Quantum Dots and Single-Electron Transistors have been used over the past decade to model Fermi liquid or Kondo states in and near equilibrium. The universal features of the Kondo effect and its significance in describing strongly correlated electron materials have led to a strong interest both from a technological as well as scientific point of view. Here, we introduce what we believe to be the first realistic system – a quantum dot attached to ferromagnetic leads – that models non-Fermi liquid states near a quantum phase transition. We theoretically demonstrate a gate-voltage induced quantum phase transition. At the transition the Kondo effect becomes quantum critical, leading to distinct, universal properties. We find a fractional-power-law dependence of the conductance on temperature (T). The AC conductance and thermal noise spectrum have related power-law dependences on frequency (omega) and, in addition, show an (omega/T) scaling. Our results imply that the ferromagnetic nanostructure constitutes a robust and realistic model system to elucidate magnetic quantum criticality that is central to the heavy fermions and other strongly correlated electron systems with non-Fermi liquid behavior.

Rice University, Houston