Room-temperature superconductivity?

[WizWom]

No, even with an average free path length >> length, you will see measurable resistivity; because the mean path is a mean, with path lengths above and below. Those that fall within the length will generate resistivity.

Do you not know that one can generate a permanently flowing curent around a mesoscopic ring of gold when the circumference of the ring is less than the mean free path? How do you explain this?

Level Correlations and Persistent Currents in Mesoscopic Metals (Sitotaw, Serota 1999) seems to be the most cited work in this field. And, honestly, it's over my head. It appears the material forms a single quantum state, and the current flow is an effect of resonance.
Also, this occurs below a transition temperature only, that is, it is a superconducting effect.
As a first approximation, I would say that if the scale is a half or full wavelength of the ionization photon, then the system would tend to be stable, with photons exciting another atom within the structure.

This applies even when you are getting into the realm of quantum effects, its been observed in the 20 nm research parts, where the uncertainty principle contributes significant leakage in FETs.

Leakage or resistvity?

Leakage current in a semiconductor is typically modeled as a resistive load to the leak drain.

R=0 implies I(in) = I(out) and V(in) = V(out).
However, it allows one to call a vacuum tube "zero resistivity" and a "superconductor" for the vacuum travel portion, which is somewhat absurd...
A superconductor is a conductor that exhibits no voltage differential across its entirety.

You have an inkling but it is still woolly. So let me hone it a bit: A superconductor is a phase through which a current can flow while the applied electric-field is cancelled everywhere within the superconductor.

Ah, now, you're trying to lead me down the garden path. The exclusion of magnetic fields is a side effect; electric fields are necessary for current flow. Electric fields are normalized throughout a conductor (for distances << speed of light), and this should be the case for a superconductor, also.

Which, of course, merely adds "conductor" to the zero resistivity definition.

An "implication of zero resistivity" to which you merely add the word "conductor" can in no ways be a rational argument and does not constitute any physics-logic.

I beg to differ. The concept of a conductor is an important physics idea. I am somewhat out of my depth, but I understand a conductor to be a physical material which current flows through.

And, of course, the model for resistivity is NOT unknown.

I have NOT asked for a model for resistivity: I have asked for a definition of zero resistivity which you could not give me. An "implication" is NOT a "definition".

Since a superconductor is, by definition, a material in which resistivity is 0, you're just being strange. To say what a resistivity of 0 is, you need to say what resistivity is.

In the band gap theory, a superconductor is a conductor in which the electrons continue to be excited above the valence energy of the material, and so don't undergo the resistivity mechanism.

The same happens in a semimetal, but the metal still has a resistivity. How are the electrons excited within a superconductor and why is the energy they gain by being excited not deposited as heat within the superconductor?

Again, I'm out of my depth. It is clear that there is a definite energy transition at which superconductivity starts. It ALSO is clear that this effect is not evident above a transition temperature; that is, above T(c) the material exhibits a resistivity to any voltage, below T(c) the material will only conduct voltages above the excitation energy, but then it does so without losses.
As near as I can tell, this is because in that realm, the material is stable enough for quantum effects to dominate.

[GIThruster]

While I agree in general with the notion of agreeing upon a definition of the term "superconductor" before developing the 500-700 word brief that Dave has called for, I see three troubles developing.

First is, Johan is defining his terms in unusual ways that others don't recognize, where he writes:

"A superconductor is a phase through which a current can flow while the applied electric-field is cancelled everywhere within the superconductor. "

Why are we seeing the term "phase" here? It's not a phase, it'a a material and like all materials it exists in some specific phase. Without defining the phase, the point of using the term is lost on me.

My second trouble is that this definition is focused very sharply on the e-field inside the SC--that the field is "cancelled". That seems an odd perspective as well. Why define in such odd terms? If the material has no resistivity, and no voltage drop, then no field differential or gradient develops. That doesn't mean there's no field! Talking about the field being cancelled is therefore going to raise a host of issues that no field theorist is able to agree with. I think the definition is wrong in spirit at this point, and technically wrong as well. It's appearing more and more that the specific definition Johan has in mind is chosen for wrong reasons, and this is going to be apparent to anyone who knows field theory.

