Magnetoelastic coupling in a quantum spin ladder

In this thesis, I present magnetic and optical property studies of a S = 1/2 Heisenberg antiferromagnetic quantum system. The compound under investigation is a copper halide spin ladder, (2,3-dmpyH)₂CuBr₄. This material is attractive not only because of its rarity as a spin ladder with strong magnetic exchange along the rail, but because copper halides are good model materials for the copper oxide systems. Both temperature and magnetic field were used to tune the system. Magnetization was performed, allowing us to observe the experimental critical fields. These critical fields were in reasonable agreement with the theoretical models we extrapolated from strong rung spin ladder systems. Variable temperature optical experiments allowed us to assign the electronic excitations and showed that certain features (especially in the visible range) change shape or "sharpen" at low temperature. According to theoretical calculations, these excitations are caused by the CuBr₄²⁻ chromophore of the system. Using the magnetic field, we were able to drive the antiferromagnetic (AFM) to ferromagnetic (FM) transition and see a spectral shift, resulting in a color change. This shift is too large to be a result of either Zeeman splitting or g factor effects, and is interpreted as an effect of magnetoelastic coupling. Finally, the magneto-optical data was correlated with the magnetization data. By plotting the absorption contrast (the integral of the absolute absorption difference) with the magnetization, we saw that the optical properties track the magnetization data with a small lag. The lag that we see is attributed to a slower lattice response in the system. Although we also discuss the results in terms of Cu²⁺ moment saturation, spin canting, and fluctuation canting. The optics, thus, are sensitive to the antiferromagnetic to ferromagnetic transition, and distortion of the "ladder" couples the intramolecular effects with the intermolecular effects.