Dear Jose,
I appreciate your kind explanation.
It was really helpful to understand the system.
Regards,
Kyoung
Quoting "Jose A. Torres" <[email protected]>:
Dear Kyoung:
When it comes to I-V characteristics of molecular
size devices, comparisons to experimental results
is not straight forward.
Even if there is only ballistic transport in the
experimental device (which is so for metallic point
contacts and low biases to a 99% accuracy), there
are many unknowns in the experimental devices, which
imply that a direct comparison might not be
completely meaningful.
Some of these difficult (almost impossible) to control
characteristics of the experimental objects are
the following:
1- The specific (down to the atomic detailed) geometry
of the nano-thing connected to the electrodes.
2- The specific atomic detail of the contact-regions
(regions between the central object and the electrodes).
3- The influence of (uncontrolled) tensile (or compressive)
stress in the contact region.
4- The influence of (thermal or mechanical) vibrations
of the central region.
5- The nano-scale structure of the electrodes near
the contact, local crystaline orientation.
6- The presence of uncontrolled, unwanted, chemical
species at the contact (typically originated from the
residual gas present event under high vaccuum conditions)...
7,8,9... - Influence of spurious electrical fields , ... magnetic
fields ..., local heating , ...
...
...
All the former may affect notably your computed I-V
results.
So this amounts to say that:
--> A simple comparison of a set of calculations with
a set of experiments sometimes is usually not enough
to conclude that the computation is correct or not.
Rather, when facing a big discrepancy, one should wonder
about the following:
--> What is the difference between the computed (model)
system and the real system?
- In some cases the answer to this question may be:
"The real system does not present the molecule attached
in such or such way (or such site) to the electrodes."
- In other cases, the answer may be: "There are likely some
residual gas atoms glued to the nano-object that we did
not take into account."
...
...
...
- Yet, in some other cases the answer may be: "Ballistic
transport only accounts for a fraction of the current here,
most of it is in the diffusive regime."
But the latter is usually not the answer found, given
the atomic scale sizes of the system usually studied.
(The diffusive regime does not usually appear for
very small devices)
Sorry about the long explanation, the summary is that
comparison to experiments in the case of I-V characteristics
of atomic scale things is usually much trickier than
other typical comparisons in DFT (e.g., stable structures
of molecules)
Yours
Jose A.
On 30/04/2010 1:21, [email protected] wrote:
Dear Siesta users,
I've followed tutorials for TranSiesta (which was done TS school 2009)
For one of the exercise, Gold_111, I have some general questions.
I calculated I-V characteristic for a given strcture and the electrical
resistivity is estimated to be ~ 58 *10^(-7)ohm m, which is much larger
than experimental value of 22*10^(-9)ohm m(at T=20C).
Is the difference from only considering ballistic transport and not
including the diffusive transport part? (if yes, then calculated number
should be smaller then experimental value. doesn't it?)
Or temperature factor?
Can you explain clearly about my questions.?
I appreciate any help.
Thanks.
Kyoung
