| Summary: | 碩士 === 國立臺灣大學 === 物理研究所 === 103 === Frustrated spin system is a popular and challenging field in condensed matter physics. The complicated interplay between interactions and special geometry of the lattice leads to macroscopically degenerate ground states even at the temperatures closed to absolute zero. Among frustrated magnets, the classical spin ice is very noticeable because it is the magnetic analogue to the water ice system. Ground state degeneracy, residual entropy, and the ice-rule configurations are found in both water ice and spin ice.
In this thesis, we numerically study the dipolar spin ice model under slightly tilted magnetic field in [111] direction by classical Monte Carlo simulation. The Ewald summation and parallel tempering algorithm are applied for calculating the long-range interaction and equilibrating the system at low temperatures, respectively.
The magnetic scattering maps are produced in our simulation and are comparable to the results of neutron scattering experiment. Also, we find a phase transition and a long-range ordered state which is responsible for the anomalous critical scattering observed in single crystal of Ho2Ti2O7. The order parameter is calculated to show that the origin of ordering comes from antiferromagnetic alignment of spins on the kagome plane. The recovery of thermal entropy indicates the broken degeneracy of kagome ice phase under the perturbation of tilted field. Finally, we plot the phase diagram in different magnetic fields and temperatures, and discuss several spin configurations accounted for different phase regions.
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