ABX 3 -type compounds (A=Rb or Cs; B=Mn, Fe, Co, Ni, Cu, or V; X=Cl, Br, or I) provide an ideal platform for investigating low-dimensional phase transitions. Within that family, CsCuCl 3 has attracted much attention with regard to two-dimensional triangular antiferromagnets and chirality. The crystal structure of CsCuCl 3 at low temperatures (below 423 K) is hexagonal with the space group P 6 1 22 or P 6 5 22. In this phase, Cu chains form helices along the c axis with six Cu atoms per unit cell. These chains form a triangular lattice in the ab plane. 1) The main exchange interactions between the Cu ions are the intra-chain ferromagnetic interactions (coupling constant J 0 ∼ 28 K), inter-chain antiferromagnetic interaction (coupling constant J 1 ∼ 4.9 K), and Dzyaloshinskii-Moriya (DM) interaction with the D vector pointing along the c axis (|D| ∼ 5 K). 1, 2) Because of these interactions, below T N = 10.7K , this compound displays a helical 120 • spin structure along the c axis with a pitch angle of θ = 5.1 • . 1)The ground state of CsCuCl 3 in a longitudinal magnetic field (H c) at a low temperature and under ambient pressure is well understood within spin-wave theory. Nikuni and Shiba showed that the quantum-phase transition from an umbrella phase to a 2-1 coplanar phase occurs when the magnetic field increases. 3) This theoretical prediction was confirmed by neutron diffraction and specific heat measurements. 4, 5) On the other hand, a new magnetization plateau was recently found under high pressure. 6) Although this plateau is expected to be an up-up-down (uud) phase showing the 1/3 plateau of the saturation magnetic field, its existence has so far not been explained within spin-wave theory. 3) In this paper, we predict the existence of the uud phase theoretically by considering the pressure dependence of exchange interactions, on the basis of spin-wave theory. We also predict a Y coplanar phase under high pressure, which has not been confirmed experimentally. We then also examine thermal fluctuations for each phase, and the H-T phase diagram.We write the Hamiltonian of CsCuCl 3 in H c asHere, S i,n ≡ (S x i,n , S y i,n , S z i,n ) is a spin operator at the i-th site in the n-th ab plane, and the summation ij covers nearest-neighbor sites in the ab plane. The z axis is parallel to the magnetic field H and D n,n+1 ≡ (0, 0, D) is the DM interaction between the i-th spins in the n-th and (n + 1)-st planes. The quantity η is an anisotropic exchange interaction of the easy-plane type, and g and µ B are the g-factor (g = 2) and Bohr magneton, respectively.We can eliminate the DM interaction term by rotating the xy plane about z axis by an angle q = tan −1 (D/2J 0 (1 + η)). 3) Then, the Hamiltonian is rewrittenwhereJ 0 = J 0 (1 + η) 2 + (D/2J 0 ) 2 . Note that the DM interaction is incorporated into the easy-plane anisotropy and we define the anisotropy parameter asWe consider the five spin configurations shown in Fig. 1 as candidates for the ground state. The umbrella con-Fig. 1. (Color online) Five spin config...
