Large mass enhancement in RbOs2O6

Heat capacity measurements on the recently discovered geometrically frustrated β-pyrochlore superconductor RbOs2O6 (Tc=6.4K) yield a Sommerfeld coefficient of 44mJ/molf.u./K2. This is about four times larger than the one found in band structure calculations. In order to specify the enhancement due to electron-electron interactions, we have measured the electron-phonon enhancement. By a suitable analysis, an electron-phonon coupling constant λep=1+/-0.1 is derived from the specific heat jump at Tc. This leaves a significant additional λadd=2.1+/-0.3 for enhancement due to other mechanisms, possibly related to the triangular lattice. To arrive at these results, an appropriate analysis method for bulk thermodynamic data based on the condensation energy was applied