Optical properties of TlNi2Se2: Observation of pseudogap formation

The quasi-two-dimensional nickel chalcogenides $TlN{i}_{2}S{e}_2$ is a newly discovered superconductor. We have performed optical spectroscopy study on $TlN{i}_{2}S{e}_2$ single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure $BaN{i}_{2}A{s}_2$. Both the suppression in $R(\omega )$ and the peaklike feature in ${\sigma }_1(\omega )$ suggest the progressive formation of a pseudogap feature in the midinfrared range with decreasing temperatures, which might be originated from the dynamic local fluctuation of charge-density-wave (CDW) instability. We propose that the CDW instability in $TlN{i}_{2}S{e}_2$ is driven by the saddle points mechanism, due to the existence of van Hove singularity very close to the Fermi energy.