dimensions reveal an intricate property of the Berry term in the former as compared to the Chern-Simons term of the latter. Guided by the Bogoliubov theory of superfluidity, we discuss the measurable consequences for the Berry term, which manifest themselves in a quantized response in any spatial dimension and distinguishes between both time directions. We argue that the same basic principles lay behind the topological $d$-wave superconductivity in two spatial dimensions. The same ideas applied to the model with elastic pseudogauge fields reveal a plethora of possible condensed phases, both with preserved and violated spatial isotropy and, consequently, different topological responses. The ground state of these phases is degenerate, hence all of them are equally probable. The spectral anisotropy raises parallels to the Lifshitz transitions, which occur due to static strain.

${\mathrm{KTaO}}_3$ presents a rich hyper-Raman spectrum originating from two-phonon processes at the Brillouin zone boundary, indicating the possibility of driving these phonon modes using intense midinfrared laser sources. We obtained the coupling of light to the highest-frequency longitudinal optic phonon mode $Q_{\text{HY}}$ at the $X(0,0,\frac{1}{2})$ point by first principles calculations of the total energy as a function of the phonon coordinate $Q_{\text{HY}}$ and electric field $E$. We find that the energy curve as a function of $Q_{\text{HY}}$ softens for finite values of electric field, indicating the presence of $Q_{\text{HY}}^2{E}^2$ nonlinearity with negative coupling coefficient. We studied the feasibility of utilizing this nonlinearity to transiently break the translation symmetry of the material by making the $Q_{\text{HY}}$ mode unstable with an intense midinfrared pump pulse. We also considered the possibility that nonlinear phonon-phonon couplings can excite the lowest-frequency phonon coordinates $Q_{\text{LZ}}$ and $Q_{\text{LX}}$ at $X$ when the $Q_{\text{HY}}$ mode is externally driven. The nonlinear phonon-phonon couplings were also obtained from first principles via total-energy calculations as a function of the phonon coordinates, and these were used to construct the coupled classical equations of motion for the phonon coordinates in the presence of an external pump term on $Q_{\text{HY}}$. We numerically solved them for a range of pump frequencies and amplitudes and found three regimes where the translation symmetry is broken: (i) rectification of the lowest-frequency coordinates due to large amplitude oscillation of the $Q_{\text{HY}}$ coordinate about its equilibrium position, (ii) rectification of only the $Q_{\text{HY}}$ coordinate without displaced oscillations of the lowest-frequency coordinates, and (iii) rectification of all three coordinates. Due to the small magnitude of the coupling constant between the $Q_{\text{HY}}$ mode and the electric field, the smallest value of the pump amplitude that manages to transiently break the the translation symmetry of ${\mathrm{KTaO}}_3$ is 270 MV/cm. Such a large value of electric field will likely cause a dielectric breakdown of the material. However, our paper shows that light-phonon coupling with negative sign can exist in real materials and motivates the search of other materials with a larger magnitude of the coupling.

Single-layer $\alpha$-ruthenium trichloride $\left(\alpha \text{−}{\mathrm{RuCl}}_3\right)$ has been proposed as a potential quantum spin liquid. Graphene/${\mathrm{RuCl}}_3$ heterobilayers have been extensively studied with a focus on the large interlayer electron transfer that dopes both materials. Here we examine the interplay between the competing magnetic state of ${\mathrm{RuCl}}_3$ layer and graphene electronic properties. We perform self-consistent Hartree-Fock calculations on a Hubbard-Kanamori model of the $4d^5{t}_{2g}$ electrons of $\alpha \text{−}{\mathrm{RuCl}}_3$ and confirm that out-of-plane ferromagnetic and zigzag antiferromagnetic states are energetically competitive. We show that the influence of hybridization between graphene and ${\mathrm{RuCl}}_3$ bands is strongly sensitive to the magnetic configuration of ${\mathrm{RuCl}}_3$ and the relative orientations of the two layers. We argue that strong hybridization leads to graphene magnetoresistance and that it may tilt the balance between closely competing magnetic states. Our analysis can be applied to any van der Waals heterobilayer system with weak interlayer hybridization and allows for arbitrary lattice constant mismatch and relative orientation.

We obtain the Seebeck coefficient or thermopower $S$, which determines the conversion efficiency from thermal to electrical energy, for the two-dimensional Hubbard model on different geometries (square, triangular, and honeycomb lattices) for different electronic densities and interaction strengths. Using determinantal Quantum Monte Carlo we find the following key results: (a) the bi-partiteness of the lattice affects the doping dependence of $S$; (b) strong electronic correlations can greatly enhance $S$ and produce nontrivial sign changes as a function of doping, especially in the vicinity of the Mott insulating phase; and (c) $S\left(T\right)$ near half-filling can show nonmonotonic behavior as a function of temperature. That is, our results show that electronic interactions lead to unexpected behavior for the thermopower for given fillings, even at high temperatures, which may vary depending on the geometry. We emphasize the role of strong interaction effects in engineering better devices for energy storage and applications, as captured by our calculations of the power factor $\text{PF}=S^2\sigma$, where $\sigma$ is the dc conductivity.

