Superconductivity at room temperature??

[Skwinty]

I was under the impression that in a superconductor, the magnetic field is removed from the superconductor.

The electric field is confined to within the superconductor.
I could of course be wrong though.


ETA: Marouchkine:
"

From a classical point of view, the superconducting state is characterized by two distinctive properties:
perfect electrical conductivity (ρ = 0) and perfect diamagnetism ​

(B = 0 inside the superconductor), as shown in Figs 2.1 and 2.2, respectively. However, this definition of the superconducting state can soon be

changed because, as recently found in an unconventional organic superconductor, the applied magnetic field induces superconductivity. Therefore, B =/= 0


inside this organic superconductor"

​

 

[sol invictus]

There are about 8,7 million more links to this topic.

Actually there are "only" 50,000 links if you put the phrase in quotes (otherwise you're counting links that mention both terms somewhere, not necessarily together).

But in any case, what I meant was simply that dielectrics reduce the energy density when they polarize in response to an applied field.

I cannot agree more since you will rather die than admit that when a wave intensity gives the "most probable position" of an electron at a position where the intensity is zero, the intensity distribution cannot be a probability distribution with an uncertainty in position around a position at which the probability to find an electron is zero and nothing else but zero. It is a waste of time to even try and reason with a person who argues that the most probable position of any object is at a piont where this probability is zero..

A [probability wave that gives the "most probable" position for an electron where the wave intensity is zero? Come on!

I have no idea what you're talking about in those two quotes - it sounds as though you've misunderstood something basic again. If I had to guess, I'd say you've confused the expectation value with the most probable value (they're not the same), despite the admittedly misleading terminology.

It was not my intention to call Sol a Nazi but if that is the way he experienced it I apologize unreservedly. I just felt that he was twisting around what I was saying and accused him of that . Since it turned out that I was the donkey, I also unreservedly apologize to him for any comment I made that he found insulting.

Apology accepted.

Now - when can we expect your new explanation of the energy budget?

 

[sol invictus]

Furthermore, the defining characteristic of a superconducting charge-carrier is that it can NEVER be accelerated.

I'm pretty sure (without thinking very carefully, and having never done this experiment) that if I take a toroidal piece of superconductor and move a magnet through the hole, I will induce a current flowing around the torus. Similarly for a disk of superconductor passing between the horns of a horseshoe magnet.

Since in neither of those cases can I possible have "injected" any charges, is it your claim that I am wrong about that?

 

[sol invictus]

The field inside a conductor scales linearly with the resistivity. Take the limit of zero resistivity (a perfect conductor), and you get exactly that: a cancelled electric field even in the presence of current. The only intrinsic difference between a superconductor and an ideal conductor is the Meisner effect.

Actually I think one can understand part of the Meissner effect simply from zero resistance. If the resistance is zero, the electric field must be as well. But exactly zero electric field implies dB/dt=0. So if there was no magnetic field at t=0 there cannot ever be one, and therefore a perfect conductor will repel magnetic fields.

Of course this argument doesn't fully explain the Meissner effect - it simply says the magnetic field cannot change with time, not that it must be zero.

 

[Ziggurat]

Of course this argument doesn't fully explain the Meissner effect - it simply says the magnetic field cannot change with time, not that it must be zero.

The expulsion of an existing magnetic field upon cooling into the superconducting phase is the Meissner effect. If you're just talking about no change of field once you're in the superconducting phase, that's just persistent eddy currents, not the Meissner effect. It's a common mistake to consider the latter as being part of the former, but it's not. That's partly just a terminology issue (there is still a connection between them), but the term "Meissner effect" refers specifically and exclusively to expulsion of the field upon transition into the superconducting state.

 

[ben m]

If you're just talking about no change of field once you're in the superconducting phase, that's just persistent eddy currents, not the Meissner effect. It's a common mistake to consider the latter as being part of the former, but it's not. That's partly just a terminology issue (there is still a connection between them), but the term "Meissner effect" refers specifically and exclusively to expulsion of the field upon transition into the superconducting state.

Thanks, that had confused me in the past.

 

[ben m]

Now back to the Josephson junction. Without an applied electric field over the junction, the orbitals barrier jump it as if it is not there, since it is thin enough to allow this to happen. With the electric field the orbital can only jump without encountering this electric-field by increasing its internal energy by qV. Thus this is the extra potential energy it has when it reaches the other side; and which it must have to move without acceleration as the charge-carriers within a superconductor must move or else they do not form a superconducting state. Since there is plenty of stored electric-field energy within a superconductor, this added energy is extracted from this stored energy and therefore there is no violation of the conservation of energy.

