The doping and disorder dependent variation of the isotope exponent in hole doped cuprates: A non-superconducting perspective

TL;DR

This paper proposes a non-superconducting perspective on the isotope exponent variation in hole-doped cuprates, emphasizing the role of lattice-charge interactions and stripe states over traditional phonon-mediated pairing.

Contribution

It introduces a scenario where isotope effects influence stripe states via lattice coupling, explaining the doping and disorder dependence of the isotope exponent.

Findings

Isotope substitution affects charge/spin stripe states.

The isotope exponent is linked to stripe modulation, not just phonon energies.

The model explains doping and disorder dependence of isotope effects.

Abstract

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.