Quantum magnetism and quantum phase transitions

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Direct Measurement of the Spin Hamiltonian and Observation of Condensation of Magnons in the 2D Frustrated Quantum Magnet Cs2CuCl4

(2001)

Authors:

R Coldea, DA Tennant, K Habicht, P Smeibidl, C Wolters, Z Tylczynski
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Correlations and fluctuations in the 2D Heisenberg antiferromagnet

Journal of Magnetism and Magnetic Materials 236:1-2 (2001) 4-5

Authors:

HM Ronnow, DF McMorrow, A Harrison, ID Youngson, R Coldea, TG Perring, G Aeppli, O Syljuåsen

Abstract:

The correlations and fluctuations in Cu(DCOO)2·4D2O, which is a good physical realization of the 2D S = 1/2 Heisenberg antiferromagnet on a square lattice, have been measured by neutron scattering measurements. The quantum fluctuations cause a non-uniform renormalization of the spin-wave dispersion with a zone boundary dispersion of 7%, while the spin wave amplitude is reduced to 51±4% of its classical value. The temperature dependence of the correlation length, the spin-wave damping and the spin-wave softening agrees with theoretical predictions over a large temperature range. © 2001 Elsevier Science B.V. All rights reserved.
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Spin dynamics of the 2D spin 1/2 quantum antiferromagnet copper deuteroformate tetradeuterate (CFTD).

Phys Rev Lett 87:3 (2001) 037202

Authors:

HM Rønnow, DF McMorrow, R Coldea, A Harrison, ID Youngson, TG Perring, G Aeppli, O Syljuåsen, K Lefmann, C Rischel

Abstract:

The magnetic excitation spectrum in the two-dimensional (2D) S = 1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate has been measured for temperatures up to T approximately J/2, where J = 6.31+/-0.02 meV is the 2D exchange coupling. For T<
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Multiparticle states in the S = 1 chain system CsNiCl3.

Phys Rev Lett 87:1 (2001) 017201

Authors:

M Kenzelmann, RA Cowley, WJ Buyers, R Coldea, JS Gardner, M Enderle, DF McMorrow, SM Bennington

Abstract:

A continuum of magnetic states has been observed by neutron scattering from the spin-1 chain compound CsNiCl3 in its disordered gapped one-dimensional phase. Results using both triple-axis and time-of-flight spectrometers show that around the antiferromagnetic point Qc = pi, the continuum lies higher in energy than the Haldane gapped excitations. At 6 K the integrated intensity of the continuum is about 12(2)% of the total spectral weight. This result is considerably larger than the 1%-3% weight predicted by the nonlinear sigma model for the three-particle continuum.
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Spin waves and electronic interactions in La2CuO4.

Phys Rev Lett 86:23 (2001) 5377-5380

Authors:

R Coldea, SM Hayden, G Aeppli, TG Perring, CD Frost, TE Mason, SW Cheong, Z Fisk

Abstract:

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.
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