Nowadays, programmable arrays with thousands of in-series Josephson junctions (JJs) are the most common devices used at metrological institutes to represent the voltage standard. Whatever basic element they use, all of them are based on the Nb and Al technology, which implies the need for liquid helium temperatures for their operation, with corresponding complications in the experimental setup and consequent remarkable costs. On the other hand, high temperature superconductors show a layered crystal structure, which can be viewed as a series of atomically flat intrinsic JJs. Therefore, a new technology providing thousands of highly packed, in-series JJs working at liquid nitrogen temperatures is theoretically possible. In the present work we explored such a possibility by fabricating stacks of intrinsic JJs out of high-quality BSCCO and YBCO single crystals with high aspect ratios (whiskers). In the case of BSCCO, stacks have been produced by means of a photolithographic process exploiting both a single cleaving and a double cleaving technique. Although these two methods differ in the typical cross section area of the stacks, both require the use of a Focused Ion Beam (FIB) to etch trenches forcing the current to flow normal to the superconducting planes. Stacks with about 100 junctions have been produced and characterized. However, their properties show a shift in the doping regime compared to pristine crystals. This seems to be related to the presence of laterally implanted Ga ions in the stack sidewalls, as nano-XRF characterization by means of synchrotron radiation has pointed out. In the case of YBCO, stacks were fabricated by means a of a process involving no photolithography and no cleaving, but the use of FIB could not be avoided. Also in this case a decrease in the critical temperature has been observed, but this seems to depend on the cross-section area in the sense of a larger decrease for smaller stacks, which sets a lower limit for stack fabrication able to work at liquid nitrogen temperature. Attempts of observing junction switching and Shapiro steps for this material are currently under way.
Fabrication of stacks of intrinsic Josephson junctions for metrological applications
TRUCCATO, Marco;AGOSTINO, Angelo;PASCALE, LISE;
2013-01-01
Abstract
Nowadays, programmable arrays with thousands of in-series Josephson junctions (JJs) are the most common devices used at metrological institutes to represent the voltage standard. Whatever basic element they use, all of them are based on the Nb and Al technology, which implies the need for liquid helium temperatures for their operation, with corresponding complications in the experimental setup and consequent remarkable costs. On the other hand, high temperature superconductors show a layered crystal structure, which can be viewed as a series of atomically flat intrinsic JJs. Therefore, a new technology providing thousands of highly packed, in-series JJs working at liquid nitrogen temperatures is theoretically possible. In the present work we explored such a possibility by fabricating stacks of intrinsic JJs out of high-quality BSCCO and YBCO single crystals with high aspect ratios (whiskers). In the case of BSCCO, stacks have been produced by means of a photolithographic process exploiting both a single cleaving and a double cleaving technique. Although these two methods differ in the typical cross section area of the stacks, both require the use of a Focused Ion Beam (FIB) to etch trenches forcing the current to flow normal to the superconducting planes. Stacks with about 100 junctions have been produced and characterized. However, their properties show a shift in the doping regime compared to pristine crystals. This seems to be related to the presence of laterally implanted Ga ions in the stack sidewalls, as nano-XRF characterization by means of synchrotron radiation has pointed out. In the case of YBCO, stacks were fabricated by means a of a process involving no photolithography and no cleaving, but the use of FIB could not be avoided. Also in this case a decrease in the critical temperature has been observed, but this seems to depend on the cross-section area in the sense of a larger decrease for smaller stacks, which sets a lower limit for stack fabrication able to work at liquid nitrogen temperature. Attempts of observing junction switching and Shapiro steps for this material are currently under way.
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