Coexistence of topological bands and a charge density wave (CDW) in topological materials has attracted immense attention because of their fantastic properties, such as an axionic CDW, the three-dimensional quantum Hall effect, etc. In this work, the nodal-line semimetal ${\mathrm{In}}_x{\mathrm{TaS}}_2$ characterized by a CDW and superconductivity is successfully synthesized, whose structure and topological bands (two separated Weyl rings) are similar to ${\mathrm{In}}_{0.58}{\mathrm{TaSe}}_2$. A $2\times 2$ commensurate CDW is observed at low temperature in ${\mathrm{In}}_x{\mathrm{TaS}}_2$, identified by transport properties and scanning tunneling microscopy measurements. Moreover, superconductivity emerges below 0.69 K, and the anisotropy ratio of the upper critical field $[\mathrm{\Gamma }=H_{c2}^{\left|\right|ab}\left(0\right)/H_{c2}^{\left|\right|c}\left(0\right)]$ is significantly enhanced compared to 2H-${\mathrm{TaS}}_2$, which shares the same essential layer unit. According to the Lawrence-Doniach model, the enhanced $\mathrm{\Gamma }$ may be explained by the reduced effective mass in the $k_x\text{−}{k}_y$ plane, where Weyl rings locate. Therefore, this type of layered topological systems may offer a platform to investigate highly anisotropic superconductivity and to understand the extremely large upper critical field in the bulk or in the two-dimensional limit.

We examine the performance of the density matrix embedding theory (DMET) recently proposed in Knizia and Chan [Phys. Rev. Lett. 109, 186404 (2012)]. The core of this method is to find a proper one-body potential that generates a good trial wave function for projecting a large-scale original Hamiltonian to a local subsystem with a small number of bases. The resultant ground state of the projected Hamiltonian can locally approximate the true ground state. However, the lack of the variational principle makes it difficult to judge the quality of the choice of the potential. Here we focus on the entanglement spectrum (ES) as a judging criterion; accurate evaluation of the ES guarantees that the corresponding reduced density matrix well reproduces all physical quantities on the local subsystem. We apply the DMET to the Hubbard model on the one-dimensional chain, zigzag chain, and triangular lattice, and test several variants of potentials and cost functions. It turns out that ES serves as a more sensitive quantity than the energy and double occupancy to probe the quality of the DMET outcomes. A symmetric potential reproduces the ES of the phase that continues from a noninteracting limit. The Mott transition as well as symmetry-breaking transitions can be detected by the singularities in the ES. However, the details of the ES in the strongly interacting parameter region depends much on these variants, meaning that the present DMET algorithm allowing for numerous variant is insufficient to fully characterize the particular phases that require characterization by the ES.

Within the position-dependent translation operator formalism for quantum systems, we obtain analytical expressions for the eigenstates and the Landau level spectrum of Dirac fermions in graphene in the presence of a perpendicularly applied magnetic field and, as a consequence of the formalism, with a generalized form of the momentum operator. Moreover, we explore the behavior of wave packet dynamics in such a system by considering different initial pseudospin polarizations and metric parameters. Our findings show that the Landau levels, the wave packet trajectories, and velocities are significantly affected by the choice of the metric in the non-Euclidean space of the deformed momentum operator, exhibiting a tunable energy level spacing. In the dynamics analysis, one observes an enhancement of the oscillation amplitude of the average positions for all investigated pseudospin polarizations due to the nonsymmetric evolution of the wave packet induced by the different metrics in the system. The present formalism is shown to be a theoretical platform to describe the effects of two scenarios due to (i) a lattice deformation in graphene, giving rise to a natural Fermi velocity renormalization, or even (ii) a nonuniform mass term, induced by a specific substrate, that varies on a length scale much greater than the magnetic field length.

We study the acoustoelectric effect in two-dimensional materials like transition metal dichalcogenide monolayers located on a nonpiezoelectric substrate and exposed to the Rayleigh surface acoustic waves. We investigate the behavior of the Hall component of the electric current density which appears due to the trigonal warping of the valleys in k space. We calculate the spectrum of the current density and study its dependence on the electron effective lifetime and density in the sample. We distinguish between the drift and diffusive components of the current and show which components turn out predominant. Furthermore, we compare the effect of the Rayleigh and Bluestein-Gulyaev acoustic waves, which appear if the sample is located on a piezoelectric substrate.

