Search

During the last decade the cubic perovskite oxide EuTiO3 (ETO) has attracted enormous novel research activities due to possible multiferroicity, hidden magnetism far above its Néel temperature at TN = 5.5 K, structural instability at TS = 282 K, possible application as magneto-electric optic device, and strong spin–lattice coupling. Here we address a novel highlight of this compound by showing that well below TS a further structural phase transition occurs below 210 K without the application of an external magnetic field, and by questioning the assumed tetragonal symmetry of the structure below TS where tiny deviations from true tetragonality are observed by birefringence and XRD measurements. It is suggested that the competition in the second nearest neighbor spin–spin interaction modulated by the lattice dynamics is at the origin of these new observations.

Superconductivity was discovered in 1911 by Kamerlingh Onnes and Holst in mercury at the temperature of liquid helium (4.2 K). It took almost 50 years until in 1957 a microscopic theory of superconductivity, the so-called BCS theory, was developed. Since the discovery a number of superconducting materials were found with transition temperatures up to 23 K. A breakthrough in the field happened in 1986 when Bednorz and M\"uller discovered a new class of superconductors, the so-called cuprate high-temperature superconductors with transition temperatures as high as 135 K. This surprising discovery initiated new efforts with respect to fundamental physics, material science, and technological applications. In this brief review the basic physics of the conventional low-temperature superconductors as well as of the high-temperature superconductors are presented with a brief introduction to applications exemplified from high-power to low-power electronic devices. Finally, a short outlook and future challenges are presented, finished with possible imaginations for applications of room-temperature superconductivity., Comment: 13 pages, 15 figures

The basic features of multi-band superconductivity and its implications are derived. In particular, it is shown that enhancements of the superconducting transition temperature take place due to interband interactions. In addition, isotope effects differ substantially from the typical BCS scheme as soon as polaronic coupling effects are present. Special cases of the model are polaronic coupling in one band as realized e.g., in cuprates, coexistence of a flat band and a steep band like in MgB2, crossovers between extreme cases. The advantages of the multiband approach as compared to the single band BCS model are elucidated and its rather frequent realization in actual systems discussed, Comment: 16 pages, 9 figures

While it is known that the amplification of the superconducting critical temperature Tc is possible in a system of multiple electronic components in comparison with a single component system, many different road maps for room temperature superconductivity have been proposed for a variety of multicomponent scenarios. Here we focus on the scenario where the first electronic component is assumed to have a vanishing Fermi velocity corresponding to a case of the intermediate polaronic regime, and the second electronic component is in the weak coupling regime with standard high Fermi velocity using a mean field theory for multiband superconductivity. This roadmap is motivated by compelling experimental evidence for one component in the proximity of a Lifshitz transition in cuprates, diborides and iron based superconductors. By keeping a constant and small exchange interaction between the two electron fluids, we search for the optimum coupling strength in the electronic polaronic component which gives the largest amplification of the superconducting critical temperature in comparison with the case of a single electronic component., 13 pages, 5 figures

The oxygen isotope ($^{16}\mathrm{O}$/$^{18}\mathrm{O}$) effect (OIE) on the pseudogap (charge-stripe ordering) temperature ${T}^{*}$ is investigated for the cuprate superconductor ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ as a function of doping $x$ by means of x-ray absorption near edge structure studies. A strong $x$ dependent and sign reversed OIE on ${T}^{*}$ is observed. The OIE exponent ${\ensuremath{\alpha}}_{{T}^{*}}$ systematically decreases from ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}0.6(1.3)$ for $x=0.15$ to ${\ensuremath{\alpha}}_{{T}^{*}}=\ensuremath{-}4.4(1.1)$ for $x=0.06$, corresponding to increasing ${T}^{*}$ and decreasing superconducting transition temperature ${T}_{c}$. Both ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ exhibit a linear doping dependence with different slopes and critical end points [where ${T}^{*}(^{16}\mathrm{O})$ and ${T}^{*}(^{18}\mathrm{O})$ fall to zero] at ${x}_{c}(^{16}\mathrm{O})=0.201(4)$ and ${x}_{c}(^{18}\mathrm{O})=0.182(3)$, indicating a large positive OIE of ${x}_{c}$ with an exponent of ${\ensuremath{\alpha}}_{{x}_{c}}=0.84(22)$. The remarkably large and strongly doping dependent OIE on ${T}^{*}$ signals a substantial involvement of the lattice in the formation of the pseudogap, consistent with a polaronic approach to cuprate superconductivity and the vibronic character of its ground state.

