Tunneling Spectra Of Underdoped Bi₂Sr₂CaCu₂O_8+δ Instrinsic Josephson Junctions

Superconducting mechanism and origin of high-temperature superconductors (HTSC) material cuprates have been widely studied for about two decades since 1986 when the HTS were found, and are of much interest for their promising practical application. In the active devices field, study of Josephson devices is one of the most active issue. The SQUID (Superconducting Quantum Interference Device) which is from Josephson effect is used in the earth's magnetic, heart magnetic, brain magnetic measurement, Nondestructive testing and digital circuits. We can analyse Josephson effect, superconducting gap (SG) and pseudogap (PG) by measuring the tunneling spectra of Bi₂Sr₂CaCu₂O_8+δ (called Bi-2212 in brief). Bi-2212 is a hole-doped HTSC with highly anisotropic properties, and Intrinsic Josephson Junctions (IJJs) made of Bi-2212 are a unique type of tunnel junctions Nowadays, almost all the experiment study focused on the optimal doped and overdoped Bi-2212, so we expect to learn more about the characteristic of the superconductor material by the intrinsic tunneling spectra of underdoped Bi-2212.Along c-axis direction, Bi-2212 has many CuO2-double-layers, we could fabricate IJJs by cleaving single crystal along the BiO plane always at the liquid Nitrogen temperature and evaporating metal film at high speed in vacuum, then lithography, reaction ion etching (RIE) ,Ar+ etching the stack mesas, lift-off, sputtering and so on. After these, we can measure the R - T and I - V characteristic. In the course of the series experiment, we got the main findings as follows:(1) By exploring the technology and alalysing the self-heating, the reproducible micron scale area and a well-controlled given number junctions were fabricated successfully, which could avoid the self-heating problem at very low temperature.(2) Based on the actual experimental condition, we have developed a set of measurement program for the R - T and I - V characteristics in LabVIEW language, and the tunneling spectra have been obtained successfully by numerical differentiation of the I - V curves. However, at the room temperature, the spectra shows serious hysteresis and distortion due to the remaining heating when bias current I is sweeping up and down at high voltage value. The short-pulse measurement technique is then used to avoid the self-heating problem in this case.(3) The R - T curve of the underdoped Bi-2212 shows that the resistance of the single crystal becomes bigger along with the temperature lower above Tc , which is different from the overdoped and optimal doped Bi-2212. It is a way to distinguish the unerdoped between the overdoped single crystal.(4) Recently, much attention has been drawn to the pseudogap, the superconducting gap and their interplay in HTSC. With the decreasing doping, the SG value 2Δand the PG value increase, but the critical temperature gets lower. At the same time, we have observed a clear"peak-dip-hump"structure in intrinsic tunneling spectroscopy at low temperatures, which is similar to the other tunneling measurement (STM, Break junctions etc.)Our most results are consistent with previous studies and some are good agreement with the experimental dates and the phase diagram. So our experimental and measurement methods are feasible. Meanwhile, some of the results are different from other material, but we believe that they are helpful further understanding the even less scale mesas and the tunneling spectra..