We investigate the ultrafast optical response of PbTe to an intense single-cycle terahertz pulse, resonant with the soft transverse optical (TO) phonon mode of the crystal. We detect multifrequency oscillations of the reflectance anisotropy, which we associate with nonlinear motion of the TO phonon oscillator excited directly by the terahertz pulse. Our observation of monotonically decaying optical anisotropy together with second harmonic oscillations of the TO mode is evidence of a transient noncentrosymmetric state of the crystal lattice that can be accompanied by the ferroelectric order. We suppose that this state is induced in the PbTe crystal by the intense terahertz pulse via alignment of the local polar nanodomains. This hypothesis is partially supported by the observation of coherent phonons near the satellite phonon mode frequency that are impulsively generated by a femtosecond laser pulse and are considerably enhanced by a synchronous terahertz pulse.

We use the density matrix renormalization group (DMRG) and a hard-core boson map to investigate the quantum phase transitions present in the phase diagram of the frustrated Heisenberg ladder in a magnetic field. The quantum bicritical point is observed at the end of a first-order transition line, which is at the meeting of the two second-order transition lines that bound the fully polarized plateau. The characterization of the bicritical point was made using a hard-core boson mapping of the low-energy excitations from the fully polarized phase and through DMRG by studying the probability density of finding the rung spins in a singlet or a triplet state with zero spin component along the magnetic field. In particular, we give conditions for the exchange couplings for the presence of the first-order transition line and the bicritical point in the phase diagram. Moreover, we unveil the phase-separated states for magnetization values inside the magnetization jump and, in particular, the dependence on the system size of the energy curve as a function of magnetization. Finite-size scaling analysis of the transverse spin correlation functions has been used to estimate the critical points of the Kosterlitz-Thouless transitions from the fractional magnetization plateau $m=1/2$ to the respective gapless Luttinger liquid phases for some sets of parameters.

We reconsider magnetic properties of face-centered cubic (fcc) and hexagonal close-packed (hcp) cobalt within the density functional theory plus dynamical mean-field theory $(\mathrm{DFT}+\mathrm{DMFT})$ approach in the paramagnetic phase. Using the recently proposed approach of calculation of exchange interactions in the paramagnetic phase, we extract exchange interaction parameters of fcc and hcp cobalt and show that the hcp phase possesses larger spin stiffness, in agreement with the experimental data, showing stronger tendency to ferromagnetism. Accordingly, the DMFT Curie temperature of the hcp phase appears to be higher than that of the fcc phase. The disappearance of magnetic order in the fcc phase well below the cobalt Curie temperature is expected to affect its structural stability and make this phase energetically unfavorable near the experimental Curie temperature. This may explain the revival of the hcp phase near the Curie temperature of cobalt, observed in the recent experimental results of perturbed angular correlation study [Sewak et al., Sci. Rep. 12, 10054 (2022)].

We present a first-principles study of the low-temperature rhombohedral phase of ${\mathrm{BaTiO}}_3$ using Hubbard-corrected density-functional theory. By employing density-functional perturbation theory, we compute the onsite Hubbard $U$ for $\mathrm{Ti}\left(3d\right)$ states and the intersite Hubbard $V$ between $\mathrm{Ti}\left(3d\right)$ and $\mathrm{O}\left(2p\right)$ states. We show that applying the onsite Hubbard $U$ correction alone to $\mathrm{Ti}\left(3d\right)$ states proves detrimental, as it suppresses the $\mathrm{Ti}\left(3d\right)–\mathrm{O}\left(2p\right)$ hybridization and drives the system towards a cubic phase. Conversely, when both onsite $U$ and intersite $V$ are considered, the localized character of the $\mathrm{Ti}\left(3d\right)$ states is maintained, while also preserving the $\mathrm{Ti}\left(3d\right)–\mathrm{O}\left(2p\right)$ hybridization, restoring the rhombohedral phase of ${\mathrm{BaTiO}}_3$. The generalized $\text{PBEsol}+U+V$ functional yields good agreement with experimental results for the band gap and dielectric constant, while the optimized geometry is slightly less accurate compared to PBEsol. Zone-center phonon frequencies and Raman spectra are found to be significantly influenced by the underlying geometry. PBEsol and $\text{PBEsol}+U+V$ provide satisfactory agreement with the experimental Raman spectrum when the PBEsol geometry is used, while $\mathrm{PBEsol}\text{+}U$ Raman spectrum diverges strongly from experimental data highlighting the adverse impact of the $U$ correction alone in ${\mathrm{BaTiO}}_3$. Our findings underscore the promise of the extended Hubbard $\text{PBEsol}+U+V$ functional with first-principles $U$ and $V$ for the investigation of other ferroelectric perovskites with mixed ionic-covalent interactions.