It is known that in pressurized H2S the complex electronic structure in the energy range of 200 meV near the chemical potential can be separated into two electronic components, the first characterized by steep bands with a high Fermi velocity and the second by flat bands with a vanishing Fermi velocity. Also the phonon modes interacting with electrons at the Fermi energy can be separated into two components: hard modes with high energy around 150 meV and soft modes with energies around 60 meV. Therefore we discuss here a multiband scenario in the standard BCS approximation where the effective BCS coupling coefficient is in the range 0.1- 0.32. We consider a first (second) BCS condensate in the strong (weak) coupling regime 0.32 (0.15). We discuss different scenario segregated in different portions of the material. The results show the phenomenology of unconventional superconducting phases in this two-gap superconductivity scenario where there are two electronic components in two Fermi surface spots, the pairing is mediated by either by a soft or a hard phonon branch where the inter-band exchange term, also if small, plays a key role for the emergence of high temperature superconductivity in pressurized sulfur hydride., 11 pages, 4 figures

Magnetoelectric coupling phenomena in EuTiO3 are of considerable fundamental interest and are also understood to be key to reported multiferroic behavior in strained films, which exhibit distinctly different properties to the bulk. Here, the magnetoelastic coupling of EuTiO3 is investigated by resonant ultrasound spectroscopy with in situ applied magnetic field and stress as a function of temperature ranging from temperatures above the structural transition temperature Ts to below the antiferromagnetic ordering temperature Tn. One single crystal and two polycrystalline samples are investigated and compared to each other. Both paramagnetic and diamagnetic transducer carriers are used, allowing an examination of the effect of both stress and magnetic field on the behavior of the sample. The properties are reported in constant field/variable temperature and in constant temperature/variable field mode where substantial differences between both data sets are observed. In addition, elastic and magnetic poling at high fields and stresses at low temperature has been performed in order to trace the history dependence of the elastic constants. Four different temperature regions are identified, characterized by unusual elastic responses. The low-temperature phase diagram has been explored and found to exhibit rich complexity. The data evidence a considerable relaxation of elastic constants at high temperatures, but with little effect from magnetic field alone above 20 K, in addition to the known low-temperature coupling. NRF (Natl Research Foundation, S’pore) Published version

On the basis of first principles density functional calculations, we investigated the effect of spin flip and hole doping on the spin, charge and lattice degrees of freedom in the square planar CuO2 layers of the high-Tc cuprate superconductors. The local responses of the CuO2 layers to these point defects appear as spatially extended spin-charge-lattice coupled polarons in ferromagnetic clusters and hence introduce heterogeneity. The results demonstrate the need to treat the spin, charge and lattice degrees of freedom on an equal footing in describing the high-Tc superconductivity.

Strontium titanate is an incipient ferroelectric where the freezing of the soft polar mode is inhibited by quantum fluctuations (Muller K. A. and Burhard H., Phys. Rev. B, 19 (1979) 3593). Exchanging 16O by its isotope 18O induces ferroelectricity at Tc = 25 K (Itoh M., Wang R., Inaguma Y., Yamaguchi T., Shan Y. J. and Nakamura T., Phys. Rev. Lett., 82 (1999) 3540). This finding is unexpected and its origin remains unclear since seemingly contradictory data have been obtained. Here we suggest that SrTi18O3 (STO18) exhibits self-induced intrinsic inhomogeneity due to optic–acoustic mode coupling, whereby dynamical clusters are formed with a lower symmetry than the embedding matrix. With decreasing temperature these clusters grow in size and volume fraction and remain present in the ferroelectric state leading to an incomplete ferroelectric ground state. The remaining lattice shows soft-mode behaviour. Since the length and time scales of the coexisting components differ considerably, experiments which test these scales can reach contradictory conclusions concerning the phase transition mechanism.