That's still a deeply unclear explanation, Johan.

I suppose you could describe a discharging capacitor the way you have described the junction.

"In order to have a voltage on the capacitor you must store energy in its internal field. When a charge crosses the junction, it withdraws energy from the stored field energy." (Well, sort of. If the charge withdraws energy from the field, that means that the field strength has dropped, whcih means the voltage has dropped.) So you have to add: "... and energy is immediately put back into the capacitor by the battery, which notices the voltages drop and injects charge to fix it."

Even if this is not the case, you still don't double-count; when one e- leaks through a disconnected capacitor at charge V, it gets 1 x eV worth of work done on it, as you can tell from the potential drop, and also the capacitor's stored energy decreases by 1 x eV.

Perhaps I misunderstand you entirely. If this is just a pedagogical approximation to a DC current, I have no idea what the point is.

And I'm still not clear on whether you think that each electron emits qV (as everyone else does) or whether you think each electron emits 2qV ("one qV from the potential, plus one qV extracted from the stored")? Perhaps your forthcoming "simpler explanation" will clear it up.

I also want to add: you keep coming back to "charge carriers cannot accelerate in a superconductor". It's wrong in any case, most obviously because superconductors have nonzero AC impedance. It sounds like you learned a casual (and imprecise) description of how superconductors work, and assumed it was the primary or defining law from which everything else should be derived.

In any case: the charge carriers in an SIS junction are not always in a superconductor. There is nothing forbidding charge carriers from accelerating in an insulator, and we're talking about an insulator---the "I" in the SIS. That's where the voltage drops, that's where the electric field may be nonzero, and the transport of an electron across it is not the same thing (and would not follow the "cannot accelerate" dictum, even if it were true within superconductors) as the transport of an electron within a superconductor.

 

[ben m]

I will have to also change the cover since the Vatican asked me not to use the image of God that is on the ceiling of the Sistine Chapel.

If I understand correctly, it is only photographs of the Chapel that are technically under copyright. (Not the chapel itself; Michelangelo has been dead for too long.) If you can find an old-enough photograph of the ceiling, by a dead-enough photographer, then that photo is public domain even if recent photos are not. You could photocopy something out of an old art book.

 

[sol invictus]

The expulsion of an existing magnetic field upon cooling into the superconducting phase is the Meissner effect. If you're just talking about no change of field once you're in the superconducting phase, that's just persistent eddy currents, not the Meissner effect. It's a common mistake to consider the latter as being part of the former, but it's not. That's partly just a terminology issue (there is still a connection between them), but the term "Meissner effect" refers specifically and exclusively to expulsion of the field upon transition into the superconducting state.

What that simple argument shows is that perfect conductors must repel applied external magnetic fields, so that the field inside doesn't change - but you're right, it doesn't say anything about expelling pre-existing fields as the resistance is lowered to zero. That's not a phenomenon that can be understood from Maxwell's equations.

Incidentally, does either of you know what the origin of the friction is when you levitate a magnet above a superconductor?

That is, levitate the magnet above a fair sized piece of flat superconductor, then push it parallel to the surface. It slows and stops rapidly (much more rapidly than air resistance alone would do).

I assume that's some kind of dissipation associated with flux pinning and the field lines jumping from impurity to impurity - is that correct? Or is it resistance in the induced eddy currents as they pass through the non-zero magnetic field regions? Or something else?

 

[Uncayimmy]

Incidentally, does either of you know what the origin of the friction is when you levitate a magnet above a superconductor?

That is, levitate the magnet above a fair sized piece of flat superconductor, then push it parallel to the surface. It slows and stops rapidly (much more rapidly than air resistance alone would do).

I assume that's some kind of dissipation associated with flux pinning and the field lines jumping from impurity to impurity - is that correct? Or is it resistance in the induced eddy currents as they pass through the non-zero magnetic field regions? Or something else?

I don't know if it's related, but one humid summer in high school when we were about week into the two-a-day football practices, I was showering in the nasty locker room showers. I dropped my heavy-duty, anti-bacterial soap. It didn't actually hit the floor but rather slid around levitating just above the floor and then quickly settled into place, just hovering there. I suppose something similar is going on considering the opposing forces in play.

Note: Before anyone asks, No, I don't have the math.

 

[Ziggurat]

Incidentally, does either of you know what the origin of the friction is when you levitate a magnet above a superconductor?

