Polarized optical reflectivity is studied in single crystals of BaCo1−xNixS2 over a wide compositional range 0 ≤ x ≤ 0.28 from the antiferromagnetic insulating to the paramagnetic metallic phase. BaCoS2 is a charge-transfer-type Mott insulator with an anisotropic optical gap of about (or less than) 1 eV for E⊥c (parallel to the CoS plane) and 3.5 eV for Ec (perpendicular to the CoS plane). Ni substitution for Co has different effects on the in-plane σab(ω) and out-of-plane σc(ω) conductivities. As the Ni substitution proceeds, a Drude-like peak develops in σab(ω) above x=0.18, while a mid-infrared absorption band develops and a Drude-like peak is not confirmed in σc(ω). This indicates that the two-dimensionality is stronger than predicted from band calculations. However, the mid-infrared spectral weight induced in σc(ω) is much larger than that of high-Tc cuprates. This suggests that the charge carriers are not so tightly confined within the plane as in high-Tc cuprates. The looseness of the charge confinement probably originates from a strong interlayer-coupling effect via the 3d3z2−r2 orbital.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2001|
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics