Type of Document	Dissertation
Author	Yildirim, Yucel
Author's Email Address	yy03@fsu.edu, can_671@msn.com,yildirim@ornl.gov
URN	etd-07032007-142651
Title	Numerical Study of Spin-Fermion Models for Diluted Magnetic Semiconductors and High Tc Cuprates
Degree	Doctor of Philosophy
Department	Physics, Department of
Advisory Committee	Advisor Name Title Nicholas E. Bonesteel Committee Chair Jorge Piekarewicz Committee Member Naresh Dalal Committee Member Oskar Vafek Committee Member Peng Xiong Committee Member
Keywords	Spin-Fermion Models Realistic Lattice Model High Temperature Cuprates Multi Band Model Diluted Magnetic Semiconductors (DMS) Electron-phonon Interactions
Date of Defense	2007-05-30
Availability	unrestricted
Abstract In this dissertation, Spin-Fermion (SF) models for diluted magnetic semiconductors and high temperature superconducting cuprates are constructed and studied with unbiased numerical techniques. A microscopic model to describe magnetically doped III-V semiconductors is proposed. This model includes the appropriate lattice geometry, as well as, magnetic, spin-orbit, and Coulomb interactions and contains no free parameters. Its study using state-of-the-art numerical techniques provides results in excellent agreement with experimental data for $Mn$ doped GaAs. For the first time, Curie-Weiss behavior of the magnetization is obtained numerically and the values of the Curie temperature are reproduced in a wide range of $Mn$ doping and compensations. We observed that for $xge 3\%$, the holes are doped into the valence band and uniformly distributed in the material. This could support the ``valence band' scenario regarding this material. Phononic degrees of freedom, which are often neglected in studies of high $T_c$ cuprates, are considered in a numerical study of a spin-fermion model. Both diagonal and off-diagonal electron-phonon interactions are considered. While diagonal terms tend to stabilize ordered structures such as stripes, the off-diagonal terms introduce disorder making this structures more dynamical. Our results indicate that phonons play a role in the stabilization of stripe-like states.
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