109 research outputs found
Constraints on the Quasiparticle Density of States in High- Superconductors
In this Letter we present new tunneling data on YBaCuO thin films
by low temperature scanning tunneling spectroscopy. Unusual peak-dip-hump
features, previously reported in BiSrCaCuO, are also
found in YBaCuO. To analyse these common signatures we propose a
new heuristic model in which, in addition to the d-wave symmetry, the gap
function is energy dependent. A simple expression for the quasiparticle density
of states is derived, giving an excellent agreement with the experiment. The
dynamics of the quasiparticle states and the energy scales involved in the
superconducting transition are discussed.Comment: 4 page Letter with 3 figure
Local tunneling spectroscopy of the electron-doped cuprate Sm1.85Ce0.15CuO4
We present local tunneling spectroscopy in the optimally electron-doped
cuprate Sm2-xCexCuO4 x=0.15. A clear signature of the superconducting gap is
observed with an amplitude ranging from place to place and from sample to
sample (Delta~3.5-6meV). Another spectroscopic feature is simultaneously
observed at high energy above \pm 50meV. Its energy scale and temperature
evolution is found to be compatible with previous photoemission and optical
experiments. If interpreted as the signature of antiferromagnetic order in the
samples, these results could suggest the coexistence on the local scale of
antiferromagnetism and superconductivity on the electron-doped side of cuprate
superconductors
Proximity effect between two superconductors spatially resolved by scanning tunneling spectroscopy
We present a combined experimental and theoretical study of the proximity
effect in an atomic-scale controlled junction between two different
superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb
nanocrystal with an energy gap of 1.2 meV, connected to a crystalline atomic
monolayer of lead with a gap of 0.23 meV. Using in situ scanning tunneling
spectroscopy we probe the local density of states of this hybrid system both in
space and in energy, at temperatures below and above the critical temperature
of the superconducting monolayer. Direct and inverse proximity effects are
revealed with high resolution. Our observations are precisely explained with
the help of a self-consistent solution of the Usadel equations. In particular,
our results demonstrate that in the vicinity of the Pb islands, the Pb
monolayer locally develops a finite proximity-induced superconducting order
parameter, well above its own bulk critical temperature. This leads to a giant
proximity effect where the superconducting correlations penetrate inside the
monolayer a distance much larger than in a non-superconducting metal.Comment: 13 pages, 14 figures, accepted for publication in Physical Review
Nanometer Scale Mapping of the Density of States in an Inhomogeneous Superconductor
Using high speed scanning tunneling spectroscopy, we perform a full mapping
of the quasiparticle density of states (DOS) in single crystals of
BiPbSrCaCuO(2212). The measurements carried out at 5 K showed a complex spatial
pattern of important variations of the local DOS on the nanometer scale.
Superconducting areas are co-existing with regions of a smooth and larger
gap-like DOS structure. The superconducting regions are found to have a minimum
size of about 3 nm. The role of Pb-introduced substitutional disorder in the
observed spatial variations of the local DOS is discussed.Comment: 4 page Letter with 3 figures (2 color figures
Proximity effect model of ultra-narrow NbN strips
We show that narrow superconducting strips in superconducting (S) and normal
(N) states are universally described by the model presenting them as lateral
NSN proximity systems in which the superconducting central band is sandwiched
between damaged edge-bands with suppressed superconductivity.The width of the
superconducting band was experimentally determined from the value of magnetic
field at which the band transits from the Meissner state to the static vortex
state. Systematic experimental study of 4.9 nm thick NbN strips with widths in
the interval from 50 nm to 20 m, which are all smaller than the Pearl's
length, demonstrates gradual evolution of the temperature dependence of the
critical current with the change of the strip width
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
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