Local Atomic Effects in the Phase Transition of TiSe2-x Tex and NaMnBi Semiconductors

Studies of the effects of disorder in semiconducting systems has been central to the search for materials with novel properties. By introducing atomic inhomogeneities through doping or by controlling the growth conditions, the ground state of the system can change significantly even when the average symmetry of the system does not. Intimately connected with this are the local interactions involving the charge, lattice, and spin degrees of freedom that are crucial to understanding the physical properties of materials. Two systems were investigated in this work to explore the effects of the lattice disorder on the properties. One is TiSe2, the other is NaMnBi.

Two exemplary systems have been explored in this thesis: the transition metal dichalcogenides (TMD) and the I-Mn-V semiconductors. The TMDs are a widely studied class of materials. Due to their quasi-two dimensional nature, they offer a window into the physics of low dimensional systems. At the same time, TMDs are promising for use in lightweight, flexible thin film applications. Many TMDs exhibit what is commonly referred to as a charge density wave (CDW) instability in which charge modulations bring about a periodic lattice distortion. The CDW typically occurs in competition with other phases, such as superconductivity. TiSe2 is such a TMD with a 2 × 2 × 2 CDW order that sets in below 200 K. Upon further cooling it becomes superconducting under pressure and with Cu intercallation. Many mechanisms have been proposed to explain the CDW transition that include Fermi surface nesting, exciton condensation, and Jahn-Teller distortions. In this work, we show that a pseudo Jahn-Teller mechanism is most likely responsible for the CDW transition. The intrinsic distortions that arise from the local symmetry breaking due to Jahn-Teller serve as a signature of the charge modulation.

A second system investigated is NaMnBi. This is a semiconductor belonging to the I-Mn-V class of antiferromagnets. The I-Mn-V class is promising for spintronics applications due to their high Néel transition temperatures, their compatibility with existing substrates, and high anisotropy that could lead to spin-orbit torque, crucial to spintronic devices. Moreover, NaMnBi exhibits extreme magnetoresistance never observed before in this class. Such a property can be of tremendous importance in GMR devices. Therefore, this work has opened up a venue where the physics of antiferromagnetic semiconductors can be explored.

Key to this work are the effects from the lattice distortions on phase transitions in these two classes of materials. Neutron scattering has been used to study the magnetic and atomic structures. In the case of TiSe2, doping with Te as in TiSe2-xTex leads to suppression of the CDW transition between x = 0.2 and 0.25. The local distortions in the CDW phase are found to be inconsistent with the existing models, and a local model with breathing distortions that significantly shorten a fraction of the Ti-Se nearest neighbor bonds is found. This indicates that a pseudo Jahn-Teller effect that lowers the Se p bands can explain the CDW formation. In NaMnBi, with a Néel temperature of 340 K, a positive magnetoresistance of up to 10000% at 2 K and 9 T when vacancies are introduced to the lattice, the highest reported for this class of material. In addition to the high magnetoresistance, a structural phase transition is observed leading to a superlattice with a polaron formation and a change in the coordination geometry of the Mn atoms. The temperature of the superlattice transition is suppressed upon reducing the Bi vacancy content. The structural phase transition leads to a broken inversion symmetry, which is useful for spin transfer torques. The large magnetoresistance and broken inversion symmetry make NaMnBi a promising candidate material for spintronic applications.