119 research outputs found
Local Tunneling Study of Three-Dimensional Order Parameter in the -band of Al-doped MgB Single Crystals
We have performed local tunneling spectroscopy on high quality
MgAlB single crystals by means of Variable Temperature Scanning
Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap
conductance spectra due to c-axis tunneling were extensively measured, probing
different amplitudes of the three-dimensional as a function of Al
content. Temperature and magnetic field dependences of the conductance spectra
were studied in S-I-N configuration: the effect of the doping resulted in a
monotonous reduction of the locally measured down to 24K for x=0.2. On
the other hand, we have found that the gap amplitude shows a maximum value
meV for x=0.1, while the ratio increases
monotonously with doping. The locally measured upper critical field was found
to be strongly related to the gap amplitude, showing the maximum value
for x=0.1 substituted samples. For this Al concentration the
data revealed some spatial inhomogeneity in the distribution of on
nanometer scale.Comment: 4 pages, 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
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
Two Gap State Density in MgB: A True Bulk Property or A Proximity Effect?
We report on the temperature dependence of the quasiparticle density of
states (DOS) in the simple binary compound MgB2 directly measured using
scanning tunneling microscope (STM). To achieve high quality tunneling
conditions, a small crystal of MgB2 is used as a tip in the STM experiment. The
``sample'' is chosen to be a 2H-NbSe2 single crystal presenting an atomically
flat surface. At low temperature the tunneling conductance spectra show a gap
at the Fermi energy followed by two well-pronounced conductance peaks on each
side. They appear at voltages V mV and V mV. With rising temperature both peaks disappear at the Tc of the bulk
MgB2, a behavior consistent with the model of two-gap superconductivity. The
explanation of the double-peak structure in terms of a particular proximity
effect is also discussed.Comment: 4 pages, 3 figure
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
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
Probing the superfluid velocity with a superconducting tip: the Doppler shift effect
We address the question of probing the supercurrents in superconducting (SC)
samples on a local scale by performing Scanning Tunneling Spectroscopy (STS)
experiments with a SC tip. In this configuration, we show that the tunneling
conductance is highly sensitive to the Doppler shift term in the SC
quasiparticle spectrum of the sample, thus allowing the local study of the
superfluid velocity. Intrinsic screening currents, such as those surrounding
the vortex cores in a type II SC in a magnetic field, are directly probed. With
Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe_2, reveals
both the vortex cores, on the scale of the SC coherence length , and the
supercurrents, on the scale of the London penetration length . A
subtle interplay between the SC pair potential and the supercurrents at the
vortex edge is observed. Our results open interesting prospects for the study
of screening currents in any superconductor.Comment: 4 pages, 5 figure
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