Polarity reversal of the charge carrier in tetragonal TiH_{x}(x=1.6−2.0) at low temperatures

Abstract

We present a combined experimental and theoretical study of the charge transport properties of TiH_{x}(x=1.6–2.0) epitaxial thin films. We found that the Hall coefficient of TiH_{x} strongly depends on hydrogen content and unit-cell volume: Nearly stoichiometric TiH_{x}(x≈2.0) films with large unit-cell volumes showed positive Hall coefficients at 4 K, whereas TiH_{x} samples with x<∼1.7 and small unit-cell volumes showed negative Hall coefficients at 4 K. Our density functional theory calculations reveal that the volume change leads to the change in the aspect ratio of the tetragonal lattice, thereby lifting the degeneracy of Ti t_{2g} states, and alters the contributions of electrons and holes at the Fermi surface and the sign of the Hall coefficient. The present study clarifies the important role of the lattice symmetry in determining the charge carrier polarity, and we suggest that electronic properties of metal hydrides can be tuned by the lattice parameters via the hydrogen contents.