Search

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.

Lemon-yellow single crystals of Eu2[SiO4] were obtained by reaction of elemental europium with Eu2O3 in the presence of SiO2 carried out in evacuated silica ampules at 1373 K for 48 h followed by constant cooling to room temperature with 120 K per hour. Eu2[SiO4] crystallizes at room temperature in the larnite-type structure of β-Ca2[SiO4] (monoclinic, P21/n) with parameters a = 565.02(5), b = 709.15(6), c = 975.84(8) pm, β = 92.614(3)° for Z = 4. At 452 K it undergoes a reversible phase transition to an incommensurate structure similar to the isotypic compounds Sr2[SiO4] and K2[SeO4]. The phase transition energy was determined to 0.54(2) J·g–1 by DSC measurements and temperature dependent birefringence measurements show that the phase transition exhibits a significant hysteresis. Refinements of the structure of Eu2[SiO4] based on single crystal X-ray diffraction data show that the high temperature modification crystallizes in the orthorhombic space group Pnma(α00)0ss with a = 710.16(6), b = 566.93(5), c ...

We have investigated in detail the lattice dynamics of five different perovskite titanates ATiO3 (A = Ca, Sr, Ba, Pb, Eu) where the A sites are occupied by +2 ions. In spite of the largely ionic character of these ions, the properties of these compounds differ substantially. They range from order/disorder like, to displacive ferroelectric, quantum paraelectric, and antiferromagnetic. All compounds crystallize in the cubic structure at high temperature and undergo structural phase transitions to tetragonal symmetry, partly followed by further transitions to lower symmetries. Since the TiO6 moiety is the essential electronic and structural unit, the question arises, what makes the significant difference between them. It is shown that the lattice dynamics of these compounds are very different, and that mode-mode coupling effects give rise to many distinct properties. In addition, the oxygen ion nonlinear polarizability plays a key role since it dominates the anharmonicity of these perovskites and determines the structural instability.

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.

Perovskite oxides ABO3 offer a vast playground for applications and basic research. Especially multifunctionality is a frequently targeted aim, since the replacement of the A and/or B sublattice ions easily changes the properties of the system. Here we concentrate on the perovskite EuTiO3 (ETO) which was ignored by the scientific community for a long time. We review recent results on it and show that this compound provides many surprises which may offer novel device techniques and applications. In particular we demonstrate that far above its Neel temperature TN = 5.7 K magnetism exists and new magneto-dielectric-elastic coupling is at work. From capacitance and muon spin rotation (μSR) data we conclude that the phase diagram of ETO is much more complex than believed and a variety of new phases are described.

In many cases, functionality is connected with the response of a material to the action of an external electric field. In oxidic perovskites, so widely used in practice, a disadvantage is their surface chemical instability when subjected to an electric field. That is why investigations of the influence of a high intensity alternating electric field on the phase transition temperatures and chemical properties of the surface and bulk of single crystals of PbZrO3 intentionally made defective by doping with niobium have been undertaken. It has been demonstrated that defects produced through the electro-degradation process make the low-field dielectric response a highly diffusive one, but they do not obscure the sharp phase transitions detected by temperature changes in the bulk piezoelectric and electrostrictive deformation around the critical temperature. Based on XPS spectra, the types of defects at the surface and in the bulk of as-grown single crystals caused by the electric field action are identified. It has been shown that the surface–bulk interrelation in ferroic lead zirconate and solid solutions based on this material has to be taken into account while trying to use them as high quality functional materials.

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.

The research in perovskite oxides is ever increasing since their discovery in 1945. The number of compounds with this rather simple structure is growing continuously since then and fascinating properties ranging from magnetic via dielectric and superconducting have been reported for them. While an extreme research activity has been devoted to SrTiO 3 from its discovery on, its analog EuTiO 3 has attracted rather little interest until recently. In this review we concentrate on the recent results from studies of this perovskite and its mixed crystals with SrTiO 3 and demonstrate that novel interesting properties are related to it. Specifically we provide evidence for the existence of hybrid-paramagnon–phonon coupling at elevated temperatures and its manifestations in bulk sensitive experiments. In addition, the phase diagram of the mixed crystal series is presented where pressure and temperature effects are included.

Acoustic properties of antiferroelectric (AFE) PbZr0.72Sn0.28O3 single crystals were investigated in a wide temperature range by Brillouin light scattering spectroscopy. The Brillouin frequency shift of the longitudinal acoustic (LA) mode exhibited three abrupt changes corresponding to the successive phase transitions between the paraelectric, intermediate, antiferroelectric (AFE2) and antiferroelectric (AFE1) phases upon cooling. The change in the LA mode frequency and the damping maximum of PbZr0.72Sn0.28O3 were much higher than those of pure PbZrO3. This was attributed to enhanced polarization fluctuations caused by increased ferroelectric distortions due to Sn substitution and the proximity to the tricritical point on the phase diagram of PbZr1-xSnxO3.

We explored under what conditions perovskite oxides ABO3 adopt a polar or an antiferrodistortive structure from the viewpoint of the tolerance factor τ and the orbital interactions of the empty orbitals of the A and B cations with the filled orbitals of their surrounding O2– ions. Polar structures require the presence of a substantial size-mismatch as well as strong cation-anion orbital interactions at A or B cation sites. A tetragonal polar structure is preferred when the A cation empty orbitals are extended or when the B-cation empty orbitals are contracted. A trigonal polar structure is favored in opposite cases. An antiferrodistortive structure is found for perovskites with weak size-mismatch or those with substantial size-mismatch if both A and B cations do not generate strong cation-anion orbital interactions. Antiferrodistortive structures result from interactions involving the A cations with the O4 squares of the face-sharing AO8 square-prisms in a two-in-two-out manner, and are found only when the A/O cation-anion interactions are substantial.

The dynamical properties of EuTiO3 are investigated to gain insight into its precursor effects analogous to those observed in many other perovskite oxides. In spite of the fact that a competition between the long wave length transverse optic mode and the zone boundary acoustic mode is absent, which makes the mode-mode coupling inessential, acoustic instabilities on limited spatial regions appear well above TS. This confirms the notion that dynamic forerunners are inherent and precede any phase transition in cubic perovskite oxides.

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

Using a hybrid Hartree–Fock (HF)-DFT method combined with LCAO basis set and periodic supercell approach, the atomic, electronic structure and phonon properties of oxygen vacancies in ZnO and SrTiO3 were calculated and compared. The important role of a ghost basis function centered at the vacant site and defect spin state for SrTiO3 is discussed. It is shown that the use of hybrid functionals is vital for correct reproduction of defects basic properties. The Gibbs free energy of formation of oxygen vacancies and their considerable temperature dependence has been compared for the two oxides. These calculations were based on the polarizability model for the soft mode temperature behavior in SrTiO3. The supercell size effects in the Gibbs free energy of formation of oxygen vacancies in the two oxides are discussed. The major factors for the quite different behavior of the two oxides and the degree of electron delocalization nearby the oxygen vacancy have been identified.

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.

At ultrahigh pressure (

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