We make stacks of intrinsic Josephson junctions (IJJs) imbedded in the bulk of very thin (d ≤ 100 nm) Bi2Sr2CaCu2O8+x single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height (0 − 9 nm < d), i.e. enclosing a specified number of IJJ (0 − 6), including the important case of a single junction. We discuss reproducible gap-like features in the current-voltage characteristics of the samples at high bias. [1,2]. A BSCCO single crystal in the c-axis direction can thus be viewed as a large array of very thin (1.5 nm) intrinsic Josephson junctions (IJJs) in series. At present, this is perhaps the only reliable way to obtain HTS SIS junctions. These junctions appear to have high potential for high-frequency applications like heterodyne receivers and quantum voltage standards [3,4].It is difficult to isolate and study individual IJJ. Most of applications and research on IJJs use therefore stacks containing many junctions. The stacks can be formed either on the surfaces of single crystals (mesas) or etched out in the middle of BSCCO whiskers using focussed ion beams [5]. Earlier we developed a method for making a single intrinsic Josephson junction (SIJJ) enclosed in a U-shaped mesa structure [6]. An effective SIJJ could be seen in the 4-probe measurements on such a mesa. However, extra junctions under the electrodes can give rise to unwanted local heating [7,8].In this letter, we report on reproducible fabrication of uniform stacks formed inside the very thin (d ∼ 100 nm) pieces of BSCCO single crystals by using a double-sided etching technique [9]. The number of junctions in the stacks can be tailor-made to any small number N < 10 including the important case of N = 1 (a single-junction stack). As the stacks are imbedded in the superconducting material, i.e. there are no direct resistive contacts to them, heating effects can be minimized in such stacks allowing the genuine current-voltage (I-V) characteristics to be traced out. Devices like SQUIDs are feasible to make using the developed technique.The stack-fabrication routines start with evaporation of an Au thin film on a freshly cleaved crystal glued onto a sapphire substrate using polyimide. Using the conventional photolithography and Ar-ion etching, a bow-tieshaped mesa with a micro-bridge in the center is formed on the crystal. The overall thickness of this mesa d is typically about 100 nm which is controlled by etching time and rate. Then, a 40 nm d/2 deep slit is made across the bridge. In the next step, we flip the sample and glue it onto another sapphire substrate, sandwiching the single crystal between the two substrates. Separating the substrates cleaves the single crystal into two pieces, one with the mesa being flipped upside down and glued to the second substrate. We subsequently remove all material but the mesa by iteratively cleaving the former with the aid of Scotch tape and inspecting the resulting sample in optical microscope. A ne...

In stacks of intrinsic Josephson junctions (IJJs) with lateral sizes of several microns, the current is non-uniform in many cases. In certain geometries a significant part of the current flows along the superconducting planes and can reach the critical value. The current-driven superconductivity breakdown within a single Cu 2 O 4 plane can be seen as an extra branch structure of the c-axis current-voltage characteristics. This allows us to deduce the sheet critical current of a single Cu 2 O 4 plane in different measurement configurations. The conditions for the observation of such a current-induced transition in different IJJ geometries are discussed.

We use the temperature dependent c axis critical current and characteristic gap voltage as temperature gauges to assess the self-heating in stacks of intrinsic Josephson junctions etched out from the ultrathin Bi2Sr2CaCu2O8+δ (BSCCO) single crystals. The thermal resistance which characterizes the overall cooling efficiency is found to be ∼120–160 K mW−1 for the two stacks studied. This value is in agreement with earlier reports for stacks of similar geometry, while it is about two to three times higher than for simple mesas made on the surfaces of mm-size BSCCO single crystals. The suggested method can guarantee the validity of the temperature estimations inside the stacks of various geometries and different cooling efficiencies without the need for attaching additional thermometers.

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