We develop a conformal field theory approach for investigation of the quantum charge, heat, and thermoelectric transport through a quantum impurity fine-tuned to a non-Fermi liquid regime. The non-Fermi-liquid operational mode is associated with the overscreened spin Kondo effect and controlled by the number of orbital channels. The universal low-temperature scaling and critical exponents for Seebeck and Peltier coefficients are investigated for the multichannel geometry. We discuss the universality of Lorenz ratio and power factor beyond the Fermi-liquid paradigm. Different methods of verifying our findings based on the recent experiments are proposed.

Floquet states have been used to describe the impact of periodic driving on lattice systems, either using a tight-binding model or by using a continuum model where a Kronig-Penney-like description has been used to model spatially periodic systems in one dimension. A number of these studies have focused on finite systems, and results from these studies are distinct from those of infinite lattice systems as a consequence of boundary effects. In the case of a finite system, there remains a discrepancy in the results between tight-binding descriptions and continuous lattice models. Periodic driving by a time-dependent field in tight-binding models results in a collapse of all quasienergies within a band at special driving amplitudes. In the continuum model, however, a pair of nearly degenerate edge bands emerge and remain gapped from the bulk bands as the field amplitude increases. We resolve these discrepancies and explain how these edge bands represent Schrödinger cat-like states with effective tunneling across the entire lattice. Moreover, we show that these extended cat-like states become perfectly localized at the edge sites when the external driving amplitude induces a collapse of the bulk bands.

The wavelength scale confinement of light offered by photonic crystal (PhC) cavities is one of the fundamental features on which many important on-chip photonic components are based, opening silicon photonics to a wide range of applications from telecommunications to sensing. This trapping of light in a small space also greatly enhances optical nonlinearities and many potential applications build on these enhanced light-matter interactions. In order to use PhCs effectively for this purpose it is necessary to fully understand the nonlinear dynamics underlying PhC resonators. In this work, we derive a first principles thermal model outlining the nonlinear dynamics of optically pumped silicon two-dimensional (2D) PhC cavities by calculating the temperature distribution in the system in both time and space. We demonstrate that our model matches experimental results well and use it to describe the behavior of different types of PhC cavity designs. Thus, we demonstrate the model's capability to predict thermal nonlinearities of arbitrary 2D PhC microcavities in any material, only by substituting the appropriate physical constants. This renders the model critical for the development of nonlinear optical devices prior to fabrication and characterization.

The interplay between superconductivity, magnetic field, and spin-orbit coupling can lead to the realization of nontrivial topological phases. Recent experiments have found signatures of such phases in magnetic nanoflakes formed by nanostructures coupled to a superconducting substrate. These heterostructures comprise a topologically nontrivial region surrounded by a trivial one due to the finite magnetic exchange field induced by the magnetic nanoflake. The analysis of the topological phase diagram of such a system shows that a similar phase separation occurs by tuning the chemical potential of the nanoflake. In this paper, we study such a possibility in detail, analyzing the spatial extent of the edge modes circulating around the nanoflake and discussing some practical implementations. We also show how the chirality of Majorana edge states can be probed using scanning tunneling spectroscopy with a double-tip setup.

Ferroelectric lead-free ${\mathrm{K}}_x{\mathrm{Na}}_{1-x}{\mathrm{NbO}}_3$ (KNN) perovskite, as the representative of the preferred oxides with good dielectric and piezoelectric behaviors, has attracted broad attentions in recent years. Here we systematically investigate the complete process of temperature dependent structural evolution in manganese (Mn)-doped ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3\text{−}{\mathrm{LiBiO}}_3$ single crystals, presenting the ultrahigh piezoelectric coefficient of about 1050 pC ${\mathrm{N}}^{-1}$ and the excellent dielectric and ferroelectric performances, by demonstrating phonon thermodynamics associated with all Raman active modes at a wide temperature range of 80–800 K. All-round unpolarized and polarized scattering characteristics reflecting various phonon and structure properties are specified in detail. Symmetry difference, symmetry breaking, and structure rearrangement among the molecular vibrations have been proven to be related to a multiphase coexistence and the discontinuity of first-order phase transition. In comparison with the lattice dynamics, a complete phase transition ordering and the shift of transition point have been observed in $\mathrm{KNN}\text{−}{\mathrm{LiBiO}}_3$ crystals with the different Mn contents. This work aims at deeply revealing the details of good ferroelectric/dielectric performance, structure, and phonon thermodynamics, as well as understanding the mechanisms of first-order phase transition under the doping manipulation in KNN systems.

