We show that tilted Weyl semimetals with a spatially varying tilt of the Weyl cones provide a platform for studying analogs to problems in anisotropic optics as well as curved spacetime. Considering particular tilting profiles, we numerically evaluate the time evolution of electronic wave packets and their current densities. We demonstrate that electron trajectories in such systems can be obtained from Fermat's principle in the presence of an inhomogeneous, anisotropic effective refractive index. On the other hand, we show how the electron dynamics reveal gravitational features and use them to simulate gravitational lensing around a synthetic black hole. These results bridge optical and gravitational analogies in Weyl semimetals, suggesting different pathways for experimental solid-state electron optics.

We have calculated thermodynamic properties of four bcc refractory elements—V, Ta, Mo, and W—up to the melting point with full density-functional-theory accuracy, using the recently developed direct-upsampling method [J. H. Jung et al., npj Comput. Mater. 9, 3 (2023)]. The direct-upsampling methodology takes full account of explicit anharmonic vibrations and electron-vibration coupling very efficiently using machine-learning potentials. We have thus been able to compute highly converged free-energy surfaces for the PBE exchange-correlation functional, from which accurate temperature dependencies of various thermodynamic properties such as the heat capacity, thermal expansion coefficient, and bulk modulus are accessible. For all four elements, the electronic contribution is large, including a strong coupling with the thermal vibrations. The atomic forces in W are even affected by the temperature-consistent Fermi broadening, which alters the free energy by around 3 meV/atom at the melting point. Trends within group V and group VI refractory elements are observed and explained by qualitative differences in the electronic density of states. We also provide an estimate of the Gibbs energies of vacancy formation and the vacancy contribution to the thermodynamics. Lastly and most importantly, our results are analyzed in terms of the homologous temperature scale relative to theoretically predicted melting points (for the PBE functional). The homologous temperature dependencies show a remarkable agreement with experiments and reveal the predictive power of self-consistently determined ab initio thermodynamic properties.

Using a diffusion Monte Carlo technique, we calculated the phase diagrams of ${}^4\mathrm{He}$ and ${\mathrm{H}}_2$ adsorbed on a single (5,5) carbon nanotube, one of the narrowest that can be obtained experimentally. For a single monolayer, when the adsorbate density increases, both species undergo a series of first-order solid-solid phase transitions between incommensurate arrangements. Remarkably, the ${}^4\mathrm{He}$ lowest-density solid phase shows supersolid behavior, in contrast to the normal solid that we found for ${\mathrm{H}}_2$. The nature of the second layer is also different for both adsorbates. Contrary to what happens on graphite, the second layer of ${}^4\mathrm{He}$ on the tube is a liquid, at least up to the density corresponding to a third-layer promotion on a flat substrate. However, the second layer of ${\mathrm{H}}_2$ is a solid that, at its lowest stable density, has a small, but observable, superfluid fraction.

Employing an autocorrelation analysis of angle resolved photoemission spectroscopy data, we have investigated the evolution of the joint density of states (JDOS) in the pseudogap phase of slightly underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }\left(\mathrm{Bi}2212\right)$ cuprate high temperature superconductors for different temperatures $(T\text{'}\mathrm{s})$ and carrier concentrations $(\delta \text{'}\mathrm{s})$. Our JDOS analysis show that the Fermi arcs expand with increasing $T$ at a fixed $\delta$ and with increasing $\delta$ at a specific $T$. Qualitatively, these observations are consistent with the theoretical scenarios of the pseudogap phase being a phase incoherent $d$-wave superconductor, in which the Fermi arcs are described in terms of a subtle balance between temperature-dependent lifetime broadening of the single-particle excitations and the magnitude of the pairing gap.

The operating principle of traveling-wave parametric amplifiers is typically understood in terms of the standard coupled mode theory, which describes the evolution of forward propagating waves without any reflections, i.e., for perfect impedance matching. However, in practice, superconducting microwave amplifiers are unmatched nonlinear finite-length devices, where the reflecting waves undergo complex parametric processes, not described by the standard coupled mode theory. Here, we present an analytical solution for the TWPA gain, which includes the interaction of reflected waves. These reflections result in corrections to the well-known results of the standard coupled mode theory, which are obtained for both three-wave and four-wave mixing processes. Due to these reflections, the gain is enhanced and unwanted nonlinear phase modulations are suppressed. Predictions of the model are experimentally demonstrated on two types of unmatched TWPA, based on coplanar waveguides with a central wire consisting of (i) a high kinetic inductance superconductor, and (ii) an array of 2000 Josephson junctions.

