Local structural distortions due to isolated atomic defects and defect complexes strongly affect the macroscopic properties of oxide ceramics. While the characterization of local defect structures is more common in simple $\mathrm{AB}{\mathrm{O}}_3$ perovskites, unambiguous determination of the same in layered perovskites is more difficult due to their complex crystal structures. Here, we combined x-ray pair distribution function and density functional theory calculations to characterize the structure of cation-oxygen divacancy pairs in a Dion-Jacobson (D-J) layered perovskite. Our results indicate that local incommensurate dipole moments with polarization density in the range of $\sim 0.1–17\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ are created due to divacancy-induced structural distortions in the D-J phase $\mathrm{K}{\mathrm{Ca}}_2{\mathrm{Nb}}_3{\mathrm{O}}_{10}$. This is comparable with defect dipole moments observed in well-known perovskite ferroelectrics. The current results imply that controlling the atomic defects can potentially lead to significant control of dielectric properties in D-J layered perovskites.

The discovery of lead-free piezoelectric materials is crucial for future information and energy storage applications. Enhanced piezoelectric and other physical properties are commonly observed near the morphotropic phase boundary (MPB) composition of ferroelectric solid solutions. The $(1-x\left)\mathrm{Ba}\right({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\left){\mathrm{O}}_3\text{−}x\right({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_3$ $\left(\mathrm{BZT}\text{−}x\mathrm{BCT}\right)$ system exhibits a large electromechanical response around its MPB region at $x=0.5$. We report experimental and theoretical results of $\mathrm{BZT}\text{−}x\mathrm{BCT}$ over a wide composition range $(0.3\le x\le 1.0)$. X-ray diffraction and Raman spectroscopy studies indicate a composition-induced structural phase transition from a rhombohedral $\left(R3m\right)$ phase at $x\le 0.4$ to a tetragonal $\left(P4mm\right)$ phase at $x\ge 0.6$ through a multiphase coexistence region at 0.45 ≤ $x\le 0.55$ involving orthorhombic + tetragonal (Amm2 + $P4mm$) phases. First-principles calculations elucidate the phase competition in the coexistence region. The critical composition $(x=0.5)$ displays enhanced dielectric, ferroelectric, and piezoelectric properties, where notably $d_{33}\sim 320$ pC/N. This paper provides clear evidence of Amm2 + $P4mm$ crystallographic phases in the MPB region, which is responsible for the improved functional properties.

We determine the Hall conductivity of light-driven graphene, with a specific focus on its frequency dependence, and compare it to the static effective approximation, based on Floquet states. This approximation gives the Haldane model as the effective model for light-driven graphene, with a gapped spectrum and a quantized Hall conductivity of $-2e^2/h$. We simulate both the light-driven and the effective model, and explicitly include the dissipative environment in our simulations. We investigate the effect of different driving regimes and dissipation strengths on the Hall conductivity in graphene. As a central result, the Hall conductivity of the light-driven system is not well approximated by the effective model, except for a regime of intermediate driving frequencies and small dissipation where the Hall conductivity contribution of the Dirac point approximately recovers the quantized value of $-2e^2/h$, as well as in the transient dynamics for weak dissipation.

