941 research outputs found
Singularities of equidistants and global centre symmetry sets of Lagrangian submanifolds
We define the Global Centre Symmetry set (GCS) of a smooth closed
m-dimensional submanifold M of R^n, , which is an affinely invariant
generalization of the centre of a k-sphere in R^{k+1}. The GCS includes both
the centre symmetry set defined by Janeczko and the Wigner caustic defined by
Berry. We develop a new method for studying generic singularities of the GCS
which is suited to the case when M is lagrangian in R^{2m} with canonical
symplectic form. The definition of the GCS, which slightly generalizes one by
Giblin and Zakalyukin, is based on the notion of affine equidistants, so, we
first study singularities of affine equidistants of Lagrangian submanifolds,
classifying all the stable ones. Then, we classify the affine-Lagrangian stable
singularities of the GCS of Lagrangian submanifolds and show that, already for
smooth closed convex curves in R^2, many singularities of the GCS which are
affine stable are not affine-Lagrangian stable.Comment: 26 pages, 2 figure
Observation of a superconducting glass state in granular superconducting diamond
The magnetic field dependence of the superconductivity in nanocrystalline
boron doped diamond thin films is reported. Evidence of a glass state in the
phase diagram is presented, as demonstrated by electrical resistance and
magnetic relaxation measurements. The position of the phase boundary in the H-T
plane is determined from resistance data by detailed fitting to
zero-dimensional fluctuation conductivity theory. This allows determination of
the boundary between resistive and non-resistive behavior to be made with
greater precision than the standard ad hoc onset/midpoint/offset criterion
Fluctuation spectroscopy as a probe of granular superconducting diamond films
We present resistance versus temperature data for a series of boron-doped
nanocrystalline diamond films whose grain size is varied by changing the film
thickness. Upon extracting the fluctuation conductivity near to the critical
temperature we observe three distinct scaling regions -- 3D intragrain,
quasi-0D, and 3D intergrain -- in confirmation of the prediction of Lerner,
Varlamov and Vinokur. The location of the dimensional crossovers between these
scaling regions allows us to determine the tunnelling energy and the Thouless
energy for each film. This is a demonstration of the use of \emph{fluctuation
spectroscopy} to determine the properties of a superconducting granular system
Dynamic behavior of magnetic avalanches in the spin-ice compound DyTiO
Avalanches of the magnetization, that is to say an abrupt reversal of the
magnetization at a given field, have been previously reported in the spin-ice
compound DyTiO. This out-of-equilibrium process, induced by
magneto-thermal heating, is quite usual in low temperature magnetization
studies. A key point is to determine the physical origin of the avalanche
process. In particular, in spin-ice compounds, the origin of the avalanches
might be related to the monopole physics inherent to the system. We have
performed a detailed study of the avalanche phenomena in three single crystals,
with the field oriented along the [111] direction, perpendicular to [111] and
along the [100] directions. We have measured the changing magnetization during
the avalanches and conclude that avalanches in spin ice are quite slow compared
to the avalanches reported in other systems such as molecular magnets. Our
measurements show that the avalanches trigger after a delay of about 500 ms and
that the reversal of the magnetization then occurs in a few hundreds of
milliseconds. These features suggest an unusual propagation of the reversal,
which might be due to the monopole motion. The avalanche fields seem to be
reproducible in a given direction for different samples, but they strongly
depend on the initial state of magnetization and on how the initial state was
achieved.Comment: 11 pages, 14 figure
Magnetic order and the electronic ground state in the pyrochlore iridate Nd2Ir2O7
We report a combined muon spin relaxation/rotation, bulk magnetization,
neutron scattering, and transport study of the electronic properties of the
pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic
behavior in the temperature dependent magnetization below 120 K, and an abrupt
increase in the temperature dependent resistivity below 8 K. Zero field muon
spin relaxation measurements show that the hysteretic magnetization is driven
by a transition to a magnetically disordered state, and that below 8 K a
complex magnetically ordered ground state sets in, as evidenced by the onset of
heavily damped spontaneous muon precession. Our measurements point toward the
absence of a true metal-to-insulator phase transition in this material and
suggest that Nd2Ir2O7 lies either within or on the metallic side of the
boundary of the Dirac semimetal regime within its topological phase diagram.Comment: 21 pages, 7 figure
Precession of a Freely Rotating Rigid Body. Inelastic Relaxation in the Vicinity of Poles
When a solid body is freely rotating at an angular velocity ,
the ellipsoid of constant angular momentum, in the space , has poles corresponding to spinning about the minimal-inertia and
maximal-inertia axes. The first pole may be considered stable if we neglect the
inner dissipation, but becomes unstable if the dissipation is taken into
account. This happens because the bodies dissipate energy when they rotate
about any axis different from principal. In the case of an oblate symmetrical
body, the angular velocity describes a circular cone about the vector of
(conserved) angular momentum. In the course of relaxation, the angle of this
cone decreases, so that both the angular velocity and the maximal-inertia axis
of the body align along the angular momentum. The generic case of an asymmetric
body is far more involved. Even the symmetrical prolate body exhibits a
sophisticated behaviour, because an infinitesimally small deviation of the
body's shape from a rotational symmetry (i.e., a small difference between the
largest and second largest moments of inertia) yields libration: the precession
trajectory is not a circle but an ellipse. In this article we show that often
the most effective internal dissipation takes place at twice the frequency of
the body's precession. Applications to precessing asteroids, cosmic-dust
alignment, and rotating satellites are discussed.Comment: 47 pages, 1 figur
- …