Observation of Andreev bound states inYBa2Cu3O7−x∕Au

Abstract

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.

Upper bound on the Andreev states induced second harmonic in the Josephson coupling ofYBa2Cu3O7−δ/NbChesca1, Smilde2, Hilgenkamp3 2008Phys. Rev. BSelf Cite320130View full textAdd to dashboardCite

“…1 19 It was found that all observed features are consistent with a convolution of density of states with broadened ZES formed at the YBa 2 Cu 3 O 7−␦ / Au/ Nb junction interfaces. 19 Here, we only summarize some of the most important findings from a qualitative point of view. We observed the same qualitative picture independent of the tunneling direction.…”

“…As our previous report showed, 19 as far as quasiparticle tunneling is concerned, there is a good quantitative agreement between the measured conductance spectra and calculations made on the basis of an S d IS s tunnel junction model using quasiclassical techniques. Looking into the Josephson tunneling, in the frame of a Green's function formalism J 2 is calculated by integrating over all transverse wave vectors, 9 and YBCO, ␦Ј͑E͒ is the derivative of the Dirac delta function, and ␣ is the angle between a reflected wave and the normal to the junction interface.…”

“…That is due to a junction interface that consists of a multitude of small facets having different sizes and orientations and characterized by alternating signs of the dc Josephson current. 21 Within this faceted d-wave junction model, 21,22 one can evaluate various J c ͑x͒ distributions along the junction, 19 as well as various combinations ͑J c1 , J c2 ͒ until the simulated I c ͑⌽ / ⌽ 0 ͒ given by …”

“…The junctions are fabricated on the same chip, and the angle with the YBa 2 Cu 3 O 7−␦ crystal b axis is varied in units of 5°, so that tunneling can be probed in 360°/ 5°= 72 different directions in the ab plane ͑see Fig. 1 19 It was found that all observed features are consistent with a convolution of density of states with broadened ZES formed at the YBa 2 Cu 3 O 7−␦ / Au/ Nb junction interfaces. 19 Here, we only summarize some of the most important findings from a qualitative point of view.…”

Topology of Andreev bound states with flat dispersion

“…The ABS shows up as a zero bias conductance peak of tunneling spectroscopy [6][7][8]10 , and through the study of tunneling spectroscopy of high T c cuprates [11][12][13][14][15][16][17][18][19][20] , it has been established that a dispersionless zero energy ABS 7 is generated for d xy -wave superconductors. The existence of the ABS influences seriously interface and surface properties [21][22][23][24][25][26] .…”