My third trouble is that if what we want is to disseminate Johan's short essay about his work, we ought not be developing our own definition for superconductor at all. We ought to be grabbing the simplest and most common definition we can find anywhere, say for example on wiki, or the most authoritative definition from a time-honored text. Making up your own definition for superconductivity may have benefits, but it's entirely wrong so far as convincing others is concerned. I wholly recommend against it. Rather, I suggest you choose a time honored definition and cite that definition in the short essay--you'll raise fewer eyebrows in the wrong places and get a lot further, IMHO.

[johanfprins]

Level Correlations and Persistent Currents in Mesoscopic Metals (Sitotaw, Serota 1999) seems to be the most cited work in this field. And, honestly, it's over my head.

Not just yours but this is the case for ALL the people working in that field. It is really pitifull.

It appears the material forms a single quantum state, and the current flow is an effect of resonance.

It does NOT form a single quantum state in the case of mesoscopic rings

Also, this occurs below a transition temperature only, that is, it is a superconducting effect.

Again wrong! The "transition temperature" in this case is determined by the increase in mean free path when lowering the temperature.

As a first approximation, I would say that if the scale is a half or full wavelength of the ionization photon, then the system would tend to be stable, with photons exciting another atom within the structure.

If you think this is physics, you still have a loooooong way to go my boy!

Leakage current in a semiconductor is typically modeled as a resistive load to the leak drain.

. Modelled as if it is a resistive-load because some charge-carriers escape is not modelling real resistance.

Ah, now, you're trying to lead me down the garden path. The exclusion of magnetic fields is a side effect; electric fields are necessary for current flow. Electric fields are normalized throughout a conductor (for distances << speed of light), and this should be the case for a superconductor, also.

Which garden path? Are you saying that there is an electric-field driving the current? Then it will be physically impossible to measure zero voltage over two contacts.

I beg to differ. The concept of a conductor is an important physics idea. I am somewhat out of my depth, but I understand a conductor to be a physical material which current flows through.

And you obviously do not understand what a conductor means or else you would have known that it is physically impossible for a conductor to register a zero voltage over two contacts while a current is flowing through it.

Since a superconductor is, by definition, a material in which resistivity is 0, you're just being strange

. Here you are again defining superconduction in terms of something else you have to define. The fact is that before Onnes discovered superconduction, "zero" resistivity never existed and has therefore NEVER been defined. Furthermore, Onnes did NOT measure resistivity but voltage. Thus what he measured is that a current flows through a material while there is NO electric field within the material. If he defined this behaviour as "zero resistivity", it means that he deduced zero resistivity from superconduction; NOT the other way around.

To say what a resistivity of 0 is, you need to say what resistivity is.

And the ONLY definition of resistivity we have had and still has before superconduction was discovered, and Onnes defined it as zero resistivity, was through Ohm's law: And Ohm's law is NOT valid for zero resistivity. Thus zero resistivity is just another nomenclature for superconduction. It cannot be used to define superconduction.

Again, I'm out of my depth.

Then why do you post when you clearly do not know what you are talking about?

It is clear that there is a definite energy transition at which superconductivity starts. It ALSO is clear that this effect is not evident above a transition temperature; that is, above T(c) the material exhibits a resistivity to any voltage, below T(c) the material will only conduct voltages above the excitation energy, but then it does so without losses.
As near as I can tell, this is because in that realm, the material is stable enough for quantum effects to dominate.

Yes, when you are out of your depth, blame the "peculiarities of quantum mechanics" and close your eyes to reality. This is not reasoning physics but practising voodoo. We have now had more than eighty years of these Alice in Wonderland stories. The time has come that we rediscover reality and logic.

[WizWom]

Level Correlations and Persistent Currents in Mesoscopic Metals (Sitotaw, Serota 1999) seems to be the most cited work in this field. And, honestly, it's over my head.

Not just yours but this is the case for ALL the people working in that field. It is really pitifull.
Yes, when you are out of your depth, blame the "peculiarities of quantum mechanics" and close your eyes to reality. This is not reasoning physics but practising voodoo. We have now had more than eighty years of these Alice in Wonderland stories.

As an undergraduate engineer, I'm not surprised it's past my ken. but you don't get far insulting people.

In any case, I shall address the points I DO know.

Ah, now, you're trying to lead me down the garden path. The exclusion of magnetic fields is a side effect; electric fields are necessary for current flow. Electric fields are normalized throughout a conductor (for distances << speed of light), and this should be the case for a superconductor, also.

