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In this paper, the development in the field of ferroelectrics is reviewed by summarizing the topics in the field as presented at the International and European conferences on Ferroelectrics. While ...

We demonstrate how ordered arrangements of oxygen vacancies can be engineered during the growth of superconducting La2CuO4 films by oxide molecular-beam epitaxy. These arrangements are seen using in situ reflection high-energy electron diffraction. Based on qualitative real-time observations, we propose a surface reconstruction mechanism emphasizing the active role of dopants and oxygen vacancies at the film surface. Due to the specific atomic arrangement induced by dopant positions, characteristic surface “stripes” are generated, and they determine the intrinsically heterogeneous structure characterized by distorted checkerboard patterns on the surface. Not only can the surface motif during growth be monitored via characteristic surface reconstructions, but it can also be customized by altering strain, doping, and oxygen activity.

Polycrystalline ceramic samples and a single crystal of EuTiO3 have been investigated by Raman spectroscopy in the temperature range 80-300 K. Although synchrotron XRD data clearly indicated the cubic to tetragonal phase transition around 282 K, no mode from the symmetry allowed Raman active phonons was found in the tetragonal phase, contrary to the case of the homologous SrTiO3. In order to study the evolution of this unique characteristic, ceramics of EuxSr1-xTiO3 (x=0.03-1.0) characterized by synchrotron XRD for the structural phase transition have been also investigated by Raman spectroscopy, verifying the very strong influence on the Raman yield by Eu substitution. By applying an external magnetic field or alternatively hydrostatic pressure modes are activated in the Raman spectra. The temperature dependence of the main mode that is activated shows remarkable agreement with theoretical predictions. We attribute the puzzling absence of the Raman modes to a mechanism related to strong spin-lattice interaction that drives the cubic to tetragonal structural phase transition and makes the Raman tensor antisymmetric. On the contrary, the external perturbations induce a symmetric Raman tensor allowing even symmetry modes to be present in the spectra. Previous EPR, muon scattering and magnetic measurements indicated the presence of small magnetic interactions deep inside the paramagnetic phase. In order to probe those magnetic interactions in our EuTiO3 polycrystalline sample and test our hypothesis, we have performed temperature dependant XAS/XMCD, which support the existence of magnetic nanodomains even close to room temperature.

Influence of heterovalent Nb5+ doping of antiferroelectric PbZrO3 single crystal has been investigated. This kind of doping, producing defects in the perovskite matrix, influences the lattice instabilities responsible for phase transition at TC, and for inducing pre- and post-transitional effects. The dielectric, electromechanical, and optical properties were investigated. Opposite to their ceramic analogues, the investigated single crystals show a new phase transition and enlarged temperature ranges of lattice instabilities.

The influence of an external static magnetic field (up to 480 mT) on the structural properties of EuTiO3 (ETO) polycrystalline samples was examined by powder XRD at the Elettra synchrotron facilities in the temperature range 100–300 K. While the cubic to tetragonal structural phase transition temperature in this magnetic field range remains almost unaffected, significant lattice effects appear at two characteristic temperatures (∼200 K and ∼250 K), which become more pronounced at a critical threshold field. At ∼200 K a change in the sign of magnetostriction is detected attributed to a modification of the local magnetic properties from intrinsic ferromagnetism to intrinsic antiferromagnetism. These data are a clear indication that strong spin–lattice interactions govern also the high temperature phase of ETO and trigger the appearance of magnetic domain formation and phase transitions.

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

The almost multiferroic perovskite EuTiO3 (ETO) has been prepared as films on substrates of SrTiO3. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at TN = 5.1 K and a structural transition at TS = 282 K. The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5 eV and larger than observed before for any bulk and thin film samples.

The measurements of Sr K-edge XAFS were performed under the approval of Proposal No. 97G042 of Photon Factory (KEK) and partially supported by the Research Grants of Hirosaki University. This work was supported by Bruce Ravel providing data for BTO. Boby Joseph acknowledges IISc Bangalore and ICTP Trieste for financial support through the award of the IISc-ICTP fellowship., The ferroelectric distortions in perovskites were a subject of numerous investigations for a long time. However, some controversial results still exist, coming from the analysis of diffraction (X-ray, neutron or electron) data and X-ray absorption spectra. In this study, our goal was to revisit these classical materials using recently developed methods without imposing any predefined structural model. Local environment around A-type atom in ABO3 perovskites (SrTiO3, BaTiO3, EuTiO3) was studied by X-ray absorption spectroscopy (XAS) in a wide range of temperatures (20–400 K). Using reverse Monte Carlo method enhanced by evolutionary algorithm, the 3D structure was extracted from the extended X-ray absorption fine structure (EXAFS) and interpreted in terms of the radial distribution functions (RDFs). Our findings show that both diffraction and XAS data are consistent, but reflect the structure of the material from different points of view. In particular, when strong correlations in the motion of certain atoms are present, the information obtained by XAS might lead to a different from expected shape of the RDF. At the same time, the average positions of all atoms are in good agreement with those given by diffraction. This makes XAS an important technique for studying interatomic correlations and lattice dynamics., Abdus Salam International Centre for Theoretical Physics; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

