Berlin 2005 – scientific programme

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TT: Tiefe Temperaturen

TT 32: Correlated Electrons - Low-dimensional Materials II

TT 32.8: Talk

Tuesday, March 8, 2005, 18:00–18:15, TU H2053

DIMER FORMATION IN THE Cu (S=1/2) SPIN CHAINS OF Sr₁₃LaCu₂₄O₄₁ — •H.-H. Klauss¹, H. Gerdes¹, A. Bosse¹, H.-J. Grafe^1,2, D. Mienert¹, J. Litterst¹, R. Klingeler², and B. Büchner² — ¹Institut für Metallphysik und Nukleare Festkörperphysik, TU Braunschweig, 38106 Braunschweig — ²Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden, Helmholtzstr. 20, 01069 Dresden

We present a comparitive ⁶³Cu-NMR study on the Cu chain site in Sr₁₃LaCu₂₄O₄₁ and Sr₁₄Cu₂₄O₄₁. The experiments on the ≈ 60 % hole doped antiferromagnetic (AFM) spin chains in Sr₁₄Cu₂₄O₄₁ confirm the existence of local dimers with a spin excitation gap of Δ ≈ 130 K in good agreement with [1]. The formation of weakly interacting dimers is explained by a specific charge order model. In Sr₁₃LaCu₂₄O₄₁ the hole doping is reduced to ≈ 50 %. In a static alternating spin/hole charge order model an AFM S=1/2 spin chain with a gapless Bonner-Fisher behavior is expected. Susceptibility measurements indeed verify the absence of a spin gap. We present NMR experiments which reveal a strongly temperature dependent Knight shift and T₁ relaxation rate below 150 K which can be described by a thermal activation over a finite spin excitation gap. Possible reasons for this discrepancy will be discussed. [1] M. Takigawa et al., PRB 57 (1998) 1124