[Pw_forum] Pw_forum Digest, Vol 48, Issue 68

Brad Malone brad.malone at gmail.com
Tue Jun 28 01:08:59 CEST 2011

Dear Stefano,

I appreciate your additional remarks and Claudia's thesis. They are both
very helpful. I'll continue to look into this possible Kohn anomaly and its
relationship with the lattice instability as the pressure is lowered.

Thanks again,
UC Berkeley

> Dear Brad & Nicola,
> I was going to reply to Brad, when I noticed that Nicola already did so,
> and quite appropriately. Let me just elaborate a little bit on Nicola's
> remarks.
> The reason why Kohn anomalies require so many k-points to be properly
> captured on a computer is the same why they are so sensitive to temperature
> in nature. They are due to different portions of the Fermi surface to be
> quasi-parallel (i.e. connected by a same q vector, the "nesting" vector). By
> the way, this is why Kohn anomalies are more important on low dimensions:
> the lower the dimension, the easier it is to have nesting. When this occurs,
> perturbations with the periodicity of the nesting vector will be strongly
> screened, hence phonons with that periodicity will be soft or "quasi soft".
> When the temperature increases, the Fermi surfaces becomes "blurred", and
> the very concept of nesting breaks down. Computationally, in a metal the
> energy smearing (Gaussian or other) plays the role of an effective
> temperature. The smaller the smearing, the larger the number of k-points
> necessary to sample the Brillouin zone to an energy resolution compatible
> with the smearing.
> An early paper studying a system where these effects show dramatically is:
> Claudia Bungaro et al. PRL 77, 2491 (1996)
> http://link.aps.org/doi/10.1103/PhysRevLett.77.2491
> You may also find Claudia's PhD thesis worth some attention:
> http://www.sissa.it/cm/thesis/1995/bungaro.ps.gz
> Hope this may help.
> Stefano
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