Wednesday, March 5, 2014

Large orbital magnetoresistance in layered metals I.

There is a very interesting PRL
Extremely Large Magnetoresistance in the Nonmagnetic Metal PdCoO2
Hiroshi Takatsu, Jun J. Ishikawa, Shingo Yonezawa, Harukazu Yoshino, Tatsuya Shishidou, Tamio Oguchi, Keizo Murata, and Yoshiteru Maeno

This is a highly anisotropic metal with a layered crystal structure.
The authors measure the interlayer resistance as a function of the magnitude and direction of a magnetic field parallel to the layers.

In the abstract of the paper I have highlighted statements for which I give a different perspective, below.
Extremely large magnetoresistance is realized in the nonmagnetic layered metal PdCoO2. In spite of a highly conducting metallic behavior with a simple quasi-two-dimensional hexagonal Fermi surface, the interlayer resistance reaches up to 35 000% for the field along the [1-10] direction. Furthermore, the temperature dependence of the resistance becomes nonmetallic for this field direction, while it remains metallic for fields along the [110] direction. Such severe and anisotropic destruction of the interlayer coherence by a magnetic field on a simple Fermi surface is ascribable to orbital motion of carriers on the Fermi surface driven by the Lorentz force, but seems to have been largely overlooked until now.
1. The material is always metallic.

The figure below shows how the magnetoresistance varies with temperature for fields of fixed direction and magnitude. The key observation is different temperature dependences in the top and bottom panel In the top panel [field in the [1-10] direction] the resistance increases with decreasing temperature and in the bottom panel it decreases.

The authors refer to the top as "non-metallic" and the bottom as "metallic". I think this is confusing terminology because it hints at the idea that there is metal-insulator transition as a function of field direction.
However,  the material is always in a metallic phase., i.e. it is a Fermi liquid with a well-defined Fermi surface. It is just that the magnitude of the orbital magnetoresistance [which only depends on omega_c tau, as stated by the authors since Kohler's rule is obeyed] varies with temperature because the scattering time tau varies with temperature.

This issue has come up before. More than a decade ago, Perez Moses and I wrote a paper trying to elucidate similar confusion.
Temperature dependence of the interlayer magnetoresistance of quasi-one-dimensional Fermi liquids at the magic angles
It contains the figure below. The context is different but the physics is the same.

2. The interlayer transport is coherent.

The authors state
As another aspect of the field dependence, it is known that such a H^1.5 dependence can originate from the out-of-plane incoherent transport [29], in which a large number of in-plane scatterings of conduction electrons occur before electrons hop or tunnel to a neighboring plane.
I don't think Ref. 29 establishes that claim. Rather it just speculates about the effects of incoherent transport. Very little is known about what really happens in that regime. A PRB by Moses and I discusses definitive signatures of coherent transport. One is that as the field is slightly tilted away from the layer direction, the magnetoresistance should drop significantly. The authors do see such an effect. So I would say the material is actually in a coherent regime (at least at low temperatures). Also the interlayer hopping integral t_perp is estimated to be about 40 meV which should be compared to a scattering rate of about 4 meV (corresponding to the scattering time of 1 psec used in their simulations). The approximate H^1.5 dependence can have a more mundane explanation: it reflects a crossover from H^2 at low fields to H at high fields. This is actually what happens for a circular Fermi surface, as discussed in the PRB above and the Schofield-Cooper paper mentioned next.

3. Large orbital magnetoresistance in layered metals has been observed before and discussed theoretically.

There are many measurements in clean organic charge transfer salts which show a large magnetoresistance. For example, Figure 5 of this paper by Kartsovnik and Laukhin shows a magnetoresistance of order 100.
A nice theoretical paper is
Quasilinear magnetoresistance in an almost two-dimensional band structure
A. J. Schofield and J. R. Cooper
Indeed Ref. 31, by two of the co-authors of the PRL, calculates magnetoresistances as large as 60, motivated by experiments on quasi-one-dimensional organic metals.
What is new is the suggestion that one could actually do something technologically useful with this magnetoresistance. I always assumed that the problem was the requirement of low temperatures to keep omega_c tau large.

In a later post I will discuss the essential physics of the orbital magnetoresistance, including the extreme anisotropy. As discussed by the authors the key issue is that the Fermi surface is approximately hexagonal and so has large "flat" sections that can be exactly parallel or perpendicular to an intralayer magnetic field.

I thank Luis Balicas for bringing the paper to my attention.

No comments:

Post a Comment