The mineral linarite $\left[\mathrm{Pb}\mathrm{Cu}\mathrm{S}{\mathrm{O}}_4{\left(\mathrm{O}\mathrm{H}\right)}_2\right]$ forms a monoclinic structure where a sequence of $\mathrm{Cu}{\left(\mathrm{O}\mathrm{H}\right)}_2$ units forms a spin-$\frac{1}{2}$ chain. Competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest-neighbor interactions ($J_2$) in this quasi-one-dimensional spin structure imply magnetic frustration and lead to magnetic ordering below $T_N=2.8\mathrm{K}$ in a multiferroic elliptical spin-spiral ground state. Upon the application of a magnetic field along the spin-chain direction, distinct magnetically ordered phases can be induced. We studied the thermal conductivity $\kappa$ in this material across the magnetic phase diagram as well as in the paramagnetic regime in the temperature ranges 0.07–1 K and 9–300 K. We found that in linarite the heat is carried mainly by phonons but shows a peculiar nonmonotonic behavior in field. In particular, $\kappa$ is highly suppressed at the magnetic phase boundaries, indicative of strong scattering of the phonons off critical magnetic fluctuations. Even at temperatures far above the magnetically ordered phases, the phononic thermal conductivity is reduced due to scattering off magnetic fluctuations. The mean free path due to spin-phonon scattering ($l_{\text{spin-phonon}}$) was determined as a function of temperature. A power law behavior was observed mainly above $500\mathrm{m}\mathrm{K}$ indicating the thermal activation of spin fluctuations. In the critical regime close to the saturation field, $l_{\text{spin-phonon}}$ shows a $1/T$ dependence. Furthermore, a magnon thermal transport channel was verified in the helical magnetic phase. We estimate a magnon mean free path which corresponds to about 1000 lattice spacings.

Moiré potential has become a powerful tool to modulate electronic structures and realize quantum phases. Authors of previous studies mainly focused on the first-order moiré effect in systems with small lattice mismatches and rotation angles. Here, we report evidence of a high-order moiré effect and multiple Dirac cone replicas by performing angle-resolved photoemission spectroscopy on a graphene-SiC heterostructure. Despite that the first-order moiré effect is weak due to large lattice mismatch and rotation angle, a high-order moiré effect with a 1.9 nm periodicity in real-space is experimentally observed. Different from previously studied first-order moiré systems, in which only replicas with momentum transfer by the primitive reciprocal lattice vectors were observed, Dirac cone replicas with momentum transfer by second- and third-order reciprocal lattice vectors are experimentally identified. Our results not only demonstrate the high-order moiré effect in a graphene-SiC heterostructure but also provide opportunities to engineer moiré systems without the limitation of small lattice constant mismatches.

In this paper, we show how to calculate analytical atomic forces within the self-consistent density functional theory $+$ dynamical mean-field theory $(\mathrm{DFT}+\mathrm{DMFT})$ approach in the case when ultrasoft or norm-conserving pseudopotentials are used. We show how to treat the nonlocal projection terms arising within the pseudopotential formalism and circumvent the problem of nonorthogonality of the Kohn-Sham eigenvectors. Our approach is, in principle, independent of the DMFT solver employed and was tested with the Hubbard I solver. We benchmark our formalism by comparing it against the forces calculated for ${\mathrm{Ce}}_2{\mathrm{O}}_3$ and ${\mathrm{PrO}}_2$ by numerical differentiation of the total free energy as well as by comparing the energy profiles against the numerically integrated analytical forces.

We study the topological physics in nonlinear Schrödinger systems on lattices. We employ the quench dynamics to explore the phase diagram, where a pulse is given to a lattice point and we analyze its time evolution. There are two system parameters $\lambda$ and $\xi$, where $\lambda$ controls the hoppings between the neighboring links and $\xi$ controls the nonlinearity. The dynamics crucially depends on these system parameters. Based on analytical and numerical studies, we derive the phase diagram of the nonlinear Su-Schrieffer-Heeger (SSH) model in the ($\lambda ,\xi$) plane. It consists of four phases. The topological and trivial phases emerge when the nonlinearity $\xi$ is small. The nonlinearity-induced localization phase emerges when $\xi$ is large. We also find a dimer phase as a result of a cooperation between the hopping and nonlinear terms. A similar analysis is made of the nonlinear second-order topological system on the breathing Kagome lattice, where a trimer phase appears instead of the dimer phase.

