Field induced phase transitions in the helimagnet Ba2CuGe2O7

We present a theoretical study of the two-dimensional spiral antiferromagnet Ba2CuGe2O7 in the presence of an external magnetic field. We employ a suitable nonlinear sigma model to calculate the T=0 phase diagram and the associated low-energy spin dynamics for arbitrary canted fields, in general agreement with experiment. In particular, when the field is applied parallel to the c axis, a previously anticipated Dzyaloshinskii-type incommensurate-to-commensurate phase transition is actually mediated by an intermediate phase, in agreement with our earlier theoretical prediction confirmed by the recent observation of the so-called double-k structure. The sudden pi/2 rotations of the magnetic structures observed in experiment are accounted for by a weakly broken U(1) symmetry of our model. Finally, our analysis suggests a nonzero weak-ferromagnetic component in the underlying Dzyaloshinskii-Moriya anisotropy, which is important for quantitative agreement with experiment.Comment: 17 pages, 14 figures. Corrected typos in the abstrac

Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors

We develop a theory of the magnetization dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. We describe the effects of the strong carrier spin relaxation on the nonlinear optical response by using the Lindblad semigroup method. We demonstrate magnetization control during femtosecond timescales via the interplay between circularly polarized optical excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin interactions. Our results show a light-induced magnetization precession and relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure

Photonic Integrated Circuits for Communication Systems

Photonic Integrated Circuits allow to meet the increasing demand of communication systems for internet which is growing at about 40% per year. This growth is driven mainly by increasing video traffic in the internet network. This growth is now further accelerated by mobile access, with video clients shipping on an all smart phones and tablets, enabling video to be consumed more conveniently via network connections anywhere and anytime. This paper reviews several material platform of photonic integrated circuits a compares their performance. This paper also describes the new approaches in the design and fabrication of optical transceivers based on photonic integrated circuits for next terabit era

Intermediate phase in the spiral antiferromagnet Ba_2CuGe_2O_7

The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear sigma model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H_1 and H_2 such that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.Comment: 15 pages, 11 figures, 2 table