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Quantum and thermal ionic motion, oxygen isotope effect, and superexchange distribution in La$_2$CuO$_4$

  • Häfliger, P. S.
  • Gerber, S.
  • Pramod, R.
  • Schnells, V. I.
  • Piazza, B. dalla
  • Chati, R.
  • Pomjakushin, V.
  • Conder, K.
  • Pomjakushina, E.
  • Dreau, L. Le
  • Christensen, N. B.
  • Syljuåsen, O. F.
  • Normand, B.
  • Rønnow, H. M.
Published Article
Publication Date
Dec 02, 2013
Submission Date
Dec 02, 2013
DOI: 10.1103/PhysRevB.89.085113
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We study the zero-point and thermal ionic motion in La$_2$CuO$_4$ by means of high-resolution neutron diffraction experiments. Our results demonstrate anisotropic motion of O and to a lesser extent of Cu ions, both consistent with the structure of coupled CuO$_6$ octahedra, and quantify the relative effects of zero-point and thermal contributions to ionic motion. By substitution of $^{18}$O, we find that the oxygen isotope effect on the lattice dimensions is small and negative ($-0.01\%$), while the isotope effect on the ionic displacement parameters is significant ($-6$ to $50\%$). We use our results as input for theoretical estimates of the distribution of magnetic interaction parameters, $J$, in an effective one-band model for the cuprate plane. We find that ionic motion causes only small ($1\%$) effects on the average value $\langle J\rangle$, which vary with temperature and O isotope, but results in dramatic ($10$-$20\%$) fluctuations in $J$ values that are subject to significant ($8$-$12\%$) isotope effects. We demonstrate that this motional broadening of $J$ can have substantial effects on certain electronic and magnetic properties in cuprates.

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