XPS and NMR as Complementary Probes of Pseudogaps and Spin–charge Separation

The possibility that strongly correlated many–electron systems may exhibit spin–charge separation has generated great excitement, particularly in the light of recent experiments on low dimensional conductors and high temperature superconductors. However, finding experimental support for this hypothesis has been made difficult by the fact that most commonly used probes couple simultaneously to spin and charge excitations. We argue that core hole photoemission (XPS) and nuclear magnetic resonance (NMR) couple independently and in exactly comparable ways to the local spin and charge susceptibilities of the system being measured. The explicit comparison of XPS and NMR data, particularly in systems which exhibit a pseudogap, may therefore yield fresh evidence for the existence (or non–existence) of spin–charge separation. Application of these ideas to the normal state of high temperature superconductors is discussed, and illustrated in some detail in the context of quasi one–dimensional charge density wave systems. PACS numbers: 71.10.Pm, 71.30.+h, 82.80 Short title: XPS ’v’ NMR