Quantum phase transitions of quantum dots in the Kondo regime – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2007 > Quantum phase transiti...

Quantum phase transitions of quantum dots in the Kondo regime

Speaker: Prof. Chung-Hou Chung, National Chiao-Tung University,  HsinChu, Taiwan

Quantum phase transitions (zero temperature phase transitions due to quantum fluctuations) in mesoscopic systems form a growing area of condensed matter research due to high tunibility in these devices. In particular, quantum dots, which can be used to build single-electron transistors and spin-based quantum qubits, serve equally interestingly as artificial atoms allowing access to strongly correlated states of matter leading often to quantum phase transitions. In quantum dot devices, the strong antiferromagnetic spin-spin interactions between electrons on the dots and in the leads give rise to the well-known Kondo effect. In this talk, I will present two examples of my recent works on quantum phase transitions in quantum dots associated with the Kondo effect.

1. Quantum criticality in a double-quantum-dot system. We discuss the realization of the quantum-critical non-Fermi liquid state, originally discovered within the two-impurity Kondo model, in double quantum-dot systems where the spins of the two dots are coupled antiferromagnetically. Contrary to the common belief, the corresponding fixed point separating the Kondo phase and the local spin-singlet phase is robust against particle-hole and various other asymmetries, and is only unstable to charge transfer between the two dots. We propose an experimental set-up where such charge transfer processes are suppressed, allowing a controlled approach to the quantum critical state. We also discuss transport and scaling properties in the vicinity of the critical point. cf. G. Zarand, CH Chung, P. Simon, M. Vojta, Phys. Rev. Lett. 97 166802 (2006)

2. Quantum phase transitions in a dissipative quantum dot. We study a spinless level (quantum dot) that hybridizes with a fermionic band and is also coupled via its charge to a dissipative bosonic bath. We consider the general case of a power-law hybridization function. This Bose-Fermi quantum impurity model features a continuous zero-temperature transition between a delocalized phase, with tunneling between the impurity level and the band, and a localized phase, in which dissipation suppresses tunneling in the low-energy limit. The phase diagram and the critical behavior of the model are elucidated using perturbative and numerical renormalization-group techniques, between which there is excellent agreement in the appropriate regimes. In the case of metallic leads, this model's critical properties coincide with those of the spin-boson and Ising Bose-Fermi Kondo models, as expected from bosonization. The relevance of our paradigmic model for describing a dissipative quantum dot is discussed. cf. C.H. Chung, M. Glossop, L. Fritz, M. Kircan, K. Ingersent, M. Vojta, PRB 76, 235103 (2007)