We explore the elastic, dielectric, piezoelectric, and flexoelectric phenomenological coefficients as functions of microscopic model parameters such as ionic positions and spring constants in the two-dimensional square-lattice model with rocksalt-type ionic arrangement. Monte Carlo simulation reveals that a difference in the given elastic constants of the diagonal springs, each of which connects the same cations or anions, is responsible for the linear flexoelectric effect in the model. We show the quadratic flexoelectric effect is present only in noncentrosymmetric systems, and it can overwhelm the linear effect in feasibly large strain gradients. It can also be seen that the linear flexoelectric effect is suppressed by increasing the degree of inversion symmetry breaking due to a rigid dipolar feature.

The $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ antisite was reported to be a possible deep-level recombination-center defect in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ solar cells with a concentration higher than ${10}^{15}\mathrm{c}{\mathrm{m}}^{-3}$ under I-poor conditions. However, whether it is really an effective nonradiative recombination center and limits the photovoltaic efficiency depends also on its cross sections for capturing the electron or hole carriers, which is difficult to determine in both experiment and theory. Here we use a recently developed method to overcome the high computational cost of electron-phonon coupling calculation and implement it in the standard first-principles code quantum espresso so that we can calculate the carrier-capture cross sections effectively for the point defects in complicated semiconductors, such as $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$. The calculation showed that both the hole- and the electron-capture cross sections and capture rates of $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ are large relative to those of the point defects in conventional semiconductors, such as Si or GaP, which is attributed mainly to the heavy Pb-I atoms and the soft phonon modes in this Pb halide. Nonradiative recombination increases the thermal energy by exciting the phonons in the soft Pb-I lattice, whereas the contribution of the organic $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3$ group is negligible. Since $\mathrm{P}{\mathrm{b}}_{\mathrm{I}}$ has a higher concentration under I-poor conditions, especially when the semiconductor is $p$-type, our results suggest that the minority-carrier lifetime and thus the photovoltaic efficiency should be very limited in $p$-type $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ and $p$-type doping should be avoided for fabricating high-efficiency $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ solar cells under I-poor conditions. Similar calculations can be used for studying the influence of various defects on the photovoltaic performance of other organic-inorganic hybrid and inorganic halide perovskites, shedding light on the design of high-efficiency solar cells.

Thermal and many-body localized phases are separated by a dynamical phase transition of a new kind. We analyze the distribution of off-diagonal matrix elements of local operators across this transition in two different models of disordered spin chains. We show that the behavior of matrix elements can be used to characterize the breakdown of thermalization and to extract the many-body Thouless energy. We find that upon increasing the disorder strength the system enters a critical region around the many-body localization transition. The properties of the system in this region are: (i) the Thouless energy becomes smaller than the level spacing, (ii) the matrix elements show critical dependence on the energy difference, and (iii) the matrix elements, viewed as amplitudes of a fictitious wave function, exhibit strong multifractality. This critical region decreases with the system size, which we interpret as evidence for a diverging correlation length at the many-body localization transition. Our findings show that the correlation length becomes larger than the accessible system sizes in a broad range of disorder strength values and shed light on the critical behavior near the many-body localization transition.

The magnetic excitation spectrum of the magnetocaloric compound ${\mathrm{MnFe}}_4{\mathrm{Si}}_3$ has been investigated by means of polarized and unpolarized inelastic neutron scattering on single crystals. Spectra were collected in the ferromagnetic phase ($T_C\approx 305\mathrm{K}$), as well as in the paramagnetic state, in order to understand the nature of the magnetism in ${\mathrm{MnFe}}_4{\mathrm{Si}}_3$. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the $q$-dependent linewidths in the paramagnetic state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of $k_B{T}_C$ pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin fluctuations are found to be isotropic around $T_C$ and can be suppressed by a magnetic field of 2 T.

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies ($V_{\mathrm{O}}$) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through $V_{\mathrm{O}}$ variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning both magnetism and ferroelectricity simultaneously by using $V_{\mathrm{O}}$. Combining experimental and density-functional theory studies of $\mathrm{E}{\mathrm{u}}_{0.5}\mathrm{B}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3-\delta }$, we demonstrate that oxygen vacancies create $\mathrm{T}{\mathrm{i}}^{3+}3d^1$ defect states, mediating the ferromagnetic coupling between the localized Eu $4f^7$ spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the $\mathrm{E}{\mathrm{u}}_{0.5}\mathrm{B}{\mathrm{a}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3-\delta }$.

