Total Angular Momentum Dichroism of the Terahertz Vortex Beams at the Antiferromagnetic Resonances.

Terahertz vortex beams with different superposition of the orbital angular momentum l=±1, ±2, ±3, and ±4 and spin angular momentum σ=±1 were used to study antiferromagnetic (AFM) resonances in TbFe_{3}(BO_{3})_{4} and Ni_{3}TeO_{6} single crystals. In both materials we observed a strong vortex beam dichroism for the AFM resonances that are split in external magnetic field. The magnitude of the vortex dichroism is comparable to that for conventional circular dichroism due to σ. The selection rules at the AFM resonances are governed by the total angular momentum of the vortex beam: j=σ+l. In particular, for l=±2, ±3, and ±4 the sign of l is shown to dominate over that for conventional circular polarization σ.

Terahertz Vortex Beam as a Spectroscopic Probe of Magnetic Excitations.

Circularly polarized light with spin angular momentum is one of the most valuable probes of magnetism. We demonstrate that light beams with orbital angular momentum (OAM), or vortex beams, can also couple to magnetism exhibiting dichroisms in a magnetized medium. Resonant optical absorption in a ferrimagnetic crystal depends strongly on both the handedness of the vortex and the direction of the beam propagation with respect to the sample magnetization. This effect exceeds the conventional dichroism for circularly polarized light. Our results demonstrate the high potential of the vortex beams with OAM as a new spectroscopic probe of magnetism in matter.

Bifurcations of Coupled Electron-Phonon Modes in an Antiferromagnet Subjected to a Magnetic Field.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

Synchrotron radiation-based far-infrared spectroscopic ellipsometer with full Mueller-matrix capability.

We developed far-IR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and full-Mueller matrix spectra using rotating retarders, and wire-grid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm(-1). Fourier-transform infrared (FT-IR) spectrometer is used for multi-wavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of θ-2θ angular rotation, χ tilt angle adjustment, and X-Y-Z translation. A LabVIEW-based software controls the motors, sample temperature, and FT-IR spectrometer and also allows to run fully automated experiments with pre-programmed measurement schedules. Data analysis is based on Berreman's 4 × 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability µ ≠ 1. A nonlinear regression of the rotating analyzer ellipsometry and∕or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thin-film samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with µ ≠ 1 are illustrated with experimental results and simulations for TbMnO3 and Dy3Fe5O12 single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO3.

Phase transitions and crystal-field and exchange interactions in TbFe3(BO3)4 as seen via optical spectroscopy.

High-resolution polarized broadband (1800-23 000 cm(-1)) optical absorption spectra of Tb(3+) in TbFe(3)(BO(3))(4) single crystals are studied between room temperature and 4.2 K. The spectral signatures of the structural (R32-P3(1)21, T(S ) = 192 K) and magnetic (T(N ) = 41 K) phase transitions are found and analyzed. Energies and symmetries of the Tb(3+) crystal-field (CF) levels were determined for both the high-temperature R32 and the low-temperature P3(1)21 structures of TbFe(3)(BO(3))(4) and compared with the calculated ones. It follows unambiguously from the spectral data that the ground state is the Γ(1) + Γ(2) quasi-doublet of the local D(3) point symmetry group for Tb(3+) in the R32 high-temperature structure. The CF calculations revealed the CF parameters and wavefunctions for Tb(3+) in TbFe(3)(BO(3))(4). The value of the Tb-Fe exchange integral and of the effective magnetic field created by the ordered Fe subsystem were estimated as J(fd) = 0.26 K and B(eff) = 3.92 T, using the observed splitting Δ = 32 cm(-1) of the Tb(3+) ground quasi-doublet at the temperature 5 K. The reliability of the obtained parameters was proven by modeling the literature data on the magnetic susceptibility of TbFe(3)(BO(3))(4). Lattice distortions below T(S) were evidenced by the observed changes of probabilities of the forced electric dipole transitions of Tb(3+).

Breaking of the selection rules for optical transitions in the dielectric PrFe3(BO3)4 crystal by a praseodymium-iron exchange interaction.

We report on the emergence of new lines in the optical spectrum of the PrFe3(BO3)4 single crystal at the magnetic ordering temperature. The transitions between singlet crystal-field sublevels of Pr3+ ion with the same transformational properties, strictly forbidden for the trigonal D3 point symmetry of this ion in PrFe3(BO3)4, appear below the Néel temperature and grow in intensity as a square of the order parameter. We show that the phenomenon originates from the mixing of wave functions of different Pr3+ sublevels by the Pr-Fe exchange interaction.