We have gone through the detailed microscopic calculation of the shear viscosity of a 2-dimensional graphene system in the relaxation time approximation–based kinetic theory framework. After getting its final expressions, we compared it with the shear viscosity expressions of other possible 2-dimensional as well as 3-dimensional nonrelativistic and ultrarelativistic fluid systems. The aim of the comparison is to reveal how their different one-body dispersion relations affect their many-body fluid properties like shear viscosity and the viscosity to entropy density ratio. It is also aimed to reveal the 3-dimensional to the 2-dimensional transformation of their mathematical structures. We have numerically explored the differences in their order of magnitude and dependence on thermodynamical parameters—temperature and chemical potential. Marking two thermodynamical domains—Dirac fluid and Fermi liquid—for a 2-dimensional graphene system, we have noticed that shear viscosity, entropy density, as well as their ratios decrease toward saturated values when one goes from Fermi liquid to Dirac fluid domain. When one shifts from milli–electron volt scales of temperature and chemical potential in the condensed-matter physics location to their mega–electron volt scales in the high-energy physics location, then the same results may be expected for the hot quark matter case, where the transition from the neutron star to early universe domains may be considered as a Fermi liquid to Dirac fluid transition.

We study metallic transport in an effective model that describes the coupling of electrons to fluctuating magnetic moments with full SU(2) symmetry, exhibiting characteristic behavior of metals at the approach of the Mott transition. We show that scattering by fluctuating local moments causes a fully incoherent regime of electron transport with $T$-linear resistivity. This strange metal regime is characterized by almost universal, nearly Planckian slope and a finite zero-temperature intercept, that we can associate respectively with the amplitude fluctuations and with the random orientations of local magnetic moments. Our results indicate a route for understanding the microscopic origin of strange metal behavior that is unrelated to quantum criticality and does not rely on the existence of quasiparticles.

We report a comprehensive study of magnetotransport properties, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations on self-flux grown ${\mathrm{LaAuSb}}_2$ single crystals. Resistivity and Hall measurements reveal a charge density wave (CDW) transition at 77 K. MR and de Haas-van Alphen measurements indicate that the transport properties of ${\mathrm{LaAuSb}}_2$ are dominated by Dirac fermions that arise from Sb square nets. ARPES measurements and DFT calculations reveal an electronic structure with a common feature of the square-net-based topological semimetals, which is in good agreement with the magnetotransport properties. Our results indicate the coexistence of CDW and Dirac fermion in ${\mathrm{LaAuSb}}_2$, both of which are linked to the bands arising from the Sb square net, suggesting that the square net could serve as a structural motif to explore various electronic orders.

Raman spectroscopy uses light scattering to extract information on low-energy excitations of solids. The Raman process is described by diagrams which are fourth order in the light-matter interaction and in particular the resonant contribution, which involves four different space-time arguments, is difficult to evaluate. If one instead simulates explicitly the incoming (classical) light pulse, the Raman signal is given by the outgoing photon flux and can be determined from a two-point correlation function. Such a formalism can be used to compute the time-resolved Raman spectrum of nonequilibrium systems, as well as nonlinear signals which are higher order in the incoming field, such as hyper-Raman scattering. Here we explain how to implement this time-dependent formalism within the dynamical mean field theory framework. The method is illustrated with applications to the Holstein-Hubbard model in the strong electron-phonon coupling regime. We demonstrate hyper-Raman scattering in measurements with strong probe fields and frequency mixing signals in the presence of a pump field and simulate the evolution of Stokes and anti-Stokes features after photoexcitations of metallic and Mott insulating systems.

