The interaction between localized $f$ and itinerant conduction electrons is crucial in the electronic properties of heavy fermion and valence fluctuating compounds. Using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structure of the archetypical valence fluctuating compound ${\mathrm{EuNi}}_2{\mathrm{P}}_2$ that hosts multiple $f$ electrons. At low temperatures, we reveal the hybridization between Eu $4f$ and Ni $3d$ states, which contributes to the electron mass enhancement, consistent with the periodic Anderson model. With increasing temperature, interestingly, we observe opposite temperature evolution of electron spectral function above and below the Kondo coherence temperature near 110 K, which is in contrast to the monotonic valence change and beyond the expectation of the periodic Anderson model. We argue that both f-d hybridization and correlation are imperative in the electronic properties of ${\mathrm{EuNi}}_2{\mathrm{P}}_2$. Our results shed light on the understanding of novel properties, such as heavy fermion behaviors and valence fluctuation of rare-earth transition-metal intermetallic compounds with multiple $f$ electrons.

A canted ferromagnetic arrangement with a long-range-ordered antiferromagnetic component is predicted for the metallic ${\mathrm{SrRuO}}_3$ orthoperovskite based on local-density-approximation-type electronic-structure calculation including on-site Coulomb repulsion $U$ and spin-orbit coupling. The electronic state of ${\mathrm{Ru}}^{4+}$ ions and the nature of nonzero orbital momentum are analyzed in detail. With regard to the density of states at the Fermi level, the value of the magnetic moment, and the easy axis along the $a$-axis in a Pbnm setting, a good agreement with the experimental data is achieved for $U=1$ eV. The calculated parameters of magnetic exchange, including the anisotropic and antisymmetric terms, are used to determine the magnon spectra. It is shown that notable anisotropic exchange in ${\mathrm{SrRuO}}_3$ is responsible for a gap of 1.7 meV at the $\mathrm{\Gamma }$ point of the magnon spectra, found previously by inelastic neutron diffraction. The origin of the spin canting that is confined within the $ab$-plane follows from the presence of the Dzyaloshinskii-Moriya interaction associated with the antisymmetric exchange.

Kramers' theorem ensures double degeneracy in the energy spectrum of a time-reversal symmetric fermionic system with half-integer total spin. Here, we are trying to go beyond the closed system and discuss Kramers' degeneracy in open systems out of equilibrium. A natural way to extend the Kramers' degeneracy in open quantum systems is by the degeneracy of different spins' spectra together with the vanishing interspin spectrum. We find the violation of Kramers' degeneracy in time-reversal symmetric open quantum systems is locked with whether the system reaches thermal equilibrium. After a general coupling to an environment in a time-reversal symmetry-preserving way, the Kramers doublet experiences an energy splitting at a short time and then a recovery process. We verified the violation and revival of Kramers' degeneracy in a concrete model of interacting fermions and we find Kramers' degeneracy is restored after the local thermalization time. By contrast, for time-reversal symmetry $\tilde{\mathcal{T}}$ with ${\tilde{\mathcal{T}}}^2=1$, we find that although there is a violation and revival of spectral degeneracy for different spins, the inter-spin spectral function is always nonzero. We also prove that the degeneracy in spectral function protected by unitary symmetry can always be maintained.

As a prototypical light metal, lithium shows various and intriguing behaviors including electride character under high pressure. It has a pronounced melting line depression from 40 to 60 GPa, yet a precise and detailed understanding of this melting minimum remains elusive. Here, using computations that include metadynamics, crystal structure searching, and molecular dynamics with machine learned potentials, we demonstrate a temperature-induced electride (pocketlike to tubelike) transition accompanied by complex structural phase transitions in dense premelting lithium. The possible rearrangement of nearest neighbors from three-coordinated net to dimers and the premelting collective atomic motion affected by this electronic transition demonstrates the combined influence of temperature and electronic structure on the high-pressure behaviors of metals.

Starting from recent advances in the first-principles modeling of polarons, variational polaron equations in the strong-coupling adiabatic approximation are formulated in Bloch space. In this framework, polaron formation energy as well as individual electron, phonon, and electron-phonon contributions are obtained. We suggest an efficient gradient-based optimization algorithm and apply these equations to the generalized Fröhlich model with anisotropic nondegenerate electronic bands, both in two- and three-dimensional cases. The effect of the divergence of Fröhlich electron-phonon matrix elements at $\mathrm{\Gamma }$ point is treated analytically, improving the convergence with respect to the sampling in reciprocal space. The whole methodology is validated by obtaining the known asymptotic solution of the standard Fröhlich model in isotropic scenario and also by comparing our results with the Gaussian ansatz approach, showing the difference between the numerically exact and Gaussian trial wavefunctions. Additionally, decomposition of the energy into individual terms allows one to recover the Pekar's 1:2:3:4 theorem, which is shown to be valid even in the anisotropic case. We expect that the improvements in the formalism and numerical implementation will be applicable beyond the large polaron hypothesis inherent to Fröhlich model.

