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some puzzles on Tc,lambda and Fermi energy
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Joined: Tue Sep 06, 2016 2:14 am
Posts: 12
University: USTC
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 λ 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 (λnk on the Fermi surface). MgB2 has four bands crossing the Fermi level, i.e. two σ and two π bands. But in the tutorial of MgB2, I found the file “MgB2.lambda_FS” only contains two σ bands, where are the two π bands? And I also found that the shapes of λnk (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

Sun Sep 11, 2016 4:37 pm Joined: Fri Jan 22, 2016 6:48 pm
Posts: 144
University: SUNY-Binghamton
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

_________________
Roxana Margine
Associate Professor
Department of Physics, Applied Physics and Astronomy
Binghamton University, State University of New York
PO Box 6000
Binghamton, NY 13902-6000

Sun Sep 11, 2016 5:38 pm Joined: Tue Sep 06, 2016 2:14 am
Posts: 12
University: USTC
Dear Roxana,
(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 Δnk(ω=0) like the Fig 4 in the tutorial of MgB2? I didn’t find the MgB2.pade_aniso_gap0_XX file in my calculation.

Ruichun Xiao

Mon Sep 12, 2016 8:31 am  Joined: Wed Jan 13, 2016 7:25 pm
Posts: 575
University: Oxford
Dear Ruichun,

There you should find an input file named epw_aniso.in

Best,

Samuel

_________________
Dr. Samuel Poncé
Department of Materials
University of Oxford
Oxford OX1 3PH, UK

Mon Sep 12, 2016 11:34 am Joined: Tue Sep 06, 2016 2:14 am
Posts: 12
University: USTC
But the third question in my first post haven’t 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.   Wed Sep 14, 2016 5:22 pm  Joined: Thu Jan 14, 2016 10:52 am
Posts: 153
University: University of Vienna
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

Wed Sep 14, 2016 5:45 pm Joined: Tue Sep 06, 2016 2:14 am
Posts: 12
University: USTC
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

Sat Sep 17, 2016 3:09 am Joined: Tue Sep 06, 2016 2:14 am
Posts: 12
University: USTC
I attached the parameter of epw.in below. What's wrong with these parameters?
Code:
--
&inputepw
prefix      = 'TiTe2',
amass(1)    = 47.86700
amass(2)    = 127.60
outdir      = './'

wepexst     = .false.
ep_coupling = .true.
elph        = .true.
kmaps       = .false.
epbwrite    = .false.
epexst  = .false.
epwwrite = .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

Sat Sep 17, 2016 4:01 am Joined: Fri Jan 22, 2016 6:48 pm
Posts: 144
University: SUNY-Binghamton
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

_________________
Roxana Margine
Associate Professor
Department of Physics, Applied Physics and Astronomy
Binghamton University, State University of New York
PO Box 6000
Binghamton, NY 13902-6000

Sat Sep 17, 2016 1:14 pm
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