Electronic structure ofY2−xCaxBaNiO5from photoemission and inverse photoemission

We investigate the electronic structure of a one-dimensional spin-1 system, ${\mathrm{Y}}_2{\mathrm{BaNiO}}_5,$ and hole-doped ${\mathrm{Y}}_{2-x}{\mathrm{Ca}}_x{\mathrm{BaNiO}}_5$ for $x=0.2\mathrm{}$ and 0.4, employing photoemission and inverse photoemission spectroscopies. ${\mathrm{Y}}_2{\mathrm{BaNiO}}_5$ has a charge excitation gap of about 2.3 eV. Hole doping via Ca substitution in this system introduces new localized states within this gap. The existence of a finite gap $(\sim 0.6$ eV), even in doped compounds with a variable-range-hopping transport, indicates the importance of correlation effects and disorder in determining the electronic structure in this system.