Based on first-principles calculations and symmetry analysis, we discovery that the ${\mathrm{Ag}}_2\mathrm{S}$ with $Cmce$ symmetry is a topological nodal-line semimetal in the absence of spin-orbit coupling. A single nodal loop as protected by the glide symmetry exists around the center of the Brillouin zone, dispersing slightly in the momentum space to form both electron and hole pockets around the loop. Moreover, a nearly flat drumheadlike surface state appears on the (001) surface of this material. The nodal-line semimetal phase and its drumheadlike surface states are expected to be experimentally detectable in $Cmce$-phase ${\mathrm{Ag}}_2\mathrm{S}$ because spin-orbit coupling will only open a negligible gap comparing to the energy dispersion of the nodal loop. Our finding provides a different member to the growing family of nodal-line semimetals with a single nodal loop structure.

The entanglement properties of the time-periodic Kitaev chain with nearest-neighbor and next-nearest-neighbor hopping is studied. The cases of the exact eigenstate of the time-periodic Hamiltonian, referred to as the Floquet ground state (FGS), as well as a physical state obtained from time evolving an initial state unitarily under the influence of the time-periodic drive are explored. Topological phases are characterized by different numbers of Majorana zero (${\mathbb{Z}}_0$) and $\pi$ (${\mathbb{Z}}_{\pi }$) modes, where the zero modes are present even in the absence of the drive, while the $\pi$ modes arise due to resonant driving. The entanglement spectrum (ES) of the FGS as well as the physical state show topological Majorana modes whose number is different from that of the quasienergy spectrum. The number of Majorana edge modes in the ES of the FGS vary in time from $|{\mathbb{Z}}_0-{\mathbb{Z}}_{\pi }|$ to ${\mathbb{Z}}_0+{\mathbb{Z}}_{\pi }$ within one drive cycle, with the maximal ${\mathbb{Z}}_0+{\mathbb{Z}}_{\pi }$ modes appearing at a special time-reversal-symmetric point of the cycle. For the physical state, on the other hand, only the modes inherited from the initial wave function, namely the ${\mathbb{Z}}_0$ modes, appear in the ES. The ${\mathbb{Z}}_{\pi }$ modes are absent in the physical state as they merge with the bulk excitations that are simultaneously created due to resonant driving. The topological properties of the Majorana zero and $\pi$ modes in the ES are also explained by mapping the parent wave function to a Bloch sphere.

We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the nonmonotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers.

Since the discovery of high-temperature superconductivity in the thin-film $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron selenide if the structural parameters could be tuned at liberty. We calculate the momentum dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a nontrivial way. These results are consistent with the known experimental behavior of binary iron chalcogenides and, at the same time, reveal two promising ways of tuning superconducting transition temperatures in these materials: on one hand by expanding the iron lattice of FeSe at constant iron-selenium distance and, on the other hand, by increasing the iron-selenium distance with unchanged iron lattice.

Understanding the microscopic mechanism of coexisting long-range orders (such as lattice supersolidity) in strongly correlated systems is a subject of immense interest. We study the possible manifestations of long-range orders, including lattice-supersolid phases with differently broken symmetry, in a two-dimensional square lattice system of hard-core bosons (HCBs) coupled to archetypal cooperative/coherent normal-mode distortions such as those in perovskites. At strong HCB-phonon coupling, using a duality transformation to map the strong-coupling problem to a weak-coupling one, we obtain an effective Hamiltonian involving nearest-neighbor, next-nearest-neighbor, and next-to-next-nearest-neighbor hoppings and repulsions. Using stochastic series expansion quantum Monte Carlo, we construct the phase diagram of the system. As coupling strength is increased, we find that the system undergoes a first-order quantum phase transition from a superfluid to a checkerboard solid at half-filling and from a superfluid to a diagonal striped solid [with crystalline ordering wave vector $\vec{Q}=(2\pi /3,2\pi /3)$ or $(2\pi /3,4\pi /3)]$ at one-third filling without showing any evidence of supersolidity. On tuning the system away from these commensurate fillings, checkerboard supersolid is generated near half-filling whereas a rare diagonal striped supersolid is realized near one-third filling. Interestingly, there is an asymmetry in the extent of supersolidity about one-third filling. Within our framework, we also provide an explanation for the observed checkerboard and stripe formations in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_4$ at $x=1/2$ and $x=1/3$.

In this work, we present an expression for the near-field thermal radiative transfer between two spheres with an arbitrary numbers of coatings. We numerically demonstrate that the spectrum of heat transfer between layered spheres exhibits novel features due to the newly introduced interfaces between coatings and cores. These features include broad super-Planckian peaks at nonresonant frequencies and near-field selective emission between metallic spheres with polar material coatings. Spheres with cores and coatings of two different polar materials are also shown to exceed the total conductance of homogeneous spheres in some cases.