Accurate predictions of the nature of defects in low-dimensional materials are essential to enable and promote their applications in micro- and optoelectronic devices. However, when dealing with charged defects in low-dimensional semiconductors, conventional first-principles calculations based on the jellium model encounter the divergence problem, where the formation energy diverges with the vacuum layer thickness. Two promising approaches, the supercell extrapolation method based on the jellium model [Wang et al., Phys. Rev. Lett. 114, 196801 (2015)] and the real charge model [Xiao et al., Phys. Rev. B 101, 165306 (2020)], have been proposed to address this divergence issue. However, these two models disagree when extrapolated to an infinitely large supercell following the first-order Makov-Payne correction. Whether they are unified or physically different remains unknown. In this work, we analyze the scaling behaviors of charged defect formation energy in these two computational models. We find that when considering the real charge distribution of ionized charge in two-dimensional systems, an extra quadratic finite-size correction $L_S^{-2}$ ($L_S$ is the lateral supercell size) is required for the formation energy calculations. After this correction, the results of these two models can be unified when extrapolated to infinity following the derived scaling behaviors. Meanwhile, based on the understanding of the differences between these two models, we propose a simple and convenient approach to correct the jellium model error for charged defect calculations in low-dimensional semiconductors. Our work thus provides a practical understanding of the charged defect calculations in low-dimensional semiconductors.

The onset of nonequilibrium in a driven Anderson insulator is identified by monitoring the system with two thermometers. Features of nonequilibrium appear at surprisingly weak drive intensity demonstrating, among other things, that conductivity may not be a reliable thermometer for ensuring linear-response conditions. In addition, the spectral contents of the applied field could be more important to take the system out of equilibrium than its absorbed power. Ensuing hot-electron transport effects and the nontrivial role phonons play in driven quantum systems are pointed out.

We demonstrate that a class of stable ${\mathbb{Z}}_2$ monopole charge Dirac point (${\mathbb{Z}}_2\mathrm{DP}$) phases can robustly exist in real materials, which is counterintuitive: that is, a ${\mathbb{Z}}_2\mathrm{DP}$ is unstable and generally considered to be only the critical point of a ${\mathbb{Z}}_2$ nodal line (${\mathbb{Z}}_2\mathrm{NL}$) characterized by a ${\mathbb{Z}}_2$ monopole charge (the second Stiefel-Whitney number $w_2$) with space-time inversion symmetry but no spin-orbital coupling. We explicitly reveal the higher-order bulk-boundary correspondence in the stable ${\mathbb{Z}}_2\mathrm{DP}$ phase. We propose the alternating-twist multilayer graphene, which can be regarded as 3D twisted bilayer graphene (TBG), as the first example to realize such stable ${\mathbb{Z}}_2\mathrm{DP}$ phase and show that the Dirac points in the 3D TBG are essentially degenerate at high-symmetry points protected by crystal symmetries and carry a nontrivial ${\mathbb{Z}}_2$ monopole charge ($w_2=1$), which results in higher-order hinge states along the entire Brillouin zone of the $k_z$ direction. By breaking some crystal symmetries or tailoring interlayer coupling we are able to access ${\mathbb{Z}}_2\mathrm{NL}$ phases or other ${\mathbb{Z}}_2\mathrm{DP}$ phases with hinge states of adjustable length. In addition, we present other 3D materials which host ${\mathbb{Z}}_2\mathrm{DPs}$ in the electronic band structures and phonon spectra. We construct a minimal eight-band tight-binding lattice model that captures these nontrivial topological characters and furthermore tabulate all possible space groups to allow the existence of the stable ${\mathbb{Z}}_2\mathrm{DP}$ phases, which will provide direct and strong guidance for the realization of the ${\mathbb{Z}}_2$ monopole semimetal phases in (among others) electronic materials, metamaterials, and electrical circuits.

A first-principles study of the insulator-to-metal transition under pressure in ${\mathrm{NbS}}_3$ is reported. In contrast with previous proposals, it is found that the transition is not due to a progressive closing of the initial band gap but to the inversion of the relative stability of two different polymorphs under pressure. The Fermi surface of the new metallic state under low pressure (polymorph V′) is discussed, and the possible occurrence of as yet unknown incommensurate phases is suggested. The reason for the inversion of stability of the two polymorphs is discussed.

