Boris Chesca

Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron-doped superconductor La2-xCexCuO4-y

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron-doped cuprate superconductor La2-xCexCuO4-y. An applied magnetic field reduces the spectral weight around zero energy and shifts it nonlinearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.

Radio Frequency SQUIDs and their Applications

Radio frequency superconducting quantum interference devices (rf SQUIDs) made of conventional superconductors had been very popular some fifteen to thirty years ago. Later on, they had been replaced by dc SQUIDs, which then offered higher sensitivity. However, the operation of rf SQUIDs at microwave bias frequencies has greatly improved their sensitivity, so that now rf SQUIDs can be fabricated which are comparable to the best dc SQUIDs in terms of field sensitivity, as well as energy resolution. The discovery of high temperature superconductors (HTS) has further increased interest in rf SQUIDs, mainly because the inductance of rf SQUIDs and thus their loop area, can be made much larger than that of dc SQUIDs before a significant reduction in signal voltage is observed. Today, rf SQUIDs are again used in numerous applications, such as biomagnetism, geophysical measurements and nondestructive evaluation.

Observation of Andreev bound states in YBa2Cu3O7-x/Au/Nb ramp-type Josephson junctions

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling is studied in the ab-plane of YBCO either in the (100) or (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperatures and magnetic fields. With Nb superconducting, the proximity gap induced in the Au layer appears in the quasiparticle conductance spectra as well defined coherence peaks and a dip at the center of a broadened zero-bias conductance peak (ZBCP). The voltage position of the coherence peaks varies with Au layer thickness. As we increase the temperature or an applied magnetic field both the coherence peaks and the dip get suppressed and the ZBCP fully develops, while states are conserved. With Nb in the normal state the ZBCP is observed up to about 77 K and is almost unaffected by an increasing field up to 7 T. The measurements are consistent with a convolution of density of states with broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces. Since junctions with different geometries are fabricated on the same substrate under the same conditions one expects to extract reliable tunneling information that is crystallographic direction sensitive. In high contrast to Josephson tunneling, however, the quasiparticle conductance spectra are crystallographic orientation insensitive: independent whether the tunneling occurs in the (100) or (110) directions, a pronounced ZBCP is always observed, consistent with microscopic roughness of the junction interfaces. Qualitatively, all these particularities regarding quasiparticle spectra hold regardless whether the YBCO thin film is twinned or untwinned. This suggests that the formation of Andreev bound states is, to a first approximation, insensitive to twinning.

Josephson coupling in untwinned YBa2Cu3O7-x/Nb d-wave junctions

Theory predicts that d-wave superconductivity induces a significant second harmonic J2 in the Josephson current, as a result of zero-energy Andreev states (ZES) formed at the junction interface. Consequently, anomalies such as half-integer Shapiro steps should be observed. Both ZES formation and J2 are expected to be highly anisotropic as we change the tunneling orientation in the ab plane reaching their maximum for tunneling close to [110] direction and their minimum for [100] or [010] directions. We performed experiments on junctions between untwinned d-wave YBa2Cu3O7-x and Nb and found clear evidence of ZES formation for all 72 different tunneling directions in the ab plane investigated. However, in contrast to the theoretical predictions, we found no trace of half-integer Shapiro steps. That suggests J2 is insignificantly small compared to the first Josephson harmonic for all of the orientations. We believe that microscopic scale roughness, or diffusive reflection at a scale that is much smaller than the Fermi wavelength, dramatically suppresses J2 due to scattering processes. Our findings suggest that YBa2Cu3O7-x/Nb d-wave junctions have a purely sinusoidal current-phase relation which is essential to take into consideration for their implementation as qubits or π-junctions in digital circuits.

Order parameter phase sensitive experiments and SDJ dc SQUIDs

Spontaneous generation of persistent dc currents is one of the most striking aspects of the physics of superconducting samples in a frustrated geometry. Trapped vortices, however, are also sources of induced persistent circulating dc currents. In practice, usually, it is very difficult to establish which of the two is the real source of the observed persistent circulating dc currents and their only presence without additional and, very often, complicated approaches cannot serve as an unambiguous demonstration of frustration in a specific geometry. Recently, spontaneous generation of circulating ac currents in a frustrated geometry has been predicted. Experimentally observed in spatially distributed junctions dc SQUIDs, the effect cannot be reproduced by trapped vortices in the SQUID. Consequently, detection of spontaneous circulating ac currents is a straightforward demonstration of frustration in a given superconducting configuration. For that reason this effect can serve as an excellent tool to be used in order parameter phase sensitive experiments to determine the pairing symmetry of high transition temperature superconductors.

Electronic behavior of spatially distributed junction small inductance dc π-SQUIDs

Abstract As investigated theoretically [Annalen der Physik (Leipzig) 8 (1999) 511; Phys. B, in press] spatially distributed junction small inductance dc π-SQUIDs exhibit unusual electric transport properties in both zero-voltage and voltage states. The purpose of this paper is to summarize the peculiarities of this electronic behavior in order to (a) reveal some of the advantages the device has over other known Josephson junctions based configurations when used as a tool to investigate the order parameter symmetry in high- T c superconductors, and (b) to emphasize its potential for applications in superconducting electronics.

Tuning the current-voltage characteristics of Josephson junctions by strong microwave fields

We observe some unusual resonance-like structures on the current-voltage characteristics (CVC) of thin film ramp-edge Josephson junctions between YBCO and Nb when applying strong microwave fields (MW). The unusual resonances have their voltage position on the CVC dependent on the MW power rather than the MW frequency, while their amplitude is largely unaffected by an increase in the MW power. Such behaviour indicates an intriguing Josephson dynamics associated with the switching from a parametric excitation regime induced by the magnetic field of the MW via oscillations of the Josephson critical current to an ac-current excitation regime triggered by the electric field of the MW. We propose a model, which describes the experimentally observed features on the CVC in terms of MW-induced multiple switching between running and locked solutions of sine-Gordon equation. Such tuning of the CVC via MW could be used to optimize the output of THz emitters.