We use a van der Waals pickup technique to fabricate different heterostructures containing ${\mathrm{WSe}}_2\left({\mathrm{WS}}_2\right)$ and graphene. The heterostructures were structured by plasma etching, contacted by one-dimensional edge contacts, and a top gate was deposited. For $\mathrm{graphene}/{\mathrm{WSe}}_2/{\mathrm{SiO}}_2$ samples we observe mobilities of $\sim 12000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Magnetic-field-dependent resistance measurements on these samples show a peak in the conductivity at low magnetic fields. This dip is attributed to the weak antilocalization (WAL) effect, stemming from spin-orbit coupling. Samples where graphene is encapsulated between ${\mathrm{WSe}}_2\left({\mathrm{WS}}_2\right)$ and hexagonal boron nitride show a much higher mobility of up to $\sim 120000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. However, in these samples no WAL peak can be observed. We attribute this to a transition from the diffusive to the quasiballistic regime. At low magnetic fields a resistance peak appears, which we ascribe to a size effect due to boundary scattering. Shubnikov–de Haas oscillations in fully encapsulated samples show all integer filling factors due to complete lifting of the spin and valley degeneracies.

The ferroelectric to paraelectric phase transition of multiferroic ${\mathrm{CaMnTi}}_2{\mathrm{O}}_6$ has been investigated at high pressures and ambient temperature by second-harmonic generation (SHG), Raman spectroscopy, and powder and single-crystal x-ray diffraction. We have found that ${\mathrm{CaMnTi}}_2{\mathrm{O}}_6$ undergoes a pressure-induced structural phase transition ($P{4}_{2}mc\to P{4}_2/nmc$) at $\sim 7\mathrm{GPa}$ to the same paraelectric structure found at ambient pressure and $T_c=630\mathrm{K}$. The continuous linear decrease of the SHG intensity that disappears at 7 GPa and the existence of a Raman active mode at 244 ${\mathrm{cm}}^{-1}$ that first softens up to 7 GPa and then hardens with pressure are used to discuss the nature of the phase transition of ${\mathrm{CaMnTi}}_2{\mathrm{O}}_6$, for which a $d{T}_c/dP=-48$ K/GPa has been found. Neither a volume contraction nor a change in the normalized pressure on the Eulerian strain is observed across the phase transition with all the unit-cell volume data following a second-order Birch-Murnaghan equation of state with a bulk modulus of $B_0=182.95\left(2\right)\mathrm{GPa}$.

High-throughput first-principles calculations based on density functional theory (DFT) are a powerful tool in data-oriented materials research. The choice of approximation to the exchange-correlation functional is crucial as it strongly affects the accuracy of DFT calculations. This study compares performance of seven approximations, six of which are based on Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) with and without Hubbard U and van der Waals corrections (PBE, PBE+U, PBED3, PBED3+U, PBEsol, and PBEsol+U), and the strongly constrained and appropriately normed (SCAN) meta-GGA on the energetics and crystal structure of elementary substances and binary oxides. For the latter, only those with closed-shell electronic structures are considered, examples of which include $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$, $\mathrm{A}{\mathrm{g}}_2\mathrm{O}$, MgO, ZnO, CdO, SnO, PbO, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$, $\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$, $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$, $\mathrm{Si}{\mathrm{O}}_2$, $\mathrm{Sn}{\mathrm{O}}_2$, $\mathrm{Pb}{\mathrm{O}}_2$, $\mathrm{Ti}{\mathrm{O}}_2$, $\mathrm{Zr}{\mathrm{O}}_2$, $\mathrm{Hf}{\mathrm{O}}_2$, ${\mathrm{V}}_2{\mathrm{O}}_5$, $\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5$, $\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$, $\mathrm{Mo}{\mathrm{O}}_3$, and $\mathrm{W}{\mathrm{O}}_3$. Prototype crystal structures are selected from the Inorganic Crystal Structure Database (ICSD) and cation substitution is used to make a set of existing and hypothetical oxides. Two indices are proposed to quantify the extent of lattice and internal coordinate relaxation during a calculation. The former is based on the second invariant and determinant of the transformation matrix of basis vectors from before relaxation to after relaxation, and the latter is derived from shifts of internal coordinates of atoms in the unit cell. PBED3, PBEsol, and SCAN reproduce experimental lattice parameters of elementary substances and oxides well with few outliers. Notably, PBEsol and SCAN predict the lattice parameters of low dimensional structures comparably well with PBED3, even though these two functionals do not explicitly treat van der Waals interactions. SCAN gives formation enthalpies and Gibbs free energies closest to experimental data, with mean errors (MEs) of 0.01 and −0.04 eV, respectively, and root-mean-square errors (RMSEs) are both 0.07 eV. In contrast, all GGAs including those with Hubbard U and van der Waals corrections give 0.1 to 0.2 eV MEs and at least 0.11 eV RMSEs. Phonon contributions of solid phases to the formation enthalpies and Gibbs free energies are estimated to be small at less than ∼0.1 eV/atom within the quasiharmonic approximation. The same crystal structure appears as the lowest energy polymorph with different approximations in most of the investigated binary oxides. However, there are some systems where the choice of approximation significantly affects energy differences between polymorphs, or even the order of stability between phases. SCAN is the most reasonable regarding relative energies between polymorphs. The calculated transition pressure between polymorphs of ZnO and $\mathrm{Sn}{\mathrm{O}}_2$ is closest to experimental values when PBED3, PBEsol (also PBED3+U and PBEsol+U for ZnO), and SCAN are employed. In summary, SCAN appears to be the best choice among the seven approximations based on the analysis of the energetics and crystal structure of binary oxides, while PBEsol is the best among the GGAs considered and shows a comparably good performance with SCAN for many cases. The use of PBEsol+U alongside PBEsol is also a reasonable choice, given that U corrections are required for several materials to qualitatively reproduce their electronic structures.