We analyzed the publication output of one of the 1987 Nobel Prize awardees, K. Alex Müller, using bibliometric methods. The time-dependent number of publications and citations and the network with respect to the coauthors and their affiliations was studied. Specifically, the citation history of the Nobel Prize awarded 1986 article on “Possible high-temperature superconductivity in the Ba-La-Cu-O system” has been evaluated in terms of the overall number of articles on superconductivity and the corresponding citations of other most frequently referenced articles. Thereby, a publication with “delayed recognition” was identified.

The problem of intermediate phase in pure single PbZrO3 crystal, appearing below the main transformation from the paraelectric to antiferroelectric state, was investigated and analysed based on changes in the birefringence, secondary ion mass spectroscopy, and x-ray photoelectron spectroscopy. Controlled changes in stoichiometry of the crystal were achieved by thermal treatment of the crystal in ultra-high vacuum and at high temperatures, thereby creating defects in all sublattices which influence the appearance of this intermediate phase. The results obtained have allowed correlating changes in the distribution of individual elements in surface and crystal volume, as well as their concentration and their electron states, with changes in the intermediate phase temperature range.

H2S is converted under ultrahigh pressure (> 110 GPa) to a metallic phase that becomes superconducting with a record Tc of 200 K. It has been proposed that the superconducting phase is body-centered cubic H3S ( Im3m , a = 3.089 ��) resulting from a decomposition reaction 3H2S --> 2H3S + S. The analogy of H2S and H2O leads us to a very different conclusion. The well-known dissociation of water into H3O+ and OH- increases by orders of magnitude under pressure. An equivalent behavior of H2S is anticipated under pressure with the dissociation, 2H2S --> H3S+ + SH- forming a perovskite structure (SH-)(H3S+), which consists of corner-sharing SH6 octahedra with SH- at each A-site (i.e., the center of each S8 cube). Our DFT calculations show that the perovskite (SH-)(H3S+) is thermodynamically more stable than the Im3m structure of H3S, and suggest that the A-site H atoms are most likely fluxional even at Tc., 17 pages, 6 figures, 5 tables The mistakes for the enthalpy and the free energy changes for Eq. 1 (namely, half the energy of H2 was used instead of the full energy of H2) was corrected. Our corrections of this error plus other discussion were given in a corrigendum instead of fixing the text. We added a corrigendum

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.

The temperature dependence of the elastic properties of antiferroelectric PbHfO3 was investigated by Brillouin scattering. The two structural phase transitions of antiferroelectric-antiferroelectric-paraelectric phases were clearly identified by discontinuous changes in the acoustic mode frequencies and the hypersonic damping. The substantial softening of the mode frequency along with the remarkable increase in the acoustic damping observed in the paraelectric phase indicated the formation of precursor noncentrosymmetric (polar) clusters and their coupling to the acoustic waves. This was corroborated by the observation of quasi-elastic central peaks, the intensity of which grew upon cooling toward the Curie point. The obtained relaxation time exhibited a slowing-down behavior, suggesting that the dynamics of precursor clusters becomes more sluggish on approaching the phase transition temperature., Comment: 16 pages, 5 figures

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

The magneto-optical activity of high quality transparent thin films of insulating EuTiO3 (ETO) deposited on a thin SrTiO3 (STO) substrate, both being non-magnetic materials, are demonstrated to be a versatile tool for light modulation. The operating temperature is close to room temperature and allows for multiple device engineering. By using small magnetic fields birefringence of the samples can be switched off and on. Similarly, rotation of the sample in the field can modify its birefringence Δn. In addition, Δn can be increased by a factor of 4 in very modest fields with simultaneously enhancing the operating temperature by almost 100 K.

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

The recent report of superconductivity under high pressure at the record transition temperature of Tc=203K in sulfur hydride has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms can neither account for the observed values of Tc nor the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high values of Tc together with the anomalous pressure dependent isotope effect. In addition, it is shown that anharmonicity of the hydrogen bonds vanishes under pressure whereas anharmonic phonon modes related to sulfur are still active, Comment: 11 pages, 5 figures

The structural instability of SrTiO3 at T S = 105 K is reinvestigated experimentally and theoretically by birefringence measurements and within the self-consistent phonon approximation. Analogous to BaTiO3 the phase transition shows precursor dynamics starting approximately 60 K above the phase transition temperature. These rotational dynamic clusters compete with precursor effects stemming from the polar soft mode and appear on different length scales as compared to BaTiO3.

