62 research outputs found
A Study of Carbon Substitutions in MgB_2 within the two-band Eliashberg theory
We study the effects of C substitutions in MgB_2 within the two-band model in
the Eliashberg formulation. We use as input the B-B stretching-mode frequency
and the partial densities of states N_{sigma}(EF) and N_{pi}(EF), recently
calculated for Mg(B_{1-x}C_{x})_2 at various x values from first-principles
density functional methods. We then take the prefactor in the Coulomb
pseudopotential matrix, mu, and the interband scattering parameter,
Gamma^{sigma pi}, as the only adjustable parameters. The dependence on the C
content of Tc and of the gaps (Delta_{sigma} and Delta_{pi}) recently measured
in Mg(B_{1-x}C_{x})_2 single crystals indicate an almost linear decrease of mu
on increasing x, with an increase in interband scattering that makes the gaps
merge at x=0.132. In polycrystals, instead, where the gap merging is not
observed, no interband scattering is required to fit the experimental data.Comment: 7 pages, 8 figures, RevTex4. Detailed discussion of the results adde
Phonon dispersion and lifetimes in MgB2
We measure phonon dispersion and linewidth in a single crystal of MgB_2 along
the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We
use Density Functional Theory to compute the effect of both electron-phonon
coupling and anharmonicity on the linewidth, obtaining excellent agreement with
experiment. Anomalous broadening of the E_2g phonon mode is found all along
Gamma-A. The dominant contribution to the linewidth is always the
electron-phonon coupling.Comment: 4 pages, 3 figure
The low-frequency response in the surface superconducting state of ZrB single crystal}
The large nonlinear response of a single crystal ZrB to an ac field
(frequency 40 - 2500 Hz) for has been observed. Direct
measurements of the ac wave form and the exact numerical solution of the
Ginzburg-Landau equations, as well as phenomenological relaxation equation,
permit the study of the surface superconducting states dynamics. It is shown,
that the low frequency response is defined by transitions between the
metastable superconducting states under the action of an ac field. The
relaxation rate which determines such transitions dynamics, is found.Comment: 7 pages, 11 figure
Point-contact spectroscopic studies on normal and superconducting AFe_2As_2-type iron-pnictide single crystals
Point-contact Andreev reflection spectroscopy (PCARS) is applied to
investigate the gap structure in iron pnictide single crystal superconductors
of the AFe_2As_2 (A=Ba, Sr) family ("Fe-122"). The observed point-contact
junction conductance curves, G(V), can be divided into two categories: one
where Andreev reflection is present for both (Ba_{0.6}K_{0.4})Fe_2As_2 and
Ba(Fe_{0.9}Co_{0.1})_2As_2, and the other with a V^{2/3} background conductance
universally observed extending even up to 100 meV for Sr_{0.6}Na_{0.4}Fe_2As_2
and Sr(Fe_{0.9}Co_{0.1})_2As_2. The latter is also observed in point-contact
junctions on the nonsuperconducting parent compound BaFe_2As_2. Mesoscopic
phase-separated coexistence of magnetic and superconducting orders is
considered to explain distinct behaviors in the superconducting samples. For
Ba_{0.6}K_{0.4}Fe_2As_2, double peaks due to Andreev reflection with
strongly-sloping background are frequently observed for point-contacts on
freshly-cleaved c-axis surfaces. If normalized by a background baseline and
analyzed by the Blonder-Tinkham-Klapwijk model, the data show a gap size
~3.0-4.0 meV with 2\Delta_0/k_BT_c ~ 2.0-2.6, consistent with the smaller gap
size reported in the LnFeAsO family ("Fe-1111"). For the
Ba(Fe_{0.9}Co_{0.1})_2As_2, G(V) curves typically display a zero-bias
conductance peak.Comment: 12 pages, 6 figure
Directional point-contact Andreev-reflection spectroscopy of Fe-based superconductors: Fermi surface topology, gap symmetry, and electron-boson interaction
Point-contact Andreev reflection spectroscopy (PCAR) has proven to be one of
the most powerful tools in the investigation of superconductors, where it
provides information on the order parameter (OP), a fundamental property of the
superconducting state. In the past 20 years, successive improvements of the
models used to analyze the spectra have continuously extended its capabilities,
making it suited to study new superconductors with "exotic" properties such as
anisotropic, nodal and multiple OPs. In Fe-based superconductors, the complex
compound- and doping-dependent Fermi surface and the predicted sensitivity of
the OP to fine structural details present unprecedent challenges for this
technique. Nevertheless, we show here that PCAR measurements in Fe-based
superconductors carried out so far have already greatly contributed to our
understanding of these materials, despite some apparent inconsistencies that
can be overcome if a homogeneous treatment of the data is used. We also
demonstrate that, if properly extended theoretical models for Andreev
reflection are used, directional PCAR spectroscopy can provide detailed
information not only on the amplitude and symmetry of the OPs, but also on the
nature of the pairing boson, and even give some hints about the shape of the
Fermi surface.Comment: 27 pages, 14 figures. Review article to appear in a special issue of
Reports on Progress in Physic
Ivermectin reduces in vivo coronavirus infection in a mouse experimental model
