Nonequilibrium dynamics of a singlet-triplet Anderson impurity near the quantum phase transition

P. ROURA BAS AND A. A. ALIGIA

JOURNAL OF PHYSICS CONDENSED MATTER

IOP PUBLISHING LTD

Lugar: Londres; Año: 2010 vol. 22 p. 25 - 602

0953-8984

We study the singlet–triplet Anderson model (STAM) in which a configuration with a doublet ishybridized with another containing a singlet and a triplet, as a minimal model to describetwo-level quantum dots coupled to two metallic leads in effectively a one-channel fashion. Themodel has a quantum phase transition which separates regions of a doublet and a singlet groundstate. The limits of integer valence of the STAM (which include a model similar to theunderscreened spin-1 Kondo model) are derived and used to predict the behavior of theconductance through the system on both sides of the transition, where it jumps abruptly. At aspecial quantum critical line, the STAM can be mapped to an infinite-U ordinary Andersonmodel (OAM) plus a free spin 1/2. We use this mapping to obtain the spectral densities of theSTAM as a function of those of the OAM at the transition. Using the non-crossingapproximation (NCA), we calculate the spectral densities and conductance through the systemas a function of temperature and bias voltage, and determine the changes that take place at thequantum phase transition. The separation of the spectral density into a singlet and a triplet partallows us to shed light on the underlying physics and to explain a shoulder observed recently inthe zero bias conductance as a function of temperature in transport measurements through asingle fullerene molecule (Roch et al 2008 Nature 453 633). The structure with three peaksobserved in nonequilibrium transport in these experiments is also explained.