Current controlled high Tc superconducting switch

Current controlled high Tc superconducting switch Q . Y . M a and E.S. Y a n g Microelectronics Sciences Laboratories and Center for Telecommunications Research, 1312 Mudd, Department of Electrical Engineering, Columbia University, New York, NY 10027, USA When a current is applied above the critical current/c of a superconductor, the material is in its normal state and has a finite resistance. Below Ic the material becomes a superconductor with zero resistance. Switching between these t w o states can be achieved by modulating a current through the sample. Various high Tc superconducting (HTS) line structures have been made. In the normal state these structures are ordinary resistors with resistances ranging from 10 ~ to 100 kfl. The critical currents are in the range 10 #A - 100 mA. Switching behaviour has been observed in a simple divider circuit using the HTS lines at 77 K. Applications of the current controlled HTS switch to digital and logic circuits are discussed.

Keywords: high T c superconductors; critical currents; low temperature electronics

The high Tc superconductor (HTS) l has unique characteristics which can be used in electronics. Unlike a conventional metallic superconductor, HTS material has very large normal state resistivity. The typical resistivity of YBaCuO material at 100 K is of the order of 2 mfl cm, which is approximately three orders of magnitude higher than that of a metal. The large difference in resistances between normal and superconducting states is suitable for making a high Tc superconducting switch. Switching can be controlled in several ways, e.g. by an optical pulse 2-5, by a magnetic field or by an electrical current. In this paper, a current controlled superconductor switch (CCSS) is presented. Switching between normal and superconductor states is achieved by modulating a current below or above the critical current Ic of a sample. To realize a better switch, the normal state resistance of a sample has to be compatible with that of a conventional resistor. By using the patterning techniques developed 6'7, it was possible to pattern the YBCO films into various line structures with resistances ranging from 10 ~2 to 100 kfl. Accordingly, the critical currents in these line structures were in the range 10 # A - 100 mA. Switching was performed in a simple current divider circuit at 77 K by modulating the applied current to a patterned superconducting line. The current controlled switch can be modified to make AND and OR logic circuits. Such circuits can be applied to low temperature digital electronics.

Normal state resistance The normal state resistance of a superconductor depends on the sample geometry. For electronic applications, thin 0011 - 2 2 7 5 / 9 0 / 1 2 1 1 4 6 - 0 3 © 1 9 9 0 B u t t e r w o r t h - Heinernann Ltd

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film HTS materials are preferred s-t°. The YBaCuO superconducting films were made by rapid thermal annealing of Cu/BaO/Y203 layered structures, which were deposited on a magnesium oxide substrate by sequential electron-beam evaporation j~-~3. The films were polycrystalline and = 0 . 4 / z m thick. The films showed a typical critical temperature Tc of 80 K and a critical current density Jc of 3 0 0 - 5 0 0 A cm -2 at 77 K. Normal state resistivity of the film was ---2 m~2 cm at 100 K, corresponding to a sheet resistance of 50 ~ per square measured by a four-point probe technique. The sheet resistance mentioned above is not suitable for connection with measurement circuits. To achieve a large resistive switch, the normal state resistance of a sample has to be adjusted to a value compatible with that of a conventional resistor. A new method of using a S i - Y B a C u O intermixed system for patterning the HTS films 6'7 has been developed. This method allows the fabrication of various superconducting line structures with different lengths and widths. The resistances of the patterned structures were in the range 10 f l - 1 0 0 k~. Figure 1 shows the temperature-dependent resistance of a typical line, 10 #m wide and 0.5 mm long. This line has a normal state resistance of 1.4 kfl at 100 K and zero resistance at 79 K.

CCSS and applications A current controlled superconducting switch (CCSS) is constructed by using a patterned superconducting line (S) as a resistor in parallel with an external load resistor Re, as illustrated in Figure 2a. The input is an a.c. current source and the output is the voltage across RL. If the

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input current is smaller than the critical current of the line structure, Jr,., at 77 K, then all of the current will go through the S loop since S = 0. Thus the output is zero, corresponding to an OFF state. When the input current exceeds 1~, the line becomes a normal resistor. The input current is divided into two loops and the output from the divider has a voltage value. The switch is turned ON. The voltage output of such a switch depends on the ratio of two resistances, S and Rt+, i.e. V,+, = IRLS/(S + RL). In general, S has to be very large to exhibit measurable switching behaviour. Ideally, it is expected that S = 0 at 77 K. However, the contact resistance of metal leads to contact pads of the line structure was = 0 . 1 - 1 ~2 for a

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A u - Y B a C u O contact ~4. The normal state resistance of S must be much larger than this. Usually, we choose 1 _ l0 n A cm 2) a smaller value of S can be chosen for the switching measurement. The CCSS can easily be modified to make A N D and OR logic circuits. Figure 3 shows the two circuits. For an OR circuit, whenever input Id or Ib is larger than the critical current I~ of the CCSS element, the switch is ON. An AND circuit requires two CCSS elements in parallel, as shown in Figure 3b. To turn the switch ON, the input must have 1, + Ib > 2L. If input /, or I b > /~ but < 21,., the switch is OFF. An extension of the above circuits can be applied to many integrated circuits to make a superconducting electronic system operated at 77 K. Conclusion

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A current controlled high T, superconductor switch operated at 77 K has been constructed. Switching between normal state and superconducting states has been demonstrated by modulation of current in a simple current divider circuit. This CCSS is expected to switch at a very high speed. Recent studies 2-5 show that YBaCuO film material has a very fast response time ( < 1 ns) to an optical pulse. This is a non-thermal response, which may be due to hot electron transport effects. The measurement

Cryogenics 1990 Vol 30 December

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Superconducting switch: Q.Y. Ma and E.S. Yang

technique will be further improved to detect the fast switching phenomenon.

Acknowledgements The authors would like to thank Dr Chin-An Chang and C.E. Farrell for deposition of the films and Dr X. Wu, L. F. Luo, C. Shu, V. Treyz and Professor R.M. Osgood for their assistance. This work was supported by the National Science Foundation through the Center for Tel~ommunications Research, and by the Office of Naval Research.

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3 Frenkel, A., Saifi, M.A., Venkatesan, T., Lin, C., Wu, X.D. and Inam, A. Appl Phys Lett (1989) 54 1594

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