1,152 research outputs found
A Dual Four Dimensional Superstring
The 26 dimensional bosonic string, first suggested by Nambu and Goto, is
reduced to a four dimensional superstring by using two species of 6 and 5
Majorana fermions as proposed by Deo. These two species of fermions differ in
their 'neutrino-like' phase, and are vectors in the bosonic representation
SO(d-1,1).Using Polchinski's equivalence between operators and states, we can
write the Virasoro generators for 4 dimensional string theory. The theory is
shown to give the same results as given by other superstrings and also reveals
the well known aspects of four dimensional string theory.The bosons and the
fermions are found to be the basis for constructing this string theory which
includes gravity and exhibits strong-weak coupling duality as well as the usual
electric-magnetic duality. This formalism is used to calculate the metric
tensor as well as the entropy area relation for a black hole.Comment: 10 page
Large diamagnetic persistent currents
In multichannel rings, evanescent modes will always co-exist with propagating
modes. The evanescent modes can carry a very large diamagnetic persistent
current that can oscillate with energy and are very sensitive to impurity
scattering. This provides a natural explanation for the large diamagnetic
persistent currents observed in experiments.Comment: 5 figure
Persistent Currents in the Presence of a Transport Current
We have considered a system of a metallic ring coupled to two electron
reservoirs. We show that in the presence of a transport current, the persistent
current can flow in a ring, even in the absence of magnetic field. This is
purely a quantum effect and is related to the current magnification in the
loop. These persistent currents can be observed if one tunes the Fermi energy
near the antiresonances of the total transmission coefficient or the two port
conductance.Comment: To appear in Phys. Rev. B. Three figures available on reques
Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect
We present a simple model of transmission across a metallic mesoscopic ring.
In one of its arm an electron interacts with a single magnetic impurity via an
exchange coupling. We show that entanglement between electron and spin impurity
states leads to reduction of Aharonov-Bohm oscillations in the transmission
coefficient. The spin-conductance is asymmetric in the flux reversal as opposed
to the two probe electrical conductance which is symmetric. In the same model
in contradiction to the naive expectation of a current magnification effect, we
observe enhancement as well as the suppression of this effect depending on the
system parameters. The limitations of this model to the general notion of
dephasing or decoherence in quantum systems are pointed out.Comment: Talk presented at the International Discussion Meeting on Mesoscopic
and Disordered systems, December, 2000, at IISc Bangalore 17 pages, 8figure
Renormalization group study of the conductances of interacting quantum wire systems with different geometries
We examine the effect of interactions between the electrons on the
conductances of some systems of quantum wires with different geometries. The
systems include a wire with a stub in the middle, a wire containing a ring
which can enclose a magnetic flux, and a system of four wires which are
connected in the middle through a fifth wire. Each of the wires is taken to be
a weakly interacting Tomonaga-Luttinger liquid, and scattering matrices are
introduced at all the junctions. Using a renormalization group method developed
recently for studying the flow of scattering matrices for interacting systems
in one dimension, we compute the conductances of these systems as functions of
the temperature and the wire lengths. We present results for all three regimes
of interest, namely, high, intermediate and low temperature. These correspond
respectively to the thermal coherence length being smaller than, comparable to
and larger than the smallest wire length in the different systems, i.e., the
length of the stub or each arm of the ring or the fifth wire. The
renormalization group procedure and the formulae used to compute the
conductances are different in the three regimes. We present a
phenomenologically motivated formalism for studying the conductances in the
intermediate regime where there is only partial coherence. At low temperatures,
we study the line shapes of the conductances versus the electron energy near
some of the resonances; the widths of the resonances go to zero with decreasing
temperature. Our results show that the conductances of various systems of
experimental interest depend on the temperature and lengths in a non-trivial
way when interactions are taken into account.Comment: Revtex, 17 pages including 15 figure
Friedel Sum Rule for single channel quantum wire
Elastic scattering in a quantum wire has several novel features not seen in
1D, 2D or 3D. In this work we consider a single channel quantum wire as its
application is inevitable in making devices based on quantum interference
effects. We consider a point defect or a single delta function impurity in such
a wire and show how some of these novel features affect Friedel-sum-rule (FSR)
in a way, that is quite unlike in 1D, 2D and 3D.Comment: shortene
Friedel phases and phases of transmission amplitudes in quantum scattering systems
We illustrate the relation between the scattering phase appearing in the
Friedel sum rule and the phase of the transmission amplitude for quantum
scatterers connected to two one-dimensional leads. Transmission zero points
cause abrupt phase changes of the phase of the transmission amplitude.
In contrast the Friedel phase is a continuous function of energy. We
investigate these scattering phases for simple scattering problems and
illustrate the behavior of these models by following the path of the
transmission amplitude in the complex plane as a function of energy. We verify
the Friedel sum rule for these models by direct calculation of the scattering
phases and by direct calculation of the density of states.Comment: 12 pages, 12 figure
Quantum current magnification in a multi-channel mesoscopic ring
We have studied the current magnification effect in a multi-channel open
mesoscopic ring. We show that the current magnification effect is robust even
in the presence of several propagating modes inspite of mode mixing and
cancellation effects. The magnitude of circulating currents in the
multi-channel regime can be much larger than that in a single channel case.
Impurities can enhance or degrade the current magnification effect depending
sensitively on the system parameters. Circulating currents are mostly
associated with Fano resonances in the total transport current. We further show
that system-lead coupling qualitatively changes the current magnification
effect.Comment: 12 pages, 11 figure
Aharonov-Bohm oscillations and spin transport in a mesoscopic ring with a magnetic impurity
We present a detailed analysis of the Aharonov-Bohm (AB) interference
oscillations manifested through transmission of an electron in a mesoscopic
ring with a magnetic impurity atom inserted in one of its arms. The spin
polarization transport is also studied. The electron interacts with the
impurity through the exchange interaction leading to exchange spin-flip
scattering. Transmission in the spin-flipped and spin-unflipped channels are
explicitly calculated. We show that the entanglement between electron and
spin-flipper states lead to a reduction of AB oscillations in spite of absence
of any inelastic scattering. The spin-conductance (related to spin-polarized
transmission coefficient) is asymmetric in the flux reversal as opposed to the
two probe conductance which is symmetric under flux reversal. We point out
certain limitations of this model in regard to the general notion of dephasing
in quantum mechanics.Comment: 6 pages RevTeX, 9 eps figures included, enlarged version of
cond-mat/000741
Measuring the transmission of a quantum dot using Aharonov-Bohm Interferometers
The conductance G through a closed Aharonov-Bohm mesoscopic solid-state
interferometer (which conserves the electron current), with a quantum dot (QD)
on one of the paths, depends only on cos(phi), where Phi= (hbar c phi)/e is the
magnetic flux through the ring. The absence of a phase shift in the
phi-dependence led to the conclusion that closed interferometers do not yield
the phase of the "intrinsic" transmission amplitude t_D=|t_D|e^{i alpha}
through the QD, and led to studies of open interferometers. Here we show that
(a) for single channel leads, alpha can be deduced from |t_D|, with no need for
interferometry; (b) the explicit dependence of G(phi) on cos(phi) (in the
closed case) allows a determination of both |t_D| and alpha; (c) in the open
case, results depend on the details of the opening, but optimization of these
details can yield the two-slit conditions which relate the measured phase shift
to alpha.Comment: Invited talk, Localization, Tokyo, August 200
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