Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 32: Correlated Electrons: Poster Session

TT 32.69: Poster

Thursday, February 28, 2008, 14:00–18:00, Poster B

Effective Models for Undoped and Doped Spin Ladders from Self-Similar Continuous Unitary Transformations — •Sebastian Duffe and Götz Uhrig — Technische Universität Dortmund

Spin ladder systems, occuring e.g. in Sr₁₄Cu₂₄O₄₁ or La₆Ca₈Cu₂₄O₄₁, are important as model systems for 2D high-Tc superconductors. We are generating effective Hamiltonians for these systems systematically by using continuous unitary transformations (CUT). The CUT is performed self-similarly in real space for the coefficients of operator monomials in second quantization. The finite correlation length allows us to omit farther reaching processes.

The triplet excitations on the rungs with their dressing are our quasiparticles called "triplons". The CUT yields a block-diagonal Hamiltonian conserving the quasiparticle number. This effective Hamilton is used to calculate the dispersion for one triplon and the multi-triplon continua. The CUT has been improved decisively by using a new basis for the triplet states that takes advantage of the spin symmetry of the Hamiltonian. Furthermore, the generator is adapted such that it is possible to achieve convergence for much higher values of the coupling constant parallel to the ladder. In this way, the treatment of doping has become also possible.