$\left(\mathrm{Mg},\mathrm{Fe}\right)\mathrm{Si}{\mathrm{O}}_3$ post-perovskite is the highest-pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond post-perovskite. In this work, we have explored the high-pressure phase relations in ${\mathrm{Mg}}_2\mathrm{Si}{\mathrm{O}}_4$ using computations based on density functional theory. We find that a partially disordered $I\overline{4}2d$-type structure would be stable under the conditions expected for the interiors of super-Earth planets. We have explored the mechanism of the phase transition from the ordered ground state and the effect of disordering on the electronic properties of the silicate phase. The discovery of a structure where two very dissimilar cations, ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Si}}^{4+}$, occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy.

Epitaxial ${\mathrm{PbZrO}}_3/{\mathrm{SrRuO}}_3/{\mathrm{SrTiO}}_3$ heterostructures are among the most widely studied thin-film antiferroelectrics. This paper explores their temperature-induced phase transitions and the characteristics of the domain structure by means of high-resolution synchrotron x-ray diffraction. The antiferroelectric transition order appears to change from the first to the second; peculiar in-plane $M$-point superstructures develop at high temperatures, manifesting a new phase; the $R$- and $\mathrm{\Sigma }$-point superstructure reflections demonstrate much reduced splitting as compared to the demands of the mechanical compatibility. We discuss the energetics of the reduced around-the-normal antiferroelectric domain tilts, its relation to the observed changes in phase transitions, and point to the possible contribution to the observed effect from an unusual diffraction mechanism related to the partial coherence between domains in a random nanodomain structure.

Bound states in the continuum (BICs) are wave modes that remain in the continuous spectrum of radiating waves that carry energy; however, they remain perfectly localized and nonradiating. BICs, or embedded eigenmodes, exhibit high quality factors that have been observed in optical and acoustic waveguides, photonic structures, and other physical systems. However, there are limited means to manipulate these BICs in terms of the quality factor and their excitation. In this work, we show that quasi-BICs (QBICs) in open resonators can be tailored by introducing embedded scatters. Using microwave cavities and dielectric scatters as an example, QBICs are shown to be capable of being repeatedly manipulated by tuning the geometry of the structure and the specific locations of the dielectric scatters. Using coupled mode theory and numerical simulations, we demonstrate by altering dielectric and structural parameters that tuning the quality factor as well as selective excitation and suppressing of specific QBIC modes can be achieved. These results provide an alternative means to control BICs in open structures and may be beneficial to applications including sensors and high-Q resonators that need confined fields and selectivity in frequency.

The $\eta$-pairing is a type of Cooper pairing state in which the phase of the superconducting order parameter is aligned in a staggered manner, in contrast to the usual BCS superconductors with a spatially uniform phase. In this study, we search for a characteristic $\eta$-pairing state in a triangular lattice where a simple staggered alignment of the phase is not possible. As an example, we consider the attractive Hubbard model on both the square and triangular lattices under a strong external Zeeman field. Using the mean-field approximation, we have identified several $\eta$-pairing states. Additionally, we have examined the electromagnetic stability of the pairing state by calculating the Meissner kernel. Odd-frequency pairing plays a crucial role in achieving a diamagnetic response if the electrons experience a staggered superconducting phase during the propagation of current.

Violet phosphorus, as a kind of two-dimensional material, has become an excellent candidate for optoelectronic devices as it makes up for the drawbacks of black phosphorus applications. This study explores the structural and electrical properties of violet phosphorus under high-pressure conditions. Two structure phase transitions are revealed by the spectra and electromagnetic transport measurements. At $\sim 8$ GPa, a structural transition from monoclinic to A7 phase is observed with the appearance of superconductivity. The transition from the A7 phase to the simple cubic phase starts at $\sim 13$ GPa and completes after $\sim 30$ GPa at low temperatures. In addition, the band gap of violet phosphorus decreases linearly with the increase of pressure until it disappears after the transition to the A7 phase. The anisotropic upper magnetic field of superconductivities in A7 and simple cubic phases showed the relative crystal orientations of the high-pressure phases compared with initial violet phosphorus crystal phase.

