[Wien] Value of relaxation time in Boltztrap code

[Gavin Abo]

The merit of Boltztrap is that it can get the thermopower without the 
relaxation time (tau) 
[http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg05038.html], 
but this is under the assumption that tau acts as an 'arbitrary' 
constant [http://arxiv.org/abs/cond-mat/0602203v1].

The code can also give the electrical conductivity per relaxation time 
(sigma/tau).  However, if a calculated electrical conductivity (sigma) 
is wanted, then an experimental tau or calculated tau is needed 
[https://sites.google.com/site/cta4rbk/home/tips4vasp].  It is believed 
that the experimental tau is more commonly used.

It seems that the latest version, Boltztrap V1.2.3, might be able to 
estimate tau given the right input.

In the Boltztrap package (BoltzTraP_V1.2.3.tar.bz2) 
[http://www.icams.de/content/departments/ams/madsen/boltztrap.html], the 
following line can be seen in Table 1.1 of doc/UserGuide.pdf:

0 0 0 0 0 
tau-model. Not documented

In the code (src/boseintegrals.F90), it appears that 'tau' might be the 
variable 'lifetime' in Boltztrap.

It can be determined from the documented code (src/m_input.F90) that the 
description of the parameters for the Table 1.1 line should be:

1 0 0 0 -1 
#tau-model: tauref tauexp taurefen tempexp taureftemp

As seen in the line above, the default values if the line is not given 
in case.intrans is:

tauref = 1
tauexp = 0
taurefen = 0
tempexp = 0 <= This value is not currently initialized with the other 
parameters, so it might be uninitialized if the compiler does not set it 0.
taureftemp = -1

 From the equation in the article Okuda et al. PRB 63 113104 (2001), it 
can be seen that tau can be calculated if tauref, the energy (ene), and 
tauexp are known.

tauref is the reference lifetime tau0 (femtoseconds) 
[src/fermiintegrals.F90] <= Not sure why tauref in src/boseintegrals.F90 
uses picoseconds
tauexp is the scattering parameter r (unitless) <= For r values, see 
Durczewski et al. PRB 61 5303 (2000)
taurefen is the reference energy (Ry), e.g. Fermi level, at which tauref 
is given
tempexp is the exponent for the temperature dependence (unitless) <= see 
Table 1 of Asen-Palmer et al. PRB 56 9431 (1997)
         -4 -> isotope effects phonon
         -3 or -4 -> normal phonon
taureftemp is the reference temperature (K) for the given tauref

On 12/17/2013 4:10 AM, sonusharma at iitmandi.ac.in wrote:
> Dear all
> I want to know the value relaxation time &#964;, which is used as a
> constant in the Boltztrap code.
>
> Thanking you
> Dr. Sonu Sharma
> IIT Mandi (H.P.) India.