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Journal of Applied Physics

Magnetic field dependence of micromachined Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions with a submicron loop

S. J. Kim, J. Chen, Kenji Nakajima, Takashi Yamashita, S. Takahashi, Toshihisa Hatano

Published May 15, 2002• 19 citations

Abstract

Two-stacked intrinsic Josephson junctions (IJJs) on a Bi2Sr2CaCu2O8+δ(Bi-2212) whisker with a submicron hole are micromachined by a 3D focused ion beam (FIB) etching method. The junction area, the number of elementary junctions, and the loop area of the SQUIDs are 3, 20, and 0.42 μm2, respectively. The SQUIDs show typical I–V characteristics of IJJs with a critical current IC of 12 μA at 4.2 K. The modulation parameter βL(=2ICL/Φ0) is 6.4, and the inductance L of the SQUID is 1120 pH. That includes the inductances of the elementary junction around a SQUID loop. In a strong magnetic field, the Fraunhofer pattern is obtained and well fitted to the calculated value from the junction size, ΔB=Φ0/tW; t is the interlayer distance, and W is the junction width. In a low magnetic field, approximately 10% of the IC modulation corresponding to the value of βL is observed. Eighty G of the modulation period approximately agrees with the value calculated from the SQUID.

DOI: 10.1063/1.1448792

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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.

The fabrication of ultrasmall high-quality intrinsic Josephson junctions (IJJs) using a cuprate superconductor is critical for the realization of a qubit. We investigated the mechanism of damage induced by a Ga + beam in a Bi 2 Sr 2 CaCu 2 O 8+δ IJJ during focused ion beam (FIB) processing. On the basis of the results, we developed a process that allowed the successful fabrication of an ultrasmall and high-quality IJJ. The damage induced by the FIB is reduced by restricting the direction of the Ga + beam so that the junction area is not directly irradiated by the ion beam. The fabricated ultrasmall IJJ device has a junction area of 0.3 µm 2 and shows excellent current–voltage characteristics.

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.

For the observation of the charging effect on a Josephson junction, the charging energy (E c) must be much larger than the Josephson coupling energy (E J) and thermal energy (k B T). We investigated the properties of submicron-sized Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions (IJJs) at low temperature. The current–voltage curve of IJJs with a junction area of 0.06 µm2 showed a much higher resistance state near the zero bias voltage regions when the measurement temperature decreased to 72.6 mK. The ratios of E c/E J and E c/k B T were 7.04 and 5.78, respectively, which indicates that the charging effect is dominant. In addition, a characteristic voltage structure due to the charging effect in one-dimensional junction array, which is composed of the offset voltage and the threshold voltage of the junction, was observed.

Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La2−xSrxCuO4 (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multibranches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method.