Prof Radu Coldea

Abstract:

We report on results of sound-velocity and sound-attenuation measurements in the triangular-lattice spin-1/2 antiferromagnet Cs 2CuCl 4 (T N =0.6 K), in external magnetic fields up to 14 T, applied along the b axis, and at temperatures down to 300 mK. The results are analyzed with a quasi-two-dimensional hard-core boson theory based on exchange-striction coupling. There is a good qualitative agreement between theoretical and experimental results.

Abstract:

Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of eight particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by using strong transverse magnetic fields to tune the quasi-one-dimensional Ising ferromagnet CoNb2O6 (cobalt niobate) through its critical point. Spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviors.

Abstract:

We report on results of sound-velocity and sound-attenuation measurements in the low-dimensional spin-1/2 antiferromagnet Cs2 CuCl4 (TN =0.6 K), in external magnetic fields up to 15 T, applied along the b axis, and at temperatures down to 300 mK. The experimental data are analyzed with a theory based on exchange-striction coupling resulting in a qualitative agreement between theoretical results and experimental data. © 2009 The American Physical Society.