Which garden path? Are you saying that there is an electric-field driving the current? Then it will be physically impossible to measure zero voltage over two contacts.
The fact is that before Onnes discovered superconduction, "zero" resistivity never existed and has therefore NEVER been defined.
And the ONLY definition of resistivity we have had and still has before superconduction was discovered, and Onnes defined it as zero resistivity, was through Ohm's law: And Ohm's law is NOT valid for zero resistivity.

Of course there is an electric field, a voltage potential IS an electric field. With no voltage, there would be no motive force for the electrons, that is, they would have 0 speed.
<Edit>Now i believe i'm getting closer to your understanding. If electrons move, then power must be used; if that's the case, then it should be dissipated as heat, or some other effect. If electrons move in a superconductor, then it would not be able to keep a magnetic field indefinitely.</edit>

I grasp what you are saying, though:
Ohm's law is V=IR; if R=0 then V=0. But Ohm's Law is a statistical effect of absurd numbers of quantum effects; just as many other physical "Laws" are.
Plank and Einstein and a host of others have clearly demonstrated, it seems, that such laws become untenable at describing the behavior in realms where the statistical effects no longer dominate. You can't define the resistance of a single atom of something.

[johanfprins]

First is, Johan is defining his terms in unusual ways that others don't recognize,

Not true at all!

where he writes:
"A superconductor is a phase through which a current can flow while the applied electric-field is cancelled everywhere within the superconductor. " Why are we seeing the term "phase" here? It's not a phase

Wrong. It is clearly accepted that the critical temperature Tc defines a second order phase transition at which SOME of the valence electrons within THE MATERIAL starts to form a superconducting phase. It is also on this concept that Ginzberg and Landau based their model.

it'a a material and like all materials it exists in some specific phase. Without defining the phase, the point of using the term is lost on me.

the phase IS the superconducting charge-carriers; which, as we all know, is a phase of the valence electrons. So I am NOT using a term which is unusual and cannot be recognised.

My second trouble is that this definition is focused very sharply on the e-field inside the SC--that the field is "cancelled". That seems an odd perspective as well.

Why is it odd? This is exactly what Onnes measured! He DID NOT measure resistance but voltage; and if the voltage went to zero this surely means in simple terms that there is no net electric field. So above the critical temperature there is a field and below it one cannot neasure it. This means that at the critical temperature the field is being cancelled: It cannot mean ANYTHING else.

Why define in such odd terms?

What is odd about a phase and an electric-field? What is odd when one deduces that zero voltage means that there is no resultant electric-field between the contacts?

If the material has no resistivity,

As I have pointed out above "zero resistivity" has NEVER existed and cannot be used to define superconduction since it is really superconduction that defines zero resistivity: AND the reason why it is defining zero resistivity is because the applied electric field is cancelled as soon as superconduction sets in. So it is NOT zero resistivity that cancels the electric field but the cancellation of the electric field which we use to define zero resistivity.

That doesn't mean there's no field!

Unless I am VERY stupid, I have always concluded that when I measure zero voltage over two points there is no net electric field between the two points.

Talking about the field being cancelled is therefore going to raise a host of issues that no field theorist is able to agree with.

So a field theorist believes that there IS an electric field when you measure no voltage. No wonder they are in a quagmire! No wonder they also believe in magnetic monopoles.

I think the definition is wrong in spirit at this point, and technically wrong as well.

Why? I am defining it completely in terms of the experimental results that Onnes measured in 1911. He did NOT directly measure resistivity; he defined zero resistivity from observing the voltage going to zero, and the electric field being cancelled.

It's appearing more and more that the specific definition Johan has in mind is chosen for wrong reasons, and this is going to be apparent to anyone who knows field theory.

Well if it is apparently wrong tell me in straight normal physics how an electric field is present above Tc while it cannot be measured below Tc without it having been cancelled at Tc.

My third trouble is that if what we want is to disseminate Johan's short essay about his work, we ought not be developing our own definition for superconductor at all. We ought to be grabbing the simplest and most common definition we can find anywhere, say for example on wiki, or the most authoritative definition from a time-honored text.

I did not ask you to help me develop our own definition, but to give me definitions where ever they come from: wiki or what have you.

