The Mysterious Pseudogap in High Temperature Superconductivity, an Infrared View

Abstract

We review the contribution of infrared spectroscopy to the study of the pseudogap in high temperature superconductors. The pseudogap appears as a depression of the frequency dependent conductivity in the c-axis direction and seems to be related to a real gap in the density of states. It can also be seen in the Knight shift, photoemission and tunneling experiments. In underdoped samples it appears near room temperature and does not close with temperature. Another related phenomenon that has been studied by infrared is the depression in the ab-plane scattering rate. Two separate effects can be discerned. At high temperatures there is broad depression of scattering below 1000 cm-1 which may be related to the gap in the density of states. At a lower temperature a sharper structure is seen, which appears to be associated with scattering from a mode at 300 cm-1, and which governs the carrier life time at low temperatures. This mode shows up in a number of other experiments, as a kink in ARPES dispersion, and a resonance at 41 meV in magnetic neutron scattering. Since the infrared technique can be used on a wide range of samples it has provided evidence that the scattering mode is present in all high temperature cuprates and that its frequency in optimally doped materials scales with the superconducting transition temperature. The lanthanum and neodymium based cuprates do not follow this scaling and appear to have depressed transition temperatures.

arXiv comment12 pages, 7 figures, APS Frank Isakson Prize paper

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• The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50
• Effects of energy dependence in the quasiparticle density of states on far-infrared absorption in the pseudogap state
• Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta
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• Lower Pseudogap Phase: A Spin/Vortex Liquid State
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