Regensburg 2002 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 13: Systeme korrelierter Elektronen: Theorie I

TT 13.9: Vortrag

Dienstag, 12. März 2002, 16:45–17:00, H20

Doping dependence of the spin gap in a 2-leg ladder — •Lorenzo Campos Venuti and Alejandro Muramatsu — Institut f”ur Theoretische Physik, Universit”at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart

A proper theoretical description of doped antiferromagnets is until now an unresolved issue. In particular in the case of spin liquids, where the ground state of the undoped system has no classical analogue, a contradiction arises between analytical treatment [1,2] on the one hand and TDMRG simulations [3] or NMR experiments [4] on the other hand. Whereas in the first case, a magnon gap increasing with doping is predicted, a decrease is observed in simulations and experiments.

Starting from the Spin-Fermion model the contradiction is solved by using a controlled analytical treatment that properly takes into account fluctuations in the continuum limit. Integrating out the fermions we are left with a Fermi-determinant which we can evaluate exactly in that limit. The result is an effective Non Linear Sigma Model with doping dependent parameters. The spontaneously generated mass gap of this theory is seen to decrease as holes are introduced. Once physical value for the parameters are given, we obtain very good agreement with NMR experiments performed on Sr_14−xCa_xCu₂₄O₄₁.

[1] M. Sigrist, T. M. Rice, F. C. Zhang, Phys. Rev. B49, (1994), 12058

[2] Y. L. Lee et. al. , Phys. Rev. B60, (1999), 13418

[3] B. Ammon et. al. , Phys. Rev. Lett. 82, (1999), 3855

[4] K. Kumagai et. al. , Phys. Rev. Lett. 78, (1997), 1992