Growth And Magnetism Of Cs₂CuCl_4-xBr_x Single Crystals

Geometrically frustrated materials have received much attention in condensed matter physics due to their exotic ground-state properties and quantum-phase-transition behaviours. The triangular-lattice antiferromagnet (TLA) is a simple but important geometrical frustrated system. TLA is expected to be excellent material to investigate exotic quantum phenomena, such as fractional magnetization plateau and quantum spin liquids. In this paper, a detailed study on the synthesis and low-temperature magnetic properties of Br-doped Cs2CuCl4 single crystals has been performed to get a better understanding of spin-liquid ground state in Cs2CuCl4.The first part of chapter one is the review on the research progress of TLA materials. The physical properties of TLA materials such as quantum spin liquid and fractional magnetization plateau, are briefly introduced. In the second part, several typical TLA materials are introduced. Cs2CuCl4 is a typical TLA material with spatially anisotropic couplings and shows a pronounced spin-liquid behavior at low temperature.In chapter two, we mainly introduce the synthesis and characterization of Cs2CuCl4 and Cs2CuCl4-xBrx, single crystals. Cs2CuCl4 and Cs2CuCl4-xBrx single crystals are grown from aqueous solutions. The structure of single crystal is characterized by X-ray diffraction. The growth conditions including the solvents and temperature have been carefully optimized to obtain Cs2CuCl4-xBrx single crystals with high quality and good morphology.In chapter three, the magnetic susceptibility and specific heat of Cs2CuCl4-xBrx single crystals is measured. Firstly, the low-field susceptibility x(T) of Cs2CuCl4 single crystal shows a broad maximum around 2.5 K characteristic of short-range antiferromagnetic correlation. The in-plane and out-of-plane magnetic corralations, probed by the low-temperature broad peak in the magnetic susceptibility, do not show a smooth variation with x. Instead, two distinct concentration regimes can be identified, which are separated by critical concentration x=1. It can be explained by site-selective replacement of Cl" by Br- ions due to the structural peculiarities of the distorted Cu-halide tetrahedral. For x< 1, Br- ions prefer to replace Cl- ions out of be plane, so the 2D magnetic correlations remain unaffected for Br substitutions. For x > 1, Br-ions will replace Cl- ions in plane, which leads a reduction of the 2Dmagnetic correlations. The magnetic specific heat data of Cs2CuCl4-xBrx single crystals shows broad peak around 5 K as well as the magnetic susceptibility. Meanwhile, the magnetic specific heat data for x< 1 can be fitted very well by using one-dimensional S=1/2 Heisenberg antiferromagnetic chain model.