Photoconductance of a one-dimensional quantum dot

The ac-transport properties of a one-dimensional quantum dot with non-Fermi liquid correlations are investigated. It is found that the linear photoconductance is drastically influenced by the interaction. Temperature and voltage dependences of the sideband peaks are treated in detail. Characteristic Luttinger liquid power laws are founded.Comment: accepted in European Physical Journal

Experimental confirmation of the low B isotope coefficient in MgB2

Recent investigations have shown that the first proposed explanations of the disagreement between experimental and theoretical value of isotope coefficient in MgB2 need to be reconsidered. Considering that in samples with residual resistivity of few mu-Ohm cm critical temperature variations produced by disorder effects can be comparable with variations due to the isotopic effect, we adopt a procedure in evaluating the B isotope coefficient which take account of these effects, obtaining a value which is in agreement with previous results and then confirming that there is something still unclear in the physics of MgB2.Comment: 8 pages, 3 figures Title has been changed A statement has been added in page 7 of the pdf file "Finally we would..." Reference 21 has been added Figure 1 anf Figure 2 have been change

Seebeck effect in the conducting LaAlO_{3}/SrTiO_{3} interface

The observation of metallic behavior at the interface between insulating oxides has triggered worldwide efforts to shed light on the physics of these systems and clarify some still open issues, among which the dimensional character of the conducting system. In order to address this issue, we measure electrical transport (Seebeck effect, Hall effect and conductivity) in LaAlO_{3}/SrTiO_{3} interfaces and, for comparison, in a doped SrTiO_{3} bulk single crystal. In these experiments, the carrier concentration is tuned, using the field effect in a back gate geometry. The combined analysis of all experimental data at 77 K indicates that the thickness of the conducting layer is ~7 nm and that the Seebeck effect data are well described by a two-dimensional (2D) density of states. We find that the back gate voltage is effective in varying not only the charge density, but also the thickness of the conducting layer, which is found to change by a factor of ~2, using an electric field between -4 and +4MV/m at 77K. No enhancement of the Seebeck effect due to the electronic confinement and no evidence for two-dimensional quantization steps are observed at the interfaces.Comment: 15 pages, 5 figure

Thermal properties of MgB2: the effect of disorder on gap amplitudes and relaxation times of p and s bands

We present thermal conductivity and specific heat measurements on MgB2 and Mg-AlB2 samples. Thermal properties have been analysed by using a two-gap model in order to estimate the gap amplitudes, D(0)p and D(0)s and the intra-band scattering rates, Gss and Gpp. As a function of Al doping and disorder D(0)s rapidly decreases, while D(0)p is rather constant. Gss and Gpp are increased by the disorder, being Gpp more affected than Gss.Comment: 2 pages, 3 figures, presented to the conference M2S-HTSC, 25-30 May 2003, Rio de Janeir

Magneto Seebeck effect in REFeAsO (RE=rare earth) compounds: probing the magnon drag scenario

We investigate Seebeck effect in REFeAsO (RE=rare earth)compounds as a function of temperature and magnetic field up to 30T. The Seebeck curves are characterized by a broad negative bump around 50K, which is sample dependent and strongly enhanced by the application of a magnetic field. A model for the temperature and field dependence of the magnon drag contribution to the Seebeck effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts for the magnitude and scaling properties of such bump feature in our experimental data. This analysis allows to extract precious information on the coupling between electrons and AFM spin fluctuations in these parent compound systems, with implications on the pairing mechanism of the related superconducting compounds

Thermal properties of SmFeAs(O1-xFx) as probe of the interplay between electrons and phonons

A comparative study of thermal properties of SmFeAsO, SmFeAs(O0.93F0.07) and SmFeAs(O0.85F0.15) samples is presented. Specific heat and thermal conductivity show clear evidences of the spin density wave (SDW) ordering below TSDW 135 K in undoped SmFeAsO. At low level of F-doping, SmFeAs(O0.93F0.07), SDW ordering is suppressed and superconducting features are not yet optimally developed in both specific heat and thermal conductivity. At optimal level of F-doping SmFeAs(O0.85F0.15) anomalies related to the superconducting transition are well noticeable. By a compared analysis of doped and undoped samples we conclude that, despite F-doping modifies definitely the electronic ground state, it does not substantially alter phonon and electron parameters, like phonon modes, Sommerfeld coefficient, electro-phonon coupling. The analysis of the thermal conductivity curves provides an evaluation of SDW and superconducting energy gap, showing that phonons can suitably probe features of electronic ground state

Stress transmission in granular matter

The transmission of forces through a disordered granular system is studied by means of a geometrical-topological approach that reduces the granular packing into a set of layers. This layered structure constitutes the skeleton through which the force chains set up. Given the granular packing, and the region where the force is applied, such a skeleton is uniquely defined. Within this framework, we write an equation for the transmission of the vertical forces that can be solved recursively layer by layer. We find that a special class of analytical solutions for this equation are L\'evi-stable distributions. We discuss the link between criticality and fragility and we show how the disordered packing naturally induces the formation of force-chains and arches. We point out that critical regimes, with power law distributions, are associated with the roughness of the topological layers. Whereas, fragility is associated with local changes in the force network induced by local granular rearrangements or by changes in the applied force. The results are compared with recent experimental observations in particulate matter and with computer simulations.Comment: 14 pages, Latex, 5 EPS figure

Thermal conductivity of MgB2_{2} in the superconducting state

We present thermal conductivity measurements on very pure and dense bulk samples, as indicated by residual resistivity values as low as 0.5 mW cm and thermal conductivity values higher than 200 W/mK. In the normal state we found that the Wiedemann Franz law, in its generalized form, works well suggesting that phonons do not contribute to the heat transport. The thermal conductivity in the superconducting state has been analysed by using a two-gap model. Thank to the large gap anisotropy we were able to evaluate quantitatively intraband scattering relaxation times of $\pi$ and $\sigma$ bands, which depend on the disorder in different way; namely, as the disorder increases, it reduces more effectively the relaxation times of $\pi$ than of $\sigma$ bands, as suggested by a recent calculation [1].Comment: 12 pages, 5 figure

Probing the electron-phonon coupling in MgB2 through magnetoresistance measurements in neutron irradiated thin films

We report magnetoresistance (MR) measurements on MgB2 and the corresponding full account from ab-initio calculations; we suggest that this combination can be a useful tool to probe electron- phonon coupling. We obtain good quantitative agreement between high field measurements on neutron irradiated epitaxial thin films and calculations within Bloch-Boltzmann transport theory over a wide range of magnetic fields (0-28 T) and temperatures (40-300 K), and as a function of the field orientation. The crossovers between in-plane and out-of-plane MR, experimentally observed as a function of either disorder or temperature are well reproduced indicating that disorder and interaction with phonons strongly affect the scattering rate of s-carriers.Comment: 13 pages, 4 figures, 1 tabl