some puzzles on Tc,lambda and Fermi energy

[Ruichun]

Dear all,
I have some puzzles when using the EPW to calculate the superconductivity based on the Allen-Dynes theory. I list them one by one.
1. As we know, each smearing value corresponds to one ? and Tc. But I found only one Tc value estimated by Allen-Dynes equation in epw.out file. So which smearing value does this Tc value corresponds to?
2. This question is about the lambda_FS (?nk on the Fermi surface). MgB2 has four bands crossing the Fermi level, i.e. two ? and two ? bands. But in the tutorial of MgB2, I found the file MgB2.lambda_FS only contains two ? bands, where are the two ? bands? And I also found that the shapes of ?nk (prefix.lambda_YY.cube file plotted by VESTA) have no relationship with the Fermi surface in my case (TiTe2, space group P-3m).
3. The Fermi energy calculated by the fine k-mesh (by wannier?) is different with the Fermi energy calculated by coarse grid (by DFT?) in my calculation(TiTe2), the difference is as large as 1.1 eV. So in the epw.out it writes Warning: check if difference with Fermi level fine grid makes sense But the spread is well converged and the bands calculated by wannier90 coincide well with DFT. I used enough k mesh in nscf calculation(12*12*8). So why it happens?
These problems have puzzled me for a long time.

Yours,
Ruichun Xiao

[roxana]

Dear Ruichun,

1. The estimated Tc corresponds to the first smearing value.
2. EPW doesn't take into account the band character. The *.lamda_FS files contain the eigenvalues close to the Fermi surface (within the some energy window). Please look at the eliashberg_setup.f90 file for more details.
3. Can you check if you get the same Fermi level is the fine mesh is the same as the coarse mesh?

Best,
Roxana

[Ruichun]

Dear Roxana,
Thanks very much for your reply! And I have some questions based on your reply.
(1) Which smearing value does the data in the *.lamda_FS file corresponds to? also the first smearing value? How can I estimate the Tc for other smearing value ? Using the data in the prefix.a2f?
(2) In my calculation, the Fermi level calculated by the fine mesh is different with the coarse mesh, and the difference is as large as 1.1 eV. So I got above warning in the epw.out files. How can I solve this problem?
By the way, how can I plot the anisotropic superconducting gap ?nk(?=0) like the Fig 4 in the tutorial of MgB2? I didnt find the MgB2.pade_aniso_gap0_XX file in my calculation.
Looking forward to your reply! Thanks!

Ruichun Xiao

[sponce]

Dear Ruichun,

To answer your second question, please look at EPW/tests/Inputs/t05

There you should find an input file named epw_aniso.in

This should help you produce the anisotropic superconducting gap.

Best,

Samuel

[Ruichun]

Thanks for your reply!
But the third question in my first post havent solved until now, i.e. the Fermi energy calculated by the fine k-mesh is different with the Fermi energy calculated by coarse grid, the difference is very large. All the calculated results is very strange owing to this.
This question has puzzled me nearly two weeks! And I tried a lot of ways to solve the problem, however none is successful.
Looking forward to your guidance!

[carla.verdi]

Dear Ruichun,

I would suggest you try and plot the band structure obtained from the wannierization, you can do that by including "band_plot=.true." in the input and using a k-path in filkf. How is it compared to the dft band structure?

Best
Carla

[Ruichun]

Dear Carla,
Thanks very much for your reminder. The band structure obtained from the wannierization is indeed different with the DFT.
But in previous calculations, the bands obtained by two methods are the same. I used the same parameters in all calculations, and the only differences are the lattice constants due to pressure. Why would lead to such in current calculation?
Yours,
Ruichun Xiao

[Ruichun]

I attached the parameter of epw.in below. What's wrong with these parameters?

