Xindi Wu

Correlation of surface topography and flux pinning in superconducting YBaCuO films

The surface topography of YBa2Cu3O7 thin films prepared by pulsed laser deposition is studied by atomic force microscopy and scanning tunneling microscopy. The possible contributions from observed structural features to the critical current density Jc are discussed. It is shown that the contribution of the screw dislocations associated with some structures can be either repulsive or attractive, depending on the dominant pinning mechanism. At low magnetic fields (to about 0.25 kOe) these dislocations can account for the observed critical current density values (Jc≳1011 A/m2) while for larger fields the higher‐density steplike growth structures can account for significant values of Jc. It is shown that the pinning strength associated with pinning at steps varies inversely with thickness.

Transport and magnetism correlations in thin‐film ferromagnetic oxides

To determine the Tc dependence of the colossal magnetoresistance (CMR) exhibited by the ferromagnetic La0.7A0.3MnO3+δ (A=Ba, Ca, Sr) system, we examine the magnetic‐field and temperature‐dependent resistivity and magnetization of a series of thin films that were grown via pulsed‐laser deposition. The films had magnetic ordering temperatures (Tc) ranging from 150 to 350 K. All samples display a large negative MR that is largest near Tc, and samples with a low Tc display significantly larger MR values than do samples with large Tc’s. The quantity ρ(Tc)/ρ(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, varies as exp(Ea/Tc) with an activation energy Ea=0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by ρ(Tc), but by Tc via the ratio ρ(Tc)/ρ(4 K). Phenomenological scaling relationships are also reported that link ρ(H,T) to both H and M(H,T).

Penetration depth in oxygen depleted YBCO thin films

Abstract The effect of oxygen depletion on YBCO film is investigated via penetration depth measurements. The T -dependence is measured in high precision for each oxygen concentration. λ ( T / T c )/ λ (0) vs . T / T c for our films is almost identical for different oxygen content. For a fully oxygenated film, it agrees extremely well with the crystal data for T > 25K. 1/λ 2 (0) which is proportional to superfluid density is reduced by factor 10 as T c is reduced to 48K. T c vs . 1/ λ 2 (0) in our result basically reproduces μSR result on bulk by Uemura et al. except the approximately 25% smaller value in 1/λ 2 (0) of film, which can be interpreted as impurity effect in a d -wave superconductor.

Comparison of chain (Co) and plane (Zn) dopants on charge transport in YBa2Cu3O7 − δ

We report the penetration depth, A, and infrared reflectance of a YBa 2 (Cu 0.97 Co 0.03 ) 3 O 7-δ film from 35 cm -1 to 600 cm -1 and from 10 K to 300 K. Co is a chain dopant in contrast to previous work on Zn which is a plane dopant. The effects of the chain dopants and the plane dopants are similar in the normal state. For example, the reflectance of the 3% Co sample is very similar to 4% Zn sample at 100 K and 300 K. However, in the superconducting state, 3% Co has a high reflectance similar to pure YBa 2 Cu 3 O 7-δ while 4% Zn has a gapless reflectance. The penetration depth of the Co doped sample is close to that of pure YBa 2 Cu 3 O 7-δ while that of the Zn doped film is a factor of five larger.

Transport and magnetism correlations in thin-film ferromagnetic oxides

In order to determine the {Tc}-dependence of the colossal magnetoresistance (MR) exhibited by the ferromagnetic La{sub 0.7}M{sub 0.3}MnO{sub 3+{sigma}} (M = Ba, Ca, Sr) system, the authors examine the magnetic-field and temperature dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (T{sub C}) ranging from 150 to 350 K; all samples displayed a large negative MR that is largest near {Tc}. The magnitude of a given sample`s MR at {Tc} inversely correlates with {Tc}; samples with a low {Tc} display significantly larger MR values than do samples with large {Tc}`s. The quantity {rho}({Tc})/{rho}(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, is an activated function of {Tc} with an activation energy E{sub a} = 0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by {rho}({Tc}), but by {Tc} via the ratio {rho}({Tc})/{rho}(4 K). Phenomenological scaling relationships are also reported that link {rho}(H,T) to both H and M(H, T).

Measurement of penetration depth λ(T) in YBa2Cu3O7−δ thin film

Abstract We present the magnetic penetration depth λ(T) in two laser ablated YBa2Cu3O7−δ thin films, determined by the two-coil method. The slope of 1 λ 2 (T) near Tc, the only exactly measurable quantity, extrapolates to 930A at T = 0, which compares well with 830A in the high quality single crystal recently reported by Hardy , and which exhibited a linear T dependence at low T. For T