We present a non-perturbative study of an extended Falicov-Kimball model in one dimension. Working within the binary alloy interpretation, we include the spin of the itinerant electrons and a Hubbard interaction to model the inter-electron correlations. We derive an effective Ising model for the atomic configuration in order to show how the Hubbard term affects the stability of the phase separated states. Furthermore, we investigate the competition between the Mott insulator state of the itinerant electrons and the checkerboard phase of the spinless Falicov-Kimball model.

Superconductivity in the heavy-fermion compound CeCoIn5 has recently been found to be due to multiple-band effects (Phys. Rev. Lett., 94 (2005) 107005) with a distinct two-gap structure. It is shown here that the enormously enhanced values of the gap ratios with respect to Tc have an unusual origin with the electron dynamics mainly governed by f-p hybridization.

Unconventional isotope effects (Phys. Rev. Lett., 92 (2004) 057602) as observed in high-temperature superconducting cuprates (HTSC), are explained by polaron formation which leads to a renormalization of the single-particle energies and the formation of density-density interactions. It is shown that the dominant contribution to these effects stems from the coupling to the quadrupolar Q2-type phonon mode.

Two-component or two-channel superconductivity has been proposed since long to be a source of raising the superconducting transition temperature T c considerably. The first observation of two-component superconductivity was made in Nb-doped SrTiO 3 , yet without the hope for T c enhancement. An unambiguous observation of two superconducting gaps has been made in the recently discovered high- T c compound MgB 2 where the two component interband interaction amplifies T c by a factor of 1.6 as compared to a single band system. Another candidate for such a scenario has been speculated to be verified in the cuprate superconductors, but the overwhelming experimental evidence for a d-wave order parameter in the CuO 2 planes has largely suppressed this aspect. Here it is shown that the coexistence of an s-wave and d-wave order parameter in cuprates is a scenario in which the observed high T c s can be realized. The crucial quantity is here the pairwise exchange interaction of holes between the two channels caused by quantum interference. Lattice effects on superconductivity are explicitly incorporated and shown to be of major relevance to superconductivity including various exotic isotope effects.

We suggest that ferroelectric relaxor glasses represent an important class of materials in which intrinsic spatial heterogeneity and multiscale dynamics can arise from the formation of local modes due to inherent nonlinearity in the polarizable medium. Specifically, the phenomenology of relaxor ferroelectrics in terms of the spherical random bond–random field (Blinc et al 1999 Phys. Rev. Lett. 83 424) model is explained microscopically by the formation of discrete breathers embedded in a soft but silent medium, which form in-gap local modes (IGLM) where charge and lattice are intrinsically coupled. Complete mode softening is inhibited by the IGLM and soft elasticity a prerequisite for their existence.

The magnetic properties of a system of coexisting localized spins and conduction electrons are investigated within an extended version of the one dimensional Kondo lattice model in which effects stemming from the electron-lattice and on-site Coulomb interactions are explicitly included. After bosonizing the conduction electrons, is it observed that intrinsic inhomogeneities with the statistical scaling properties of a Griffiths phase appear, and determine the spin structure of the localized impurities. The appearance of the inhomogeneities is enhanced by appropriate phonons and acts destructively on the spin ordering. The inhomogeneities appear on well defined length scales, can be compared to the formation of intrinsic mesoscopic metastable patterns which are found in two-fluid systems., Comment: 9 pages, to appear in Jour. Superconductivity

The two component model of coexisting local electron pairs and itinerant fermions coupled via charge exchange mechanism, which mutually induces superconductivity in both subsystems, is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s and d_{x^2-y^2} -wave symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. We determined the phase diagrams and superconducting characteristics as a function of the position of the local pair (LP) level and the total electron concentration. The model exhibits several types of interesting crossovers from BCS like behavior to that of LP's. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for exotic superconductors., Proceedings of the 3rd Polish-US Workshop on Magnetism and Superconductivity of Advanced Materials, July 14-19, 2002, Ladek Zdroj (Poland) to appear in Physica C