Making up your own definition for superconductivity may have benefits, but it's entirely wrong so far as convincing others is concerned. I wholly recommend against it. Rather, I suggest you choose a time honored definition and cite that definition in the short essay--you'll raise fewer eyebrows in the wrong places and get a lot further, IMHO.

I have found all the definitions in text books not to be correct. So now you want to force me to use a wrong definition as a starting point? Pleaase!!!

[GIThruster]

Johan, I don't know any engineers nor physicists who believe in magnetic monopoles. They were proposed decades ago and no one believes in them--they're ridiculous.

Yes, all kinds of fields exist at all points in space. They are never "cancelled". When you measure no field, what you are in fact measuring is no field gradient. Just because there is no gradient does not mean there is no field.

You need to use a standard definition. Anything else is going to lead you off into wasted time and effort, and added opportunities for your detractors to dispute asinine issues.

[johanfprins]

As an undergraduate engineer, I'm not surprised it's past my ken. but you don't get far insulting people.

I have not tried to insult you but find it insulting that you as an undergraduate engineer think that you know enough physics to correct my logic. Maybe you should use a more enquiring tone. Even so I apologise if you feel insulted: I especially do not want to insult young enthusiastic persons like you!

Of course there is an electric field, a voltage potential IS an electric field. With no voltage, there would be no motive force for the electrons, that is, they would have 0 speed.
Now i believe i'm getting closer to your understanding. If electrons move, then power must be used; if that's the case, then it should be dissipated as heat, or some other effect. If electrons move in a superconductor, then it would not be able to keep a magnetic field indefinitely.

By Jove! You've got it!!! But not completely! The electrons have to move or else there will NOT be a current BUT this movement CANNOT be generated by acceleration of the electrons since such kinetic energy MUST be dissipated. This means that the charge carriers must acquire the kinetic energy in another way, and then "give it back" before it can dissipate to generate entropy.

I grasp what you are saying, though:
Ohm's law is V=IR; if R=0 then V=0. But Ohm's Law is a statistical effect of absurd numbers of quantum effects; just as many other physical "Laws" are.
Plank and Einstein and a host of others have clearly demonstrated, it seems, that such laws become untenable at describing the behavior in realms where the statistical effects no longer dominate. You can't define the resistance of a single atom of something.

No it is far simpler that this: The fcat is that Ohm's law is only valid when the acceleration-scattering events are so numerous that you can approximate the movement of the charge-carriers by a drift speed.: i.e Ohm's law is only valid for high resistivities at which, after switching on the current, the current settles down to a constant equiilibrium current. Now comes the additional surprise which one has to explain in a superconductor. The charge-carriers are also moving with a constant drift speed even though they are NOT being accelerated and scatterred. Only my model can explain the absence of an electric field AND the average drift speed even though there is NO acceleration-scattering events.

[BenTC]

Why are we seeing the term "phase" here? It's not a phase, it'a a material and like all materials it exists in some specific phase. Without defining the phase, the point of using the term is lost on me.

Would I be correct in assuming you are referring to generic phases Solid/Liquid/Gas whereas I believe Johan is using the more specific materials science definition ...

Phase. A homogeneous portion of a system that has uniform physical and chemical characteristics.

As temperature reduces, solid materials change "phase." The most common example used is the phase diagram of Steel , where the physcial structure of the solid changes with temperature, for example between body-centred-cubic and face-centred-cubic.

[johanfprins]

Johan, I don't know any engineers nor physicists who believe in magnetic monopoles. They were proposed decades ago and no one believes in them--they're ridiculous.

I apologsie that I have to kep on disagreeing with you: Blas Cabrera has been looking for monopoles for many years, as well as a group of physicists in in Italy (I believe).

Yes, all kinds of fields exist at all points in space. They are never "cancelled". When you measure no field, what you are in fact measuring is no field gradient. Just because there is no gradient does not mean there is no field.

Although I can now ask you to define a point and how a field manifests "at a point" this is will lead us into esoteric nonsense which is beside the point. The fact is that if you measure zero voltage over two contacts there is NO NET FIELD between these contacts which can accelerate charge carriers. That is an experimental fact and this is what Onnes measured happens when superconduction sets in. Thus a current flows through a superconductor while there is no electric-field accelerating the charge-carriers. The effect of an electric field "disappears". And the only way in which this is possible physically is when the applied electric-field is cancelled by an opposite polarisation-field. There is NO OTHER mechanism that can do this. This is the only valid definition of superconduction. To say that this behaviour is zero resistance does not help one iota in understanding superconduction.

