Raman response of magnetic excitations in cuprate ladders and planes

K. P. Schmidt, A. Gössling, U. Kuhlmann, C. Thomsen, A. Löffert, C. Gross, and W. Assmus
Phys. Rev. B 72, 094419 – Published 15 September 2005

Abstract

A unified picture for the Raman response of magnetic excitations in cuprate spin-ladder compounds is obtained by comparing calculated two-triplon Raman line shapes to those of the prototypical compounds SrCu2O3 (Sr123), Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the two-leg ladder contains Heisenberg exchange couplings J and J plus an additional four-spin interaction Jcyc. Within this model Sr123 and Sr14 can be described by xJJ=1.5, xcycJcycJ=0.2, JSr123=1130cm1 and JSr14=1080cm1. The couplings found for La6Ca8 are x=1.2, xcyc=0.2, and JLa6Ca8=1130cm1. The unexpected sharp two-triplon peak in the ladder materials compared to the undoped two-dimensional cuprates can be traced back to the anisotropy of the magnetic exchange in rung and leg direction. With the results obtained for the isotropic ladder, we calculate the Raman line shape of a two-dimensional square lattice using a toy model consisting of a vertical and a horizontal ladder. A direct comparison of these results with Raman experiments for the two-dimensional cuprates R5CuO4(R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O6+δ yields a good agreement for the dominating two-triplon peak. We conclude that short-range quantum fluctuations are dominating the magnetic Raman response in both ladders and planes. We discuss possible scenarios responsible for the high-energy spectral weight of the Raman line shape, i.e., phonons, the triple-resonance, and multiparticle contributions.

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  • Received 22 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.094419

©2005 American Physical Society

Authors & Affiliations

K. P. Schmidt*

  • Institut für Theoretische Physik, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany

A. Gössling

  • II. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany

U. Kuhlmann and C. Thomsen

  • Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany

A. Löffert, C. Gross, and W. Assmus

  • Physikalisches Institut, J.W. Goethe-Universität, Robert-Mayer-Strasse 2-4, D-60054 Frankfurt a. M., Germany

  • *Electronic address: ks@thp.uni-koeln.de

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Issue

Vol. 72, Iss. 9 — 1 September 2005

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