3,064 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
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
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
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
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
Topographic controls on dike injection in volcanic rift zones
Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 246 (2006): 188-196, doi:10.1016/j.epsl.2006.04.005.Dike emplacement in volcanic rift zones is often associated with the injection of “bladelike”
dikes, which propagate long distances parallel to the rift, but frequently remain
trapped at depth and erupt only near the tip of the dike. Over geologic time, this style of
dike injection implies that a greater percentage of extension is accommodated by magma
accretion at depth than near the surface. In this study, we investigate the evolution of
faulting, topography, and stress state in volcanic rift zones using a kinematic model for
dike injection in an extending 2-D elastic-viscoplastic layer. We show that the intrusion
of blade-like dikes focuses deformation at the rift axis, leading to the formation of an
axial rift valley. However, flexure associated with the development of the rift topography
generates compression at the base of the plate. If the magnitude of these deviatoric
compressive stresses exceeds the deviatoric tensile stress associated with far-field
extension, further dike injection will be inhibited. In general, this transition from tensile
to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is
greater than 50-60% of the far-field extension rate. These results indicate that over
geologic time-scales the injection of blade-like dikes is a self-limiting process in which
dike-generated faulting and topography result in an efficient feedback mechanism that
controls the time-averaged distribution of magma accretion within the crust.Funding
for this research was provided by NSF Grants OCE 04-43246, OCE 05-50147, OCE 02-42597 and OCE 04-26575, and a Carnegie Postdoctoral Fellowship to M.B
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
Auto-immune haematological complications occurring during the treatment of malignant lymphoproliferative diseases
Auto-immune haematological complications occurring during treatment for malignant Iymphoproliferative diseases are described in 5 patients. There appeared to be a temporal relationship between the development of these complications and the administration of chemotherapeutic drugs or extensive radiotherapy.S. Afr. Med. J., 48, 2143 (1974)
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