That is, levitate the magnet above a fair sized piece of flat superconductor, then push it parallel to the surface. It slows and stops rapidly (much more rapidly than air resistance alone would do).

I assume that's some kind of dissipation associated with flux pinning and the field lines jumping from impurity to impurity - is that correct? Or is it resistance in the induced eddy currents as they pass through the non-zero magnetic field regions? Or something else?

My understanding is that the macroscopic effect you're describing is primarily flux pinning on defects (not just impurities in the sense of contaminants, grain boundaries will do it too), but I believe there is some resistance to vortex motion due to the normal state inside the vortex as well.

 

[Skwinty]

Actually I think one can understand part of the Meissner effect simply from zero resistance. If the resistance is zero, the electric field must be as well. But exactly zero electric field implies dB/dt=0. So if there was no magnetic field at t=0 there cannot ever be one, and therefore a perfect conductor will repel magnetic fields.

Of course this argument doesn't fully explain the Meissner effect - it simply says the magnetic field cannot change with time, not that it must be zero.

Question.

If there is no electric or magnetic field in the superconductor, what force field is responsible for accelerating the paired electrons round bends in the conductor?

 

[Ririon]

I don't know if it's related, but one humid summer in high school when we were about week into the two-a-day football practices, I was showering in the nasty locker room showers. I dropped my heavy-duty, anti-bacterial soap. It didn't actually hit the floor but rather slid around levitating just above the floor and then quickly settled into place, just hovering there. I suppose something similar is going on considering the opposing forces in play.

Note: Before anyone asks, No, I don't have the math.

I thought stories about dropping the soap in the common shower was restricted to Forum Community.

 

[johanfprins]

I am looking forward to it, as well.

Yes; and the last few days I have been trying as fast as I can to put it in words that will not lead to further animosity on this thread. I found it difficult without also doing the mathematics carefully. I have thus decided that the only way to do this is to write a decent erratum to my book and post it on my website. I will at the same time post the section in my book dealing with all the aspects of Josephson tunneling.

But I will have to request some indulgence from the readers of this thread, since I cannot do this overnight. It is also Xmas time, and my grandchildren are becoming very unhappy that I spend all my time in front of my computer. Thus accept that the information will appear on my website before the middle of January. I will also post on this thread once the information is loaded, or if I could not make the deadline.

In the mean time I want you to contemplate the following outline: Consider the SIS before the regions become superconducting and assume that the thickness of the SC regions are the same value D and that I is so thin that we can at first ignore its presence. With voltage V_main over the SIS you will have an electric field energy proportional to E² within the material, which is divided half-half between the two SIS regions.

When these regions become superconducting, this electric field-energy becomes totally stored within the charge-carriers which polarize to increase their internal energies to accommodate it. But, the internal energy of each orbital is still the same, no matter on what side of the junction it is. Now put a voltage-drop V within the junction. The energy levels shift by qV owing to the shift in Fermi-levels: But in addition the voltage over the lower energy S region also decreases by V_main/2-V. Thus the electric-field energy that these orbitals have to store internally decreases and when you calculate this, one finds that it adds another energy drop qV to the energy drop caused by the shift in Fermi-levels.

I know you might now want to ask more questions which will just extend this thread with arguments being thrown without having studied the full calculation. I thus implore you to be patient. If I am a crackpot, you will have enough time to rub it in after you have seen the full calculation.

 

[johanfprins]

I'm pretty sure (without thinking very carefully, and having never done this experiment) that if I take a toroidal piece of superconductor and move a magnet through the hole, I will induce a current flowing around the torus. Similarly for a disk of superconductor passing between the horns of a horseshoe magnet.

Since in neither of those cases can I possible have "injected" any charges, is it your claim that I am wrong about that?

My, my the postings are coming so fast and furious that it is impossible to attend to each one individually. I will have to be highly selective to maintain my sanity; which has already been severely impaired by listening to the arguments of main stream physicists during the past 10 years.

But this argument by Sol is so incisive (congratulations Sol) and important that I must comment. I treat it in detail in my book. The fact is that a circular current around a superconducting ring is not caused by the induced circular electric-field accelerating the charge-carriers. If it were so, then when next switching off the magnetic field, the reverse induced electric-field must decelerate the current to become zero. But this is not what happens.

When switching off the magnetic field, the current proceeds merrily as if this reverse electric field is not there, since the superconducting charge-carriers cannot even experience this field; just as they did not experience it when the magnetic field was switched on. You thus end up with a perpetual current trapping its own magnetic field through the ring.