We calculate the electrical conductivity and the equation of state of dense fluid nitrogen for high pressures up to several megabars by using ab initio molecular dynamics simulations. We determine the instability region of a first-order liquid-liquid phase transition which results from an abrupt dissociation of nitrogen molecules. This transition is accompanied by a nonmetal-to-metal transition (metallization) of the fluid and corresponding structural changes from a molecular to a polymeric phase. We compare our new data with earlier theoretical results and available experiments.

Computing finite temperature properties of a quantum many-body system is key to describing a broad range of correlated quantum many-body physics from quantum chemistry and condensed matter to thermal quantum field theories. Quantum computing, which has seen rapid developments in recent years, has a huge potential to impact the computation of quantum thermodynamics. To fulfill these potential impacts, it is crucial to design quantum algorithms that utilize the computation power of quantum computing devices. Here, we present a quantum kernel function expansion (QKFE) algorithm for solving thermodynamic properties of quantum many-body systems. In this quantum algorithm, the many-body density of states is approximated by a kernel Fourier expansion, whose expansion moments are obtained by random state sampling and quantum interferometric measurements. Compared to its classical counterpart, namely, the kernel polynomial method (KPM), QKFE has an exponential advantage in the cost of both time and memory. In computing low temperature properties, QKFE becomes inefficient, similar to classical KPM. To resolve this difficulty, we further construct a thermal ensemble iteration (THEI) protocol which starts from the trivial limit of an infinite temperature ensemble and approaches the low temperature regime step by step. For quantum Hamiltonians, whose ground states are preparable with polynomial quantum circuits, THEI has an overall polynomial complexity. We demonstrate its efficiency with applications to one- and two-dimensional quantum spin models and a fermionic lattice. With our analysis of the realization with digital and analog quantum devices, we believe the quantum algorithm is accessible to current quantum technology.

Charge transfer in type-II heterostructures plays important roles in determining device performance for photovoltaic and photocatalytic applications. However, current theoretical studies of charge transfer process do not consider the effects of operating conditions such as illuminations and yield systemically larger interlayer transfer time of hot electrons in ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ compared to experimental results. Here in this work, we propose a general picture that illumination can induce interfacial dipoles in type-II heterostructures, which can accelerate hot carrier transfer by reducing the energy difference between the electronic states in separate materials and enhancing the nonadiabatic couplings. Using the first-principles calculations and the ab initio nonadiabatic molecular dynamics, we demonstrate this picture using ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ as a prototype. The calculated characteristic time for the interlayer transfer (60 fs) and the overall relaxation (700 fs) processes of hot electrons is in good agreement with the experiments. We further find that illumination mainly affects the ultrafast interlayer transfer process but has little effect on the relatively slow intralayer relaxation process. Therefore, the overall relaxation process of hot electrons has a saturated time with increased illumination strengths. The illumination-accelerated charge transfer is expected to universally exist in type-II heterostructures.

We report the emergence of time-crystalline behavior in the $\pi$-Berry phase protected edge states of a Heisenberg ferromagnet in the presence of an external driving field. The magnon amplification due to the external field spontaneously breaks the discrete time-translational symmetry, resulting in a discrete time crystal with a period that is twice that of the applied electromagnetic field. We discuss the nature and symmetry protection of the time crystalline edge states and their stability against various perturbations that are expected in real quantum magnets. We propose an experimental signature to unambiguously detect the time crystalline behavior and identify two recently discovered quasi-two-dimensional magnets as potential hosts. We present a realization of time crystals at topological edge states, which can be generalized and extrapolated to other bosonic quasiparticle systems that exhibit parametric pumping and topological edge states.

