Annette Bussmann-Holder

The polarizability model for ferroelectricity in perovskite oxides

This article reviews the polarizability model and its applications to ferroelectric perovskite oxides. The motivation for the introduction of the model is discussed and nonlinear oxygen ion polarizability effects and their lattice dynamical implementation outlined. While a large part of this work is dedicated to results obtained within the self-consistent-phonon approximation, nonlinear solutions of the model are also handled, which are of interest to the physics of relaxor ferroelectrics, domain wall motions, and incommensurate phase transitions. The main emphasis is to compare the results of the model with experimental data and to predict novel phenomena.

Experimental evidence for two gaps in the high-temperature La1.83Sr0.17CuO4 superconductor

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. The temperature dependence of \lambda^{-2}_{ab} has an inflection point around 10-15K, suggesting the presence of two superconducting gaps: a large gap (\Delta_1^d) with d-wave and a small gap (\Delta_2^s) with s-wave symmetry. The zero-temperature values of the gaps at \mu_0H=0.02T were found to be \Delta_1^d(0)=8.2(2)meV and \Delta_2^s(0)=1.57(8)meV.

Jahn–Teller physics and high-Tc superconductivity

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

Raising the diboride superconductor transition temperature using quantum interference effects

The model of two (σ and π) channels superconductivity known to be necessary to explain the superconductivity in MgB2 has been applied to the Al1-xMgxB2 diborides by tuning x from MgB2 to AlMgB4. The evolution of the interband coupling parameter (probing the strength of the interchannel pairing due to quantum interference effects) and the two gaps in the σ and π channel as a function of x have been calculated. While in MgB2 the quantum interference effects gives an amplification of Tc by factor 1.5 in comparison with the dominant intra σ band single channel pairing, in AlMgB4 the amplification is about 100, in comparison with the dominant intra π band single channel pairing. For correspondence email addresses: bianconi@superstripes.com A.Bussmann-Holder@fkf.mpg.de The interest in enhancing the superconducting temperature Tc by quantum interference between the pairing in two channels is receiving renewed attention even though its theory has been pointed out in the fifties [1] and extended later by several authors for conventional [2,3] and non conventional superconductors [4-6] including multi channels interaction. In fact the recent experimental results for MgB2 shows that the two band model is needed to explain both the normal and the superconducting properties [7-19]. Here the key ingredient that allows the physical realization of the process of two channels Tc amplification is the fact that the two gaps referring to two different parts in k space (a large gap in the σ Fermi surface and a small gap in the π Fermi surface) are well separated in real space, one for the σ holes in the boron layers, and the other for π electrons in the interstitial space (Mg layers between the boron layers). Therefore the system can be described as a multi layer made of alternated layers of metallic and superconducting planes. While in most of the materials (like in conventional isotropic 3D superconductors) the impurity interband scattering suppresses the quantum interference, here it is very low and does not suppress the two band superconductivity enhancement [19]. It has been found that in the Al1-xMgxB2 alloys [20-31] show a continuos evolution through a complicated mixed phase from MgB2 (at x=1) to the end member AlMgB4 (at x=0.5) where a well ordered superlattice structure of boron layers intercalated by alternated layers of Al and Mg is formed [20-24]. In spite of the fact that the alloys with intermediate x are rather disordered, the superconducting transition temperature is still well defined and it drops systematically with decreasing x as it is shown in Fig. 1a [from ref. 25,29,30 in agreement with several independent experiments ref. 21-24]. It shows a a kink around x=0.7 where a dimensionality crossover of the Fermi surface takes place [31,25-26] and reach 3K at x=0.5 for AlMgB4 and the partial density of states of the σ band (Fig.1b) shows a kink. The superconducting phase in the ordered phase AlMgB4 is a highly interesting case since the Fermi level is droven near the top of the σ band, the partial density of states (PDOS) in the σ band is strongly reduced and the Fermi energy for the σ holes Ef is only about 100-200 meV [2527]. The two-gap scenario has already been invoked for pure MgB2 and electronphonon interactions together with Coulomb potentials are well known. The interesting question here is whether such an approach can be used for the alloys up to the ordered AlMgB4 phase and how the gap structure and interband couplings develop with doping. Going from x=1 to x=.5 a dramatic increase in the E2g phonon mode energy ωE2g takes place [29-31] going from 70 meV to 115 meV as shown in Fig. 2a (from the data in ref. 30) indicating a strong reduction in the electron-phonon interaction which reflects itself also in a reduction of the phonon damping as shown in Fig. 2b (from the data in ref. 30) while the average phonon frequencies remains nearly constant ωln =59-62 meV .Since the E2g phonon mode strongly couples with the modulation of the top of the σ band the electron-phonon coupling

