2,072 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
Fractional Periodicity of Persistent Currents: A Signature of Broken Internal Symmetry
We show from the symmetries of the many body Hamiltonian, cast into the form
of the Heisenberg (spin) Hamiltonian, that the fractional periodicities of
persistent currents are due to the breakdown of internal symmetry and the spin
Hamiltonian holds the explanation to this transition. Numerical
diagonalizations are performed to show this explicitely. Persistent currents
therefore, provide an easy way to experimentally verify broken internal
symmetry in electronic systems.Comment: minor correction
S-Matrix Formulation of Mesoscopic Systems and Evanescent Modes
The Landauer-Butikker formalism is an important formalism to study mesoscopic
systems. Its validity for linear transport is well established theoretically as
well as experimentally. Akkermans et al [Phys. Rev. Lett. {\bf 66}, 76 (1991)]
had shown that the formalism can be extended to study thermodynamic properties
like persistent currents. It was earlier verified for simple one dimensional
systems. We study this formula very carefully and conclude that it requires
reinterpretation in quasi one dimension. This is essentially because of the
presence of evanescent modes in quasi one dimension.Comment: non
Landauer-B\"uttiker approach for hyperfine mediated electronic transport in the integer quantum Hall regime
The interplay of spin-polarized electronic edge states with the dynamics of
the host nuclei in quantum Hall systems presents rich and non-trivial transport
physics. Here, we develop a Landauer-B\"uttiker approach to understand various
experimental features observed in the integer quantum Hall set ups featuring
quantum point contacts. The approach developed here entails a phenomenological
description of spin resolved inter-edge scattering induced via hyperfine
assisted electron-nuclear spin flip-flop processes. A self-consistent
simulation framework between the nuclear spin dynamics and edge state
electronic transport is presented in order to gain crucial insights into the
dynamic nuclear polarization effects on electronic transport and in turn the
electron-spin polarization effects on the nuclear spin dynamics. In particular,
we show that the hysteresis noted experimentally in the conductance-voltage
trace as well as in the resistively detected NMR lineshape results from a lack
of quasi-equilibrium between electronic transport and nuclear polarization
evolution. In addition, we present circuit models to emulate such hyperfine
mediated transport effects to further facilitate a clear understanding of the
electronic transport processes occurring around the quantum point contact.
Finally, we extend our model to account for the effects of quadrupolar
splitting of nuclear levels and also depict the electronic transport signatures
that arise from single and multi-photon processes.Comment: 21 pages, 8 figure
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
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