K. Alex Müller

Earlier and Recent Aspects of Superconductivity

Contemporary knowledge of superconductivity is set against its historical background in this book. First, the highlights of superconductivity research in the twentieth century are reviewed. Further contributions then describe the basic phenomena resulting from the macroscopic quantum state of superconductivity (such as zero resistivity, the Meissner-Ochsenfeld effect, and flux quantization) and review possible mechanisms, including the classical BCS theory and the more recent alternative theories. The main categories of superconductors - elements, intermetallic phases, chalcogenides, oxides and organic compounds - are described. Common features and differences in their structure and electronic properties are pointed out. This overview of superconductivity is completed by a discussion of properties related to the coherence length.

The discovery of a class of high-temperature superconductors.

The exceptional interest in the new class of oxide superconductors and the importance of these materials are discussed together with the concepts that led to their discovery. The discovery itself and its early confirmation are summarized, including the work until the beginning of 1987. The observation of a superconductive glass state in percolative samples is also discussed.

Indication for a novel phase in the quantum paraelectric regime of SrTiO3

Electron paramagnetic resonance of Fe3+ gives evidence for a phase-transition-like feature in SrTiO3 belowTq=37±1 K, both in the tetragonal and the pressure-induced trigonal phase. In the latter,Tq shifts to lower temperatures as a function of uniaxial stressp111. The new phenomenon is very tentatively discussed in terms of a possible transition to a novel coherent quantum state.

On the oxygen isotope effect and apex anharmonicity in high-T c cuprates

Emphasis is put on aprecise knowledge of the oxygen isotope shift exponent αO is the so far synthesized high-Tc superconductors in order to elucidate the basic mechanism. The known data, see αa in Table 6, indicate the presence of a presumably anharmonic coupling of pyramidal apex oxygen motion along thec-axis in these highly in-plane correlated superconductors. αa is computed from αO=rαa, based on the conjecture that the ratior of apex to total oxygen-ion content per unit cell is relevant. The related experimental and theoretical literature is reviewed.

Superconductivity in Alkaline Earth-Substituted La2CuO4-y

La2CuO4-y ceramics containing a few percent of Ca2+, Sr2+, and Ba2+ ions have been prepared. Resistivity and susceptibility measurements exhibit superconductive onsets (as in earlier Ba2+-containing samples). The onset temperature La2CuO4-y with Sr2+ is higher and its superconductivity-induced diamagnetism larger than that found with Ba2+ and Ca2+. This is proof that the electronic change resulting from alkaline earthdoping, rather than the size effect, is responsible for superconductivity. The ionic radius of Sr2+ is close to that of La3+ for which it presumably substitutes.

Electron spin and paramagnetic resonance in KH2PO4 and its isomorphs

Abstract This paper reviews magnetic resonance of radicals and paramagnetic ions in KH2PO4-type crystals up to 1985. In two sections, dedicated to each category of impurities, their site, symmetry and static particularities are summarized. Then the application of spin resonance to monitor fast ferroelectric domain polarization reversal is exposed. Finally, in the last section, slow local re-orientations of certain magnetic defects as well as information on intrinsic soft-mode dynamics from a static defect are described.

Macroscopic quantum phenomena

Abstract Condensation of fermions occurs in superconductivity, and superfluid 3He condensation of bosons is found in superfluid 4He and in the laser. These phenomena are described. Then it will be asked whether a coherent quantum state could occur in a quantum paraelectric such as SrTiO3. Electron paramagnetic resonance, recent high-energy X-ray diffraction and accurate elastic constant measurements in this double oxide would be compatible with the existence of such a state and will be reviewed. If such a coherent state exists, second sound perpendicular to the tetragonal c-axis in SrTiO3 could be observed.

Quantum Para-, Ferro-, and Random Electric Behaviors in Oxide Perovskites

An introduction to and a survey of quantum para- and ferro-electric behavior in oxide perovskites are given. In the paraelectric case, quantum fluctuations stabilize the disordered state with a high, temperature-independent dielectric constant. Application of uniaxial stress or ion substitution can induce a quantum ferroelectric state at a special quantum multicritical point (QMCP). The occurrence of or crossover to a random-field induced domain state in partially substituted crystals is reviewed. The emphasis is on SrTiO3 studies and on Sr1-xCaxTiO3 mixed crystals, but contact is made to stressed KTaO3 and substitution of K+ or Ta5+ ions

Apex anharmonicity observed by Raman scattering in 18O substituted YBa2Cu3O6+x

Abstract IR excited (1.16 eV) Raman spectra of several YBa 2 Cu 3 O 6+ x metallic samples (0.48 x 16 O and 96% substituted 18 O have been performed. The dominant features of the spectra are two phonon bands around 340 and 490 cm -1 in 16 O samples. In comparing the energy shift of these two Raman bands upon isotopic substitution with the calculated ones, the “harmonic” approximation accounts well for the shift of the lower of the two, the O(2)/O(3) “dimpling” vibration, whereas the higher O(4) band behavior is consistent with the existence of a highly anharmonic potential along the c -axis, a property otherwise only found in ferroelectrics.

High-resolution x-ray diffraction study of structural phase transition in srtio3

Recent experiments on the 105 K transition in SrTiO3 by means of the high-angle double-crystal X-ray diffractometry (HADOX) are presented and discussed. The temperature dependence of the Bragg peak-width shows two anomalies: a peak in the temperature range Tc±2 K and a gradual increase over a range of about 10 K below Tc. The former is due to critically enhanced fluctuation and is connected with the precursor behaviour of the lattice constant. The latter is attributed to a change in mosaicity and is consistent with an anomaly in the intensity vs. temperature relation. By combining HADOX with the use of white X-rays and an energy sensitive detector, all the results mentioned above are found being common in the region with the depth from the specimen surface 1 to 57 µm, and regarded as representing bulk behaviours.