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Superconductor Science and Technology

Flavours of intrinsic d-wave induced effects in Y Ba2Cu3O7 grain boundary Josephson junctions

Francesco Tafuri, John R. Kirtley, Floriana Lombardi, Thilo Bauch, E. Ii'chev, F. Miletto Granozio, D. Stornaiuolo, U. Scotti di Uccio

Published Mar 2, 2004• 5 citations
Publisher

Abstract

The quality of Y Ba 2 Cu 3 O 7−δ biepitaxial grain boundary Josephson junctions has been improved and 'tunnel-like' characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favourable to isolate intrinsic d-wave induced effects. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for π-circuitry and qubits.

DOI
10.1088/0953-2048/17/5/022

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The quality of Y Ba 2 Cu 3 O 7−δ biepitaxial grain boundary Josephson junctions has been improved and 'tunnel-like' characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favourable to isolate intrinsic d-wave induced effects. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for π-circuitry and qubits.
We have investigated the properties of biepitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grain boundary Josephson junctions, which appear to be characterized by low barrier transmission probabilities. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for $\ensuremath{\pi}$-circuitry and qubits. The absence of spontaneous magnetization and of the consequent additional noise, in a wide range of experimental conditions, seems to be encouraging for designs of ``quiet'' devices.
The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.
The predominant d-wave pairing symmetry in high temperature superconductors leads to an unconventional current–phase relation in Josephson junctions. This circumstance may induce new effects in the dynamics of dc SQUIDs. In this contribution we report on the measurements of the dependence of the SQUID Josephson current on the external magnetic field taken at very low temperatures, down to 20 mK. Different grain boundaries have been fabricated by using the biepitaxial and the bicrystal technique. Some of the effects which are induced by a nonsinusoidal current–phase relation can be clearly identified in the dynamics of the SQUIDs. The experimental data are also compared with theoretical simulations taking into account the inductance of the loop. The data show that, in specific conditions, a non-negligible inductance of the loop can induce effects similar to an unconventional current–phase relation, with a pronounced second harmonic sin(2φ) term. This fact has to be taken into account when designing d-wave SQUIDs for quantum circuitry.
A flux qubit is designed and fabricated that capitalizes on intrinsic properties of submicron YBCO (yttrium-barium-copper oxide high-Tc{T}_{c}Tc​ superconductor) grain boundary junctions. The operating point is protected from the fluctuations of the external fields, already on the classical level. The effects of external perturbations only appear in the third or fourth order, depending on the character of the coupling. The estimates of the decoherence due to fluctuations of the external flux show that an experimental observation of coherent quantum tunneling and Rabi oscillations in the system is feasible.
We present electric noise measurements of nanoscale biepitaxial YBa2{}_{2}2​Cu3{}_{3}3​O${}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) Josephson junctions fabricated by two different lithographic methods. The first (conventional) technique defines the junctions directly by ion milling etching through an amorphous carbon mask. The second (soft patterning) method makes use of the phase competition between the superconducting YBCO (Y123) and the insulating Y2{}_{2}2​BaCuO5{}_{5}5​ (Y211) phase at the grain boundary interface on MgO (110) substrates. The voltage noise properties of the two methods are compared in this study. For all junctions (having a thickness of 100 nm and widths of 250--500 nm), we see a significant amount of individual charge traps. We have extracted an approximate value for the effective area of the charge traps from the noise data. From the noise measurements, we infer that the soft-patterned junctions with a grain-boundary (GB) interface manifesting a large ccc-axis tunneling component have a uniform barrier and a superconductor-insulator-superconductor (SIS) -like behavior. The noise properties of soft-patterned junctions having a GB interface dominated by transport parallel to the ababab planes are in accordance with a resonant tunneling barrier model. The conventionally patterned junctions, instead, have suppressed superconducting transport channels with an area much less than the nominal junction area. These findings are important for the implementation of nanosized Josephson junctions in quantum circuits.