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IEEE Transactions on Magnetics

Single Cooper-pair tunneling junctions using high-T/sub c/ superconducting materials

S.-J. Kim, Yu. I. Latyshev, Takashi Yamashita

Published Jan 1, 1999• 1 citation
Publisher

Abstract

We introduce the single electron (Cooper-pair) tunneling junctions using c-axis Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+d/ (Bi-2212) superconducting single crystal whiskers. Focused-ion-beam (FIB) etching patterned the Bi-2212 whiskers. The fabricated small stacked junctions have in-plane area S smaller than <1 /spl mu/m/sup 2/. The junctions showed current-voltage (I-V) characteristics with the periodic structure of current peaks. The stacking layered structure of Bi-2212 works as a multi-junction array which decreases the effective capacitance, C/sub /spl Sigma//=C/sub 0//N, where C/sub 0/ is the capacitance of the junction and N is the layer number of elementary junctions. The period of current peaks of I-V curves corresponds to the charging energy of the single Cooper pair, 2Ec (=e/sup 2//C/sub /spl Sigma//).

DOI
10.1109/20.800766

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We introduce the single electron (Cooper-pair) tunneling junctions using c-axis Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+d/ (Bi-2212) superconducting single crystal whiskers. Focused-ion-beam (FIB) etching patterned the Bi-2212 whiskers. The fabricated small stacked junctions have in-plane area S smaller than <1 /spl mu/m/sup 2/. The junctions showed current-voltage (I-V) characteristics with the periodic structure of current peaks. The stacking layered structure of Bi-2212 works as a multi-junction array which decreases the effective capacitance, C/sub /spl Sigma//=C/sub 0//N, where C/sub 0/ is the capacitance of the junction and N is the layer number of elementary junctions. The period of current peaks of I-V curves corresponds to the charging energy of the single Cooper pair, 2Ec (=e/sup 2//C/sub /spl Sigma//).
We fabricated submicron-sized intrinsic Josephson junctions of Bi-2212 by the focused-ion-beam (FIB) etching method. The main result was a reduction of the in-plane junction area to 0.3 µm 2 by direct FIB etching with no degradation in the critical transition temperature ( T c ). At the current ( I )-voltage ( V ) characteristic of these stacks, the gap structure and the normal-state resistance are clearly observed by a reduction of the Joule heating. The critical current density is considerably suppressed, the hysteresis and multibranch structure are eliminated and a periodic structure of current peaks appears reproducibly on the I - V curves. The period V of the structure is consistent with the Coulomb charging energy of a single pair, V = e / C , where C is the effective capacitance of the stack. It is considered that this behaviour originates from the Coulomb blockade of intrinsic Josephson tunnelling in submicron Bi-2212 stacks. We demonstrated the junction operating up to 10 K for the stack with an in-plane size of 0.3 µm 2 and an elementary junction number of 50.
We investigate critical current (Ic) modulation of stacked Josephson junctions (SJJs) dc superconducting quantum interference device ( dc SQUID) on a Bi 2 Sr 2 CaCu 2 O 8+δ ( Bi -2212) whisker. A coupled SJJs with a completely perforated hole of 0.1 μ m × 0.2 μ m were fabricated by using a 3-D focused-ion-beam (FIB) etching method. Those dc SQUIDs exhibited uniform multi-branch structures and clearly oscillated critical current under external magnetic fields (He). With applying He, the clear periodic modulation patterns of Ic are obtained and well fitted to the calculated value from the loop area of 0.02 μm2.
We fabricated submicron-sized intrinsic Josephson junctions by the focused-ion-beam (FIB) etching method. The principal result was a reduction of the in-plane junction area to 0.3 μm2 by direct FIB etching with no degradation in the critical transition temperature (Tc). In the current (I)–voltage (V) characteristics of these stacks, the gap structure and the normal state resistance are clearly observed together with a reduction of the Joule heating and disappearance of the branch structure. The Coulomb staircase structure was found in the I–V curves of submicron junctions as a result of their small effective capacitance of fF order.
Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.
In view of future macroscopic quantum experiments, we have fabricated prototypes of charge qubits (with tunable Josephson coupling and capacitive gate) provided with capacitively coupled readout single-electron transistor electrometers. The devices are prepared by electron beam direct-writing in conjunction with material deposition by sputtering. All leads are made of Al, and the AlO/sub x/ tunnel barriers are generated by local thermal oxidizing of the surface of the base striplines. The island electrodes are made of Al or Nb in a subsequent deposition step. In the case of Nb it is suitable to protect the barriers by a thin sputtered Al film first. The small-area (100 nm/spl times/100 nm) tunnel junctions have capacitances in the order of a few 10/sup -16/ F; their resistances can be adjusted in a wide range from k/spl Omega/ to M/spl Omega/ with an on-chip spread of less than 10%. The (double) superconducting gap energy was determined to be about 0.4 meV for Al and 1.4 meV for Nb, respectively. Measuring the so-called Coulomb staircase, we have demonstrated the operation of both qubit boxes and sub-e electrometers.
We present observations of charging effects for Cooper pairs in short linear arrays of small-capacitance Josephson junctions. Current-voltage characteristics show a Coulomb gap for Cooper-pair tunneling when the charging energy exceeds the Josephson coupling energy. In a double junction a zero-voltage current is observed that is modulated by a gate voltage applied to the metal island between the junctions. For longer arrays a crossover from Coulomb blockade of Cooper-pair tunneling to a supercurrent is observed when the ratio of Josephson coupling to charging energy is increased.