I will discuss exceptions to all of these. These exceptions occur for lower dimensional systems. For quasi-one dimensional materials, it is "normal" that the occupied electronic states exhibit no Fermi-Dirac cutoff. It is also common that there is no Fermi surface of quasiparticle Bloch states. Instead, the electrical resistivity (R) can exhibit metallic behavior (dR/dT > 0) while the carriers that participate in electrical conduction are incoherent- not quasiparticle Bloch states at all.
The quasi-one dimensional materials do exhibit the opening of a real bandgap when undergoing a metal-insulator phase transition. Above this phase transition, however, the materials exhibit a pseudogap. Since the idea of a pseudogap is significant for the cuprate superconductors, I will spend considerable time explaining how and what one observes a pseudogap using photoemission. The materials I will discuss include the Bechgaard salts,[1] TTF-TCNQ,[2] and (TaSe_4)_2I.[3]
Unlike three-dimensional materials, for which disorder typically leads to localization, in quasi-one dimensional materials disorder often stabilizes the metallic state and suppresses metal-insulator phase transitions. I present an example of this behavior, using the TaS_2 system, in which a small amount (<1%) Nb substitution for Ta changes a first order metal-insulator transition to a "metallic" state.[4] Come see what the quotation marks signify!
In addition, we have recently investigated TaTe_4 and NbTe_4. These materials possess some aspects of quasi-one dimensional structural properties, but the interchain coupling is so strong that the electrical resistivity has only a small anisotropy (~ 2:1) and looks like a slightly anisotropic three dimensional metal. We find an amusing coexistence of quasi-one dimensional properties and three-dimensional properties.[5]
1. F. Zwick et.al., Phys. Rev. Lett., Vol. 79, 3982 (1997).
2. F. Zwick et.al., submitted.
3. A. Terrasi et.al., Phys. Rev. B, Vol. 52, 5592 (1995).
4. F. Zwick et.al., Phys. Rev. Lett., in press (1998).
5. F. Zwick et.al., submitted.
Work done in collaboration with Prof. G. Margaritondo and colleagues at EPFL, Switzerland. Financial support provided by the Wisconsin Alumni Research Foundation.
Host: Chris Lobb