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arXiv · 1312.0633

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

P. S. Häfliger·S. Gerber·R. Pramod·V. I. Schnells·B. dalla Piazza·R. Chati·V. Pomjakushin·K. Conder·E. Pomjakushina·L. Le Dreau·N. B. Christensen·O. F. Syljuåsen·B. Normand·H. M. Rønnow
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Abstract

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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P. S. Häfliger, S. Gerber, R. Pramod, V. I. Schnells, B. dalla Piazza, R. Chati, V. Pomjakushin, K. Conder, E. Pomjakushina, L. Le Dreau, N. B. Christensen, O. F. Syljuåsen, B. Normand, H. M. Rønnow. 2013-12-02. Quantum and thermal ionic motion, oxygen isotope effect, and superexchange distribution in La$_2$CuO$_4$. https://doi.org/10.1103/physrevb.89.085113

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