Quantum magnetism and quantum phase transitions

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Anomalous high-energy spin excitations in the high-Tc superconductor-parent antiferromagnet Laâ‚‚CuOâ‚„.

Phys Rev Lett 105:24 (2010) 247001

Authors:

NS Headings, SM Hayden, R Coldea, TG Perring

Abstract:

Inelastic neutron scattering is used to investigate the collective magnetic excitations of the high-temperature superconductor-parent antiferromagnet La2CuO4. We find that while the lower energy excitations are well described by spin-wave theory, including one- and two-magnon scattering processes, the high-energy spin waves are strongly damped near the (1/2, 0) position in reciprocal space and merge into a momentum dependent continuum. This anomalous damping indicates the decay of spin waves into other excitations, possibly unbound spinon pairs.

Anomalous High-Energy Spin Excitations in La2CuO4

(2010)

Authors:

NS Headings, SM Hayden, R Coldea, TG Perring

Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering

(2010)

Authors:

GL Pascut, R Coldea, PG Radaelli, A Bombardi, G Beutier, II Mazin, MD Johannes, M Jansen

Magnetoacoustics of the low-dimensional quantum antiferromagnet Cs 2CuCl 4 with spin frustration

Journal of Low Temperature Physics 159:1-2 (2010) 109-113

Authors:

A Sytcheva, O Chiatti, J Wosnitza, S Zherlitsyn, AA Zvyagin, R Coldea, Z Tylczynski

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.

Quantum criticality in an Ising chain: experimental evidence for emergent E8 symmetry.

Science 327:5962 (2010) 177-180

Authors:

R Coldea, DA Tennant, EM Wheeler, E Wawrzynska, D Prabhakaran, M Telling, K Habicht, P Smeibidl, K Kiefer

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.