We report the magnetic and magnetoelectric properties of two isostructural polar compounds $\mathrm{Lu}M\mathrm{W}{\mathrm{O}}_6(M=\mathrm{Fe}$ and Cr) that were synthesized at high pressure and high temperatures. Both compounds have a polar orthorhombic aeschynite-type structure (space group $Pna{2}_1)$ with the ordered $M^{3+}$ and ${\mathrm{W}}^{6+}$ ions. Magnetization measurements show that the ${\mathrm{Fe}}^{3+}$ spins in $\mathrm{LuFeW}{\mathrm{O}}_6$ and ${\mathrm{Cr}}^{3+}$ spins in $\mathrm{LuCrW}{\mathrm{O}}_6$ exhibit antiferromagnetic (AFM) ordering at $T_{\mathrm{N}}=11.8$ and 19.2 K, respectively. Powder neutron diffraction analysis at 1.5 K reveals that $\mathrm{LuFeW}{\mathrm{O}}_6$ has a noncollinear AFM structure with k $=$ (0, $\frac{1}{2},\frac{1}{2})$, whereas $\mathrm{LuCrW}{\mathrm{O}}_6$ has a collinear AFM arrangement with k $=$ (0, 0, 0). The noncollinear AFM structure in $\mathrm{LuFeW}{\mathrm{O}}_6$ causes an additional electric polarization, whereas the collinear magnetic order in $\mathrm{LuCrW}{\mathrm{O}}_6$ does not. Our density functional theory calculations indicate that $\mathrm{LuFeW}{\mathrm{O}}_6$ exhibits spin frustration, which is absent in $\mathrm{LuCrW}{\mathrm{O}}_6$. This finding suggests that the induced electric polarization at $T_{\mathrm{N}}$ in $\mathrm{LuFeW}{\mathrm{O}}_6$ occurs due to a noncollinear spin structure resulting from magnetic frustration.

We investigate the thermal phase transitions of the $J_1\text{−}{J}_2\text{−}{J}_3$ Ising model on the honeycomb lattice. This model exhibits the zigzag (ZZ) antiferromagnetic phase, which is observed in the Ising honeycomb compound ${\mathrm{FePS}}_3$. By employing a cluster mean-field approach, we describe the role of exchange couplings on phase transitions and thermodynamics of the model. We found first- and second-order transitions between ZZ and paramagnetic phases. Moreover, the model hosts thermal order-by-disorder state selection and, for weak third-neighbor couplings, signatures of strong frustration, such as a round maximum in the specific heat. Our results provide a picture for the nature of thermally driven phase transitions in a wide range of exchange interactions. We show that the first-order phase transition observed in ${\mathrm{FePS}}_3$ is hosted by the present microscopic model by adopting exchange couplings from literature.

Motivated by the possible triple-$\mathbf{Q}$ classical order in the Kitaev candidate material ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, we investigate microscopic models that may stabilize the triple-$\mathbf{Q}$ order by studying an extended Kitaev honeycomb model with ring-exchange interactions (namely, the $K\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }\text{−}{J}_{\mathrm{R}}$ model) using the variational Monte Carlo method. It turns out that with a positive ring-exchange interaction ($J_{\mathrm{R}}>0$) there indeed appears an exotic noncoplanar triple-$\mathbf{Q}$ ordered state featured by three Bragg peaks at symmetry-related M points in the crystallographic Brillouin zone. A magnetic field in the honeycomb plane can suppress the triple-$\mathbf{Q}$ order and induce a gapless quantum spin liquid (QSL) with eight cones. Furthermore, with the increase of $J_{\mathrm{R}}$ a proximate Kitaev spin liquid with eight Majorana cones labeled “PKSL8” is found, which is very stable over a large range of $\mathrm{\Gamma }$ interactions. The PKSL8 state shares the same projective symmetry group with the Kitaev spin liquid (KSL) which is located at small $\mathrm{\Gamma }$ and $J_{\mathrm{R}}$. In a weak magnetic field applied normal to the honeycomb plane, the PKSL8 turns into an Abelian chiral spin liquid with Chern number $\nu =-4$, unlike the KSL, which yields a chiral spin liquid with $\nu =1$. Since the triple-$\mathbf{Q}$ phase is adjacent to two QSLs in the phase diagram, our work suggests that it is more hopeful to experimentally realize the exotic QSL phases starting from the triple-$\mathbf{Q}$ order.

Recent reports in the literature suggest that emerging magnetic phenomena, like magnetic compensation, magnetic reorientation, etc., in correlated complex oxide systems arise due to the coupling of electronic spin with phonons in the lattice. Here, we investigate the interlinkage of electronic spin with phonon dynamics across tetrahedral and octahedral magnetic sublattices in ${\mathrm{Li}}_{0.5}{\mathrm{Fe}}_{2.5-x}{\mathrm{Cr}}_x{\mathrm{O}}_4$ ($x=1.1$, 1.3, 1.5, and 1.6) that shows magnetic compensation. Temperature-dependent micro-Raman spectroscopic measurements exhibit the anomalies in phonon wave number vs temperature plots below the magnetic-ordering temperature. The estimated spin-phonon coupling strengths in all compounds over the entire temperature range of interest reveal a dual nature of magnetoelastic coupling at both sublattice sites in this system. The coupling strength for the tetrahedral sublattice site is found to be an order of magnitude higher in the regime between near-compensation and phase-transition temperatures than over the low-temperature regime, while the same nearly vanishes above the compensation temperature for the octahedral site. The study suggests the possible role of phonon in establishing the magnetic compensation phenomenon involving two magnetic sublattices. Additionally, the Fano line shape of the $A_{1\mathrm{g}}$ mode in the tetrahedral site, especially the variation of the Fano asymmetry parameter with temperature, carries the signature of a cross talk between spin and lattice degrees of freedom in this system. The anomaly in the variation of Raman spectral width of the same mode with temperature suggests a relatively strong electron-phonon coupling in these compounds. The study demonstrates a complex interplay between electronic spin dynamics, electrical and lattice degrees of freedom in two magnetic sublattices of ${\mathrm{Li}}_{0.5}{\mathrm{Fe}}_{2.5-x}{\mathrm{Cr}}_x{\mathrm{O}}_4$.