We study a model of a multiband Fermi-surface structure to investigate its effect on the superconducting transition temperature in the limit of a high number of bands. We consider a simple limit consisting of an infinite number of identical locally pairwise coupled bands with intraband and interband hopping and a multiband generalized Bardeen-Cooper-Schrieffer Hamiltonian. The self-consistent mean-field system of equations which determines the intraband and interband order parameters decouples to two independent equations, unless the interband hopping integral is non-zero, in which case an energetically stable superconducting phase appears, where both the intraband and the interband gaps are non-zero. We demonstrate that for all values of the interband coupling constant the critical transition temperature is enhanced compared with the pure intraband critical transition temperature. The model is equivalent to a multiple momentum exchange originating from the interband coupling and thus modelling a...

A variety of different experimental results show substantial evidence that the order parameter in high-temperature superconducting copper oxides is not of pure d-wave symmetry, but that an s-wave component exists, which especially shows up in experiments that test the c-axis properties. These findings are modeled theoretically within a two-band model with interband interactions, where the superconducting order parameters in the two bands are allowed to differ in symmetry. It is found that the coupling of order parameters with different symmetries (s+d) leads to substantial enhancements of the superconducting transition temperature Tc as compared to order parameters with only s-wave symmetry. An additional enhancement factor of Tc is obtained from the coupling of the bands to the lattice where moderate couplings favor superconductivity while too strong couplings lead to electron (hole) localization and consequently suppress superconductivity.

EuTiO_3, which is a G-type antiferromagnet below T_N = 5.5 K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substantiated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation (${\mu}$SR) experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field., Comment: 5 pages, 6 figures

Despite the general belief that hydrogen-bonded ferro- and antiferroelectrics undergo a pure order/disorder transition at the structural instability, new NMR data and a new theoretical concept yield convincing evidence that a pronounced displacive component is present in these systems, which modifies substantially the temperature dependencies of the tunnel and lattice mode frequencies. The experiments and their interpretation are presented. (C) 1999 Elsevier Science Ltd. All rights reserved.

The phase diagram of high-Tc superconducting copper oxides (HTSC) exhibits an unusual complexity which is, at present, beyond any microscopic modeling. Motivated by very recent experimental findings—(i) a giant isotope effect on T* and (ii) doping-dependent phonon anomalies and “extra” modes—the role played by the lattice is reinvestigated and the consequences arising for the striped phase are discussed. By including nonlinear higher-order phonon–phonon and density–density interactions in the Hamiltonian, an isotope effect on T* is obtained, as well as substantial q-dependent phonon anomalies, where both effects are strongly doping dependent.

From various theoretical approaches it seems to be rather well established that the Zhang–Rice (ZR) singlet state should play an important role in doped high-temperature superconducting (HTSC) oxides. Yet the conclusion that a one-band model is capable of describing many of the unusual physics observed in HTSC compounds has been questioned because the pz states of the oxygen atoms in the planes as well as those of the apical oxygen atoms hybridize with the Cu d3z2-r2 orbitals and induce a mixing of the in-plane states with the out-of-plane bands. It is shown that electron–lattice coupling and polarizability effects modify the electronic structure of the effective Hamiltonian considered here.

Perovskite oxides show various structural instabilities which are related to ferroelectricity, antiferroelectricity, Jahn-Teller effects, ferromagnetism, charge- and spin-density instabilities. The electronic, spin and lattice instabilities are described within an anharmonic lattice model extended by electron (spin) phonon interactions and higher-order multiphonon density–density interactions. The model yields within the self-consistent phonon approximation (SPA) mode softening at arbitrary q-values, charge and spin ordering. Exact solutions of the nonlinear equations of motion have been obtained numerically and show highly anomalous q-dependences of the ionic displacements. In real space correlated dynamic displacement patterns are obtained which are mostly related to the oxygen ions which define new length and time scales. The results are related to recent local structure studies.

