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d-wave Josephson Junctions

T. Lindström

Published Jan 1, 2005• 0 citations
OpenAlex

Abstract

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-T$_c$ 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.

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Physical Review Letters•Mar 3, 2005•158 citations
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 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 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.
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.
The topic of the PhD project is a comparative study of phase dynamics and macroscopic quantum phenomena in moderately damped NbN, YBaCuO grain boundary (GB) Josephson junctions (JJs) and hybrid devices. This type of research activity responds to the needs of better identifying phase dynamics in JJs in the moderately damped regime, which is going to be more and more common in hybrid nanostructures. Issues on a more detailed understanding of coherence, dissipation and noise in the various devices have a relevant role in the progress of quantum circuits. In the last few years, studies of phase dynamics and macroscopic quantum phenomena have been extended to junctions composed of materials other than the traditional low critical temperature superconductors (LTS) and to novel types of structures with unconventional barriers composed for instance of graphene sheets or of grain boundaries. Progress in engineering new materials into junctions and in understanding and controlling the physics of interfaces may offer novel solutions for junctions of superior quality and complementary functionalities, and therefore may lead in the long run to improve also specific qubit performances. For LTS JJs, once the barrier thickness and the critical current density (Jc) have been fixed, a reduction in its size unavoidably leads to a lowering of the critical current and determines a quite different phase dynamics re-normalized to the new scaling energy. Lower critical currents Ic result in lower Josephson energies EJ, and higher levels of dissipation are expected. The range of the energy dynamical parameters is significantly enlarged, and it is technologically easier to reproducibly realize nontrivial configurations. These devices are characterized by intermediate levels of dissipation (moderately damped regime) and by phase diffusion phenomena. The low Jc limit seems to be characteristic also of all futuristic nanohybrids devices incorporating nanowires, and the moderately damped regime (MDR) is intrinsically more common than it could be expected. Measurements of switching current distribution (SCD) in these last years have turned to be standard tools to investigate phase dynamics in unconventional and hybrid systems and nanostructures. High critical temperature superconductors (HTS) are an example of unconventional systems, because of the d-wave order parameter symmetry and of the presence of low-energy quasiparticles, which are expected to induce high level of dissipation and as a consequence to spoil macroscopic quantum coherence. A comparative study in systems so different and complementary is of great interest. During the PhD program, we have performed measurements of SCD, both in the thermal and quantum regime down to 20 mK, on moderately damped NbN/MgO/NbN JJs and on YBCO biepitaxial GB JJs. The NbN JJs exhibit a crossover from thermal activation (TA) to macroscopic quantum tunneling (MQT) regime at about 90 mK. After the MQT saturation, the width of the SCD follows the expected dependence in the TA regime. Deviations are evident in proximity and above the crossover temperature T* (about 1.6K) where we find effects of anti-correlation between the bath temperature and thermal fluctuations since the width of the histograms starts to collapse, and indications of a transition to a third regime called phase diffusion (PD) are evident, such as the change of the switching distributions symmetry. Experimental data have been analyzed through numerical fitting of the switching probability and Monte Carlo simulations, and we found a good agreement with theoretical expectations based on multiple re-trapping processes, which strongly depend on the junction damping. The combined analysis of these three regimes allowed us to extract the fundamental parameters, which completely characterize the dynamics of such moderately damped NbN JJs. In addition, clear signatures of the PD regime have been detected and numerical methods have been developed to estimate the dissipation level in the MDR. As a term of comparison, the studies on moderately damped systems have been also carried out on HTS junctions, which have different levels of intrinsic dissipation induced by low energy quasi-particles, and are for this reason of great impact as a reference. In such JJs we have clearly observed a direct transition from the MQT to the PD regime. The width of the SCDs is constant below 100mK, which is a clearly indication of a quantum activation regime. Above 100mK the negative temperature derivative of the width is consistent with a diffusive motion due to multiple escape and re-trapping in the potential wells. Thermal behavior of the width has been fitted through numerical simulations, thus providing the estimate of the damping factor. These conclusions of a PD regime also in YBaCuO JJs are supported by data in presence of an externally applied magnetic field, and are consistent with the numerical outcomes. Numerical simulations of the thermal behavior of the SCD width for different values of the quality factor ranging from 1 to 10 have been carried out. Such numerical simulations allow us to reconstruct a (Q, kBT/EJ) phase diagram summarizing the various activation regimes. The transition curve between the PD regime and the running state following thermal or quantum activation has been determined numerically by varying the damping factor Q as function of the ratio between the thermal energy and the Josephson energy. The result of the calculation is a universal curve. A phase diagram valid in a large range of dissipation conditions emerges as a functional guide to classify the switching behavior and to settle the fundamental junction parameters and energies in the MDR. It is therefore a reference for phase dynamics of novel types of junction and system for which the nature of the current induced transition from the superconducting to the normal state has not been completely clarified.
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 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.