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The microwave power dependence of high quality Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films grown on 1'' diameter LSAT substrates using the Amorphous Phase Epitaxy method has been investigated in this paper. We have performed surface resistance (R/sub S/) measurements of Tl-1223 thin films on LSAT using the Hakki-Coleman sapphire dielectric resonator technique in the temperature range 15-94 K. High accuracy in measurements was achieved by using the multi-frequency Transmission Mode Q-Factor Technique. The Tl-1223 films exhibited low surface resistance comparable to YBCO films. When RF power levels were increased the onset of nonlinearity was observed at 0 dBm for all measured temperatures (17, 50, 70 and 84 K).

Epitaxial Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub y/ and Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films have been prepared on CeO/sub 2/ buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the Tl-2212 films showed well connected and smaller sized grains of 1 /spl mu/m or less. In the case of Tl-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm/sup 2/ at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm /spl times/ 10 mm. The temperature dependence of surface resistance of Tl-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the Tl-1223 exhibits slightly higher values resulting from an inferior surface morphology.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUID's.

A change of electronic structure and chemical bond nature of high pressure synthesized (Cu,Tl)Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [(Cu,Tl)-1223] and TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [Tl-1223] with reduction-annealing, which leads to a significant rise of their superconducting transition temperature T/sub c/ above 130 K, has been investigated by in-situ photoemission spectroscopy. Their intrinsic surface nature has been achieved by fracturing in vacuum. In (Cu,Tl)-1223, a Fermi edge is observed for the first time. Trivalent Tl and heavily over-doped states of Cu-O clusters are dominant in as-grown specimens of both systems. The annealing resulted in remarkable shifts of photoemission spectra of Ba and Tl core levels towards higher binding energy with an increase of the annealing temperature, which means a selective oxygen-depletion around their charge-reservoirs and a decrease of valency of Tl. In contrast, there are quite little shape-changes and movements in valence band spectra and core signals of Cu and O. These results reveal that, in the high temperature-treated specimens, major portions of the electrons left should be absorbed into the cations in their charge reservoir, which consequently should cause a much more moderate change of the hole-concentration of the CuO/sub 2/ planes. It suggests that the utilization of this charge-redistribution mechanism in over-doped specimens should be an advantageous way to optimize the electronic structure of the 1223 phases for high T/sub c/.

The bi-axially-oriented thin films of Cu/sub 1-x/Tl/sub x/(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10-y/(Cu/sub 1-x/Tl/sub x/-1223: x=0.2-1.0) were prepared by an amorphous phase epitaxy (APE) technique on SrTiO/sub 3/ substrates. The amorphous phase was deposited by sputtering on the substrate and annealed in a thallium atmosphere at /spl sim/885/spl deg/C for /spl sim/50 min. Their T/sub c/ values were 100-113 K and higher than those of the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y-123, that is, Cu-1212) system. Their J/sub c/ values were 2/spl times/10/sup 7/ A/cm/sup 2/ (77 K, 0 T) and 4/spl times/10/sup 5/ A/cm/sup 2/ (77 K, 10 T) at their maximum for x/spl sim/0.5. The J/sub c/ values were higher than the maximum values of Y-123 thin films by a factor of 2. The high J/sub c/ values are consistent with the low superconducting anisotropy (/spl gamma/=/spl xi//sub ab///spl xi//sub c/=2.5-4) of Cu/sub 1-x/Tl/sub x/-1223 and its high T/sub c/.

We have examined the relationship between the surface resistance and the depairing current density of superconductors. From the two-fluid model and the relationship between the depairing current density and the thermodynamic critical field, we can show that the surface resistance is in inverse proportion to the depairing current density. We also experimentally examined the correlation between the surface resistance and the critical current density of YBCO thin films, and found that the surface resistance is in inverse proportion to the critical current density. The results mean that our theoretical analysis is suitable for predicting the correlation between the surface resistance and the critical current density.