Starting from a Hamiltonian with anharmonic coupling between in-plane acoustic displacements and out-of-plane (flexural) modes, we derived coupled equations of motion for in-plane displacements correlations and flexural mode density fluctuations. Linear response theory and time-dependent thermal Green's functions techniques are applied in order to obtain different response functions. As external perturbations we allow for stresses and thermal heat sources. The displacement correlations are described by a Dyson equation where the flexural density distribution enters as an additional perturbation. The flexural density distribution satisfies a kinetic equation where the in-plane lattice displacements act as a perturbation. In the hydrodynamic limit this system of coupled equations is at the basis of a unified description of elastic and thermal phenomena, such as isothermal versus adiabatic sound motion and thermal conductivity versus second sound. The general theory is formulated in view of application to graphene, two-dimensional h-BN, and 2H-transition metal dichalcogenides and oxides.

We report the structural and magnetic properties of CoFeMnSi equiatomic quaternary Heusler alloy thin films. The epitaxial growth of the single-crystalline films with full $B2$ and partial $L{2}_1$ ordering on the Cr-buffered MgO(001) substrate was achieved using the in situ postannealing at temperature ($T_{\mathrm{a}}$) of $500-600{}^{\circ }\mathrm{C}$. A saturation magnetization value of about $3.5{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (where f.u. represents formula unit) was obtained at room temperature for the sample with $T_{\mathrm{a}}=600{}^{\circ }\mathrm{C}$, which is very close to the value of $3.7{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ reported previously for a bulk sample. Ferromagnetic resonance unveiled that negligible extrinsic relaxation due to magnetic softness as well as small intrinsic spin relaxation, i.e., the Gilbert damping constant of 0.005, which is smaller than that of permalloy. The spin magnetic moments for Co, Fe, and Mn atoms for the sample with $T_{\mathrm{a}}=600{}^{\circ }\mathrm{C}$ deduced from the x-ray magnetic circular dichroism were in agreement with those previously reported in the bulk sample, the effects of Fe-Mn swapping and Mn antisite disorder were discussed.

We have systematically studied the evolution of magnetic properties, especially the coercivity and the remanence ratio in the vicinity of the Verwey transition temperature $\left(T_V\right)$, of high-quality epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin films grown on MgO (001), ${\mathrm{MgAl}}_2{\mathrm{O}}_4$ (MAO) (001), and ${\mathrm{SrTiO}}_3$ (STO) (001) substrates. We observed rapid change of magnetization, coercivity, and remanence ratio at $T_V$, which are consistent with the behaviors of resistivity versus temperature $\left[\rho \right(T\left)\right]$ curves for the different thin films. In particular, we found quite different magnetic behaviors for the thin films on MgO from those on MAO and STO, in which the domain size and the strain state play very important roles. The coercivity is mainly determined by the domain size but the demagnetization process is mainly dependent on the strain state. Furthermore, we observed a reversal of remanence ratio at $T_V$ with thickness for the thin films grown on MgO: from a rapid enhancement for 40-nm- to a sharp drop for 200-nm-thick film, and the critical thickness is about 80 nm. Finally, we found an obvious hysteretic loop of coercivity (or remanence ratio) with temperature around $T_V$, corresponding to the hysteretic loop of the $\rho \left(T\right)$ curve, in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin film grown on MgO.

We performed a Raman scattering study of thin films of ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ spinel oxide superconductor. We detected four out of five Raman active modes, with frequencies in good accordance with our first-principles calculations. Three ${\mathrm{T}}_{2g}$ modes show a Fano lineshape from 5 K to 295 K, which suggests an electron-phonon coupling in ${\mathrm{LiTi}}_2{\mathrm{O}}_4$. Interestingly, the electron-phonon coupling shows an anomaly across the negative to positive magnetoresistance transition at 50 K, which may be due to the onset of other competing orders. The strength of the electron-phonon interaction estimated from the Allen's formula and the observed lineshape parameters suggest that the three ${\mathrm{T}}_{2g}$ modes at 0 momentum are not sufficient to drive superconductivity.