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Applied Physics Letters

Design and realization of an all d-wave dc π-superconducting quantum interference device

Robert Schulz, Boris Chesca, B. Goetz, Christof W. Schneider, A. Schmehl, H. Bielefeldt, Hans Hilgenkamp, J. Mannhart, Chang C. Tsuei

Published Feb 14, 2000• 125 citations
Publisher

Abstract

The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.

DOI
10.1063/1.125627

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The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.
A dc pi SQUID consists of a superconducting ring interrupted by two Josephson junctions, one of which carries in equilibrium a pi phase difference, caused, for example, by the d-wave pairing symmetry of the high- T(c) cuprates. If this phase shift is maintained in the voltage state, anomalous resonance currents are expected in the SQUIDs transport characteristics. Here we report the observation of such resonances for high- T(c) dc pi SQUIDs, providing evidence for the influence of the d-wave symmetry on the voltage state of a Josephson junction for frequencies of several tens of GHz.
Superconducting quantum interference devices (SQUIDs) have been realized with a built-in π∕2 phase shift of the superconducting wave function. Its design is inspired by the phase-shifter concept, first proposed by Ioffe et al. [Nature 398, 679 (1999)] as the key element in a solid-state qubit configuration. It exploits the dx2−y2 order parameter symmetry in the high-Tc cuprates in multilayer structures based on YBa2Cu3O7-Nb ramp-type Josephson contacts. By reversing the sign of the persistent circulating current in the phase-shifting element, the π∕2 SQUID can be controllably switched between two energetically degenerate ground states, with complementary magnetic flux dependencies.
The optimal design of a direct current superconducting quantum interference device (DC-SQUID) with high sensitivity requires two identical Josephson junctions, which is difficult to achieve. The design of a DC-SQUID using controlled Josephson junction technology provides a mechanism for modifying the characteristics of the device. Here we report on the nano-fabrication and measurement of a DC-SQUID utilizing two Dayem bridge weak-link Josephson junctions fabricated with a width ∼100 nm and a length of ∼200 nm. These junctions are controlled via the irradiation of hot phonons from nearby titanium/chromium normal metal constrictions which are current biased. The devices were measured at a temperature of 4.2 K and control over the critical current oscillations was observed.
A thin-film dc superconducting quantum interference device (SQUID) consisting of quasiparticle-injected superconducting weak links was fabricated and successfully operated. This SQUID was made on a silicon substrate and it consists of a ∼100-nm-thick lead alloy with two injector junctions. The characteristics of this dc SQUID are controllable by varying the injecting rate of quasiparticles; therefore, it can be operated at arbitrary temperatures below the transition temperature of the superconducting material.
We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic π phase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic π phase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π–π–π superconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the π phase shifter. The intrinsic circulating currents induced by the π phase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (Φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π–π–π SQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future.
The direct current superconducting quantum interference device (DC-SQUID), using controlled Josephson junction technology, provides a mechanism to modify the characteristics of the device post-fabrication. We report on the fabrication and measurement of a micron sized DC-SQUID using two Dayem bridge weak-link Josephson junctions with integrated “heaters.” The weak link critical current is controlled by hot phonons from the current biased titanium, normal metal heater. By using the heaters, control over the critical current oscillations of the SQUID was observed at 4.2K.