2,406 research outputs found
Gapless Singlet modes in the Kagome strips: A study through DMRG and strong coupling analysis
Recently Azaria et al have studied strips of the Kagome-lattice in the
weak-coupling limit, where they consist of two spin-half chains on the outside
weakly coupled to an array of half-integer spins in the middle. Using a number
of mappings they have arrived at the interesting result that in this system all
spin excitations are gapped but there are gapless spinless modes. Here we study
these Kagome strips in the limit where the interchain couplings are comparable
to the coupling to the middle spins by density matrix renormalization group and
by a strong coupling analysis. In the limit when the coupling to the
middle-spin dominates, the 5-spins of the unit-cell reduce to a single S=3/2
spin, and the overall system has well known gapless spin excitations. We study
the phase transition from this phase to the weak-coupling phase. We also carry
out a strong coupling analysis away from the S=3/2 limit, where the five-spin
blocks have four degenerate ground states with S=1/2, which can be thought of
as two spin and two pseudospin degrees of freedom. A numerical study of this
strong coupling model also suggests a finite spin-gap.Comment: 4 pages, 4 PS figure
Fluctuation, time-correlation function and geometric Phase
We establish a fluctuation-correlation theorem by relating the quantum
fluctuations in the generator of the parameter change to the time integral of
the quantum correlation function between the projection operator and force
operator of the ``fast'' system. By taking a cue from linear response theory we
relate the quantum fluctuation in the generator to the generalised
susceptibility. Relation between the open-path geometric phase, diagonal
elements of the quantum metric tensor and the force-force correlation function
is provided and the classical limit of the fluctuation-correlation theorem is
also discussed.Comment: Latex, 12 pages, no figures, submitted to J. Phys. A: Math & Ge
Minimum cbits for remote preperation and measurement of a qubit
We show that a qubit chosen from equatorial or polar great circles on a Bloch
spehere can be remotely prepared with one cbit from Alice to Bob if they share
one ebit of entanglement. Also we show that any single particle measurement on
an arbitrary qubit can be remotely simulated with one ebit of shared
entanglement and communication of one cbit.Comment: Latex, 7 pages, minor changes, references adde
Mediation of Long Range Charge Transfer by Kondo Bound States
We present a theory of non-equilibrium long range charge transfer between
donor and acceptor centers in a model polymer mediated by magnetic exciton
(Kondo) bound states. Our model produces electron tunneling lengths easily
exceeding 10, as observed recently in DNA and organic charge transfer
systems. This long ranged tunneling is effective for weak to intermediate
donor-bridge coupling, and is enhanced both by weak to intermediate strength
Coulomb hole-electron attraction (through the orthogonality catastrophe) and by
coupling to local vibrational modes.Comment: Revised content (broadened scope, vibrations added), submitted to
Phys Rev Lett, added autho
Exclusion Principle for Quantum Dense Coding
We show that the classical capacity of quantum states, as quantified by its
ability to perform dense coding, respects an exclusion principle, for arbitrary
pure or mixed three-party states in any dimension. This states that no two
bipartite states which are reduced states of a common tripartite quantum state
can have simultaneous quantum advantage in dense coding. The exclusion
principle is robust against noise. Such principle also holds for arbitrary
number of parties. This exclusion principle is independent of the content and
distribution of entanglement in the multipartite state. We also find a strict
monogamy relation for multi-port classical capacities of multi-party quantum
states in arbitrary dimensions. In the scenario of two senders and a single
receiver, we show that if two of them wish to send classical information to a
single receiver independently, then the corresponding dense coding capacities
satisfy the monogamy relation, similar to the one for quantum correlations.Comment: v2: 6 pages, RevTeX 4, title changed, previous results unchanged, new
results adde
Exploring the possibility of enhancing the figure-of-merit ( 2) of NaCoO: A combined experimental and theoretical study
Search of new thermoelectric (TE) materials with high
\textit{figure-of-merit} (ZT) is always inspired the researcher in TE field.
Here, we present a combined experimental and theoretical study of TE properties
of NaCoO compound in high-temperature region. The experimental
Seebeck coefficient (S) is found to vary from 64 to 118 V/K in the
temperature range K. The positive values of S are indicating the
dominating p-type behaviour of the compound. The observed value of thermal
conductivity () is 2.2 W/m-K at 300 K. In the temperature region
K, the value of increases up to 2.6 W/m-K and then
decreases slowly till 620 K with the corresponding value of 2.4 W/m-K.
