In this dissertation, two exactly solvable models from the Kitaev class [Ann. Phys. 321, 2 (2006)] of exactly solvable models are analysed. In the second chapter, Kitaev models and their generic properties are reviewed. Majorana fermions are introduced and discussed. Then their relationship with the solution of Kitaev models are discussed which involves the emergence of a Z₂ gauge symmetry and anyonic particles of both Abelian and non-Abelian varieties. The third chapter, which is based on the research article [Phys. Rev. B (Rapid Comm.) 83, (2011)], examines the Kitaev model on the kagome lattice. A rich phase diagram of this model is found to include a topological (gapped) chiral spin liquid with gapless chiral edge states, and a gapless chiral spin liquid phase with a spin Fermi surface. The ground state of the current model contains an odd number of electrons per unit cell which qualitatively distinguishes it from previously studied exactly solvable models with a spin Fermi surface. Moreover, it is shown that the spin Fermi surface is stable against weak perturbations. The fourth chapter is based on the article [Phys. Rev. B 84,(2011)] and analyses a disordered generalisation of the Yao-Kivelson [Phys. Rev. Lett. 99,247203 (2007)] chiral spin-liquid on the decorated honeycomb lattice. The model is generalised by the inclusion of random exchange couplings. The phase diagram was determined and it is found that disorder enlarges the region of the topological non-Abelian phase with finite Chern number. A study of the energy level statistics as a function of disorder and other parameters in the Hamiltonian show that the phase transition between the non-Abelian and Abelian phases of the model at large disorder can be associated with pair annihilation of extended states at zero energy. Analogies to integer quantum Hall systems, topological Anderson insulators, and disordered topological Chern insulators are discussed. / text

This dissertation is devoted to investigation of quantum chaos in the Sachdev-Ye-Kitaev (SYK) and supersymmetric SYK models. First, a unified minimal scheme is developed to classify quantum chaos in the SYK and supersymmetric SYK models and also work out the structure of the energy levels in one periodic table. The SYK with even q-body or supersymmetric SYK with odd q-body interaction, with N even or odd number of sites, are put on an equal footing in the minimal Hilbert space; N (mod 8), q (mod 4) double Bott periodicity, and a reflection relation are identified. Then, exact diagonalizations are performed to study both the bulk energy level statistics and hard-edge behaviors. Excellent agreements between the exact diagonalization results and the symmetry classifications are demonstrated. This compact and systematic method can be transformed to map out more complicated periodic tables of SYK models with more degrees of freedom, tensor models, or symmetry protected topological phases.