In this paper, a detailed comprehensive magnetic study on the origin of the Griffiths phase (GP) in the nanocrystalline ${\mathrm{La}}_{0.4}{\left({\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{0.5}\right)}_{0.6}{\mathrm{MnO}}_3$ compound has been presented. The system exhibits a ferromagnetic (FM)-like transition around $T_{\mathrm{C}}\sim$ 274 K. Interestingly, above $T_{\mathrm{C}}$, the state is not pure paramagnetic, but a short-range FM interaction or GP is observed and the interaction persists up to the Griffiths transition temperature, $T_G\sim$ 350 K. The detailed investigation of isothermal magnetization and the magnetocaloric effect along with its theoretical analysis have been employed besides the conventional magnetic measurements to probe the GP. Disorder in the system is introduced via increasing an A-site cationic size mismatch along with an additional defect originated owing to the particle size reduction. The disorder-induced magnetic phase transition has also been analyzed by critical analysis. A correlation between the unusual critical parameters with the GP has been furnished.

The van der Waals materials are a new platform to study two-dimensional systems, including magnetic order. Since the number of spins is relatively small, measuring the magnetization is challenging. Here, we report spin Hall magnetoresistance (SMR) up to room temperature caused by the magnetic surface texture of exfoliated flakes of magnetic van der Waals materials. For the antiferromagnet ${\mathrm{FePS}}_3$ the SMR amounts to $0.1\%$ for an applied magnetic field of 7 T at 5 K, which implies a substantial canting of the magnetic moments in a perpendicular antiferromagnetic order. The canting is substantial even for a magnetic field along the Néel vector, which illustrates the unique power of the SMR to detect magnetic surface textures in van der Waals magnets.

We performed high-pressure electrical transport and Raman measurements on a two-dimensional rhombohedral semiconductor γ-InSe. Our results confirm two structural phase transitions at high pressure. More interestingly, a domelike superconducting transition with maximum $T_c$ around 2.3 K is discovered when the compound transforms to the cubic CsCl phase above 40 GPa. Our first-principles calculations indicate that the high-pressure superconducting phase possesses nontrivial topological band structure in the vicinity of the Fermi level. These results show that the physical properties in this material strongly depend on its structure, which provides insights into the interplay between superconductivity and topological physics. Our work suggests promising emergent phenomena in this material and other related III-VI semiconductors under high-pressure conditions.

Recently, square-net materials have attracted lots of attention for the Dirac semimetal phase with negligible spin-orbit coupling (SOC) gap, e.g., ZrSiS/LaSbTe and ${\mathrm{CaMnSb}}_2$. In this paper, we demonstrate that the Jahn-Teller effect enlarges the nontrivial SOC gap in the distorted structure, e.g., LaAsS and ${\mathrm{SrZnSb}}_2$. Its distorted $X$ square-net layer ($X=$ P, As, Sb, Bi) resembles a quantum spin Hall (QSH) insulator. Since these QSH layers are simply stacked in the $\widehat{x}$ direction and weakly coupled, three-dimensional QSH effect can be expected in these distorted materials, such as insulating compounds ${\mathrm{CeAs}}_{1+x}{\mathrm{Se}}_{1-y}$ and ${\mathrm{EuCdSb}}_2$. Our detailed calculations show that it hosts two twisted nodal wires without SOC [each consists of two noncontractible time-reversal symmetry- and inversion symmetry-protected nodal lines touching at a fourfold degenerate point], while with SOC it becomes a topological crystalline insulator with symmetry indicators $(000;2)$ and mirror Chern numbers $(0,0)$. The nontrivial band topology is characterized by a generalized spin Chern number $C_{s+}=2$ when there is a gap between two sets of ${\widehat{s}}_x$ eigenvalues. The nontrivial topology of these materials can be well reproduced by our tight-binding model and the calculated spin Hall conductivity is quantized to ${\sigma }_{yz}^x=\left(\frac{\hslash }{e}\right)\frac{G_x{e}^2}{\pi h}$ with $G_x$ a reciprocal lattice vector.

