多层交替沉积后退火处理MgB_2超导薄膜上约瑟夫森结的制备

Josephson junctions fabrication in multi-layer deposition and post annealing MgB_2 thin films

在多层交替(SiC/[Mg/B]5)沉积后退火处理的MgB2薄膜上用紫外光刻和Ar离子刻蚀制作出SQUID环路膜条,然后用聚焦离子束(FIB)刻蚀方法在SQUID的环路上制作了150～300nm之间不同尺寸的纳米微桥结构,并测量了其电阻温度(R-T)曲线和电流电压(I-V)曲线.膜条的R-T曲线与薄膜基本相同,表明薄膜没有受到膜条制备过程中潮湿的影响.对SQUID的R-T关系测量发现电阻有较大升高,并看到由纳米微桥的存在而具有的结构.SQUID的I-V曲线表明,纳米微桥形成了弱连接,超流主要体现为约瑟夫森耦合电流.其中一个150nm宽纳米微桥的SQUID,其回滞消失的温度约为10K,在此温度下,得到临界电流Ic约为4.5mA,IcRN～2.25mV,单个纳米微桥结的临界电流密度约为1.5×107A/cm2.临界电流Ic随温度以幂指数关系变化,也验证了纳米微桥的弱连接特性.我们的实验对基于MgB2薄膜的约瑟夫森器件制备具有参考价值.

Josephson junctions are fabricated by focused ion beam in MgB2 thin films. The films were prepared by deposite multi-layer Mg and B films at room temperature first and then annealed at about 700℃. These films have Tc of 32 K. Strips and SQUID loops were defined by photolithography and Ar ion beam milling. Then nanobridges of 150–300 nm wide were made by focused ion beam etching. The R-T characteristics of the strip is the same as that of the raw film while SQUIDs have a ＂foot＂ near the critical temperature. The I-V curve of one of the SQUIDs is hysteretic below about 10 K. Nanobridges show features of the resistively shunted junction like, and the temperature dependence of the critical current turns out to be of SINS weaklink. At 9.8 K, the critical current density is about 1.5×107 A/cm2.