Hans Schumacher
from Physikalisch-Technische Bundesanstalt Braunschweig, German metrology center, gave a talk today. The main direction of his research is to make a practical electric-current standard that uses electron counting in Coulomb blockade regime. Many teams, including that in Delft, have attempted this 20-15 years ago but have to stop. Hans Schumacher boldly took the road that has considered to be a dead end and has demonstrated to us a significant progress as well as a hope for even further progress. I like his strive very much and wish him best sucsess.
That was positive and encouraging part of the talk. There was also a negative and dissapoinig part. It appears that the physical factor that stopped the research 15 years ago – undesired co-tunneling processes – has been completely forgotten today. At least the speaker could not give correct esimations of the precision limitations owing to this factor, and I suspect that people helping him did not think about it too. Find this outrageous since the factor has been discussed in the literature in quite some detail and presents common knowledge in quantum transport. I suggest that the authors, invoved editors and especially anonymous referees of
Phys. Rev. Lett. 104, 186805 (2010)should immediately attend my master course in quantum transport. This seems the only way to restore the continuity of scientific progress:)
1 comment
Dear Yuli:
Really appreciate your comments, especially the sharp critique for complete disregard of co-tunneling in the theory part backing the research program as presented by Hans. I happen to belong to the narrow community you have kindly invited to your master course.
Co-tunneling contribution to the current in these devices is negligible (under favorable conditions) because at any point in the time the product of tunneling amplitudes to the left and to the right remains abysmally low. Loading and unloading parts of the cycle can and should be treated separately, like, e.g., Flensberg , Pustilnik and Niu did for the surface acoustic waves in PRB 60, 16291 (1999). Elastic level broadening is surely relevant unless it is smaller than the temperature around the time of adiabaticity breakdown (which is the fortunate case for the devices of Hans et al.).
Please, shoot these arguments down if you find them misguided.
What has been holding back research progress for at least a decade is the missed realization that the barrier modulation may act simultaneously as a plunger, and act strongly. The effect of simultaneous pinch-off and rise of the potential-well bottom introduces a new energy scale that can trump other scales (like temperature or “quantum shake-up” non-adiabatic excitation al la Flensberg & Co.). Writing sequential tunneling equations a la Beenakker may seem naive to the extreme but the point is in the parametric dependencies of the rates and energies and extracting measurable and falsifiable claims from them.
If in your view this theory agenda is wrong beyond repair, a counterexample or, perhaps, a comment on the PRL in question would be helpful indeed to return our small community to sanity and mend the continuity of scientific progress.
Respectfully yours,
Slava Kashcheyevs