Berlin 2012 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 16: Correlated Electrons: Low-dimensional Systems - Models 2
TT 16.1: Vortrag
Dienstag, 27. März 2012, 09:30–09:45, H 0104
The flat-band ferromagnetic transition as a Pauli-correlated percolation transition — •Mykola Maksymenko1, Andreas Honecker2, Roderich Moessner3, Johannes Richter4, and Oleg Derzhko1 — 1ICMP, 79011 Lviv, Ukraine — 2ITP Georg-August-Universität Göttingen, 37077 Göttingen, Germany — 3MPIPKS, 01187 Dresden, Germany — 4Universität Magdeburg, Germany
It is known that flat bands yield a route to ferromagnetism in the Hubbard model, but the actual location and nature of the para-ferro transition remain unknown. We study the N-site Hubbard model on the 2D Tasaki lattice. For electron densities n≤N=N/3 many-electron ground states can be constructed from one-particle states localized in trapping cells. Occupation of neighboring traps may lead to an increase of energy due to on-site repulsion U. However, if electrons in neigboring traps are in a symmetric spin state, due to the Pauli principle on-site repulsion is not active, i.e. electrons can form a ferromagnetic cluster. This problem can be analyzed as a new Pauli-correlated site-percolation problem on the square lattice, where due to the Kramers degeneracy of independent ferromagnetic clusters different cluster coverings of electrons obtain different weights. We provide an exact solution for the corresponding 1D case and a numerical algorithm for the 2D case of the new percolation problem. In 2D the para-ferro transition takes place at concentration p=n/N=pf=0.66±0.01 that is well above the threshold for the standard percolation pc≈0.59. Moreover, there exists a region above pf where ferromagnetism is unsaturated.