Biomedical subjects

R Coldea

Publications and source records attributed to R Coldea.

9 recordsLinked to original sources

PubMed2005

Bose-Einstein condensation of magnons in Cs2CuCl4.

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.

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PubMed2005

Three-dimensional spin fluctuations in Na0.75CoO2.

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. The data confirm ferromagnetic correlations within the cobalt layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of approximately 12 meV. From a minimal spin-wave model, containing only nearest-neighbor Heisenberg exchange interactions, we estimate the interlayer and intralayer exchange constants to be 12.2+/-0.5 meV and -6+/-2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three dimensional.

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PubMed2004

Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering.

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies ( approximately 10 meV), which is localized in wave vector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.

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PubMed2002

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

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.

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PubMed2001

Spin dynamics of the 2D spin 1/2 quantum antiferromagnet copper deuteroformate tetradeuterate (CFTD).

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<<J, a dispersion of the zone boundary energy is observed, which is attributed to a wave vector dependent quantum renormalization. At higher temperatures, spin-wavelike excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behavior of the model system is found over the whole temperature interval investigated.

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PubMed2001

Multiparticle states in the S = 1 chain system CsNiCl3.

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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PubMed2001

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.

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PubMed2001

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.

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PubMed2000

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 CeCu(6-x)Au(x), 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 (x(c) = 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.

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