On the Optical Properties of the Misfit Layer Compound (PbSe)1+δ(NbSe2)n

Abstract

Misfit layer compounds (MLCs) are layered materials that consist of two different sublattices with a mismatch in at least one of their lattice parameters. This incommensurate mismatch along with charge transfer between the constituent sublattices gives rise to interesting properties and can potentially be tuned for practical applications. These systems can be understood using the framework of rigid band model and the changes in band filling depending on the extent of interlayer charge transfer. This thesis presents the optical properties of (PbSe)1+δ(NbSe2)n and their evolution as the number of NbSe2 layers, n, are varied. NbSe2 is a known transition metal dichalcogenide superconductor and PbSe is a semiconductor with a rocksalt structure. NbSe2 and n=1-3 samples were synthesized via chemical vapour transport and characterized via X-ray diffraction, energy dispersive X-ray spectroscopy, and atomic force microscopy. Further characterization from resistivity and magnetization measurements showed a decrease in the superconducting transition temperature, Tc, with decreasing n. Reflectivity measurements were analyzed using the Drude-Lorentz model and reveal a shift in the plasma frequency to lower energies as n decreases along with the broadening of their interband transitions when compared to pure NbSe2. This behaviour is consistent with increased band filling due to charge transfer from PbSe to NbSe2. Optical conductivity functions were obtained using Kramers-Kronig analysis confirm decreased conductivity for the MLCs and follow a similar pattern found in the resistivity measurements. Finally, frequency-dependent scattering rate and effective mass were obtained from the extended Drude model. Results show a region of purely Drude response in the scattering rate for the MLCs and a trend of decreasing mass enhancement with decreasing n at low wavenumber.