G. Heine

Superconductivity and flux pinning in Y and heavily Pb codoped Bi-2212 single crystals

Studies of superconductivity and flux pinning were carried out on (Bi1.64Pb0.36)Sr2Ca1−xYxCu2O8+y (x=0, 0.05, 0.11, 0.33) single crystals grown by the self-flux method. X-ray diffraction, transport, and magnetic measurements were performed for purposes of characterization. X-ray analysis revealed that the c lattice parameter systemically decreases as the Y doping level increases. The superconducting transition temperature Tc decreases from 80 to 30 K as x increases. A strong annealing effect on Tc and superconducting volume has been observed. Resistance measurements show that x=0.33 samples are semiconductive over a wide temperature range between 4.2 and 300 K for the as-grown state, but become metallic with Tc of 65–70 K after air or oxygen annealing. Flux pinning was studied by measuring the hysteresis loop at different temperatures and different fields. A peak effect was observed in all the codoped samples. Results show that at low temperatures, the peak field is smaller than in solely Pb doped crystal...

Magnetoresistance of the new ceramic “Cernox” thermometer from 4.2 K to 300 K in magnetic fields up to 13 T

Measurements of the resistance and the magnetoresistance of a new ceramic thin-film temperature sensor (Cernox-1080, Lake Shore Cryotronics Inc.) are reported for magnetic fields up to 13 T in the temperature range 4.2 to 300 K. The zero-field resistance changes monotonically over more than three orders of magnitude with a negative temperature coefficient. The magnetoresistance is small at temperatures above 10 K and exhibits partially compensating positive and negative components. The magnetic-field induced temperature reading error at 13 T is about 2% at 4.2 K and less than 0.3% at 10 K and above. In the temperature range 80 to 300 K, the magnetoresistance can be approximated by a fourth-order polynomial, allowing for an estimation and correction of the apparent temperature reading error of the Cernox thermometer in high magnetic fields.

Study of anisotropic magnetoresistance in the thermodynamic fluctuation regime of single crystals of Bi2Sr2CaCu2O8+x

Measurements of the in-plane and out-of-plane magnetoresistance in Bi2Sr2CaCu2O8+x single crystals with a critical temperature Tc=93 K are reported for temperatures between Tc and 200 K and in magnetic fields up to 13 T. Both the transverse and the longitudinal in-plane magnetoresistance are positive and originate entirely from the suppression of superconducting fluctuations by the magnetic field. The longitudinal out-of-plane magnetoresistance is negative at all temperatures and magnetic fields. As possible origins two recent models are discussed, the reduction of the quasiparticle density of states due to condensation into superconducting fluctuations and thermally activated transport over an interplane pseudogap.

Superconducting fluctuations in YBaCuO thin films

Abstract Measurements of the electrical conductivity, the magnetoconductivity and the Hall effect on thin films of YBaCuO in the regime of thermodynamic fluctuations of the superconducting order parameter are reported. The fluctuation conductivity is in good agreement with the 2-dimensional Aslamazov-Larkin theory. Excess magnetoconductivity and excess Hall effect diverge by higher exponents near the critical temperature indicating a pronounced effect of the magnetic field on the fluctuations.

Out-of-plane magnetoresistance of Bi2Sr2CaCu2O8+x single crystals: Probing the fluctuation-induced pseudogap in the density of states

Abstract We report on measurements of the out-of-plane resistivity and magnetoresistance in the normal state region above the critical temperature T c in optimally doped and overdoped Bi 2 Sr 2 CaCu 2 O 8+ x single crystals. The temperature dependence of the out-of-plane resistivity indicates thermally activated hopping of carriers over an interplane pseudogap. In the overdoped samples this behavior graduall changes to a metallic-like resistivity at higher temperatures. The out-of-plane magnetoresistance (measured with the magnetic field parallel to the current) is negative, indicating a shrinking of the pseudogap in magnetic fields. Our experimental results are in good accordance with a recent model for the reduction of the quasiparticle density of states due to condensation of quasiparticles into superconducting fluctuations. Such a fluctuation-induced pseudogap is expected to be suppressed in high magnetic fields according to a reduction of the in-plane fluctuation amplitude.

