Infrared Absorption from Charge Density Waves in Magnetic Manganites

The infrared absorption of charge density waves coupled to a magnetic background is first observed in two manganites ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ with $x=0.5\mathrm{}$ and $x=0.67\mathrm{}$. In both cases a BCS-like gap $2\Delta \left(T\right)$, which for $x=0.5\mathrm{}$ follows the hysteretic ferromagnetic-antiferromagnetic transition, fully opens at a finite $T_0<T_N$, with $2\Delta \left(T_0\right)/k_B{T}_c\simeq 5$. These results may also explain the unusual coexistence of charge ordering and ferromagnetism in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$.