## [SOLVED] High temperature superconductors

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nWhat is the highest known temperature of superconductors these days?\n\nHans Aberg\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>What is the highest known temperature of superconductors these days?

Hans Aberg
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haberg.not.this@matematik.su.se (Hans Aberg) wrote in message news:... > What is the highest known temperature of superconductors these days? The ambient pressure record is 138K. It is held by $Hg_{1-x} Tl_x Ba_2 Ca_2 Cu_3 O_{8$.$33},$ also referred to as $Hg-1223$. Under pressures of about 300 000 atm, the transition temperature goes up to 160K. This is the current world record. http://www.ceramics.nist.gov/srd/hts/A00373.htm Hope this helps, Igor



Hans Aberg wrote: > What is the highest known temperature of superconductors these days? > > Hans Aberg 135K -- Dirk The Consensus:- The political party for the new millenium http://www.theconsensus.org

## [SOLVED] High temperature superconductors

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\nWhy does increasing the pressure increase the temperature at which materials\ncan be superconductors? And could sound waves passed through\na superconductor raise the temperature at which it superconducts\ngiven that sound causes regions of high pressure in solids?\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>Why does increasing the pressure increase the temperature at which materials
can be superconductors? And could sound waves passed through
a superconductor raise the temperature at which it superconducts
given that sound causes regions of high pressure in solids?


alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405061532.3d60059b@p...google.com>... > Why does increasing the pressure increase the temperature at which materials > can be superconductors? And could sound waves passed through > a superconductor raise the temperature at which it superconducts > given that sound causes regions of high pressure in solids? I think a naive argument would go as follows. In the standard theory of superconductivity (BCS theory), the transition temperature is proportional to $\exp(-1/g),$ where g is a dimensionless constant proportional to both the density of states of the electrons at the Fermi surface and to the strength of the electron-phonon coupling. Simply speaking, putting pressure on a solid decreases its volume and thus increases the density of states, which in turn increases g, which in turn increases the transition temperature. In high temperature superconductors, the BCS electron-phonon theory does not seem to appy directly. However, many people believe that electron pairing into Cooper pairs still takes place, although phonons may not be responsible for the attractive interaction between them. In this case the constant g should still be proportional to the charge carrier density of states at the Fermi surface, hence pressure can still increase the transition temperature. As for the effect of sound, I doubut it would affect the properties of the material much. I would guess that pressure produced by sound waves in solids are is tiny compared to the amount of hydrostatic pressure needed to change the transition temperature. And if you could inject a sound wave with a great enough amplitude, the crystal would most probably not be able to sustain it. This argument is a product of a moment's reflection and I don't have solid references to back it up. So I hope someone corrects me if I'm wrong. Hope this helps. Igor



Thanks Igor -your post was very helpful. Presumably, if positrons were fed into a superconductor, their positive electric charge could hold electron pairs together at higher temperatures?



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405111151.1d88a2ce@p...google.com>... > Thanks Igor -your post was very helpful. > > Presumably, if positrons were fed into a superconductor, > their positive electric charge could hold electron pairs together > at higher temperatures? If positrons were to be injected into any material, all they would do is annihilate some electrons and produce some photons. Believe it or not, this technique is actually useful for styding the distribution of electron states inside a material. But it has nothing to do with superconductivity. Igor



alistair wrote: > > Thanks Igor -your post was very helpful. > > Presumably, if positrons were fed into a superconductor, > their positive electric charge could hold electron pairs together > at higher temperatures? Presumably, positrons injectd into a superconductor will annihilate with electrons to give 511 keV photons, within nanoseconds. If they pair to positronium, the singlet has a half-life of $10(-10)$ seconds and the triplet $10^(-7)$ seconds. http://rockpile.phys.virginia.edu/mod23.pdf -- Uncle Al http://www.mazepath.com/uncleal/qz.pdf http://www.mazepath.com/uncleal/eotvos.htm (Do something naughty to physics)



k_{igor_k}@lycos.com (Igor Khavkine) wrote in message news:... > alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405061532.3d60059b@p...google.com>... > > Why does increasing the pressure increase the temperature at which materials > > can be superconductors? And could sound waves passed through > > a superconductor raise the temperature at which it superconducts > > given that sound causes regions of high pressure in solids? > > I think a naive argument would go as follows. In the standard theory of > superconductivity (BCS theory), the transition temperature is proportional > to $\exp(-1/g),$ where g is a dimensionless constant proportional to both > the density of states of the electrons at the Fermi surface and to the > strength of the electron-phonon coupling. Simply speaking, putting pressure > on a solid decreases its volume and thus increases the density of states, > which in turn increases g, which in turn increases the transition temperature. > > In high temperature superconductors, the BCS electron-phonon theory does > not seem to appy directly. However, many people believe that electron pairing > into Cooper pairs still takes place, although phonons may not be responsible > for the attractive interaction between them. In this case the constant > g should still be proportional to the charge carrier density of states > at the Fermi surface, hence pressure can still increase the transition > temperature. > This argument is a product of a moment's reflection and I don't > have solid references to back it up. So I hope someone corrects me > if I'm wrong. Alas, as most of such arguments go, it's not entirely correct. I should have also mentioned that in BCS theory, the transition temperature is proportional to the so-called Debye temperature. Both it and the value of g depend on the phonon spectrum and structural properties of the material. High pressure can change structural properties and even induce structural transitions (change from one crystal lattice type to another), so it definitely affects the superconducting transition temperature. However, the effect itself is difficult to predict a priori. For example, this experiment reports decreasing transition temperature with increasing pressure http://www.arxiv.org/abs/cond-mat/0105475. As for high temperature superconductors, people don't actually know what makes the transition temperature increase under pressure. But it is belived that the layered nature of these materials is important, especially when pressure is applied to bring the layers closer together. Igor



