82 research outputs found
Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1-x}Co_x)_2As_2 thin films
The dependence of the superconducting gaps in epitaxial
Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq
0.15) was studied by means of point-contact Andreev-reflection spectroscopy.
The normalized conductance curves were well fitted by using the 2D
Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although
the possible presence of gap anisotropies cannot be completely excluded. The
amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic
trends as a function of the local critical temperature T_{c}^{A} (measured in
the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x
is well correlated to the trend of the critical temperature, i.e. to the shape
of the superconducting region in the phase diagram. When analyzed within a
simple three-band Eliashberg model, this trend turns out to be compatible with
a mechanism of superconducting coupling mediated by spin fluctuations, whose
characteristic energy scales with T_{c} according to the empirical law
\Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength
\lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly
decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15).Comment: 8 pages, 5 color figure
Resistivity in Co-doped Ba-122: comparison of thin films and single crystals
The temperature dependence of the resistivity of epitaxial
Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15)
has been analyzed and compared with analogous measurements on single crystals
taken from literature. The rho(T) of thin films looks different from that of
single crystals, even when the cobalt content is the same. All rho(T) curves
can be fitted by considering an effective two-band model (with holes and
electrons bands) in which the electrons are more strongly coupled with the
bosons (spin fluctuations) than holes, while the effect of impurities is mainly
concentrated in the hole band. Within this model the mediating boson has the
same characteristic energy in single crystals and thin films, but the shape of
the transport spectral function at low energy has to be very different, leading
to a "hardening" of the electron-boson spectral function in thin films,
associated with the strain induced by the substrate.Comment: 13 pages, 4 figure
ab-plane resistivity and possible charge stripe ordering in strongly underdoped LaSrCuO single crystals
We have measured the ab-plane resistivity of LaSrCuO single
crystals with small Sr content (x=0.052 0.075) between 4.2 and 300 K by
using the AC Van der Pauw technique. As recently suggested by Ichikawa et al.,
the deviation from the linearity of the curve starting
at a temperature T can be interpreted as due to a progressive
slowing down of the fluctuations of pre-formed charge stripes. An electronic
transition of the stripes to a more ordered phase could instead be responsible
for some very sharp anomalies present in the of
superconducting samples just above .Comment: M2S-HTSC-VI Conference paper (2 pages, 2 figures), using Elsevier
style espcrc2.st
Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating
The electrochemical gating technique is a powerful tool to tune the surface conduction properties
of various materials by means of pure charge doping, but its efficiency is thought to be hampered in
materials with a good electronic screening. We show that, if applied to a metallic superconductor
(NbN thin films), this approach allows observing reversible enhancements or suppressions of the bulk
superconducting transition temperature, which vary with the thickness of the films. These results
are interpreted in terms of proximity effect, and indicate that the effective screening length depends
on the induced charge density, becoming much larger than that predicted by standard screening
theory at very high electric fields
Energy-efficient adaptive machine learning on IoT end-nodes with class-dependent confidence
Energy-efficient machine learning models that can run directly on edge devices are of great interest in IoT applications, as they can reduce network pressure and response latency, and improve privacy. An effective way to obtain energy-efficiency with small accuracy drops is to sequentially execute a set of increasingly complex models, early-stopping the procedure for 'easy' inputs that can be confidently classified by the smallest models. As a stopping criterion, current methods employ a single threshold on the output probabilities produced by each model. In this work, we show that such a criterion is sub-optimal for datasets that include classes of different complexity, and we demonstrate a more general approach based on per-classes thresholds. With experiments on a low-power end-node, we show that our method can significantly reduce the energy consumption compared to the single-threshold approach
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
The effect of magnetic impurities in a two-band superconductor: A point-contact study of Mn-substituted MgB2 single crystals
We present the first results of directional point-contact measurements in
Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3
K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting
the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model.
Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the
junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once
analyzed within the Eliashberg theory, the results indicate that spin-flip
scattering is dominant in the sigma band, as also confirmed by first-principle
band structure calculations.Comment: 4 pages, 5 eps figures. New theoretical results added, text and some
figures changed. References adde
Huge field-effect surface charge injection and conductance modulation in metallic thin films by electrochemical gating
The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached more than 4 × 10 15 charges/cm 2 . At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (≥25 nm), results can be nicely explained within a free-electron model with parallel resistive channels. The huge charge injections achieved make this particular field-effect technique very promising for a vast variety of materials such as unconventional superconductors, graphene and 2D-like materials. © 2012 Elsevier B.V
Ultra-compact binary neural networks for human activity recognition on RISC-V processors
Human Activity Recognition (HAR) is a relevant inference task in many mobile applications. State-of-the-art HAR at the edge is typically achieved with lightweight machine learning models such as decision trees and Random Forests (RFs), whereas deep learning is less common due to its high computational complexity. In this work, we propose a novel implementation of HAR based on deep neural networks, and precisely on Binary Neural Networks (BNNs), targeting low-power general purpose processors with a RISC-V instruction set. BNNs yield very small memory footprints and low inference complexity, thanks to the replacement of arithmetic operations with bit-wise ones. However, existing BNN implementations on general purpose processors impose constraints tailored to complex computer vision tasks, which result in over-parametrized models for simpler problems like HAR. Therefore, we also introduce a new BNN inference library, which targets ultra-compact models explicitly. With experiments on a single-core RISC-V processor, we show that BNNs trained on two HAR datasets obtain higher classification accuracy compared to a state-of-the-art baseline based on RFs. Furthermore, our BNN reaches the same accuracy of a RF with either less memory (up to 91%) or more energy-efficiency (up to 70%), depending on the complexity of the features extracted by the RF
Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling
We report on a study of the superconducting order parameter in
Fe(TeSe) thin films (with different Se contents: x=0.3, 0.4, 0.5)
by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS
spectra show reproducible evidence of multiple structures, namely two clear
conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we
show, are the signature of the strong interaction of charge carriers with a
bosonic mode whose characteristic energy coincides with the spin-resonance
energy. The details of some PCARS spectra at low energy suggest the presence of
a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS
measurements in Fe(TeSe) single crystals. The values of the two
gaps and , once plotted as a function of the local
critical temperature , turn out to be in perfect agreement with the
results obtained by various experimental techniques reported in literature.Comment: 8 pages, 6 figures. This is an author-created, un-copyedited version
of an article published in Supercond. Sci. Technol. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
10.1088/0953-2048/27/12/12401
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