Universal Nodal Fermi Velocity in High Temperature Superconductors
Abstract
The physical properties of cuprate superconductors vary dramatically as a function of doping, evolving from antiferromagnetic insulator to superconductors, and to normal metal upon doping. They also vary among different families of compounds, most prominent being the superconducting transition temperature (Tc), which ranges from 38 K for optimally-doped (La2-xSrx)CuO4 (x=0.15) to 135 K for Hg2Ba2Ca2Cu3O10. Such dramatic changes with doping and material family have been observed in transport properties, optical response, magnetic excitation spectra, the superconducting condensation energy and superfluid density. All these seem to imply that the underlying microscopic quantities of cuprates are generally non-universal. We will present a striking exception by providing experimental evidence that the nodal Fermi velocity, a quantity that governs the low-energy quasiparticle dynamics along the (0,0)-(pi,pi) direction where the d-wave superconducting gap is zero in cuprate superconductors, is actually universal. This conclusion is based on extensive measurements from a wide range of doping, and from five families of hole-doped cuprates whose maximum Tc varies by a factor of three or more. The invariance of the nodal Fermi velocity all the way to the Mott insulator boundary clearly signals the breakdown of the conventional Fermi liquid theory where the metal-insulator transition is realized by the divergence of the effective mass near the insulator boundary. A possible way to understand this behaviour is the nanoscale phase separation where doped holes tend to create a preferred local environment so that the behaviour of the individual hole is more or less the same for low energy dynamics.
- Publication:
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APS March Meeting Abstracts
- Pub Date:
- March 2003
- Bibcode:
- 2003APS..MARA20006Z