In this paper, MgS is evaluated to be a suitable qubit host material due to its wide bandgap, being nuclear spin free, and its weak spin-orbit coupling. The point defects including vacancies, substitutions, and defect pairs were systematically investigated using first-principles calculations. Results show that the neutral ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ defect pair has $C_{3v}$ symmetry with the defect level sequence of $E_1>E_{exy}>E_2$. In addition, the net spin and defect levels of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ can be tuned by charge states. The ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2-}$ defects are relatively stable with the charge transition level of ∼2.53 eV. The four charged defect states of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{-}$, and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2-}$ may be plausible qubits with nonzero spin, and their zero-phonon lines indicate the corresponding fluorescence wavelengths fall within the infrared band. Furthermore, the zero field splitting of the ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{-}$ shows that the values of parameter $D$ are in the range of microwaves. The hyperfine constants are roughly proportional to the spin difference charge density and exponentially decay as a function of the distance from the P site.

Recent experimental and theoretical progress as well as the prospect of commercially viable quantum technologies have inspired great interest in the study of open quantum systems and their dynamics. Many open quantum systems are well described by an effective non-Hermitian Hamiltonian generating a time evolution that allows eigenstates to decay and dissipate to the environment. In this framework, quantum coherent scaling is traditionally tied to the appearance of dark states, where the effect of dissipation becomes negligible. Here, we discuss the universal dynamical scaling after a sudden quench of the non-Hermitian $O\left(N\right)$ model Hamiltonian. While universality is generally spoiled by non-Hermiticity, we find that for a given set of internal parameters short-time scaling behavior is restored with an initial slip exponent profoundly different from that of closed quantum systems. This result is tied to the compensation of dissipation by interaction effects at short times leading to a prethermal dark state, where coherent many-body dynamics can be still observed.

We investigated the effects of cubic and quartic anharmonicity on lattice dynamics and thermal transport in highly anharmonic ${\mathrm{BaZrO}}_3$ crystal over a wide temperature range (300–2000 K) by combining the first-principles-based self-consistent phonon theory and a unified theory of thermal transport including population and coherence contributions. By considering the effects from bubble and loop diagrams, the contributions of both three-phonon (3ph) and four-phonon (4ph) interaction processes to phonon scattering rates and energy shifts were clarified. Anharmonic phonon renormalization is found to play a crucial role in determining the finite-temperature phonon energies and lattice thermal conductivity ${\kappa }_{\mathrm{L}}$ in ${\mathrm{BaZrO}}_3$. Specifically, the lattice anharmonicity induces significant low-frequency optical phonon hardening at elevated temperatures, which is correlated with the U-shaped potential energy surfaces for these modes. The low-frequency optical phonon hardening significantly suppresses phonon scattering rates by altering the phonon weighted phase space of both 3ph and 4ph interaction processes, thereby leading to significant enhancement in the ${\kappa }_{\mathrm{L}}$ and weaker temperature dependence of ${\kappa }_L\sim T^{-0.75}$ than traditional harmonic treatments. Moreover, although the coherent thermal transport channel is suppressed by anharmonic phonon renormalization, it is enhanced by the 4ph scattering processes. The coherence contribution becomes nonnegligible at elevated temperatures and may contribute up to 17.38% of the total ${\kappa }_{\mathrm{L}}$ at 1500 K. In this paper, we highlight the strong influence of the lattice anharmonicity on thermal conductivity in severely anharmonic systems and the importance of coherent thermal transport channel at elevated temperatures.

In this paper we present a self-consistent cumulant expansion (SC-CE) and investigate its accuracy for the one-dimensional Holstein model with and without phonon dispersion. We show that for finite lattice sizes, the numerical integration of the SC-CE equations becomes unstable at long times. This defect is partially ameliorated when studying systems in the thermodynamic limit, enabling the demonstration that the SC-CE corrects many deficits of the standard perturbative CE in the (nondispersive) Holstein model. The natural phonon damping that arises in the more realistic dispersed Holstein model renders the SC-CE stable, allowing for a complete assessment of the method. Here, we find that self-consistency dramatically corrects many of the failures found in the perturbative CE, but also introduces some unphysical features. Finally, we comment on the potential use of SC-CE as a tool for calculating Green's functions in generic many-body problems.

