Charge dynamics of doped holes in one-dimensional $S=1\mathrm{}$ Haldane-gap system ${\mathrm{Y}}_{2-x}{\mathrm{Ca}}_x{\mathrm{BaNiO}}_5$

The evolution of the anisotropic optical spectra of ${\mathrm{Y}}_{2-x}{\mathrm{Ca}}_x{\mathrm{BaNiO}}_5$ with x demonstrates that holes are doped into the parent insulator and that the dynamics of the holes is one dimensional. In the conductivity for $\mathbf{E}\Vert$ chain a single peak develops with doping, centered at $E\sim 1\hspace{0.5em}\mathrm{eV}$ and within the charge-transfer gap of the parent insulator. The peak energy is almost unchanged and its spectral weight increases in proportion to x. This feature indicates that the doped holes form local bound states rather than a charge ordered state as is realized in the two-dimensional ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_{4+\delta }.$