tag:blogger.com,1999:blog-13869903.post3853677997352526865..comments2024-03-29T02:45:10.096-05:00Comments on nanoscale views: What is a quantum point contact? What is quantized conductance?Douglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-13869903.post-76970402399514159542018-05-17T10:58:21.914-05:002018-05-17T10:58:21.914-05:00Hey Prof. Douglas,
Thanks for blogging and giving...Hey Prof. Douglas,<br /><br />Thanks for blogging and giving us physical insights.<br /><br />A relevant topic and which I think is a bit confusing is where is the resistance/voltage drop in a 1D ballistic channel. As a ballistic channel doesn't have dissipation and has no potential drop in it. As far as I understand the answer to 'where the voltage drop is?' depends if the contacts to the 1D channel are resistive (allow for inelastic scattering) or tunneling (no energy dissipation/elastic scattering). In the first case, I think there should be two voltage drops on each of the contacts, while for the tunneling case all the voltage drop will be in the drain electrode (elastic tunneling into the 1D channel, no energy loss in the channel, elastic tunneling out of the 1D channel and then inelastic processes in the drain electrode).<br /><br />Maybe the confusing thing is what is the correct question to ask. Where is the resistance/voltage drop? or How much current can I have in a 1D channel for a given bias?. The 2nd question seems less confusing and will give I=e^2/h*V. The 1st is a bit confusing as it gives a finite resistance for a non-dissipative system. <br /><br />If it is about the contacts, the main point may be the difference between a resistor and a tunnel junction. I think it would be nice if you could comment on this and shed light on your blog :)<br /><br />Best,<br />Avishai BenyaminiAnonymoushttps://www.blogger.com/profile/06161743553261116535noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-21543179471842358702018-05-02T09:09:22.301-05:002018-05-02T09:09:22.301-05:00Hi Prof. Natelson, thank you very much for the tho...Hi Prof. Natelson, thank you very much for the thorough overview on the current research of the 0.7 anomaly. Some familiar names in the author lists that you presented. Glad to see it's still active research.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-56248508483269360312018-04-30T09:21:53.431-05:002018-04-30T09:21:53.431-05:00Hi Anon - Good question. (For those not familiar ...Hi Anon - Good question. (For those not familiar with this story, the "0.7 anomaly" is the name given to a plateau in the conductance of a quantum point contact that appears at about 0.7 G0 as the QPC is pinched off.). As Anon recalls, the explanation proposed by Cronenwett et al. back in 2002 (<a href="https://arxiv.org/abs/cond-mat/0201577" rel="nofollow">https://arxiv.org/abs/cond-mat/0201577</a>) was that this feature may result from Kondo physics - that a single spin can be localized in the point contact during pinch-off, and that the 0.7 feature (and how the conductance evolves as the bias voltage is cranked up) is a signature of resonant conduction via coherent tunneling processes involving that spin (a many-body correlation effect). Meir has a more recent discussion <a href="https://arxiv.org/abs/0710.3307" rel="nofollow">here</a>. Another proposal (<a href="https://www.nature.com/articles/nature12421" rel="nofollow">here</a>) is that this feature comes from enhanced density of states at pinch-off effectively due to the remnants of a van Hove singularity at the 1d band edge (in principle a single-electron effect). Recently, the argument is that the latter scenario is closely <a href="https://arxiv.org/abs/1409.3415" rel="nofollow">connected</a> to the former (different limits), and that <a href="https://arxiv.org/abs/1703.02734" rel="nofollow">spin fluctuations</a> are the key. There is some <a href="https://physics.aps.org/articles/v9/32" rel="nofollow">evidence</a> in the form of phase shifts through the QPC that many-body (Kondo) physics is at work. Still a subtle business!Douglas Natelsonhttps://www.blogger.com/profile/13340091255404229559noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-38063045291809479652018-04-29T14:18:23.276-05:002018-04-29T14:18:23.276-05:00I haven't worked in this field for quite some ...I haven't worked in this field for quite some time now, but has there been an accepted explanation for the 0.7 anomaly since I left 15 years ago?Anonymousnoreply@blogger.com