Wednesday, February 17, 2010

High Tc, pseudogaps, broken symmetries

What distinguishes one phase of matter from another?  A physicist would probably say that different phases possess different symmetries.  More specifically, transitions between phases can be described (when going in the right direction) by the breaking of a symmetry.  For example, when water freezes, the continuous rotational and translational symmetry of the liquid (liquid water looks, on average, the same in every direction and at different points within the liquid) are broken, because crystalline ice has a specific lattice (and therefore certain preferred lattice directions, as well as a spatial periodicity).  Solid ice instead has discrete rotational and translational symmetries, rather than continuous ones.

High temperature superconductors have been confounding physicists for 24 years now.  Progress has been made in understanding these complicated materials (typically layered, multicomponent copper oxides with weird oxygen stoichiometries to control the number of mobile charge carriers), but the situation is still a mess.  These compounds have a complicated phase diagram as a function of, e.g., temperature and chemical doping.  The undoped parent compounds are antiferromagnetic insulators.  Over a range of chemical compositions, the ground state is a d-wave superconductor.  Within a good part of that range of composition, at temperatures above the superconducting transition, these materials show a "pseudogap" below some higher temperature, T*.  That is, the number of available electronic states near the Fermi level is depressed compared to what you'd expect for a metal, but not vanishing as you'd expect for a superconductor.  People have been arguing for years about what the pseudogap is - is this a distinct phase?  Is it a precursor to superconductivity (e.g., pairing of electrons w/o long-range coherence), or does it compete with superconductivity?

This recent paper by Louis Taillefer reports the observation of broken rotational symmetry in the pseudogap phase (mainly via the Nernst effect).  The claim is that below T*, the four-fold rotational symmetry (because it's a square lattice) of the electronic properties of the CuO2 planes is broken, and the system becomes electronically anisotropic.  This is important, because it firmly argues that the pseudogap state is a real thermodynamic phase of some kind, and that kind of broken symmetry apparently places strong constraints on possible theories of high Tc.  Not my direct area of expertise, but it looks very interesting.  I'll admit, though, I was surprised by the strong statements made here.  Unless there's way more to this than meets the eye, it's not clear to me why it's justified to claim that we're now much closer to room temperature superconductivity....

13 comments:

Uncle Al said...

BCS supercons (MgB2 surprise) to LN2 supercons was "punctuated evolution" (Bednorz and Mueller were insubordinate). The best minds spend 20 years not cracking that nut. Ambient temp supercons will likely be heterodox again.

WA Little at Stanford proposed elegant (beyond synthesis) high temp exciton supercons as doped polyacetylenes with pendant chromophores, Phys. Rev. 134 A1416-A1424 (1964). Said syntheses are trivial today. Make it an advanced undergrad project. The worst it can do is succeed.

http://www.mazepath.com/uncleal/pave1.png
Stereogram; hydrogens and pi-bonds omitted for ease of viewing. Add pendant chains for (liquid crystal) solubility.

Confused Guy said...

One thing I am wondering about. I have heard that the idea that a phase transition necessarily involves a broken symmetry is only in general true for second-order phase transitions, not first-order ones. As I understand it, this is the entire basis of Landau's theory of second order phase transitions.

My confusion arises because as I understand it, the liquid-solid phase transition is first order. Is it just a specific example of a first order phase transition that just happens to involve broken symmetry? Or am I wrong, and broken symmetry is characteristic of both first and second-order phase transitions?

Doug Natelson said...

Confused Guy, first-order phase transitions have broken symmetries, too. Only second-order transitions can have critical fluctuations, though. Wikipedia actually has quite a decent summary of the issue, here.

Heumpje said...

I also don't see how this helps in the search for room T SC. Having spend some time in this field, I think the pseudogap phase is not helping SC. When doping is decreased, correlations between the electrons become stronger and this seems to lead to a new, possibly striped/nematic phase. I would think that this is detrimental to SC, since it induces localization of some the electrons. This does not necessarily destroy SC, since it is the phase of the wave function that is carrying the current. However, scattering of electrons on this static order will scramble the phases. Another trivial observation is that the pseudogap temperature decreases when SC appears. Most scenarios also show that T* dissapears when SC is at its strongest.

My conclusion would therefore be that they have given a bit more evidence for a competing phase scenario (there was already quite some evidence for that from other experiments on the same and other compounds). And that kills at least a few of the proposed scenarios.

I liked the remark by the high school teacher:
"I'm excited by it," he said. "Unleashing the power of magnets will make a lot of things more affordable and more practical. It's really easy to excite young students with these kinds of possibilities."

Shows how much actually gets through to the public...O well, as long as they are excited. They are paying for a 10 million dollar magnet...

Anonymous said...

"$10 million in new funding just received ... Taillefer plans to buy a coil of superconducting wire"

priceless!

Taillefer must be pretty pissed about the article.

About the Nernst effect measurement itself - I think the interpretation is very simplified. YBCO has 1 d chains which can contribute to the Nernst signal (in spite of Taillefer's arguments against it) below T*.

Doug Natelson said...

Thanks for the useful comments, everyone. Al, people have synthesized Little's materials (and ones very similar). They're not superconductors. We know much, much more about charge carriers in organic compounds now than we did 46 years ago.

Heumpje said...

I have never had the opportunity to see my work featured in a news paper so I don't know how these things go, but do you get a change to correct the article before it is published? I would guess that it would help somewhat if the scientist involved would check that there are no wrong claims made.

CarlBrannen said...

I don't see why it isn't possible to have a phase change with no change in symmetry. I'm thinking of something two different choices of unit cell but with some rearrangement of the atoms, but with the same symmetry classes. Or would that be included as a symmetry change?

I'm guessing that last comment was spam.

Peter Armitage said...

You can have a phase transition without a symmetry change. One of the most familiar examples of a phase transition - water turning to steam - has no change of symmetry and is 1st order. But 1st order transitions can also have symmetry changes ... the given melting example is the most obvious one.

If there is no associated latent heat with the transition (so not 1st order) then usually it will be 2nd order and there will be a symmetry change. This is because if there no associated latent heat and no symmetry change than there is usually no difference between one phase and another and hence ... no transition at all. There are counter examples to this reasoning in the form of transitions to "topological" orders (KTB transitions, transitions between QH plateus) however.

Anonymous said...

it seems that room temperature superconductivity, much like fusion power, is always 20 years off...

:)

Confused Guy said...

Thanks to both Professors Natelson and Armitage for the helpful clarifications.

Here's my follow up question. Has it been observed, or is it theoretically possible, for there to be a change in symmetry without there being a phase transition?

JJ said...

Hi CG,
The important point is the definition of the word "phase." We define phases, mostly, according to their symmetries, and we define any change in that symmetry as being a "phase transition." So, by definition, any change in symmetry is a phase transition. But not every phase transition involves a change in symmetry.

Physicists like the symmetry description, because it's easy to define the symmetry of a phase and thus to know whether the phase has changed. The liquid-gas phase transition is harder to define (despite the common observation that it's easy to tell a liquid from a gas), because the difference between the two phases is just density. It's possible to move all the way from liquid to gas without going through a phase transition, which makes the definitions slightly harder.

Charudatta Galande said...

Somehow, the talk about room temperature superconductivity sounds suspiciously similar to carbon nanotube space elevators. :)