The fullerene C(60) can be converted into two different structures by high pressure and temperature. They are metastable and revert to pristine C(60) on reheating to 300 degrees C at ambient pressure. For synthesis temperatures between 300 degrees and 400 degrees C and pressures of 5 gigapascals, a nominal face-centered-cubic structure is produced with a lattice parameter a(o) = 13.6 angstroms. When treated at 500 degrees to 800 degrees C at the same pressure, C(60) transforms into a rhombohedral structure with hexagonal lattice parameters of a(o) = 9.22 angstroms and c(o) = 24.6 angstroms. The intermolecular distance is small enough that a chemical bond can form, in accord with the reduced solubility of the pressure-induced phases. Infrared, Raman, and nuclear magnetic resonance studies show a drastic reduction of icosahedral symmetry, as might occur if the C(60) molecules are linked.
Frequency moments of the light-scattering spectrum from spin-pair excitations are calculated for the spin-y planar Heisenberg antiferromagnet. The quantitative agreement with the experimental B\ g spectrum in La2CuC>4 demonstrates that the observed linewidth is dominated by quantum fluctuations and yields a value for the exchange parameter J of 1030 ± 50 cm ~1. Quantum fluctuations also permit light scattering due to diagonal-next-neighbor spin-pair excitations. Observed spectral features in the A \ g and Big symmetries are consistent with those calculated for this process.PACS numbers: 75.40. Gb, 75.30.Ds, 75.50.Ee The effects of quantum fluctuations on the static and dynamic properties of two-dimensional spin-y quantum antiferromagnets have been the subject of numerous theoretical studies ] over the past thirty years. Quantum corrections, for example, to the sublattice magnetization, M + , the spin-wave velocity, C s , and the perpendicular susceptibility, %±, for the nearest-neighbor square lattice Heisenberg antiferromagnet were originally studied via a l/S expansion in the spin-wave theory. 2 In contrast to these expansions, which are asymptotic, convergent expansions have now been developed for these quantities by perturbing around the Ising limit. 3 These latter expansions open up the possibility of directly computing the experimentally observable static and dynamic properties of Heisenberg antiferromagnets in a controlled and systematic manner.The quantum corrections are expressed most appropriately 4 in terms of multiplicative renormalizations of the classical values. For example, the spin-wave velocity becomes Cs^Zc^/SSJa/ h, where J is the exchange constant for the Heisenberg Hamiltonian and the classical value of C s for S*= j is -JlJal h. In most experiments the renormalization parameter (such as Z c ) is not independently determined, so that the role of quantum fluctuations, embodied in Z, is not directly tested.In this Letter we show that the light-scattering spectrum from spin-pair excitations in La2CuC>4 provides dramatic evidence for the role of quantum fluctuations in these systems as well as a quantitative determination of the microscopic exchange parameter. The Ising expansions 3 are used to estimate the various frequency moments of the spectrum (for B\ g symmetry) for comparison with those observed experimentally. The predicted first moment can be used to deduce the exchange parameter J, while the ratios of the higher moments to the first one, or the ratio of the peak width to its position, provide a quantitative, parameter-free check on the theoretical prediction for the effect of quantum fluctuations. We find that the agreement between the theory and the experiments is excellent. This demonstrates clearly that the observed B\ g peak width is intrinsic to the S=j Heisenberg system. Furthermore, the ratio of the peak width to its position differs from the purely classical prediction 5 by a factor of 3, thus highlighting the role of quantum fluctuations.In addition we show that light scatt...
Raman scattering studies of La2CuO~have revealed a high-frequency component {-3000 cm ') which we interpret as due to scattering by spin pairs. Comparison of the spectra with those of K2Nipg leads to a value of J -1100 cm ' for the exchange interaction. The twodimensional nature of the fluctuations suggested by earlier neutron scattering work is indicated here by the insensitivity of the spectra to temperature as well as to departures from stoichiometry which strongly inhuence the three-dimensional ordering temperature T~.A common feature of the high-temperature
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