Show simple item record

dc.contributor.authorTabei, Seyed Mohiaddeen Ali
dc.date.accessioned2008-08-25 18:29:50 (GMT)
dc.date.available2008-08-25 18:29:50 (GMT)
dc.date.issued2008-08-25T18:29:50Z
dc.date.submitted2008
dc.identifier.urihttp://hdl.handle.net/10012/3888
dc.description.abstractThe LiHo_xY_{1-x}F_4 magnetic material in a transverse magnetic field B_x perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in pure and random disordered systems. We first present analytical and numerical evidences for the validity of an effective spin-1/2 approach to the description of a general dipolar spin glass model with strong uniaxial Ising anisotropy and subject to weak B_x. We relate this toy model to the LiHo_xY_{1-x}F_4 transverse field Ising material. We show that an effective spin-1/2 model is able to capture both the qualitative and quantitative aspects of the physics at small B_x. After confirming the validity of the effective spin-1/2 approach, we show that the field-induced magnetization along the x direction, combined with the local random dilution-induced destruction of crystalline mirror symmetries generates, via the predominant dipolar interactions between Ho^{3+} ions, random fields along the Ising z direction. This identifies LiHo_xY_{1-x}F_4 in B_x as a new random field Ising system. We show that the random fields explain the smearing of the nonlinear susceptibility at the spin glass transition with increasing B_x. In this thesis, we also investigate the phase diagram of non-diluted LiHoF_4 in the presence of B_x, by performing Monte-Carlo simulations. A previous quantum Monte Carlo (QMC) simulation found that even for small B_x where quantum fluctuations are small, close to the classical critical point, there is a discrepancy between experiment and the QMC results. We revisit this problem, focusing on weak B_x close to the classical T_c, using an alternative approach. For small B_x, by applying a so-called cumulant expansion, the quantum fluctuations around the classical T_c are taken into account perturbatively. We derived an effective perturbative classical Hamiltonian, on which MC simulations are performed. With this method we investigate different proposed sources of uncertainty which can affect the numerical results. We fully reproduce the previous QMC results at small B_x. Unfortunately, we find that none of the modifications to the microscopic Hamiltonian that we explore are able to provide a B_x-T phase diagram compatible with the experiments in the small semi-classical B_x regime.en
dc.language.isoenen
dc.publisherUniversity of Waterlooen
dc.subjectMagnetismen
dc.subjectQuantum Magnetismen
dc.subjectIsingen
dc.subjectQuantum Phase Transitionen
dc.subjectSpin Glassen
dc.subjectRandom Fielden
dc.subjectMean Fielden
dc.subjectMonte Carloen
dc.subjectEffective Hamiltonianen
dc.subjectPerturbative Monte Carloen
dc.subjectQuantum Monte Carloen
dc.titleInterplay of Disorder and Transverse-Field Induced Quantum Fluctuations in the LiHo_xY_{1-x}F_4 Ising Magnetic Materialen
dc.typeDoctoral Thesisen
dc.pendingfalseen
dc.subject.programPhysicsen
uws-etd.degree.departmentPhysics and Astronomyen
uws-etd.degreeDoctor of Philosophyen
uws.typeOfResourceTexten
uws.peerReviewStatusUnrevieweden
uws.scholarLevelGraduateen


Files in this item

Thumbnail

This item appears in the following Collection(s)

Show simple item record


UWSpace

University of Waterloo Library
200 University Avenue West
Waterloo, Ontario, Canada N2L 3G1
519 888 4883

All items in UWSpace are protected by copyright, with all rights reserved.

DSpace software

Service outages