Photoelectron Spectroscopy of Transition Metal Chalcogenides: Charge Density Wave Phase and Thermoelectric Performance

The successful exfoliation of graphene in 2004 ignited the interest in two-dimensional materials within the scientific community. Since then, the study of atomically thin and ultrathin layered materials has grown as an interdisciplinary field and over the years, hundreds of new two-dimensional materials have been discovered. This dissertation focuses on the transition metal dichalcogenides, which is the most commonly studied family of two-dimensional materials after graphene. Among a wide range of physical phenomena exhibited by these compounds, the incommensurate charge density wave phase of the 2H- polytype of TaS_2 is primarily analyzed in comparison to the similar compounds, 2H-NbSe_2 and 2H-TaSe_2. Angle resolved photoemission spectroscopy (ARPES) is used as the primary experimental probe for the presented band structure studies due to its unique ability to resolve both energy and momentum in order to map intricate details of the reciprocal space.

The momentum space of a 2H- transition metal dichalcogenide consists of concentric double-walled Fermi barrels about both K and Gamma high symmetry points in its normal state. Using this momentum space landscape as a blueprint, the presented work identifies both generalizable and compound-specific features of the incommensurate charge density wave order of these compounds. Reported experiments are conducted in energy, momentum, and temperature domains.

In general, the Gamma- centric Fermi surface barrels exhibit no charge density wave energy gap, while K- centric barrels are preferentially gapped. However, the details of the momentum specificity of the gap change among compounds. These variations can be explained using the differences in orientations of transition metal d-orbitals. Further, this gap is particle-hole asymmetric with respect to the chemical potential throughout the momentum space. Contrary to the expectations, a gap is observed at temperatures higher than the charge density wave transition temperature. Comparable pseudogaps have been observed in several compounds under the scientific spotlight, including high temperature cuprate superconductors. In the case of charge density waves, this pseudogap can be related to short-range ordering in the sample which remains even at higher temperatures, despite the long-range charge density wave phase coherence being diminished. When comparing momentum space maps among different samples, the sizes of the Gamma- centric Fermi surface barrels stay the same while the K- barrel sizes vary. Within the samples studied, these variations can be correlated with charge density wave transition temperatures of each compound.

Formulating a model applicable for the entire material class is of utmost importance as a basis for further understanding of these materials. However, according to preceding literature, initial attempts of explaining the formation of charge density waves using the traditional ‘Fermi surface nesting’ model have been persistently unsuccessful. On the other hand, in the case of presented work, an alternate tight binding model with strong electron-collective mode coupling is successful in explaining the reported observations. In support of this model, observed band dispersions of the samples show distinct renormalization signatures due to electrons getting coupled to some collective modes. 2H- TaS_2 was further analyzed in order to probe the underlying cause of these collective modes where they are identified as phonons. The work presented suggests that the mechanism behind the incommensurate charge density wave phase of the 2H- polytype of transition metal dichalcogenides is the electron-phonon coupling, rather than the Fermi surface nesting.

Lead chalcogenide, which is another class of transition metal chalcogenides with a three-dimensional structure is studied in order to understand their band structure and the mechanism leading to the outstanding thermoelectric performance observed above a characteristic crossover temperature. The mechanism behind this phenomenon is controversial, mainly in terms of predicted crossover temperature values. The study of n and p doped PbTe, PbSe, and PbS reveal an upper valence band with lighter holes and a lower valence band with heavier holes. This is the first experimental observation of the lower valence band of these compounds.

Upon the increase of the temperature, the lower valence band which lies below the upper valence band rises in energy and eventually crosses over to become the topmost valence band. The heavier holes in the lower valence band increase the thermal carrier density, leading to superior thermoelectric performance at higher temperatures. Additionally, the indirect nature of the bandgap helps in mitigating any adverse effects due to intrinsic carrier activation. Apart from demonstrating the details of this mechanism via a series of temperature dependent ARPES measurements, crossover temperature values are also predicted for the compounds under study. The superior thermoelectric efficiency of lead chalcogenides can be explained by the temperature dependent convergence of light and heavy hole valence bands.