20 Years of High Temp Superconductors

[natski]

So we've had 20 years of high temperature superconductors and I cannot help but ask myself why isn't my power cord made out it? In all this time you would expect some kind of applications to be coming through (excepting the odd industrial application).

I assume the main prolem is with the engineering side of superconductors. What are the main problems in this field preventing superconductors appearing more often in our lives?

Natski

 

[ZapperZ]

So we've had 20 years of high temperature superconductors and I cannot help but ask myself why isn't my power cord made out it? In all this time you would expect some kind of applications to be coming through (excepting the odd industrial application).
I assume the main prolem is with the engineering side of superconductors. What are the main problems in this field preventing superconductors appearing more often in our lives?
Natski

Before answering directly your question, let me point out a few things that many people do not realize. The study of high-Tc superconductors (HTS) isn't JUST about high-Tc superconductors. This needs to be emphasized clearly. The whole condensed matter community realized very quickly that this material is providing a direct study of THE most fundamental aspect of condensed matter physics - strongly-correlated systems. This is the study of how huge amount of particles behave when they are strongly interacting with each other beyond just simple Fermi Liquid-type mean-field interactions. Many of our approximations when dealing with regular materials no longer work for HTS's. This has implications in areas beyond just HTS. The study of magnetism, Mott insulators, low-dimensional conductors, magnetoresistance, quantum critical point, and many more, all hinges on our understanding of correlated systems.

So while we're making progress in our understanding of HTS, we are also making progress in OTHER fields of study. There is no doubt in my mind that many areas benefited from the progress made in HTS. Mott insulators, for instance, would have not had that much interest beyond just curiosity if it weren't for HTS. Even if we stop and make no more progress in our understanding of the physics of HTS, what we have learned so far MORE than justify the studies already done and results obtained at this point.

Having said that, here are the reasons, from my perspective, why the physics of HTS is still daunting, and why you don't have it in your house.

1. Every time we probe the material, it gives us new and unexpected results. In the beginning, it was the non-conventional d-wave pairing symmetry. Then the discovery of the pseudogap in the underdoped phase, etc.. etc. In other words, this material is very complex and has a wealth of exotic properties waiting to still to be discovered. This is a very rich system that will keep us busy for a very long time.

2. There is no one ideal material to be studied across the board with a range of techniques. Photoemission techniques are mainly concentrated on the BiSrCaCuO family because one can easily cleave the layered surface in situ, exposing a clean, pristine surface that is required for accurate photoemission studies. Other HTS material are not that easily cleaved and requires extra effort into getting a clean surface. On the other hand, inelastic neutron scattering studies require large single crystals and have a majority of their work on the YBCO family. Only with the past 5 years or so have there been a large overlap in various techniques and materials being studied and a more coherent picture is beginning to develop.

3. Too many possible red herrings. Is the pseudogap a precursor to the paring formation or is it simply competing with the pairing formation? Is the magnetic background in the HTS material a hinderance to superconductivity, or does it actually provide the glue? Does phonon plays a major role in the pairing formation, or is it simply a supporting player?

4. Equal but not the same? Is the electron doped HTS identical to the hole doped HTS? Are they based on the same mechanism, or do they have nothing to do with each other other than a similar crystal structure?

Oy.. I can go on and on and on... but you get the idea.

Why don't you already have them in your house?

1. You STILL need at least liquid He temperatures to achieve superconductivity. But at these temperatures, the superconductors have a lot of "noise". Not may applications would want this.

2. HTS doesn't carry a large current, at least not as large as conventional superconductor. Remember that in the normal state, HTS's are VERY bad conductors. So they don't have a lot of free charges to start with. In the superconducting state, the current density is not as large as conventional superconductors. If you try to pump MORE than it can, the material goes normal.

3. It is a type II superconductor and notorious for having vortices. If you have meandering vortices, you lose power.

4. Most HTS are brittle. YBCO have been made into flexible tapes, but I don't see others achieving the same technical success any time soon.

