821 research outputs found
Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films
a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been
structurally characterized by TEM (Transmission Electron Microscopy)
measurements. As-deposited or annealed films are shown to be continuous and
homogeneous in composition and thickness, up to an annealing temperature of
500{\deg}C. We have carried out low temperature transport measurements on these
films close to the superconductor-to-insulator transition (SIT), and shown a
qualitative difference between the effect of annealing or composition, and a
reduction of the film thickness on the superconducting properties of a-NbSi.
These results question the pertinence of the sheet resistance R_square as the
relevant parameter to describe the SIT.Comment: 9 pages, 12 figure
Observation of the Nernst signal generated by fluctuating Cooper pairs
Long-range order is destroyed in a superconductor warmed above its critical
temperature (Tc). However, amplitude fluctuations of the superconducting order
parameter survive and lead to a number of well established phenomena such as
paraconductivity : an excess of charge conductivity due to the presence of
short-lived Cooper pairs in the normal state. According to an untested theory,
these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous
superconducting films, the lifetime of Cooper pairs exceeds the elastic
lifetime of quasi-particles in a wide temperature range above Tc; consequently,
the Cooper pairs Nernst signal dominate the response of the normal electrons
well above Tc. In two dimensions, the magnitude of the expected signal depends
only on universal constants and the superconducting coherence length, so the
theory can be unambiguously tested. Here, we report on the observation of a
Nernst signal in such a superconductor traced deep into the normal state. Since
the amplitude of this signal is in excellent agreement with the theoretical
prediction, the result provides the first unambiguous case for a Nernst effect
produced by short-lived Cooper pairs
Optimization of a Classical Stamping Progression by Modal Correction of Anisotropy Ears
This work is a development from the Inetforsmep European project. We proposed
to realize a global optimization of a deep drawing industrial progression (made
of several stages) for a cup manufacture. The objectives of the process were
the thickness decrease and the geometrical parameters (especially the height).
This paper improves on this previous work in the aim of mastering the contour
error. From the optimal configuration, we expect to cut down the amount of the
needed material and the number of forming operations. Our action is focused on
the appearance of unexpected undulations (ears) located on the rim of the cups
during forming due to a nonuniform crystallographic texture. Those undulations
can cause a significant amount of scraps, productivity loss, and cost during
manufacture. In this paper, this phenomenon causes the use of four forming
operations for the cup manufacture. The aim is to cut down from four to two
forming stages by defining an optimal blank (size and shape). The advantage is
to reduce the cost of the tool manufacturing and to minimize the needed
material (by suppressing the part flange). The chosen approach consists in
defining a particular description of the ears' part by modal decomposition and
then simulating several blank shapes and sizes generated by discrete cosine
transformation (DCT). The use of a numerical simulation for the forming
operation and the design of an experiment technique allow mathematical links
between the ears' formation and the DCT coefficients. An optimization is then
possible by using mathematical links. This original approach leads the ears'
amplitude to be reduced by a factor of 10, with only 15 numerical experiments.
Moreover, we have limited the number of forming stages from 4 to 2 with a
minimal material use
Defect detection and characterisation in composite materials using active IR thermography coupled with SVD analysis and thermal quadrupole modeling
Abstract In t his s tudy, a ctive i nfrared t hermography is us ed t o det ect and c haracterize def ects i n c arbon/epoxy c omposite plates. Defects are polymeric discs inserted between plies at different depths of the sample. The thermal excitation consists in a f inite t ime s tep us ing h alogen l amps. The t ransient t hermal m odeling pr ovides a one-dimensional analytical s olution through thermal quadrupoles. Finally an inversion procedure is carried out to estimate modeling unknown parameters, especially the depth and thermal resistance of the defect
Nernst effect as a probe of superconducting fluctuations in disordered thin films
In amorphous superconducting thin films of and ,
a finite Nernst coefficient can be detected in a wide range of temperature and
magnetic field. Due to the negligible contribution of normal quasi-particles,
superconducting fluctuations easily dominate the Nernst response in the entire
range of study. In the vicinity of the critical temperature and in the
zero-field limit, the magnitude of the signal is in quantitative agreement with
what is theoretically expected for the Gaussian fluctuations of the
superconducting order parameter. Even at higher temperatures and finite
magnetic field, the Nernst coefficient is set by the size of superconducting
fluctuations. The Nernst coefficient emerges as a direct probe of the ghost
critical field, the normal-state mirror of the upper critical field. Moreover,
upon leaving the normal state with fluctuating Cooper pairs, we show that the
temperature evolution of the Nernst coefficient is different whether the system
enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting
regime.Comment: Submitted to New. J. Phys. for a focus issue on "Superconductors with
Exotic Symmetries
Design of a Gd-DOTA-Phthalocyanine Conjugate Combining MRI Contrast Imaging and Photosensitization Properties as a Potential Molecular Theranostic
Cataloged from PDF version of article.The design and synthesis of a phthalocyanine - Gd-DOTA conjugate is presented to open the way to novel molecular theranostics, combining the properties of MRI contrast imaging with photodynamic therapy. The rational design of the conjugate integrates isomeric purity of the phthalocyanine core substitution, suitable biocompatibility with the use of polyoxo water-solubilizing substituents, and a convergent synthetic strategy ended by the use of click chemistry to graft the Gd-DOTA moiety to the phthalocyanine. Photophysical and photochemical properties, contrast imaging experiments and preliminary in vitro investigations proved that such a combination is relevant and lead to a new type of potential theranostic agent
Spectral Properties of Quasiparticle Excitations Induced by Magnetic Moments in Superconductors
The consequences of localized, classical magnetic moments in superconductors
are explored and their effect on the spectral properties of the intragap bound
states is studied. Above a critical moment, a localized quasiparticle
excitation in an s-wave superconductor is spontaneously created near a magnetic
impurity, inducing a zero-temperature quantum transition. In this transition,
the spin quantum number of the ground state changes from zero to 1/2, while the
total charge remains the same. In contrast, the spin-unpolarized ground state
of a d-wave superconductor is found to be stable for any value of the magnetic
moment when the normal-state energy spectrum possesses particle-hole symmetry.
The effect of impurity scattering on the quasiparticle states is interpreted in
the spirit of relevant symmetries of the clean superconductor. The results
obtained by the non-self-consistent (T matrix) and the self-consistent
mean-field approximations are compared and qualitative agreement between the
two schemes is found in the regime where the coherence length is longer than
the Fermi length.Comment: to appear in Phys. Rev. B55, May 1st (1997
Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR
We present electron spin resonance data of Ti (3) ions in single
crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g
tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is
predominantly occupied and owing to the occurrence of orbital order a linear
spin chain forms along the crystallographic b axis. This result corroborates
recent theoretical LDA+U calculations of the band structure. The temperature
dependence of the parameters of the resonance signal suggests a strong coupling
between spin and lattice degrees of freedom and gives evidence for a transition
to a nonmagnetic ground state at 67 K.Comment: revised version, accepted for publication in Phys. Rev. B, Rapid Com
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