2,978 research outputs found
Quantum Waveguide Transport in Serial Stub and Loop Structures
We have studied the quantum transmission properties of serial stub and loop
structures. Throughout we have considered free electron networks and the
scattering arises solely due to the geometric nature of the problem. The band
formation in these geometric structures is analyzed and compared with the
conventional periodic potential scatterers. Some essential differences are
pointed out. We show that a single defect in an otherwise periodic structure
modifies band properties non trivially. By a proper choice of a single defect
one can produce positive energy bound states in continuum in the sense of von
Neumann and Wigner. We also discuss some magnetic properties of loop structures
in the presence of Aharonov-Bohm flux.Comment: Revtex 3.2 version, 27 pages, figures available on request, To appear
in Phys. Rev.
Phase of Aharonov-Bohm oscillations in conductance of mesoscopic systems
Motivated by a recent experiment we analyze in detail the phase of
Aharonov-Bohm oscillations across a 1D ring with a stub coupled to one of its
arms, in the presence of a magnetic flux. We demonstrate that there are two
kinds of conductance extremas. One class of them are fixed at particular flux
values and can only change abruptly from a maxima to a minima as incident
energy is varied. We show a different mechanism for such abrupt phase change in
conductance oscillation. We demonstrate that these extremas can exhibit
phase locking". However, the second kind of extremas can shift continuously
as the incident energy is varied.Comment: Figure available on reques
Novel interference effects and a new Quantum phase in mesoscopic systems
Mesoscopic systems have provided an opportunity to study quantum effects
beyond the atomic realm. In these systems quantum coherence prevails over the
entire sample. We discuss several novel effects related to persistent currents
in open systems which do not have analogues in closed systems. Some phenomena
arising simultaneously due to two non-classical effects namely, Aharonov-Bohm
effect and quantum tunneling are presented. Simple analysis of sharp phase
jumps observed in double-slit Aharonov-Bohm experiments is given. Some
consequences of parity violation are elaborated. Finally, we briefly describe
the dephasing of Aharonov-Bohm oscillations in Aharonov-Bohm ring geometry due
to spin-flip scattering in one of the arms. Several experimental manifestations
of these phenomena and their applications are given.Comment: A revie
Study of quantum current enhancement, eigenenergy spectra and magnetic moments in a multiply connected system at equilibrium
A multiply connected system in both its open and closed form variations but
in equilibrium is studied using quantum waveguide theory. The system exhibits
remarkable features, in its open form variation we see current enhancement,
hitherto seen only in non-equilibrium systems in absence of magnetic flux. In
its closed form analog parity effects are broken. Further we analyse the global
and local current densities of our system and also show that the orbital
magnetic response of the system calculated from the current densities (and
inherently linked to the topological configuration) is qualitatively not same
as that calculated from the eigenenergy spectra.Comment: 10 pages, 15 figures, 3 table
Particle Spectrum of the Supersymmetric Standard Model from the Massless Excitations of a Four Dimensional Superstring
A superstring action is quantised with Neveu Schwarz(NS) and Ramond(R)
boundary conditions. The zero mass states of the NS sector are classified as
the vector gluons, W-mesons, -mesons and scalars containing Higgs. The
fifteen zero mass fermions are obtained from the Ramond sector. A space time
supersymmetric Hamiltonian of the Standard Model is presented without any
conventional SUSY particles
Aharonov-Bohm effect in the presence of evanescent modes
It is known that differential magnetoconductance of a normal metal loop
connected to reservoirs by ideal wires is always negative when an electron
travels as an evanescent modes in the loop. This is in contrast to the fact
that the magnetoconductance for propagating modes is very sensitive to small
changes in geometric details and the Fermi energy and moreover it can be
positive as well as negative. Here we explore the role of impurities in the
leads in determining the magnetoconductance of the loop. We find that the
change in magnetoconductance is negative and can be made large provided the
impurities do not create resonant states in the systems. This theoretical
finding may play an useful role in quantum switch operations.Comment: 9 figures available on reques
Directional dependence of spin currents induced by Aharonov-Casher phase
We have calculated the persistent spin current of an open ring induced by the
Aharonov-Casher phase. For unpolarized electrons there exist no persistent
charge currents, but persistent spin currents. We show that, in general, the
magnitude of the persistent spin current in a ring depends on the direction of
the direct current flow from one reservoir to another. The persistent spin
current is modulated by the cosine function of the spin precession angle. The
nonadiabatic Aharonov-Casher phase gives anomalous behaviors. The
Aharonov-Anandan phase is determined by the solid angle of spin precession.
When the nonadiabatic Aharonov-Anandan phase approaches a constant value with
the increase of the electric field, the periodic behavior of the spin
persistent current occurs in an adiabatic limit. In this limit the periodic
behavior of the persistent spin current could be understood by the effective
spin-dependent Aharonov-Bohm flux.Comment: 16pages, 5 postscript figures. Submitted to Phys. Rev.
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