טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentTal Kirzhner
SubjectCoherence-Length and Energy-Gap Scales in Cuprate and
Topological Superconductors with a Pseudogap
DepartmentDepartment of Physics
Supervisor Professor Emeritus Koren Gad
Full Thesis textFull thesis text - English Version


Abstract

This thesis is based on three published research papers in the broad field of superconductivity. Two of these papers deal with phenomena occurring in the pseudogap regime of the cuprates high temperature superconductors, and one deals with topological superconductivity.

The first paper, which comprises the main part of this thesis, describes the phase diagram of the normal coherence length 𝝃(T, x) above  TC of La2-xSrxCuO4, where T is the temperature and x is the hole-doping level. This diagram was obtained by measuring the Josephson supercurrents in superconducting-normal-superconducting (SNS) c-axis junctions where S was the YBa2Cu3O7-𝜹 superconductor below its TC (~90 K) and N was La2-xSrxCuO4 in its pseudogap regime, above its TC  (? 25 K). Besides observing long ranged proximity effect which has already been reported in the literature, the new result in the present study is that for T ? 55 K we found that 𝝃(T, x=0.1) in the underdoped regime of La2-xSrxCuO4 is higher than 𝝃(T, x=0.18) in the overdoped regime, contrary to common understanding that SNS junctions with more resistive N barrier (that with x=0.1) should have lower 𝝃 values. This finding supports the pre-formed pairs scenario for the high temperature superconductors in the pseudogap regime, where uncorrelated pairs above TC, reach global phase coherence below it.

The second paper presents conductance spectroscopy results where two energy scales were observed in SNS ramp-type junctions with La2-xSrxCuO4 as the superconducting (S) electrodes and La1.65Sr0.35CuO4 as a non-superconducting, normal (N) barrier. The first energy scale was identified as the superconducting energy gap Δ1 of La2-xSrxCuO4, while the second energy scale Δ2 was clearly in the pseudogap regime of La2-xSrxCuO4, but is of an unknown origin as of yet. In the paper, various possible origins for this Δ2 energy scale are discussed. On the phase diagram, Δ2(T, x) is similar to the superconducting dome, but with about twice its energy. The third paper, describes point contact studies of conductance spectra in a Cu0.2Bi2Se3 single crystal which is believed to be a topological superconductor. In this material a zero bias conductance peak as observed here, could be due to Majorana fermions, but also to result from the more common zero energy Andreev bound states. Analysis of the results could not ascertain which of the two is dominant here, but it clearly indicates unconventional superconductivity and signature of a pseudogap regime.