We theoretically and experimentally study the stability of the unconventional fractional antiferromagnetic skyrmion lattice (AF-SkL) in ${\mathrm{MnSc}}_2{\mathrm{S}}_4$ spinel under magnetic fields applied along the [1-10] crystal direction. By performing numerical Monte Carlo simulations for the minimal effective spin model that we proposed in S. Gao et al., Nature 586, 37 (2020), we show that the lattice is aligned within the equivalent and symmetric [1-11] or [1-1-1] planes, which are equally inclined to the applied magnetic-field $H$. We attribute this behavior to the magnetic anisotropy of the host material. Neutron single-crystal diffraction presents a very good agreement with the predictions of the effective model. It reveals that the topological spin texture gets destabilized at low temperatures and moderate magnetic fields and is replaced by a conical phase for $H$// [1-10]. The present study elucidates the central role of the magnetic anisotropy in the stabilization of AF-Sk states.

Magnetic adatoms on a superconducting substrate undergo a quantum phase transition as their exchange coupling to the conduction electrons increases. For quantum spins, this transition is accompanied by screening of the adatom spin. Here, we explore the consequences of this screening for the phase diagrams and subgap excitation spectra of dimers of magnetic adatoms coupled by hybridization of their Yu-Shiba-Rusinov states and spin-spin interactions. We specifically account for higher spins, single-ion anisotropy, Ruderman-Kittel-Kasuya-Yosida coupling, and Dzyaloshinsky-Moriya interactions relevant in transition-metal and rare-earth systems. Our flexible approach based on a zero-bandwidth approximation provides detailed physical insight and is in excellent qualitative agreement with available numerical-renormalization group calculations on monomers and dimers. Remarkably, we find that even in the limit of large impurity spins or strong single-ion anisotropy, the phase diagrams for dimers of quantum spins remain qualitatively distinct from phase diagrams based on classical spins, highlighting the need for a theory of quantum Yu-Shiba-Rusinov dimers.

The Si(553) surface, covered with half a monolayer of gold, forms a double strand of well-ordered atomic Au chains in each miniterrace. It represents one of the smallest possible realizations of quasi-one-dimensional (1D) systems. In this prototype system, by combining DC conductance and low-energy electron diffraction measurements with density functional calculations and ab initio Monte Carlo simulations, we demonstrate that nonlocal electronic correlations, together with thermal excitations, lead to peculiar phase transitions without long-range order. They are characterized by marginal (average) geometric relaxations, but with huge variations of the electronic band structure and concomitant strong temperature-dependent modifications of the density of states close to the Fermi level. Similar phenomena are expected in the large class of quasi-1D conductors and open a wide range of possibilities for their controlled manipulation. It is the increasing hybridization between spin-polarized Au and Si edge states that makes the Si dangling bond states at the step edge conducting, first as a transient between two insulating phases and finally opening a permanent new 1D conduction channel at high temperatures.

A study of spinless matter fermions coupled to a constrained ${\mathbb{Z}}_2$ lattice gauge theory on a triangular ladder is presented. The triangular unit cell and the ladder geometry strongly modify the physics, as compared with a previous analysis on the square lattice. In the static case, the even and odd gauge theories for the empty and filled ladder are identical. The gauge field dynamics due to the electric coupling is drastically influenced by the absence of periodic boundary conditions, rendering the deconfinement-confinement process a crossover in general and a quantum phase transition (QPT) only for decorated couplings. At finite doping and in the static case, a staggered flux insulator at half-filling and vanishing magnetic energy competes with a uniform flux metal at elevated magnetic energy. As for the square lattice, a single QPT into a confined fermionic dimer gas is found vs electric coupling. Dimer resonances in the confined phase are, however, a second-order process only, likely reducing the tendency to phase separate for large magnetic energy. The results obtained employ a mapping to a pure spin model of ${\mathbb{Z}}_2$ gauge-invariant moments, adapted from the square lattice, and density matrix renormalization group calculations thereof for numerical purposes. Global scans of the quantum phases in the intermediate coupling regime are provided.

Ultrafast optical control of phase change materials is of great importance from fundamental and practical points of view. Transition-metal dichalcogenides are of significant interest in this research field because of the photoinduced phase transition originating from the nonthermal melting of their charge density wave (CDW) orders. In this work, we investigated the ultrafast optical response of ${\mathrm{VTe}}_2$ by using broadband coherent optical phonon spectroscopy with subpicosecond time resolution. With an increase of laser-excitation fluence, a characteristic oscillation around 4.4 THz appeared in the transient reflectivity change. This suggests photoinduction of the high-temperature phase, i.e., a CDW-melt state, of ${\mathrm{VTe}}_2$ within a picosecond. Such an ultrafast transition would be induced by a purely electronic effect, but as with thermodynamic melting, it is strongly suggested that the phase transition dynamics induced spatially by laser irradiation proceeds inhomogeneously via a nucleation mode.