Ph.D Student | Kirzhner Tal |
---|---|

Subject | Coherence-Length and Energy-Gap Scales in Cuprate and Topological Superconductors with a Pseudogap |

Department | Department of Physics |

Supervisor | Professor Emeritus Gad Koren |

Full Thesis text |

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 T_{C} of La_{2-x}Sr_{x}CuO_{4},
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 YBa_{2}Cu_{3}O_{7-}_{𝜹} superconductor below its T_{C}
(~90 K) and N was La_{2-x}Sr_{x}CuO_{4 }in its
pseudogap regime, above its T_{C} (? 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 La_{2-x}Sr_{x}CuO_{4
}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 T_{C}, 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 La_{2-x}Sr_{x}CuO_{4 }as the
superconducting (S) electrodes and La_{1.65}Sr_{0.35}CuO_{4 }as
a non-superconducting, normal (N) barrier. The first energy scale was
identified as the superconducting energy gap Δ_{1} of La_{2-x}Sr_{x}CuO_{4},
while the second energy scale Δ_{2 }was clearly in the pseudogap
regime of La_{2-x}Sr_{x}CuO_{4}, but is of an unknown
origin as of yet. In the paper, various possible origins for this Δ_{2 }energy
scale are discussed. On the phase diagram, Δ_{2}(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 Cu_{0.2}Bi_{2}Se_{3}
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.