Author Topic: Discrepancy in bulk and device transmission  (Read 1252 times)

0 Members and 1 Guest are viewing this topic.

Offline pshinyeong

  • Heavy QuantumATK user
  • ***
  • Posts: 33
  • Country: kr
  • Reputation: 0
    • View Profile
Discrepancy in bulk and device transmission
« on: September 6, 2024, 08:01 »
Hello,

I’ve come across previous discussions regarding differences between bulk and device transmission calculations, but none have helped solve my issue.

I am working with a bulk configuration that consists of 3-10 repeated unit cells in the b direction, along with 12.5 Å of vacuum in the top and bottom of the b direction (as shown in the first screenshot attached).

For the device configuration, it is similar to the bulk configuration, but with 10 repeated unit cells in the c direction (the transport direction).

After performing transmission calculations for both the bulk and the device configurations, I noticed a significant discrepancy between the two results. Theoretically, the bulk and device transmission values should be at least somewhat similar, but that’s not the case in my results.

Could you help me understand the possible reasons for this difference?

Thank you!
« Last Edit: September 6, 2024, 08:08 by pshinyeong »

Offline pshinyeong

  • Heavy QuantumATK user
  • ***
  • Posts: 33
  • Country: kr
  • Reputation: 0
    • View Profile
Re: Discrepancy in bulk and device transmission
« Reply #1 on: September 6, 2024, 08:02 »
+ additional attachment for device configuraiton
« Last Edit: September 6, 2024, 08:08 by pshinyeong »

Offline Anders Blom

  • QuantumATK Staff
  • Supreme QuantumATK Wizard
  • *****
  • Posts: 5541
  • Country: dk
  • Reputation: 91
    • View Profile
    • QuantumATK at Synopsys
Re: Discrepancy in bulk and device transmission
« Reply #2 on: September 9, 2024, 21:32 »
Thanks for sharing all relevant input and output up front!

Maybe this is just a numerical issue. In principle we are of course just counting the number of states at the Fermi level in both cases, so they should agree. (We can see this very clearly from the k-point resolved graph, where the band edges give rise to sharp jumps.)

But the algorithms used in the two cases are quite different, so sampling a single point that might be a sharp peak could mean the results differ numerically but on the whole, if you computed the transmission for several energy points in an internal around the Fermi level, they might look similar qualitatively (and integrate to the same current).

The old algorithm for bulk transmission required the system to be repeated in C until it was as long as an electrode should be. The new algorithm I think doesn't need this, and similarly we now can use minimal electrodes and instead set the electrode copy to be sufficiently "deep" to capture all interactions. That is why the dashed line contains two repetitions. Now, that mean 8.8 Angstrom which still is a bit on the short side... You could retry the device config with the electrode copy length set to 3x the electrode.

Finally, k-points work a bit differently in bulk and device. You took care to use the same density etc, and it's a high number, so I guess it will not make such a drastic difference.

Finally, you might want to double-check that no disturbance of the coordinates was accidentally made to the device after it was built. Sometimes when you click to select some atoms, they move a tiny bit not seen by the eye, but if the Undo button lights up, so know some change was made unintentionally.

Offline pshinyeong

  • Heavy QuantumATK user
  • ***
  • Posts: 33
  • Country: kr
  • Reputation: 0
    • View Profile
Re: Discrepancy in bulk and device transmission
« Reply #3 on: September 13, 2024, 07:18 »
Hello, I have tried several methods to match the results between bulk and device transmission.

Increasing the number of energy point for calculation for both bulk and device have almost no effect in transmission result.

Increasing the extended electrode size for device configuration from 8.8Å to 13.2Å very slightly increases the device transmission from 2.1552 to 2.1641 (I assume this value will converge when increasing the extended elecrode size)

Repeating the bulk configuration x2 in the c direction (transport direction) changes bulk transmission from 4.94 to 2.1667, further repeating it to x3 result in 2.1665.

So repeating the bulk configuration in the transport direction solved the issue. Thank you for your suggestions! :)

Offline Anders Blom

  • QuantumATK Staff
  • Supreme QuantumATK Wizard
  • *****
  • Posts: 5541
  • Country: dk
  • Reputation: 91
    • View Profile
    • QuantumATK at Synopsys
Re: Discrepancy in bulk and device transmission
« Reply #4 on: September 13, 2024, 23:18 »
Yes, this makes sense. I had thought the bulk transmission did not need repetitions in C anymore, but it appears I was wrong. It looks like your value 2.16 is half of the device result, did you expect that (repeated 2x in transverse plane?).

Offline pshinyeong

  • Heavy QuantumATK user
  • ***
  • Posts: 33
  • Country: kr
  • Reputation: 0
    • View Profile
Re: Discrepancy in bulk and device transmission
« Reply #5 on: September 19, 2024, 06:45 »
I realize there may have been some confusion. The screenshots I attached represent the total transmission, while the transmission values I mentioned earlier refer to the per-spin transmission. I apologize for any misunderstanding this may have caused. So per-spin or total transmission, both matches the device and bulk transmission when bulk transmisison are repeated 2x in transverse plane.

Offline Anders Blom

  • QuantumATK Staff
  • Supreme QuantumATK Wizard
  • *****
  • Posts: 5541
  • Country: dk
  • Reputation: 91
    • View Profile
    • QuantumATK at Synopsys
Re: Discrepancy in bulk and device transmission
« Reply #6 on: September 19, 2024, 21:17 »
Cool, then all is clear now, and we learned something about the repetition in C (which probably should be included in the manual...)!