R. Coldea

Onset of antiferromagnetism in heavy-fermion metals

There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second description, based on the physics of electron fluids or ‘Fermi liquids’, states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is at present unknown which view is appropriate at a ‘quantum critical point’, where the antiferromagnetic transition temperature vanishes. Here we report neutron scattering and bulk magnetometry measurements of the metal CeCu6-xAux, which allow us to discriminate between the two models. We find evidence for an atomically local contribution to the magnetic correlations which develops at the critical gold concentration (xc = 0.1 ), corresponding to a magnetic ordering temperature of zero. This contribution implies that a Fermi-liquid-destroying spin-localizing transition, unanticipated from the spin density wave description, coincides with the antiferromagnetic quantum critical point.

Spin waves and electronic interactions in La2CuO4

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Quantum Criticality in an Ising Chain: Experimental Evidence for Emergent E8 Symmetry

Hidden Symmetry Revealed It is not often that an exact theory can describe a many-particle quantum-mechanical system, but one of the few exceptions is the behavior of a string of ferromagnets—an Ising chain—at magnetic field strengths that separate different types of ordering. Its excitations were predicted 20 years ago to be governed by the symmetry group E8, one of the most intriguing objects in mathematics. Now, Coldea et al. (p. 177) report direct experimental confirmation of this result in a quasi-one-dimensional Ising ferromagnet CoNb2O6, which they probed by neutron scattering. Two of the eight predicted excitations could be observed. Moreover, the ratio of the two lowest excitations is in quantitative agreement with the so-called “golden ratio” predicted by theory. A long-predicted hidden symmetry in spin ordering has been observed experimentally at temperatures near absolute zero. 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.

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3  K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.

Experimental Realization of a 2D Fractional Quantum Spin Liquid

The ground-state ordering and dynamics of the two-dimensional S = 1/2 frustrated Heisenberg antiferromagnet Cs(2)CuCl(4) are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of S = 1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.

From incommensurate to dispersive spin-fluctuations : The high-energy inelastic spectrum in superconducting YBa2Cu3O6.5

We have investigated the spin-fluctuations at energy transfers up to

Extended scattering continua characteristic of spin fractionalization in the two-dimensional frustrated quantum magnet Cs 2 CuCl 4 observed by neutron scattering

\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{meV}

Direct measurement of the spin Hamiltonian and observation of condensation of magnons in the 2D frustrated quantum magnet Cs2CuCl4.

, well above the resonance energy

Monoclinic crystal structure of α-RuCl3 and the zigzag antiferromagnetic ground state

(33\phantom{\rule{0.3em}{0ex}}\mathrm{meV})

Kitaev interactions between j = 1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations

in the