Quantum phase transition in a single-molecule quantum dot

Abstract

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, can be observed in several strongly correlated materials such as heavy fermion compounds or possibly high-temperature superconductors, and is believed to govern many of their fascinating, yet still unexplained properties. In contrast to these bulk materials with very complex electronic structure, artificial nanoscale devices could offer a new and simpler vista to the comprehension of quantum phase transitions. This long-sought possibility is demonstrated by our work in a fullerene molecular junction, where gate voltage induces a crossing of singlet and triplet spin states at zero magnetic field. Electronic tunneling from metallic contacts into the C60 quantum dot provides here the necessary many-body correlations to observe a true quantum critical behavior.

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

“…The slow saturation of the low temperature conductance for the even charge spin S = 1 regime, associated to the logarithmic approach to the underscreening fixed point, is readily contrasted to the rapid Fermi liquidlike behavior at low temperature for the odd charge spin S = 1/2 Kondo effect. This quantitative comparison gives also more convincing evidence for our observation [34] of a quantum phase transition when singlet and triplet states meet at the critical gate voltage.…”

“…Such phenomena, not fully elucidated in our original work [34], has received a lot of experimental and theoretical attention recently [65,66,67,68,69] and has a simple explanation in terms of molecular levels renormalized by virtual charge fluctuations.…”

“…Kondo physics has been widely reported in a variety of quantum dot systems, based on semiconducting heterostructures [9,10], carbon nanotubes [11] and molecular devices [12,13,34,18,14], showing the great universality of this phenomenon. Usually the Kondo effect is best observed for odd charge quantum dots, because the formation of a net spin S = 1/2 is always guaranteed in that case (for even charge dots, the ground state turns out often to be a non-magnetic singlet).…”

“…In both plots, arrows denote the dispersive singlet/triplet excitations. The upper panel shows the experimental measurement [34] and the lower panel the theory [68,69] In the experimental plot (upper panel), the four lower curves (associated to low conductance at small bias) are taken on the singlet side, while the upper curves correspond to the triplet side.…”

“…Several different theoretical proposals have also been made for the unscreening QPT [28,32,37,38], which was shown to be even robust to valence fluctuations. Our detailed experimental observation [34] of this singlet-triplet quantum phase transition in a two-electron molecular quantum dot will be one of the main topics of this review, and the various scenarios presented above will be reviewed with more details in the concluding Section 4.…”