Study on the Physical Properties of High-Tc Superconductors by Interlayer Tunneling Spectroscopy

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1. Epitaxial Growth of La_<1.9>Sr_<0.1>CuO_4 Superconducting Thin Films by Magnetron SputteringLa_<1.9>Sr_<0.1>CuO_4 (001) superconducting thin films are grown on SrTiO3 (100) substrates by rf magnetron sputtering at substrate temperatures of 730 to 770 C. The films are metallic and exhibit superconducting transition at 29 K.2. Reduction in Contact Resistance of Au Electrodes onto La_<1.9>Sr_<0.1>CuO_4 (001) Thin FilmsBy means of in situ sputter-cleaning and Au evaporation combined with rapid thermal annealing, the contact resistance is reduced to, 1x10^<-50>Ωcm^2, which is applicable to the small mesa upper electrode.3. Fabrication and Tunneling Characteristics of Small MesasSmall mesas are fabricated by Ar ion milling and photolithography on the surface of thin films with a lateral dimension of 10x10mm and a thickness of 13 nm, which corresponds to 20 CuO_2 layers. The Mesas exhibit metallic behavior with a resistivity of 13 Ωcm at 300 K. The mesas show current-voltage characteristics of a resistively shunted junction.4. Short Pulse Tunneling Spectroscopy Using Bi_2Sr_2CaCu_2O_<8+8> Intrinsic Josephson JunctionsUsing small mesas fabricated on top of cleaved surface of Bi_2Sr_2CaCu_2O_<8+8> single crystals with different doping levels, it is found that the superconducting gap and the pseudogap is definitely different. This is particularly clear in the underdoped specimens. This experimental result implies that the fluctuation model of high-T_c superconductivity-is ruled out.5. Extension of Energy Range for Short Pulse Interlayer Tunneling SpectroscopyPulse wave form reformation from a rectangular to a combined wave form consisting of rectangular and half-period sinusoidal wave form leads to a significant improvement of break-down voltage from 100 mV for a single junction in the conventional case to greater than 200 mV. Almost no breakage of specimen takes place during measurements within this range.