Low-temperature scanning electron microscopy (LTSEM) has been used to image the supercurrent distribution in ramp-type Josephson junctions between Nb and either the electron-doped cuprate Nd 2−x Ce x CuO 4−y or the hole-doped cuprate YBa 2 Cu 3 O 7 . For zigzag-shaped devices in the short junction limit the critical current is strongly suppressed at zero applied magnetic field. The LTSEM images show, that this is due to the Josephson current counterflow in neighboring 0 and π facets, which is induced by the d x 2 −y 2 order parameter in the cuprates. Thus, LTSEM provides imaging of the sign change of the superconducting order parameter, which can also be applied to other types of Josephson junctions. One of the most controversial topics on high-T c cuprate superconductors has been the determination of their order parameter symmetry (OPS). A myriad of experiments have been performed, indicating a predominant d x 2 −y 2 OPS, which implies important consequences for the microscopic mechanism of Cooper pairing in these materials. Obviously, it was quite difficult to identify an unambiguous experiment for the determination of the cuprate OPS. Among the most convincing experiments is the observation of half-integer magnetic flux quanta in tricrystal grain boundary Josephson junctions (JJs) by scanning SQUID microscopy [1]. These experiments, and related integral measurements of critical current I c vs applied magnetic field B, rely on the difference π of the phase of the order parameter between orthogonal directions in (k x , k y )-space, which can be detected by interferometertype configurations, such as corner junctions [2], tricrystal rings and long JJs [3,4,5,6,7,8], and dc π SQUIDs [9,10,11,12], or by the angular dependence of I c in biepitaxial JJs [13]. High-quality hybrid ramp-type JJs, combining an s-wave superconductor (Nb) with either the hole-doped cuprate YBa 2 Cu 3 O 7−δ (YBCO) [14,15,16] or the electrondoped cuprate Nd 2−x Ce x CuO 4−y (NCCO) [17] have also been realized. Arranging such JJs in a zigzag geometry with the facets oriented along the a-and b-axis of the cuprate, one obtains alternating facets of 0 and π JJs [15,17]. π JJs [18] have negative I c , i. e., j s = − j c sin φ = j c sin(φ + π), instead of j s = j c sin φ, where j s is the supercurrent density; j c > 0 is the maximum supercurrent density, and φ is the Josephson phase. Realizations include JJs with magnetic barriers [19,20,21,22,23] JJs containing both, 0-and π-parts have also been realized using ferromagnetic barriers [28,29,30] or current injectors [31].A striking property of s-d-wave zigzag JJs in the long JJ limit (facet length a by scanning SQUID microscopy. In the short JJ limit (neglecting selffield effects), for a JJ with N facets, the supercurrent density in the n th facet can be described as [15] Here,x is the coordinate along the zigzag (withx = 0 at the JJ edge), and Φ f is the magnetic flux per facet. As the prefactor (−1) n changes sign at every corner of the zigzag, as a direct consequence of the d-wave OPS, I c (B) i...

We report on Josephson and quasiparticle tunneling in YBa 2 Cu 3 O 7-x (YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling is studied in the ab-plane of YBCO either in the (100) and (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 zerobias 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.

We present a novel method, based on vortex imaging by low-temperature scanning electron microscopy (LTSEM), to directly image the sheet-current distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex signals are simply related to the scalar stream function describing the vortex-free circulating sheet-current distribution J . Unlike previous inversion methods that infer the current distribution from the measured magnetic field, our method uses pinned vortices as local detectors for J . Our experimental results are in very good agreement with numerical calculations of J .PACS numbers: 68.37. Hk, 74.25.Op, 85.25.Dq Spatially resolved techniques can provide important insight into current flow, arrangement of vortices, flux pinning, and noise in superconductors and their mutual interactions. So far there has been only one method of imaging the current distribution in superconductors: The magnetic field distribution on top of a superconducting thin film is measured, e.g. by magneto-optics, from which the current distribution can then be calculated by inverting the Biot-Savart law [1].In this paper we present a novel method to directly image the sheet-current distribution in a YBa 2 Cu 3 O 7 thin film. We use low-temperature scanning electron microscopy (LTSEM) [2,3,4,5] to image vortices in dc SQUID washers [6,7]. Most techniques for vortex imaging, such as Lorentz microscopy [8], scanning SQUID microscopy [9,10], scanning Hall microscopy [11] or magneto-optics[12] rely on the detection of the stray magnetic field produced in close proximity to a vortex. In contrast, vortex imaging by LTSEM is different from those techniques, as it is based on the electron-beaminduced apparent displacement of a vortex, pinned at position r in the (x, y)-plane of a SQUID washer, which is detected as a change of stray magnetic flux Φ(r) coupled to the SQUID. Hence, the contrast of the LTSEM vortex signals directly senses ∇Φ(r). Recently, Clem and Brandt [13] have shown that Φ(r) is proportional to the scalar stream function G(r) that describes the circulating sheet-current density J(r) flowing in the vortex-free case at position r in the SQUID washer. In this paper we show that this relationship allows us to use the vortices as local detectors for J(r): At each position a vortex has been imaged, we can directly determine J(r) without complicated calculations.In our experiments, we investigated several dc SQUID washers [see Fig. 1(a)] fabricated from epitaxially grown d=80 nm thick c-axis oriented YBa 2 Cu 3 O 7 (YBCO) thin * Electronic address: koelle@uni-tuebingen.de films. We will present an analysis of LTSEM data obtained from one representative device with washer size 120 µm × 305 µm, with a 100 µm long and 4 µm wide slit. The 1 µm wide Josephson junctions are formed by a 24 • symmetric grain boundary in the underlying SrTiO 3 substrate. For imaging by LTSEM, the YBCO SQUIDs are mounted on a magnetically shielded, liquid nitrogen cooled cryostage of an SEM [14] and read out by a standard flux-locked loop (FLL) w...

We use low-temperature scanning electron microscopy combined with SQUID detection of magnetic flux to image vortices and to investigate low-frequency flux noise in YBa2Cu3O7 thin film SQUIDs. The low-frequency flux noise shows a nonlinear increase with magnetic cooling field up to 60 µT. This effect is explained by the surface potential barrier at the SQUID hole. By correlating flux noise data with the spatial distribution of vortices, we obtain information on spatial fluctuations of vortices on a microscopic scale, e.g. an average vortex hopping length of approximately 10 nm.

We present low-temperature scanning electron microscopy (LTSEM) investigations of superconducting microbridges made from ultrathin NbN films as used for hot electron bolometers. LTSEM probes the thermal structure within the microbridges under various dc current bias conditions, either via electron-beam-induced generation of an unstable hotspot, or via the beam-induced growth of a stable hotspot. Such measurements reveal inhomogeneities on a micron scale, which may be due to spatial variations in the NbN film or film-interface properties. Comparison with model calculations for the stable hotspot regime confirm the basic features of common hot spot models.