محاسبه خواص فونوني، مكانيكي و گرمايي بلور و نانوسيم MnAs

چكيده انگليسي :

Calculation of Phonon Mechanical and Thermal Properties of MnAs Bulk and Nanowire Elahe Teimouri e teimouri@ph iut ac ir Date of Submission 2011 02 16 Department of Physics Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisors Hadi Akbarzadeh Abstract 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 All these calculations are based on Density Functional theory DFT and Density Functional Perturbation theory DFPT by using PWscf Software pseudo potential computations with Generalized Gradient Approximation GGA for the exchange correlation term Density Functional Perturbation theory is employed to study the phonon based properties of MnAs in the bulk and nanowire structure 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 modes 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 Keywords Density Functional Perturbation Theory Phonon Spectra Gruneisen Parameter Elastic Constant Heat Capacity Bulk MnAs MnAs Nanowire

تيموري، الهه

محاسبه خواص فونوني، مكانيكي و گرمايي بلور و نانوسيم MnAs

نظريه تابعي چگالي اختلالي , ضريب گرونيس , ثابت سختي كشساني , ظرفيت گرمايي , بلور a-MnAs