Nanotube Quantum Dot Transport With Spin-Orbit Coupling and Interacting Leads

TL;DR

This paper investigates the transport properties of a nanotube quantum dot with spin-orbit coupling and multiple quantum states, considering Coulomb interactions and their effects on conductance and density of states.

Contribution

It extends the NEGF formalism to four quantum states in a Coulomb-interacting nanotube quantum dot, revealing effects on conductance peaks and density of states.

Findings

Conductance peaks near zero are unaffected by Coulomb interaction strength.

Coulomb interaction shifts the density of states to higher energies.

Presence of four quantum states reduces current by about ten times compared to two states.

Abstract

We analyzed the effects of a spin voltage as well as a conventionally applied voltage in a QD system with a different number of quantum states in the dot region in presence of Coulombic interaction between the quantum dot and two leads. We extended the NEGF treatment developed for noninteracting leads onto the case of four quantum states $m =\{\sigma,\lambda\}=\{\pm,\pm\}$ interacting with leads. Our derivation is based on the equation-of-motion technique and Langreth's theorem. For a Coulombic repulsion between the contacts and QD we obtain an expression for the current through QD for the four quantum states. To determine the parameters of the model Hamiltonian we used our previous calculations [1] of the electronic properties of a symmetrical nanotube QD (5,5)/(10,0)\_1/(5,5) in a tight binding model, where \_1 denotes the length of the middle QD segment of a (10,0) zigzag nanotube. We calculated the density of electronic states with spin up and down for the case of a single QD without pseudospin states for an infinite Coulomb repulsion, in good agreement with the calculations of Yuan Li, et al. [2]. Our calculation showed that the position of the conductance peaks nearest to zero is not affected by the strength of the QD-lead Coulombic interaction parameters. We also demonstrated that this interaction shifts the density of states to higher energies. The interplay between the Kondo effect and the bias is highly temperature-dependent and becomes significant only at low temperatures. Lastly, we found that the existence of four quantum states $m=\{\sigma,\lambda\}$ leads to abrupt changes in the density of states. In this case the values of the current are approximately ten times lower than for QD with only two quantum states $m =\{\sigma\}=\{\pm\}$. However, in the case of a conventional bias the current amplitudes in both cases are approximately the same.