A new excitation is observed at 201 meV in the doped-hole ladder cuprate Sr$_{ 14}$Cu$_{ 24}$O$_{ 41}$, using ultraviolet resonance Raman scattering with incident light at 3.7 eV polarized along the direction of the rungs. The excitation is found to be of charge nature, with a temperature independent excitation energy, and can be understood via an intra-ladder pair-breaking process. The intensity tracks closely the order parameter of the charge density wave in the ladder (CDW$_L$), but persists above the CDW$_L$ transition temperature ($T_{ CDW_L}$), indicating a strong local pairing above $T_{ CDW_L}$. The 201 meV excitation vanishes in La$_{ 6}$Ca$_{ 8}$Cu$_{ 24}$O$_{ 41+\delta}$, and La$_{ 5}$Ca$_{ 9}$Cu$_{ 24}$O$_{ 41}$ which are samples with no holes in the ladders. Our results suggest that the doped holes in the ladder are composite bosons consisting of paired holons that order below $T_{ CDW}$.
Categories: cond-mat.str-el