Magnetoresistance anisotropy and transport properties of tungsten ditelluride
Thoutam, Laxman Raju
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The quest for the materials showing high magnetoresistance values is ongoing and scientists come up with new methods and materials to harness their vast potential in modern electronic and sensing applications. Extreme large magnetoresistance (XMR) was recently discovered in tungsten ditelluride (WTe₂), triggering extensive research on this material regarding the electronic properties and the origin of XMR. WTe₂ being a layered compound with metal layers sandwiched between adjacent insulating chalcogenides layers is considered to be electronically two dimensional (2D), but the findings of this dissertation reveal interesting three dimensional (3D) electronic properties with a small temperature dependent mass anisotropy. The 3D electronic properties are revealed by 3D scaling behavior of the resistance R(H,θ)=R(εθH) with εθ=(cos²θ+γ⁻²sin²θ)¹/², θ being the magnetic field angle with respect to the c-axis of the crystal and γ being the mass anisotropy. The mass anisotropy γ has a value of 2 at high temperatures (T≥100K) and increases to a value of 5 as temperature is lowered and follows the magnetoresistance behavior of the Fermi liquid state. The general scaling behavior introduced to understand the anisotropical magnetoresistance in WTe₂ can be applied to many other systems to study the angle dependence of the magnetoresistance. The dissertation also focuses on explaining the origin of the turn-on temperature behavior and XMR in WTe₂ based on a simple scaling behavior MR~(H/ρ₀)^m with m≈2 and ρ₀ being the zero field resistivity, that is derived from the semi-classical two band model. The scaling behavior can be accounted for the observed temperature and the magnetic field dependence of the XMR in WTe2 and leads to a simple quantitative expression for the resistivity ρ*≈2ρ₀ at the onset of XMR behavior. The experimental results unambiguously demonstrate that the turn-on temperature behavior is not indicative of a metal-insulator transition, but in fact of a high quality and low charge carrier density sample with a small residual resistivity, high mobility of the charge carriers and large residual resistance ratio following Kohler's rule in a magnetic field. This work resolves the origin of the turn-on behavior observed in several XMR materials and also provides a general route for a quantitative understanding of the temperature dependence of MR in both XMR and non-XMR materials.