Effect of impurity scattering on the upper critical field of MgB2
single crystals
B. Kanga*, M.-S. Parkb, H.-S. Leeb, S.-I. Leec, and M.-H. Jungc
a
BK21 physics program and department of Physics, Chungbuk National University, Cheongju, 361-763, Korea
b Department of Physics, Pohang University of Science and Technology, Pohang, 790-784, Korea
c Department of Physics, Sogang University, Seoul, 121-742, Korea
We investigated the temperature dependence of the upper critical fields Hc2(T) of Al- and C-substituted
MgB2 single crystals along the two main crystallographic directions. Despite similar suppression of T c
with the Al and C concentrations, the effects of the two substitutions on Hc2(T) are found to be
different. Hc2(T) of the Al- and the C-doped MgB2 can be well explained within the dirty-limit two-gap
theory although Hc2(0) of two substitutions show an opposite behavior, indicating very weak interband
scattering. For the C-doping case, the ratio of in-plane electron diffusivities D/D is much smaller
than that for the Al-doping case indicating that C-doping enhances impurity scattering mainly in the 
bands as anticipated. The ratio of Hc2 along the two directions systematically decreases with both Al
and C doping, however, the ratio of Hc2 for the C-doped shows a stronger temperature dependence
than that for the Al-doped. Based on parameters obtained by fitting experimental data to the dirty-limit
two-gap theory, we discussed difference between the two substitutions in view of impurity scattering.
keywords : MgB 2 single crystals, upper critical field, Al doping, C doping, impurity scattering
Title : Electron fractionalization in heavy fermion quantum criticality
Abstract : We discuss electron fractionalization in heavy fermion quantum
criticality. First, we discuss possible situations for electron fractionalization
in addition to one dimensional systems and fractional quantum Hall states. We propose
Mott criticality as one candidate. Second, we try to explain various experiments
in the heavy fermion quantum criticality of YbRh2Si2 based on this fractionalization
picture, referred as the Kondo breakdown scenario. We emphasize novel energy scales
in the Kondo breakdown theory and z = 3 quantum criticality, where z is the dynamical
critical exponent. Third, we revisit a recently discussed controversial issue on
the large N (spin degeneracy) calculation (Eliashberg framework). We discuss the
role of vertex corrections in various critical exponents. Finally, we discuss a
future direction.