Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnetCs2CuCl4

We report magnetization and specific heat measurements in the two-dimensional frustrated spin-$1∕2$ Heisenberg antiferromagnet ${\mathrm{Cs}}_2\mathrm{Cu}{\mathrm{Cl}}_4$ at temperatures down to $0.05\mathrm{K}$ and high magnetic fields up to $11.5\mathrm{T}$ applied along $a$, $b$, and $c$ axes. The low-field susceptibility $\chi \left(T\right)\simeq M∕B$ shows a broad maximum around $2.8\mathrm{K}$ characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with $J=4.46\mathrm{K}$ and $J^{\prime }∕J=1∕3$. At much lower temperatures $(⩽0.4\mathrm{K})$ and in in-plane field (along $b$ and $c$ axes) several new intermediate-field ordered phases are observed in between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields.