Zero energy bound states in tunneling conductance spectra at the interface of an s-wave superconductor and a topological insulator in NbN-Bi2Se3Bi_2Se_3-Au thin film junctions

Measurements of conductance spectra in a superconductor - topological insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below $\rm T_c$ of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the $\rm Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.Comment: Published versio

Critical current measurements in superconductor - ferromagnet - superconductor junctions of YBa2Cu3OyYBa_2Cu_3O_y-SrRuO3SrRuO_3-YBa2Cu3OyYBa_2Cu_3O_y: No evidence for a dominant proximity induced triplet superconductivity in the ferromagnetic barrier

Transport measurements in ramp-type junctions of $YBa_2Cu_3O_y-SrRuO_3-YBa_2Cu_3O_y$ with $T_c$ values of either 80-90 K or 60-70 K are reported. In both type of junctions but without a barrier ("shorts"), the supercurrent densities at 4.2 K reached 7.5 and 3.5 MA/cm$^2$, respectively, indicating the high quality of the fabrication process. Plots of the critical current versus thickness of the ferromagnetic barrier at 4.2 K show exponential decays with decay lengths of 1.1 nm for the 90 K phase and 1.4 nm for the 60 K phase, which are much shorter than the relevant coherence lengths $\xi_F\sim 5-6$ nm or $\xi_N\sim$16 nm of $SrRuO_3$. We thus conclude that there is no dominant proximity induced triplet superconductivity in the ferromagnet in our junctions.Comment: 5 figure

Observation of two distinct pairs fluctuation lifetimes and supercurrents in the pseudogap regime of cuprate junctions

Pairs fluctuation supercurrents and inverse lifetimes in the pseudogap regime are reported. These were measured on epitaxial c-axis junctions of the cuprates, with a PrBa[subscript 2]Cu[subscript 3]O[subscript 7−δ] barrier sandwiched in between two YBa[subscript 2]Cu[subscript 3]O[subscript 7−δ] or doped YBa[subscript 2]Cu[subscript 3]O[subscript y] electrodes, with or without magnetic fields parallel to the a-b planes. All junctions had a T[subscript c](high)≈85–90 K and a T[subscript c](low)≈50–55 K electrodes, allowing us to study pairs fluctuation supercurrents and inverse lifetimes in between these two temperatures. In junctions with a pseudogap electrode under zero field, an excess current due to pair fluctuations was observed which persisted at temperatures above T[subscript c](low), in the pseudogap regime, and up to about T[subscript c](high). No such excess current was observed in junctions without an electrode with a pseudogap. The measured conductance spectra at temperatures above T[subscript c](low) were fitted using a modified fluctuations model by Scalapino [Phys. Rev. Lett. 24, 1052 (1970)] of a junction with a serial resistance. We found that in the pseudogap regime, the conductance vs voltage consists of a narrow peak sitting on top of a very broad peak. This yielded two distinct pairs fluctuation lifetimes in the pseudogap electrode which differ by an order of magnitude up to about T[subscript c](high). Under in-plane fields, these two lifetime values remain separated in two distinct groups, which varied with increasing field moderately. We also found that detection of Amperian pairing [Phys. Rev. X 4, 031017 (2014)] in our cuprate junctions is not feasible, due to Josephson vortices penetration into the superconducting electrodes which drove the necessary field above the depairing field.National Science Foundation (U.S.) (Grant DRM-1522575