35 research outputs found
Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
We present a method for characterizing the propagation of the magnetic flux
in an artificially drilled bulk high-temperature superconductor (HTS) during a
pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical
sample, the magnetic flux density is measured simultaneously in 16 holes by
means of microcoils that are placed across the median plane, i.e. at an equal
distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes
during a pulse and measure the time taken by the external magnetic flux to
reach each hole. Our data show that the flux front moves faster in the median
plane than on the surface when penetrating the sample edge; it then proceeds
faster along the surface than in the bulk as it penetrates the sample further.
Once the pulse is over, the trapped flux density inside the central hole is
found to be about twice as large in the median plane than on the surface. This
ratio is confirmed by modelling
Electron Standing Wave Formation in Atomic Wires
Using the Landauer formulation of transport theory and tight binding models
of the electronic structure, we study electron transport through atomic wires
that form 1D constrictions between pairs of metallic nano-contacts. Our results
are interpreted in terms of electron standing waves formed in the atomic wires
due to interference of electron waves reflected at the ends of the atomic
constrictions. We explore the influence of the chemistry of the atomic
wire-metal contact interfaces on these standing waves and the associated
transport resonances by considering two types of atomic wires: gold wires
attached to gold contacts and carbon wires attached to gold contacts. We find
that the conductance of the gold wires is roughly for the
wire lengths studied, in agreement with experiments. By contrast, for the
carbon wires the conductance is found to oscillate strongly as the number of
atoms in the wire varies, the odd numbered chains being more conductive than
the even numbered ones, in agreement with previous theoretical work that was
based on a different model of the carbon wire and metal contacts.Comment: 14 pages, includes 6 figure
Coherence in the Quasi-Particle 'Scattering' by the Vortex Lattice in Pure Type-II Superconductors
The effect of quasi-particle (QP) 'scattering' by the vortex lattice on the
de-Haas van-Alphen oscillations in a pure type-II superconductor is
investigated within mean field,asymptotic perturbation theory. Using a 2D
electron gas model it is shown that, due to a strict phase coherence in the
many-particle correlation functions, the 'scattering' effect in the asymptotic
limit () is much weaker than what is predicted
by the random vortex lattice model proposed by Maki and Stephen, which destroys
this coherence . The coherent many particle configuration is a collinear array
of many particle coordinates, localized within a spatial region with size of
the order of the magnetic length. The amplitude of the magnetization
oscillations is sharply damped just below because of strong
out of phase magnetic oscillations in the superconducting
condensation energy ,which tend to cancel the normal electron oscillations.
Within the ideal 2D model used it is found, however, that because of the
relative smallness of the quartic and higher order terms in the expansion, the
oscillations amplitude at lower fields does not really damp to zero, but only
reverses sign and remains virtually undamped well below . This
conclusion may be changed if disorder in the vortex lattice, or vortex lines
motion will be taken into account. The reduced QP 'scattering' effect may be
responsible for the apparent crossover from a strong damping of the dHvA
oscillations just below to a weaker damping at lower fields observed
experimentally in several 3D superconductors.Comment: 26 pages, Revtex no Figure
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR
Magnetoresistivity in MgB2 as a probe of disorder in p- and s-bands
In this paper we present normal state magnetoresistivity data of magnesium
diboride epitaxial thin films with different levels of disorder, measured at
42K in magnetic fields up to 45 Tesla. Disorder was introduced in a controlled
way either by means of neutron irradiation or by carbon doping. From a
quantitative analysis of the magnetoresistivity curves with the magnetic field
either parallel or perpendicular to the plane of the film, we extract the ratio
of the scattering times in p- and s-bands. We demonstrate that the undoped
unirradiated thin film has p scattering times smaller than s ones; upon
irradiation, both bands become increasingly more disordered; eventually the
highly irradiated sample (neutron fluence 7.7X1017 cm-2) and the C-doped sample
have comparable scattering times in the two types of bands. This description of
the effect of disorder in the two kinds of bands on transport is consistent
with the residual resistivity values and with the temperature dependence of the
resistivity.Comment: 19 pages, 3 tables, 2 figure
Quantum Point Contacts and Coherent Electron Focusing
I. Introduction
II. Electrons at the Fermi level
III. Conductance quantization of a quantum point contact
IV. Optical analogue of the conductance quantization
V. Classical electron focusing
VI. Electron focusing as a transmission problem
VII. Coherent electron focusing (Experiment, Skipping orbits and magnetic
edge states, Mode-interference and coherent electron focusing)
VIII. Other mode-interference phenomenaComment: #3 of a series of 4 legacy reviews on QPC'
Quantum oscillations and the Fermi surface in an underdoped high-Tc superconductor
Despite twenty years of research, the phase diagram of high transition-
temperature superconductors remains enigmatic. A central issue is the origin of
the differences in the physical properties of these copper oxides doped to
opposite sides of the superconducting region. In the overdoped regime, the
material behaves as a reasonably conventional metal, with a large Fermi
surface. The underdoped regime, however, is highly anomalous and appears to
have no coherent Fermi surface, but only disconnected "Fermi arcs". The
fundamental question, then, is whether underdoped copper oxides have a Fermi
surface, and if so, whether it is topologically different from that seen in the
overdoped regime. Here we report the observation of quantum oscillations in the
electrical resistance of the oxygen-ordered copper oxide YBa2Cu3O6.5,
establishing the existence of a well-defined Fermi surface in the ground state
of underdoped copper oxides, once superconductivity is suppressed by a magnetic
field. The low oscillation frequency reveals a Fermi surface made of small
pockets, in contrast to the large cylinder characteristic of the overdoped
regime. Two possible interpretations are discussed: either a small pocket is
part of the band structure specific to YBa2Cu3O6.5 or small pockets arise from
a topological change at a critical point in the phase diagram. Our
understanding of high-transition temperature (high-Tc) superconductors will
depend critically on which of these two interpretations proves to be correct
A Review on the Mechanical Modeling of Composite Manufacturing Processes
© 2016, The Author(s). The increased usage of fiber reinforced polymer composites in load bearing applications requires a detailed understanding of the process induced residual stresses and their effect on the shape distortions. This is utmost necessary in order to have more reliable composite manufacturing since the residual stresses alter the internal stress level of the composite part during the service life and the residual shape distortions may lead to not meeting the desired geometrical tolerances. The occurrence of residual stresses during the manufacturing process inherently contains diverse interactions between the involved physical phenomena mainly related to material flow, heat transfer and polymerization or crystallization. Development of numerical process models is required for virtual design and optimization of the composite manufacturing process which avoids the expensive trial-and-error based approaches. The process models as well as applications focusing on the prediction of residual stresses and shape distortions taking place in composite manufacturing are discussed in this study. The applications on both thermoset and thermoplastic based composites are reviewed in detail