The terahertz (THz) optical conductivity of charge-ordered nickelate ${\mathrm{SmNiO}}_3$ thin films depicts a unique dependence of the charge-density-wave (CDW) type instability on the film thickness. Here, we report a rare observation of two dominant resonance modes in the THz conductivity spectrum. Corroborated by structural and electrical conductivity data, we demonstrate that these doublet excitations are the attribution of the CDW type pinned and bound modes. Both the peak position and the peak strength can be controlled by the film thickness and the defect density associated with oxygen stoichiometry. The oxygen vacancies in these films act like defects/impurities and induce charge oscillation around them which get coupled with unperturbed CDW mode of the charge-ordered phase. Thus, an additional bound mode emerges alongside the pinned mode. The defect density as controlled by the film thickness allows modulation of the resonance excitations in the desired THz frequency range.

We investigate the possibility to control dynamically the interactions between repulsively bound pairs of fermions (doublons) in correlated systems with off-resonant ac fields. We introduce an effective Hamiltonian that describes the physics of doublons up to second order in the high-frequency limit. It unveils that the doublon interaction, which is attractive in equilibrium, can be completely suppressed and then switched to repulsive by varying the power of the ac field. We show that the signature of the dynamical repulsion between doublons can be found in the single-fermion density of states averaged in time. Our results are further supported by nonequilibrium dynamical mean-field theory simulations for the half-filled Fermi-Hubbard model.

Spin-$\frac{1}{2}$ chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement as the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb lattice compound ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$, but to confirm this picture a comprehensive understanding of the exchange interactions including terms outside of the idealized model is required. Here, we employ neutron scattering to study the spin dynamics in ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$ and accurately determine the coupling strengths through the random phase approximation and density functional theory approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative. This hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths. Our results establish ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$ as a weakly coupled alternating AF-FM chain compound and reveal the robustness of the gapped ground state in alternating chains under weak interchain couplings.

Selective excitation of coherent high-amplitude vibrations of atoms in a solid can induce exotic nonequilibrium states, in which the character of interactions between electronic, magnetic and lattice degrees of freedom is considerably altered and the underlying symmetries are broken. Here we use intense single-cycle terahertz pulses to drive coherently the dipole-active $E_u^1$ phonon mode of a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal. As a result, several Raman-active modes are simultaneously excited in a nonlinear process, while one of them, having the $E_g^2$ symmetry, experiences dynamical splitting during the first two picoseconds after excitation. The corresponding angular scattering pattern is modified indicating the coexistence of two phonon modes characteristic of a nonequilibrium state with a lower crystal symmetry. We observe also a short-lived frequency splitting of the original $E_g^2$ mode that immediately after excitation amounts to $\sim 25\%$ of the unperturbed value. This transient state relaxes with a characteristic time of $\sim 1$ ps, that is close to the decay time of the squared amplitude of the resonantly excited infrared-active $E_u^1$ mode. We discuss possible mechanisms of the dynamical splitting: nonlinear lattice deformation caused by the intense $E_u^1$ vibrations and excitation of anisotropic electronic distribution due to nonlinear electron-phonon interaction. Our data also contain an evidence in favor of the sum-frequency Raman mechanism of generation of the coherent $E_g^2$ phonons in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ excited by terahertz pulses.

