講演会のお知らせ （12/4：Javier Campo 氏 ( Material Science Institute of Aragon, CSIC-University of Zaragoza,Department of Condensed Matter Physics, University of Zaragoza, Zaragoza))

講演題目：Stability and thermal fluctuations in cubic helimagnets
講師：Javier Campo 氏 ( Material Science Institute of Aragon, CSIC-University of Zaragoza,Department of Condensed Matter Physics, University of Zaragoza, Zaragoza )
日時：2018年12月4日（火）14:30～
場所：理学部 本館 21講義室
要旨：
　The role of thermal fluctuations in the stabilization of the magnetic states that appear in cubic helimagnets is theoretically analyzed, using a saddle point expansion to obtain the free energy. It is shown that in the low temperature region thermal fluctuations destabilize the conical state and stabilize the skyrmion lattice (SKL) near the transition to the forced ferromagnetic state. Also, a new phase, whose features cannot be determined with our method, appears in this region of the phase diagram (Figure left) [1]. This new state, and the SKL at low temperature, have been recently found in Cu₂OSeO₃ [2, 3]. It is also shown that at high temperature and low field, near the phase boundary, thermal fluctuations stabilize the SKL, which becomes the equilibrium state (Figure right) [4]. Comparisons with recent experimental results [2, 3] and with the computations of Mühlbauer et al. [5] are given.



Figure: left, theoretical phase diagram of the cubic helimagnet, including the effect of Gaussian fluctuations, in the low T region; right, same in the high T region (the parameter a controls the variation of temperature)

References
[1] V. Laliena and J. Campo, Phys. Rev. B96, 134420 (2017)
[2] F. Qian et al.,arXiv:1802.02070v2.
[3] A. Chacon et al., Nature Physics, June 2018.
[4] V. Laliena, G. Albalate, and J. Campo, in preparation.
[5] S. Mühlbauer et al., Science 323, 915 (2009).

連絡先：異分野基礎科学研究所 小林 夏野（内線8631）