Femtosecond electron and spin dynamics in Gd(0001) studied by time-resolved photoemission and magneto-optics

Femtosecond electron and spin dynamics of the Gd(0001) surface are investigated by time-resolved photoemission and second harmonic generation. Upon optical excitation the spin polarization of the surface state is reduced by half while its exchange splitting remains nearly unchanged. Electron-magnon interaction is proposed to facilitate electron-spin-flip scattering among spin-mixed surface and bulk states, which provides a mechanism for ultrafast demagnetization.     

Quantum vacuum magneto-optics

In this article we report on the recent experimental activities concerning the magneto-optics of quantum vacuum performed by our team based at the LNCMI of Toulouse. In particular, we will deal with quantum vacuum Cotton–Mouton and inverse Cotton–Mouton effects which have been predicted in the framework of Quantum electrodynamics. Finally, we will present our experiment on photon oscillations into massive particles in the presence of a magnetic field, an effect that could be a signature of physics beyond the standard model.     

Null ellipticity in magneto-optics

We determine the relationship between the photon, cyclotron, collision and plasma frequencies which ensures a null ellipticity for electromagnetic wave propagation in a free-carrier magnetoplasma. A comparison is made with the condition for null Faraday rotation.     

X-ray magneto-optics in lanthanides

Magneto-optical methods in the visible light regime generally lack element specificity, which has become a considerable shortcoming in research on advanced heteromagnetic systems. Using circularly polarized soft x rays tuned to a 4d-4f core-level transition of a lanthanide element, the specularly reflected x-ray intensity changes proportionally to the magnetization of this element and, e.g., hystereses are easily measured element specifically. In contrast to the case of visible light, temperature dependent 4d-4f magneto-optical signals are not influenced by the thermal lattice expansion.     

Spatial dispersion phenomena in magneto-optics

An experimental study of nonreciprocal spatial-dispersion effects in para-(Cd_1−x Mn_xTe), ferro-(LiFe₅O₈), and antiferromagnetic (Cr₂O₃) crystals caused by an external magnetic field or magnetic order is reported. Fiz. Tverd. Tela (St. Petersburg) 40, 946–948 (May 1998)     

On possible conical singularities in magneto-optics

The conditions for the appearance of conical singularities in a magnetically ordered crystal in the presence of dichroism, birefringence, and magneto-optical activity have been considered. The possibility of the existence of singularities such as a nodal point, cusp, and point of contact has been demonstrated. The relation of the topological index of the closed singular line to the indices of the C points lying inside the contour has been revealed. Estimates have been made for the YIG:Bi crystal.     

Multiresolution spectrotemporal analysis of complex sounds

A computational model of auditory analysis is described that is inspired by psychoacoustical and neurophysiological findings in early and central stages of the auditory system. The model provides a unified multiresolution representation of the spectral and temporal features likely critical in the perception of sound. Simplified, more specifically tailored versions of this model have already been validated by successful application in the assessment of speech intelligibility [Elhilali et al., Speech Commun. 41(2-3), 331-348 (2003); Chi et al., J. Acoust. Soc. Am. 106, 2719-2732 (1999)] and in explaining the perception of monaural phase sensitivity [R. Carlyon and S. Shamma, J. Acoust. Soc. Am. 114, 333-348 (2003)]. Here we provide a more complete mathematical formulation of the model, illustrating how complex signals are transformed through various stages of the model, and relating it to comparable existing models of auditory processing. Furthermore, we outline several reconstruction algorithms to resynthesize the sound from the model output so as to evaluate the fidelity of the representation and contribution of different features and cues to the sound percept.     

Quantum magneto-optics of the graphite family

The optical conductivity of graphene, bilayer graphene, and graphite in quantizing magnetic fields is studied. Both dynamical conductivities, longitudinal and Hall’s, are evaluated analytically. The conductivity peaks are explained in terms of electron transitions. Correspondences between the transition frequencies and the magneto-optical features are established using the theoretical results. We show that trigonal warping can be considered within the perturbation theory for strong magnetic fields larger than 1 T. The semiclassical approach is applied for weak fields when the Fermi energy is much larger than the cyclotron frequency. The main optical transitions obey the selection rule with Δn = 1 for the Landau number n, but the Δn = 2 transitions due to the trigonal warping are also possible. The Faraday/Kerr rotation and light transmission/reflection in quantizing magnetic fields are calculated. Parameters of the Slonczewski-Weiss-McClure model are used in the fit taking the previous de Haas-van Alphen measurements into account and correcting some of them in the case of strong magnetic fields.     