We compute the diagrammatic expansion of the particle-particle susceptibility via algorithmic Matsubara integration and compute the correlated pairing susceptibility in the thermodynamic limit of the two-dimensional Hubbard model. We study the static susceptibility and its dependence on the pair momentum $\mathbf{q}$ for a range of temperature, interaction strength, and chemical potential. We show that $d_{x^2-y^2}$-wave pairing is expected in the model in the $U/t\to 0^{+}$ limit from direct perturbation theory. From this, we identify key second- and third-order diagrams that support pairing processes and note that the diagrams responsible are not a part of charge or spin susceptibility expansions. We find two key components for pairing at momenta (0,0) and $(\pi ,\pi )$ that can be well fit as separate bosonic modes. We extract amplitudes and correlation length scales where we find a predominantly local $(\pi ,\pi )$ pairing and nonlocal $\mathbf{q}=(0,0)$ pairs and present the relative weights of these modes for variation in temperature, doping, and interaction strength.

Spontaneous symmetry breaking is a ubiquitous phenomenon in nature. One of the defining features of symmetry-broken phases is that the large system size limit and the vanishing external field limit do not commute. In this work, we study a family of extensions of the $N$-state clock model. We find that the exact symmetry and the ground state degeneracy under the periodic boundary condition heavily depend on the system size, although the model has the manifest translation symmetry. In particular, the ground state can be unique and all excitations are gapped even when the phase exhibits noncommutativity of the two limits. Our model hence poses a question on the standard understanding of spontaneous symmetry breaking.

We propose a magnetic laser in a subwavelength system consisting of a high-refractive-index dielectric cavity and an active medium formed by magnetic quantum emitters. Stimulated emissions of magnetic quantum emitters induced by their coherent interactions with quantized magnetic fields of a cavity are theoretically considered. The condition to achieve such a magnetic laser is obtained. Numerical results show that magnetic lasers are feasible in some realistic systems, for example, a silicon disk of high-quality whispering gallery modes with embedded emitters. Furthermore, the competitions between the electric interaction and magnetic one in terms of their Purcell factors are also considered in some magnetic laser achievable systems. In a wavelength-scale silicon block of a high-order magnetic mode, the ratio of magnetic Purcell factor to the electric one can reach more than $\sim {10}^3$ large. Our results open up ways to enhanced magnetic light-matter interactions.

This study investigates the mechanisms driving optical activity and quantum transport in twisted bilayer graphene systems. We demonstrate that optical activity results from spatial dispersion, making it inadequate to consider the system purely two-dimensional. Therefore, we utilize the transfer matrix method to describe the propagation of electromagnetic waves through multilayer systems, which is compatible with our treatment of spatial dispersion. Using an effective continuum model, we analyze the formation of electron states in the bilayer systems and the correlation of transverse and longitudinal motions of electrons in the two graphene layers. The chiral structure of the atomic lattices results in a finite drag component of the optical conductivity tensor due to incomplete cancellation of these correlations. We show the decisive role of this drag component for optical activity by analyzing the circular dichroism spectrum of the system. Additionally, we calculate the dc conductivity, which shows that the twisted bilayer graphene supports a quantum conductivity value proportional to $e^2/h$ at the intrinsic Fermi energy.

A general strategy of alternated slide construction to craft topological metals is proposed, where there is a relative slide between the odd and even chains in the trivial spinless quantum wire array. Firstly, taking the three-leg ladder as an example, we find that alternated slide can induce a topological phase transition from the normal metal to topological metal phases, which are protected by inversion symmetry. Remarkably, topological metal without nontrivial edge states is found, and the bulk-boundary correspondence breaks down. Secondly, the two-dimensional quantum wire array with alternated slide manifests similar physical behaviors. Two types of topological metal phases emerge, where there are gapless bulk bands with and without nontrivial edge states. These results could be confirmed by current experimental techniques.

