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

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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)/nuclear magnetic resonance (NMR) couple independently and in exactly comparable ways to the local charge/spin 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 the application is further illustrated in some detail for quasi one-dimensional systems with charge density waves.