Giant dielectric permittivity in Nb-doped rutile crystals

Dielectric properties of Nb-doped (~1.5 at.%) rutile single crystals were studied in the 10-300 K temperature range (at frequencies below the MHz range down to 0.3 K) in a broad frequency range, including terahertz and infrared range, to understand the origin of its giant permittivity. The results were fitted, modelled and compared with those of the undoped rutile crystal measured in the terahertz and infrared ranges. The primary effect originates from the near-electrode depletion layer of lower conductivity compared to the bulk (surface barrier-layer capacitor effect), which causes a strong thermally activated relaxation in the MHz dielectric spectra. In the higher frequency range, the main difference between doped and undoped crystals is the presence of an overdamped microwave excitation (central mode) in the doped crystal for both polarizations, persisting down to 10 K and not thermally activated. This accounts for the previously reported increased permittivity values down to 2 K – where all lower-frequency relaxations are frozen – compared to undoped crystals. It also explains why our low-frequency permittivity at 0.3 K exceeds its THz value. In similar Nb-doped rutile single crystals it was quite recently ascribed [V. A. Dinh et al., Phys. Rev. B 114, 024112, (2026)] to microwave tunneling of molecular polarons. Polar phonons are affected only by a slightly increased damping.