688,695 research outputs found
Generalized Ricci solitons and Einstein metrics on weak -contact manifolds
We study metric structures on a smooth manifold (introduced in our recent
works and called a weak contact metric structure and a weak K-structure) which
generalize the metric contact and K-contact structures, and allow a new look at
the classical theory. First, we characterize weak K-contact manifolds among all
weak contact metric manifolds by the property well known for K-contact
manifolds, and find when a Riemannian manifold endowed with a unit Killing
vector field forms a weak K-contact structure. Second, we find sufficient
conditions for a weak K-contact manifold with parallel Ricci tensor or with a
generalized Ricci soliton structure to be an Einstein manifold.Comment: 10 page
Contact structures on principal circle bundles
We describe a necessary and sufficient condition for a principal circle
bundle over an even-dimensional manifold to carry an invariant contact
structure. As a corollary it is shown that all circle bundles over a given base
manifold carry an invariant contact structure, only provided the trivial bundle
does. In particular, all circle bundles over 4-manifolds admit invariant
contact structures. We also discuss the Bourgeois construction of contact
structures on odd-dimensional tori in this context, and we relate our results
to recent work of Massot, Niederkrueger and Wendl on weak symplectic fillings
in higher dimensions.Comment: 14 pages, 1 figure; v2: changes to exposition, Sections 5.2, 5.3 and
6 are ne
Electrostatic tailoring of magnetic interference in quantum point contact ballistic Josephson junctions
The magneto-electrostatic tailoring of the supercurrent in quantum point
contact ballistic Josephson junctions is demonstrated. An etched InAs-based
heterostructure is laterally contacted to superconducting niobium leads and the
existence of two etched side gates permits, in combination with the application
of a perpendicular magnetic field, to modify continuously the magnetic
interference pattern by depleting the weak link. For wider junctions the
supercurrent presents a Fraunhofer-like interference pattern with periodicity
h/2e whereas by shrinking electrostatically the weak link, the periodicity
evolves continuously to a monotonic decay. These devices represent novel
tunable structures that might lead to the study of the elusive Majorana
fermions.Comment: 4.5 pages, 4 color figure
Mechanical Stretching of Proteins: Calmodulin and Titin
Mechanical unfolding of several domains of calmodulin and titin is studied
using a Go-like model with a realistic contact map and Lennard-Jones contact
interactions. It is shown that this simple model captures the experimentally
observed difference between the two proteins: titin is a spring that is tough
and strong whereas calmodulin acts like a weak spring with featureless
force-displacement curves. The difference is related to the dominance of the
alpha secondary structures in the native structure of calmodulin. The tandem
arrangements of calmodulin unwind simultaneously in each domain whereas the
domains in titin unravel in a serial fashion. The sequences of contact events
during unraveling are correlated with the contact order, i.e. with the
separation between contact making amino acids along the backbone in the native
state. Temperature is found to affect stretching in a profound way.Comment: To be published in a special bio-issue of Physica A; 14 figure
The topology of Stein fillable manifolds in high dimensions II
We continue our study of contact structures on manifolds of dimension at
least five using complex surgery theory. We show that in each dimension 2q+1 >
3 there are 'maximal' almost contact manifolds to which there is a Stein
cobordism from any other (2q+1)-dimensional contact manifold. We show that the
product M x S^2 admits a weakly fillable contact structure provided M admits a
weak symplectic filling. We also study the connection between Stein fillability
and connected sums: we give examples of almost contact manifolds for which the
connected sum is Stein fillable, while the components are not.
Concerning obstructions to Stein fillings, we show that the
(8k-1)-dimensional sphere has an almost contact structure which is not Stein
fillable once k > 1. As a consequence we deduce that any highly connected
almost contact (8k-1)-manifold (with k > 1) admits an almost contact structure
which is not Stein fillable. The proofs rely on a new number-theoretic result
about Bernoulli numbers.Comment: We corrected mistakes in the proofs of Lemma 2.9 and Corollary 2.10.
This lead to an assumption being removed from the statement of Theorem 1.3.
The paper is now published in Geometry and Topology. The appendix was written
by Bernd C. Kellne
Patterned Irradiation of YBa_2Cu_3O_(7-x) Thin Films
We present a new experiment on YBa_2Cu_3O_{7-x} (YBCO) thin films using
spatially resolved heavy ion irradiation. Structures consisting of a periodic
array of strong and weak pinning channels were created with the help of metal
masks. The channels formed an angle of +/-45 Deg with respect to the symmetry
axis of the photolithographically patterned structures. Investigations of the
anisotropic transport properties of these structures were performed. We found
striking resemblance to guided vortex motion as it was observed in YBCO single
crystals containing an array of unidirected twin boundaries. The use of two
additional test bridges allowed to determine in parallel the resistivities of
the irradiated and unirradiated parts as well as the respective current-voltage
characteristics. These measurements provided the input parameters for a
numerical simulation of the potential distribution of the Hall patterning. In
contrast to the unidirected twin boundaries in our experiment both strong and
weak pinning regions are spatially extended. The interfaces between
unirradiated and irradiated regions therefore form a Bose-glass contact. The
experimentally observed magnetic field dependence of the transverse voltage
vanishes faster than expected from the numerical simulation and we interpret
this as a hydrodynamical interaction between a Bose-glass phase and a vortex
liquid.Comment: 7 pages, 8 Eps figures included. Submitted to PR
Patterned Irradiation of YBa_2Cu_3O_(7-x) Thin Films
We present a new experiment on YBa_2Cu_3O_{7-x} (YBCO) thin films using
spatially resolved heavy ion irradiation. Structures consisting of a periodic
array of strong and weak pinning channels were created with the help of metal
masks. The channels formed an angle of +/-45 Deg with respect to the symmetry
axis of the photolithographically patterned structures. Investigations of the
anisotropic transport properties of these structures were performed. We found
striking resemblance to guided vortex motion as it was observed in YBCO single
crystals containing an array of unidirected twin boundaries. The use of two
additional test bridges allowed to determine in parallel the resistivities of
the irradiated and unirradiated parts as well as the respective current-voltage
characteristics. These measurements provided the input parameters for a
numerical simulation of the potential distribution of the Hall patterning. In
contrast to the unidirected twin boundaries in our experiment both strong and
weak pinning regions are spatially extended. The interfaces between
unirradiated and irradiated regions therefore form a Bose-glass contact. The
experimentally observed magnetic field dependence of the transverse voltage
vanishes faster than expected from the numerical simulation and we interpret
this as a hydrodynamical interaction between a Bose-glass phase and a vortex
liquid.Comment: 7 pages, 8 Eps figures included. Submitted to PR
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