Dynamical properties of uniform electron fluids are studied within a nonperturbative approach consisting in the combination of the self-consistent version of the method of moments (SCMM) involving up to nine sum rules and other exact relations, the two-parameter Shannon information entropy maximization procedure, and the $\mathit{ab}\mathit{initio}$ path integral Monte Carlo (PIMC) simulations of the imaginary-time intermediate scattering function. The explicit dependence of the dynamic structure factor (DSF) on temperature and density is studied in a broad realm of variation of the dimensionless parameters ($2\le r_s\le 36$ and $1\le \theta \le 8$). When the coupling is strong ($r_s\ge 16$) we clearly observe a bimodal structure of the excitation spectrum with a lower-energy mode possessing a well pronounced rotonlike feature ($\theta \le 2$) and an additional high-energy branch within the roton region which evolves into the strongly overdamped high-frequency shoulder when the coupling decreases ($r_s\le 10$). We are not aware of any reconstruction of the DSF at these conditions with the effects of dynamical correlations included here via the intermediate scattering and the dynamical Nevanlinna parameter functions. The standard static-local-field approach fails to reproduce this effect. The reliability of our method is confirmed by a detailed comparison with the recent $\mathit{ab}\mathit{initio}$ dynamic local field approach by T. Dornheim et al. [Phys. Rev. Lett. 121, 255001 (2018)] available for high/moderate densities ($r_s\le 10$). Moreover, within the SCMM we are able to construct the modes' dispersion equation in a closed analytical form and find the decrements (lifetimes) of the quasiparticle excitations explicitly. The physical nature of the revealed modes is discussed. Mathematical details of the method are complemented in Appendix. The proposed approach, due to its rigorous mathematical foundation, can find numerous diverse applications in the physics of Fermi and Bose liquids.

A copper-based plasmonic system is presented to provide optical-to-microwave photon conversion. The process uses highly excited levels in ${\mathrm{Cu}}_2\mathrm{O}$ Rydberg excitons and takes advantage of spoof plasmons, which allow for significant enhancement of the transition probability between specific excitonic energy levels. The theoretical results are verified with numerical simulations. The proposed system is very flexible, allowing for emission of microwave wavelengths from 0.1 to 10 mm.

The hierarchical equations of motion (HEOM) approach is an accurate method to simulate open system quantum dynamics, which allows for systematic convergence to numerically exact results. To represent effects of the bath, the reservoir correlation functions are usually decomposed into summation of multiple exponential terms in the HEOM method. Since the reservoir correlation functions become highly non-Markovian at low temperatures or when the bath has complex band structures, a present challenge is to obtain accurate exponential decompositions that allow efficient simulation with the HEOM. In this work, we employ the barycentric representation to approximate the Fermi function and hybridization functions in the frequency domain. This method, by approximating these functions with optimized rational decomposition, greatly reduces the number of basis functions in decomposing the reservoir correlation functions, which further allows the HEOM method to be applied to ultralow temperature and general band structures. We demonstrate the efficiency, accuracy, and long-time stability of this decomposition scheme by applying it to the Anderson impurity model in the low-temperature regime with the Lorentzian and tight-binding hybridization functions.

We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_2$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_2$ show a systematic increase in frequencies under hydrostatic pressure up to $\sim 7.0\mathrm{GPa}$. Interestingly, the linewidth of the $A_{1g}, N$, and $E_g$ phonon modes show clear anomalies at $\sim 2.5\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\lambda$, anomalies in the Raman linewidth of $E_g$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number $n_M$ and ${\mathbb{Z}}_2$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number $n_M$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_2$ at $\sim 2.5\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\sim 7.0\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_2$ ternary chalcogenide $\mathrm{TlBi}{X}_2$ ($X=\mathrm{S}$, Se, Te) family.

Isolated quantum many-body systems are often well described by the eigenstate thermalization hypothesis. There are, however, mechanisms that cause different behavior: many-body localization and quantum many-body scars. Here, we show how one can find disordered Hamiltonians hosting a tower of scars by adapting a known method for finding parent Hamiltonians. Using this method, we construct a spin-$\frac{1}{2}$ model which is both partially localized and contains scars. We demonstrate that the model is partially localized by studying numerically the level spacing statistics and bipartite entanglement entropy. As disorder is introduced, the adjacent gap ratio transitions from the Gaussian orthogonal ensemble to the Poisson distribution and the entropy shifts from volume-law to area-law scaling. We investigate the properties of scars in a partially localized background and compare with a thermal background. At strong disorder, states initialized inside or outside the scar subspace display different dynamical behavior but have similar entanglement entropy and Schmidt gap. We demonstrate that localization stabilizes scar revivals of initial states with support both inside and outside the scar subspace. Finally, we show how strong disorder introduces additional approximate towers of eigenstates.