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IEEE Transactions on Microwave Theory and Techniques

Terahertz Shapiro steps in high temperature SNS Josephson junctions

Peter A. Rosenthal, Erich N. Grossman

Published Apr 1, 1994• 47 citations
Publisher

Abstract

We have studied the far infrared behavior of high -T/sub c/ superconductor-normal metal-superconductor (SNS) microbridges with T/sub c/>85 K and critical current-resistance products (I/sub c/R/sub N/) as high as 10 mV at 4 K. These are the highest I/sub c/R/sub N/ products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R/sub N/ between 6 and 38 /spl Omega/, reasonably well matched to the antenna's estimated RF impedance of 53 /spl Omega/. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T/sub c/ junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics.>

DOI
10.1109/22.285085

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We have studied the far infrared behavior of high -T/sub c/ superconductor-normal metal-superconductor (SNS) microbridges with T/sub c/>85 K and critical current-resistance products (I/sub c/R/sub N/) as high as 10 mV at 4 K. These are the highest I/sub c/R/sub N/ products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R/sub N/ between 6 and 38 /spl Omega/, reasonably well matched to the antenna's estimated RF impedance of 53 /spl Omega/. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T/sub c/ junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics.>
We have studied the far infrared behavior of high -T/sub c/ superconductor-normal metal-superconductor (SNS) microbridges with T/sub c/>85 K and critical current-resistance products (I/sub c/R/sub N/) as high as 10 mV at 4 K. These are the highest I/sub c/R/sub N/ products reported to date for microfabricated Josephson junctions of any material. The junctions were integrated at the feeds of planar log-periodic antennas made from Au thin films. The junctions had dc normal state resistances R/sub N/ between 6 and 38 /spl Omega/, reasonably well matched to the antenna's estimated RF impedance of 53 /spl Omega/. Far infrared laser radiation at 404, 760, and 992 GHz induced distinct Shapiro steps (i.e. constant voltage steps at voltages n(hf/2e), n=1,2,...) in the current voltage characteristics as well as modulation of the critical current. Steps were observed at voltages up to 17 mV and 6 mV, at temperatures of 9 K and 57 K respectively. This corresponds to maximum Josephson oscillation frequencies of 8 and 3 THz at these temperatures. These are the first far infrared measurements performed on high T/sub c/ junctions. Measurements of the power, frequency, and temperature dependence of the Shapiro steps are presented and discussed in the context of a resistively and capacitively shunted junction (RCSJ) model. A value of 4.5 fF for the junction capacitance is inferred from the hysteresis of the slightly underdamped current-voltage characteristics. >
We have fabricated step edge superconductor-normal metal-superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current-normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor-normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current-voltage characteristics with a 760 GHz beam from a far infrared laser.
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Superconducting step-edge junctions were fabricated on step-edges in (100) LaAlO/sub 3/ substrates by sputter-deposited epitaxial YBaCuO and TlBaCaCuO thin films. The substrate steps were patterned by ion beam etching using metal film as a mask. The transport properties of the junctions were characterized by four-probe electrical measurements. The critical current-normal resistance product (I/sub c/R/sub N/) was larger than 0.2 mV (77 K) for most of the junctions. Clear Shapiro steps were observed in the current-voltage characteristics of the junctions with 36 GHz or 95 GHz millimeter-wave irradiation. The high frequency properties of these high T/sub c/ Josephson junctions showed that they were quite suitable for the Josephson harmonic mixer used in the digital phase locked millimeter and sub-millimeter wave sources.>
We present experiments on the far infrared response of intrinsic Josephson junctions in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+x/ (BSCCO). We detected first order Shapiro steps on resistive branches in a frequency range between 584 GHz and 2.5 THz. Due to the very low power coupled into the junctions the suppression of the critical current was only weak. With increasing number of junctions in the resistive state a higher amplitude of the Shapiro steps was measured. As the frequencies are well above the plasma frequency of our junctions, the steps are expected to be stable at any power level. To verify this, step heights were studied at various power levels as a function of frequency. The impedance matching of the system improved considerably when several junctions have switched into the resistive state.
A simple process has been developed for microfabricating high transition temperature superconductor-normal-metal-superconductor (SNS) Josephson devices that operate up to 80 K and are reasonably ideal at 43 K. Bridge resistances greater than 10 Omega and critical-current-normal-state resistance (I/sub c/R/sub N/) products greater than 1 mV have been achieved. Clearly defined RF steps have been observed, with power dependence qualitatively similar to theoretical predictions. The fabrication process and the device characteristics are suitable for superconducting integrated circuit applications such as millimeter-wave Josephson oscillators, parametric amplifiers, and single-flux quantum digital logic.