We have also carried out the theoretical calculations and the best matching
between experimental and calculated values of transport properties are observed
in spin-polarized calculation within DFT+\textit{U} by chosen \textit{U} = 4
eV. The maximum calculated value of ZT is found to be 0.67 at 1200 K for
p-type conduction. Our computational study suggests that the possibility of
n-type behaviour of the compound which can lead to a large value of ZT at
higher temperature region. Electron doping of 5.110
cm is expected to give rise the high ZT value of 2.7 at 1200 K.
Using these temperature-dependent ZT values, we have calculated the maximum
possible values of efficiency () of thermoelectric generator (TEG) made
by p and n-type NaCoO. The present study suggests that one can
get the efficiency of a TE cell as high as 11 when the cold and hot
end temperature are fixed at 300 K and 1200 K, respectively. Such high values
of ZT and efficiency suggest that NaCoO can be used as a
potential candidate for high-temperature TE applications
General impossible operations in quantum information
We prove a general limitation in quantum information that unifies the
impossibility principles such as no-cloning and no-anticloning. Further, we
show that for an unknown qubit one cannot design a universal Hadamard gate for
creating equal superposition of the original and its complement state.
Surprisingly, we find that Hadamard transformations exist for an unknown qubit
chosen either from the polar or equatorial great circles. Also, we show that
for an unknown qubit one cannot design a universal unitary gate for creating
unequal superpositions of the original and its complement state. We discuss why
it is impossible to design a controlled-NOT gate for two unknown qubits and
discuss the implications of these limitations.Comment: 15 pages, no figures, Discussion about personal quantum computer
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Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study
Due to the advantage of tunability via size, shape, doping and relatively low
level of loss and high extent of spatial confinement, graphene quantum dots
(GQDs) are emerging as an effective way to control light by molecular
engineering. The collective excitation in GQDs shows both high energy plasmon
frequency along with frequencies in the terahertz (THz) region making these
systems powerful materials for photonic technologies. Here, we report a
systematic study of the linear and nonlinear optical properties of large
varieties of GQDs (400 systems) in size and topology utilizing the strengths of
both semiempirical and first-principles methods. Our detailed study shows how
the spectral shift and trends in the optical nonlinearity of GQDs depends on
their structure, size and shape. Among the circular, triangular, stripe, and
random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always
possess lower HOMO-LUMO gap, broadband absorption and high nonlinear optical
coefficients. Also, we find that for majority of the GQDs with interesting
linear and nonlinear optical properties have zigzag edges, although reverse is
not always true. We strongly believe that our findings can act as guidelines to
design GQDs in optical parametric oscillators, higher harmonic generators and
optical modulators.Comment: 21 pages, 11 figures, 4 table
Quantum Information Paradox: Real or Fictitious?
One of the outstanding puzzles of theoretical physics is whether quantum
information indeed gets lost in the case of Black Hole (BH) evaporation or
accretion. Let us recall that Quantum Mechanics (QM) demands an upper limit on
the acceleration of a test particle. On the other hand, it is pointed out here
that, if a Schwarzschild BH would exist, the acceleration of the test particle
would blow up at the event horizon in violation of QM. Thus the concept of an
exact BH is in contradiction of QM and quantum gravity (QG). It is also
reminded that the mass of a BH actually appears as an INTEGRATION CONSTANT of
Einstein equations. And it has been shown that the value of this integration
constant is actually zero. Thus even classically, there cannot be finite mass
BHs though zero mass BH is allowed. It has been further shown that during
continued gravitational collapse, radiation emanating from the contracting
object gets trapped within it by the runaway gravitational field. As a
consequence, the contracting body attains a quasi-static state where outward
trapped radiation pressure gets balanced by inward gravitational pull and the
ideal classical BH state is never formed in a finite proper time. In other
words, continued gravitational collapse results in an "Eternally Collapsing
Object" which is a ball of hot plasma and which is asymptotically approaching
the true BH state with M=0 after radiating away its entire mass energy. And if
we include QM, this contraction must halt at a radius suggested by highest QM
acceleration. In any case no EH is ever formed and in reality, there is no
quantum information paradox.Comment: 8 pages in Pramana Style, 6 in Revtex styl
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