Striking similarities between the pseudogap phenomena in cuprates and in layered organic and dichalcogenide superconductors

Underdoped high temperature superconductors (HTS) exhibit a 'normal' state for energies E > E-g and/or temperatures T > T-0, and a pseudogap in their electronic spectrum for E < E-g and/or T-0 > T > T-c. Strikingly similar behavior occurs in the transition metal dichalcogenides (TMD) 2H-MX2, where M = Ta, Nb, and X = S, Se, both in the 'normal' (T > T-0) and in the incommensurate charge-density wave (T-10DW > T > T-c) states. Such strikingly similar behavior has also been seen in the organic layered superconductors (OLS) kappa-(ET)(2)X, 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 'normal' region T > T-SDW > T-c and in the spin-density wave region 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 HTS arises from charge- and/or spin-density waves, and not from either superconducting fluctuations or "preformed" charged quasiparticle pairs.