[SOLVED] High temperature superconductors

Hans Aberg

<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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>What is the highest known temperature of superconductors these days?

Hans Aberg

Igor Khavkine

<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>haberg.not.this@matematik.su.se (Hans Aberg) wrote in message news:<haberg.not.this-0305041935390001@du129-86.ppp.su-anst.tninet.se>...\n> What is the highest known temperature of superconductors these days?\n\nThe ambient pressure record is 138K. It is held by\nHg_{1-x} Tl_x Ba_2 Ca_2 Cu_3 O_{8.33}, also referred to\nas Hg-1223. Under pressures of about 300 000 atm, the transition\ntemperature goes up to 160K. This is the current world record.\n\nhttp://www.ceramics.nist.gov/srd/hts/A00373.htm\n\nHope this helps,\n\nIgor\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>haberg.not.this@matematik.su.se (Hans Aberg) wrote in message news:<haberg.not.this-03050419353900....tninet.se>...
> What is the highest known temperature of superconductors these days?

The ambient pressure record is 138K. It is held by
[itex]Hg_{1-x} Tl_x Ba_2 Ca_2 Cu_3 O_{8[/itex].[itex]33},[/itex] also referred to
as [itex]Hg-1223[/itex]. 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

Dirk Bruere at Neopax

<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>Hans Aberg wrote:\n\n> What is the highest known temperature of superconductors these days?\n>\n> Hans Aberg\n\n135K\n\n--\nDirk\n\nThe Consensus:-\nThe political party for the new millenium\nhttp://www.theconsensus.org\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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

alistair

<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">  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?

Igor Khavkine

<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\nalistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405061532.3d60059b@posting.google.com>...\n> Why does increasing the pressure increase the temperature at which materials\n> can be superconductors? And could sound waves passed through\n> a superconductor raise the temperature at which it superconducts\n> given that sound causes regions of high pressure in solids?\n\nI think a naive argument would go as follows. In the standard theory of\nsuperconductivity (BCS theory), the transition temperature is proportional\nto exp(-1/g), where g is a dimensionless constant proportional to both\nthe density of states of the electrons at the Fermi surface and to the\nstrength of the electron-phonon coupling. Simply speaking, putting pressure\non a solid decreases its volume and thus increases the density of states,\nwhich in turn increases g, which in turn increases the transition temperature.\n\nIn high temperature superconductors, the BCS electron-phonon theory does\nnot seem to appy directly. However, many people believe that electron pairing\ninto Cooper pairs still takes place, although phonons may not be responsible\nfor the attractive interaction between them. In this case the constant\ng should still be proportional to the charge carrier density of states\nat the Fermi surface, hence pressure can still increase the transition\ntemperature.\n\nAs for the effect of sound, I doubut it would affect the properties of\nthe material much. I would guess that pressure produced by sound waves\nin solids are is tiny compared to the amount of hydrostatic pressure\nneeded to change the transition temperature. And if you could inject\na sound wave with a great enough amplitude, the crystal would most\nprobably not be able to sustain it.\n\nThis argument is a product of a moment\'s reflection and I don\'t\nhave solid references to back it up. So I hope someone corrects me\nif I\'m wrong.\n\nHope this helps.\n\nIgor\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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 [itex]\exp(-1/g),[/itex] 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

alistair

<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\nThanks Igor -your post was very helpful.\n\nPresumably, if positrons were fed into a superconductor,\ntheir positive electric charge could hold electron pairs together\nat higher temperatures?\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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?

