Sequence of Multipolar Transitions : Scenarios for URu_2Si_2(Yukawa International Seminar 2004 (YKIS2004) Physics of Strongly Correlated Electron Systems)
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概要
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d-and f-shells support a large number of local degrees of freedom : dipoles, quadrupoles, octupoles, hexadecapoles, etc. Usually, the ordering of any multipole component leaves the system sufficiently symmetrical to allow a second symmetry breaking transition. Assuming that a second continuous phase transition occurs, we classify the possibilities. We construct the symmetry group of the first ordered phase, and then re-classify the order parameters in the new symmetry. While this is straightforward for dipole or quadrupole order, it is less familiar for octupole order. We give a group theoretical analysis, and some illustrative mean field calculations, for the hypothetical case when a second ordering transition modifies the primary T_<xyz> octupolar ordering in a tetragonal system like URu_2Si_2. If quadrupoles appear in the second phase transition, they must be accompanied by a time-reversal-odd multipole as an induced order parameter. For O_<xy>, O_<zx>, or O_<yz> quadrupoles, this would be one of the components of J, which should be easy either to check or to rule out. However, a pre-existing octupolar symmetry can also be broken by a transition to a new octupole-hexadecapole order, or by a combination of O^2_2 quadrupole and triakontadipole order. It is interesting to notice that if recent NQR results on URu_2Si_2 are interpreted as a hint that the onset of octupolar hidden order at T_0=17K is followed by quadrupolar ordering at T^*≈13.5K, this sequence of events may fit several of the scenarios found in our general classification scheme. However, we have to await further evidence showing that the NQR anomalies at T^*≈13.5K are associated with an equilibrium phase transition.
- 理論物理学刊行会の論文
- 2006-02-06
著者
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KISS Annamaria
Department of Physics, Tohoku University
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Kiss Annamaria
Department Of Physics Tohoku University
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FAZEKAS Patrik
Research Institute for Solid State Physics and Optics
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RADNOCZI Katalin
Research Institute for Solid State Physics and Optics
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