IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989 88 zy 1 zyxw PROPERTIES OF THE MICROWAVE SQUID WITH AN YBaCuO POINT CONTACT JUNCTION T. Ryhinen and H. Seppa Electrical Engineering Laboratory, VTT, Technical Research Centre of Finland and Laboratory of Metrology, HUT, Helsinki University of Technology Otakaari 7B, SF-02150 ESPOO, Finland Abstract A high sensitivity microwave SQUID, suitable for rf attenuation measurements, is constructed by taking advantage of the low den- sity of states of current carriers in high-T, materials. In this article the dynamics, the noise properties and the sensitivity of the mi- crowave SQUID are theoretically analyzed. The ultimate energy resolution of the waveguide SQUID is inversely proportional to the characteristic frequency zyxwvutsrq U, zyxwvutsrqp = zyxwvutsr Rq/L of the loop. zyxwvutsrq w, is high for YBaCuO junctions at zyxwvutsrqp 4.2 K because of small leakage. A flux sensitivity of 4.10-'@0/& has been achieved, which is in good agreement with the theoretlcal predictions. Introduction After the discovery of new high-T, superconducting materials [I] a great deal of effort has been made to find out the ways to pro- duce reliable SQUID magnetometers capable of operating at liq- uid nitrogen temperature. Not only the high but also the low temperature applications of these materials are quite promising; the large gap and the low density of current carriers can be uti- lized in special SQUIDs. All SQUID theories suggest that the resolution is inversely proportional to the shunt resistance of the Josephson junction; tunnel junctions are favored because the damping of the junction can be set by an extra resistor. In point contacts, zyxwvutsrqpo pc parameter remains very low because of a strong normal electron leakage through the weak link. The point con- tacts are, however, used in SQUIDs and especially in microwave SQUIDs because the junction capacitance is low and the critical current can be adjusted. These properties are very tempting at least in rf attenuation measurements [2] and in rf biased R-SQUID [3] used to establish an absolute temperature scale at mK-range. In this article we discuss the possibilities to produce a low- noise microwave SQUID to be used in accurate rf measurements employing high-Tc ceramic materials. It is shown in Ref. [4] that the dependence of the point contact on voltage and power restricts the improvement of the rf attenuation measurements by the mi- crowave SQUID. Both these limitations will be strongly reduced by replacing metallic superconductor by ceramic one. Because of a high leakage through a point contact, the microwave SQUID operates usually in the mode where the pump frequency wp ex- ceeds the cut-off frequency U, = Rq/L of the SQUID loop. If wp > we, the SQUID operates in the quasi-nonhysteretic mode and its dynamics can be examined by separating the dynamics into the low and high frequency parts. The analysis shows that also the response of the high pL device can be tuned to produce a nearly sinusoidal flux-to-voltage response, which is a very im- portant property regarding to the accurate attenuation measure- ments. It should be noted here that an ordinary inductive SQUID exhibits sinusoidal response only if the ,& parameter is very low. Therefore, the sensitivity cannot be maximized. The low density of normal electron states at low temperatures in ceramic materi- als shifts the cut-off frequency of the practical SQUID over the range of 10 GHz. Hence, the X-band SQUID no more operates in the quasi-nonhysteretic mode. The performance of the SQUID formed by the X/4 section of the waveguide resembles the SQUID operating in the mode wp > w,. In other words, a waveguide SQUID enables the producing of a sensitive gauge for accurate rf measurements. We have constructed a waveguide SQUID operating at 9.3 GHz including a point contact junction of YBaCuO, whose construc- tion is discussed first. The operation of the microwave SQUID is discussed and the noise performance is analyzed. Finally attenu- ation measurements and error sources are briefly discussed. Apparatus The microwave SQUID structure used in our measurements is shown in Fig. 1. The SQUID loop is formed by the last X/4 sec- tion of the X-band waveguide. A point-contact Josephson junc- tion is placed at the distance X/4 from the end of the waveguide. The waveguide is tapered to match the 50 n line to the 10 load. The low-frequency signals are inductively coupled to the SQUID loop by separate coils placed inside the waveguide as shown in Fig. 1. The geometric inductance of the SQUID loop is L M 0.15 nH, and the mutual inductance between the closer coil and the SQUID loop is M x 100 pH. r 5OR COAXIAL CABLE A/Z Z550R z =(OR SOR FLEXIBLE COAXIAL CABLES BIAS SIGNAL COIL \ WOOD-METAL Figure 1: 9.3 GHz. The structure of the X-band SQUID operating a' 0018-9464/89/0300-0881$01 .000 1989 IEEE