N\'eel and disordered phases of coupled Heisenberg chains with S=1/2S=1/2 to S=4

We use the two-step density-matrix renormalization group method to study the effects of frustration in Heisenberg models for $S=1/2$ to S=4 in a two-dimensional anisotropic lattice. We find that as in $S=1/2$ studied previously, the system is made of nearly disconnected chains at the maximally frustrated point, $J_d/J_{\perp}=0.5$, i.e., the transverse spin-spin correlations decay exponentially. This leads to the following consequences: (i) all half-integer spins systems are gapless, behaving like a sliding Luttinger liquid as in $S=1/2$; (ii) for integer spins, there is an intermediate disordered phase with a spin gap, with the width of the disordered state is roughly proportional to the 1D Haldane gap.Comment: 13 pages, 22 figure

Critical Exponents in a Quantum Phase Transition of an Anisotropic 2D Antiferromagnet

I use the two-step density-matrix renormalization group method to extract the critical exponents $\beta$ and $\nu$ in the transition from a N\'eel $Q=(\pi,\pi)$ phase to a magnetically disordered phase with a spin gap. I find that the exponent $\beta$ computed from the magnetic side of the transition is consistent with that of the classical Heisenberg model, but not the exponent $z\nu$ computed from the disordered side. I also show the contrast between integer and half-integer spin cases.Comment: 4 pages, 2 figure

A Matrix Kato-Bloch Perturbation Method for Hamiltonian Systems

A generalized version of the Kato-Bloch perturbation expansion is presented. It consists of replacing simple numbers appearing in the perturbative series by matrices. This leads to the fact that the dependence of the eigenvalues of the perturbed system on the strength of the perturbation is not necessarily polynomial. The efficiency of the matrix expansion is illustrated in three cases: the Mathieu equation, the anharmonic oscillator and weakly coupled Heisenberg chains. It is shown that the matrix expansion converges for a suitably chosen subspace and, for weakly coupled Heisenberg chains, it can lead to an ordered state starting from a disordered single chain. This test is usually failed by conventional perturbative approaches.Comment: 4 pages, 2 figure

On the universality class of the Mott transition in two dimensions

We use the two-step density-matrix renormalization group method to elucidate the long-standing issue of the universality class of the Mott transition in the Hubbard model in two dimensions. We studied a spatially anisotropic two-dimensional Hubbard model with a non-perfectly nested Fermi surface at half-filling. We find that unlike the pure one-dimensional case where there is no metallic phase, the quasi one-dimensional modeldisplays a genuine metal-insulator transition at a finite value of the interaction. The critical exponent of the correlation length is found to be $\nu \approx 1.0$. This implies that the fermionic Mott transition, belongs to the universality class of the 2D Ising model. The Mott insulator is the 'ordered' phase whose order parameter is given by the density of singly occupied sites minus that of holes and doubly occupied sites.Comment: 9 pages, 8 figure