UC Riverside

We explore the excitonic, electronic, phononic and thermal properties of low-dimensional materials, specifically the two-dimensional and quasi-one-dimensional transition metal chalcogenides. The possibility of observing Bose-Einstein exciton condensation (BEC) in transition metal dichalcogenides (TMDs) has been analyzed at three different levels of theory. We find that, in the strong coupling regime, mean field theory with either an unscreened or screened interlayer interaction predicts a room-temperature condensate. However, intralayer interactions can essentially renormalize the quasiparticle dispersion, which can be captured by many-body GW formalism. In the strong coupling regime, the improved BEC theory predicts that intralayer interactions have a large impact on the condensate order parameter, as well as on its functional dependencies on effective mass and carrier density. We also explore the thermal properties of 2D materials, specifically in the misoriented bilayer graphene (m-BLG) system, using ab initio density functional theory (DFT) and phonon Boltzmann transport equation (BTE). we find that the lattice thermal conductivity of m-BLG reduces to almost half of its unrotated counterpart. To explain the phonon dynamics, we analyze the phonon dispersions, phonon velocity distributions, occupations, density of states and heat capacity, both before and after misorientation. Detailed calculation of the phonon-phonon scattering lifetime reveals that, the increased umklapp scattering in the acoustic and quasi-acoustic phonon branches is the main reason for the reduced thermal conductivity in m-BLG system. We also explore the thermal conductivity of quasi-1D materials, specifically TaSe3 and NbS3, using ab initio DFT and phonon BTE. We find that both materials exhibit highly anisotropic thermal transport. A thermal conductivity of 6.3 W/mK (70.6 W/mK) is observed for metallic TaSe3 (semiconducting NbS3) along the chain direction. In-depth study of velocity and lifetime distribution shows that lower scattering and higher phonon velocity in NbS3 are the reasons behind such higher thermal conductivity. The umklapp scattering process is found to be the dominant phonon scattering mechanism in this family of low-dimensional materials. We also investigate the electronic and vibrational properties of different phases of the quasi-1D material NbS3. We find that the dimerized phase NbS3-IV is a semiconductor, whereas the undimerized phase NbS3-V is a metal. Similarity between the band dispersions of phase-I and phase-IV arises from the similarity in their structures, in spite of some stacking and chiral faults. Both phase-I and phase-IV are dynamically stable, whereas the phonon dispersion in phase-V exhibits instability along the inter-chain and growth direction, indicating a possible charge density wave ground state. Finally, we explore the band alignment properties of different quasi-1D transition metal trichalcogenides (TMTs). From the DFT calculations, we can identify several TMTs as promising candidates for ohmic contacts and tunnel FET devices.