Local lattice dynamics and isotope effect in yttrium diboride probed by extended x-ray absorption fine structure spectroscopy

The lattice vibration properties and the boron isotope effect have been studied in YB2 by using temperature-dependent extended x-ray absorption fine structure (EXAFS). The data show anomalous behavior of the Debye-Waller factor of the Y-B pair due to the superposition of an optical mode associated with the boron sublattice and an acoustic mode corresponding to the yttrium sublattice. We claim that the observed decoupling between metal and boron vibrations is responsible for the lower transition temperature compared to MgB2. The analysis of the boron isotope effect confirms also that the B-B vibration mode plays a key role in the electron-phonon coupling.