Temperature dependent percolation mechanism for conductivity in ${{\rm{Y}}}_{0.63}$ ${\mathrm{Ca}}_{0.37}$TiO₃ revealed by a microstructure study

Abstract

We have performed optical microscopy, micro-photoelectron spectroscopy, and micro-Raman scattering measurements on Y0.63Ca0.37TiO3 single crystals in order to clarify the interplay between the microstructure and the temperature dependent electronic transport mechanisms in this material. Optical microscopy observations reveal dark and bright domain patterns on the surface with length scales of the order of several to a hundred micrometers showing a pronounced temperature dependent evolution. Spatially resolved photoelectron spectroscopy measurements show the different electronic character of these domains. Using micro-Raman spectroscopy, we observe a distinct temperature dependence of the crystal structure of these domains. On the basis of these findings the different domains are assigned to insulating and metallic volume fractions, respectively. By decreasing the temperature, the volume fraction of the conducting domains increases, hence allowing the electrons to percolate through the sample at temperatures lower than ∼150 K.

Two-component electronic phase separation in the doped Mott insulator Y1−xCaxTiO3

“…Signatures of electronic phase separation have indeed been observed experimentally in Seebeck and magnetization measurements of bulk LaTiO 3+δ [18]. In bulk Y 1−x Ca x TiO 3 , low-temperature structural phase separation has been reported in the doping range x = 0.37 − 0.41 that is in the vicinity of the IMT [12,19,20], which is likely a consequence of possible electronic phase separation. However, no coupling of the IMT to a specific lattice symmetry has been observed in films of Sm 1−x Sr x TiO 3 [11].…”

“…This would then lead to a change in volume fraction of the metallic p = p * phase with temperature, and hence of the pre-edge peak intensity. Alternatively, the intensity enhancement could also be due to the temperaturedependent changes in lattice symmetry that have been reported in this doping range [12,19,20]. Note that no temperature dependence is observed within experimental error in the Ti L-edge (see [22]), indicating the absence of a change in the average Ti valence-state with temperature.…”

“…The finding clearly demonstrates the first-order nature of the IMT. As noted in the introduction, structural phase-separation into orthorhombic and monoclinic domains at low temperature is known to occur for Y 1−x Ca x TiO 3 in the doping range x = 0.37 − 0.41 [12,19,20]. This is likely the result of large strains associated with the electronic phase separation.…”

“…Prior structural studies of samples with x ≤ 0.38 showed that, whereas the system adopts an orthorhombic crystal structure at 300 K, it transitions to a monoclinic phase at ∼ 200 K [20]. Upon decreasing the sample's temperature further, a structural separation into a monoclinic phase and an orthorhombic phase was observed for x = 0.37 − 0.41 and the low temperature orthorhombic phase was argued to be metallic in character [12,19,20]. Such inhomogeneity manifested itself in our transport measurements as well.…”

“…In contrast to the wellknown Ti 4+ systems SrTiO 3 and BaTiO 3 , the Ti 3+ ion in RTiO 3 has a spin- 1 2 3d 1 electronic configuration. Recent years have seen renewed interest in the RTiO 3 materials, particularly concerning the electronic and magnetic structure of the insulating state [7,8] and the nature of the Mott transition [9][10][11][12]. The rare-earth ion R in RTiO 3 is an important factor in determining the ground Depending on the rare-earth ion, the doping level at which the system becomes metallic varies widely, from x c = 0.05 in La 1−x Sr x TiO 3 [13] to x c = 0.15 and x c = 0.35, respectively, in Nd 1−x Ca x TiO 3 [14] and Y 1−x Ca x TiO 3 [15].…”