You need to use a standard definition. Anything else is going to lead you off into wasted time and effort, and added opportunities for your detractors to dispute asinine issues.

I cannot use definitions based on incorrect physics-logic!

[johanfprins]

Would I be correct in assuming you are referring to generic phases Solid/Liquid/Gas whereas I believe Johan is using the more specific materials science definition ...

Phase. A homogeneous portion of a system that has uniform physical and chemical characteristics.

As temperature reduces, solid materials change "phase." The most common example used is the phase diagram of Steel , where the physcial structure of the solid changes with temperature, for example between body-centred-cubic and face-centred-cubic.

Exactly!

[Giorgio]

How about we go back to the definition of Superconductor?

A much better definition than one usually finds in text books; but it is still flawed. Assume that the charge-carriers in a metal have an average free pathlength L and you have a wire of length smaller than L: There will then be no heat generation even though the wire is NOT a superconductor.

If I remember correctly collision times of electrons in a medium is in the range of tenths of picoseconds. For semplicity let's make 0,5 ps. We might calculate it if we want to be precise, but is not necessary.
Now, considering an electron mobility of 10^5 m/s, we get an average free path lenght of 50 nanometer.

We can consider for our superconductor a lenght of several mm, being safe that our wire will never be shorter than the average free pathlength.

Another problem: By defining a superconductor as a material with zero resistivity, you are defining one unknown in terms of another unknown. Zero resistivity has NEVER been defined in physics EVER. Thus to define a superconductor in terms of zero resistivity you must first give an independent definition for zero resistivity.

That's a point. Let's skip resistivity at all than.


Defining a superconducting wire:
A piece of material (in the shape of a wire several mm long) that will transport a given current from its start to its end with no heat dissipation and no power dissipation generated by the current flowing into the wire itself.

From a mathematical point of view it must satisfy the following criteria:
Joule Effect equal to zero : P=RI^2=0

Edited:
If we want to skip the resistivity at all, we can say that it must satisfy the following criteria: P=VI=0

[BenTC]

My second trouble is that this definition is focused very sharply on the e-field inside the SC--that the field is "cancelled". That seems an odd perspective as well.

The following on just occured to me in response, but my niave layman's response is that we observe superconductors excluding magnetic fields. So why not electric fields being excluded by the same mechanism (what ever that is.) It "kind of" makes sense intuitively. Wasn't there commentary recently (not sure if it was this thread) about how physics education improperly teaches spearately about electric and magnetic fields when in fact they are inexorably interlinked.

And so some the charge is moved through the material by some other mechanism than an electric field. The previous mentioned analogy of Newtons Cradle comes to mind, whereas an electric field is like balls rolling down from the top of a slope.

[Giorgio]

My second trouble is that this definition is focused very sharply on the e-field inside the SC--that the field is "cancelled". That seems an odd perspective as well.

The following on just occured to me in response, but my niave layman's response is that we observe superconductors excluding magnetic fields. So why not electric fields being excluded by the same mechanism (what ever that is.) It "kind of" makes sense intuitively. Wasn't there commentary recently (not sure if it was this thread) about how physics education improperly teaches spearately about electric and magnetic fields when in fact they are inexorably interlinked.

This is exactly what makes me consider and deeply think about the hypothesis of johanfprins.

[johanfprins]

How about we go back to the definition of Superconductor?
We can consider for our superconductor a lenght of several mm, being safe that our wire will never be shorter than the average free pathlength.

Correct, superconduction has NOTHING to do with an average pathlength generated by acceleration-scattering events.

That's a point. Let's skip resistivity at all than.
Defining a superconducting wire:
A piece of material (in the shape of a wire several mm long) that will transport a given current from its start to its end with no heat dissipation and no power dissipation generated by the current flowing into the wire itself.
If we want to skip the resistivity at all, we can say that it must satisfy the following criteria: P=VI=0

Exactly: And this still requires that V=0 and therefore the applied electric-field within the superconductor must be cancelled to be zero: i.e. no acceleration-scattering events.

[Giorgio]

So, we have a definition?