Specific-Heat of YBa₂Cu₃O_x up to 400 K: High-Resolution Adiabatic Measurements and Ab-initio LDA Phonon Calculations

Abstract

The specific heat of YBa₂Cu₃O_x (x=6.7, 6.9, and 7.0) has been determined to within 0.1% accuracy from 5 to 400 K using adiabatic calorimetry. The phonon specific heat contribution for x=7.0 is calculated using ab-initio LDA and subtracted from the experimental data, yielding estimates of the oxygen-ordering and electronic heat capacity contributions. The oxygen-ordering contribution agrees well with theoretical predictions. Although the LDA phonon heat capacity is found to be very accurate, it is not quite precise enough to reliably determine the electronic term over the whole temperature range. We demonstrate how small changes in the phonon density of states can affect the results, which underlines the extreme difficulty in accurately determining the electronic heat capacity of YBa₂Cu₃O_x.