tag:blogger.com,1999:blog-13869903.post6128477654522818674..comments2024-03-15T21:47:07.684-05:00Comments on nanoscale views: High Tc, pseudogaps, broken symmetriesDouglas Natelsonhttp://www.blogger.com/profile/13340091255404229559noreply@blogger.comBlogger13125tag:blogger.com,1999:blog-13869903.post-72205606319143540092010-03-06T01:08:44.844-06:002010-03-06T01:08:44.844-06:00Somehow, the talk about room temperature supercond...Somehow, the talk about room temperature superconductivity sounds suspiciously similar to carbon nanotube space elevators. :)Zaphod Boozlebroxhttps://www.blogger.com/profile/07626188058236019155noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-50313387874901098622010-02-22T11:30:43.264-06:002010-02-22T11:30:43.264-06:00Hi CG,
The important point is the definition of th...Hi CG,<br />The important point is the definition of the word "phase." We define phases, mostly, according to their symmetries, and we define any change in that symmetry as being a "phase transition." So, by definition, any change in symmetry is a phase transition. But not every phase transition involves a change in symmetry.<br /><br />Physicists like the symmetry description, because it's easy to define the symmetry of a phase and thus to know whether the phase has changed. The liquid-gas phase transition is harder to define (despite the common observation that it's easy to tell a liquid from a gas), because the difference between the two phases is just density. It's possible to move all the way from liquid to gas without going through a phase transition, which makes the definitions slightly harder.Jhttps://www.blogger.com/profile/10294551872667818548noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-75974313568014441742010-02-20T17:43:09.955-06:002010-02-20T17:43:09.955-06:00Thanks to both Professors Natelson and Armitage fo...Thanks to both Professors Natelson and Armitage for the helpful clarifications.<br /><br />Here's my follow up question. Has it been observed, or is it theoretically possible, for there to be a change in symmetry without there being a phase transition?Confused Guynoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-86729414221372191462010-02-19T11:22:38.921-06:002010-02-19T11:22:38.921-06:00it seems that room temperature superconductivity, ...it seems that room temperature superconductivity, much like fusion power, is always 20 years off...<br /><br />:)Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-64575196505367801552010-02-19T05:46:28.241-06:002010-02-19T05:46:28.241-06:00You can have a phase transition without a symmetry...You can have a phase transition without a symmetry change. One of the most familiar examples of a phase transition - water turning to steam - has no change of symmetry and is 1st order. But 1st order transitions can also have symmetry changes ... the given melting example is the most obvious one.<br /><br />If there is no associated latent heat with the transition (so not 1st order) then usually it will be 2nd order and there will be a symmetry change. This is because if there no associated latent heat and no symmetry change than there is usually no difference between one phase and another and hence ... no transition at all. There are counter examples to this reasoning in the form of transitions to "topological" orders (KTB transitions, transitions between QH plateus) however.Peter Armitagehttps://www.blogger.com/profile/11567089164372083820noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-84096229297526960582010-02-18T18:36:17.567-06:002010-02-18T18:36:17.567-06:00I don't see why it isn't possible to have ...I don't see why it isn't possible to have a phase change with no change in symmetry. I'm thinking of something two different choices of unit cell but with some rearrangement of the atoms, but with the same symmetry classes. Or would that be included as a symmetry change?<br /><br />I'm guessing that last comment was spam.CarlBrannenhttps://www.blogger.com/profile/17180079098492232258noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-57896593613030212232010-02-18T09:33:20.555-06:002010-02-18T09:33:20.555-06:00I have never had the opportunity to see my work fe...I have never had the opportunity to see my work featured in a news paper so I don't know how these things go, but do you get a change to correct the article before it is published? I would guess that it would help somewhat if the scientist involved would check that there are no wrong claims made.Heumpjehttps://www.blogger.com/profile/01993462463863993238noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-18979218701688592172010-02-18T08:17:24.701-06:002010-02-18T08:17:24.701-06:00Thanks for the useful comments, everyone. Al, peo...Thanks for the useful comments, everyone. Al, people have synthesized Little's materials (and ones very similar). They're not superconductors. We know much, much more about charge carriers in organic compounds now than we did 46 years ago.Douglas Natelsonhttps://www.blogger.com/profile/13340091255404229559noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-5302236724394393172010-02-18T07:53:07.657-06:002010-02-18T07:53:07.657-06:00"$10 million in new funding just received ......"