Urmas Nagel

Quantum rotation of ortho and para-water encapsulated in a fullerene cage

Inelastic neutron scattering, far-infrared spectroscopy, and cryogenic nuclear magnetic resonance are used to investigate the quantized rotation and ortho–para conversion of single water molecules trapped inside closed fullerene cages. The existence of metastable ortho-water molecules is demonstrated, and the interconversion of ortho-and para-water spin isomers is tracked in real time. Our investigation reveals that the ground state of encapsulated ortho water has a lifted degeneracy, associated with symmetry-breaking of the water environment.

Rotor in a cage: infrared spectroscopy of an endohedral hydrogen-fullerene complex

We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C(60). Narrow infrared absorption lines at the temperature of 6 K correspond to vibrational excitations in combination with translational and rotational excitations and show well-resolved splittings due to the coupling between translational and rotational modes of the endohedral H(2) molecule. A theoretical model shows that H(2) inside C(60) is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the relative concentration of ortho- and para-H(2).

Interaction potential and infrared absorption of endohedral H2 in C60

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C(60) cage, H(2)@C(60), in the temperature range from 6 to 300 K and analyzed the excitation spectrum by using a five-dimensional model of a vibrating rotor in a spherical potential. The electric dipole moment is induced by the translational motion of endohedral H(2) and gives rise to an infrared absorption process where one translational quantum is created or annihilated, ΔN = ±1. Some fundamental transitions, ΔN = 0, are observed as well. The rotation of endohedral H(2) is unhindered but coupled to the translational motion. The isotropic and translation-rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H(2). The vibrational frequency and the rotational constant of endohedral H(2) are smaller than those of H(2) in the gas phase. The assignment of lines to ortho- and para-H(2) is confirmed by measuring spectra of a para enriched sample of H(2)@C(60) and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone et al., J. Chem. Phys. 130, 081103 (2009)].

The dipolar endofullerene HF@C60

The cavity inside fullerenes provides a unique environment for the study of isolated atoms and molecules. We report the encapsulation of hydrogen fluoride inside C60 using molecular surgery to give the endohedral fullerene HF@C60. The key synthetic step is the closure of the open fullerene cage with the escape of HF minimized. The encapsulated HF molecule moves freely inside the cage and exhibits quantization of its translational and rotational degrees of freedom, as revealed by inelastic neutron scattering and infrared spectroscopy. The rotational and vibrational constants of the encapsulated HF molecules were found to be redshifted relative to free HF. The NMR spectra display a large (1)H-(19)F J coupling typical of an isolated species. The dipole moment of HF@C60 was estimated from the temperature dependence of the dielectric constant at cryogenic temperatures and showed that the cage shields around 75% of the HF dipole.

Chirality of matter shows up via spin excitations

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet.

Infrared spectroscopy of endohedral HD and D2 in C60

We report on the dynamics of two hydrogen isotopomers, D(2) and HD, trapped in the molecular cages of a fullerene C(60) molecule. We measured the infrared spectra and analyzed them using a spherical potential for a vibrating rotor. The potential, vibration-rotation Hamiltonian, and dipole moment parameters are compared with previously studied H(2)@C(60) parameters [M. Ge, U. Nagel, D. Hüvonen, T. Rõõm, S. Mamone, M. H. Levitt, M. Carravetta, Y. Murata, K. Komatsu, J. Y.-C. Chen, and N. J. Turro, J. Chem. Phys. 134, 054507 (2011)]. The isotropic part of the potential is similar for all three isotopomers. In HD@C(60), we observe mixing of the rotational states and an interference effect of the dipole moment terms due to the displacement of the HD rotation center from the fullerene cage center.

One-way transparency of four-coloured spin-wave excitations in multiferroic materials

The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.

Far-infrared study of the two-dimensional dimer spin system SrCu 2 ( BO 3 ) 2

Using far-infrared spectroscopy in magnetic fields up to 12 T we have studied a two-dimensional dimer spin gap system

Spin-stretching modes in anisotropic magnets: Spin-wave excitations in the multiferroic ba2coge2o7

{\mathrm{SrCu}}_{2}({\mathrm{BO}}_{3}{)}_{2}.

Optical Diode Effect at Spin-Wave Excitations of the Room-Temperature Multiferroic BiFeO_{3}.

We found several infrared-active modes in the dimerized state (below 10 K) in the frequency range from 3 to