Raman studies in low-dimensional spin systems

Presented in this thesis are theoretical studies of the two-magnon Raman scattering in a quasi-I1D two-leg spin ladder Heisenberg antiferromagnet Sr14Cu24O 41 and in a 2D Heisenberg antiferromagnet LaCuO 4. In Raman scattering using visible light the transferred momentum is negligible in comparison to the momenta in the Brillouin zone. When the experiment is performed with X-rays it is possible to achieve non-zero momentum transfers and it becomes necessary to have a more general expression for the Raman matrix element than previously available. Such a Raman matrix element for a two magnon scattering in a 2D antiferromagnet with non-zero transferred momentum is derived here. The typical scattering polarization geometries of the incident and scattered light are considered. In each case, the intensity profiles are given with and without magnon-magnon interaction taken into account. The effect of the magnon-magnon interaction is evaluated through a series (RPA) type approximation. The RPA renormalized Raman profile shows a peak intensity at transferred momentum (pi, 0) in both scattering geometries. The relevance of this peak to the experimental data is discussed. The two-magnon Raman scattering experiments using visible light in a two-leg spin ladder have shown a remarkably sharp two-magnon peak, with one order of magnitude smaller width than that of a two-magnon Raman profile in a typical 2D antiferromagnet. The Raman matrix element is derived here. The Heisenberg exchange integral J2 between the legs of the ladder is the reason for a non-zero value of the Raman matrix element. The resultant RPA renormalized Raman intensity has a resonant type profile. The width of the resonant feature is controlled by a parameter in the calculations and is rather small. By using the experimental data in comparison to the theoretical curve the values of the Heisenberg exchange integrals are estimated. Finally, we present a derivation of the spin wave spectrum in a theoretical quasi-1D antiferromagnet with a Kagome stripe spin lattice. This is a first step, non-trivial for such highly frustrated spin arrangement, in studying the order by disorder effect.