SARS-CoV2 is a single strand RNA virus member of the type 2 coronavirus family, responsible for causing COVID-19 disease in humans. The objective of this study was to test the ivermectin drug in a murine model of coronavirus infection using a type 2 family RNA coronavirus similar to SARS-CoV2, the mouse hepatitis virus (MHV). BALB/cJ female mice were infected with 6,000 PFU of MHV-A59 (Group Infected; n=20) and immediately treated with one single dose of 500 μg/kg of ivermectin (Group Infected + IVM; n=20), or were not infected and treated with PBS (Control group; n=16). Five days after infection/treatment, mice were euthanized to obtain different tissues to check general health status and infection levels. Overall results demonstrated that viral infection induces the typical MHV disease in infected animals, with livers showing severe hepatocellular necrosis surrounded by a severe lymphoplasmacytic inflammatory infiltration associated with a high hepatic viral load (52,158 AU), while ivermectin administration showed a better health status with lower viral load (23,192 AU; p<0.05) and few livers with histopathological damage (p<0.05), not showing statistical differences with control mice (P=NS). Furthermore, serum transaminase levels (aspartate aminotransferase and alanine aminotransferase) were significantly lower in treated mice compared to infected animals. In conclusion, ivermectin seems to be effective to diminish MHV viral load and disease in mice, being a useful model for further understanding new therapies against coronavirus diseases.Fil: Arevalo, A. P.. Instituto Pasteur de Montevideo; UruguayFil: Pagotto, R.. Instituto Pasteur de Montevideo; UruguayFil: Pórfido, Jorge Luis. Instituto Pasteur de Montevideo; Uruguay. Universidad de la República; Uruguay. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Daghero, H.. Instituto Pasteur de Montevideo; UruguayFil: Segovia, Alcira Mercedes. Instituto Pasteur de Montevideo; Uruguay. Universidad de la República; UruguayFil: Yamasaki, K.. Universidad de la Republica. Facultad de Veterinaria.; UruguayFil: Varela, B. Universidad de la Republica. Facultad de Veterinaria.; UruguayFil: Hill, Marcelo. Instituto Pasteur de Montevideo; Uruguay. Universidad de la República; UruguayFil: Verdes, J. M.. Universidad de la Republica. Facultad de Veterinaria.; UruguayFil: Duhalde Vega, Maite. Instituto Pasteur de Montevideo; Uruguay. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de QuÃmica y FÃsico-QuÃmica Biológicas "Prof. Alejandro C. Paladini". Universidad de Buenos Aires. Facultad de Farmacia y BioquÃmica. Instituto de QuÃmica y FÃsico-QuÃmica Biológicas; ArgentinaFil: Bollati Fogollin, M.. Instituto Pasteur de Montevideo; UruguayFil: Crispo, Martina. Instituto Pasteur de Montevideo; Urugua
Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy
We have studied current-voltage characteristics of Andreev contacts in
polycrystalline GdOFFeAs samples with bulk critical
temperature = (52.5 \pm 1)K using break-junction technique. The data
obtained cannot be described within the single-gap approach and suggests the
existence of a multi-gap superconductivity in this compound. The large and
small superconducting gap values estimated at T = 4.2K are {\Delta}L = 10.5 \pm
2 meV and {\Delta}S = 2.3 \pm 0.4 meV, respectively.Comment: 5 pages, 4 figures, submitted to JETP Letter
Effect of transport-induced charge inhomogeneity on point-contact Andreev reflection spectra at ferromagnet-superconductor interfaces
We investigate the transport properties of a ferromagnet-superconductor
interface within the framework of a modified three-dimensional
Blonder-Tinkham-Klapwijk formalism. In particular, we propose that charge
inhomogeneity forms via two unique transport mechanisms, namely, evanescent
Andreev reflection and evanescent quasiparticle transmission. Furthermore, we
take into account the influence of charge inhomogeneity on the interfacial
barrier potential and calculate the conductance as a function of bias voltage.
Point-contact Andreev reflection (PCAR) spectra often show dip structures,
large zero-bias conductance enhancement, and additional zero-bias conductance
peak. Our results indicate that transport-induced charge inhomogeneity could be
a source of all these anomalous characteristics of the PCAR spectra.Comment: 9 pages, 6 figure
Single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2: growth, structure and superconducting properties
A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr,
Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been
applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals
were grown using quartz ampoule method. Single crystals were used for
electrical transport, structure, magnetic torque and spectroscopic studies.
Investigations of the crystal structure confirmed high structural perfection
and show less than full occupation of the (O, F) position in superconducting
LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx crystals show a
significant broadening of the transition in high magnetic fields, whereas the
resistive transition in Ba1 xRbxFe2As2 simply shifts to lower temperature.
Critical current density for both compounds is relatively high and exceeds
2x109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth,
measured on LnFeAsO1-xFx crystals by torque magnetometry is temperature
dependent and apparently larger than the anisotropy of the upper critical
field. Ba1-xRbxFe2As2 crystals are electronically significantly less
anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the
existence of two energy gaps in LnFeAsO1-xFx. Scanning Tunneling Spectroscopy
reveals in addition to a superconducting gap, also some feature at high energy
(~20 meV).Comment: 27 pages, 19 figures, 2 tables, accepted to the special issue of the
Physica C on superconducting pnictide
Observation of multiple superconducting gaps in Fe1+yTe1-xSex via a nano-scale approach to point-contact spectroscopy
We report a distinct experimental approach to point-contact Andreev
reflection spectroscopy with diagnostic capability via a unique design of
nano-scale normal metal/superconductor devices with excellent thermo-mechanical
stability, and have employed this method to unveil the existence of two
superconducting energy gaps in iron chalcogenide Fe1+yTe1-xSex which is crucial
for understanding its pairing mechanism. This work opens up new opportunities
to study gap structures in superconductors and elemental excitations in solids.Comment: 18 pages (5 figures) plus supplementary materials (5 pages). J.
Phys.: Cond. Matter, in pres
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