Julien Robert

Probing complex RNA structures by mechanical force

Abstract.RNA secondary structures of increasing complexity are probed combining single molecule stretching experiments and stochastic unfolding/refolding simulations. We find that force-induced unfolding pathways cannot usually be interpreted by solely invoking successive openings of native helices. Indeed, typical force-extension responses of complex RNA molecules are largely shaped by stretching-induced, long-lived intermediates including non-native helices. This is first shown for a set of generic structural motifs found in larger RNA structures, and then for Escherichia coli’s 1540-base long 16S ribosomal RNA, which exhibits a surprisingly well-structured and reproducible unfolding pathway under mechanical stretching. Using out-of-equilibrium stochastic simulations, we demonstrate that these experimental results reflect the slow relaxation of RNA structural rearrangements. Hence, micromanipulations of single RNA molecules probe both their native structures and long-lived intermediates, so-called “kinetic traps”, thereby capturing -at the single molecular level- the hallmark of RNA folding/unfolding dynamics.

Long-range order and low-energy magnetic excitations in CeRu2Al10 studied via neutron scattering

= 27 K wasinvestigated by neutron scattering. Powder diffraction patterns show clear superstructure peakscorresponding to forbidden (h+ k)-odd reflections of the Cmcm space group. Inelastic neutronscattering experiments further reveal a pronounced magnetic excitation developing in the orderedphase at an energy of 8 meV.

Spin dynamics in the geometrically frustrated multiferroic CuCrO2

The spin dynamics of the geometrically frustrated triangular antiferromagnet multiferroic CuCrO2 have been mapped out using inelastic neutron scattering. The relevant spin Hamiltonian parameters modelling the incommensurate modulated helicoid have been determined, and correspond to antiferromagnetic nearest and next-nearest neighbour interactions in the ab plane, with a strong planar anisotropy. The weakly dispersive excitation along c reflects the essentially two-dimensional character of the magnetic interactions and according to classical energy calculations it is weakly ferromagnetic. Our results clearly point out the relevance of the balance between a ferromagnetic coupling between adjacent planes and a weakly antiferromagnetic next-nearest neighbour interaction in stabilising the three-dimensional ferroelectric magnetic order in CuCrO2. This novel insight on the interplay between magnetic interactions in CuCrO2 should provide a useful basis in the design of new delafossite-based multiferroic materials.

Anisotropic Spin Dynamics in the Kondo Semiconductor CeRu 2 Al 10

Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T0=27.3  K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8±0.3  meV and a pronounced dispersion up to ≈8.5  meV. Comparison with random-phase approximation magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybridization between 4f and conduction band states in this compound.

Spin-liquid correlations in the Nd-langasite anisotropic kagomé antiferromagnet.

Dynamical magnetic correlations in the geometrically frustrated Nd(3)Ga(5)SiO(14) compound were probed by inelastic neutron scattering on a single crystal. A scattering signal with a ring shape distribution in reciprocal space and unprecedented dispersive features was discovered. Comparison with calculated static magnetic scattering from models of correlated spins suggests that the observed phase is a spin liquid inherent to an antiferromagnetic kagomé-like lattice of anisotropic Nd moments.

Magnetic frustration on a Kagomé lattice in R3Ga5SiO14 langasites with R = Nd, Pr

In the R3Ga5SiO14 compounds, the network R of rare earth cations forms well separated planes of corner sharing triangles topologically equivalent to a Kagome lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails suggests that the Nd and Pr magnetic moments can be modelled as coplanar elliptic rotators perpendicular to the threefold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature, which is about 25 times smaller than the energy scale for the exchange interactions.

Spin liquid correlations, anisotropic exchange, and symmetry breaking in Tb2Ti2O7

We have studied the low energy spin dynamics between 4.6K and 0.07K in a Tb2Ti2O7 single crystal sample, by means of inelastic neutron scattering experiments. The spectra consist in a dual response, with a static and an inelastic contribution, showing striking Q-dependences. We propose an interpretation involving an anisotropic exchange interaction in combination with a breaking of the threefold symmetry at the rare earth site. Simulations of the Q-dependent scattering in the Random Phase Approximation account well for the inelastic response.

Magnetic properties of the honeycomb oxide Na2Co2TeO6

We have studied the magnetic properties of

Spin dynamics in the presence of competing ferromagnetic and antiferromagnetic correlations in Yb2Ti2O7

{\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}

Propagation and ghosts in the classical kagome antiferromagnet.

, which features a honeycomb lattice of magnetic