Anisotropic Electrical Conductivity of Oxygen-Deficient Tungsten Oxide Films with Epitaxially Stabilized 1D Atomic Defect Tunnels

Modulation of electrical and thermal transports through lattice distortion in BaTi1-xNbxO3solid solutions

The electron and heat transports in solids are through the movement of carrier electrons and quantized lattice vibrations (phonons), which are sensitive to the lattice distortion and ionized impurities, and are essential aspects for the development of novel thermoelectric materials. In this study, we systematically investigated the modulations of electrical and thermal conductivities of BaTi_1-xNb_xO₃solid solution (BTNO, 0 ≤ x ≤ 1) epitaxial films. At room temperature, BaTiO₃belongs to tetragonal perovskite and exhibits electron conduction through doubly degenerated Ti 3d-t_2gorbitals upon doping, while BaNbO₃belongs to cubic perovskite and exhibits metallic electron conduction through partially filled triply degenerate Nb 4d-t_2gorbitals. By controlling the Ti/Nb ratio, we found a dual modulation effect on both the lattice structures and conduction band, which affects the electrical and thermal conductivities. Similar to the SrTi_1-xNb_xO₃solid solution (STNO, 0 ≤ x ≤ 1) system, a phase transition was detected atx ∼ 0.5, at which both the electron and heat transports exhibit abrupt changes. Unlike the transition in STNO, which was attributed to a polaronic phase transition, the transition in BTNO was due to contributions from both the lattice distortion and polaron effect. By controlling the lattice distortion, conduction band, and polaronic phase transitions, the electrical and thermal conductivity of BTNO epitaxial films are modulated within a much greater range than those of the STNO epitaxial films. Due to the double contribution of electron carriers and phonon to thermal conductivity (κ), the maximumκmodulation ratio of BTNO epitaxial films was ∼6.9. Our research provides an effective route to design electrical/thermal management materials.

Anisotropic magnetoresistance and memory effect in bulk systems with extended defects

To describe kinetic phenomena in disordered conductors, various acts of scattering of electrons can be often considered as independent, that is captured by the Boltzmann equation. However, in some regimes, especially, in a magnetic field, it becomes necessary to take into account the correlations between different scattering events of electrons on defects at different times in the past. Such memory effects can have a profound impact on the resistivity of 2D semiconductor systems, resulting in giant negative magnetoresistance and microwave-induced resistance oscillations phenomena. This work opens the discussion of the memory effects in 3D conducting systems featured by the presence of extended one-dimensional defects, such as screw dislocations or static charge stripes. We demonstrate that accounting for the memory effect, that is the capture of electrons on collisionless spiral trajectories winding around extended defects, leads to the strong negative magnetoresistance in case when the external magnetic field direction becomes parallel to the defects axis. This effect gives rise to a significant magnetoresistance anisotropy already for an isotropic Fermi surface and no spin-orbit effects. The proposed resistivity feature can be used to detect one-dimensional scattering defects in these systems.