Showing papers by "Roman V. Pisarev published in 1998"

Nonlinear Optical Spectroscopy of the Two-Order-Parameter Compound YMnO 3

Abstract: We have observed two types of optical second harmonic spectra of ${\mathrm{Mn}}^{3+}$ ions in hexagonal ${\mathrm{YMnO}}_{3},$ one of which is caused by the noncentrosymmetric ferroelectric ordering of charges, whereas the other is due to the centrosymmetric antiferromagnetic ordering of spins. Partial overlapping between the electronic transitions gives rise to a new kind of nonlinear optical polarization $\mathbf{P}(2\ensuremath{\omega})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\mathbf{P}}^{\mathrm{FE}}(2\ensuremath{\omega})+{\mathbf{P}}^{\mathrm{AFM}}(2\ensuremath{\omega}),$ which depends on two order parameters. The magnitude and phase of $\mathbf{P}(2\ensuremath{\omega})$ can be changed by varying the ratio between the two contributions, as demonstrated by changing the contrast between $180\ifmmode^\circ\else\textdegree\fi{}$ antiferromagnetic domains, which are indistinguishable in linear optics.

Topography of antiferromagnetic domains using second harmonic generation with an external reference

Abstract: O3, in which the time-invariant second-harmonic reference can be generated either intrinsically by the crystal or externally by use of the new technique, so that both methods may be compared. The technique is then used to visualize the domain structure of YMnO3, an antiferromagnetic crystal which does not provide the time-invariant reference wave intrinsically. The experiments demonstrate the wide applicability of the method.

Second Harmonic Spectroscopy and Control of Domain Size in Antiferromagnetic YMnO3

Abstract: A novel technique for the visualization of antiferromagnetic domains making use of second harmonic generation is used to study the easy-plane antiferromagnet YMnO3. Domain states characterized by small, medium, or large domains are observed. Switching from one state to another can be achieved by a heat treatment above TN and subsequent cooling below TN. Contributions to the mechanism of this unusual behavior of domains are discussed.

Magnetospatial dispersion effect in magnetic semiconductors Cd 1-x Mn x Te

Abstract: We show that a magnetic field induces a strong birefringence in noncentrosymmetric cubic crystals of ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ in the Voigt configuration $\mathbf{k}\ensuremath{\perp}\mathbf{B}$. The induced birefringence can be separated into the linear magnetospatial dispersion birefringence of a $\mathrm{kB}$ type, and the quadratic Voigt birefringence of ${B}^{2}$ type. The $\mathrm{kB}$ effect is strikingly anisotropic, whereas the ${B}^{2}$ effect is fully isotropic. A microscopic theory is proposed for explaining the dispersion of symmetric and antisymmetric contributions to the $\mathrm{kB}$ birefringence near the absorption band edge.

Dispersion of the Voigt effect in the magnetic semiconductors Cd 1-x Mn x Te

Abstract: Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd1−xMnxTe (0≤x≤0.52) in order to investigate how the exchange interaction of Mn2+ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x2 is independent of x and the magnetic field direction, i.e., the effect is due to the Mn2+ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(Eg−ℏω) −3.5. This can be explained by a decrease of the exchange interaction of Mn2+ ions with itinerant electrons with increasing distance from the center of the Brillouin zone.

Magnetically induced spatial dispersion in the cubic magnetic semiconductors Cd1−x MnxTe

Abstract: In the transverse geometry we have detected birefringence that is linear in the magnetic field B and the light wave vector k in the cubic magnetic semiconductors Cd1−xMnxTe (0⩽x⩽0.52). The effect was found to be large, ∼1 (deg cm−1 T−1), and highly anisotropic, in contrast to the Faraday and Voigt effects. The phenomenon is represented by terms of type γijklBkkl in the permittivity tensor eij and can be described by two parameters, A and g. Spectral studies have shown that the normalized parameters A/x and g/x are independent of x, i.e., the effect can be related to the Mn2+ ions. Below the edge Eg of the forbidden band, the dispersion of A is described by a (Eg−E)−1.4-dependence, while the dispersion of g is nil. Theoretical analysis has shown that the spectral curves for A and g can be explained by the special features of the dispersion laws for electrons and holes (features related to the fact that there is no inversion center) and by the dependence of the parameters of the exchange interaction on the electron wave vector.

Nonlinear optical spectroscopy of electronic transitions and domains in ferroelectric antiferromagnet YMnO3

Abstract: Hexagonal YMnO3 belongs to a very interesting class of materials, which are ferroelectric below Tc (P. G. 6mm) and antiferromagnetic below TN (magnetic P. G. 6mm). The optical second harmonic generation spectra have been studied in a wide spectral range from 1.6 to 3.2eV. Two types of spectra related to electronic transitions within Mn3+ ions in a strong trigonal field have been observed. One is caused by the ferroelectric ordering of charges, whereas another is due to the antiferromagnetic ordering of spins. Partial overlapping between two types of spectra gives rise to a new kind of nonlinear optical polarization P(2ω), which depends on two order parameters. We show that this nonlinear polarization allows a new kind of internal phasesensitive spectroscopy and the observation of 180° domains, which are indistinguishable in linear optics.

Spatial dispersion phenomena in magneto-optics

Abstract: An experimental study of nonreciprocal spatial-dispersion effects in para-(Cd1−xMnxTe), ferro-(LiFe5O8), and antiferromagnetic (Cr2O3) crystals caused by an external magnetic field or magnetic order is reported.