Abstract
MoTe2 is a versatile transition metal dichalcogenide known for its intriguing electronic properties, including a high electron mobility and significant magnetoresistance, making it a promising material for advanced electronic and spintronic applications. It is also a type II Weyl semimetal with three different polytype forms; monoclinic, orthorhombic, and hexagonal. Its crystalline structure is composed of two dimensional layers, each layer made of Mo4+ ions and Te-2 ions connected by strong covalent bonding in a zig-zag pattern. The weak interlayer coupling, known as van der Waals (vdW), enables the layers to slide over each other, rearranging the structure and allowing transitions across the different polytypes. Electronic and topological properties have been found to differ among the three polytypes.
MoTe2 undergoes a topological phase transition when cooled from approximately 350 K, transitioning from a monoclinic 1T′ to an orthorhombic Td structure at lower temperatures, with the phase transition nominally occurring around 250 K. The topological properties of MoTe2 can be influenced by external factors such as external fields, defects, disorder, chemical doping, and pressure. In this thesis, the structural phase transition of MoTe2 under strain is explored using elastic neutron scattering. Moreover transport across the vdW layers is measured for the first time. Transport measurements were conducted in two planes: the in-plane (layer growth plane) and the out-of-plane (layer stacking plane). In-plane transport measurements revealed a sudden increase in the gradient of the resistivity curve, as a function of temperature, indicating the point at which the structural transition occurs. The inplane resistivity curve forms a closed hysteresis loop between 200 and 270 K, within which the structural transition occurs. In contrast, the out-of-plane resistivity measurements, displayed a broad, open hysteresis loop from 200 up to 350 K. This observation accounts for layer sliding in the out-of-plane direction, where each layer must be perfectly positioned after sliding to form a specific crystal structure. Broad diffuse scattering occurs within this temperature range. Applying pressure to MoTe2 alters the temperature at which phase transitions occur, affecting its electronic structure and potentially leading to new topological phases. In this thesis, uniaxial strain was applied using a mechanical pressure cell in the out-of-plane direction of a cubic MoTe2 crystal, followed by elastic neutron scattering. An increase in applied pressure/force led to lowering of the structural phase transition temperature. Neutron scattering experiment revealed changes in the intensities of the out-of-plane 201 and 201 peaks under strain. Variation in the intensity of these peaks as a function of pressure was observed, and further measurements will be conducted to precisely clarify the origin of these results. Moreover the broad diffuse scattering spectrum will be discussed by analyzing the diffuse scattering along the h0l and 0kl planes in contrast to the hk0 plane, where there is no diffuse scattering using the data acquired from TOPAZ, a high-resolution single-crystal diffractometer.
