ISOTOPE EFFECT AND ELECTRONIC MECHANISMS

The observed very small value for the isotope effect coefficient beta in YBa2Cu3O7 and the Bi and Tl copper oxides is consistent with a mechanism that is mainly electronic in origin and is widely regarded as inconsistent with pure phonons. More recently, however, values of beta have been found in La2-xSrxCu O4 to vary widely with impurity concentration and to sometimes take on a value even above the canonical BCS value of 0.5. Another example is Y1-xPrxBa2Cu3 O7 in which beta rises to almost 0.5 as the critical temperature is reduced to 30K on Pr doping. We will argue that such striking variation in beta as well as large values, do not necessarily imply a large phonon contribution to the T(c) value of the parent stoichiometric compound and do not invalidate electronic mechanisms such as the Marginal Fermi Liquid model (MFL) augmented, of course, with a small phonon contribution. Here we will treat this electronic model more explicitly than others because it has had considerable recent success in explaining some striking experimental results.