Temperature Dependence of the Spin-Peierls Energy Gap and Anomalous Line Shapes in CuGeO$_3$

TL;DR

This study investigates how the spin-Peierls energy gap in CuGeO$_3$ varies with temperature, revealing deviations from conventional behavior and identifying anomalous scattering features.

Contribution

It provides the first detailed measurement of the temperature dependence of the spin-Peierls energy gap in CuGeO$_3$, highlighting unusual scaling exponents and scattering phenomena.

Findings

Energy gap decreases with temperature but with a smaller exponent than expected.

Superlattice peak intensity follows a power law with exponent 0.33.

Anomalous scattering continuum observed at higher energies.

Abstract

Neutron scattering measurements on a large single crystal of CuGeO$_3$ have been used to determine the temperature-dependence of the spin-Peierls energy gap. While the power law behavior of the intensity of structural superlattice peaks is well fit by $I({\rm T})\propto ({\rm T}_c - {\rm T})^{2\beta}$ with an exponent of $\beta =0.33$, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as $\Delta({\rm T}) \propto I^a$ with $a=1/3$; the present results suggest an exponent of $a\approx 1/6$ for CuGeO$_3$. Additional scattering cross-section is observed in constant-$q$ and constant-$E$ scans creating a long `tail' extending to higher energies relating to a proposed scattering continuum.