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

1. 磁控溅射生长La_<1.9>Sr_<0.1>CuO_4超导薄膜在730 ~ 770℃的温度下，在SrTiO3（100）衬底上生长出La_<1.9>Sr_<0.1>CuO_4（001）超导薄膜，薄膜呈金属状，在29 K.2的温度下表现出超导转变。降低Au电极在La_<1.9>Sr_<0.1>CuO_4（001）薄膜上的接触电阻通过原位溅射清洗和Au蒸发结合快速热退火，使接触电阻降低到，1 × 10^<-50>Ωcm^2，适用于小台面上电极。在横向尺寸为10 × 10mm，厚度为13 nm的薄膜表面，采用氩离子铣削和光刻技术制备了20层CuO_2薄膜。Mesas在300 K时表现出金属行为，电阻率为13 Ωcm。台面显示电阻分流结的电流-电压特性。利用不同掺杂水平的Bi_2Sr_2CaCu_2O_<8+8>单晶的裂面顶部制作小平台，发现超导隙和赝隙明显不同。这在未掺杂的样品中尤为明显。这一实验结果意味着排除了高t_c超导性的涨落模型。短脉冲层间隧穿光谱能量范围的扩大脉冲波形由矩形波形转变为矩形波形和半周期正弦波形的组合波形，击穿电压由传统情况下的单个结击穿电压100 mV显著提高到200 mV以上。在此范围内，几乎不发生试样断裂。

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.