2,704 research outputs found
Solvable Examples of Drift and Diffusion of Ions in Non-uniform Electric Fields
The drift and diffusion of a cloud of ions in a fluid are distorted by an
inhomogeneous electric field. If the electric field carries the center of the
distribution in a straight line and the field configuration is suitably
symmetric, the distortion can be calculated analytically. We examine the
specific examples of fields with cylindrical and spherical symmetry in detail
assuming the ion distributions to be of a generally Gaussian form. The effects
of differing diffusion coefficients in the transverse and longitudinal
directions are included
Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2
The recent discovery that superconductivity occurs in several clean itinerant
ferromagnets close to low temperature magnetic instabilities naturally invites
an interpretation based on a proximity to quantum criticality. Here we report
measurements of the pressure dependence of the low temperature magnetisation in
one of these materials, UGe_2. Our results show that both of the magnetic
transitions observed in this material as a function of pressure are first order
transitions and do not therefore correspond to quantum critical points. Further
we find that the known pressure dependence of the superconducting transition is
not reflected in the pressure dependence of the static susceptibility. This
demonstrates that the spectrum of excitations giving superconductivity is not
that normally associated with a proximity to quantum criticality in weak
itinerant ferromagnets. In contrast our data suggest that instead the pairing
spectrum might be related to a sharp spike in the electronic density of states
that also drives one of the magnetic transitions.Comment: to appear in Phys. Rev. Let
A scanning drift tube apparatus for spatio-temporal mapping of electron swarms
A "scanning" drift tube apparatus, capable of mapping of the spatio-temporal
evolution of electron swarms, developing between two plane electrodes under the
effect of a homogeneous electric field, is presented. The electron swarms are
initiated by photoelectron pulses and the temporal distributions of the
electron flux are recorded while the electrode gap length (at a fixed electric
field strength) is varied. Operation of the system is tested and verified with
argon gas, the measured data are used for the evaluation of the electron bulk
drift velocity. The experimental results for the space-time maps of the
electron swarms - presented here for the first time - also allow clear
observation of deviations from hydrodynamic transport. The swarm maps are also
reproduced by particle simulations
Microscopic theories for cubic and tetrahedral superconductors: application to PrOs_4Sb_{12}
We examine weak-coupling theory for unconventional superconducting states of
cubic or tetrahedral symmetry for arbitrary order parameters and Fermi surfaces
and identify the stable states in zero applied field. We further examine the
possibility of having multiple superconducting transitions arising from the
weak breaking of a higher symmetry group to cubic or tetrahedral symmetry.
Specifically, we consider two higher symmetry groups. The first is a weak
crystal field theory in which the spin-singlet Cooper pairs have an approximate
spherical symmetry. The second is a weak spin orbit coupling theory for which
spin-triplet Cooper pairs have a cubic orbital symmetry and an approximate
spherical spin rotational symmetry. In hexagonal UPt_3, these theories easily
give rise to multiple transitions. However, we find that for cubic materials,
there is only one case in which two superconducting transitions occur within
weak coupling theory. This sequence of transitions does not agree with the
observed properties of PrOs_4Sb_{12}. Consequently, we find that to explain two
transitions in PrOs_4Sb_{12} using approximate higher symmetry groups requires
a strong coupling theory. In view of this, we finally consider a weak coupling
theory for which two singlet representations have accidentally nearly
degenerate transition temperatures (not due to any approximate symmetries). We
provide an example of such a theory that agrees with the observed properties of
PrOs_4Sb_{12}.Comment: 11 pages,1 figur
An Experimental Investigation of the Scaling of Columnar Joints
Columnar jointing is a fracture pattern common in igneous rocks in which
cracks self-organize into a roughly hexagonal arrangement, leaving behind an
ordered colonnade. We report observations of columnar jointing in a laboratory
analog system, desiccated corn starch slurries. Using measurements of moisture
density, evaporation rates, and fracture advance rates as evidence, we suggest
an advective-diffusive system is responsible for the rough scaling behavior of
columnar joints. This theory explains the order of magnitude difference in
scales between jointing in lavas and in starches. We investigated the scaling
of average columnar cross-sectional areas due to the evaporation rate, the
analog of the cooling rate of igneous columnar joints. We measured column areas
in experiments where the evaporation rate depended on lamp height and time, in
experiments where the evaporation rate was fixed using feedback methods, and in
experiments where gelatin was added to vary the rheology of the starch. Our
results suggest that the column area at a particular depth is related to both
the current conditions, and hysteretically to the geometry of the pattern at
previous depths. We argue that there exists a range of stable column scales
allowed for any particular evaporation rate.Comment: 12 pages, 11 figures, for supporting online movies, go to
http://www.physics.utoronto.ca/nonlinear/movies/starch_movies.htm
Force Dependence of the Michaelis Constant in a Two-State Ratchet Model for Molecular Motors
We present a quantitative analysis of recent data on the kinetics of ATP
hydrolysis, which has presented a puzzle regarding the load dependence of the
Michaelis constant. Within the framework of coarse grained two-state ratchet
models, our analysis not only explains the puzzling data, but provides a
modified Michaelis law, which could be useful as a guide for future
experiments.Comment: 4 pages, 3 eps figures, accepted for publication on Physical Review
Letter
The Magnetic Phase Diagram and the Pressure and Field Dependence of the Fermi Surface in UGe
The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe are
measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B}
parallel to the {\it a} axis, which is the easy axis of magnetization. Two
anomalies are observed at {\it B}({\it P}) and {\it B}({\it P}) ({\it
B} {\it B} at any {\it P}), and the {\it P}-{\it B} phase diagram
is presented. The Fermi surface and quasiparticle mass are found to vary
smoothly with pressure up to 17.7 kbar unless the phase boundary {\it
B}({\it P}) is crossed. The observed dHvA frequencies may be grouped into
three according to their pressure dependences, which are largely positive,
nearly constant or negative. It is suggested that the quasiparticle mass
moderately increases as the boundary {\it B}({\it P}) is approached. DHvA
effect measurements are also performed across the boundary at 16.8 kbar.Comment: to be published in Phys. Rev.
Nodal domains of Maass forms I
This paper deals with some questions that have received a lot of attention
since they were raised by Hejhal and Rackner in their 1992 numerical
computations of Maass forms. We establish sharp upper and lower bounds for the
-restrictions of these forms to certain curves on the modular surface.
These results, together with the Lindelof Hypothesis and known subconvex
-bounds are applied to prove that locally the number of nodal domains
of such a form goes to infinity with its eigenvalue.Comment: To appear in GAF
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