Transport properties of YBaCuO and BiSrCaCuO thin films in the thermodynamic fluctuation range

Abstract Thin films of YBaCuO have been fabricated by pulsed-laser deposition in oxygen atmosphere on MgO substrates. Films of the 2223-phase of BiSrCaCuO, stabilized by Pb-addition in the ceramic targets, were prepared by DC-magnetron sputtering. The effect of thermodynamic fluctuations of the superconducting order parameter on the normal state transport properties was investigated by measurements of the electrical resistivity, the magnetoresistance and the Hall effect. Analyzing the results in terms of Aslamazov-Larkin and Maki-Thompson theories and their extensions for 2-dimensional, layered superconductors, we obtain a consistent set of parameters from which the c-axis coherence lengths ξ c (0) = 1.5 A in YBaCuO and ξ c (0) = 1.7 A in 2223-(Bi,Pb)SrCaCuO are deduced. A significant contribution from the Maki-Thompson process was found in YBaCuO, but no such effects in the (Bi,Pb)SrCaCuO films.

Origin and characterisation of peak effect in pure and Pb doped Bi-2212 single crystals

Abstract The peak effect in pure and heavily Pb doped Bi2212 single crystals with different oxygen doping levels were studied by measuring M-H loops. The peak effect in pure Bi2212 crystals was obtained only for crystals with optimum oxygen doping and overdoping but not for oxygen underdoping. For Pb-doped Bi2212 crystals, a strong peak effect appeared in higher field than in pure crystals and it persisted up to T c . Formation and decomposition of Bi 5+ and/or Pb 4+ rich clusters, which caused the reduction of c-axis and ϱ c , were proposed to be responsible for the appearance and disappearance of the peak effect in pure and Pb doped crystals. The peak effect was characterised by plotting (H max -H min )/H max vs T/T c , where H max and H min represents the field in which the magnetisation reaches maximum and starts to increase around secondary peak position, respectively. Results showed that the evolution of the peak effect with temperature in the Pb doped crystals was similar to that in Y123.

Sign Change of the Out-of-plane Magnetoresistance in Oxygen-deficient YBCO

The out-of-plane magnetoresistance in optimally doped (Tc ≈ 90 K) and oxygen-deficient (Tc ≈ 58 K) thin films of YBa2Cu3O7−δgrown on vicinal SrTiO3substrates was investigated. The magnetoresistance of both compounds changes sign above the critical temperature. The results are compared to theoretical predictions for the magnetic-field suppression of superconducting order parameter fluctuations in anisotropic materials. It is suggested that the competition of Aslamazov-Larkin and density-of-states fluctuations, which contribute with different sign to the magnetoresistance, are responsible for the observed sign change.

Photoinduced changes of the in-plane and out-of-plane electronic properties in YBa2Cu3Ox

We report on the photoinduced changes of the electronic properties of YBa2Cu3Ox (x ≈ 6.6) thin films, focussing on the electrical anisotropy in the normal and the superconducting state. The latter was determined from magnetoconductivity measurements, performed before and after light excitation, and fitting the experimental results to the Aslamazov–Larkin theory of superconducting order-parameter fluctuations. The normal-state anisotropy σab/σc was calculated from resistance measurements carried out on samples grown on vicinal substrates. We observed that the photodoping process enhanced superconducting coherence lengths ξc(0) and ξab(0), but reduced the superconducting anisotropy ξab(0)/ξc(0) at all studied temperatures. In contrast, σab/σc was enhanced by photodoping at low temperatures, but reduced at high temperature. Our results strongly suggest that two different mechanisms contribute to the photodoping process, with one affecting only the normal-state properties.

Superconducting transition and magnetoresistance in Bi2Sr2CaCu2O8+x single crystals

Abstract The in-plane resistivity of overdoped Bi2Sr2CaCu2O8+x single crystals was investigated in various magnetic fields up to 13 T in the temperature range from 4.2 K to T c +20 K . The characteristic broadening of the resistive transition in a magnetic field oriented perpendicular to the CuO2 planes is compared to recent theories of critical order-parameter fluctuations. The experimental results agree well with the theoretical predictions in the upper part of the superconducting transition. In the lower part of the transition dissipative flux motion prevails.