  Since the early days of the discovery of hole doped high-Tc cuprates, the
variation of the oxygen isotope exponent (IE) with the number of doped holes,
p, in the CuO2 planes has been a source of considerable debate. There is a
growing acceptance in the high-Tc community that the mediating bosons leading
to Cooper pairing have largely electronic character. At the same time there is
an increasing acknowledgement that at some level the lattice-charge
interactions might be important in cuprates. The experimentally observed
substantially large IE over certain doping range always casts a shadow over any
proposal where non-phononic mechanism is invoked. Besides, the various existing
theoretical schemes, based on electron-phonon interactions, cannot describe the
anomalous features shown by the IE as a function of hole
concentration/disorder, either quantitatively or qualitatively. Based on a
recent experiment relating the possibility of Fermi-surface reconstruction to
the thermoelectric transport measurements (Nature Commun. 2:432 doi:
10.1038/ncomms1440 (2011)), we propose here a simple scenario where isotope
substitutions affect the charge/spin stripe state via the coupling to the
underlying lattice and thereby change the superconducting Tc. In this picture,
significant part of the IE, over an extended p-range, actually originates from
the isotope induced stripe modulation and is not directly related to the
characteristic energy scale of the pairing phonons as is the case in
conventional BCS theory. Our proposal qualitatively explains all the disorder-
and p-dependent features of the IE seen in hole doped high-Tc cuprates. We also
provide with an outline of some experiments that can verify the degree of
validity of the proposed scenario.
