Dynamics of high-angle grain boundary YBCO Josephson junctions

This thesis describes experimental investigations of properties of high-angle YBa2Cu3O7−δ(YBCO)bicrystal Josephson junctions and SQUIDs fabricated on SrTiO3-substrates. The main focus of theinvestigation has been on the effects of the predominant d-wave symmetry of the superconductingwavefunction in YBCO on transport properties and dynamics.At a high-angle grain-boundary interface between two high-temperature superconductors Andreevstates can form, the current carried by these states hasπ-periodic component which flows in a direc-tion opposite of the usual Josephson current. Asymmetric high-angle grain boundaries also exhibit acritical current which is four orders of magnitude lower than symmetric lower-angle junctions, thiscan be attributed to the node-lobe arrangement of the the superconducting order parameter.High-angle grain-boundary dc-SQUIDs that have been studied which exhibit unusual dynamicssuch as a relative ”shift” of the positions of the positive and negative modulation and a highly non-sinusoidal dependence on the external field. This behavior vanished when moving to very narrowjunctions. These result are explained using a semi-classical model which assumes the presence ofa2ndharmonic in the current-phase relation. Numerical simulations confirm that this model is inqualitative agreement with experimental results.The properties of sub-micron sized junctions have also been studied. These exhibit some un-usual behavior. These junctions have been used to study the tunnelling spectra since the high normalresistance means that it is possible to study energies close to the gap.Finally, some general properties of high-angle Josephson Junctions are discussed. It is arguedthat some seemingly inconsistent experimental results can be explained using a multi-channel modelwhich accounts for the wiggling and faceting of the interface

Dynamics of high-angle grain boundary YBCO Josephson junctions

This thesis describes experimental investigations of properties of high-angle YBa2Cu3O7−δ(YBCO)bicrystal Josephson junctions and SQUIDs fabricated on SrTiO3-substrates. The main focus of theinvestigation has been on the effects of the predominant d-wave symmetry of the superconductingwavefunction in YBCO on transport properties and dynamics.At a high-angle grain-boundary interface between two high-temperature superconductors Andreevstates can form, the current carried by these states hasπ-periodic component which flows in a direc-tion opposite of the usual Josephson current. Asymmetric high-angle grain boundaries also exhibit acritical current which is four orders of magnitude lower than symmetric lower-angle junctions, thiscan be attributed to the node-lobe arrangement of the the superconducting order parameter.High-angle grain-boundary dc-SQUIDs that have been studied which exhibit unusual dynamicssuch as a relative ”shift” of the positions of the positive and negative modulation and a highly non-sinusoidal dependence on the external field. This behavior vanished when moving to very narrowjunctions. These result are explained using a semi-classical model which assumes the presence ofa2ndharmonic in the current-phase relation. Numerical simulations confirm that this model is inqualitative agreement with experimental results.The properties of sub-micron sized junctions have also been studied. These exhibit some un-usual behavior. These junctions have been used to study the tunnelling spectra since the high normalresistance means that it is possible to study energies close to the gap.Finally, some general properties of high-angle Josephson Junctions are discussed. It is arguedthat some seemingly inconsistent experimental results can be explained using a multi-channel modelwhich accounts for the wiggling and faceting of the interface

Suppression of low-frequency charge noise in superconducting resonators by surface spin desorption

Noise and decoherence due to spurious two-level systems located at material interfaces are long-standing issues for solid-state quantum devices. Efforts to mitigate the effects of two-level systems have been hampered by a lack of knowledge about their chemical and physical nature. Here, by combining dielectric loss, frequency noise and on-chip electron spin resonance measurements in superconducting resonators, we demonstrate that desorption of surface spins is accompanied by an almost tenfold reduction in the charge-induced frequency noise in the resonators. These measurements provide experimental evid ence that simultaneously reveals the chemical signatures of adsorbed magnetic moments and highlights their role in generating charge noise in solid-state quantum devices

YBCOSQUIDs with unconventional current phase relation

We have studied the dynamics of YBa2Cu3O7-delta,5 (YBCO) dc sperconducting quantum interference devices (SQUIDs) characterized by an unconventional Josephson current phase relation (CPR). We have focused on SQUID configurations with Josephson junctions where the lobe of the order parameter in one electrode is facing a node in the other electrode. This order parameter arrangement should enhance the appearance of a sin(2(p) term in the CPR. The response of the critical current of the dc SQUID, under the effect of an external magnetic field, has been measured in temperature, down to 20 mK. Our experimental data have been compared with numerical simulations of the SQUIDs dynamics by considering a CPR of a single junction of the form I(phi) = I-I sin(phi) - I-II sin(2 phi) where I-I, and I-II are, respectively, the first and second harmonic component. In our devices the values of the sin(2 phi) term are such that the fundamental state of the SQUID is naturally double degenerate. This is of great relevance for applications of d-wave SQUIDs in quantum information processing. (C) 2007 Elsevier B.V. All rights reserved