A novel negative oxygen-isotope (16O/18O) effect (OIE) on the low-temperature tetragonal phase transition temperature T_{LTT} was observed in La_{2-x}Ba_xCuO_4 (x = 1/8) using high-resolution neutron powder diffraction. The corresponding OIE exponent alpha(T_{LTT}) = - 0.36(5) has the same sign as alpha(T_{so}) = -0.57(6) found for the spin-stripe order temperature T_so. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature T_co. Furthermore, a temperature dependent shortening of the c-axis with the heavier isotope is observed. These results combined with model calculations demonstrate that anharmonic electron-lattice interactions are essential for all transitions observed in the stripe phase of cuprates.

The structural transformation of multiferroic ${\mathrm{EuTiO}}_{3}$ has been intensively investigated by synchrotron x-ray diffraction at pressures up to 50.3 GPa and temperatures from 50 to 500 K. A pressure-induced antiferrodistortive phase transition from cubic $Pm\overline{3}m$ to tetragonal $I4/mcm$ space group has been observed, identical to the one that has been previously explored at ambient pressure and low temperatures. Several compression/decompression cycles at different temperatures have been carried out to accurately map the transition, and as a result a $P\text{\ensuremath{-}}T$ phase diagram for ${\mathrm{EuTiO}}_{3}$ has been constructed. The observed phase transition exhibits a positive $d{P}_{c}/dT$ (${P}_{c}$ being the critical pressure of the transition) and has many similarities with isostructural ${\mathrm{SrTiO}}_{3}$, although the absence of magnetoelectric interactions in the latter accounts for the different phase boundaries between the two materials.

High temperature superconducting copper oxides (HTSC) have been shown to be intrinsically heterogeneous. Consequently multiple ground states coexist, which are either in competition with or of support to superconductivity. Here it is shown that spin, charge and lattice are intrinsically coupled to form a composite of locally charge carrying distorted areas in an antiferromagnetic matrix on the regular lattice. The dynamical interplay between both induces level shifts together with band narrowing of the spin and charge related bands, by means of which interband interactions are possible which lead to a substantial increase in the superconducting transition temperature T c . Further consequences of this multicomponent approach as, for example, isotope effects are discussed.

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

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

The phase diagram of superconducting copper oxides is calculated as a function of doping based on a theory of dynamic stripe induced superconductivity. The two major conclusions from the theory and the numerical analysis are that T* (the pseudogap onset) and Tc (superconductivity onset) are correlated through the pseudogap, which induces a gap in the single particle energies that persists into the superconducting state. By decreasing the doping the pseudogap Delta* increases and T* increases, but when Delta* exceeds a certain critical value the superconducting transition temperature Tc is suppressed. A mixed s- and d-wave pairing symmetry is also examined as function of doping., Comment: 8 pages, 4 figures

The antiferromagnetic (AFM) phase transition temperature T_N of EuTiO_3 has been studied as a function of pressure p. The data reveal a nonlinear dependence of T_N on p with T_N increasing with increasing pressure. The exchange interactions exhibit an analogous dependence on p as T_N (if the absolute value of the nearest neighbor interaction is considered) and there is evidence that the AFM transition is robust with increasing pressure. The corresponding Weiss temperature {\Theta}_W remains anomalous since it always exhibits positive values. The data are analyzed within the Bloch power law model and provide excellent agreement with experiment., Comment: 9 pages, 4 figures article was selected for inclusion in IOPselect

EuTiO_3 is shown to exhibit novel strong spin-charge-lattice coupling deep in the paramagnetic phase. Its existence is evidenced by an, until now, unknown response of the paramagnetic susceptibility at temperatures exceeding the structural phase transition temperature T_S = 282K. The "extra" features in the susceptibility follow the rotational soft zone boundary mode temperature dependence above and below T_S. The theoretical modeling consistently reproduces this behavior and provides reasoning for the stabilization of the soft optic mode other than quantum fluctuations., 8 pages, 4 figures

The phase diagram of Sr_1-xEu_xTiO_3 is determined experimentally by electron paramagnetic resonance and resistivity measurements and analyzed theoretically within the self-consistent phonon approximation as a function of x ([0.03-1.0]). The transition temperature of the structural instability of the system increases nonlinearly to higher temperatures with increasing x. This is interpreted theoretically by a substantial alteration in the dynamics caused by a change in the double-well potential from broad and shallow to narrow and deep.