| speaker: | Kevin Ingersent Department of Physics, University of Florida, Gainesville, Florida 32611, USA |
| time: | Th. 10.04.03, 11:20 - 12:10 |
The pseudogap Kondo model couples a localized spin to a noninteracting Fermi bath described by a density of states that vanishes in power-law fashion at the Fermi energy. A quantum critical point (QCP) at finite Kondo coupling separates the strong- and weak-coupling phases of the model. In the vicinity of this QCP, the local magnetic susceptibility exhibits omega/T scaling with a fractional frequency/temperature exponent. These and other properties will be discussed in relation to two sets of experiments: (1) NMR on Li impurities in YBCO suggests that a magnetic moment induced around the impurity site exhibits Kondo-like behavior, both above and below the superconducting transition tempersture. (2) The dynamical spin susceptibility of the heavy-fermion metal CeCu5.9Au0.1 close to its T=0 antiferromagnetic ordering transition exhibits omega/T scaling with a fractional exponent over essentially the entire Brillouin zone, pointing to anomalous local physics. This observation has led to the notion of a "locally critical" QCP in the Kondo lattice, at which critical long-wavelength spin fluctuations coexist with critical local-moment fluctuations.