The origin of the intermediate anomalous metallic state in two-dimensional superconductor materials remains enigmatic. In the present paper, we observe such a state in a series of $\sim 9.0$-nm-thick ${\mathrm{Ta}}_x{\left({\mathrm{SiO}}_2\right)}_{1-x}$ ($x$ being the volume fraction of Ta) nanogranular films. At zero field, the $x\gtrsim$ 0.75 films undergo a Berezinskii-Kosterlitz-Thouless transition as transform from normal to superconducing states upon cooling. For the $x\lesssim$ 0.71 films, the resistance increases with decreasing temperature from 2 K down to 40 mK. A normal state to anomalous metallic state transition is observed in the $x\simeq 0.73$ film, i.e., near the transition temperature, the resistance of the film decreases sharply upon cooling as if the system would cross over to superconducting state, but then saturates to a value far less than that in normal state. When a small magnetic field perpendicular to the film plane is applied, the anomalous metallic state occurs in the $x\gtrsim 0.75$ films. It is found that both disorder and magnetic field can induce the transition from superconductor to anomalous metal and their influences on the transition are similar. For the magnetic field induced case, we find the sheet resistance $R_{\square }(T,H)$ ($T$ and $H$ being the temperature and the magnitude of the magnetic field, respectively) data near the crossover from the anomalous metal to superconductor and in the vicinity of the anomalous metal to insulator transition, respectively, obey unique scaling laws deduced from the Bose-metal model. Our results strongly suggest that the anomalous metallic state in the ${\mathrm{Ta}}_x{\left({\mathrm{SiO}}_2\right)}_{1-x}$ granular films is bosonic and dynamical gauge field fluctuation resulting from superconducting quantum fluctuations plays a key role in its formation.

Studying manganite superlattices ${\left[{\left(\mathrm{LCMO}\right)}_{2n}/{\left(\mathrm{PCMO}\right)}_n\right]}_t$ made of ${\mathrm{La}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_3$ (LCMO) and ${\mathrm{Pr}}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_3$ (PCMO), we found an unexpected behavior varying the period $n$. At small $n$, the ensemble is a three-dimensional ferromagnetic metal due to interfacial charge transfer. At large $n$, the LCMO layers dominate transport. However, rather than a smooth interpolation between these limits a sharp transport and magnetic anomaly is found at an intermediate critical PCMO thickness $n^{*}$. Magnetic force microscopy reveals that the phase-separation length scale also maximizes at $n^{*}$ where, unexpectedly, it becomes comparable to that of the ${\left({\mathrm{La}}_{1–y}{\mathrm{Pr}}_y\right)}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_3$ (LPCMO) alloy. We conjecture the phenomenon originates in a disorder-related length scale: Large charge-ordered clusters as in LPCMO can only nucleate when Pr-rich regions reach a critical size related to $n^{*}$.

The electronic structure of a pulsed laser deposited epitaxial $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ (LVO) thin film grown on a $\mathrm{LaAl}{\mathrm{O}}_3$ (001) (LAO) substrate has been studied at room temperature and at 130 K, which is below the structural (141 K) and magnetic transition temperatures (143 K) of single-crystal LVO. Significant modification in spectral intensity, largely around the $\mathrm{V}3d\text{−}\mathrm{O}2p$ hybridized region, is observed in the valence band spectrum (VBS) of a LVO film at 130 K. Resonant photoemission study at 130 K confirms the presence of a charge-transfer screened $3d^n\underline{\mathrm{L}}$ ($\underline{\mathrm{L}}$: hole in the $\mathrm{O}2p$) final state along with the dominant $3d^{n-1}$ final state at 1.5 eV binding energy in the valence band. On the contrary, in the room temperature VBS, dominant $\mathrm{V}3d$ states with only the $3d^{n-1}$ kind of final state are accentuated. To understand this difference, density functional theory calculations are employed. The changes in crystal structure from room temperature orthorhombic to low temperature monoclinic ($M$ LVO) symmetry leads to remarkable change in the electronic structure around the Fermi level, including transition from the direct to the indirect nature of the band gap. Our calculations also confirm an enhanced $\mathrm{O}2p$ character in the valence band edge of $M$ LVO that is hybridized with $\mathrm{V}3d$. These results are further corroborated with octahedral distortions associated with the structural transition.

The $p\text{−}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{−}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.