<p> An energy gap or "pseudogap", where there is spin pairing without phase coherence, has been observed in a number of unconventional superconductors that has been detected even above the superconducting transition. It has been proposed that this "pseudogap" region is intimately related to the appearance of high Tc superconductivity. Comparable spin gaps have been observed in a number of low dimensional quantum spin systems with a spin-singlet ground state. Therefore quantum magnets which show collective singlet ground states have been receiving much interest recently. </p> <p> As an example of these; CuGe03 is one of the few quasi-one dimensional magnetic insulators which displays a Spin-Peierls transition. This novel transition results from the coupling of the lattice with S=l/2 spin degrees of freedom to break transitional symmetry below some characteristic phase transition temperature, and a collective singlet magnetic ground state with a characteristic energy gap is observed. </p> <p> In my thesis I have studied the critical phenomena associated with the SpinPeierls transition which occurs in the inorganic compound Cul-xCdxGe03by means of Xray Scattering. I also conducted inelastic neutron scattering experiments and studied the temperature dependence of the singlet-triplet excitation of this system. I applied different theoretical methods to determine the best model describing the behavior observed for this material and compared the results with those obtained on the pure compound. </p> <p> There are few quasi two dimensional experimental examples of interacting dimers such as SrCu2(B03)2 which has been proposed as a realization of the Shastry-Sutherland model. This system has been modeled as Heisenberg spins in a square lattice with two exchange coupling constants of magnitudes J and J' along the diagonal and the edges of the lattice. Its ground state is known to be a collective singlet state. It is known theoretically that its ground state changes from a gapped singlet to a gapless antiferromagnetic state as a function of J/J'. Recently, subleading terms in the Hamiltonian have been considered, such as Dzyaloshinski-Moriya interactions, which are needed to understand the precise physical properties of this material. </p> <p> I performed high resolution, inelastic neutron scattering measurements on this material aim at clarifying the nature of the singlet-triplet excitation spectrum. The results revealed the dispersion relations along with the Q-dependence of the excitations. Finally, neutron powder diffraction measurements were also performed in order to investigate any possible structural phase transition in this material. </p> / Thesis / Master of Science (MSc)