Universal transport signatures in two-electron molecular quantum dots: gate-tunable Hund's rule, underscreened Kondo effect and quantum phase transitions,

S. FLORENS, A, FREYN, N. ROCH, W. WERNSDORFER, F. BALESTRO, P. ROURA-BAS AND A. A. ALIGIA

JOURNAL OF PHYSICS CONDENSED MATTER

IOP PUBLISHING LTD

Lugar: Londres; Año: 2011 vol. 23 p. 24 - 3202

0953-8984

We review here some universal aspects of the physics of two-electron molecular transistors inthe absence of strong spin–orbit effects. Several recent quantum dot experiments have shownthat an electrostatic backgate could be used to control the energy dispersion of magnetic levels.We discuss how the generally asymmetric coupling of the metallic contacts to two differentmolecular orbitals can indeed lead to a gate-tunable Hund’s rule in the presence of singlet andtriplet states in the quantum dot. For gate voltages such that the singlet constitutes the(non-magnetic) ground state, one generally observes a suppression of low voltage transport,which can yet be restored in the form of enhanced cotunneling features at finite bias. Moreinterestingly, when the gate voltage is controlled to obtain the triplet configuration, spin S = 1Kondo anomalies appear at zero bias, with non-Fermi liquid features related to theunderscreening of a spin larger than 1/2. Finally, the small bare singlet–triplet splitting in ourdevice allows fine-tuning with the gate between these two magnetic configurations, leading toan unscreening quantum phase transition. This transition occurs between the non-magneticsinglet phase, where a two-stage Kondo effect occurs, and the triplet phase, where the partiallycompensated (underscreened) moment is akin to a magnetically ‘ordered’ state. Theseobservations are put theoretically into a consistent global picture by using new numericalrenormalization group simulations, tailored to capture sharp finite-voltage cotunneling featureswithin the Coulomb diamonds, together with complementary out-of-equilibrium diagrammaticcalculations on the two-orbital Anderson model. This work should shed further light on thecomplicated puzzle still raised by multi-orbital extensions of the classic Kondo problem.