Nano-SQUID中方波脉冲测量方法对于临界电流的精确表征  

ACCURATE CHARACTERIZATION OF THE CRITICAL CURRENT OF NANO-SQUIDS BY A SQUARE-WAVE-PULSE MEASUREMENT METHOD

作　　者：柳西西 刘晓宇[1] 汪浩[1] 陈垒[1] 王镇[1] 

机构地区：[1]中科院上海微系统与信息技术研究所超导中心,上海200050

出　　处：《低温物理学报》2015年第4期276-282,共7页Low Temperature Physical Letters

基　　金：中科院先导B类(批准号:XDB04010000);国家自然科学基金(批准号:61306151)资助的课题~~

摘　　要：Nano-SQUID因为具备可能探测到单电子自旋的能力而受到广泛关注.为进一步提高该器件的灵敏度,不但要优化器件制备工艺,而且也要发展低噪音高效率的测量系统.通常,由于nano-SQUID存在回滞现象,导致在测量方面nano-SQUID无法使用标准的SQUID读出电路.本文设计了一种全新的方波脉冲测量方法,由信号发生器串联一个大电阻发送电流脉冲,注入nano-SQUID,并采用ADWin数据采集卡读取器件两端的电压.该方法不仅可以消除电流—电压回滞现象带来的测量上的不便,而且可以通过缩短脉冲周期的方法进一步提高系统的测量速度.在此基础上,通过降低系统方波脉冲占空比ton:toff,增加超导器件因受到脉冲电流激发后的冷却时间,减小热噪音对临界电流准确测量的影响,从而获得准确的临界电流—磁通调制曲线,调制深度△Ic/Ic=7.9%.此外,本文对采用本测量方法的nano-SQUID磁通噪音表征方法进行了讨论,测得器件磁通噪声为72μΦ0/Hz1/2,等效磁性测量精度为1.19×10-16emu/Hz1/2.Nano-SQUIDs （superconducting quantum interference devices） have attracted many attentions in research because they are able to detect a few or even one Bohr magnetons. In order to improve the sensitivity of the devices, one needs to not only optimize the fabrication process of nano-SQUIDs, but also develop a measurement system with extreme low noises. In general, the hysteretic critical-voltage curve hinders the nano-SQUID from being read-out by the traditional SQUID electronics. In this paper, we developed a new square-wave-pulse method for the nano-SQUID low-noise measurement. The square-wave current pulses are injected into the nano-SQUID by a function-wave generator in series with a large resistor. Then, the corresponding voltage pulse developed over the nano-SQUID are picked up by a data-acquisition card. In this arrangement, not only the inconvenience of the hysteretic critical-voltage curve is avoided during the measurement, but also the speed of the measurement system is greatly increased by reducing the pulse cycle. Furthermore, we achieved a more accurate measurement in the critical current of the nano SQUID by tuning the duty ratio to..- toff of the square-wave pulse. The thermal noise generated during the device switching were properly dissipated during the to~ time. Therefore, a more precise modulation curve of the critical-current vs magnetic flux were measured. In addition, we deduce the way of characterizing the magnetic flux noise in this measurement method. The flux noise of tested device was 72μФ0/Hz1/2. Based on its size, it corresponded to 1.19× 10-16 emu/Hz1/2.

关 键 词：nano-SQUID 方波脉冲测量 临界电流 热效应 

分 类 号：O441.5[理学—电磁学]