>Sorry about the long explanation, Dear Jose, THANK YOU VERY MUCH FOR SUCH A DESCRIPTIVE ANSWER. KEEP IT UP. (if it does not consume much of your precious time) This post are very useful to us. They are like summary of some review articles... Thanks again.
Sagar Ambavale The M.S. University of Baroda India ________________________________ From: Jose A. Torres <[email protected]> To: [email protected] Sent: Fri, 30 April, 2010 7:12:18 PM Subject: Re: [SIESTA-L] Calculated Gold resistivity 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
