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arXiv:cond-mat/0506677 · International Journal of Modern Physics B

EFFECTS OF THE MAGNETIC FLUX AND OF THE ELECTRON MOMENTUM ON THE TRANSMISSION AMPLITUDE IN THE AHARONOV–BOHM INTERFEROMETER

M. V. AMARESH KUMAR, DEBENDRANATH SAHOO

Published Sep 7, 2005• 0 citations

Abstract

A characterization of the two-terminal open-ring Aharonov–Bohm interferometer is made by analyzing the phase space plots in the complex transmission amplitude plane. Two types of plots are considered: type 1 plot uses the magnetic flux as the variable parameter and type 2 plot which uses the electron momentum as the variable parameter. In type 1 plot, the trajectory closes upon itself only when the ratio R of the arm lengths (of the interferometer) is a rational fraction, and the shape and the type of the generated flower-like pattern is sensitive to the electron momentum. For momenta corresponding to discrete eigenstates of the perfect ring (i.e., the ring without the leads), the trajectory passes through the origin a certain fixed number of times before closing upon itself, whereas for arbitrary momenta it never passes through the origin. Although the transmission coefficient is periodic in the flux with the elementary flux quantum as the basic period, the phenomenon of electron transmission is shown not to be so when analyzed via the present technique. The periodicity is seen to spread over several flux units whenever R is a rational fraction whereas there is absolutely no periodicity present when R is an irrational number. In type 2 plot, closed trajectories passing through the origin a number of times are seen for R being a rational fraction. The case R = 1 (i.e., a symmetric ring) with zero flux is rather pathological — it presents a closed loop surrounding the origin. For irrational R values, the trajectories never close.

DOI: 10.1142/s0217979205032218

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A characterization of the two-terminal open-ring Aharonov–Bohm interferometer is made by analyzing the phase space plots in the complex transmission amplitude plane. Two types of plots are considered: type 1 plot uses the magnetic flux as the variable parameter and type 2 plot which uses the electron momentum as the variable parameter. In type 1 plot, the trajectory closes upon itself only when the ratio R of the arm lengths (of the interferometer) is a rational fraction, and the shape and the type of the generated flower-like pattern is sensitive to the electron momentum. For momenta corresponding to discrete eigenstates of the perfect ring (i.e., the ring without the leads), the trajectory passes through the origin a certain fixed number of times before closing upon itself, whereas for arbitrary momenta it never passes through the origin. Although the transmission coefficient is periodic in the flux with the elementary flux quantum as the basic period, the phenomenon of electron transmission is shown not to be so when analyzed via the present technique. The periodicity is seen to spread over several flux units whenever R is a rational fraction whereas there is absolutely no periodicity present when R is an irrational number. In type 2 plot, closed trajectories passing through the origin a number of times are seen for R being a rational fraction. The case R = 1 (i.e., a symmetric ring) with zero flux is rather pathological — it presents a closed loop surrounding the origin. For irrational R values, the trajectories never close.

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We consider the phase-coherent transport of electrons passing through an Aharonov–Bohm ring while interacting with a tunnel charge in a double quantum dot (representing a charge qubit) which couples symmetrically to both arms of the ring. For Aharonov–Bohm flux Φ AB = h /2 e we find that electrons can only be transmitted when they flip the charge qubit's pseudospin parity an odd number of times. The perfect correlations of the dynamics of the pseudospin and individual electronic transmission and reflection events can be used to entangle the charge qubit with an individual passing electron.

The Aharonov-Bohm oscillations in the transmission probability of a one-dimensional mesoscopic ring between two electrodes are considered numerically where a time-periodical flux \ensuremath{\gamma}(t)=\ensuremath{\gamma}+\ensuremath{\lambda} cos\ensuremath{\omega}t is applied. Such a flux simulates a phase-coherent inelastic scattering due to the absorption/emission processes of modulation quanta. An increase of alternating flux amplitude \ensuremath{\lambda} drastically changes the Aharonov-Bohm oscillations of the transmission though the exact period remains ${\mathrm{\ensuremath{\varphi}}}_{0}$=hc/e. A frequency dependence of the transmission has abrupt steps at \ensuremath{\omega}=(E-

{\mathit{E}}_{\mathit{F}}

)/n, caused by the locking of the transmission in the nth channel where

{\mathit{E}}_{\mathit{F}}

is the Fermi level. Moreover, for transmission of a narrow wave packet the time-periodical flux gives rise to two effects. The first is a fractioning of the input wave packet by the ring in such a way that the output wave function consists of a few separate wave packets. Their number, distance apart, and height essentially depend on the static flux \ensuremath{\gamma}. The second effect is a strong space squeezing of the output wave packet.

We report on a study of complementarity in a two-terminal closed-loop Aharonov-Bohm interferometer. In this interferometer, the simple picture of two-path interference cannot be applied. We introduce a nearby quantum point contact to detect the electron in a quantum dot inserted in the interferometer. We found that charge detection reduces but does not completely suppress the interference even in the limit of perfect detection. We attribute this phenomenon to the unique nature of the closed-loop interferometer. That is, the closed-loop interferometer cannot be simply regarded as a two-path interferometer because of multiple reflections of electrons. As a result, there exist indistinguishable paths of the electron in the interferometer and the interference survives even in the limit of perfect charge detection. This implies that charge detection is not equivalent to path detection in a closed-loop interferometer. We also discuss the phase rigidity of the transmission probability for a two-terminal conductor in the presence of a detector.

The Aharonov-Bohm effect is measured in a four-terminal open ring geometry. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. Phase lapses are found only in certain ranges of the magnetic field.