Magnetic‐field‐controlled quantum critical points in the triangular antiferromagnetic Cs$_2$ CuCl$_{4‐x}$ Br$_x$ mixed system

Overview & Synopsis

The triangular antiferromagnetic Cs$_2$ CuCl$_{4‐x}$ Br$_x$ mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field $B_c$ is found for a Br concentration x = 0.8 at $B_c$ = 8.10(1) T and for x = 1.1 at B$_c$ = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at $B_c$ = 0.99(3) T. For magnetic fields larger than the respective $B_c$, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature TN = 0.51(1) K for Cs$_2$CuCl$_{3.2}$Br$_{0.8}$, and q = (0, 0.418, 0) below $T_N$ = 1.00(6) K for Cs$_2$CuCl$_{0.3}$Br$_{3.7}$. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at $B_c$ = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.