Layered multiferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ spontaneously phase segregates to form ferrielectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ and paraelectric ${\mathrm{In}}_{4/3}{\mathrm{P}}_2{\mathrm{S}}_6$ (IPS) domains in a two-dimensional self-assembled heterostructure. Here, we study the effect of hydrostatic pressure on the structure of Cu-deficient $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ by Raman spectroscopy measurements up to 20 GPa. Detailed analysis of the frequencies, intensities, and linewidths of the Raman peaks reveals four discontinuities in the spectra around 2, 10, 14, and 17 GPa. At ∼2 GPa, we observe a structural transition initiated by the diffusion of IPS domains, which culminates in a drastic reduction of the number of peaks around 10 GPa. We attribute this to a possible monoclinic-trigonal phase transition at 10 GPa. At higher pressures (∼14 GPa), significant increases in peak intensities and sharpening of the Raman peaks suggest a band gap lowering and an isostructural electronic transition, with a possible onset of metallization at pressures above 17 GPa. When the pressure is released, the structure again phase separates into two distinct chemical domains within the same single crystalline framework—however, these domains are much smaller in size than the as-synthesized material resulting in suppression of ferroelectricity through nanoconfinement. Hydrostatic pressure can thus be used to tune the electronic and ferrielectric properties of Cu-deficient layered $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$.

We report a first-principles investigation of electronic structure, topological bands, and electron-phonon interactions in metallic biphenylene sheets. Biphenylene is a recently synthesized $s{p}^2$-bonded carbon allotrope. We find coupling of electrons at the Fermi surface to very high frequency carbon-derived phonons, analogous to superconducting ${\mathrm{MgB}}_2$. This leads to low-temperature weak coupling superconductivity due to an unusual combination of exceptionally large logarithmically averaged phonon frequency ${\omega }_{\text{log}}$=1369 K and moderate electron-phonon coupling. The electronic structure shows a two-band Fermi surface dominated by C $p_z$ orbitals and a pair of type-II tilted Dirac cones along the $\mathrm{\Gamma }$-Y line at the Brillouin zone boundary. Berry curvature and edge-state calculations show that monolayer biphenylene is a two-dimensional ${\mathbb{Z}}_2$ topological material. Thus, monolayer biphenylene is predicted to be a topological superconductor based on C $p$ orbitals and high-frequency phonons.

The optical conductivity contains information about energy absorption and the underlying physical processes. In finite-dimensional systems, vertex corrections to the bare bubble need to be considered, which is a computationally challenging task. Recent numerical studies showed that in the weak-coupling limit, near an ordering instability with wave-vector $\pi$, the vertical ladder describing particle-hole pairs interacting via the exchange of this wave vector becomes the dominant vertex correction. The corresponding Maki-Thompson-like diagram has been dubbed $\pi$-ton. Here we add the $\pi$-ton ladder vertex correction to dynamical mean-field theory estimates of the optical conductivity. By performing calculations on the Kadanoff-Baym contour, we reveal the characteristic spectral signatures of the $\pi$-tons and their evolution under nonequilibrium conditions. We consider interaction quenches of the weakly correlated Hubbard model near the antiferromagnetic phase boundary and analyze the evolution of the Drude and $\pi$-ton features. While the bubble contribution to the optical conductivity is found to thermalize rapidly, after some oscillations with frequencies related to the local spectral function, the $\pi$-ton contribution exhibits a slower evolution. We link this observation to the prethermalization phenomenon which has been previously studied in weakly interacting, quenched Hubbard models.

The recently discovered layered kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$=K, Rb, and Cs) attract intensive interest due to their intertwining with superconductivity, charge-density-wave state, and nontrivial band topology. In this work, we show by first-principles calculations and symmetry arguments that unconventional type-II Dirac nodal line fermions close to the Fermi level are present in another latest class of experimentally synthesized kagome compounds $A{\mathrm{V}}_6{\mathrm{Sb}}_6$ ($A$=K, Rb, and Cs). These compounds possess a unique kagome ${\left({\mathrm{V}}_3\mathrm{Sb}\right)}_2$ bilayer that dominates their electronic and topological properties, instead of the kagome ${\mathrm{V}}_3\mathrm{Sb}$ monolayer in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$. Crystal symmetry guarantees that the type-II Dirac nodal lines with quantized Berry phase lie in reflection-invariant planes of the Brillouin zone. We further reveal that the type-II Dirac nodal lines remain nearly intact in the presence of spin-orbital coupling and can be categorized as a ${\mathbb{Z}}_2$ classification. The findings establish $A{\mathrm{V}}_6{\mathrm{Sb}}_6$ as a class of new fascinating prototypes, which will extend the knowledge of interplay between unconventionally topological fermions and exotic quantum ordered states in kagome systems.

The interaction between bulk Dirac points and $s$-wave bulk superconductivity can sustain an exotic quasiparticle excitation, namely, a gapless one-dimensional helical Majorana mode. Herein, based on first-principles calculations, we predict that noncentrosymmetric superconductors LaPtSi and LaPtGe are Dirac superconducting semimetal candidates. Without spin-orbit coupling (SOC), LaPtSi and LaPtGe show three-dimensional metallic behavior with a normal band order. SOC induces energy level splitting and reordering, resulting in two pairs of bulk Dirac points with type-II and type-III characteristics and a topological insulator state in the $\mathrm{\Gamma }\text{−}Z$ direction. The obtained nonzero topological ${\mathbb{Z}}_2$ invariants and multiple nontrivial surface states on the (001) surface directly demonstrate the nontrivial topological features of LaPtSi and LaPtGe. Thus, these predicted compounds provide realistic platforms for the interplay of bulk Dirac points and superconductivity.

A quantitative correlation between the Urbach energy $\left(E_u\right)$ and asymmetry parameter $q$ of Raman spectra has been derived. For this purpose, the effect of electronic disorder $(\sim E_u)$ on the possible interference between the electronic continuum states and discrete phononic states is reinvestigated. The equation of the form $\frac{I}{q^2}\propto \xi E_u\pm \lambda$ ($I$: intensity of phonon mode $;\xi ,\lambda :$ some material-dependent parameters) has been obtained and verified experimentally on different semiconductors. The experimental results reveal an offset which has been understood as intrinsic or disorder-induced contribution to electron-phonon interactions in the system. The obtained equations quantitatively explain the changes in the Raman line shape due to disorder induced by atomic substitutions, temperature, etc., and provide understanding of the superposition between electronic and phononic states and of electron-phonon coupling in semiconductors with disorder.

Two-dimensional (2D) ferromagnetic (FM) semiconductors have attracted increasing interest for advanced spintronics. Here, we report that the monolayer perovskite ${\mathrm{Rb}}_2{\mathrm{CuCl}}_4$ (RCC) exhibits an intrinsic 2D FM semiconductor feature, emergent Jahn-Teller (JT) effect, and related orbital order (OO) with the diversity and tunability of physical behaviors. First-principles calculations and Monte Carlo simulations reveal the crucial changes of OO type, related mechanism of ferromagnetism, and electronic structure during the transition of JT distortion. Finally, we examine the significant tunable effect of biaxial strain on electronic and magnetic properties. Our comprehensive work on magnetism, structural distortion, and the electronic properties of monolayer RCC provides a guideline for studying the physical phenomena of $3d^9$ Cu(II) in 2D halide perovskites.

Coherent coupling between excitons is at the heart of many-body interactions and quantum information with transition metal dichalcogenide heterostructures as an emergent platform for the investigation of these interactions. We employ multidimensional coherent spectroscopy on monolayer ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructures and observe coherent coupling between excitons spatially localized in monolayer ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$. Through many-body spectroscopy, we further observe the absorption state arising from free interlayer electron-hole pairs. This observation yields a spectroscopic measurement of the interlayer exciton binding energy of about 250 meV.

The electronic wave functions in moiré materials are highly sensitive to the details of the local atomic configuration enabling Bloch band geometry and topology to be controlled by stacking and strain. Here, we predict that large injection currents (under circularly polarized irradiation) can develop in strained twisted bilayer graphene (TBG) heterostructures with broken sublattice symmetry; such bulk photovoltaic currents flow even in the absence of a $p\text{−}n$ junction and can be controlled by the helicity of incident light. As we argue, large injection current rates proceed from strong and highly peaked interband Berry curvature dipole distributions (arising from the texturing of Bloch wave functions in strained TBG heterostructures). Strikingly, we find that TBG injection current displays pronounced responses in the THz regime and can be tuned by chemical potential. These render injection currents a useful photocurrent probe of symmetry breaking in TBG heterostructures and make TBG a promising material for THz technology.

