Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/16422
Title: QUANTUM MOLECULAR DYNAMICS STUDY OF THERMAL TRANSPORT IN SPIN JUNCTION SYSTEM.
Authors: IMAM MAKHFUDZ
Keywords: quantum molecular dynamics thermal transport spin
Issue Date: 7-Apr-2009
Source: IMAM MAKHFUDZ (2009-04-07). QUANTUM MOLECULAR DYNAMICS STUDY OF THERMAL TRANSPORT IN SPIN JUNCTION SYSTEM.. ScholarBank@NUS Repository.
Abstract: This thesis studies the thermal transport in spin junction with one dimensionalferromagnetic Heisenberg model. The system consists of a junction in the central part andtwo semi-infinite leads on the right and left. The leads are harmonic while the junctioncan have linear or nonlinear interactions. The dynamics is expressed in terms of a set ofequations for a dynamical variable to be solved using Greenb s function from which thefrictional and colored noise forces of the leads on the junction are calculated. Theequation will be cast into a form which resembles generalized Langevin equation. Thetransport of heat energy in spin junction is then evaluated using classical moleculardynamics.The quantum mechanical nature of spin system is approximately implemented intwo steps. First, the simulation evaluates the dynamics of quantum mechanical operatorsrather than position coordinate and momentum. Holstein b Primakoff transformation isused to convert Pauli spin matrices to Bosonic operators. Secondly, quantum mechanicalheat bath obeying quantum Bose-Einstein statistics is used for the noise spectrum of heatbath as given by the noise correlation functions.Spin wave approximation will be used extensively as linear approximation to theoriginally nonlinear Hamiltonian. The Hamiltonian of linear spin system is expandedwith respect to basis eigenstates consisting of the first excited states of ferromagneticspin. A non-diffusive, ballistic transport is obtained from simulation and this simplestcase can directly be compared with other methods such as NEGF which is found to givethe same result. The quantum molecular dynamics with exact and fully nonlinear force ofspin-spin interaction is shown to produce non-ballistic, diffusive thermal transport. Wethen study possible thermal rectification effect in this spin junction by imposingasymmetry on the coupling constants between junction and leads on the two sides.Thermal rectification does occur and the variation of thermal rectification against degreeof asymmetry in the coupling constant is investigated.Several variants of spin junctions that can be used as spin-based thermal switchdevices are studied. The first model relies on field strength as switching parameter. Thesecond model uses change in the orientation of magnetic field to change thermalconductance provided the spin-spin coupling is weak relative to the magnetic field. Thesemodels therefore have the potential to be used as thermal switches with magnetic fieldstrength and orientation as the respective switching parameters.
URI: http://scholarbank.nus.edu.sg/handle/10635/16422
Appears in Collections:Master's Theses (Open)

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