טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentEva Sagi
SubjectFrustration Driven Lattice Distortion in Y2M02O7
DepartmentDepartment of Physics
Supervisor Full Professor Keren Amit


Abstract

A frustrated system is one whose symmetry precludes the possibility that every pairwise interaction in the system can be satisfied at the same time.  Such systems can have a disordered ground state with "macroscopic" degeneracy; that is, one that comprises a huge number of equivalent states of the same energy, thus preventing the system from settling into a single ground state at zero temperature. In real materials the ground state is selected by perturbations to the Hamiltonian whose ground state is degenerate. In this work we investigate the possibility of removing the frustration of the Heisenberg AF Hamiltonian on the pyrochlore lattice, a lattice of corner sharing tetrahedra, via a magnetoelastic coupling of the spins to the lattice, which enables the lattice to distort, thus relieving the degeneracy. We chose to look for frustration-driven distortion in Y2Mo2O7, since the spin-glass transition it undergoes hints at disorder in the exchange integral, whose origin might be a lattice distortion. We studied the compound using μSR and DC magnetization, and since we obtained inconclusive magnetic evidence for a lattice distortion, we further investigated the nature of a magnetoelastic distortion in the pyrochlore lattice using computer simulations. We modeled the pyrochlore lattice in the presence of a distortion inducing term in the Hamiltonian, and sought its minimum energy state at T=0. Then we looked into the transformation this state undergoes as temperature is increased, in order to find the similarities between the idealized computer simulation and our real-life compound. We discovered that the magnetic characteristics of the ground state persist above the temperature at which the spatial signature of the distortion disappears, and this might explain why we see only magnetic evidence for a lattice distortion.