When you analyze the situation correctly, you find that a circular current within a superconducting ring only starts to manifest when it is required to keep the phase in a lower-energy superconducting state. This is why a current does not even flow before a critical magnetic field is reached equal to a flux quantum. Thus the circular current around a superconducting ring is not caused by the acceleration of the charge carriers under the action of the induced circular electric field. Superconducting charge carriers cannot be accelerated by an electric-field ever!

 

[johanfprins]

If I understand correctly, it is only photographs of the Chapel that are technically under copyright. (Not the chapel itself; Michelangelo has been dead for too long.) If you can find an old-enough photograph of the ceiling, by a dead-enough photographer, then that photo is public domain even if recent photos are not. You could photocopy something out of an old art book.

That is what I also thought and that is why I used the oldest photograph taken before the chapel was retouched. But to play safe I wrote to the Vatican Museum who took such a long time to answer that I had to go ahead with the first printing.

They then came back and asked how many books etc. etc. etc. It was clear that they wanted me to pay royalties. I thus told them that the book is on physics and that I will most probably not sell many of them since the main stream physicists do not like to think past their blinkered eyes; and therefore if they do not want to give permission I will have to use an alternative image of God.

Last week I received a letter from them requesting that should not use the image.

 

[johanfprins]

Actually I think one can understand part of the Meissner effect simply from zero resistance. If the resistance is zero, the electric field must be as well. But exactly zero electric field implies dB/dt=0. So if there was no magnetic field at t=0 there cannot ever be one, and therefore a perfect conductor will repel magnetic fields.

Of course this argument doesn't fully explain the Meissner effect - it simply says the magnetic field cannot change with time, not that it must be zero.

I have my hands so full that I do not want to be drawn into the Meissner effect which is also explained in my book. The Meissner effect is not caused as it is modeled by the main stream models. What happens is that the stationary orbitals absorb magnetic energy (similar to the way that they absorb electric-field energy). When the absorbed energy becomes too high there is not enough orbitals left to allow superconduction to proceed. It is this absorption of a magnetic field along a penetration depth lambda that screens the interior of the superconductor from the externally applied magnetic field. There are no surface currents within this penetration depth since the magnetic field cannot induce such currents by acceleration the charge carriers.

But in order to keep my sanity, I will not answer further questions on the Meissner effect since it involves quite a bit of mathematics; which I could never get published anywhere.

 

[sol invictus]

But this argument by Sol is so incisive (congratulations Sol) and important that I must comment. I treat it in detail in my book. The fact is that a circular current around a superconducting ring is not caused by the induced circular electric-field accelerating the charge-carriers. If it were so, then when next switching off the magnetic field, the reverse induced electric-field must decelerate the current to become zero. But this is not what happens.

I'm not so sure that isn't what happens, nor can I make any sense of your explanation.

But regardless, Zig and Ben made an even more simple and incisive point - that since currents circulate essentially forever in toroidal superconductors, the charge carriers are obviously accelerating all the time. That immediately falsifies your assertion.

 

[sol invictus]

Question.

If there is no electric or magnetic field in the superconductor, what force field is responsible for accelerating the paired electrons round bends in the conductor?

I think the answer is that the current flows very close to the surface of the superconductor where the magnetic field isn't zero.

 

[johanfprins]

I'm not so sure that isn't what happens, nor can I make any sense of your explanation.

Can you not understand that when the charge-carriers are accelerated by the induced circular electric-field around a ring while the magnetic field increases to reach a constant value at which a constant current I flows, then when switching off this magnetic field to decay to zero, the concomitant induced circular electric-field, which now works in the opposite direction than the original field when the magnetic field increased, must now decelerate the charge-carriers back to zero speed? I thought it is so simple that my goldfish will be able to understand this.

But regardless, Zig and Ben made an even more simple and incisive point - that since currents circulate essentially forever in toroidal superconductors, the charge carriers are obviously accelerating all the time. That immediately falsifies your assertion.

Wrong again! If they did accelerate they would have emitted EM radiation; and they do not do this. They are not accelerated towards the center of the ring since they barrier-jump along straight links around the hole. The smallest arrangement of orbitals around a hole for which this can happen is for six orbitals sitting around the hole on the apexes of a hexagon. This is why the fluxoids in a type II superconductor always try to form a hexagonal arrangement, independent of the crystal symmetry of the material.