A structure specific four-band model Hamiltonian is used to investigate the various doping dependences of the superconducting temperature ${\mathrm{T}}_{\mathrm{c}}$ and the related isotope effect for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (YBCO). The model considers explicitly multiphonon-mediated attractive electron (hole) pairing which acts intra- and interband and yields a complicated three-dimensional gap structure. The doping dependence of ${\mathrm{T}}_{\mathrm{c}}$ is calculated by varying the band specific density of states, and from this the corresponding isotope effect is evaluated. Consistent agreement with experimental results is obtained and predictions with respect to site selective doping (including the isotope effect) are given.

The exact solutions of a one-dimensional nonlinear electron-photon interaction model are studied as a function of time and temperature in order to evaluate the particle displacement pattern. It is found that order-disorder effects always dominate the dynamics in the vicinity of T c , even if well resolved soft mode behaviour is observed above T c . The crucial role of the phase transition mechanism is clearly played by the mass ratio of the respective sublattices, which govern the dynamics and phase transition mechanism.

Exact solutions of a nonlinear diatomic shell model are investigated in the regime where quasiharmonic and pseudoperiodic traveling waves exist with phonon-type character. The existence regimes of these solutions are determined by the boundary conditions and the model parameters for which especially a strong time and mass dependence is observed. It is found that slowly propagating waves mostly show large displacement responses which can be associated with large dipole moments, while rapidly traveling waves carry a much smaller dipole moment, but still this is appreciably larger than that induced by bare optic-phonon modes. The case of large anharmonicity shows the opposite effect. Here high-frequency responses carry a large dipole moment. The large dipole moments can be associated with effective charges which induce high oscillator strengths in the corresponding phonon modes, incompatible with results deduced from harmonic lattice dynamics. The origin of the large dipole moments in the shell model is investigated by solving for core and shell displacements separately, where ``acoustic-type'' periodic in-phase displacements of core and shell with different amplitudes are observed, as well as pseudoperiodic out-of-phase ``optic-type'' displacements resulting from large anharmonicity, and are also found in the static limit. Besides the displacement frequency spectrum, the effective potentials are calculated which are distinctly different from ${\mathrm{\ensuremath{\varphi}}}_{4}$-type potentials: The potential height is finite with finite width which, in certain cases becomes very small, thus admitting for tunneling through the barrier. \textcopyright{} 1996 The American Physical Society.

The observation of various structural anomalies correlated with the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ provide a motivation to investigate anharmonic phonon-phonon and electron-phonon interactions, and their effect on normal state and superconducting properties. We specifically investigate c-axis-related anharmonic lattice fluctuations coupled to the electron density, which provide a charge transfer and interplanar coupling mechanism. Due to the strongly nonlinear phonon fluctuations, the effective interplanar electron (hole) hopping integral becomes temperature dependent and leads to (bi-)polaron formation at high temperatures, followed by a spin density wave instability with decreasing T, and finally interplane pair tunneling in the superconducting state. The c-axis-related infrared absorption is calculated and it is shown that due to anharmonic effects strong increases in the oscillator strengths, line shape asymmetries and broadening effects are obtained consistent with experimental data.

In this review we consider three classes of superconductors, namely cuprate superconductors, MgB(2) and the new Fe based superconductors. All of these three systems are layered materials and multiband compounds. Their pairing mechanisms are under discussion with the exception of MgB(2), which is widely accepted to be a 'conventional' electron-phonon interaction mediated superconductor, but extending the Bardeen-Cooper-Schrieffer (BCS) theory to account for multiband effects. Cuprates and Fe based superconductors have higher superconducting transition temperatures and more complex structures. Superconductivity is doping dependent in these material classes unlike in MgB(2) which, as a pure compound, has the highest values of T(c) and a rapid suppression of superconductivity with doping takes place. In all three material classes isotope effects have been observed, including exotic ones in the cuprates, and controversial ones in the Fe based materials. Before the area of high-temperature superconductivity, isotope effects on T(c) were the signature for phonon mediated superconductivity-even when deviations from the BCS value to smaller values were observed. Since the discovery of high T(c) materials this is no longer evident since competing mechanisms might exist and other mediating pairing interactions are discussed which are of purely electronic origin. In this work we will compare the three different material classes and especially discuss the experimentally observed isotope effects of all three systems and present a rather general analysis of them. Furthermore, we will concentrate on multiband signatures which are not generally accepted in cuprates even though they are manifest in various experiments, the evidence for those in MgB(2), and indications for them in the Fe based compounds. Mostly we will consider experimental data, but when possible also discuss theoretical models which are suited to explain the data.

