Softening of the lattice [#!PoirierPRB95a!#,#!LorenzoPRB94!#,#!HarrisPRB94!#,#!NishiPRB95!#,#!ChenPRB95!#]

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Softening of the lattice [#!PoirierPRB95a!#,#!LorenzoPRB94!#,#!HarrisPRB94!#,#!NishiPRB95!#,#!ChenPRB95!#]

Poirier et al. investigated elastic constants with ultrasonic velocity measurements. They observed a softening of the lattice in the chain direction, starting at $\sim 3$ K above $T_{\rm SP}$ [148]. This softening may be induced by the spin Peierls transition.

In contrast, neutron scattering measurements [149] have found a soft phonon mode perpendicular to the chain (b-axis direction). In addition, X-ray diffraction measurements [150] found a large contraction of the b-axis lattice parameter at $T_{\rm SP}$ .Neutron scattering measurements under pressure [151] showed a pronounced decrease of the b-axis lattice parameter.

Chen et al. observed diffuse scattering below $\sim$ 100 K with electron diffraction measurements [152]. They proposed a twisting mode of the CuO₂ ribbons to explain their results.