Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

The magnetotransmission, magnetoreflection, and magnetoresistance of the La_0.7Ca_0.3MnO₃ and La_0.9Ag_0.1MnO₃ epitaxial films have been investigated. It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures. It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands. The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects. As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits.     

Vibration and the Kerr effect

The theory of the Kerr effect (electric birefringence) is developed with explicit inclusion of molecular vibration. When the theory is applied to CHCl₃ and CHF₃, it is found that terms with a vibrational origin are an important component of the β-hyperpolarizability extracted from temperature-dependent Kerr studies. It is concluded that these vibrational terms partly account for the difference between β obtained from second-harmonic-generation experiments and from the Kerr constant.     

The low temperature spectral weight transfer problem in Kondo insulators

In this paper we address the problem of the spectral weight transfer in Kondo insulators (KI). We employ the X-boson approach for the periodic Anderson model, in the U →∞ limit. We calculate the two energy gaps of the system analytically: the indirect gap, Δ_ind = E_g ≃ E_mir, present in the density of states, and the direct one Δ_dir, associated with the minimum energy necessary to produce inter-band transitions. We find that the optical behavior of the system is governed by two energy scales: one of low frequency, characterized by E_g ≃ E_mir, in the mid-infrared region (MIR), which is a reminiscent of the heavy fermion E_mir peak, that appears in Kondo insulators as a broad maximum in the MIR region and that controls the low temperature transport properties, the gap opening in optical conductivity and the formation of the Drude peak, at ω = 0, in the intermediate temperature range. The other energy scale appears at high frequencies, and is characterized by the direct gap Δ_dir. According to our results, this peak controls the anomalous redistribution of spectral weight in the optical conductivity. We apply the theory in order to study the Kondo insulator FeSi, and we calculate the optical conductivity of the system and the spectral weight transfer in the optical conductivity.     

Polarons and isotope effect in manganites

Numerical estimates are carried out which show that the polaronic effects cannot be an origin of specific properties of the CMR materials. A quantum-mechanical calculation of the charge carrier band narrowing with increasing temperature is carried out for the double-exchange systems. Comparing its results with experimental data shows that the hole–phonon coupling in them is too weak for the small polaron formation. Despite this, a giant isotope effect is possible in the manganites due to an isotope dependence of the nonstoichiometry of manganites, to which lattice vibrations contribute considerably.     

Saturation of the all-optical Kerr effect

Saturation of the intensity dependence of the refractive index is directly computed from ionization rates via a Kramers-Kronig transform. The linear intensity dependence and its dispersion are found to be in excellent agreement with complete quantum mechanical orbital computations. Higher-order terms concur with solutions of the time-dependent Schr?dinger equation. Expanding the formalism to all orders up to the ionization potential of the atom, we derive a model for saturation of the Kerr effect. This model widely confirms recently published and controversially discussed experimental data and corroborates the importance of higher-order Kerr terms for filamentation.     

Weak amplification of the magnetocaloric effect in manganites

The magnetocaloric effect (MCE) has been measured by direct method in La_0.8Ag_0.15MnO₃ and La_0.85Ag_0.15MnO₃ before and after coating of Fe–Co layer on the surfaces of manganites. An evaporated film thickness has been 500 nm. The measurements have shown the MCE to be increased by 7%–8% under 26 kOe after Fe–Co coating on the flat surfaces.     

Isotope effect in manganites and hydrogenated palladium

A theory of the isotopic dependence of the hydrogen content in palladium and the excess oxygen content in manganites is formulated. The theoretical results for the first system are obtained without any fitting parameters and reproduce the experimental data within 2–3% error limits in the temperature range from 170K to 1450 K. An exact expression for the magnitude of the effect is also obtained for the second system, but owing to the lack of necessary experimental data, it can only be estimated in order of magnitude. Fiz. Tverd. Tela (St. Petersburg) 41, 2116–2120 (December 1999)     

Diffractional enhancement of the Kerr magnetooptic effect

The results of investigations of the transverse Kerr effect on an array of thin magnetic strips deposited on a silicon substrate are reported. The periodic structure of the sample gives rise to diffraction. It is observed that under certain experimental conditions the magnitude of the effect measured in diffracted beams is much greater than the maximum value for a sample with a uniform surface. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 466–469 (10 October 1997)     

