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Physical review. B, Condensed matter

Intrinsic and extrinsicd-wave effects in YBa2Cu3O7−δ grain boundary Josephson junctions: Implications for π circuitry

Francesco Tafuri, John R. Kirtley, Floriana Lombardi, F. Miletto Granozio

Published May 21, 2003• 12 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.67.174516

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We have investigated the properties of biepitaxial YBa2Cu3O7\ensuremath−\ensuremathδ{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}YBa2​Cu3​O7\ensuremath−\ensuremathδ​ 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 \ensuremathπ\ensuremath{\pi}\ensuremathπ-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.
We present various concepts and experimental procedures to produce biepitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ grain-boundary Josephson junctions. The device properties have an interesting phenomenology, related in part to the possible influence of $\mathrm{``}\ensuremath{\pi}$ loops.'' The performance of our junctions and superconducting quantum interference devices indicates significant improvement in the biepitaxial technique. Further, we propose methods for fabricating circuits in which 0 and $\ensuremath{\pi}$ loops are controllably located on the same chip.
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.
White noise properties of YBa2Cu3O7-δ (YBCO) grain boundary Josephson devices have been studied. Bias voltage (V) dependence of the white voltage noise spectral density (SV) were measured in detail for YBCO dc-SQUIDS and single Josephson junctions on different kinds of substrates with various misorientation angles. It was found that Sv was in good agreement with the thermal noise theory hypothesizing that the device noise was only due to Johnson noise. This implies that the quantum noise can be achieved with optimized YBCO dc-SQUIDS. We have designed a YBCO dc-SQUID with a cross section of junctions as small as 0.1 μm^2. It was predicted that the minimum intrinsic white energy resolution (E””'‘) should be about Planck's constant (h) at 4.2 K. The actual minimum eW, value of 1.76 x 10^-33 J/Hz = 2.7 h was obtained. This result demonstrates that E” '”of YBCO dc-SQUID s can be minimized to the order of h.
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.