We develop a scaling theory and a renormalization technique in the context of the modern theory of polarization. The central idea is to use the characteristic function (also known as the polarization amplitude) in place of the free energy in the scaling theory and in place of the Boltzmann probability in a position-space renormalization scheme. We derive a scaling relation between critical exponents which we test in a variety of models in one and two dimensions. We then apply the renormalization to disordered systems. In one dimension, the renormalized disorder strength tends to infinity, indicating the entire absence of extended states. Zero (infinite) disorder is a repulsive (attractive) fixed point. In two and three dimensions, at small system sizes, two additional fixed points appear, both at finite disorder: $W_a$ $\left(W_r\right)$ is attractive (repulsive) such that $W_a<W_r$. In three dimensions, $W_a$ tends to zero and $W_r$ remains finite, indicating a metal-insulator transition at finite disorder. In two dimensions, we are limited by system size, but we find that both $W_a$ and $W_r$ decrease significantly as system size is increased.

Nonlinearity and discreteness are two pivotal factors for an emergence of discrete breather excitations in various media. We argue that these requirements are met in the forced ferromagnetic phase of the monoaxial chiral helimagnet $\mathrm{Cr}{\mathrm{Nb}}_3\mathrm{S}{}_6$ due to the specific domain structure of the compound. The stationary, time-periodic breather modes appear as the discrete breather lattice solutions whose period mismatches with a system size. Thanks to easy-plane single-ion anisotropy intrinsic to $\mathrm{Cr}{\mathrm{Nb}}_3\mathrm{S}{}_6$, these modes are of the dark type with frequencies lying within the linear spin-wave band, close to its bottom edge. They represent cnoidal states of magnetization, similar to the well-known soliton lattice ground state, with differing but limited number of embedded $2\pi$ kinks. The linear stability of these dark breather modes is verified by means of Floquet analysis. Their energy, which is controlled by two parameters, namely, the breather lattice period and amplitude, falls off linearly with a growth of the kink number. These results may pave a path to design spintronic resonators on the base of chiral helimagnets.

To understand the unexpectedly high thermoelectric performance observed in the thin-film Heusler alloy ${\mathrm{Fe}}_2{\mathrm{V}}_{0.8}{\mathrm{W}}_{0.2}\mathrm{Al}$, we study the magnon drag effect, generated by the tungsten-based impurity band, as a possible source of this enhancement, in analogy to the phonon drag observed in ${\mathrm{FeSb}}_2$. Assuming that the thin-film Heusler alloy has a conduction band integrating with the impurity band, originated by the tungsten substitution, we derive the electrical conductivity $L_{11}$ based on the self-consistent $t$-matrix approximation and the thermoelectric conductivity $L_{12}$ due to magnon drag based on the linear response theory and estimate the temperature-dependent electrical resistivity, Seebeck coefficient, and power factor. Finally, we compare the theoretical results with the experimental results of the thin-film Heusler alloy to show that the origin of the exceptional thermoelectric properties is likely due to the magnon drag related to the tungsten-based impurity band.

Quantum spin liquids hosting Majorana excitations have recently experienced renewed interest for potential applications to topological quantum computation. Performing logical operations with reduced poisoning requires to localize such quasiparticles at specific point of a device, with energies that are well defined and inside the bulk energy gap. These are two defining features of second-order topological insulators (SOTIs). Here, we show two spin-$3/2$ models that support quantum spin liquid phases characterized by Majorana excitations. One of them, defined on a square lattice, is analytically solvable thanks to a theorem by Lieb and behaves as a known SOTI. We show that, depending on the values of spin couplings, it is possible to localize either fermions or Majorana particles at its corners. The second model, defined on a kagome lattice, exhibits coexisting topologies, which reflect in a complex pattern of edge states. For certain choices of spin couplings, edge states can localize at one of the corners of a finite kagome lattice with flat edges.