Infrared properties of the semimetal TiSe2

The infrared spectrum of the semimetal Ti${\mathrm{Se}}_2$ has been recorded over a broad range of energies, above and below its distortion temperature $T_d=202\mathrm{}°$K, a distortion for which electron-hole coupling is the likely source. Interpretation of the spectrum is immediately complex, since we have a multicarrier situation, with strong phonons, plasmons, and band-to-band transitions (including the temperature-dependent Fermi-surface gap) all in close proximity. We have, nonetheless, extracted information on the free carriers and on the Fermi-surface gapping that is in due accord with information derived from other types of measurement. Very strong precursor effects and phonon splittings are observed in the spectra. It becomes apparent that to make further progress in interpreting the optical spectra close to $E_F$ relativistic and superlattice band-structure calculations must now be undertaken. For the low-temperature state, the extent of the role played by structural defects remains unresolved. The carrier concentration (whatever the source) that persists in the low-temperature condition is small enough here for the absorption edge between 0.1 and 0.5 eV due to the condensing superlattice to stand out rather clearly. It proves not possible, however, categorically to identify any suitable temperature-dependent feature as marking the upper limit of the Fermi-surface gapping induced with the condensation, no more here in Ti${\mathrm{Se}}_2$ than has proved the case for charge-density-wave distortions in other layered dichalcogenides.