B3 Correlations in antiferromagnets Sebastian Eggert (TU Kaiserslautern) (i) Frustrated spin systems (iii) Dimensional crossover Coupled one dimensional quantum systems (chains) can be created with spin chains and ladders as well as quasi one-dimensional ultra-cold gases [P1]. We want to study the quantum phase transition from 1D behavior to a 3D interacting condensate. Topics Luttinger liquids theory and chain mean field: higher order corrections Quantum Monte Carlo simulations Mott vs. superfluid in quasi 1D: Transport Frustrated systems are of large current interest due to the possibility of new phases and exotic excitations due to the large residual entropy at low T. The most relevant realization are spin systems, ultra-cold gases, ion traps, and charge transfer salts, all of which play a central role in the TR 49. Topics Kagome lattice at incommensurate filling: Role of string excitations XXZ model on the anisotropic triangular lattice: Domain walls and possible spin liquid A5 A8 A9 A8 A10 B1 B2 B4 B6 Fig. 8: Coupled 1D boson gases and spin ladders in a magnetic field have the equivalent bosonic descritiion. Fig. 6: Triangular lattice and kagome lattice with 1/3 magnetization. Work programme First year numerical analysis of string excitations in the kagome lattice. numerical analysis of higher order critical points in coupled trimer systems. set up field theoretical calculations for higher order corrections in coupled chain systems. numerical simulations of coupled chain systems. Second year analytic analysis of string excitations for positive xy-coupling in the kagome lattice. analysis of boundary induced behavior in anisotropic triangular lattices. interacting boson theory for higher order critical points in trimer systems. Third/fourth year analysis of impurities in frustrated systems. critical behavior for larger spin clusters N > 3 and different inter-cluster couplings. analysis of transport experiments in coupled chain systems of project A9 (Ott). (ii) Coupled spin clusters Coupled spin clusters have played an important role in the TR 49 as versatile systems, where critical points can be reached either by tuning the magnetic field or the coupling strengths via the chemical composition and/or pressure. Topics Magneto-caloric effect, susceptibility, specific heat Critical scaling Higher order critical points: magnetization plateaus in larger spin clusters A8 B1 B2 B4 B5 B13N Fig. 7: Above: coupled trimers. Right: corresponding magnetization plateaus, which lead to higher order critical points Transregional Collaborative Research Centre SFB/TR 49 Frankfurt / Kaiserslautern / Mainz