1,629 research outputs found
Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes
The doping dependent isotope effect on the critical temperature (Tc) is
calculated for multi-band multi-condensate superconductivity near a 2.5
Lifshitz transition. We focus on multi-band effects that arises in
nano-structures and in density wave metals (like spin density wave or charge
density wave) as a result of the band folding. We consider a superlattice of
quantum stripes with finite hopping between stripes near a 2.5 Lifshitz
transition for appearing of a new sub-band making a circular electron-like
Fermi surface pocket. We describe a particular type of BEC (Bose-Einstein
Condensate) to BCS (Bardeen-Cooper-Schrieffer condensate) crossover in
multi-band / multi-condensate superconductivity at a metal-to-metal transition
that is quite different from the standard BEC-BCS crossover at an
insulator-to-metal transition. The electron wave-functions are obtained by
solving the Schr\"odinger equation for a one-dimensional modulated potential
barrier. The k-dependent and energy dependent superconducting gaps are
calculated using the k-dependent anisotropic Bardeen-Cooper-Schrieffer (BCS)
multi-gap equations solved joint with the density equation, according with the
Leggett approach currently used now in ultracold fermionic gases. The results
show that the isotope coefficient strongly deviates from the standard BCS value
0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for
the metal-to-metal transition. The critical temperature Tc shows a minimum due
to the Fano antiresonance in the superconducting gaps and the isotope
coefficient diverges at the point where a BEC coexists with a BCS condensate.
On the contrary Tc reaches its maximum and the isotope coefficient vanishes at
the crossover from a polaronic condensate to a BCS condensate in the new
appearing sub-band.Comment: 8 pages, 4 ps figure
Ferrocene Molecular Architectures Grafted on Si(111): A Theoretical Calculation of the Standard Oxidation Potentials and Electron Transfer Rate Constant
The standard oxidation potential and the electron transfer (ET) rate constants of two silicon-based hybrid interfaces, Si(111)/organic-spacer/Ferrocene, are theoretically calculated and assessed. The dynamics of the electrochemical driven ET process is modeled in terms of the classical donor/acceptor scheme within the framework of Marcus theory. The ET rate constants, k(ET), are determined following calculation of the electron transfer matrix element, V-RP, together with the knowledge of the energy of the neutral and charge separated systems. The recently introduced Constrained Density Functional Theory (CDFT) method is exploited to optimize the structure and determine the energy of the charge separated species. Calculated ET rate constants are k(ET) = 77.8s(-1) and k(ET) = 1.3 x 10(-9) s(-1), in the case of the short and long organic-spacer, respectively
Interface reconstruction in superconducting CaCuO2/SrTiO3 superlattices: A hard x-ray photoelectron spectroscopy study
Here we report about the interface reconstruction in the recently discovered
superconducting artificial superlattices based on insulating CaCuO2 and SrTiO3
blocks. Hard x-ray photoelectron spectroscopy shows that the valence bands
alignment prevents any electronic reconstruction by direct charge transfer
between the two blocks. We demonstrate that the electrostatic built-in
potential is suppressed by oxygen redistribution in the alkaline earth
interface planes. By using highly oxidizing growth conditions, the oxygen
coordination in the reconstructed interfaces may be increased, resulting in the
hole doping of the cuprate block and thus in the appearance of
superconductivity.Comment: 9 pages, 6 figure
Possible Fano resonance for high-T-c multi-gap superconductivity in p-Terphenyl doped by K at the Lifshitz transition
Recent experiments have reported the emergence of high temperature
superconductivity with critical temperature between 43K and 123K in a
potassium doped aromatic hydrocarbon para-Terphenyl or p-Terphenyl. This
achievement provides the record for the highest Tc in an organic superconductor
overcoming the previous record of Tc=38 K in Cs3C60 fulleride. Here we propose
that the driving mechanism is the quantum resonance between superconducting
gaps near a Lifshitz transition which belongs to the class of Fano resonances
called shape resonances. For the case of p-Terphenyl our numerical solutions of
the multi gap equation shows that high Tc is driven by tuning the chemical
potential by K doping and it appears only in a narrow energy range near a
Lifshitz transition. At the maximum critical temperature, Tc=123K, the
condensate in the appearing new small Fermi surface pocket is in the BCS-BEC
crossover while the Tc drops below 0.3 K where it is in the BEC regime. Finally
we predict the experimental results which can support or falsify our proposed
mechanism: a) the variation of the isotope coefficient as a function of the
critical temperature and b) the variation of the gaps and their ratios
2Delta/Tc as a function of Tc.Comment: 7 pages, 7 figure
Small scale rotational disorder observed in epitaxial graphene on SiC(0001)
Interest in the use of graphene in electronic devices has motivated an
explosion in the study of this remarkable material. The simple, linear Dirac
cone band structure offers a unique possibility to investigate its finer
details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has
been performed on graphene grown on metal substrates but electronic
applications require an insulating substrate. Epitaxial graphene grown by the
thermal decomposition of silicon carbide (SiC) is an ideal candidate for this
due to the large scale, uniform graphene layers produced. The experimental
spectral function of epitaxial graphene on SiC has been extensively studied.
