Growth, transport, and tunneling spectroscopy of Y1̳-̳x̳Prx̳Ba2̳Cu3̳07̳ thin films as a function of crystallographic orientation
Covington, Mark William
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https://hdl.handle.net/2142/30691
Description
Title
Growth, transport, and tunneling spectroscopy of Y1̳-̳x̳Prx̳Ba2̳Cu3̳07̳ thin films as a function of crystallographic orientation
Author(s)
Covington, Mark William
Issue Date
1997
Doctoral Committee Chair(s)
Greene, Laura H.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
tunneling spectroscopy
thin films
crystallographic orientation
Language
en
Abstract
The planar tunneling spectroscopy of superconducting, Pr-doped YBa2Cu301 thin films is
reported. The tunneling conductance is studied as a function of both crystallographic
orientation and Pr concentration. Tunnel junctions are fabricated on thin films grown by
off-axis magnetron sputter deposition from single, stoichiometric targets. Four
crystallographic orientations are grown: (100), (110), (103), and (001). Four different Pr
concentrations are studied where x=O, 0.2, 0.4, and 0.5, corresponding to zero resistance
superconducting transition temperatures (Tc's) of 90 K, 70 K, 45 K, and 20 K,
respectively. The optimization ofthe PrBa2Cu30 7 template utilized when growing (100)-
and (110)-oriented YBa2Cu301 thin films is reported. Extensive characterization of the
films by resistivity and magnetic susceptibility measurements, x-ray diffraction, and
scanning electron microscopy show they are of extremely high quality and reproducible.
The superconducting and normal state properties are comparable to those measured in
single crystals, and the (110)-oriented YBa2Cu307 films exhibit Tc's that are unsurpassed
for this orientation. The conductance of Yt-xPrxBa2Cu30711/Pb planar tunnel junctions,
where I is an insulator formed at the interface between the two materials, is measured as a
function of temperature and magnetic field. The junctions are reproducible and well
characterized. Tunneling is the dominant transport mechanism through the junctions, as
verified by the observation of a well-defined Pb superconducting density of states. For a
fixed Pr concentration, the same features are observed for (100)-, (11 0)-, and
iii
(103)-oriented (ab-oriented) films, and these features are qualitatively different than those
measured for (001)-oriented (c-axis) films. The tunneling conductance data from aboriented
films exhibit three important features. First, a gap-like feature is observed at an
energy, eV-2.2ksTc, that approximately scales with the Tc of the film. Second, there is a
temperature and magnetic field dependent zero-bias conductance peak that disappears for
the highest Pr concentration studied. Third, a conductance dip at 45 meV that is
asymmetric with respect to bias is observed in undoped films. In contrast, the tunneling
conductance data for c-axis films exhibit a gap-like feature at a fixed energy that is
independent of T c and a zero bias conductance dip. The gap-like features for c-axis and
ab-oriented films coincide at the roughly the same energy only for undoped films. There is
also a high bias dip in the c-axis conductance at roughly the same energy as that observed
in ab-oriented films. It is unknown if the high bias structure in the tunneling conductance
of ab-oriented and c-axis films is related. The tunneling conductance within the gap-like
feature energy for ab-oriented films is qualitatively different than that from conventional
superconductors. Results are compared and contrasted to predictions for a
superconducting gap with dx2
-/ symmetry.
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