Igor Khavkine

<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>alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405111151.1d88a2ce@posting.google.com>...\n> Thanks Igor -your post was very helpful.\n>\n> Presumably, if positrons were fed into a superconductor,\n> their positive electric charge could hold electron pairs together\n> at higher temperatures?\n\nIf positrons were to be injected into any material, all they would\ndo is annihilate some electrons and produce some photons. Believe\nit or not, this technique is actually useful for styding the distribution\nof electron states inside a material. But it has nothing to do with\nsuperconductivity.\n\nIgor\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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

Uncle Al

<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>alistair wrote:\n>\n> Thanks Igor -your post was very helpful.\n>\n> Presumably, if positrons were fed into a superconductor,\n> their positive electric charge could hold electron pairs together\n> at higher temperatures?\n\nPresumably, positrons injectd into a superconductor will annihilate\nwith electrons to give 511 keV photons, within nanoseconds. If they\npair to positronium, the singlet has a half-life of 10(-10) seconds\nand the triplet 10^(-7) seconds.\n\nhttp://rockpile.phys.virginia.edu/mod23.pdf\n\n--\nUncle Al\nhttp://www.mazepath.com/uncleal/qz.pdf\nhttp://www.mazepath.com/uncleal/eotvos.htm\n(Do something naughty to physics)\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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 [itex]10(-10)[/itex] seconds
and the triplet [itex]10^(-7)[/itex] 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)

Igor Khavkine

<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>k_igor_k@lycos.com (Igor Khavkine) wrote in message news:<f1ac2e6e.0405102119.46fb470f@posting.google.com>...\n> alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405061532.3d60059b@posting.google.com>...\n> > Why does increasing the pressure increase the temperature at which materials\n> > can be superconductors? And could sound waves passed through\n> > a superconductor raise the temperature at which it superconducts\n> > given that sound causes regions of high pressure in solids?\n>\n> I think a naive argument would go as follows. In the standard theory of\n> superconductivity (BCS theory), the transition temperature is proportional\n> to exp(-1/g), where g is a dimensionless constant proportional to both\n> the density of states of the electrons at the Fermi surface and to the\n> strength of the electron-phonon coupling. Simply speaking, putting pressure\n> on a solid decreases its volume and thus increases the density of states,\n> which in turn increases g, which in turn increases the transition temperature.\n>\n> In high temperature superconductors, the BCS electron-phonon theory does\n> not seem to appy directly. However, many people believe that electron pairing\n> into Cooper pairs still takes place, although phonons may not be responsible\n> for the attractive interaction between them. In this case the constant\n> g should still be proportional to the charge carrier density of states\n> at the Fermi surface, hence pressure can still increase the transition\n> temperature.\n\n> This argument is a product of a moment\'s reflection and I don\'t\n> have solid references to back it up. So I hope someone corrects me\n> if I\'m wrong.\n\nAlas, as most of such arguments go, it\'s not entirely correct. I should\nhave also mentioned that in BCS theory, the transition temperature is\nproportional to the so-called Debye temperature. Both it and the value\nof g depend on the phonon spectrum and structural properties of the\nmaterial. High pressure can change structural properties and even\ninduce structural transitions (change from one crystal lattice type\nto another), so it definitely affects the superconducting transition\ntemperature. However, the effect itself is difficult to predict a priori.\nFor example, this experiment reports decreasing transition temperature\nwith increasing pressure cond-mat/0105475.\n\nAs for high temperature superconductors, people don\'t actually know\nwhat makes the transition temperature increase under pressure. But\nit is belived that the layered nature of these materials is important,\nespecially when pressure is applied to bring the layers closer together.\n\nIgor\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>k_{igor_k}@lycos.com (Igor Khavkine) wrote in message news:<f1ac2e6e.0405102119.46fb470f@p...google.com>...
> 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 [itex]\exp(-1/g),[/itex] 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

alistair

<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>Graphite conducts well because of layers of hexagonally arranged\ncarbon atoms with overlapping p orbitals. Maybe p orbitals overlap\nmore\nwhen pressure is applied to superconducting materials. I\'ve also read\nthat in some high temperature superconductors there are cooper pairs\nin existence even before the transition temperature is reached.\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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.

Andrew

<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>An article about one interesting work on superconductors can be found\nhere http://www.physorg.com/news60.html\nIt\'s about YBa2Cu3O6.9 and it\'s superconductivity mechanism.\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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.

