Optimizing momentum space DMRG using quantum information entropy

In order to optimize the ordering of the lattice sites in the momentum space and quantum chemistry versions of the density matrix renormalization group (DMRG) method we have studied the separability and entanglement of the target state for the 1-D Hubbard model and various molecules. By analyzing the behavior of von Neumann and Neumann-Renyi entropies we have found criteria that help to fasten convergence. A new initialization procedure has been developed which maximizes the Kullback-Leibler entropy and extends the active space (AS) in a dynamical fashion. The dynamically extended active space (DEAS) procedure reduces significantly the effective system size during the first half sweep and accelerates the speed of convergence of momentum space DMRG and quantum chemistry DMRG to a great extent. The effect of lattice site ordering on the number of block states to be kept during the RG procedure is also investigated.Comment: 15 pages, 15 figure

Scaling and Decoherence in the Out-of-Equilibrium Kondo Model

We study the Kondo effect in quantum dots in an out-of-equilibrium state due to an applied dc-voltage bias. Using the method of infinitesimal unitary transformations (flow equations), we develop a perturbative scaling picture that naturally contains both equilibrium coherent and non-equilibrium decoherence effects. This framework allows one to study the competition between Kondo effect and current-induced decoherence, and it establishes a large regime dominated by single-channel Kondo physics for asymmetrically coupled quantum dots.Comment: 4 pages, 3 figures; v2: minor changes (typos corrected, esp. in Eqs. (3), (4), references updated, improved layout for figures

Unified phase diagram of models exhibiting neutral-ionic transition

We have studied the neutral-ionic transition in organic mixed-stack compounds. A unified model has been derived which, in limiting cases, is equivalent to the models proposed earlier, the donor-acceptor model and the ionic Hubbard model. Detailed numerical calculations have been performed on this unified model with the help of the density-matrix renormalization-group (DMRG) procedure calculating excitation gaps, ionicity, lattice site entropy, two-site entropy, and the dimer order parameter on long chains and the unified phase diagram has been determined.Comment: 11 pages, 8 figure

The coexistence of Swabians and Hungarians in a village in near Budapest

The study takes the contact hypothesis of social psychology as its starting point and examines a Swabian-Hungarian ethnic village as an example.The contact hypothesis suggests that contact or interaction between members of different groups under the right circumstances diminishes prejudice and hostility between those groups and mitigates stereotyping and discrimination. However, for this effect to occur, certain conditions must be met: the parties should be of equal status, have a common goal, cooperate, receive the support of authorities and maintain a personal relationship, since this is the only way long- term success can be achieved. Pursuit of mutual assimilation, physical proximity and time can also help people to accept each other and adopt coexistence. These conditions have been met in the case of the Swabian and Hungarian ethnic groups living in the village of Dunabogdány. Social and political processes, Catholicism and the fact that the German language has been added to the local school curriculum have also contributed to the successful outcome