57,144 research outputs found
Codes with Locality for Two Erasures
In this paper, we study codes with locality that can recover from two
erasures via a sequence of two local, parity-check computations. By a local
parity-check computation, we mean recovery via a single parity-check equation
associated to small Hamming weight. Earlier approaches considered recovery in
parallel; the sequential approach allows us to potentially construct codes with
improved minimum distance. These codes, which we refer to as locally
2-reconstructible codes, are a natural generalization along one direction, of
codes with all-symbol locality introduced by Gopalan \textit{et al}, in which
recovery from a single erasure is considered. By studying the Generalized
Hamming Weights of the dual code, we derive upper bounds on the minimum
distance of locally 2-reconstructible codes and provide constructions for a
family of codes based on Tur\'an graphs, that are optimal with respect to this
bound. The minimum distance bound derived here is universal in the sense that
no code which permits all-symbol local recovery from erasures can have
larger minimum distance regardless of approach adopted. Our approach also leads
to a new bound on the minimum distance of codes with all-symbol locality for
the single-erasure case.Comment: 14 pages, 3 figures, Updated for improved readabilit
Approximation of Entropy Numbers
The purpose of this article is to develop a technique to estimate certain
bounds for entropy numbers of diagonal operator on spaces of p-summable
sequences for finite p greater than 1. The approximation method we develop in
this direction works for a very general class of operators between Banach
spaces, in particular reflexive spaces. As a consequence of this technique we
also obtain that the entropy number of a bounded linear operator T between two
separable Hilbert spaces is equal to the entropy number of the adjoint of T.
This gives a complete answer to the question posed by B. Carl [4] in the
setting of separable Hilbert spaces.Comment: 10 page
Synthesis and superconductivity of new BiS2 based superconductor PrO0.5F0.5BiS2
We report synthesis and superconductivity at 3.7K in PrO0.5F0.5BiS2. The
newly discovered material belongs to the layered sulfide based REO0.5F0.5BiS2
compounds having ZrCuSiAs type structure. The bulk polycrystalline compound is
synthesized by vacuum encapsulation technique at 7800C in single step. Detailed
structural analysis has shown that the as synthesized PrO0.5F0.5BiS2 is
crystallized in tetragonal P4/nmm space group with lattice parameters a =
4.015(5) {\AA}, c = 13.362(4) {\AA}. Bulk superconductivity is observed in
PrO0.5F0.5BiS2 below 4K from magnetic and transport measurements. Electrical
transport measurements showed superconducting transition temperature (Tc) onset
at 3.7K and Tc ({\rho}=0) at 3.1K. Hump at Tc related to superconducting
transition is not observed in heat capacity measurement and rather a
Schottky-type anomaly is observed at below ~6K. The compound is slightly
semiconducting in normal state. Isothermal magnetization (MH) exhibited typical
type II behavior with lower critical field (Hc1) of around 8Oe.Comment: Short note 10 pages text+figs. First report on PrO.5F.5BiS2 Su
Gravity and Large Extra Dimensions
The idea that quantum gravity can be realized at the TeV scale is extremely
attractive to theorists and experimentalists alike. This proposal leads to
extra spacial dimensions large compared to the electroweak scale. Here we give
a very systematic view of the foundations of the theories with large extra
dimensions and their physical consequences.Comment: 26 pages, 3 diagram
Superconductivity at 5K in NdO0.5F0.5BiS2
We report appearance of superconductivity at 5K in NdO0.5F0.5BiS2 and
supplement the discovery [1] of the same in layered sulfide based ZrCuSiAs type
compounds. The bulk polycrystalline compound is synthesized by conventional
solid state route via vacuum encapsulation technique. Detailed structural
analysis showed that the studied compound is crystallized in tetragonal P4/nmm
space group with lattice parameters a = 3.9911(3) {\AA}, c = 13.3830(2) {\AA}.
Bulk superconductivity is established in NdO0.5F0.5BiS2 at 5K by both transport
and magnetic measurements. Electrical transport measurements showed
superconducting Tc onset at 5.2K and Tc ({\rho}=0) at 4.7K. Under applied
magnetic field both Tc onset and Tc ({\rho} =0) decrease to lower temperatures
and an upper critical field [Hc2(0)] of above 23kOe is estimated. Both AC and
DC magnetic susceptibility measurements showed bulk superconductivity below 5K.
Isothermal magnetization (MH) exhibited typical type II behavior with lower
critical field (Hc1) of around 15Oe. Isothermal magnetization (MH) exhibited
typical type-II behavior with lower critical field (Hc1) of around 15Oe.
Specific heat [Cp(T)] is investigated in the temperature range of 1.9-50K in
zero external magnetic field. A Schottky-type anomaly is observed at low
temperature below 7K. This low temperature Schottky can be attributed to the
change in the entropy of the system.Comment: 10 pages text + Figs (New Version):comments/suggestion welcome
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