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

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

The lattice instability versus a transverse optic mode observed in ferroelectrics is a direct consequence of a crucial electron-phonon interaction consisting of an attractive harmonic term and a stabilizing anharmonic repulsive fourth-order term. In conventional superconductors BCS-theory has proven that the electron-(hole)-pairing mechanism is phonon mediated. For high-Tc superconductors no consensus is yet achieved in explaining this phenomenon, but it is argued in the following that electron-phonon interactions play an important role here too if the complexity of the structure and Fermi surface are seriously considered. Opposite to ferroelectrics it is found that the attractive harmonic terms need no repulsive counterpart as the effective interactions are small. This finding leads to the immediate conclusion that in spite of various common features of ferroelectrics and superconductors, both are mutually exclusive phenomena.

A unified approach to superconductivity is presented using natural extensions of weak coupling BCS theory. Multiphonon-multiband interactions cause a multigap opening atTc, with cooperative pairing in all bands incorporated via interband interactions. Strong increases inTc are found due to interband-induced gap coupling. Model calculations for the experimentally observed gap distribution are presented.

Gap coupling interaction is a necessary consequence if a superconducting system has a gap distribution, as found for the high-T c oxocuprates, since the interaction has to result in a single critical temperatureT c . This paper shows with the help of BCS gap-coupling equations and values of gap distribution, including gap weights from experiments, thatT c values as observed are easily achievable with reasonable diagonal and off-diagonal coupling coefficients. Of course our phenomenologically derived coupling coefficients have to be verified from purely theoretical grounds. Most importantly, it is found that the highest gap contributes dominantly toT c in spite of its low weight. Predictions for a point-contact tunneling spectroscopy are made.

Band crossing and hybridization effects on superconductivity are investigated within a weak coupling limit of an extended BCS theory. Two-phonon modiated attractive two-band (p−d) interactions induce anisotropic coupled energy gaps, enhancement of the transition temperatureTc, and a material-dependent isotope effect. The individual gap tokBTc ratios deviate from the usual BCS value but their average recovers approximately the usual value. Effects on ultrasonic attenuation and coherence lengths are briefly discussed.

Ferroelectricity occurs when a compensation of short range attractive and long range repulsive forces is present. This leads to a lattice instability frequently accompanied by the freezing of a transverse optic polar mode at q =0. A general understanding of ferroelectricity is based on the idea that a local double well potential exists, and this triggers the lattice instability. The objective in this approach is to propose a purely ionic concept which yields a rough qualitative agreement with experimental data. It will be shown that subtle electron–phonon interaction effects play a major role in establishing the ferroelectric phase. Specifically it is found that the temperature dependence of the soft mode shows different regimes which are in addition characterized by different critical exponents.

By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multi gap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similar to MgB2 and in contrast to cuprates the two superconducting gaps in the Fe-As based materials are isotropic s-wave gaps., Comment: 8 pages, 3 figures, 1 table

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

A variety of local structural probes have demonstrated that local lattice distortions take place in cuprates, which correlate with the onset of the pseudogap phase (PG) and superconductivity (SC). We show here that these lattice responses can be a consequence of polaron formation, local coherence in the pseudogap phase, and global coherence in the superconducting phase. In addition, we demonstrate that the results are consistent with a complex s+d wave order parameter in the SC phase.

Structural instabilities and especially those which are related to polar instabilities and ferroelectric phase transitions are investigated within a modified φ4-model. Instead of describing these types of phase transitions within a rigid ion model with anharmonic ion-ion interactions the electronic degrees of freedom are seriously taken into account and shown to renormalize the temperature dependence of the soft mode frequency. Using a self-consistent phonon approximation quantitative agreement of theoretical calculations with experimental data is achieved for the temperature dependences of soft modes and related quantities.