Ronald M. Cosby

Characteristics of transmission resonance in a quantum-dot superlattice

We investigate phase-coherent electron transport through height-varying potential barriers in a quantum-dot superlattice. Due to the aspect ratio variations of two alternating potential heights in the quantum channel, well-arranged resonant peaks in the first miniband of each plateau are divided into the paired peaks of two groups, which produce an extra gap inside each miniband. In addition, for a five barrier case, the second and third resonant peaks in the miniband are no longer distinguishable at a critical aspect ratio, and the amplitude of this degenerate peak becomes smaller than one and eventually approaches zero. The mean lifetimes of the resonant peaks whose amplitudes remain unity are studied. We also examine the resonant tunneling with under-unity transmission in the one-dimensional superlattice system with alternating potential barriers. Finally, it is found that the “quasi-resonance” appears in a quantum-dot superlattice with 13 barriers consisting of 2 alternating potential heights.

Resonant tunneling in a quantum nanosystem with an attractive impurity

We present the study of the conductance of a quantum nanosystem containing a finite-size attractive impurity. A single finite-size attractive impurity introduces multiple quasi-bound states in the channel for a sufficiently strong attractive potential, and these states give rise to multiple resonant peaks before the first plateau in the conductance. These resonant peaks, arising from the resonant tunneling through the multiple quasi-bound levels, have a Lorentzian shape centered around the resonant energy and exhibit a dramatic variation in the linewidths with resonant energy. The strength of the attractive impurity in the constriction is shown to strongly affect the resonant energy and the mean lifetime of each tunneling peak. The temperature dependence of the resonant peaks of conductance is also discussed.

Conductance oscillations due to a controllable impurity in a quantum box

We present calculations of conductance in a multiply connected nanostructure with a quantum box geometry. Well‐defined conductance oscillations appear which are attributed to the quantum interference effect in the presence of a controllable impurity in the quantum box. As the strength of the impurity potential is modulated, conductance oscillations arise from the constructive and destructive interference for the two electronic paths around the centrally located impurity and a third tunneling path through the impurity. We discuss the dependence of these oscillations on the size of the impurity, in terms of circulating or bound states in the quantum box formed by multiple reflections of the phase‐coherent electron. The conductance oscillations are predicted to be strong for realistic structural parameters and robust against increasing temperature.

Quantum transport anomalies in semiconductor nanosystems

We present quantum transport anomalies in the theoretical conductance of various semiconductor nanostructures. We first investigate a quantum channel with a chain of quantum boxes connected by slits, called a superlattice structure, and study the miniband and minigap effects associated with resonances and anti‐resonances in the conductance. We also report studies of electron transport in a quantum wire containing series or parallel slits and a detector slit. In these systems, strong conductance oscillations due to quantum interference effects are predicted as a detector slit is moved across the wire. In the case of a single and multi‐series slits, we attribute these effects to multiple reflections of the phase‐coherent electron along the quantum wire. The transmission coefficients and electronic phase shifts are examined, which provide insights into the origins of these conductance oscillations. In the case of multi‐parallel slits, peaks with two‐ (four‐) fold splitting in the conductance are exhibited du...

Effects of tunneling through coupled series attractors in a mesoscopic system

We report the effects of resonant tunneling through multi-coupled attractors in series in a quantum nanosystem. We find that the multiple split-conductance peaks arise from resonant tunneling through multiple nondegenerate quasi-bound-states of the coupled attractors. We also show that the coupling can be tuned by modulating the strength of the attractor, demonstrating that the separation of peak splitting in conductance decreases as the attractor strengths increase.

Conductance oscillations through double slits in a quantum wire

Abstract We report theoretical studies of electron transport in a quantum wire containing double-paralleled or double-series slits and a “detector” slit. Strong conductance oscillations due to quantum interference are produced in both structures when the position of a detector slit in the quantum wire is changed. We interpret these interference effects in terms of multiple reflections of a phase-coherent electron along the quantum wire and circulating-bound-states in a resonant-box region. We also examine the transmission coefficients and electronic phase shifts which provide insights into the origins of conductance oscillations. For the double-paralleled system, the dependence of the interference effects on the slit width and the detector distance is discussed.

Quantum interference in multichannel systems

Quantum interference effects in four parallel channels are theoretically investigated. We attribute interference effects to the different electron paths, by calculating the difference between the conductance (electronic probability distribution) for the four channels and the sum of the conductances (probability distributions) obtained with only one channel open at a time. The large variations of the conductance difference and a periodic behavior of the difference of probability distribution indicate wave‐function phase shifts and interference due to alternative electron paths through the parallel channels.

Tuning of the transmission resonance in Aharonov-Bohm quantum rings

We investigate the total transmission probability of a nanoscale Aharonov-Bohm (AB) ring with an embedded quantum dot in one of its arms and a magnetic flux passing through its center. We find a peculiar quantum transport though this system such as a symmetric Breit- Wigner (BW) and an asymmetric Fano transmission resonance. The transition from BW to Fano resonance (or vice verse) occurs by tuning the magnetic AB flux threading through the AB ring, indicating the Fano asymmetric parameter is extended to a complex number. Unique properties of the AB phase coexistent with Fano interference are examined.

Electron transport in parallel interacting artificial molecules

The low-field conductance of interacting artificial molecular wires is simulated using a single-electron model. Coupled artificial molecules consisting of parallel chains of open quantum dots in a two-dimensional electron gas display a split-off molecular band with an energy separation that grows with the coupling strength. The position of the Fermi energy relative to the molecular band states plays a dominant role in determining the low-field conductance. The predicted conductance variation with coupling for dual five-atom molecular wires ranges from oscillatory to monotonic, depending on the Fermi energy. For electron energies near a resonant state, results imply that conductance measurements on molecules in parallel could vary significantly with the inter-molecular spacing.

Resonances in Conductance Through Tunable Attractors

We present the effects of resonant tunneling through doubly-coupled attractors in series in a quantum nanosystem. It is found that multiple split-tunneling peaks arise from resonant tunneling through multiple nondegenerate quasi-bound-states of the coupled attractors. We show that the tunability of coupling is achieved by modulating the strength of the attractor for a fixed separation length between the attractors, and by adjusting the geometrical separation length for a fixed intervening barrier strength.