However, until now the cause of an anisotropy in the spectral width of the
Fermi surface has not been determined. In the current work we show, by
comparison of the spectral function to a semi-empirical model, that the
anisotropy is due to small scale rotational disorder ( 0.15)
of graphene domains in graphene grown on SiC(0001) samples. In addition to the
direct benefit in the understanding of graphene's electronic structure this
work suggests a mechanism to explain similar variations in related ARPES data.Comment: 5 pages, 4 figure
Effects of Mediterranean diet and weight loss on blood-lipid profile in overweight adults with hypercholesterolemia
Blood cholesterol has been positively associated with increased cardiovascular risk as a modifiable risk factors together with the lifestyle and diet. Furthermore, an improvement of the blood-lipid profile seems to be able to produce a decrease in cardiovascular events. Cholesterol plasma levels are related to the body mass index (BMI) and are affected by diet. The aim of this study was to evaluate the effectiveness of a Mediterranean diet (MD) weight-loss programme to improve blood cholesterol profiles in overweight adults subjected to real-world outpatient diet. Forty-nine hypercholesteraemic, overweight adults of both sexes were subjected to a dietary weight-loss intervention. Patients were prescribed a slightly hypocaloric MD for 16 weeks, followed by an 8-week follow-up period with a normocaloric diet. Data showed significant weight loss and cholesterol blood profile improvement both under the hypocaloric diet and during the follow-up period. In particular, the decrease in both Total and LDL-cholesterol was greater than their critical differences indicating the clinical relevance of blood lipid improvement induced by MD
Resonant and crossover phenomena in a multiband superconductor tuning the chemical potential near a band edge
Resonances in the superconducting properties, in a regime of crossover from
BCS to mixed Bose-Fermi superconductivity, are investigated in a two-band
superconductor where the chemical potential is tuned near the band edge of the
second mini-band generated by quantum confinement effects. The shape resonances
at T=0 in the superconducting gaps (belonging to the class of Feshbach-like
resonances) is manifested by interference effects in the superconducting gap at
the first large Fermi surface when the chemical potential is in the proximity
of the band edge of the second mini-band. The case of a superlattice of quantum
wells is considered and the amplification of the superperconducting gaps at the
3D-2D Fermi surface topological transition is clearly shown. The results are
found to be in good agreement with available experimental data on a
superlattice of honeycomb boron layers intercalated by Al and Mg spacer layers.Comment: 13 pages, 9 image
Formation and magnetic manipulation of periodically aligned microchains in thin plastic membranes
We demonstrate the fabrication of polymeric membranes that incorporate a few layers of periodically aligned magnetic microchains formed upon the application of variable magnetic fields. A homogeneous solution containing an elastomeric polymer and a small amount of colloidal magnetic nanoparticles is spin coated on glass slides, thereby forming thin magnetic membranes of ca. 10 μm thickness. Subsequent application of a homogeneous magnetic field results in the orientation of the magnetic clusters and their further motion into the matrix along the field lines forming layers of aligned chains. The study of the kinetics of alignment demonstrates that the chains are formed in the first hour of exposure to the magnetic field. Above all, a detailed microscopy study reveals that the dimensions and the periodicity of the microchains are effectively controlled by the intensity of the magnetic field, in good agreement with the theoretical simulations. This ability to form and manipulate the size and the distribution of chains into the polymeric matrix gives the opportunity to develop multifunctional composite materials ready to be used in various applications such as electromagnetic shielding, or multifunctional magnetic membranes etc
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