Photoemission of a doped Mott insulator: spectral weight transfer and a qualitative Mott-Hubbard description

The spectral weight evolution of the low-dimensional Mott insulator TiOCl upon alkali-metal dosing has been studied by photoelectron spectroscopy. We observe a spectral weight transfer between the lower Hubbard band and an additional peak upon electron doping, in line with quantitative expectations in the atomic limit for changing the number of singly and doubly occupied sites. This observation is an unconditional hallmark of correlated bands and has not been reported before. In contrast, the absence of a metallic quasiparticle peak can be traced back to a simple one-particle effect.     

Magnetooptical Kerr effect of Fe-gratings

The magnetooptical Kerr effect in longitudinal configuration is used to measure hysteresis loops of ferromagnetic Fe-gratings grown on Al₂O₃ at different orders of diffraction. At even order of diffraction the hysteresis loops exhibit anomalies which can be attributed to the interference of the magnetic and non-magnetic parts of the grating. The Kerr angle in saturation increases linearly with the order of diffraction.     

Optical transfer functions of Kerr nonlinear cavities and interferometers

We present the optical transfer functions for third-order nonlinear cavities that involve an optical carrier frequency and its modulation sideband fields. Our approach is based on linearized transformations and provides a convenient tool to calculate squeezed light sources as well as complex interferometer topologies, containing subsystems that involve intensity dependent phase shifts, i.e., optical Kerr media. As the result we present the noise spectral density of a Michelson interferometer with Kerr nonlinear arm cavities and resonant sideband extraction and find that quantum noise can be squeezed by arbitrary amounts even outside the cavity linewidth. Such a system might apply for future gravitational wave detectors or simply for a continuous wave source of squeezed states.     

Cluster-glass state and the effect of A-site magnetism in electron-doped manganites

The magnetic properties of Y _0.35Ln_0.05Ca_0.6MnO₃ (Ln = La, Pr, Nd, Sm, Gd, Dy) have been investigated thoroughly. The small average cationic radius $〈r_A〉$ of the present system results in a short-range antiferromagnetic charge-ordering (AFM-CO) state in the manganites. With the decrease of temperature, a super-exchange interaction begins to establish itself between the remaining disordered Mn ions after the formation of the short-range CO state. Then, the reentrant cluster-glass state originates from the frustration between the FM order in the clusters and the AFM interactions present in the background matrix. However, the large magnetic moments of Gd³⁺ and Dy³⁺ ions apply an internal magnetic field on the Mn³⁺ and Mn⁴⁺ ions, which results in the suppression of the cluster-glass state.     

Measuring the Magnetooptic Kerr effect by diffraction

A method is presented to measure the Magnetooptic Kerr Effect (MOKE) by diffraction. This is accomplished by giving the magnetized surface a magnetization distribution which functions like a linear diffraction grating. It is shown that the first and higher orders of the light diffracted by such a grating are produced by MOKE whereas the zero order approximately represents the reflectivity of the surface (given by the Fresnel formulae assuming zero magnetization). The procedure used here to form a grating is restricted to thin soft magnetic films, and has been demonstrated on evaporated Fe-Ni films. It uses the magnetic field of a pulse-driven meandered stripe-conductor placed in close contact with the film in the presence of a homogeneous external magnetic field. Experimentally determined Kerr intensities were compared with those measured by ellipsometric methods. Good agreement was found in the case of films with uniaxial anisotropy. For films with no anisotropy, the relative dependence of Kerr intensity vs wavelength, polarisation and angle of incidence could be derived.     

Tunnelling effect of the non-stationary Kerr black hole

Extending Parikh and Wilczek＇s work to the non-stationary black hole, we study the Hawking radiation of the non-stationary Kerr black hole by the Hamilto-Jacobi method. The result shows that the radiation spectrum is not purely thermal and the tunnelling probability is related to the change of Bekenstein Hawking entropy, which gives a correction to the Hawking thermal radiation of the black hole.     

Saturation of the all-optical Kerr effect in solids

We discuss the influence of the higher-order Kerr effect (HOKE) in wide bandgap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore, we present experimental data from a multiwave mixing experiment in BaF2, which can be explained by the appearance of the HOKE.