Non-linear optics and magneto-optics on nano-structured interfaces

The behaviour of the reflectivity and Kerr magneto-optic effects in the non-linear second-harmonic (SH) field diffracted from nano-scale structured ferromagnetic interfaces is reported. Measurements are made in both the linear and non-linear fields at two different fundamental wavelengths (1064 nm and 800 nm) and their associated harmonics. Resonant behaviour observed as a function of angle of incidence is identified with surface-plasmon production that is known to intensify the local field within the interface. Radiation incident at angles of incidence that optimise coupling to the electron plasma produces an increase in the SH field radiated in the vicinity of those angles. Similarly, at those angles of incidence where radiation at the SH wavelengths (532 nm and 400 nm) couples optimally to the electron plasma, troughs are seen in the angular spectrum of the generated SH radiation. Kerr magneto-optic measurements taken in both the linear field and the SH field both show very significant enhancement at angles meeting the plasma-resonance condition. The totality of experimental data presented allows the conclusion that intensification of the interface electric field due to plasmon creation enhances not only the SH reflection coefficient, as was already known, but also the magneto-optic reflection coefficients in both linear and SH fields. Received: 16 October 2001 / Revised version: 14 March 2002 / Published online: 29 May 2002     

Advection-induced spectrotemporal defects in a free-electron laser

We evidence numerically and experimentally that advection can induce spectrotemporal defects in a system presenting a localized structure. Those defects in the spectrum are associated with the breakings induced by the drift of the localized solution. The results are based on simulations and experiments performed on the super-ACO free-electron laser. However, we show that this instability can be generalized using a real Ginzburg-Landau equation with (i) advection and (ii) a finite-size supercritical region.     

Interband magneto-optics in single-walled carbon nanotubes

We have carried out magneto-absorption and magneto-photoluminescence experiments on micelle-suspended single-walled carbon nanotubes in magnetic fields up to 45 T. Chirality-assigned spectral peaks exhibit significant changes with increasing magnetic field, which can be quantitatively explained in terms of the theoretically predicted splittings and redshifts of the band edge due to the Aharonov–Bohm effect combined with the magnetic-field-induced alignment of the nanotubes.     

Reflection matrix in magneto-optics of multilayer structures

An expression is given for the reflection matrix of a magnetic multilayer, each layer of which is assumed to be uniformly magnetized in an arbitrary direction. The layer permittivity tensor takes into account optical anisotropy along the direction orthogonal to boundaries and is considered in the linear approximation with respect to two magneto-optical parameters. The result obtained makes it possible to study a large number of various configurations of multilayers and magneto-optical effects in them taking into account the effect of optical anisotropy.     

Room-temperature magneto-optics of ferromagnetic transition-metal-doped ZnO thin films

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V, and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant, which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K, additional paramagnetic contributions to the MCD are observed, but above about 150 K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behavior.     

Illusory spectrotemporal ripples created with binaurally correlated noise

Binaural disparities are the primary acoustic cues employed in sound localization tasks. However, the degree of binaural correlation in a sound serves as a complementary cue for detecting competing sound sources [J. F. Culling, H. S. Colburn, and M. Spurchise, "Interaural correlation sensitivity," J. Acoust. Soc. Am. 110(2), 1020-1029 (2001) and L. R. Bernstein and C. Trahiotis, "On the use of the normalized correlation as an index of interaural envelope correlation," J. Acoust. Soc. Am. 100, 1754-1763 (1996)]. Here a random chord stereogram (RCS) sound is developed that produces a salient pop-out illusion of a slowly varying ripple sound [T. Chi et al., "Spectro-temporal modulation transfer functions and speech intelligibility," J. Acoust. Soc. Am. 106(5), 2719-2732 (1999)], even though the left and right ear sounds alone consist of noise-like random modulations. The quality and resolution of this percept is systematically controlled by adjusting the spectrotemporal correlation pattern between the left and right sounds. The prominence and limited time-frequency resolution for resolving the RCS suggests that envelope correlations are a dominant binaural cue for grouping acoustic objects.     

On the magneto-optics of orthoferrites with spin reorientation

The results of magneto-optical investigations into the Γ₂–Γ₄ spin reorientation in an Sm_0.6Tb_0.2Tm_0.2FeO₃ orthoferrite single crystal are reported. The magneto-optical parameters are determined taking into account the dichroism. The orientation of the antiferromagnetic vector in the canted phase is investigated as a function of the magnetic field and temperature. The magnetic anisotropy field strengths are determined by the processing of the results. The orientation contribution to the birefringence is estimated.     

On gyrotropic chiral sculptured thin films for magneto-optics

The planewave response of a chiral sculptured thin film with gyrotropic properties is examined, under the assumption of axial excitation. Although the reflectances and the transmittances are definitely affected by gyrotropy, the circular Bragg phenomenon is still exhibited. Most importantly, structural chirality and gyrotropy contribute cooperatively to transmission optical rotation.     

X-ray magneto-optics of lanthanide materials: principles and applications

Lanthanide metals are a particular class of magnetic materials in which the magnetic moments are carried mainly by the localized electrons of the 4f shell. They are frequently found in technically relevant systems, to achieve, e.g., high magnetic anisotropy. Magneto-optical methods in the X-ray range are well suited to study complex magnetic materials in an element-specific way. In this work, we report on recent progress on the quantitative determination of magneto-optical constants of several lanthanides in the soft X-ray region and we show some examples of applications of magneto-optics to hard-magnetic interfaces and exchange-coupled layered structures containing lanthanide elements. PACS 78.20.Ls; 78.70.Dm; 78.70.Ck; 78.20.Ci; 75.70.-i