NMR relaxation studies in doped poly-3-methylthiophene

K. Jugeshwar Singh, W. G. Clark, G. Gaidos, A. P. Reyes, P. Kuhns, J. D. Thompson, R. Menon, and K. P. Ramesh
Phys. Rev. B 91, 174421 – Published 19 May 2015

Abstract

NMR relaxation rates (1/T1), magnetic susceptibility, and electrical conductivity studies in doped poly-3-methylthiophene are reported in this paper. The magnetic susceptibility data show the contributions from both Pauli and Curie spins, with the size of the Pauli term depending strongly on the doping level. Proton and fluorine NMR relaxation rates have been studied as a function of temperature (3–300 K) and field (for protons at 0.9, 9.0, 16.4, and 23.4 T, and for fluorine at 9.0 T). The temperature dependence of T1 is classified into three regimes: (a) For T<(gμBB/2kB), the relaxation mechanism follows a modified Korringa relation due to electron-electron interactions and disorder. 1HT1 is due to the electron-nuclear dipolar interaction in addition to the contact term. (b) For the intermediate temperature range (gμBB/2kB)<T<TBPP (the temperature where the contribution from the reorientation motion to the T1 is insignificant) the relaxation mechanism is via spin diffusion to the paramagnetic centers. (c) In the high-temperature regime and at low Larmor frequency the relaxation follows the modified Bloembergen, Purcell, and Pound model. T1 data analysis has been carried out in light of these models depending upon the temperature and frequency range of study. Fluorine relaxation data have been analyzed and attributed to the PF6 reorientation. The cross relaxation among the 1H and 19F nuclei has been observed in the entire temperature range suggesting the role of magnetic dipolar interaction modulated by the reorientation of the symmetric molecular subgroups. The data analysis shows that the enhancement in the Korringa ratio is greater in a less conducting sample. Intra- and interchain hopping of charge carriers is found to be a dominant relaxation mechanism at low temperature. Frequency dependence of T11 on temperature shows that at low temperature [T<(gμBB/2kB)] the system shows three dimensions and changes to quasi one dimension at high temperature. Moreover, a good correlation between electrical conductivity, magnetic susceptibility, and NMR T1 data has been observed.

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  • Received 9 December 2014
  • Revised 27 April 2015

DOI:https://doi.org/10.1103/PhysRevB.91.174421

©2015 American Physical Society

Authors & Affiliations

K. Jugeshwar Singh1,*, W. G. Clark2, G. Gaidos2, A. P. Reyes3, P. Kuhns3, J. D. Thompson4, R. Menon1, and K. P. Ramesh1,†

  • 1Department of Physics, Indian Institute of Science, Bangalore-560012, India
  • 2Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, USA
  • 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA
  • 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

  • *jshwar@physics.iisc.ernet.in
  • kpramesh@physics.iisc.ernet.in

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Vol. 91, Iss. 17 — 1 May 2015

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