The problem of the interplay between Anderson localization and the leakage of the surface plasmon polaritons (SPPs) scattered from finite one-dimensionally (1D) rough region of the metal-dielectric interface is examined. By using the proper coordinate transformation, we reduce the problem of SPP scattering from a rough region to the problem of its scattering from a flattened region with 1D random impedance. The latter problem is then solved converting the impedance perturbation to the effective potentials in the Schrödinger-like equation, just like it was done in our previous studies [Low Temp. Phys. 42, 685 (2016); Ann. Phys. 455, 169378 (2023)]. We calculate the radiation pattern for the model of one-dimensional interface roughness and demonstrate its pronounced anisotropy arising due to the interplay between different geometrical parameters of the interface roughness. The 1D nature of the interface disturbance results in pronounced Anderson localization of the SPP penetrating a rough segment, which most significantly manifests itself in the reflection of the incident surface wave.

We study the quench and the ramp dynamics of interacting $N$-component charged bosons with dipole symmetry using Schwinger-Keldysh field theory in the large-$N$ limit. The equilibrium phase diagram of these bosons shows two phases in the large-$N$ limit. The first is a normal phase where both the global $U\left(N\right)$ and the dipole symmetries are conserved and the second is a delocalized condensed phase where both the symmetries are broken. In contrast, our explicit computation of the steady state after an instantaneous quantum quench from the condensed phase shows that an additional, novel, delocalized normal phase, where the global $U\left(N\right)$ symmetry is conserved but the dipole symmetry is broken, can exist for a range of quench parameters. A study of ramp dynamics of the model shows that the above-mentioned steady state exists only above a critical ramp rate which we estimate.

We study a one-dimensional spin-$1/2$ model with three-spin interactions and a transverse magnetic field $h$. The model is known to have a $Z_2\times Z_2$ symmetry and a duality between $h$ and $1/h$. The self-dual point at $h=1$ is a quantum critical point with a continuous phase transition. We compute the critical exponents $z, \beta , \gamma$, and $\nu$, and the central charge $c$ numerically using exact diagonalization (ED) for systems with periodic boundary conditions. We find that both $z$ and $c$ are equal to 1, implying that the critical point is governed by a conformal field theory with a marginal operator. The values obtained for $\beta /\nu , \gamma /\nu$, and $\nu$ from ED suggest that the model exhibits Ashkin-Teller criticality with an effective coupling that is intermediate between the four-state Potts model and two decoupled transverse field Ising models. A more careful analysis on much larger systems but with open boundaries using density-matrix renormalization group (DMRG) calculations, however, reveals important additive and multiplicative logarithmic corrections at and near criticality, and we present evidence that the self-dual point may be in the same universality class as the four-state Potts model. An energy level spacing analysis shows that the model is not integrable. For a system with an even number of sites and periodic boundary conditions, there are exact mid-spectrum zero-energy eigenstates whose number grows exponentially with the system size. A subset of these eigenstates have wave functions that are independent of the value of $h$ and have unusual entanglement structures; hence these can be considered to be quantum many-body scars. The number of such quantum scars scales at least linearly with system size. Finally, we study the infinite-temperature autocorrelation functions at sites close to one end of an open system. We find that some of the autocorrelators relax anomalously in time, with pronounced oscillations and very small decay rates if $h\gg 1$ or $h\ll 1$. If $h$ is close to the critical point, the autocorrelators decay quickly to zero except for an autocorrelator at the end site.

Beginning from the conventional square-lattice nearest-neighbor antiferromagnetic Heisenberg model, we allow the $J_x$ and $J_y$ couplings to be anisotropic, with their values depending on the bond orientation. The emergence of anisotropic, bond-dependent couplings should be expected to occur naturally in most antiferromagnetic compounds which undergo structural transitions that reduce the point-group symmetry at lower temperature. Using the spin-wave approximation, we study the model in several parameter regimes by diagonalizing the reduced Hamiltonian exactly and computing the edge spectrum and Berry connection vector, which show clear evidence of localized topological charges. We discover phases that exhibit Weyl-type spin-wave dispersion, characterized by pairs of degenerate points and edge states, as well as phases supporting lines of degeneracy. We also identify a parameter regime in which there is an exotic state hosting gapless linear spin-wave dispersions with different longitudinal and transverse spin-wave velocities.