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Register a new userElastic constants and ultrasound attenuation in the spin-liquid phase of Cs$_2$CuCl$_4$
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The spin excitations in the spin-liquid phase of the anisotropic triangular lattice quantum antiferromagnet Cs$_2$CuCl$_4$ have been shown to propagate dominantly along the crystallographic $b$ axis. To test this dimensional reduction scenario, we have performed ultrasound experiments in the spin-liquid phase of Cs$_2$CuCl$_4$ probing the elastic constant $c_{22}$ and the sound attenuation along the $b$ axis as a function of an external magnetic field along the $a$ axis. We show that our data can be quantitatively explained within the framework of a nearest-neighbor spin-$1/2$ Heisenberg chain, where fermions are introduced via the Jordan-Wigner transformation and the spin-phonon interaction arises from the usual exchange-striction mechanism.
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We report on electron spin resonance (ESR) studies of the spin relaxation in Cs$_2$CuCl$_4$. The main source of the ESR linewidth at temperatures $T leq 150$ K is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. Both the angular and temperature dependence of the ESR linewidth have been analyzed using a self-consistent quantum-mechanical approach. In addition analytical expressions based on a quasi-classical picture for spin fluctuations are derived, which show good agreement with the quantum-approach for temperatures $T geq 2J/k_{rm B} approx 15$ K. A small modulation of the ESR linewidth observed in the $ac$-plane is attributed to the anisotropic Zeeman interaction, which reflects the two magnetically nonequivalent Cu positions.
We report $^{133}$Cs nuclear magnetic resonance (NMR) measurements on the 2D frustrated Heisenberg antiferromagnet Cs$_2$CuCl$_4$ down to 2 K and up to 15 T. We show that $^{133}$Cs NMR is a good probe of the magnetic degrees of freedom in this material. Cu spin degrees of freedom are sensed through a strong anisotropic hyperfine coupling. The spin excitation gap opens above the critical saturation field. The gap value was determined from the activation energy of the nuclear spin-lattice relaxation rate in a magnetic field applied parallel to the Cu chains (b axis). The values of the g-factor and the saturation field are consistent with the neutron-scattering and magnetization results. The measurements of the spin-spin relaxation time are exploited to show that no structural changes occur down to the lowest temperatures investigated.
Quantum triangular-lattice antiferromagnets are important prototype systems to investigate phenomena of the geometrical frustration in condensed matter. Apart from highly unusual magnetic properties, they possess a rich phase diagram (ranging from an unfrustrated square lattice to a quantum spin liquid), yet to be confirmed experimentally. One major obstacle in this area of research is the lack of materials with appropriate (ideally tuned) magnetic parameters. Using Cs$_2$CuCl$_4$ as a model system, we demonstrate an alternative approach, where, instead of the chemical composition, the spin Hamiltonian is altered by hydrostatic pressure. The approach combines high-pressure electron spin resonance and magnetization measurements, allowing us not only to quasi-continuously tune the exchange parameters, but also to accurately monitor them. Our experiments indicate a substantial increase of the exchange coupling ratio from 0.3 to 0.42 at a pressure of 1.8 GPa, revealing a number of emergent field-induced phases.
We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ in the framework of density functional theory. Analysis of the exchange couplings J and J using the available X-ray structural data corroborates the values obtained from experimental results for Cs$_2$CuBr$_4$ but not for Cs$_2$CuCl$_4$. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs$_2$CuCl$_4$ employing different exchange-correlation functionals. We find that the exchange couplings depend on rather subtle details of the structural optimization and that only when the insulating state (mediated through spin polarization) is present in the structural optimization, we do have good agreement between the calculated and the experimentally determined exchange couplings. Finally, we discuss the effect of interlayer couplings as well as longer-ranged couplings in both systems.
The quantum-spin S = 1=2 chain system Cs$_2$CuCl$_4$ is of high interest due to competing anti-ferromagnetic intra-chain J and inter-chain exchange J interactions and represents a paramount example for Bose-Einstein condensation of magnons [R. Coldea et al., Phys. Rev. Lett. 88, 137202 (2002)]. Substitution of chlorine by bromine allows tuning the competing exchange interactions and corresponding magnetic frustration. Here we report on electron spin resonance (ESR) in single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$ with the aim to analyze the evolution of anisotropic exchange contributions. The main source of the ESR linewidth is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. The obtained results support the site selectivity of the Br substitution suggested from the evolution of lattice parameters and magnetic susceptibility dependent on the Br concentration.
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