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Correlations in Frustrated Magnets: Classical and Quantum Aspects
Stockholm University, Faculty of Science, Department of Physics.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, we study three different aspects of the physics of frustrated magnets. First, we study the interplay of thermal and quantum fluctuations in the transverse field frustrated Ising model on the triangular lattice. By means of an efficient Monte Carlo method based on a continuous time algorithm, we map out the phase diagram of the model. The unperturbed classical model (i.e. without transverse field) does not show any order at finite temperatures. Adding quantum fluctuations leads to ordering at zero temperature and adding thermal fluctuations leads to unusual melting via an extended intermediate Kosterlitz-Thouless phase.

Second, we study the magnetization properties of spin ice in a magnetic field in the [111] direction by a Monte Carlo method and a number of analytical techniques. An efficient Monte Carlo loop algorithm was developed which allows us to simulate the system at low fields with very high accuracy. We also find a giant peak in the entropy at the transition point between two plateaux. This peak is due to the crossing of an extensive number of energy levels at the transition point. The peak is well described quantitatively by a simple Bethe approximation.

Third, we study the cooperative paramagnetic correlations in a number of frustrated classical magnets with O(N) symmetry, including pyrochlore, kagome, and triangular lattices, by Monte Carlo and large-N methods. We find that correlations have a dipolar form giving rise to sharp bow-tie shape features in the structure factor. A detailed comparison of the N = 1,3 and N = ∞ correlation functions has been made. We find that the N = ∞ correlations reproduce the correlations for finite N with remarkable quantitative accuracy without any fitting parameter. We show that 1/N corrections do not alter the asymptotic behavior of the correlation functions.

Place, publisher, year, edition, pages
Stockholm: Fysikum , 2004. , 82 p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-184ISBN: 91-7265-880-0 (print)OAI: oai:DiVA.org:su-184DiVA: diva2:190643
Public defence
2004-06-03, sal FA31, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Available from: 2004-05-13 Created: 2004-05-13Bibliographically approved
List of papers
1. Interplay of quantum and thermal fluctuations in a frustrated magnet
Open this publication in new window or tab >>Interplay of quantum and thermal fluctuations in a frustrated magnet
2003 (English)In: Physical Review B, ISSN 1098-0121, Vol. 68, no 10, 104409- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate the presence of an extended critical phase in the transverse field Ising magnet on the triangular lattice, in a regime where both thermal and quantum fluctuations are important. We map out a complete phase diagram by means of quantum Monte Carlo simulations, and find that the critical phase is the result of thermal fluctuations destabilising an order established by the quantum fluctuations. It is separated by two Kosterlitz-Thouless transitions from the paramagnet on one hand and the quantum-fluctuation driven three-sublattice ordered phase on the other. Our work provides further evidence that the zero temperature quantum phase transition is in the three-dimensional XY universality class.

Place, publisher, year, edition, pages
American Physical Society, 2003
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
urn:nbn:se:su:diva-23203 (URN)10.1103/PhysRevB.68.104409 (DOI)
Note
Part of urn:nbn:se:su:diva-184Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2010-01-04Bibliographically approved
2. Magnetization process of spin ice in a [111] magnetic field
Open this publication in new window or tab >>Magnetization process of spin ice in a [111] magnetic field
2004 (English)In: Physical Review B, ISSN 1550-235X, Vol. 70, no 10, 104418- p.Article in journal (Refereed) Published
Abstract [en]

Spin ice in a magnetic field in the [111] direction displays two magnetization plateaus: one at saturation and an intermediate one with finite entropy. We study the crossovers between the different regimes from a point of view of (entropically) interacting defects. We develop an analytical theory for the nearest-neighbor spin ice model, which covers most of the magnetization curve. We find that the entropy is nonmonotonic, exhibiting a giant spike between the two plateaus. This regime is described by a monomer-dimer model with tunable fugacities. At low fields, we develop an RG treatment for the extended string defects, and we compare our results to extensive Monte Carlo simulations. We address the implications of our results for cooling by adiabatic (de)magnetization.

Place, publisher, year, edition, pages
American Physical Society, 2004
National Category
Physical Sciences
Research subject
Physics Of Matter
Identifiers
urn:nbn:se:su:diva-23204 (URN)10.1103/PhysRevB.70.104418 (DOI)
Note
Part of urn:nbn:se:su:diva-184Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2010-01-04Bibliographically approved
3. Exact dipolar correlations in pyrochlore magnets, cubic ice and related systems
Open this publication in new window or tab >>Exact dipolar correlations in pyrochlore magnets, cubic ice and related systems
Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-23205 (URN)
Note
Part of urn:nbn:se:su:diva-184Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2010-01-13Bibliographically approved

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