Graphite conducts well because of layers of hexagonally arranged carbon atoms with overlapping p orbitals. Maybe p orbitals overlap more when pressure is applied to superconducting materials. I've also read that in some high temperature superconductors there are cooper pairs in existence even before the transition temperature is reached.



An article about one interesting work on superconductors can be found here http://www.physorg.com/news60.html It's about YBa2Cu3O6.9 and it's superconductivity mechanism.



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405170315.568054bb@p...google.com>... > Graphite conducts well because of layers of hexagonally arranged > carbon atoms with overlapping p orbitals. Maybe p orbitals overlap > more > when pressure is applied to superconducting materials. I'm afraid the crystal structure of most high Tc superconductors is more complicated than that of graphite (dozens of atoms per site in some cases, plus disorder through doping). The copper-oxygen planes that are believed to be responsible for superconductivity are separated by many insulating layers layers. So there might not be much overlap between atomic orbitals between separated $Cu-O$ planes. However, electrons can still tunnel through the insulating layers. This tunneling is believed to affect superconductivity, but it is not well understood how. > I've also read > that in some high temperature superconductors there are cooper pairs > in existence even before the transition temperature is reached. This is the so-called preformed pairs hypothesis. In this scenario, electrons bind into cooper pairs at one temperature, and condense a la BEC at a lower temperature, thus creating a superconductor. Some people believe that this scenario is responsible for the presence of the mysterious pseudogap region in the phase diagram of cuprate $(Cu-O$ based) superconductors, but so far evidence is inconclusive. Igor



This is a great site for superconductors - must be one of the best science sites on the web: www.superconductors.org Apparently even a normally good insulator like a diamond shows evidence of superconductivity at high pressure.



The copper-oxygen planes that are believed to be responsible for superconductivity are separated by many insulating layers layers. So there might not be much overlap between atomic orbitals between separated $Cu-O$ planes. However, electrons can still tunnel through the insulating layers. This tunneling is believed to affect superconductivity, but it is not well understood how. In type 1 superconductors cooper pairs suddenly form as the transition temperature is reached and there is a rapid change in conductivity.Type 2 superconductors show a gradual change from normal to super conductivity .Perhaps type 2 superconductors conduct increasingly better as the temperature changes because electron movement through a lattice causes the formation of "groups" of atoms in the lattice which at a slightly lower temperature can then cause further electron movements in such a way as to increase the formation of yet more "groups" and so on.



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405201457.765b7191@p...google.com>... > In type 1 superconductors cooper pairs suddenly form as the transition > temperature is reached and there is a rapid change in > conductivity.Type 2 superconductors show a gradual change from normal > to super conductivity .Perhaps type 2 superconductors conduct > increasingly better as the temperature changes because electron > movement through a lattice causes the formation of "groups" of atoms > in the lattice which at a slightly lower temperature can then cause > further electron movements in such a way as to increase the formation > of yet more "groups" and so on. This kind of formation of clusters of particles that are in one phase while they are surrounded by a sea of particles in another phase is characteristic of first order phase transitions, such as the water-vapor transition. This behavior is due to the thermodynamic stability (or rather metastability) of both phases near the critical point. However, transition into the superconducting state is second order, meaning that at the critical pint the sample changes completely into the new phase. This happens because thermodynamically, only the new phase is stable while the old one is not. I am not aware of any cases where the normal state-superconductor transition is first order, so neither type I nor II superconductors show a slow and gradual transition. Perhaps you are referring to the transition from superconductor to normal state when a magnetic field is applied. For type II superconductors, there exists an intermediate state where the supercunducting bulk is pierced by tubes of normal metal that allow through magnetic flux. The transitions between superconducting and intermediate as well as intermediate and normal states are still second order. Hope this helps. Igor



alistair wrote: > This is a great site for superconductors - must be one of the best > science sites on the web: www.superconductors.org > > Apparently even a normally good insulator like a diamond shows > evidence of > superconductivity at high pressure. What I did not know, and found fascinating, is the class of materials known as ultraconductors, with conductivities up to a million times better than copper at room temp. http://www.superconductors.org/ultra.htm If only I could get some of that in bulk wire form, cheaply... -- Dirk The Consensus:- The political party for the new millenium http://www.theconsensus.org



electrons can still tunnel through the insulating layers. This tunneling is believed to affect superconductivity, but it is not well understood how If electrons start tunnelling at one end of a conducting layer and they spend time between conducting layers then they are leaving a defecit of negative charge in the conducting layers and creating a diffusion gradient that can make a current move across the conducting layer.As the temperature is lowered and the insulating layers increase their density,would the number of tunneling electrons increase- perhaps the insulators lower their resistance with decreasing temperature - and increase the current?