Are superconductivity and melting not understood?

[VortexLattice]

Hi all,

I've been told from a couple different people something along the lines of "we don't fully understand how melting (the phase transition) works". Same with superconductivity.

But I thought melting was fairly straightforward: The molecules in the material keep gaining energy until they break free from their crystal intermolecular bonds, and then they have some weaker bonds holding them together.

I also thought superconductivity was explained by BCS theory.

Is there any truth to this? I assume if there is, it's something much deeper and more mathematical, and that we have the general idea but there's still something more technical not understood.

 

[Drakkith]

The BCS theory works for standard superconductivity, but not high temperature superconductivity. (Or it at least doesn't fully explain high temp SC)

As for melting, I have never heard anyone say that before.

 

[Cthugha]

But I thought melting was fairly straightforward: The molecules in the material keep gaining energy until they break free from their crystal intermolecular bonds, and then they have some weaker bonds holding them together.

It is not that trivial. While describing melting from the thermodynamical point of view works reasonably well, a microscopic description of what is happening is significantly harder if you want all the details to be reproduced well. For examle liquids do not show long-range order like crystals, but are typically not completely disordered as well - they show short-range order. Trying to get this and similar effects from first principles is a major pain and takes long computation times. The role of defects may also be crucial.

See for example U. Gasser et al., "Melting of Crystals in Two Dimensions", ChemPhysChem, Volume 11, Issue 5, pages 963–970, April 6, 2010 for a review article.
You will find: 2D melting is already hard to describe - 3d melting may become a nightmare.

 

[VortexLattice]

The BCS theory works for standard superconductivity, but not high temperature superconductivity. (Or it at least doesn't fully explain high temp SC)

As for melting, I have never heard anyone say that before.

Sorry, "high temp" being the ones that can work in LN2, or fabled room temp ones?

 

[VortexLattice]

It is not that trivial. While describing melting from the thermodynamical point of view works reasonably well, a microscopic description of what is happening is significantly harder if you want all the details to be reproduced well. For examle liquids do not show long-range order like crystals, but are typically not completely disordered as well - they show short-range order. Trying to get this and similar effects from first principles is a major pain and takes long computation times. The role of defects may also be crucial.

See for example U. Gasser et al., "Melting of Crystals in Two Dimensions", ChemPhysChem, Volume 11, Issue 5, pages 963–970, April 6, 2010 for a review article.
You will find: 2D melting is already hard to describe - 3d melting may become a nightmare.

Ah, I think this is what they meant. For a bit I was looking at a colloids lab that was trying to figure out melting in 2D using small, mechanical beads or something.

 

[ZapperZ]

Sorry, "high temp" being the ones that can work in LN2, or fabled room temp ones?

These are the cuprate and pnictide superconductors.

Zz.