10,814 research outputs found
Probabilistic Super Dense Coding
We explore the possibility of performing super dense coding with
non-maximally entangled states as a resource. Using this we find that one can
send two classical bits in a probabilistic manner by sending a qubit. We
generalize our scheme to higher dimensions and show that one can communicate
2log_2 d classical bits by sending a d-dimensional quantum state with a certain
probability of success. The success probability in super dense coding is
related to the success probability of distinguishing non-orthogonal states. The
optimal average success probabilities are explicitly calculated. We consider
the possibility of sending 2 log_2 d classical bits with a shared resource of a
higher dimensional entangled state (D X D, D > d). It is found that more
entanglement does not necessarily lead to higher success probability. This also
answers the question as to why we need log_2 d ebits to send 2 log_2 d
classical bits in a deterministic fashion.Comment: Latex file, no figures, 11 pages, Discussion changed in Section
Superconductivity in Boron under pressure - why are the measured T's so low?
Using the full potential linear muffin-tin orbitals (FP-LMTO) method we
examine the pressure-dependence of superconductivity in the two metallic phases
of Boron: bct and fcc. Linear response calculations are carried out to examine
the phonon frequencies and electron-phonon coupling for various lattice
parameters, and superconducting transition temperatures are obtained from the
Eliashberg equation. In both bct and fcc phases the superconducting transition
temperature T is found to decrease with increasing pressure, due to
stiffening of phonons with an accompanying decrease in electron-phonon
coupling. This is in contrast to a recent report, where T is found to
increase with pressure. Even more drastic is the difference between the
measured T, in the range 4-11 K, and the calculated values for both bct and
fcc phases, in the range 60-100 K. The calculation reveals that the transition
from the fcc to bct phase, as a result of increasing volume or decreasing
pressure, is caused by the softening of the X-point transverse phonons. This
phonon softening also causes large electron-phonon coupling for high volumes in
the fcc phase, resulting in coupling constants in excess of 2.5 and T
nearing 100 K. We discuss possible causes as to why the experiment might have
revealed T's much lower than what is suggested by the present study. The
main assertion of this paper is that the possibility of high T, in excess
of 50 K, in high pressure pure metallic phases of boron cannot be ruled out,
thus substantiating the need for further experimental investigations of the
superconducting properties of high pressure pure phases of boron.Comment: 16 pages, 8 figures, 1 Tabl
Spin systems with dimerized ground states
In view of the numerous examples in the literature it is attempted to outline
a theory of Heisenberg spin systems possessing dimerized ground states (``DGS
systems") which comprises all known examples. Whereas classical DGS systems can
be completely characterized, it was only possible to provide necessary or
sufficient conditions for the quantum case. First, for all DGS systems the
interaction between the dimers must be balanced in a certain sense. Moreover,
one can identify four special classes of DGS systems: (i) Uniform pyramids,
(ii) systems close to isolated dimer systems, (iii) classical DGS systems, and
(iv), in the case of , systems of two dimers satisfying four
inequalities. Geometrically, the set of all DGS systems may be visualized as a
convex cone in the linear space of all exchange constants. Hence one can
generate new examples of DGS systems by positive linear combinations of
examples from the above four classes.Comment: With corrections of proposition 4 and other minor change
First principles theoretical studies of half-metallic ferromagnetism in CrTe
Using full-potential linear augmented plane wave method (FP-LAPW) and the
density functional theory, we have carried out a systematic investigation of
the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in
three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type
(NA) hexagonal. Although the ground state is of NA structure, RS and ZB are
interesting in that these fcc-based structures, which can possibly be grown on
many semiconductor substrates, exhibit half-metallic phases above some critical
values of the lattice parameter. We find that the NA structure is not
half-metallic at its equilibrium volume, while both ZB and RS structures are.
The RS structure is more stable than the ZB, with an energy that is lower by
0.25 eV/atom. While confirming previous results on the half-metallic phase in
ZB structure, we provide hitherto unreported results on the half-metallic RS
phase, with a gap in the minority channel and a magnetic moment of 4.0
per formula unit. A comparison of total energies for the
ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM)
configurations shows the lowest energy configuration to be FM for CrTe in all
the three structures. The FP-LAPW calculations are supplemented by linear
muffin-tin orbital (LMTO) calculations using both local density approximation
(LDA) and LDA+U method. The exchange interactions and the Curie temperatures
calculated via the linear response method in ZB and RS CrTe are compared over a
wide range of the lattice parameter. The calculated Curie temperatures for the
RS phase are consistently higher than those for the ZB phase.Comment: 11 pages, 14 figure
Specifications of different artisanal and mechanised fishing craft employed in marine fisheries along Tamil Nadu coast
During the period 1980-'89 Tamil Nadu
contributed an estimated annual average of 2.6
lakh tonnes of marine fish forming 16% of the
total all India fish production. Apart from the
traditional craft and gear operated along the
inshore region, expansion of the mechanised
vessels fleet especially trawlers has significantly
contributed in exploiting the productive ground
fish resources along the coast
Equally-distant partially-entangled alphabet states for quantum channels
Each Bell state has the property that by performing just local operations on
one qubit, the complete Bell basis can be generated. That is, states generated
by local operations are totally distinguishable. This remarkable property is
due to maximal quantum entanglement between the two particles. We present a set
of local unitary transformations that generate out of partially entangled
two-qubit state a set of four maximally distinguishable states that are
mutually equally distant. We discuss quantum dense coding based on these
alphabet states.Comment: 7 revtex pages, 2 eps figures, to appear in Phys. Rev. A 62, 1
November (2000
Case Study on a New Alternate Charge Material for Electric Arc Furnace
Steel production through the electric arc furnace contributes nearly 30% of the total world steel production. To-day, this process is well established as an energy efficient economic process. The likely share of steel production through the electric are furnace route is projected to go up to around 40% of the world steel production, by the turn of this century. Till the seventies, steel scrap has traditionally been used
as a main metallic source, as it has certain advantages such as low power consumption,low slag volume, less wear in the refractory lining, besides smooth furnace
operation. However, the presence of tramp elements in scrap has shortcomings in producing quality steel products. Besides the quality aspects, the phenomenal rise in steel production through the EAF and increasing adoption of continuous casting have had a great impact on the availability and cost of steel scrap
Size induced metal insulator transition in nanostructured Niobium thin films: Intragranular and intergranular contributions
With a reduction in the average grain size in nanostructured films of
elemental Nb, we observe a systematic crossover from metallic to
weakly-insulating behavior. An analysis of the temperature dependence of the
resistivity in the insulating phase clearly indicates the existence of two
distinct activation energies corresponding to inter-granular and intra-granular
mechanisms of transport. While the high temperature behavior is dominated by
grain boundary scattering of the conduction electrons, the effect of
discretization of energy levels due to quantum confinement shows up at low
temperatures. We show that the energy barrier at the grain boundary is
proportional to the width of the largely disordered inter-granular region,
which increases with a decrease in the grain size. For a metal-insulator
transition to occur in nano-Nb due to the opening up of an energy gap at the
grain boundary, the critical grain size is ~ 8nm and the corresponding grain
boundary width is ~ 1.1nm
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