Quantum phase transition in a single-molecule quantum dot

TL;DR

This paper demonstrates quantum critical behavior in a single-molecule quantum dot, revealing a controllable quantum phase transition between singlet and triplet states induced by gate voltage.

Contribution

It provides the first experimental observation of a quantum phase transition in a nanoscale device using a fullerene molecular junction.

Findings

Gate voltage induces singlet-triplet crossing at zero magnetic field.

Electronic tunneling enables many-body correlations necessary for quantum criticality.

Observation of true quantum critical behavior in a molecular 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 $\rm{C_{60}}$ quantum dot provides here the necessary many-body correlations to observe a true quantum critical behavior.