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تسجيل مستخدم جديدElectron pockets and pseudogap asymmetry observed in the thermopower of underdoped cuprates
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We calculate the diffusion thermoelectric power of high-Tc cuprates using the resonating-valence-bond spin-liquid model developed by Yang, Rice and Zhang (YRZ). In this model, reconstruction of the energy-momentum dispersion results in a pseudogap in the density of states that is heavily asymmetric about the Fermi level. The subsequent asymmetry in the spectral conductivity is found to account for the large magnitude and temperature dependence of the thermopower observed in underdoped cuprates. In addition we find evidence in experimental data for electron pockets in the Fermi surface, arising from a YRZ-like reconstruction, near the onset of the pseudogap in the slightly overdoped regime.
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We consider a model of the pseudogap specifically designed to describe the underdoped cuprates and which exhibits particle-hole asymmetry. The presence of electron pockets, besides the usual hole pockets, leads to the appearance of new vectors beyond
the usual so-called octet model in the joint density of states (JDOS), which underlies the analysis of Fourier-transform scanning tunneling spectroscopy (FT-STS) data. These new vectors are associated with distinct patterns of large amplitude in the JDOS and are expected to occur primarily at positive bias. Likewise a pseudogap Dirac point occurs at positive bias and this point can be determined either through FT-STS or through extrapolation of data from the autocorrelation function of angle-resolved photoemission spectroscopy.
The resonating valence bond spin liquid model for the underdoped cuprates has as an essential element, the emergence of a pseudogap. This new energy scale introduces asymmetry in the quasiparticle density of states because it is associated with the
antiferromagnetic Brillouin zone. By contrast, superconductivity develops on the Fermi surface and this largely restores the particle-hole symmetry for energies below the superconducting energy gap scale. In the highly underdoped regime, these two scales can be separately identified in the density of states and also partial density of states for each fixed angle in the Brillouin zone. From the total density of states, we find that the pseudogap energy scale manifests itself differently as a function of doping for positive and negative bias. Furthermore, we find evidence from recent scanning tunneling spectroscopy data for asymmetry in the positive and negative bias of the extracted $Delta(theta)$ which is in qualitative agreement with this model. Likewise, the slope of the linear low energy density of states is nearly constant in the underdoped regime while it increases significantly with overdoping in agreement with the data.
The phenomenological Greens function developed in the works of Yang, Rice and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the res
onating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here we emphasize the region of doping, $x$, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains $1+x$ hole states) to the Luttinger pocket (which contains $x$ hole states). We find that the quasiparticle effective mass renormalization increases strongly towards the edge of the Luttinger pockets beyond which it diverges.
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