【作者】 袁涛；

【导师】 王新强； 林德华； 赵士平；

【作者基本信息】 重庆大学 ， 凝聚态物理， 2010， 硕士

【摘要】 自1986年发现高温超导材料至今的二十几年,人们对铜氧化物的高温超导电性起源和机理进行了大量的研究,同时其应用前景也成为物理学的热门课题。在有源器件领域,约瑟夫森器件的研究是最活跃的方向之一,利用其原理制成的SQUID广泛地应用于诸如大地的磁测量,心磁、脑磁的测量,无损检测和数字电路中。我们通过测量Bi₂Sr₂CaCu₂O_8+δ （简称Bi-2212）隧道谱来分析超导约瑟夫森效应,超导能隙和赝能隙的性质。Bi-2212具有极端的各向异性,用它做成的本征约瑟夫森隧道结（IJJs）是一种独特类型的隧道结。但是现有的大量实验几乎都是集中在对最优掺杂和过掺杂Bi-2212的研究上,对欠掺杂Bi-2212特别是隧道谱的研究还不多,因而我们期望通过对欠掺杂Bi-2212本征结隧道谱的研究,来更多的了解该超导材料的隧道谱特性。利用Bi-2212单晶在c方向的层状性质,在高真空中进行低温（通常达到液氮温度）原位解理,同时高速蒸发金属膜,再光刻图形,反应离子刻蚀,氩离子刻蚀形成柱状mesa,剥离,磁控溅射,制作反电极等工艺,并用改进的方法测量R - T和I - V特性,分析隧道谱的性质,在这过程中,得到的主要结果如下:（1）通过对工艺的探索和对本征结热效应的分析,成功制备了结个数可控的、结面积达到亚微米尺寸的本征结,有效避免了在低温下研究隧道谱热效应的显著影响。（2）基于实验条件,编写了一套分别用于测量R - T和I - V特性的程序,并且通过对I - V曲线的数字微分得到了本征结的隧道谱。同时,在常温附近赝能隙打开的温度上,I - V曲线有较明显的回滞,采用脉冲测量方法解决了自热效应。（3）欠掺杂Bi-2212的R - T曲线显示,在超导转变温度Tc以上,样品的电阻随着温度的降低而升高,这与过掺杂和最优掺杂的R - T曲线在Tc上是随着温度的降低而减小有明显的区别,这也成为我们通过R - T曲线来分辨欠掺杂和过掺杂单晶样品的直接手段。（4）高温超导能隙和赝能隙的关系是近年来人们非常关心的问题之一。对欠掺杂样品的I - V曲线的分析,我们得到,随着掺杂浓度的降低,样品的超导能隙增大,其临界温度降低,而赝能隙的温度值则会增大。同时,隧道谱显示在低温下欠掺杂样品同样有明显的“peak-dip-hump”结构,这类似于其它隧道手段得到的结果。本文的实验结果,与已有的结果特别是最优掺杂和过掺杂在超导能隙和赝能隙的趋势上是一致的,符合超导电子相图的一般规律,并且有可重复性,说明我们的实验方法和测量方法是可行的。同时我们也得到了一些与其它材料不同的实验结果,这对进一步探讨更小尺寸结的热效应和隧道谱特性提供了参考价值和重要的指导意义。更多还原

【Abstract】 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..更多还原