Indications of proton tunneling in the proton glass (betainetphosphate)0.15(betainetphosphite)0.85from ENDOR and pulsed ESR studies

Abstract

The local proton order in the proton glass (betaine phosphate${)}_{0.15}$ (betaine phosphite${)}_{0.85}$ is studied by means of electron nuclear double-resonance and two-dimensional electron spin-echo envelope modulation (ESEEM) techniques. The experimental results allow very detailed comparisons with theoretical models such as the one-, and three-dimensional random-bond random-field (RBRF) Ising glass model. The attempt to describe the experimental behavior within the framework of the random-bond random-field (RBRF) Ising glass, which was worked out for deuteron glasses, suffers in conspicuous insufficiency at least at lower temperatures. Taking the influence of proton tunneling into account by using the stochastic model of a quantum RBRF Ising glass, the experimental results can fully be described. Furthermore, ESEEM experiments on a partially deuterated ${\mathrm{BP}}_{0.15}$${\mathrm{BPI}}_{0.85}$ crystal confirm the different local polarization behavior in the protonated and deuterated hydrogen bonds in agreement with the model expectation.