Gap Opening Transition and Fractal Ground State Phase Diagram in One Dimensional Fermions with Long Range Interaction : Mott Transition as a Quantum Phase Transition of Infinite Order

The metal-insulator transition in one dimensional fermionic systems with long-range interaction is investigated. We have focused on an excitation spectrum by the exact diagonalization technique in sectors with different momentum quantum numbers. At rational fillings, we have demonstrated gap opening transitions from the Tomonaga-Luttinger liquid to the Mott insulator associated with a discrete symmetry breaking by changing the interaction strength. Finite interaction range is crucial to have the Mott transition at a rational filling away from the half filling. It is consistent with the strong coupling picture where the Mott gap exists at any rational fillings with sufficiently strong interaction. The critical regions as a quantum phase transition are also investigated numerically. Non-analytic behavior of the Mott gap is the characteristic in the weak coupling. It is of the order of the interaction in the strong coupling. It implies that the metal-insulator transition of the model is of the infinite order as a quantum phase transition at zero temperature. Fractal nature of the ground state phase diagram is also revealed.Comment: latex209, 14 figure

Molecular-orbital representation of generic flat-band models

We develop a framework to describe a wide class of flat-band models, with and without a translational symmetry, by using "molecular orbitals" introduced in the prior work (HATSUGAI Y. and MARUYAMA I., \textit{EPL}, \textbf{95}, (2011) 20003). Using the molecular-orbital representation, we shed new light on the band-touching problem between flat and dispersive bands. We show that the band touching occurs as a result of collapse, or the linearly dependent nature, of molecular orbitals. Conversely, we can gap out the flat bands by modulating the molecular orbitals so that they do not collapse, which provides a simple prescription to construct models having a finite energy gap between flat bands and dispersive bands.Comment: 6pages, 3 figure