Magnetic heat conduction in Ca-doped SrCuO2 spin chains and related systems

We present new results on the heat conduction of the spin-chain system Sr1-xCaxCuO2. The structure of this material contains two parallel antiferromagnetic S=1/2 copper chains with a J verticalstrokeverticalstroke ∼2100 K. They are decoupled from each other due to a frustration arising from the displacement of half a Cu-Cu length between them. Additionally to the regular phonon heat conduction, this material possesses spinon contributions in the chain direction. A separation of both contributions is ambiguous, since the spinon contribution appears in form of a hump on the low-temperature phonon peak. By substituting the Sr by the isovalent Ca, we aim at a separation of both contributions. The smaller Ca leads to an increased scattering of phonons and consequently to the suppression of the phonon heat conductivity. Since strong changes of the magnetic properties by this doping are unlikely, we expect the spinon contributions to remain untouched. However, a reduction of the spinon contribution is observed upon doping. To explain this observation two scenarios are considered: on the one hand the spin-phonon-drag effect, on the other a possible Ca-induced modulation of the coupling constant due to the doping, that would lead to spinon scattering. Comparisons with earlier results on similar systems, such as the spin ladder Sr14Cu24O41 and the buckled pseudo-two-leg spin ladder CaCu2O3, are carried out. (orig.)