166,663 research outputs found
Lower bounds on concurrence and separability conditions
We obtain analytical lower bounds on the concurrence of bipartite quantum
systems in arbitrary dimensions related to the violation of separability
conditions based on local uncertainty relations and on the Bloch representation
of density matrices. We also illustrate how these results complement and
improve those recently derived [K. Chen, S. Albeverio, and S.-M. Fei, Phys.
Rev. Lett. 95, 040504 (2005)] by considering the Peres-Horodecki and the
computable cross norm or realignment criteria.Comment: 5 pages, 1 figure; minor changes, references added; final version:
minor correction in proof of lemma 1, scope of theorem 2 clarified, to appear
in PRA; mistake in proof of lemma 1 of published version corrected, results
unchange
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure
Spin torque ferromagnetic resonance with magnetic field modulation
We demonstrate a technique of broadband spin torque ferromagnetic resonance
(ST-FMR) with magnetic field modulation for measurements of spin wave
properties in magnetic nanostructures. This technique gives great improvement
in sensitivity over the conventional ST-FMR measurements, and application of
this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich
spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR
measurements with micromagnetic simulations of the spin wave spectrum allows us
to explain the character of low-frequency magnetic excitations in nanoscale
MTJs.Comment: Also see: http://faculty.sites.uci.edu/krivorotovgroup
Constructing entanglement witnesses for infinite-dimensional systems
It is shown that, every entangled state in an infinite-dimensional composite
system has a simple entanglement witness of the form with
a nonnegative number and a finite rank self-adjoint operator. We also
provide two methods of constructing entanglement witness and apply them to
obtain some entangled states that cannot be detected by the PPT criterion and
the realignment criterion.Comment: 15 page
Fermi Surface Reconstruction by Dynamic Magnetic Fluctuations
We demonstrate that nearly critical quantum magnetic fluctuations in strongly
correlated electron systems can change the Fermi surface topology and also lead
to spin charge separation (SCS) in two dimensions. To demonstrate these effects
we consider a small number of holes injected into the bilayer antiferromagnet.
The system has a quantum critical point (QCP) which separates magnetically
ordered and disordered phases. We demonstrate that in the physically
interesting regime there is a magnetically driven Lifshitz point (LP) inside
the magnetically disordered phase. At the LP the topology of the hole Fermi
surface is changed. We also demonstrate that in this regime the hole spin and
charge necessarily separate when approaching the QCP. The considered model
sheds light on generic problems concerning the physics of the cuprates.Comment: updated version, accepted to PR
Coulomb gap triptychs, effective charge, and hopping transport in periodic arrays of superconductor grains
In granular superconductors, individual grains can contain bound Cooper pairs
while the system as a whole is strongly insulating. In such cases the
conductivity is determined by electron hopping between localized states in
individual grains. Here we examine a model of hopping conductivity in such an
insulating granular superconductor, where disorder is assumed to be provided by
random charges embedded in the insulating gaps between grains. We use computer
simulations to calculate the single-electron and electron pair density of
states at different values of the superconducting gap , and we identify
"triptych" symmetries and scaling relations between them. At a particular
critical value of , one can define an effective charge that
characterizes the density of states and the hopping transport. We discuss the
implications of our results for magnetoresistance and tunneling experiments.Comment: 11 pages, 7 figure
A Note on Doubly Warped Product Contact CR-Submanifolds in trans-Sasakian Manifolds
Warped product CR-submanifolds in Kaehlerian manifolds were intensively
studied only since 2001 after the impulse given by B.Y. Chen. Immediately
after, another line of research, similar to that concerning Sasakian geometry
as the odd dimensional version of Kaehlerian geometry, was developed, namely
warped product contact CR-submanifolds in Sasakian manifolds. In this note we
proved that there exists no proper doubly warped product contact
CR-submanifolds in trans-Sasakian manifolds.Comment: 5 Latex page
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