A. Shengelaya

Detection of charge transfer processes in Cr-doped SrTIO3 single crystals

An insulator-to-metal transition is observed in Cr-doped SrTiO3 single crystals upon extended exposure to a high electric field, namely, electroconditioning (EC). Electron paramagnetic resonance (EPR) and transport measurements under laser irradiation show anticorrelation between the Cr3+ EPR signal and the electrical current. This proves that the Cr3+ ions are responsible for the photocurrent that initiates the EC process. We observe the presence of Cr3+∕Cr4+ mixed valencies in the bulk in the conducting state. The EPR characterization of the spectra in the conducting state excludes the possibility of a Cr3+-oxygen vacancy complex in the bulk as a result of the EC.

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.

Mixed order parameter symmetries in cuprate superconductors

The recent observation of an inflection point in the temperature dependence of the in-plane magnetic field dependence (λab) is investigated within a two-band model with coupled order parameters of different symmetries. While the dominant order parameter has d-wave symmetry, the smaller one is of s-wave symmetry. Superconductivity is robust in the d-wave channel and induced via interband interactions in the s-wave subsystem.

Comparison of different methods for analyzing μSR line shapes in the vortex state of type-II superconductors

A detailed analysis of muon-spin rotation (μSR) spectra in the vortex state of type-II superconductors using different theoretical models is presented. Analytical approximations of the London and Ginzburg-Landau (GL) models, as well as an exact solution of the GL model were used. The limits of the validity of these models and the reliability for extracting parameters such as the magnetic penetration depth λ and the coherence length ξ from the experimental μSR spectra were investigated. The analysis of the simulated μSR spectra showed that at high magnetic fields there is a strong correlation between λ and ξ obtained for any value of the Ginzburg-Landau parameter κ = λ/ξ. The smaller the applied magnetic field, the smaller the possibility of finding the correct value of ξ. A simultaneous determination of λ and ξ without any restrictions is very problematic, regardless of the model used to describe the vortex state. It was found that for extreme type-II superconductors and low magnetic fields, the fitted value of λ is practically independent of ξ. The second-moment method frequently used to analyze μSR spectra by means of a multi-component Gaussian fit generally yields reliable values of λ over the whole range of applied fields [Formula: see text] (H(c1) and H(c2) are the first and second critical fields, respectively). These results are also relevant for the interpretation of small-angle neutron scattering experiments on the vortex state in type-II superconductors.

Muon-spin-rotation measurements of the penetration depth of the infinite-layer electron-doped Sr0.9La0.1CuO2 cuprate superconductor

Muon-spin-rotation (muSR) measurements of the in-plane penetration depth lambda(ab) have been performed in the infinite-layer electron-doped Sr0.9La0.1CuO2 high-T(c) superconductor (HTS). Absence of the magnetic rare-earth ions in this compound allowed us to measure for the first time the absolute value of lambda(ab)(0) in electron-doped HTSs using muSR. We found lambda(ab)(0)=116(2) nm. The zero-temperature depolarization rate sigma(0) proportional, variant 1/lambda(2)(ab)(0)=4.6(1) micros(-1) is more than 4 times higher than expected from the Uemura line. Therefore, this electron-doped HTS does not follow the Uemura relation found for hole-doped HTSs.

Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3

EuTiO3 (ETO) has recently experienced a significant revival of interest because of its possible multiferroic properties, which are currently the focus of much research. Unfortunately, ETO is an unlikely candidate for enlarged multifunctionality since the mode softening—typical of ferroelectrics—remains incomplete, and the antiferromagnetic properties appear only at 5.5xa0K. However, a strong coupling between lattice and Eu spins exists and this leads to the appearance of a magnon–phonon-hybrid mode at elevated temperatures as evidenced by EPR, muon spin relaxation experiments and model predictions based on a coupled spin-polarizability Hamiltonian. This novel finding supports the notion of strong magneto-dielectric effects being realized in ETO and opens up new strategies in material design and technological applications.

Tuning the static spin-stripe phase and superconductivity in La2−xBaxCuO4 (x = 1/8) by hydrostatic pressure

Magnetization and muon spin rotation experiments were performed in La2−xBaxCuO4 (xxa0=xa01/8) as a function of hydrostatic pressure up to pxa0≃xa02.2xa0GPa. It was found that the magnetic volume fraction of the static stripe phase strongly decreases linearly with pressure, while the superconducting volume fraction increases by the same amount. This demonstrates competition between bulk superconductivity and static magnetic order in the stripe phase of La1.875Ba0.125CuO4 and that these phenomena occur in mutually exclusive spatial regions. The present results also reveal that the static spin-stripe phase still exists at pressures, where the long-range low-temperature tetragonal (LTT) structure is completely suppressed. This indicates that the long-range LTT structure is not necessary for stabilizing the static spin order in La1.875Ba0.125CuO4.

25 Mg NMR study of the MgB 2 superconductor

25Mg NMR spectra and nuclear-spin-lattice relaxation time T 1 have been measured in polycrystalline MgB 2 with a superconducting transition temperature T c = 39 K in zero magnetic field. From the first-order and second-order quadrupole perturbed NMR spectrum a quadrupole coupling frequency ν Q = 222.0(1.5) kHz is obtained. T 1 T = 1090(50) sK and Knight shift K c = 242(4) ppm are temperature independent in the normal conducting phase. The 2 5 Mg Korringa ratio equals to 0.95 which is very close to the ideal value of unity for s electrons. The comparison of the experimental ν Q , T 1 T, and K c with the corresponding values obtained by LDA calculations shows an excellent agreement for all three quantities.

25Mg NMR study of the MgB2 superconductor

25Mg NMR spectra and nuclear-spin-lattice relaxation time T 1 have been measured in polycrystalline MgB 2 with a superconducting transition temperature T c = 39 K in zero magnetic field. From the first-order and second-order quadrupole perturbed NMR spectrum a quadrupole coupling frequency ν Q = 222.0(1.5) kHz is obtained. T 1 T = 1090(50) sK and Knight shift K c = 242(4) ppm are temperature independent in the normal conducting phase. The 2 5 Mg Korringa ratio equals to 0.95 which is very close to the ideal value of unity for s electrons. The comparison of the experimental ν Q , T 1 T, and K c with the corresponding values obtained by LDA calculations shows an excellent agreement for all three quantities.

Superconductivity and Magnetism in Pure and Substituted RuSr2GdCu2O8

We describe results of several macroscopic and local measurements of magnetic and superconducting properties for pure and Ce and Cu substituted RuSr2GdCu2O8. From various experiments the physical properties phase diagram is derived as a function of the hole doping. We show that the large contribution of polarized Gd3+ magnetic moments to magnetization causes apparent absence of the Meissner state and augments the weak Ru ferromagnetism. This weak ferromagnetism in the superconducting state can be significantly enhanced by the external magnetic field. These results indicate that the low-temperature behavior can be qualitatively explained assuming a quasi-two-dimensional character of the superconducting regions.