Magnetische Phasenübergänge im Hubbard-Modell mit Frustration

In this thesis magnetic transitions accompanying the correlation-driven metal-insulator transition in the Hubbard Model with frustration are discussed. Spin systems are called frustrated if there are couplings between sites that hamper the transition to an antiferromagnetically ordered ground state. In the Hubbard Model this can be achieved by a proper choicee of the amplitudes between nearest and next nearest neighbors, generally called t and t'. In the case t'=0 one obtains an antiferromagnetically ordered ground state for every on-site interaction U. For small values of U magnetic symmetry breaking is found together with a metal-insulator transition. In the paramagnetic phase the system changes gradually with increasing U from the metallic to the insulating state. All magnetic transitions are of second order. In the case of finite t' the phase diagram is more complicated and, depending on the degree of frustration, one can find, in addition, first order transitions or more phases, e.g. an antiferromagnetic metal. The main focus of this thesis is on the perturbatively not accessible regions of intermediate coupling. The self-consistency equations for the Bethe lattice with infinite connectivity with finite t' and with total translational invariance are derived. Also, a new method to calculate the corresponding Green and correlation functions is found and solved by means of well-known Monte Carlo and dynamical mean field (DMFT) techniques. It is also discussed how thermodynamical variables depending on U and temperature are influenced by frustration and how the order of the magnetic transition could be changed from second to first order. From the present DMFT results it is concluded that for t'/t=0.5 and intermediate coupling a first order transition from the paramagnetic phase to antiferromagnetism occurs. For t'/t=0.75, even at higher coupling, no antiferromagnetic ordering was found.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2002

Author dc:creator

• Radke de Cuba, Maria Hedwig

Contributors dc:contributor

• Metzner, Walter

• info:eu-repo/classification/ddc/530
• Physik
• stark korrelierte Elektronensysteme

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-119280%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:57223