There are more, but my fingers are getting tired.

Zz.

 

[inha]

2. There is no one ideal material to be studied across the board with a range of techniques. Photoemission techniques are mainly concentrated on the BiSrCaCuO family because one can easily cleave the layered surface in situ, exposing a clean, pristine surface that is required for accurate photoemission studies. Other HTS material are not that easily cleaved and requires extra effort into getting a clean surface. On the other hand, inelastic neutron scattering studies require large single crystals and have a majority of their work on the YBCO family. Only with the past 5 years or so have there been a large overlap in various techniques and materials being studied and a more coherent picture is beginning to develop.

Do you know how well HAXPES gets around the sample quality demands of traditional PES-methods? The electron escape depth is larger but would surface roughness etc. still make processing of the spectra significantly harder than with high quality samples?

RIXS is also another technique that's gaining more popularity as instrumentation advances. And it's not terribly picky as far as the samples go.

 

[ZapperZ]

Do you know how well HAXPES gets around the sample quality demands of traditional PES-methods? The electron escape depth is larger but would surface roughness etc. still make processing of the spectra significantly harder than with high quality samples?
RIXS is also another technique that's gaining more popularity as instrumentation advances. And it's not terribly picky as far as the samples go.

I actually don't have an intimate knowledge of HAXPES. However, note that XPES typically probe the deeper band, some time called core-level photoemission. While these sometime can provide insight into crystal structures and compound, most people believe superconductivity occurs within the first 1 eV of the band. It is why typical PES or ARPES uses photons of energies ranging from 5 eV to 50 eV only.

Zz.

 

[inha]

That makes a lot of sense now that I think about it. To get rid of the surface sensitivity via increasing the photon energy would shift one out of the relevant energy region. Even if one used a higher energy photon with a suitably selected energy and momentum transfer the escape depth limit would still be there.

 

[Pieter Kuiper]

1. You STILL need at least liquid He temperatures to achieve superconductivity. But at these temperatures, the superconductors have a lot of "noise". Not may applications would want this.

You mean liquid nitrogen (at those temperatures there is noise).
Helium is required when one wants to make strong magnets of these materials.

 

[ZapperZ]

You mean liquid nitrogen (at those temperatures there is noise).
Helium is required when one wants to make strong magnets of these materials.

Yup. I meant liquid nitrogen. Thanks for catching that.

Zz.

 

[Gokul43201]

Incidentally, Campuzano was here yesterday, talking about his work on understanding the pseudogap (in the underdoped phase) and from there, the SC phase.

 

[ZapperZ]

Incidentally, Campuzano was here yesterday, talking about his work on understanding the pseudogap (in the underdoped phase) and from, there SC phase.

Good ole Juan Carlos! I like him. He cites many of my papers. :)

The pseudogap is a HUGE issue. If we can figure out if this is either a precursor to superconductivity, or simply a red herring that competes with superconductivity, we will have solved a major problem. Depending on who you talk to, there is also an even larger gap beyond the pseudogap scale that some people have mistaken for the pseudogap - this is the spin gap in the magnetic channel. This is what is seen in NMR experiments. The task of sorting this out, i.e. who sees what using which experiments, is very daunting, and rife with controveries.

I would like to add that this type of gap is seen not just in the HTS system, but other magnetic systems also. I'm not claiming that they are identical, but they do look similar in experiments. This is why I said that study of HTS has implications ALL over condensed matter, and even beyond.

Zz.

 

[ZapperZ]

I forgot about this paper. If anyone wants to read more in detail on the various issues surrounding the pseudogap in high-Tc superconductor, this is a very good paper to start.

http://arxiv.org/abs/cond-mat/0507031

Zz.

 

[inha]

Thanks! I'm always up for more info on HTSC.

 

[Locrian]

Oy.. I can go on and on and on... but you get the idea.

And you should have! Have a nearby tavern wench massage your tired fingers and get back to it.