We consider ferroelectric compounds created by relative ionic displacements between two or more sublattices (``displacive'' phase transitions). For these materials the nonbonding p orbitals of oxygen ${(0}^{2\mathrm{\ensuremath{-}}}$) chalcogen (${\mathrm{S}}^{2\mathrm{\ensuremath{-}}}$, etc.) ions play a central role, which is taken into account in dynamical theory. It turns out that fourth-order polarizability main mechanism responsible for specific properties such as phonon dispersion...

The in-plane magnetic field penetration depth (\lambda_{ab}) in single-crystal La_{1.83}Sr_{0.17}CuO_4 was investigated by means of the muon-spin rotation (\muSR) technique. temperature dependence \lambda^{-2}_{ab} has an inflection point around 10-15K, suggesting presence two superconducting gaps: a large gap (\Delta_1^d) with d-wave and small (\Delta_2^s) s-wave symmetry. zero-temperature values gaps at \mu_0H=0.02T were found to be \Delta_1^d(0)=8.2(2)meV \Delta_2^s(0)=1.57(8)meV.

The systematic analysis of high-resolution NMR data, bond lengths, and isotropic ${}^{13}\mathrm{C}$ chemical shifts crystals squaric acid ( ${\mathrm{H}}_{2}\mathrm{SQ}$) its deuterated compound ${\mathrm{D}}_{2}\mathrm{SQ}$) yields convincing evidence that order-disorder displacive features coexist at the antiferroelectric phase transition temperature. experimental data are analyzed within a coupled pseudospin-lattice model which combines dynamics consistently describes huge isotope effect...

The discovery of high-temperature superconductivity in copper oxides was not accidental, but based on the knowledge that divalent ion, Cu2+, is one strongest Jahn–Teller ions. effect a consequence interplay between electronic degeneracy and coupling to lattice, i.e. unconventional local electron–lattice interactions. search for motivated by idea polaron formation could be novel much stronger glue electron pairing than conventional Bardeen–Cooper–Schrieffer electron–phonon coupling....

The dynamical properties of ferroelectric perovskites have typically been classified as being either order-disorder or displacive type. Here we show that this simple scheme is not generally applicable but the generic and intrinsic material class always combine both aspects. Important in respect fact different length time scales are associated with two which makes it difficult to observe them a single experiment. In addition, shown independent transition temperature respective system...

We describe an extension of a lattice dynamical model for displacive-type ferroelectrics which might be applied to high-temperature superconductors. The is based on the instability oxygen ion ${\mathrm{O}}^{2\ensuremath{-}}$, dynamically described by on-site nonlinear electron-ion coupling, and valence cation [e.g., Cu(I), Cu(III)], represented electron-electron interaction provides pairwise coupling electrons. equivalent quantum-mechanical Hamiltonian describes extended Peierls-Hubbard...

Specific heat measurements and theoretical calculations reveal an intimate analogy between EuTiO${}_{3}$ SrTiO${}_{3}$. For EuTiO${}_{3}$, a hitherto unknown specific anomaly is discovered at temperatures ${T}_{A}=$ 282(1)K, which analogous to the well-known of SrTiO${}_{3}$ temperature 105K caused by antiferrodistortive transition. Because zone center soft phonon mode observed in both systems can be modeled with same parameters, we ascribe new 282(1)K instability phase The higher transition...

Using a hybrid Hartree–Fock (HF)-DFT method combined with LCAO basis set and periodic supercell approach, the atomic, electronic structure phonon properties of oxygen vacancies in ZnO SrTiO3 were calculated compared. The important role ghost function centered at vacant site defect spin state for is discussed. It shown that use functionals vital correct reproduction defects basic properties. Gibbs free energy formation their considerable temperature dependence has been compared two oxides....

Exact periodic solutions are found for three-dimensional lattices with fourth-order on-site electron-ion potentials. Numerical results presented ferroelectric SnTe. The coupling of these nonlinear waves phonons is investigated phase transitions finite critical wave vectors. For ${\mathrm{K}}_{2}$${\mathrm{SeO}}_{4}$ both the paraelectric-incommensurate and incommensurate-commensurate analyzed. cubic power force leads to a lock-in transition at vector $\frac{2\ensuremath{\pi}}{3}$.

The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal YBa_{2}Cu_{3}O_{7-delta} along three principal crystallographic directions (a, b, and c). Both in-plane components (lambda_{a};{-2} lambda_{b};{-2}) show inflection point in their which is absent component c direction (lambda_{c};{-2}). data provide convincing evidence that superconducting order parameter a mixture (s+d)-wave symmetry whereas it mainly s wave direction. In...

We apply the theory of ferroelectricity in framework polarizability model, which has been presented a previous paper, to various classes displacive-type ferroelectrics. show that experimental data temperature-dependent quantities like soft-mode frequencies and dielectric constants can be well reproduced using very limited set parameters. present results for AB${\mathrm{O}}_{3}$ perovskites, IV-VI compounds, SbSI, ${\mathrm{K}}_{2}$${\mathrm{SeO}}_{4}$, hydrogen-bonded ferroelectrics,...