$10 million in new funding just received ... Taillefer plans to buy a coil of superconducting wire"<br /><br />priceless!<br /><br />Taillefer must be pretty pissed about the article.<br /><br />About the Nernst effect measurement itself - I think the interpretation is very simplified. YBCO has 1 d chains which can contribute to the Nernst signal (in spite of Taillefer's arguments against it) below T*.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-20335792274663023162010-02-18T03:44:37.392-06:002010-02-18T03:44:37.392-06:00I also don't see how this helps in the search ...I also don't see how this helps in the search for room T SC. Having spend some time in this field, I think the pseudogap phase is not helping SC. When doping is decreased, correlations between the electrons become stronger and this seems to lead to a new, possibly striped/nematic phase. I would think that this is detrimental to SC, since it induces localization of some the electrons. This does not necessarily destroy SC, since it is the phase of the wave function that is carrying the current. However, scattering of electrons on this static order will scramble the phases. Another trivial observation is that the pseudogap temperature decreases when SC appears. Most scenarios also show that T* dissapears when SC is at its strongest. <br /><br />My conclusion would therefore be that they have given a bit more evidence for a competing phase scenario (there was already quite some evidence for that from other experiments on the same and other compounds). And that kills at least a few of the proposed scenarios.<br /><br />I liked the remark by the high school teacher:<br /><i>"I'm excited by it," he said. "Unleashing the power of magnets will make a lot of things more affordable and more practical. It's really easy to excite young students with these kinds of possibilities."</i><br /><br />Shows how much actually gets through to the public...O well, as long as they are excited. They <i>are</i> paying for a 10 million dollar magnet...Heumpjehttps://www.blogger.com/profile/01993462463863993238noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-5770899720682717112010-02-17T20:54:01.926-06:002010-02-17T20:54:01.926-06:00Confused Guy, first-order phase transitions have b...Confused Guy, first-order phase transitions have broken symmetries, too. Only second-order transitions can have critical fluctuations, though. Wikipedia actually has quite a decent summary of the issue, <a href="http://en.wikipedia.org/wiki/Phase_transition#Symmetry" rel="nofollow">here</a>.Douglas Natelsonhttps://www.blogger.com/profile/13340091255404229559noreply@blogger.comtag:blogger.com,1999:blog-13869903.post-87564775822045453342010-02-17T18:55:41.559-06:002010-02-17T18:55:41.559-06:00One thing I am wondering about. I have heard that ...One thing I am wondering about. I have heard that the idea that a phase transition necessarily involves a broken symmetry is only in general true for second-order phase transitions, not first-order ones. As I understand it, this is the entire basis of Landau's theory of second order phase transitions.<br /><br />My confusion arises because as I understand it, the liquid-solid phase transition is first order. Is it just a specific example of a first order phase transition that just happens to involve broken symmetry? Or am I wrong, and broken symmetry is characteristic of both first and second-order phase transitions?Confused Guynoreply@blogger.comtag:blogger.com,1999:blog-13869903.post-33017602649350488402010-02-17T18:40:07.057-06:002010-02-17T18:40:07.057-06:00BCS supercons (MgB2 surprise) to LN2 supercons was...BCS supercons (MgB2 surprise) to LN2 supercons was "punctuated evolution" (Bednorz and Mueller were insubordinate). The best minds spend 20 years not cracking that nut. Ambient temp supercons will likely be heterodox again.<br /><br />WA Little at Stanford proposed elegant (beyond synthesis) high temp exciton supercons as doped polyacetylenes with pendant chromophores, <i>Phys. Rev.</i> 134 A1416-A1424 (1964). Said syntheses are trivial today. Make it an advanced undergrad project. The worst it can do is succeed.<br /><br />http://www.mazepath.com/uncleal/pave1.png<br />Stereogram; hydrogens and pi-bonds omitted for ease of viewing. Add pendant chains for (liquid crystal) solubility.Uncle Alhttp://www.mazepath.com/uncleal/noreply@blogger.com