D. Di Castro

Evidence of a pressure-induced metallization process in monoclinic VO2

Raman and combined infrared transmission and reflectivity measurements were carried out at room temperature (RT) on monoclinic VO2 over the 0-19 GPa and 0-14 GPa pressure ranges. Both lattice dynamics and optical gap show a remarkable stability up to P* approximately 10 GPa whereas subtle modifications of V ion arrangements within the monoclinic lattice, together with the onset of a metallization process via band gap filling, are observed for P >P*. Differently from P=0, where the VO2 metallic phase is found only in conjunction with the rutile structure above 340 K, a new RT metallic phase within a monoclinic structure appears accessible in the high pressure regime.

Effect of the Al content on the optical phonon spectrum in Mg1-xAlxB2

Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.

The stripe critical point for cuprates

The experimental determination of the quantum critical point (QCP) that triggers the self-organization of charged striped domains in cuprate perovskites is reported. The phase diagram of doped cuprate superconductors is determined by a first variable, the hole doping ?, and a second variable, the micro-strain ? of the Cu-O bond length, obtained from the Cu K-edge extended x-ray absorption fine structure. For a fixed optimum doping, ?c = 0.16, we show the presence of the QCP for the onset of local lattice distortions and stripe formation at the critical micro-strain ?c. The critical temperature Tc(?,?) reaches its maximum at the quantum critical point (?c,?c) for the formation of bubbles of superconducting stripes. The critical charge, orbital and spin fluctuations near this strain QCP provide the interaction for the pairing.

Occurrence of a high-temperature superconducting phase in (CaCuO2)n/(SrTiO3)m superlattices

We report the occurrence of superconductivity, with maximum Tc = 40 K, in superlattices (SLs) based on two insulating oxides, namely CaCuO2 and SrTiO3. In these (CaCuO2)n/(SrTiO3)m SLs, the CuO2 planes belong only to CaCuO2 block, which is an antiferromagnetic insulator. Superconductivity, confined within few unit cells at the CaCuO2/SrTiO3 interface, shows up only when the SLs are grown in a highly oxidizing atmosphere, because of extra oxygen ions entering at the interfaces. Evidence is reported that the hole doping of the CuO2 planes is obtained by charge transfer from the interface layers, which act as charge reservoir.

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

Coexistence of stripes and superconductivity: Tc amplification in a superlattice of superconducting stripes

Abstract Here we report direct evidence for charge ordering in the oxygen doped La 2 CuO 4.1 superconductor by x-ray diffraction using a third generation synchrotron source. We observe 1D charge ordering and oxygen ordering in an optimally doped superconductor (T c =40K) well beyond the phase separation regime. The order parameter of the anharmonic 1D CDW shows a critical behavior (T co -T) 0.5 T co =190K. The oxygen ordering and the polaron 1D CDW with its associated orbital wave, realize a superlattice of quantum stripes with a wavevector that at the density of charge carriers, r s ∼6, in the superconducting stripes satifies the shape resonance condition for T c amplification.

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

High Tc superconductivity in a critical range of micro-strain and charge density in diborides

Expansion of the superlattice of boron layers, with AB2 structure, due to different intercalated A atoms has been studied to understand the emergence of high Tc superconductivity in the diborides. The structure of these metal heterostructures at the atomic limit (MEHALs) (with A = Al, Mg, Ti, Hf, Zr) has been measured by synchrotron x-ray diffraction. The increasing atomic radius of the intercalated A ions induces an increase of (1) the separation between the boron layers and (2) the tensile micro-strain e of the B–B distance within the boron layers. The results show that the superconductivity in these MEHALs appears in a critical region in a phase diagram controlled by two variables, the micro-strain and the charge density (e, ρ).

Oxygen Isotope Effects on the Superconducting Transition and Magnetic States Within the Phase Diagram of Y1-xPrxBa2Cu3O7-δ

R. Khasanov, A. Shengelaya, 2 D. Di Castro, 3 E. Morenzoni, A. Maisuradze, I.M. Savić, K. Conder, E. Pomjakushina, 7 and H. Keller Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Physics Institute of Tbilisi State University, Chavchavadze 3, GE-0128 Tbilisi, Georgia ”Coherentia” CNR-INFM and Dipartimento di Fisica, Universita’ di Roma ”La Sapienza”, P.le A. Moro 2, I-00185 Roma, Italy Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Faculty of Physics, University of Belgrade, 11001 Belgrade, Serbia and Montenegro Laboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Laboratory for Neutron Scattering, Paul Scherrer Institute & ETH Zurich,CH-5232 Villigen PSI, Switzerland Oxygen isotope (O/O) effects (OIE’s) on the superconducting transition (Tc), the spinglass ordering (Tg), and the antiferromagnetic ordering (TN) temperatures were studied for Y1−xPrxBa2Cu3O7−δ as a function of Pr content (0.0 ≤ x ≤ 1.0). The OIE on Tc increases with increasing x up to x ≈ 0.55, where superconductivity disappears. For decreasing x the OIE’s on TN and Tg increase down to x ≈ 0.7 where antiferromagnetic order and down to x ≈ 0.3 where spin-glass behavior vanish, respectively. The OIE’s on Tg and TN are found to have opposite signs as compared to the OIE on Tc. All OIE’s are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.

Pressure effects on the transition temperature and the magnetic field penetration depth in the pyrochlore superconductor RbOs2O6.

Magnetization measurements under hydrostatic pressure up to 8 kbar in the pyrochlore superconductor RbOs2O6 (T(c) approximately or equal 6.3 K at p=0) were carried out. A positive pressure effect on T(c) with dT(c)/dp=0.090(3) K/kbar was observed, whereas no pressure effect on the magnetic penetration depth lambda was detected. The pressure independent ratio 2 Delta(0)/k(B)T(c)=3.72(2) (Delta(0) is the superconducting gap at zero temperature) was found to be close to the BCS value 3.52. Magnetization and muon-spin rotation measurements of lambda(T) indicate that RbOs2O6 is an adiabatic s-wave BCS-type superconductor. The value of lambda extrapolated to zero temperature and ambient pressure was estimated to be 230(30) nm.