A central issue in the physics of relaxor ferroelectrics is origin formation polar nanoregions below some characteristic temperature scale. While it often attributed to chemical disorder, random bond--random field appearance, or local symmetry lowering, shown here that huge intrinsic nonlinearity gives rise spatially limited excitations discrete breather (DB) type, which interact strongly and self-consistently with remaining lattice. This scenario corresponds a two-component approach...

Single crystals of PbZrO${}_{3}$ have been studied by Brillouin light scattering, birefringence, and piezoelectric coefficient measurements. The structural phase transition at ${T}_{c}$ is shown to be related the softening a longitudinal acoustic (LA) mode, whereas precursor effects are deduced from central peak in scattering spectrum, finite above instability. An intermediate vicinity apparent where coexistence LA mode with high frequency appears, addition transverse (TA) becomes visible.

Taking into account the bilinear coupling between tunneling protons and displacements of electron shells surrounding ${\mathrm{PO}}_{4}$ groups, we present a new model hydrogen-bonded ferroelectrics. The is an extension both nonlinear polarizability includes geometrical aspects hydrogen bond. It leads to structural phase transition describes isotope effect due substitution $\mathrm{H}/\mathrm{D}$ in ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ pressure dependence.

A comparative analysis of the structural phase transitions EuTiO${}_{3}$ and SrTiO${}_{3}$ (at ${T}_{\mathrm{S}}$ $=$ 282 105 K, respectively) is made on basis phonon-dispersion density functional calculations. The transition predicted to arise from softening a transverse acoustic zone boundary mode caused by rotations TiO${}_{6}$ octahedra, as also found for SrTiO${}_{3}$. Although temperature dependence soft similar in both compounds, their elastic properties differ drastically because...

We have recently predicted and subsequently verified experimentally by specific heat measurements that EuTiO3 undergoes a structural phase transition at elevated temperature T S = 282 K. The origin of the has been attributed to softening transverse acoustic mode stemming from an oxygen octahedral rotation analogous SrTiO3. Here we demonstrate theoretical interpretation is correct using high-resolution laboratory X-ray powder diffraction which evidences cubic tetragonal in EuTiO3. room...

Abstract At ultrahigh pressure (>110 GPa), H 2 S is converted into a metallic phase that becomes superconducting with record T c of approximately 200 K. It has been proposed the body‐centered cubic 3 (Im m, a=3.089 Å) resulting from decomposition reaction S→2 S+S. The analogy between and O led us to very different conclusion. well‐known dissociation water + OH − increases by orders magnitude under pressure. anticipated behave similarly pressure, process S→H +SH leading perovskite...

The model of two (\ensuremath{\sigma} and \ensuremath{\pi}) channel superconductivity known to be necessary explain the in ${\mathrm{MgB}}_{2}$ has been applied ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{B}}_{2}$ diborides by tuning x from ${\mathrm{AlMgB}}_{4}.$ interband coupling parameter (probing strength interchannel pairing due quantum interference effects) gaps \ensuremath{\sigma} \ensuremath{\pi} have calculated as a function using experimental data for phonon mode...

The dynamics of a two-dimensional shell model with double-well potential in the core-shell interaction is simulated by means molecular-dynamics techniques. Snapshots lattice displacement pattern reveal presence precursor regions high-temperature phase, while low-temperature phase dynamical structure factor exhibits quasielastic component addition to ferroelectric soft-mode peak. On other hand, local particles are characterized two different time scales corresponding coexistence interwell and...

The long-standing classification scheme of ferroelectrics into either relaxor or displacive ones (the phase transition is driven by a soft phonon mode) too restrictive since smooth crossover between them exists which even admits for coexistence both phenomena. This and consequence the varying density polar nanoregions due to different doping levels respective system. formation attributed here intrinsic local modes in terms discrete breathers.

To better understand the phase transition mechanism of PbZrO3 (PZO), lattice dynamics this antiferroelectric compound are investigated within polarizability model, with emphasis on cubic to orthorhombic transition. Similarly ferroelectric transitions in ABO3 perovskites, polar dynamical clusters develop and grow size upon approaching TC from high temperature side never form a homogeneous state. Simultaneously, elastic anomalies set compete cluster dynamics. These unusual responsible for...

Magnetoelectric coupling phenomena in ${\mathrm{EuTiO}}_{3}$ are of considerable fundamental interest and also understood to be key reported multiferroic behavior strained films, which exhibit distinctly different properties the bulk. Here, magnetoelastic is investigated by resonant ultrasound spectroscopy with situ applied magnetic field stress as a function temperature ranging from temperatures above structural transition $T{}_{s}$ below antiferromagnetic ordering $T{}_{n}$. One single...