Magnetization plateaus for Cs$_{2}$CuBr$_{4}$

Cs$_{2}$CuBr$_{4}$ is a new two-dimensional spin-1/2 system, where 1/3- and 2/3-plateaus have been observed in external magnetic fields. The magnetic behaviors of the material are well explained by a two-dimensional antiferromagnetic Heisenberg model on a distorted triangular lattice. In the model, there are two types of interactions $J_{1}$ and $J_{2}$, where $J_{1}$ chains are coupled with inter chain interactions $J_{2}$. Using an exact diagonalization method, we investigated magnetic properties, especially magnetization curve. In the magnetization, 1/3-plateau appears for 0.7 $\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} J_{2 }$/$J_{1 }\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} $1.3. At the plateau, three-fold degenerate ground state, up-up-down structure, is realized. Our results indicate that the material has a more frustrated character $J_{2 }$/$J_{1 }\approx $0.7 than what has been expected from a classical theory $J_{2 }$/$J_{1 }$= 0.47. The magnetic properties at 2/3-plateau will also be discussed.