Temperature dependence of the(π,0)anomaly in the excitation spectrum of the 2D quantum Heisenberg antiferromagnet.

It is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of thepoint the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system.

Fractional excitations in the square lattice quantum antiferromagnet.

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum.

Spectroscopic effects resulting from interacting singlet and triplet excited states: vibronic structure involving the O-H stretching mode in d-d absorption bands of Ni(H2O)6(2+).

The ligand-field absorption spectrum of the Ni(H2O)6(2+) cation has been thoroughly measured and analyzed over the past sixty years, often on crystals with low symmetry at the metal site, and its absorption band maxima have been used as a benchmark for increasingly sophisticated electronic structure calculations over the last decades. We present variable-temperature absorption spectra measured on crystals with cubic Th symmetry at the site of the nickel(ii) cation. This high site symmetry is confirmed for CsNi(H2O)6PO4 by X-ray diffraction and allows for a direct comparison with ligand-field calculations in Th symmetry, at the basis of an analysis of the vibronic structure in the energy range of the lowest-energy spin-forbidden transition, the "red" or middle band of the spectrum.

Three-axis anisotropic exchange coupling in the single-molecule magnets NEt4[Mn(III)2(5-Brsalen)2(MeOH)2M(III)(CN)6] (M=Ru, Os).

We have investigated the single-molecule magnets [Mn(III)2 (5-Brsalen)2 (MeOH)2 M(III) (CN)6 ]NEt4 (M=Os (1) and Ru (2); 5-Brsalen=N,N'-ethylenebis(5-bromosalicylidene)iminate) by frequency-domain Fourier-transform terahertz electron paramagnetic resonance (THz-EPR), inelastic neutron scattering, and superconducting quantum interference device (SQUID) magnetometry. The combination of all three techniques allows for the unambiguous experimental determination of the three-axis anisotropic magnetic exchange coupling between Mn(III) and Ru(III) or Os(III) ions, respectively. Analysis by means of a spin-Hamiltonian parameterization yields excellent agreement with all experimental data.