Asymetry of critical exponents in YBCO in the overdoped, optimally doped and underdoped regime.
Résumé
We analyze very precise specific heat data of μg mass detwinned single crystals of YBa_2Cu_3O_7-δ with a relative resolution δ C/C of 10-4 and δ T/T of 10-5. Oxygen doping varies in the range δ=3D0.99 (Tc=3D89.5K, overdoped) δ=3D0.95 (Tc=3D92.3K, optimal) and δ=3D0.87 (Tc=3D9= 1.2K, underdoped). A detailed analysis of the derivative of the specific heat data reveals a pronounced asymmetry of the critical exponent α. Above Tc α ^+=3D0.5± 0.2. Below Tc α ^-=3D-0.2± 0.3. This abnormal behavior cannot be explained within the framework of regular second order phase transitions. The asymmetry of the critical exponent is robust against a change of the phonon backround or a change in the choice of Tc and persists in the overdoped, underdoped and optimally doped regime.It suggests either the existence of a 1st order phase transition in zero field or the proximity to a multicritical point at optimal doping.