In this project, we have employed density functional theory, pseudo potential approach and the generalized gradient approximation to study dynamical, structural and electronic properties of bulk MnAs and its first diameter nanowire in the hexagonal NiAs type structure (?-MnAs) and ferromagnetic state. We report vibrational properties including Gruneisen parameter at ?-point, elastic constants, thermal properties for MnAs bulk and nanowire. In the bulk structure, we obtained a metallic density of state (with no energy gap at the Fermi level) and an exchange splitting of about 4 eV. Moreover, the calculated partial density of states diagrams in the majority and minority spin channels, indicate hybridization of the As p and Mn d orbitals around the Fermi level. Next, we calculated phonon density of states as well as phonon band structure of bulk MnAs along high symmetry directions in the first Brillouin zone. These calculations are performed by using density functional perturbation theory technique implemented in PWscf. The phonon calculations were done in two steps. First, the phonon spectra of the Brillouin zone center (Gamma ponit) and, then the phonon spectra on a uniform k mesh in whole Brillouin zone was computed. We extracted Gruneisen parameter from the Gamma point phonon spectra and observed the softening of the acoustical modes under pressure, indicating the weakening of the effective force constants under pressure. The phonon spectra were used to calculate the mechanical properties such as elastic constants and the thermal properties including specific heat, entropy and the thermal expansion coefficient of bulk ?-MnAs.Then, following similar procedure to the bulk phonon study, we calculated the phonon spectra of a narrow ?-MnAs nanowire which is a linear structure repeated in the z direction and confined in the xy plane. We extracted Gruneisen parameter of MnAs nanowire that explains the acoustical mode are softening under pressure which means that the effective force constants for these modes are weakened. The specific heat of MnAs nanowire as a function of temperatures was determined to be greater than bulk structure. This behavior was attributed to some surface effects in nanowire. The calculated Young modulus of nanowire was found to be more than bulk MnAs, indicating more hardness of nanowire. Moreover, it was observed that the tensile Young moduli of bulk MnAs and MnAs nanowire are smaller than their compressive Young moduli. hyhyhyhy;Keywords: Density Functional Perturbation Theory, Phonon Spectra, Gruneise Parameter, Elastic Constant, Heat Capacity, Bulk ?-MnAs, ?-MnAs Nanowire.