Igor Khavkine

<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>alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0405170315.568054bb@posting.google.com>...\n> Graphite conducts well because of layers of hexagonally arranged\n> carbon atoms with overlapping p orbitals. Maybe p orbitals overlap\n> more\n> when pressure is applied to superconducting materials.\n\nI\'m afraid the crystal structure of most high Tc superconductors is\nmore complicated than that of graphite (dozens of atoms per site in some\ncases, plus disorder through doping). The copper-oxygen planes that\nare believed to be responsible for superconductivity are separated by\nmany insulating layers layers. So there might not be much overlap between\natomic orbitals between separated Cu-O planes. However, electrons can\nstill tunnel through the insulating layers. This tunneling is believed\nto affect superconductivity, but it is not well understood how.\n\n> I\'ve also read\n> that in some high temperature superconductors there are cooper pairs\n> in existence even before the transition temperature is reached.\n\nThis is the so-called preformed pairs hypothesis. In this scenario,\nelectrons bind into cooper pairs at one temperature, and condense\na la BEC at a lower temperature, thus creating a superconductor.\nSome people believe that this scenario is responsible for the presence\nof the mysterious pseudogap region in the phase diagram of cuprate\n(Cu-O based) superconductors, but so far evidence is inconclusive.\n\nIgor\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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 [itex]Cu-O[/itex] 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
[itex](Cu-O[/itex] based) superconductors, but so far evidence is inconclusive.

Igor

alistair

<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>This is a great site for superconductors - must be one of the best\nscience sites on the web: www.superconductors.org\n\nApparently even a normally good insulator like a diamond shows\nevidence of\nsuperconductivity at high pressure.\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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.

alistair

<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>The copper-oxygen planes that\nare believed to be responsible for superconductivity are separated by\nmany insulating layers layers. So there might not be much overlap\nbetween\natomic orbitals between separated Cu-O planes. However, electrons can\nstill tunnel through the insulating layers. This tunneling is believed\nto affect superconductivity, but it is not well understood how.\n\n\n\nIn type 1 superconductors cooper pairs suddenly form as the transition\ntemperature is reached and there is a rapid change in\nconductivity.Type 2 superconductors show a gradual change from normal\nto super conductivity .Perhaps type 2 superconductors conduct\nincreasingly better as the temperature changes because electron\nmovement through a lattice causes the formation of "groups" of atoms\nin the lattice which at a slightly lower temperature can then cause\nfurther electron movements in such a way as to increase the formation\nof yet more "groups" and so on.\n\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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 [itex]Cu-O[/itex] 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.

Igor Khavkine

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

Dirk Bruere at Neopax

<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\nalistair wrote:\n\n> This is a great site for superconductors - must be one of the best\n> science sites on the web: www.superconductors.org\n>\n> Apparently even a normally good insulator like a diamond shows\n> evidence of\n> superconductivity at high pressure.\n\nWhat I did not know, and found fascinating, is the class of materials known as\nultraconductors, with conductivities up to a million times better than copper at\nroom temp.\nhttp://www.superconductors.org/ultra.htm\n\nIf only I could get some of that in bulk wire form, cheaply...\n\n--\nDirk\n\nThe Consensus:-\nThe political party for the new millenium\nhttp://www.theconsensus.org\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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

alistair

<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\n\nelectrons can\nstill tunnel through the insulating layers. This tunneling is believed\nto affect superconductivity, but it is not well understood how\n\nIf electrons start tunnelling at one end of a conducting layer and\nthey spend time between conducting layers then they are leaving a\ndefecit of negative charge in the conducting layers and creating a\ndiffusion gradient that can make a current move across the conducting\nlayer.As the temperature is lowered\nand the insulating layers increase their density,would the number of\ntunneling\nelectrons increase- perhaps the insulators lower their resistance with\ndecreasing temperature - and increase the current?\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">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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?

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