68,953 research outputs found
Pulsars identified from the NRAO VLA Sky Survey
We identified 97 strong pulsars from the NRAO VLA Sky Survey (NVSS) at 1.4
GHz north of Dec(J2000) >-40\degr. The total flux density, linear
polarization intensity and polarization angle (PA) of all pulsars are extracted
from the NVSS catalog. The well-calibrated PA measurement of 5 pulsars can be
used for absolute PA calibrations in other observations. Comparing the source
positions with those in the pulsar catalog, we got the first measurement of the
proper motion upper limit of PSR B0031-07, which is and .Comment: 7 pages; 3 figures; one big table; To appear in A&A Supplement
Unusual persistence of superconductivity against high magnetic fields in the strongly-correlated iron-chalcogenide film FeTe:O
We report an unusual persistence of superconductivity against high magnetic
fields in the iron chalcogenide film FeTe:O below ~ 2.5 K. Instead of
saturating like a mean-field behavior with a single order parameter, the
measured low-temperature upper critical field increases progressively,
suggesting a large supply of superconducting states accessible via magnetic
field or low-energy thermal fluctuations. We demonstrate that superconducting
states of finite momenta can be realized within the conventional theory,
despite its questionable applicability. Our findings reveal a fundamental
characteristic of superconductivity and electronic structure in the
strongly-correlated iron-based superconductors.Comment: 10 pages, 3 figure
Interplay between spin density wave and phase shifted superconductivity in the Fe pnictide superconductors
We explore if the phase separation or coexistence of the spin density wave
(SDW) and superconductivity (SC) states has any relation to the
incommensurability of the SDW in the Fe pnictide superconductors. A systematic
method of determining the phase separation or coexistence was employed by
computing the anisotropy coefficient from the the 4th order terms of
the Ginzburg--Landau (GL) expansion of the free energy close to the
tricritical/tetracritical point. It was complemented by the self-consistent
numerical iterations of the gap equations to map out the boundaries between the
phase separation and coexistence of the SDW and SC phases, and between
commensurate (C) and incommensurate (IC) SDW in the temperature--doping plane.
Our principal results for the sign reversed -wave pairing SC, in terms of
the multicritical temperature, , the phase separation/coexistence boundary
between the SDW and SC, , and the boundary between C/IC SDW, , are:
(a) IC-SDW and SC coexist for and phase separate otherwise, (b) SDW
takes the C form for and IC form for , and (c) the
thermodynamic first order phase transition intervenes in between the C-SDW and
IC-SDW boundary for large , where is the SDW transition
temperature at zero doping, and . The
intervention makes the phase diagram more complicated than previously reported.
By contrast no coexistence was found for the equal sign pairing SC. These
results will be compared with the experimental reports in the Fe pnictide
superconductors.Comment: 9 pages, 4 figures, Submitted to Phys.Rev.
Experimental study of contact transition control incorporating joint acceleration feedback
Joint acceleration and velocity feedbacks are incorporated into a classical internal force control of a robot in contact with the environment. This is intended to achieve a robust contact transition and force tracking performance for varying unknown environments, without any need of adjusting the controller parameters, A unified control structure is proposed for free motion, contact transition, and constrained motion in view of the consumption of the initial kinetic energy generated by a nonzero impact velocity. The influence of the velocity and acceleration feedbacks, which are introduced especially for suppressing the transition oscillation, on the postcontact tracking performance is discussed. Extensive experiments are conducted on the third joint of a three-link direct-drive robot to verify the proposed scheme for environments of various stiffnesses, including elastic (sponge), less elastic (cardboard), and hard (steel plate) surfaces. Results are compared with those obtained by the transition control scheme without the acceleration feedback. The ability of the proposed control scheme in resisting the force disturbance during the postcontact period is also experimentally investigated
Asymptotics of Spinfoam Amplitude on Simplicial Manifold: Lorentzian Theory
The present paper studies the large-j asymptotics of the Lorentzian EPRL
spinfoam amplitude on a 4d simplicial complex with an arbitrary number of
simplices. The asymptotics of the spinfoam amplitude is determined by the
critical configurations. Here we show that, given a critical configuration in
general, there exists a partition of the simplicial complex into three type of
regions R_{Nondeg}, R_{Deg-A}, R_{Deg-B}, where the three regions are
simplicial sub-complexes with boundaries. The critical configuration implies
different types of geometries in different types of regions, i.e. (1) the
critical configuration restricted into R_{Nondeg} is degenerate of type-A in our definition of degeneracy, but implies
a nondegenerate discrete Euclidean geometry on R_{Deg-A}, (3) the critical
configuration restricted into R_{Deg-B} is degenerate of type-B, and implies a
vector geometry on R_{Deg-B}. With the critical configuration, we further make
a subdivision of the regions R_{Nondeg} and R_{Deg-A} into sub-complexes (with
boundary) according to their Lorentzian/Euclidean oriented 4-simplex volume
V_4(v), such that sgn(V_4(v)) is a constant sign on each sub-complex. Then in
the each sub-complex, the spinfoam amplitude at the critical configuration
gives the Regge action in Lorentzian or Euclidean signature respectively on
R_{Nondeg} or R_{Deg-A}. The Regge action reproduced here contains a sign
factor sgn(V_4(v)) of the oriented 4-simplex volume. Therefore the Regge action
reproduced here can be viewed a discretized Palatini action with on-shell
connection. Finally the asymptotic formula of the spinfoam amplitude is given
by a sum of the amplitudes evaluated at all possible critical configurations,
which are the products of the amplitudes associated to different type of
geometries.Comment: 54 pages, 2 figures, reference adde
Commuting Simplicity and Closure Constraints for 4D Spin Foam Models
Spin Foam Models are supposed to be discretised path integrals for quantum
gravity constructed from the Plebanski-Holst action. The reason for there being
several models currently under consideration is that no consensus has been
reached for how to implement the simplicity constraints. Indeed, none of these
models strictly follows from the original path integral with commuting B
fields, rather, by some non standard manipulations one always ends up with non
commuting B fields and the simplicity constraints become in fact anomalous
which is the source for there being several inequivalent strategies to
circumvent the associated problems. In this article, we construct a new
Euclidian Spin Foam Model which is constructed by standard methods from the
Plebanski-Holst path integral with commuting B fields discretised on a 4D
simplicial complex. The resulting model differs from the current ones in
several aspects, one of them being that the closure constraint needs special
care. Only when dropping the closure constraint by hand and only in the large
spin limit can the vertex amplitudes of this model be related to those of the
FK Model but even then the face and edge amplitude differ. Curiously, an ad hoc
non-commutative deformation of the variables leads from our new model
to the Barrett-Crane Model in the case of Barbero-Immirzi parameter goes to
infinity.Comment: 41 pages, 4 figure
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