Friday, August 5, 2016

Deducing broken rotational symmetry from angle-dependent magnetoresistance

There is an interesting preprint
Broken rotational symmetry on the Fermi surface of a high-Tc superconductor 
B. J. Ramshaw, N. Harrison, S. E. Sebastian, S. Ghannadzadeh, K. A. Modic, D. A. Bonn, W. N. Hardy, Ruixing Liang, P. A. Goddard

They measure the interlayer magnetoresistance as function of magnetic field direction (see below) and from this deduce that the C4 symmetry of the crystal is broken to C2 in the charge density wave phase that occurs in the pseudogap region.

They then compare their experimental results to a calculation that uses a Fermi surface (that is reconstructed due to the CDW), a coherent three-dimensional Fermi surface, and a Boltzmann equation.

One might be concerned about the use of a three-dimensional Fermi surface because
a. the CDW correlation length between the layers is small
b. the interlayer charge transport is not necessarily coherent.

However, based on work I did long ago with Perez Moses and Malcolm Kennett [see for example this paper]. I think the theoretical results are robust to these concerns. What we showed is that for two contrasting situations shown below, the angle-dependent magnetoresistance is identical.

The top shows a coherent three-dimensional Fermi surface.
The bottom shows two layers that are coherently coupled together. The interlayer momentum is conserved in hopping between the layers.
One does not need coherence over more than two layers.

Another minor comment is that the authors did most of their calculations numerically. However, I think a lot can be done analytically using the expression below (from the Kennett paper) and simplifying for the case an isotropic scattering time (tau) and the low field limit (omega_c tau much less than one).

I thank Sam Lederer and Steve Hayden for bringing this work to my attention and asking about these issues.

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