A single-parameter family of lattice-fermion model is constructed. It is a deformation of the Azbel-Hofstadter problem by a parameter h = ∆/t (quantum parameter). A topological number is attached to each energy band. A duality between the classical limit (h = +0) and the quantum limit (h = 1) is revealed in the energy spectrum and the topological number.The model has a close relation to the two-dimensional dwave superconductivity with a magnetic field. Making use of the duality and a topological argument, we shed light on how the quasiparticles with a magnetic field behave especially in the quantum limit.Two-dimensional Dirac fermions with a gauge field are of current interest, e.g., in the context of the vortex state in a two-dimensional d-wave superconductivity [1][2][3]. In our study, Dirac fermions with a gauge field are realized on a two-dimensional lattice. It is a single-parameter deformation of the Azbel-Hofstadter problem [4,5]. In this paper, the parameter h is called quantum parameter [6]. A topological number is assigned for each energy band [7][8][9]. As the quantum parameter is varied continuously from the classical limit (h = +0) to the quantum limit (h = 1), the energy spectrum is reconstructed through the change of each topological number (plateau transition [9][10][11]). Although the two limits are not connected adiabatically, we found that there is a duality between the classical and the quantum regime.The model has a close relation to the two-dimensional d-wave superconductivity with a magnetic field. Applying the duality and a topological argument, we provide insights into the quasiparticle spectrum. In the quantum limit, interference effects become relevant especially at zero energy [3]. The existence of edge states is discussed as well. It reflects a non-trivial topology of each energy band [9,12].Let us define a key Hamiltonian in our paper, which is a single-parameter family of lattice fermion model. It is a deformation of the Azbel-Hofstadter problem. The Hamiltonian is H = l,m c † l H lm c m with One of the basic observables is a topological number for the n-th band, C n [7][8][9]. It iswhere ∇ = ∂/∂k and |u n (k) is a Bloch vector of the n-th band. The integration dk runs over the Brillouin zone (torus). The non-zero topological number results in the existence of edge states. In order to see it, put the system on a cylinder and introduce a fictitious flux through the cylinder (it is equivalent to a twist in the boundary condition) [13]. The edge states move from one boundary to the other as the fictitious flux quanta hc/e is added adiabatically. The number of carried edge states coincides with the summation of topological numbers below the Fermi energy [9,12]. Due to the topological stability, a singularity necessarily occurs with the change of the topological number (plateau transition [9][10][11]). The singularity is identified with an energy-gap closing on some points in the Brillouin zone. In Figs. 1-3, the energy spectra are shown. As h is varied continuously from the classical l...