A resistive d.c. SQUID noise thermometer

A resistive d.c. SQUID (superconducting quantum interference device), d.c. RSQUID for short, is a superconducting loop that contains two shunted Josep hson junctions and a resistor. Our d.c. RSQUIDs are intended for noise ther mometry below 4.2 K. They are fully integrated thin film devices fabricated in standard Nb technology. The resistor is made from an evaporated silver layer. II is specially designed in order to suppress the proximity effect, and thus the resistor remains normally conductive down to mK temperatures. For read-out, modified d.c. SQUID electronics are used. It is directly coup led to the d.c. RSQUID and represents a preamplifier with a voltage noise o f 0.8 nV/Hz(1/2), a gain of 4000, and a bandwidth of 4 MHz. Due to this large bandwidth, the d.c. RSQUID can be operated at a high freq uency enabling a short averaging time for the temperature measurement. At a frequency of 2 MHz and a temperature of 4.2 K we measured the temperature with a statistical uncertainty of 0.8% for 500 s averaging time. (C) 1999 E lsevier Science Ltd. All rights reserved.