Nonequilibrium transport in the strange metal and pseudogap phases of the cuprates.

We propose that the nonequilibrium current measured in the a-b plane of an underdoped cuprate (in either the strange metal or pseudogap regime) in contact with either an overdoped cuprate or a standard Fermi liquid can be used to diagnose how different the pseudogap and strange metals are from a Fermi liquid. Naively, one expects the strange metal to be more different from a Fermi liquid than the pseudogap is. We compute the expected nonequilibrium transport signal with the three Green's functions that are available in the literature: (1) marginal Fermi-liquid theory, (2) the phenomenological ansatz for the pseudogap regime, and (3) the Wilsonian reduction of the Hubbard model which contains both the strange metal and pseudogap. All three give linear 7-V curves at low bias voltages. Significant deviations from linearity at higher voltages obtain only in the marginal Fermi-liquid approach. The key finding, however, is that I-V curves for the strange metal/Fermi-liquid contact exceed that of the pseudogap/Fermi-l iquid system. If this is borne out experimentally, this implies that the strange metal is less orthogonal to a Fermi liquid than the pseudogap is. Within the Wilsonian reduction of the Hubbard model, this result is explained in terms of a composite-particle picture. Namely, the pseudogap corresponds to a confinement transition of the charge degrees of freedom present in the strange metal. In the strange metal the composite excitations break up and electron quasiparticles scatter off bosons. The bosons here, however, do not arise from phonons but from the charge degrees of freedom responsible for dynamical spectral weight transfer. [ABSTRACT FROM AUTHOR]