Guochu Deng's Abstract

Structural and Spin Dynamics Evolution of Quasi-1D Magnet Sr_14-xCa_xCu₂₄O₄₁ with Chemical Pressure

Guochu Deng

Laboratory of Developments and Methods

Paul Scherrer Insitute, CH-5232, Villigen PSI, Switzerland

Abstract:

  The quasi one-dimensional cuprates Sr_14-xCa_xCu₂₄O₄₁, consisting of spin-chains and spin-ladders, have attracted substantial research interests because of their rich intriguing phenomena, such as spin dimerization, superconductivity under pressure, charge density wave, hole crystallization and so on. However, these interesting phenomena have not been completely understood from the point of structure view. The isovalent Ca substitution of Sr does not change the number of the self-doped 6 holes in this system, but essentially changes physical properties, such as electrical transport, magnetic susceptibility, etc. In this study, we investigated the structural evolution of this material with chemical pressure, namely, Ca substitution on Sr, by using high resolution neutron powder diffraction. The results have been fitted well with a modulated structure model and the structural evolution with chemical pressure, such as bond length, bond angle, modulation vectors,  has been well defined from the refinement. Interestingly, we found the structural evidence for the hole transfer from the chains to the ladders, consistent with the optical conductivity measurement. The Cu valence decrease from 2.45 to 2.15 on chains while it increases from 2.0 to 2.2 on ladders. On the basis of this findings,  the evolution of electrical and magnetic properties can be well understood. And we proposed the possible spin structures for the chains in different hole doped samples.

  Magnetic excitations from the two sublattices, chains and ladders, have been observed by inelastic neutron scattering on triple axis spectrometer, confirming the existence of both spin gaps on the chains and ladders. In the chains, Ca doping weakens the excitation peaks at the same energy range. This amplitude decrease should be attributed to the dilute dimer density at highly doped sample because of the charge transfer. In the ladders, however, slightly Ca doping shift the excitation energy from 32meV to 33.5meV, and half Ca substitution almost destroy the spin gap. The increase of excitation energy should be attributed to the Cu-O bond length decrease along rungs. The suppression of spin gap should be explained by the nonmagnetic Cu³⁺ introduced by the hole transfer to the ladders. The in-gap states,  caused by these nonmagnetic defects in the ladders, increase with the doping, and finally result in the collapse of the spin gap.