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Polar pumped current in quantum nano rings

Edris Faizabadi

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

By applying electric harmonic signals on two point of a quantum ring with phase difference , the induced current is calculated theoretically. The method is based on defining all the operators in the Hilbert space of side band states defined in terms of Floquet theorem. Using this method, we have calculated the electronic pumped current as a function of phase difference of applied electric signals. By controlling on phase difference, it is shown that a polar pumped current can be induced in quantum ring. Φ

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By applying electric harmonic signals on two point of a quantum ring with phase difference , the induced current is calculated theoretically. The method is based on defining all the operators in the Hilbert space of side band states defined in terms of Floquet theorem. Using this method, we have calculated the electronic pumped current as a function of phase difference of applied electric signals. By controlling on phase difference, it is shown that a polar pumped current can be induced in quantum ring. Φ
As electronic circuits get progressingly smaller to the nanometre scale, the quantum wave nature of the electrons starts to play a dominant role. It is thus possible for the devices to operate by controlling the phase of the quantum electron waves rather than the electron density as in present-day devices. This paper presents a highly accurate numerical method to treat quantum waveguides with arbitrarily complex geometry. Based on this model, a variety of quantum effects can be studied and quantified.
We apply exact-exchange spin-density functional theory in the Krieger-Li-Iafrate approximation to interacting electrons in quantum rings of different widths. The rings are threaded by a magnetic flux that induces a persistent current. A weak space and spin symmetry breaking potential is introduced to allow for localized solutions. As the electron-electron interaction strength described by the dimensionless parameter r(S) is increased, we observe-at a fixed spin magnetic moment-the subsequent transition of both spin sub-systems from the Fermi liquid to the Wigner crystal state. A dramatic signature of Wigner crystallization is that the persistent current drops sharply with increasing r(S). We observe simultaneously the emergence of pronounced oscillations in the spin-resolved densities and in the electron localization functions indicating a spatial electron localization showing ferrimagnetic order after both spin sub-systems have undergone the Wigner crystallization. The critical r(S)(c) at the transition point is substantially smaller than in a fully spin-polarized system and decreases further with decreasing ring width. Relaxing the constraint of a fixed spin magnetic moment, we find that on increasing r(S) the stable phase changes from an unpolarized Fermi liquid to an antiferromagnetic Wigner crystal and finally to a fully polarized Fermi liquid.
In this thesis we present theoretical investigations of the effects of Andreev bound states on the current transport in superconducting interferometers. We also investigate the slow dynamics of the Andreev states in a superconducting point contact, and the possible application as a quantum bit. We consider superconductor-normal metal-superconductor (SNS) and normal metalsuperconductor (NS) interferometers, where the contact region is a Y-shaped normal metal wave guide, and the two connection points to the same superconducting electrode can have different phases. The electric current in the interferometer is calculated as a function of the applied voltage and the phase difference . Andreev reflection in SNS and NS interferometers incorporates two features: interference in the arms of the Y-shaped normal region, and interplay with Andreev resonances. The latter feature yields rich phase dependent current structures in the subgap voltage region. The interference effect leads to a suppression of the current structures at . We investigate the effects on the Josephson current in NS interferometers due to current injection from the normal electrode. The two main effects of the nonequilibrium situation are: nonequilibrium population of the Andreev levels, which can result in enhancement, suppression, or even sign reversal of the Josephson current, and an anomalous interference Josephson effect, which gives rise to a long range Josephson effect, increasing with the voltage up to the superconducting gap . The two Andreev states in a superconducting quantum point contact can be accessed for manipulation and measurement by embedding the point contact in a superconducting loop. We calculate an effective Hamiltonian for the slow dynamics of the Andreev two-level system in the ring. Furthermore, we discuss methods of manipulation of the Andreev levels, and coupling of qubits. The state of the Andreev two-level system can be read out by monitoring the macroscopic quantum tunneling in a current biased Josephson junction, which is embedded in the superconducting ring of the qubit. We discuss the effects on the qubit, the readout scheme and the signal-to-noise ratio. THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Coherent processes in Superconducting quantum interferometers and qubits
In this paper, we propose a model for controlling the optical bistability in four-level [Formula: see text] quantum dot nanostructure which is embedded in a unidirectional ring cavity. [Formula: see text] quantum dot nanostructure is based on our recent paper [S. H. Asadpour, Z. Golsanamlou and H. R. Soleimani, Physica E 54 (2013) 45]. It is found that intensity threshold of optical bistability can be manipulated by signal intensity of applied fields. Moreover, we find that phase variation of terahertz signal field can also affect the behaviors of optical bistability and hysteresis loop.
Quantum operations with a charge solid-state qubit whose logical states are formed by two spatially separated localized states of an electron in the double-dot structure are studied theoretically. We show that it is possible to perform various one-qubit rotations making use of the microwave pulses tuned to the resonances between the localized states and the excited state delocalized over the nanostructure. An explicit analytic expression for the time-dependent electron state vector is derived, and the appropriate pulse parameters are determined.