Collective Spin and Charge Excitations in   (Sr,La)_{14-x}Ca_xCu_{24}O_{41} Quantum Spin Ladders
  We study magnetic and electronic properties of two-leg ladder materials. We observed a two-magnon (2M) resonance which we analyze in terms of symmetry, relaxation and resonance properties. Our findings were contrasted to 2M Raman measurements in other magnetic crystals. This comparison made us suggest that the spin-spin correlations in a self-doped two leg ladder may have a modulated component besides the exponential decay characteristic of a spin liquid ground state. We found that the 2M intensity resonates at the Mott gap energy. Interplane Sr substitution for Ca introduces strong disorder leading to inhomogeneous broadening of the 2M resonance. The doped holes in the spin liquid ground state further dilute the magnetic correlations, suppressing the spectral weight of this excitation. 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 for a wide range of Ca concentrations (x < 12) is characterized by charge density wave correlations. This state seems to be driven not by phonons but by Coulomb forces and many-body effects. We highlighted the similarity in the finite frequency Raman response as opposed to the very different behavior of the DC resistivity between undoped and doped ladders. We found that at high Ca concentrations the carrier relaxation is characterized by the same large activation energy (~2000 K) as in the self-doped compound. This observation prompted us to suggest an unconventional metallic transport driven by collective electronic response. 
A. Gozar, G. Blumberg