Now showing 1 - 2 of 2
  • Publication
    Quantum transport in interacting nanodevices: From quantum dots to single-molecule transistors
    (University College Dublin. School of Physics, 2022) ;
    The enormous interest in industrial application of semiconductor components has led to the development of unprecedented control over the manufacture of electronic devices on the nanometer scale. This allows to perform highly controllable and fine-tuned experiments in the quantum regime where exotic effects can nowadays be measured. Among those, breakthrough measurements of electrical conductance experimentally confirmed the Kondo effect - a many-body quantum effect involving macroscopic entanglement. In quantum dot devices, enhanced conductance below a characteristic energy scale is the signature of Kondo singlet formation. Precise predictions of quantum transport properties in similar nanoelectronics devices is therefore desired to design optimal functionality and control. Standard mesoscopic transport methods suffer from limitations in nanostructure specifics, set-up design, energy, temperature and voltage regime of applicability. To overcome these issues, such that we obtain modelling flexibility and accurate conductance predictions, in this thesis we analytically derive alternative and improved quantum transport formulations having as their starting point scattering theory in the Landauer-Buettiker formula, linear response theory in the Kubo formula, nonequilibrium Keldysh theory in the Meir-Wingreen formula and Fermi liquid theory in the Oguri formula. We perform a systematic benchmark of our exact expressions, comparing with the standard approaches using a state-of-the-art numerical renormalization group techniques (NRG). The new formulations not only reproduce literature results, but also show higher accuracy and computational efficiency, as well as a wider applicability under regimes and conditions out of reach by existing methods. We also derive generalized effective models for multi-orbital two-lead interacting nanostructures in both Coulomb blockade and mixed-valence regime, which yield reusable conductance predictions directly in terms of the effective model parameters. We conclude by applying our novel formulations to complex nanoelectronics systems, including a single-molecule benzene transistor, a charge-Kondo quantum dot made from graphene and a semiconductor triple quantum dot.
  • Publication
    Two-Channel Charge-Kondo Physics in Graphene Quantum Dots
    Nanoelectronic quantum dot devices exploiting the charge-Kondo paradigm have been established as versatile and accurate analogue quantum simulators of fundamental quantum impurity models. In particular, hybrid metal–semiconductor dots connected to two metallic leads realize the two-channel Kondo (2CK) model, in which Kondo screening of the dot charge pseudospin is frustrated. In this article, a two-channel charge-Kondo device made instead from graphene components is considered, realizing a pseudogapped version of the 2CK model. The model is solved using Wilson’s Numerical Renormalization Group method, uncovering a rich phase diagram as a function of dot–lead coupling strength, channel asymmetry, and potential scattering. The complex physics of this system is explored through its thermodynamic properties, scattering T-matrix, and experimentally measurable conductance. The strong coupling pseudogap Kondo phase is found to persist in the channel-asymmetric two-channel context, while in the channel-symmetric case, frustration results in a novel quantum phase transition. Remarkably, despite the vanishing density of states in the graphene leads at low energies, a finite linear conductance is found at zero temperature at the frustrated critical point, which is of a non-Fermi liquid type. Our results suggest that the graphene charge-Kondo platform offers a unique possibility to access multichannel pseudogap Kondo physics.