Friday, July 1, 2011

Interlayer magnetoresistance in a pseudogap metal

I have been working through a really nice paperFermi surface of the electron-doped cuprate superconductor Nd2–xCexCuOprobed by high-field magnetotransport by Mark Kartsovnik and collaborators.
The phase diagram of these electron-doped [in contrast to the more common hole-doped cuprates] materials is shown below. x is the Ce content. PG denotes a pseudogap phase.
(b) Shows the Fermi surface expected for x > 0.16 (e.g. from a tight binding model and DFT based calculations) and confirmed by Shubnikov de Haas (SdH) oscillations.
 (c) shows how this Fermi surface may be re-constructed due to a (pi,pi) superlattice potential (which might exist due to co-existing antiferromagnetic (AF) order.
I found the interlayer magnetoresistance measurements shown below particularly interesting. Each curve shows the interlayer resistivity as a function of magnetic field direction (theta= tilt angle from the normal to the layers) for a fixed magnetic field and temperature.
[Above some large angle the resistance goes to zero because the component of magnetic field perpendicular to the layers becomes less than the upper critical field needed to destroy the superconductivity].
What is interesting about these curves?
  • They exhibit significant qualitative differences depending on the doping x, even over the narrow range 0.13 < x < 0.17.
  • This is probably because the pseudogap has a big effect on the magnetoresistance.
  • For x=0.13, 0.15 the dependence on the azimuthal direction (phi) of the magnetic field is very weak (and opposite in sign) compared to that for x=0.16, 0.17.
The only theory of the interlayer magnetoresistance in the presence of a pseudogap that I am aware of is a paper by Michael Smith and myself in 2009. Most of that paper focuses on the case of a field parallel to the layers and shows how the azimuthal angular dependence may reveal the anisotropy of the pseudogap. We did find that the pseudogap reduces the azimuthal anisotropy compared to the anisotropy seen in the normal phase due to anisotropy of Fermi surface properties. This is basically because the pseudogap suppresses large parts of the Fermi surface from contributing to the interlayer conductivity. That physics may be relevant for understanding the data shown above, but a detailed calculation is desirable.

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