Collective Spin and Charge Excitations in (Sr,La)14-xCaxCu24O41 Quantum Spin Ladders

In this chapter we focussed on magnetic and electronic properties of two-leg ladder materials. We observed at high frequencies a two-magnon (2M) resonance characteristic of an undoped ladder which we analyze in terms of symmetry, relaxation and resonance properties. Our findings regarding the spectral properties of this excitation were contrasted to 2M Raman measurements in other magnetic crystals and existing theoretical calculations, emphasizing the sharpness of the 2M peak in the context of increased quantum fluctuations in one-dimension. This comparison made us suggest that the spin-spin correlations in an undoped two leg ladder may have a modulated component besides the exponential decay characteristic of a spin liquid ground state. We found that the 2M peak resonates with the Mott gap determined by O 2p -> Cu 3d transitions, following the behavior of the optical conductivity in the 2-3 eV region. Interplane Sr substitution for Ca in (Sr,La)14-xCaxCu24O41 introduces strong disorder leading to inhomogeneous broadening of the 2M resonance in the undoped system. The doped holes in the spin liquid ground state further dilute the magnetic correlations, suppressing considerably the spectral weight of this excitation. (Sr,La)14-xCaxCu24O41 crystals at high Ca concentrations are superconducting under pressure and hole pairing was proposed to be a robust feature of doped ladders. The measured dielectric response in the microwave region, the low energy Raman data, the non- linear transport properties along with soft X-ray scattering allowed us to conclude that the ground state in (Sr,La)14-xCaxCu24O41 for a wide range of Ca concentrations (x