Perfect Diamagnetism: Properties Necessary for Ideal State

[cesiumfrog]

What properties would an ideal ("perfect") diamagnet necessarilly* have?
(*as in in, implied by definition.)

This is a spin-off from https://www.physicsforums.com/showthread.php?t=198245", based on the common idea that superconductors are better described as perfect diamagnets than perfect conductors. If it began in zero magnetic field, a perfect diamagnet should completely repel any new field from entering. But if the material transitioned (perhaps smoothly with temperature, say) into the perfect diamagnetic state from some other phase which is only weakly magnetic, would it actually exhibit the Meissner effect (completely excluding a pre-applied magnetic field)? And would it be superconducting (I saw a three line argument that it could not have finite resistance, but could it just be an insulator)?

 

[f95toli]

Unfortunately, I think my explanations were pretty bad in that thread.
The point I was trying to make is that superconductors are best understood as materials that both exhibit perfect diamagnetism and zero dc resistance.
As I wrote in that thread, perfect conductivity (i.e. as found in a "perfect" metal) does not imply perfect diamagnetism. I am not sure if the oppositie is true (if perfect diamagnetism implies perfect conductivity), but then I guess it depends on the orgin of the diamagnetism so I suspect the "general" answer is no.

I should also point out that I was wrong about one think in that thread; liquid oxygen is paramagnetic, not diamagnetic.

*I stumbled upon a discussion about this is in an old textbook; Myers "Introduction to Solid State Physics", actually the book I used when I was an undergraduate. The "derivation" consists of solving Maxwell's equation for electrons with zero scattering (the electrons continue to be accelerated forever by an electric field) and showing that the result is not consistent with the Meissner effect.

 

[olgranpappy]

Unfortunately, I think my explanations were pretty bad in that thread.
The point I was trying to make is that superconductors are best understood as materials that both exhibit perfect diamagnetism and zero dc resistance.

yeah, I agree. Here's a quote from Crangle, "The Magnetic Properties of Solids":

"A homogeneous type I superconductor is both a perfect conductor and perfectly diamagnetic."

 

[cesiumfrog]

The first link referenced in that previous thread states "Superconductors are actually perfect diamagnets and not perfect conductors". If perfect diamagnets do exist, what would their properties be (and in what ways could their properties differ from the properties of superconductors, which may or may not have additional defining characteristics)?

 

[cesiumfrog]

A possible problem (having read the wikipedia talk section) is ambiguity of the definition of "perfect diamagnetism", but this seems widely accepted as meaning to admit zero magnetic field and hence synonymous with the full Meissner effect (regardless of field prior to any transition). With comparison to superconductors, this just leaves the question of dc resistance.

Is it possible to completely exclude magnetic fields without also having zero electrical resistance? If the fields are excluded by the creation of opposing electrical currents, then the persistence of those currents has been argued (sorry, lost the link) to prove zero resistance. But would it be impossible for the external fields to be excluded by some other mechanism (say, by magnetic moments of individual atoms) without requiring conduction between atoms?

 

[olgranpappy]

The first link referenced in that previous thread states "Superconductors are actually perfect diamagnets and not perfect conductors". If perfect diamagnets do exist, what would their properties be (and in what ways could their properties differ from the properties of superconductors, which may or may not have additional defining characteristics)?

I wasn't a part of the "previous thread", but I think I know what they mean. Probably the person
who posted was pointing out that being a perfect conductor (and defining a perfect conductor as that which alwasy has E=0 in the bulk) implies a constant magnetic field in the bulk, but being a perfect diamagnet implies not only that the field is constant but also that it is equal to zero.

A perfect diamagnet will have B=0 in the bulk always. At least, that is the definition I use for a "perfect diamagnet". So, a perfect diamagnet will always have a constant *electric* field in the bulk. If the perfect diamagnet is also a perfect conductor that constant will happen to be zero. (this is the converse of what was stated in the upper paragraph).