Magnetization plateau in triangular lattice antiferromagnet

  • Phase diagram of Cs2CuBr4 in magnetic field is remarkably different from that of isostructural Cs2CuCl4: it has magnetization plateau at M= Msat/3 (and another one, possibly, at M= 2 Msat/3). Here Msat is the magnetization of the fully polarized state.
  • Experimental data by T. Ono et al. Phys.Rev.B 67, 104431 (2003), J.Phys.:Condens. Matter 16, S773 (2004), Prog. Theor. Phys. Suppl. 159, 217 (2005). See also H. Tsuji et al., Phys.Rev.B 76, 060406 (2007).

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  • These as well as NMR measurements (Y. Fujii et al., Physica B 346-347, 45 (2004), JMMM 272-276, 861 (2004), J.Phys.:Condens. Matter 19, 145237 (2007)) indicate collinear UP-UP-DOWN (UUD) state, predicted by interacting spin wave calculations of A.V. Chubukov and D.I. Golosov, J.Phys.:Condens. Matter 3, 69 (1991). Closely related classical entropic mechanism has been analyzed by H. Kawamura and S. Miyashita, J. Phys. Soc. Jpn. 54, 4530 (1985).
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  • One of the problems with this explanation is that its key element, UUD state, becomes classically unstable for arbitrary small spatial anisotropy in the exchange, that is when J'/J < 1. It is believed that Cs2CuBr4 has J'/J = 0.75 while Cs2CuCl4 has J'/J = 0.34.
  • The resolution is that quantum fluctuations can stabilize classical unstable state. This type of phenomena is known as "order-by-disorder". In our problem one needs to work with interacting spin waves from the very beginning, and treat spatial anisotropy (J - J') as a perturbation to the isotropic UUD state which is described by interacting spin waves. It turns out that the competition between classical and quantum effects can be parametrized by a single dimensionless parameter δ=(40/3) S (J - J')2/J2. The plateau is locally stable for 0 < δ < 4. However, it is a global minimum only for 0 < δ < 2. (It is interesting to note that δ=0.6 for Cs2CuBr4 while Cs2CuCl4 has δ=2.9.) This, and numerous BEC transitions out of the UUD state, are described in Quantum stabilization of the 1/3-magnetization plateau in Cs2CuBr4, Jason Alicea, Andrey V. Chubukov, and Oleg A. Starykh, Phys. Rev. Lett. 102, 137201 (2009).
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