[Pw_forum] input parameters for lambda.x and electron-phonon coupling in MgB2

[Miao Gao]

Dear all:

If one wants to evaluate the superconducting transition temperature
for a matrial, the last step is executing lambda.x. But I have a
question about the q-points and their sequence in the input file of
lambda.x. We can see these sentences "use kpoints.x program
to calculate q-points and their weight, elph output file names in
the same order as the q-points before" in the explanatory note
of run_example in example03 of QE-5.0.2. The time sequence of elph
output files calculated by ph.x are the same as the order of
q-points generated by ph.x. But the orders of q-points generated
by ph.x and kpoints.x are different (see appendix).
There are basicly three formats to write the input of lambda.x as
follows.

1. use the sequence and weights of q-points calculated by kpoints.x,
keep the time sequence of elph files generated by ph.x.
2. use the sequence and weights of q-points calculated by kpoints.x,
adjust the sequence of elph files according to that given by kpoints.x.
3. use the sequence and weights of q-points calculated by ph.x, keep
the time sequence of elph files generated by ph.x.


First time, I choose format-1 and can reproduce the properties
of MgB2, for example the electron-phonon coupling constant lambda=0.9,
the logarithmic average frequency omega_log=60.9 meV, and 39.9 K for Tc,
which are consistent with literatures.

But according to the explanatory note of run_example in example03,
the format-2 should be chosen, am I right?
Format-2 gives lambda=0.57, omega_log=62 meV, and 11.8 K for Tc in MgB2,
which seems not correct.
So I'm confused which format should I use?

By the way, has anyone reproduced the electron-phonon coupling for MgB2?

I am greatly indebted to you for your help. Thanks again!
 
Best wishes
 
Miao Gao
------------------
Ph.D. student of Department of Physics,
Renmin University of China.
------------------
                        

 

                                     Appendix

The q-points on 6x6x6 mesh given by pw.x and ph.x are same with
each other, with the corresponding weights listed in the output of pw.x.
Following are the q-points and their weights given by pw.x/ph.x.
-------------------------------------------------------------
     Dynamical matrices for ( 6, 6, 6)  uniform grid of q-points
     (  28q-points):
       N         xq(1)         xq(2)         xq(3)
       1   0.000000000   0.000000000   0.000000000   1
       2   0.000000000   0.000000000   0.145024197   2
       3   0.000000000   0.000000000   0.290048395   2
       4   0.000000000   0.000000000  -0.435072592   1
       5   0.000000000   0.192450090   0.000000000   6
       6   0.000000000   0.192450090   0.145024197   12
       7   0.000000000   0.192450090   0.290048395   12
       8   0.000000000   0.192450090  -0.435072592   6
       9   0.000000000   0.384900179   0.000000000   6
      10   0.000000000   0.384900179   0.145024197   12
      11   0.000000000   0.384900179   0.290048395   12
      12   0.000000000   0.384900179  -0.435072592   6
      13   0.000000000  -0.577350269   0.000000000   3
      14   0.000000000  -0.577350269   0.145024197   6
      15   0.000000000  -0.577350269   0.290048395   6
      16   0.000000000  -0.577350269  -0.435072592   3
      17   0.166666667   0.288675135   0.000000000   6
      18   0.166666667   0.288675135   0.145024197   12
      19   0.166666667   0.288675135   0.290048395   12
      20   0.166666667   0.288675135  -0.435072592   6
      21   0.166666667   0.481125224   0.000000000   12
      22   0.166666667   0.481125224   0.145024197   24
      23   0.166666667   0.481125224   0.290048395   24
      24   0.166666667   0.481125224  -0.435072592   12
      25   0.333333333   0.577350269   0.000000000   2
      26   0.333333333   0.577350269   0.145024197   4
      27   0.333333333   0.577350269   0.290048395   4
      28   0.333333333   0.577350269  -0.435072592   2
-------------------------------------------------------------

The sequence and weight of q-points on 6x6x6 mesh given by kpoints.x.
-------------------------------------------------------------
0.0000000  0.0000000  0.0000000   1.00
0.1666667  0.0962250  0.0000000   6.00
0.3333333  0.1924501  0.0000000   6.00
0.5000000  0.2886751  0.0000000   3.00
0.1666667  0.2886751  0.0000000   6.00
0.3333333  0.3849002  0.0000000  12.00
0.3333333  0.5773503  0.0000000   2.00
0.0000000  0.0000000  0.1450242   2.00
0.1666667  0.0962250  0.1450242  12.00
0.3333333  0.1924501  0.1450242  12.00
0.5000000  0.2886751  0.1450242   6.00
0.1666667  0.2886751  0.1450242  12.00
0.3333333  0.3849002  0.1450242  24.00
0.3333333  0.5773503  0.1450242   4.00
0.0000000  0.0000000  0.2900484   2.00
0.1666667  0.0962250  0.2900484  12.00
0.3333333  0.1924501  0.2900484  12.00
0.5000000  0.2886751  0.2900484   6.00
0.1666667  0.2886751  0.2900484  12.00
0.3333333  0.3849002  0.2900484  24.00
0.3333333  0.5773503  0.2900484   4.00
0.0000000  0.0000000  0.4350726   1.00
0.1666667  0.0962250  0.4350726   6.00
0.3333333  0.1924501  0.4350726   6.00
0.5000000  0.2886751  0.4350726   3.00
0.1666667  0.2886751  0.4350726   6.00
0.3333333  0.3849002  0.4350726  12.00
0.3333333  0.5773503  0.4350726   2.00
-----------------------------------------------------------







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