Origin of the Pseudogap in High Temperature Superconductors

Abstract

The physical properties of hole-doped high temperature superconductors (HTCS) are characterized by a ‘normal’ state (NS) for temperatures T > T*, and a pseudogap (PG) in the electronic spectrum, for T* > T > T c . Strikingly similar behavior occurs in the charge-density wave (CDW) state, T 0 > T > T c , in the transition metal dichalcogenides (TMD) 2H-MX2, where M = Ta, Nb, and X = S, Se, both in the NS (T > T 0) and in the incommensurate charge-density wave (T ICDW > T > T c ) states. Such strikingly similar behavior has also been seen in the organic layered superconductors (OLS) κ-(ET)2X, where ET is bis(ethylenedithio)tetrathiafulvalene, and X = Cu[N(CN)2]Cl, Cu[N(CN)2]Br, and Cu(SCN)2, both in the NS, T > T SDW > T c , and in the spin-density wave regime, T SDW > T > T c . In all three materials classes, the anomalous transport and thermodynamic properties associated with the pseudogap or density-wave regime are completely independent of the applied magnetic field strength, whereas the same properties below T c are all strongly field dependent. Hence, we propose that the pseudogap in the HTSC arises from charge- and/or spin-density waves, but not from either superconducting fluctuations or charged quasiparticle “preformed pairs”.