8,456 research outputs found
Mapping the dynamic interactions between vortex species in highly anisotropic superconductors
Here we use highly sensitive magnetisation measurements performed using a
Hall probe sensor on single crystals of highly anisotropic high temperature
superconductors to study the dynamic interactions
between the two species of vortices that exist in such superconductors. We
observe a remarkable and clearly delineated high temperature regime that
mirrors the underlying vortex phase diagram. Our results map out the parameter
space over which these dynamic interaction processes can be used to create
vortex ratchets, pumps and other fluxonic devices.Comment: 7 pages, 3 figures, to be published in Supercond. Sci. Techno
On the robustness of ultra-high voltage 4H-SiC IGBTs with an optimized retrograde p-well
The robustness of ultra-high voltage (>10kV) SiC IGBTs comprising of an optimized retrograde p-well is investigated. Under extensive TCAD simulations, we show that in addition to offering a robust control on threshold voltage and eliminating punch-through, the retrograde is highly effective in terms of reducing the stress on the gate oxide of ultra-high voltage SiC IGBTs. We show that a 10 kV SiC IGBT comprising of the retrograde p-well exhibits a much-reduced peak electric field in the gate oxide when compared with the counterpart comprising of a conventional p-well. Using an optimized retrograde p-well with depth as shallow as 1 μm, the peak electric field in the gate oxide of a 10kV rated SiC IGBT can be reduced to below 2 MV.cm -1 , a prerequisite to achieve a high-degree of reliability in high-voltage power devices. We therefore propose that the retrograde p-well is highly promising for the development of>10kV SiC IGBTs
Effective Vortex Pinning in MgB2 thin films
We discuss pinning properties of MgB2 thin films grown by pulsed-laser
deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are
identified that contribute most effectively to the pinning of vortices in
randomly oriented films. The EB process produces low defected crystallites with
small grain size providing enhanced pinning at grain boundaries without
degradation of Tc. The PLD process produces films with structural disorder on a
scale less that the coherence length that further improves pinning, but also
depresses Tc
Critical Fields and Critical Currents in MgB2
We review recent measurements of upper (Hc2) and lower (Hc1) critical fields
in clean single crystals of MgB2, and their anisotropies between the two
principal crystallographic directions. Such crystals are far into the "clean
limit" of Type II superconductivity, and indeed for fields applied in the
c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough
for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the
c-direction, MgB2 has to be modified for it to become useful for high-current
applications. It should be possible to increase Hc2 by the introduction of
strong electron scattering (but because of the electronic structure and the
double gap that results, the scatterers will have to be chosen carefully). In
addition, pinning defects on a scale of a few nm will have to be engineered in
order to enhance the critical current density at high fields.Comment: BOROMAG Conference Invited paper. To appear in Supercond. Sci. Tec
A DC magnetic metamaterial
Electromagnetic metamaterials are a class of materials which have been
artificially structured on a subwavelength scale. They are currently the focus
of a great deal of interest because they allow access to previously
unrealisable properties like a negative refractive index. Most metamaterial
designs have so far been based on resonant elements, like split rings, and
research has concentrated on microwave frequencies and above. In this work, we
present the first experimental realisation of a non-resonant metamaterial
designed to operate at zero frequency. Our samples are based on a
recently-proposed template for an anisotropic magnetic metamaterial consisting
of an array of superconducting plates. Magnetometry experiments show a strong,
adjustable diamagnetic response when a field is applied perpendicular to the
plates. We have calculated the corresponding effective permeability, which
agrees well with theoretical predictions. Applications for this metamaterial
may include non-intrusive screening of weak DC magnetic fields.Comment: 6 pages, 3 figure
First order magnetic transition in CeFe alloys: Phase-coexistence and metastability
First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in
doped-CeFe alloys is studied with micro-Hall probe technique. Clear visual
evidence of magnetic phase-coexistence on micrometer scales and the evolution
of this phase-coexistence as a function of temperature, magnetic field and time
across the first order FM-AFM transition is presented. Such phase-coexistence
and metastability arise as natural consequence of an intrinsic
disorder-influenced first order transition. Generality of this phenomena
involving other classes of materials is discussed.Comment: 11 pages of text and 3 figure
Dynamic interactions between pancake vortex stacks and Josephson vortices in Bi2Sr2CaCu2O8 single crystals: relaxation and ratchets
We present a detailed study of the dynamic interactions between Josephson
vortices and stacks of pancake vortices in a Bi2Sr2CaCu2O8 single crystal,
obtained by measuring the effect of applied inplane magnetic field pulses on
the c-axis magnetisation. The predominant interaction is to relax the system
towards equilibrium. However, using a highly sensitive AC technique we are able
to measure also the forces acting to drive the system away from equilibrium,
consistent with the existence of dragging interactions between the Josephson
and pancake systems. Such forces were discussed recently as the basis of
possible flux ratchet devices.Comment: 4 pages, 4 figure
Vortices Clustering: The Origin of the Second Peak in the Magnetisation Loops of High Temperature Superconductors
We study vortex clustering in type II Superconductors. We demonstrate that
the ``second peak'' observed in magnetisation loops may be a dynamical effect
associated with a density driven instability of the vortex system. At the
microscopic level the instability shows up as the clustering of individual
vortices at (rare) preferential regions of the pinning potential. In the limit
of quasi-static ramping the instability is related to a phase transition in the
equilibrium vortex system.Comment: 11 pages + 3 figure
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