Polaron formation as origin of unconventional isotope effects in cuprate superconductors

Abstract.Various unconventional isotope effects have been reported in high-temperature superconducting copper oxides which are beyond the scheme of BCS theory. Their origin is investigated within polaron theory which leads to a renormalization of the single particle energies and introduces a level shift here. It is found that the exponential squeezing of the second nearest neighbour hopping integral carries the correct isotope effect on the superconducting transition temperature Tc, as well as the one on the penetration depth. The average superconducting gap is predicted to have an isotope effect comparable to the one on the penetration depth. The results imply that the coupling of the electronic degrees of freedom to the Jahn-Teller Q2-type mode is the origin of these isotope effects.

Multiple Gap Symmetries for the Order Parameter of Cuprate Superconductors from Penetration Depth Measurements

R. Khasanov, S. Strässle, D. Di Castro, 2 T. Masui, S. Miyasaka, S. Tajima, A. Bussmann-Holder, and H. Keller Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland CRS Coherentia, CNR-INFM and Dipartimento di Fisica, Universita’ di Roma ”La Sapienza”, P.le A. Moro 2, I-00185 Roma, Italy Department of Physics, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Relation between structural instabilities in EuTiO3and SrTiO3

Specific heat measurements and theoretical calculations reveal an intimate analogy between EuTiO3 and SrTiO3. For EuTiO3 a hitherto unknown specific heat anomaly is discovered at TA=282(1)K which is analogous to the well known specific heat anomaly of SrTiO3 at TA=105K caused by an antiferrodistortive transition. Since 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 EuTiO3 to an antiferrodistortive phase transition. The higher transition temperature of EuTiO3 as compared to SrTiO3 results from spin phonon coupling.

High-precision 31P chemical shift measurements on KH2PO4-type crystals: role of electronic instability in the ferroelectric transition mechanism

The mechanism of the ferroelectric transition in KH2PO4-type crystals has been probed using magic angle spinning 31P NMR on single crystals. The higher resolution thus obtained has enabled us to detect a marked change in the 31P isotropic chemical shift around the transition temperature Tc, with a clear jump at Tc. These results are interpreted as evidence that the transition involves a displacive component with electronic instabilities playing a role in the driving mechanism.

Precursor dynamics, incipient ferroelectricity and huge anharmonicity in antiferroelectric lead zirconate PbZrO3.

To better understand the phase transition mechanism of PbZrO3 (PZO), the lattice dynamics of this antiferroelectric compound are investigated within the polarizability model, with emphasis on the cubic to orthorhombic phase transition. Similarly to ferroelectric phase transitions in ABO3 perovskites, polar dynamical clusters develop and grow in size upon approaching T(C) from the high temperature side and never form a homogeneous state. Simultaneously, elastic anomalies set in and compete with polar cluster dynamics. These unusual dynamics are responsible for precursor effects that drive the PZO lattice towards an incipient ferroelectric state. Comparison of the model calculations with the temperature dependences of elastic coefficients measured on PZO single crystals reveals a striking similarity.

Stabilization of ferroelectricity in quantum paraelectrics by isotopic substitution

SrTiO3 and KTaO3 are incipient ferroelectrics whose phase transition temperature is suppressed by quantum fluctuations. The recent observation of induced ferroelectricity due to exchange of 16 O by 18 O (1999 Phys. Rev. Lett. 82 3540) is reproduced for the dielectric data by an anharmonic electron-phonon interaction model, and a first order phase transition is predicted. Furthermore it is shown that this isotope effect does not occur in KTaO3 since its local double-well potential is much shallower than that in SrTiO3 .