The iron-based pnictide ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ is not superconducting as synthesized, but it becomes such below $T_{\mathrm{c}}\sim 12\mathrm{K}$ upon annealing, as a consequence of a structural transition from a phase with collapsed tetragonal crystal structure to an uncollapsed phase. In this work, we carry out specific heat, Raman spectroscopy, and normal state electric and thermoelectric transport measurements in the collapsed and uncollapsed ${\mathrm{LaFe}}_2{\mathrm{As}}_2$ phases to gain insight into the electron interactions and their possible role in the superconducting pairing mechanism. Despite clear features of strong electron-phonon coupling observed in both phases, neither the low energy phonon spectra nor the electron-phonon coupling show significant differences between the two phases. Conversely, the Sommerfield constants are significantly different in the two phases, pointing to much higher electron correlation in the superconducting uncollapsed phase and confirming theoretical studies.

Polar magnet ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ has been recently attracting considerable interest due to its nonhysteretic magnetoelectricity associated with the collinear antiferromagnetic (AFM) order. Nevertheless, the magnetoelectric (ME) response in ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ remains trivial, and especially lacks sufficient tunability. Herein, we report large tunability of the ME effect in Co-substituted ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$, and special attention is paid to ${\mathrm{Ni}}_2{\mathrm{CoTeO}}_6$ and ${\mathrm{NiCo}}_2{\mathrm{TeO}}_6$ single crystals where incommensurate AFM helical structure with spins lying in the $ab$ plane is established, distinctly different from the collinear AFM structure in ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ along the $c$ axis. Our results demonstrate that the ${\mathrm{Co}}^{2+}$ substitution results in emergence of a giant electric polarization $P_c$ up to $\sim 0.52\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ induced by a magnetic field normal to the $c$ axis, i.e., the polar axis. The generated $P_c$ in ${\mathrm{NiCo}}_2{\mathrm{TeO}}_6$ is approximately 8.45 and 1.8 times larger than that of ${\mathrm{Ni}}_2{\mathrm{CoTeO}}_6$ and ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$, respectively. More importantly, the linear ME coefficient α can be remarkably modulated by the ${\mathrm{Co}}^{2+}$ substitution, evidenced with a large $\alpha \sim 323\mathrm{ps}/\mathrm{m}$ for ${\mathrm{NiCo}}_2{\mathrm{TeO}}_6$, a small $\alpha \sim 3.4\mathrm{ps}/\mathrm{m}$ for ${\mathrm{Ni}}_2{\mathrm{CoTeO}}_6$, and an intermediate $\alpha \sim 36\mathrm{ps}/\mathrm{m}$ for ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ at temperature $T\sim 45\mathrm{K}$. Such remarkable Co-substitution control of the ME effect is believed to be associated with the different occupations of the Ni/Co ions at the specific lattice sites. This work thus provides insights into the high-tunability ME effect in ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ polar magnets, a largely unexplored topic.

Pair-functional potentials are generally used for metallic solids, whereas cluster potentials are more appropriate for covalent solids; however, both face critical difficulties that cannot be solved based purely on the optimization of potential functions, e.g., the lattice stability for hcp metals with high c/a ratios and the conflict between stacking-fault energy and cleavage energy for covalent solids, which can be attributed to their respective physical foundations and approximations according to their bonding characteristics. By incorporating the long-range many-body effect in pair-functional potentials and the short-range angular-dependent terms in cluster potentials, a unified hybrid potential model is physically justified and proposed in the present study for both metallic and covalent bonding solids to resolve the aforementioned critical issues and other specific cases. The proposed model was not only successfully demonstrated for a series of elemental solids, including 20 fcc, bcc, and hcp metals and three covalent elements, but also was extended to construct cross potentials for three representative compound systems, i.e., CuNi, TiC, and BN, which suggests that the present hybrid potential model possess higher compatibility and feasibility for various metallic and covalent systems than the respective pair-functional potentials and cluster ones. Overall, the hybrid potential model not only complements the current potential library but also builds a foundation for further potential development with high flexibility.