The Magnetoelastic Distortion of Multiferroic BiFeO3_3 in the Canted Antiferromagnetic State

Using THz spectroscopy, we show that the spin-wave spectrum of multiferroic BiFeO$_3$ in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly anti-parallel spins. The revised spin model for BiFeO$_3$ contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field.Comment: 28 pages (main & supplementary), 2 figures (main article), 15 figures (supplementary material

Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene

Optical transitions between Landau levels (LL) in solids are described by selection rules associated with the LL index. Here, the authors perform photocurrent spectroscopy measurements on high-quality bilayer graphene to investigate the interband LL transitions, and observe valley-dependent optical transitions obeying unusual selection rules

Author Correction: Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene

An amendment to this paper has been published and can be accessed via a link at the top of the paper

Direct Determination of Exchange Parameters in Cs 2 CuBr 4 and Cs 2 CuCl 4 : High-Field Electron-Spin-Resonance Studies

International audienceSpin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/k(B) = 4.7(2) K, J'/k(B) = 1.42(7) K, [J'/J similar or equal to 0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/k(B) = 14.9(7) K, J'/k(B) = 6.1(3) K, [J'/J similar or equal to 0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system