X-ray photoelectron spectroscopy and nuclear magnetic resonance as complementary probes of pseudogaps and spin-charge separation

The possibility that strongly correlated many-electron systems may exhibits pin-charge separation has generated great excitement, particularly in the light of recent experiments on low-dimensional conductors and high-temperat ure superconductors. However, finding experimental support for this hypothe sis has been made difficult by the fact that most commonly used probes coup le simultaneously to spin and charge excitations. We argue that core-hole p hotoemission (XPS)/nuclear magnetic resonance (NMR) couple independently an d in exactly comparable ways to the local charge/spin susceptibilities of t he system being measured. The explicit comparison of XPS and NMR data, part icularly for systems which exhibit a pseudogap, may therefore yield fresh e vidence for the existence (or non-existence) of spin-charge separation. App lication of these ideas to the normal state of high-temperature superconduc tors is discussed, and the application is further illustrated in some detai l for quasi-one-dimensional systems with charge-density waves.