Code: Select all

--
&inputepw
  prefix      = 'TiTe2',
  amass(1)    = 47.86700
  amass(2)    = 127.60
  outdir      = './'
   
  wepexst     = .false.
  ep_coupling = .true.
  elph        = .true.
  kmaps       = .false.
  epbwrite    = .false.
  epbread     = .false.
  epexst  = .false.  
  epwwrite = .false.
  epwread  = .false.
  etf_mem     = .true.
  nbndsub     =  11,
  nbndskip    =  0
  wannierize  = .true.
  num_iter    = 500
proj(1)     = 'Ti:d'
proj(2)     = 'Te:p'
  elinterp    = .true.
  phinterp    = .true.
  tshuffle2   = .true.
  tphases     = .false.

  parallel_k  = .true.
  parallel_q  = .false.

  eps_acustic = 2.0    ! Lowest boundary for the 
  ephwrite    = .true. ! Writes .ephmat files used when wliasberg = .true.

  fsthick     = 0.4  ! eV
  eptemp      = 300  ! K
  degaussw    = 0.10 ! eV
  nsmear      = 6
  delta_smear = 0.1 ! eV

  degaussq     = 0.5 ! meV
  nqstep       = 500
 
  eliashberg  = .true.
  wscut = 0.7   ! eV   Upper limit over frequency integration/summation in the Elisashberg eq

  muc     = 0.10

  dvscf_dir   = '../elph/save'
  
  nk1         = 12
  nk2         = 12
  nk3         = 8

  nq1         = 6
  nq2         = 6
  nq3         = 4

  mp_mesh_k = .true.
  nkf1 = 36
  nkf2 = 36
  nkf3 = 24

  nqf1 = 18
  nqf2 = 18
  nqf3 = 12
 /
  24 {cartesian}
   0.000000000000000E+00   0.000000000000000E+00   0.000000000000000E+00    0.00694444444444444
   0.000000000000000E+00   0.000000000000000E+00   0.188944983169562E+00    0.0138888888888889 
   0.000000000000000E+00   0.000000000000000E+00  -0.377889966339125E+00    0.00694444444444444
   0.000000000000000E+00   0.192450089729862E+00   0.000000000000000E+00    0.0416666666666667 
   0.000000000000000E+00   0.192450089729862E+00   0.188944983169562E+00    0.0416666666666667 
   0.000000000000000E+00   0.192450089729862E+00  -0.377889966339125E+00    0.0416666666666667 
   0.000000000000000E+00   0.192450089729862E+00  -0.188944983169562E+00    0.0416666666666667 
   0.000000000000000E+00   0.384900179459723E+00   0.000000000000000E+00    0.0416666666666667 
   0.000000000000000E+00   0.384900179459723E+00   0.188944983169562E+00    0.0416666666666667 
   0.000000000000000E+00   0.384900179459723E+00  -0.377889966339125E+00    0.0416666666666667 
   0.000000000000000E+00   0.384900179459723E+00  -0.188944983169562E+00    0.0416666666666667 
   0.000000000000000E+00  -0.577350269189585E+00   0.000000000000000E+00    0.0208333333333333 
   0.000000000000000E+00  -0.577350269189585E+00   0.188944983169562E+00    0.0416666666666667 
   0.000000000000000E+00  -0.577350269189585E+00  -0.377889966339125E+00    0.0208333333333333 
   0.166666666666667E+00   0.288675134594792E+00   0.000000000000000E+00    0.0416666666666667 
   0.166666666666667E+00   0.288675134594792E+00   0.188944983169562E+00    0.0833333333333333 
   0.166666666666667E+00   0.288675134594792E+00  -0.377889966339125E+00    0.0416666666666667 
   0.166666666666667E+00   0.481125224324654E+00   0.000000000000000E+00    0.0833333333333333 
   0.166666666666667E+00   0.481125224324654E+00   0.188944983169562E+00    0.0833333333333333 
   0.166666666666667E+00   0.481125224324654E+00  -0.377889966339125E+00    0.0833333333333333 
   0.166666666666667E+00   0.481125224324654E+00  -0.188944983169562E+00    0.0833333333333333 
   0.333333333333333E+00   0.577350269189585E+00   0.000000000000000E+00    0.0138888888888889 
   0.333333333333333E+00   0.577350269189585E+00   0.188944983169562E+00    0.0277777777777778 
   0.333333333333333E+00   0.577350269189585E+00  -0.377889966339125E+00    0.0138888888888889 

[roxana]

The problem you now encounter is related to Wannier90 and not EPW. You need to make sure that the spread in the Wannier functions is reasonable and the DFT band structure well reproduced. You can post the question on the Wannier90 forum.

Roxana