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Identifier 000351520
Title Low energy magnon modes in the helimagnet Ba2CuGe2O7
Alternative Title Διεγέρσεις μαγνονίων χαμηλής ενέργειας στον ελικοειδή μαγνήτη Ba2CuGe2O7
Author Ψαρουδάκη, Χριστίνα
Thesis advisor Παπανικολάου, Νικόλαος
Abstract Low energy magnon excitations are studied in the quasi-two dimensional antiferromagnet Ba2CuGe2O7, where an incommensurate magnetic spiral structure has been observed. Symmetric exchange interactions compete with antisymmetric Dzyaloshinskii- Moriya interactions and result in helimagnetism. A continuum effective field theory is developed to provide the necessary theoretical framework for the calculation of the ground state in the commensurate and incommensurate phase. In the case of spin wave propagation along x axis, equations describing fluctuations around the ground state of the incommensurate phase admit analytic solutions obeying the classic Lam´e equation. Low energy modes appear as Goldstone modes due to broken continuous translation symmetry but exhibit a characteristic anisotropy for propagation along the y axis. Moreover, an anisotropy parameter γ determines the phase of the system. For 0 ≤ γ ≤ γ1, with γ1 = 1.419, the incommensurate phase is stable and the system is described by a flat spiral, whereas for γ ≥ γ2, with γ2 = 2, the commensurate phase is stable and the system is described by the Neel state. Therefore, there is a region γ1 ≤ γ ≤ γ2 for which neither the commensurate nor the incommensurate phase is stable. Finally, experimental results exist for the case of spin wave propagation along x axis and are in good agreement with theoretical predictions.
Language English
Subject Dryaloshinskii-Moriya anisotropy
Lame equation
Nonlinears model
Issue date 2010-03-10
Collection   School/Department--School of Sciences and Engineering--Department of Physics--Post-graduate theses
  Type of Work--Post-graduate theses
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