118 research outputs found
Point-contact spectroscopy on URuSi
Tunnel and point contact experiments have been made in a URuSi single
crystal along the c-axis. The experiments were performed changing temperature
and contact size in a low temperature scanning tunneling microscope. A
resonance develops at the Fermi level at K. This resonance splits
and becomes asymmetric when the 17.5 K phase transition is crossed. These
results are consistent with the existence of Kondo like bound states of the
U ionic configurations and the conduction electrons. Below the
transition, these configurations are split by the development of quadrupolar
ordering. The peak separation can be interpreted as a direct measurement of the
order parameter. Measurements on a policrystalline UAu_2Si_2$ sample are also
reported, with a comparative study of the behavior of both materials.Comment: 4 pages (Latex) + 2 postscript figure
Temperature and junction-type dependency of Andreev reflection in MgB2
We studied the voltage and temperature dependency of the dynamic conductance
of normal metal-MgB2 junctions obtained either with the point-contact technique
(with Au and Pt tips) or by making Ag-paint spots on the surface of
high-quality single-crystal-like MgB2 samples. The fit of the conductance
curves with the generalized BTK model gives evidence of pure s-wave gap
symmetry. The temperature dependency of the gap, measured in Ag-paint junctions
(dirty limit), follows the standard BCS curve with 2Delta/kTc = 3.3. In
out-of-plane, high-pressure point contacts we obtained almost ideal Andreev
reflection characteristics showing a single small s-wave gap Delta = 2.6 +/-
0.2 (clean limit). These results support the two-gap model of
superconductivity, the presence of a modified layer at the surface of the
crystals and an important and non-conventional role of the impurities in MgB2.Comment: 5 pages, 4 eps figures, SNS 2001 conferenc
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
Evidence for Nodal superconductivity in SrScFePO
Point contact Andreev reflection spectra have been taken as a function of
temperature and magnetic field on the polycrystalline form of the newly
discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak
which disappears at the superconducting transition temperature, dominates all
of the spectra. Data taken in high magnetic fields show that this feature
survives until 7T at 2K and a flattening of the feature is observed in some
contacts. Here we inspect whether these observations can be interpreted within
a d-wave, or nodal order parameter framework which would be consistent with the
recent theoretical model where the height of the P in the Fe-P-Fe plane is key
to the symmetry of the superconductivity. However, in polycrystalline samples
care must be taken when examining Andreev spectra to eliminate or take into
account artefacts associated with the possible effects of Josephson junctions
and random alignment of grains.Comment: Published versio
Andreev reflection in Au/La_{2-x}Sr_{x}CuO_{4} point-contact junctions: separation between pseudogap and phase-coherence gap
We made point-contact measurements with Au tips on La_{2-x}Sr_{x}CuO_{4}
samples with 0.08 < x < 0.20 to investigate the relationship between
superconducting gap and pseudogap. We obtained junctions whose conductance
curves presented typical Andreev reflection features at all temperatures from
4.2 K up to T_c^A close to the bulk T_c. Their fit with the
BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is
used. The doping dependence of the low temperature dominant isotropic gap
component Delta_{s} follows very well the T_{c} vs. x curve. These results
support the separation between the superconducting (Andreev) gap and the
pseudogap measured by angle-resolved photoemission spectroscopy (ARPES) and
tunneling.Comment: 4 pages, 5 eps figures, 1 table. SNS 2001 Conferenc
Ultrastrong poly(2âoxazoline)/poly(acrylic acid) doubleânetwork hydrogels with cartilageâlike mechanical properties
The exceptional stiffness and toughness of double-network hydrogels (DNHs) offer the possibility to mimic even complex biomaterials, such as cartilage. The latter has a limited regenerative capacity and thus needs to be substituted with an artificial material. DNHs composed of cross-linked poly(2-oxazoline)s (POx) and poly(acrylic acid) (PAA) are synthesized by free radical polymerization in a two-step process. The resulting DNHs are stabilized by hydrogen bridges even at pH 7.4 (physiological PBS buffer) due to the pKa-shifting effect of POx on PAA. DNHs based on poly(2-methyl-2-oxazoline), which have a water content (WC) of around 66 wt% and are not cytotoxic, show biomechanical properties that match those of cartilage in terms of WC, stiffness, toughness, coefficient of friction, compression in body relevant stress conditions and viscoelastic behavior. This material also has high strength in PBS pH 7.4 and in egg white as synovial liquid substitute. In particular, a compression strength of up to 60 MPa makes this material superior
Onset of dissipation in ballistic atomic wires
Electronic transport at finite voltages in free-standing gold atomic chains
of up to 7 atoms in length is studied at low temperatures using a scanning
tunneling microscope (STM). The conductance vs voltage curves show that
transport in these single-mode ballistic atomic wires is non-dissipative up to
a finite voltage threshold of the order of several mV. The onset of dissipation
and resistance within the wire corresponds to the excitation of the atomic
vibrations by the electrons traversing the wire and is very sensitive to
strain.Comment: Revtex4, 4 pages, 3 fig
Review on possible gravitational anomalies
This is an updated introductory review of 2 possible gravitational anomalies
that has attracted part of the Scientific community: the Allais effect that
occur during solar eclipses, and the Pioneer 10 spacecraft anomaly,
experimented also by Pioneer 11 and Ulysses spacecrafts. It seems that, to
date, no satisfactory conventional explanation exist to these phenomena, and
this suggests that possible new physics will be needed to account for them. The
main purpose of this review is to announce 3 other new measurements that will
be carried on during the 2005 solar eclipses in Panama and Colombia (Apr. 8)
and in Portugal (Oct.15).Comment: Published in 'Journal of Physics: Conferences Series of the American
Institute of Physics'. Contribution for the VI Mexican School on Gravitation
and Mathematical Physics "Approaches to Quantum Gravity" (Playa del Carmen,
Quintana Roo, Mexico, Nov. 21-27, 2004). Updates to this information will be
posted in http://www.lsc-group.phys.uwm.edu/~xavier.amador/anomalies.htm
Primordial magnetic fields at preheating
Using lattice techniques we investigate the generation of long range
cosmological magnetic fields during a cold electroweak transition. We will show
how magnetic fields arise, during bubble collisions, in the form of magnetic
strings. We conjecture that these magnetic strings originate from the alignment
of magnetic dipoles associated with EW sphaleron-like configurations. We also
discuss the early thermalisation of photons and the turbulent behaviour of the
scalar fields after tachyonic preheating.Comment: 7 pages. Talk presented at Lattice200
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