Prof Radu Coldea

Abstract:

Low-dimensional magnets with low-spin quatum number are ideal model systems for investigating strongly interacting macroscopic quantum ground states and their non-linear spin excitations. We present single-crystal neutron-diffraction measurements of the ordered phase of the quasi-one-dimensional spin-1/2 XY antiferromagnet Cs2CoCl4 both in zero field and in fields up to 6.5 T. In zero field the system shows long-range order below TN = 217 mK with a commensurate ordering wave-vector (0, 0.5, 0.5). With increasing magnetic field - applied perpendicular to the magnetic chain axis - the magnetic Bragg peak intensities increase monotonically, reaching a maximum at H = 1.4 T; evidence that the magnetic field suppresses quantum fluctuations in the ground state. At Hc = 2.1 T the ordered structure collapses in an apparent first-order phase transition, with no magnetic Bragg peaks being observed in the (0, k, l) scattering plane above this field. This result suggests that the magnetic field induces a phase transition to a spin-liquid ground state. Magnetic Bragg peak intensities at ferromagnetic positions increase quadratically up to about 2.8 T, corresponding to a linear increase of the magnetic moment. At higher magnetic fields, the intensity increases linearly up to 6.5 T.

Abstract:

We have mapped from the quantum to the classical limit the spin excitation spectrum of the antiferromagnetic spin-1 Heisenberg chain system CsNiCl3 in its paramagnetic phase from T=5 to 200K. Neutron scattering shows that the excitations are resonant and dispersive up to at least T=70K, but broaden considerably with increasing temperature. The dispersion flattens out with increasing temperature as the resonance energy Delta at the antiferromagnetic wave-vector increases and the maximum in the dispersion decreases. The correlation length xi between T=12 and 50K is in agreement with quantum Monte Carlo calculations. xi is also consistent with the single mode approximation, suggesting that the excitations are short-lived single particle excitations. Below T=12K where three-dimensional spin correlations are important, xi is shorter than predicted and the experiment is not consistent with the random phase approximation for coupled quantum chains. At T=200K, the structure factor and second energy moment of the excitation spectrum are in excellent agreement with the high-temperature series expansion.

Abstract:

We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperature, suggesting that the correlations perpendicular to the chain direction and associated with the interchain coupling lower the single-chain correlation length. A multi-particle continuum is observed in the quantum-disordered phase in the region in reciprocal space where antiferromagnetic fluctuations are strongest, extending in energy up to twice the maximum of the dispersion of the well-defined triplet excitations. We show that the continuum satisfies the Hohenberg-Brinkman sum rule. The dependence of the multi-particle continuum on the chain wave-vector resembles that of the two-spinon continuum in antiferromagnetic spin-1/2 Heisenberg chains. This suggests the presence of spin-1/2 degrees of freedom in CsNiCl3 for T < 12K, possibly caused by multiply-frustrated interchain interactions.