Point-contact spectroscopy on URu2_2Si2_2

Tunnel and point contact experiments have been made in a URu$_2$Si$_2$ 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 $T\sim 60$ 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$^{4+}$ 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

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 Sr2_{2}ScFePO3_{3}

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

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

Understanding acute metabolic decompensation in propionic and methylmalonic acidemias: A deep metabolic phenotyping approach

Background: Pathophysiology of life-threatening acute metabolic decompensations (AMD) in propionic acidemia (PA) and isolated methylmalonic acidemia (MMA) is insufficiently understood. Here, we study the metabolomes of PA and MMA patients over time, to improve insight in which biochemical processes are at play during AMD. Methods: Longitudinal data from clinical chemistry analyses and metabolic assays over the life-course of 11 PA and 13 MMA patients were studied retrospectively. Direct-infusion high-resolution mass spectrometry was performed on 234 and 154 remnant dried blood spot and plasma samples of PA and MMA patients, respectively. In addition, a systematic literature search was performed on reported biomarkers. All results were integrated in an assessment of biochemical processes at play during AMD. Results: We confirmed many of the metabolite alterations reported in literature, including increases of plasma valine and isoleucine during AMD in PA patients. We revealed that plasma leucine and phenylalanine, and urinary pyruvic acid were increased during AMD in PA patients. 3-hydroxyisovaleric acid correlated positively with plasma ammonia. We found that known diagnostic biomarkers were not significantly further increased, while intermediates of the branched-chain amino acid (BCAA) degradation pathway were significantly increased during AMD. Conclusions: We revealed that during AMD in PA and MMA, BCAA and BCAA intermediates accumulate, while known diagnostic biomarkers remain essentially unaltered. This implies that these acidic BCAA intermediates are responsible for metabolic acidosis. Based on this, we suggest to measure plasma 3-hydroxyisovaleric acid and urinary ketones or 3-hydroxybutyric acid for the biochemical follow-up of a patient's metabolic stability

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

Point contact spectroscopy of the electron-doped cuprate superconductor Pr{2-x}Ce{x}CuO4: The dependence of conductance-voltage spectra on cerium doping, barrier strength and magnetic field

We present conductance-voltage (G-V) data for point contact junctions between a normal metal and the electron doped cuprate superconductor Pr{2-x}Ce{x}CuO4 (PCCO). We observe a zero bias conductance peak (ZBCP) for the under-doped composition of this cuprate (x=0.13) which is consistent with d-wave pairing symmetry. For optimally-doped (x=0.15) and over-doped (x=0.17) PCCO, we find that the G-V characteristics indicate the presence of an order parameter without nodes. We investigate this further by obtaining point contact spectroscopy data for different barrier strengths and as a function of magnetic field.Comment: 13 pages, 9 figure