A ug 2 00 1 ESR study of ( Sr , La , Ca ) 14 Cu 24 O 41
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چکیده
We report an electron spin resonance (ESR) study of single crystals of the spin-chain spin-ladder compound (Sr, La, Ca) 14 Cu 24 O 41. The data suggest that in intrin-sically hole doped Sr 14−x Ca x Cu 24 O 41 only a small amount of holes is transferred from the chains to the ladders with increasing x, resulting in a crossover from spin dimerized to uniform spin chains. In the samples of La 14−x Ca x Cu 24 O 41 with reduced hole content a very broad signal is observed in the paramagnetic state, indicative of a surprisingly strong anisotropy of the nearest neighbor exchange in the chains. Spin ladders have recently attracted much attention in particular due to su-perconductivity with T c ≈ 10 K observed under pressure in Sr 14−x Ca x Cu 24 O 41 (SCCO) with x ≥ 11.5 [1]. SCCO contains 2-leg S = 1/2 Cu 2 O 3 ladders showing a large spin gap ∆ ladder ≈ 400 K [2] and S = 1/2 CuO 2 chains, both running along the c-axis. It is 'self-doped' with 6 holes per formula unit. For x = 0 almost all holes reside in the chains and show quasi-2D order [3,4]. In this charge ordered state spin dimers with a gap ∆ dimer ≈ 130 K are formed between next-nearest-neighbor Cu spins via a localized hole [3,4]. The conductivity of SCCO increases with x. The prevailing viewpoint is that the chemical pressure due to Ca doping causes a substantial hole transfer from the chains to the ladders [5], i.e., both metallic conductivity and superconductivity are confined to the ladders. However, recent x-ray absorption data indicate only a marginal increase of the hole content in the ladders with increasing x [6].
منابع مشابه
Interaction between the chain and ladder subsystems in (Ca,Sr,La)(14)Cu(24)O(41) compounds.
We investigate the influence of structural modulations on the electronic properties of incommensurate (Ca,Sr,La)(14)Cu(24)O(41) compounds by band structure calculations based on density functional theory and the local density approximation (LDA). Using a supercell approach with ten CuO(2) chain and seven Cu(2)O(3) ladder segments, we take into account the major effects of the structural incomme...
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