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Superconductor Science and Technology

Dynamics of d-wave YBa2Cu3O7−xdc SQUIDs

Thilo Bauch, Karin Cedergren, J Johansson, G. Rotoli, Francesco Tafuri, Floriana Lombardi

Published Jan 2, 2007• 4 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/20/2/s19

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Physical review. B, Condensed matter•May 21, 2003•12 citations
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.
The properties of d-wave YBCO Josephson junctions and dc-SQUIDs with high misorientation angles have been\nstudied experimentally .\nThe predominant d-wave symmetry of the pairing wavefunction in high-Tc superconductors has, to a large extent, been\na hinderance in applications, but it also makes it possible to fabricate devices with novel properties that may be suitable\nfor applications in the quantum regime; one particulary interesting property is a strong second harmonic component in the current-phase relation of\njunctions with the electrodes oriented in a node-lobe arrangement; so-called 0-45 junctions.\n\nThe work has been focused on the properties of grain-boundary 0-45 dc-SQUIDs: Their dc-properties as well\nas the effects of a strong second harmonic component on the dynamics have been studied experimentally and\nmodelled numerically. The experimental results can be explained by taking into account the meandering of\nthe grain boundary which in combination with a strong orientational dependence of the transport properties due to the\nd-wave symmetry results in interfaces with properties that change dramatically over a length scale of 50-100 nm.\n\nThe properties of grain-boundary YBCO 0-45 junctions have been studied in the quantum regime. Macroscopic\nquantum tunnelling has been demonstrated in these junctions and microwave spectroscopy is here used to verify\nthe presence of discrete energy levels. The quality factor of the junction has been experimentally determined to 40+-10, a much higher value than previously believed to be possible in high-Tc structures.\n\nFinally, the possibility of using high-Tc_cc​ structures to build qubits is discussed. There is now a renewed\ninterest in this possibility, especially in light of our recent results which\ndemonstrate that the inherent dissipation mechanisms are much less detrimental to coherent phenomena than previously feared.
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 modern physics of superconductivity can be called the physics of unconventional superconductivity. The discovery of the d-wave symmetry of the order parameter in high-temperature superconductors and the triplet superconductivity in compound Sr2RuO4 has caused a huge stream of theoretical and experimental investigations of unconventional superconductors. In this review we discuss some novel aspects of the Josephson effect which are related to the symmetry of the order parameter. The most intriguing of them is spontaneous current generation in an unconventional weak link. The example of a Josephson junction in the form of a grain boundary between two disorientated d-wave or f-wave superconductors is considered in detail. Josephson current–phase relations and the phase dependences of the spontaneous current that flows along the interface are analyzed. The spontaneous current and spontaneous phase difference are manifestations of the time-reversal symmetry (𝒯 ) breaking states in the system. We analyzed the region of appearance of 𝒯-breaking states as function of temperature and mismatch angle. A review of the basics of superconducting qubits with emphasis on specific properties of d-wave qubits is given. Recent results in the problem of decoherence in d-wave qubits, which is the major concern for any qubit realization, are presented.
We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.
We fabricated Josephson junctions (JJs) between the d-wave superconductor (SC) YBa2Cu3O7-x (YBCO) and the s-wave SC Nb (ds-JJs) on graphoepitaxially buffered MgO substrates, studied ds-JJs at temperatures down to 30 mK and employed these JJs in π-loops. Current-voltage characteristics of ds-JJs oriented along the [100] axis of YBCO exhibited up to a factor of 200 higher critical current densities than ds-JJs oriented along the [110] axis of YBCO. The critical currentIcI_{c}Ic​and theIcRnI_{c}R_{n}Ic​Rn​product of [100]-oriented 3 μm wide ds-JJs are ∼70 μA and ∼200 μV, respectively, at 4.2 K. Rectangular arrays of up to 40 000 π-loops based on such ds-JJs were investigated using a low temperature scanning superconducting quantum interference device (SQUID) microscope. We observed ordering of spontaneously generated half integer magnetic flux quanta in the π-loops correlated with minute spurious background magnetic fields, as well as with configurations and mutual coupling of the π-loops. We manipulated the magnetic states of the π-loops by the local application of magnetic fields using nearby planar coils. This paper paves the way for the use of π-loops in computations that are based on annealing processes.
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.