Monday, May 14, 2007

SCES '07, Day 1

Today was the first day of the 2007 International Conference on Strongly Correlated Electron Systems (SCES), hosted this year in Houston jointly by Rice and UH. It's a pretty big meeting, typically with between 600 and 700 participants. Traditionally the meeting has had a very strong European and Asian participation, with a focus on heavy fermion compounds and high-Tc superconductivity. This year, there's an increased inclusion of strongly correlated physics in mesoscopic systems (quantum dots, nanotubes, graphene, single-molecule devices), as well as discussion of model correlated systems based on ultracold trapped atoms and molecules.

Because I'm on an internal search committee for a dean, my semester still hasn't really ended, which means that I'm going to miss a fair bit of the meeting. However, I'll still try to blog a couple of highlights daily. Here are two neat, new (and as yet unpublished) results that I saw this morning.

First, Louis Taillefer spoke about new measurements done in extremely high quality hole-doped YBCO at high magnetic fields (pulsed up to 60 Tesla). This work has been focused on trying to suppress superconductivity with a field in this, the grand-daddy of the high-Tc compounds, and to understand the ground state and possible quantum phase transitions (as a function of doping) of the normal phase. The exciting new result is that Taillefer and collaborators have been able to see Shubnikov-deHaas oscillations in this material for the first time. This is a big deal. First, it tells you that there are some sort of excitations in the normal state that can execute closed cyclotron orbits in the presence of a magnetic field. Since the validity of weakly interacting quasiparticles in the normal state is in significant doubt, this is interesting. Second, the frequency of the oscillations in 1/B reveals the area enclosed by those orbits in k-space - essentially it tells you how big the hole pockets are in the Brillouin zone, and therefore how many mobile holes there are per copper atom. Third, the temperature dependence of the S-deH oscillations lets you infer an effective mass (in this case, about 1.9 free electron masses) for whatever's doing the cyclotron motion. Very neat!

Second, Abhay Pasupathy from Ali Yazdani's group at Princeton showed some beautiful new STM data on BSCCO. The neat thing here is that their superfancy STM is absurdly stable over a big temperature range for days. That means that they can map out the tunneling density of states of the material on the atomic scale as a function of temperature, from deep within the superconducting state to well above the resistively detected Tc. They see that the gap in the density of states indicative of pair formation vanishes nonuniformly over the surface, with local bits persisting to well above the average Tc. They also show that the temperature dependence of the gap as a function of gap size is very different than that in low-Tc materials.


david said...

The SdH oscillations are certainly unexpected.

While it is completely nuts to think of something Fermi liquid like, isn't the simplest interpretation that when the superconductivity is killed a Fermi liquid state is entered? That there are pieces of Fermi surfaces (arcs if it is 2d)? Is this really crazy given that photoemission sometimes sees Fermi arcs at T >T_c?

The Yazdani stuff is interesting because of the quality of the data and the potential to better understand the measured gap inhomogeneities. But the the non-BCS dependence of T_c and the gap has been known since quite early in the high T_c game.

Incoherent Ponderer said...

arcs/pieces of fermi surfaces are a signature of pseudogap state, which may or may not have anything to do with superconductivity. Since arcs were observed in manganites last year, it makes you wonder about connection to SC though.

I figure SdH oscillations are seen by trying to suppress superconductivity and look directly at cross over from AFM Mott insulator to normal Fermi metal through an otherwise hidden (by SC dome) critical point?

What I wonder about Yazdani's STS measurements (and work by Seamus Davis, who sort of started this whole direction) is that these are (to my knowledge anyways) the only studies that can see spatial inhomogeneities. Most other probes are essentially non-local - transport, susceptibility, even ARPES. Isn't the fact that the properties are inhomogeneous on some nanometer scale a huge finding worth investigating further? Are we sort of like proverbial drunks looking for lost keys under the street light, because it's brighter there?

For example, studying critical phenomena when the critical point is substantially smeared out by inhomogeneities is basically impossible - your exponents are going to be all over the place. First order transition may appear as second order transition due to smearing, etc. I am sure a lot of smarter people have thought about it, but as an outsider to this whole field, it's fascinating to see what the response is going to be.

Another thing I always wonder about is whether the surface states probed by ARPES and STS are EXACTLY the same as bulk properties. Yes, I know all about how nicely BISCO cleaves with no reconstruction and an oxide layer protecting superconductivity, but on intuitive level I wouldn't be surprised if the surface states were still different. Pauli (?) said surface is the work of devil, and looking at huge amounts of surface science done over the past 3 decades or so, it seems surfaces ARE very different and not well-understood.
So, would ARPES/STS people bet their lives on the fact that spectra they are looking at are truly bulk-representative? Just wondering...

david said...

IC your missing the forest for the trees. Arcs are pieces of Fermi surface. The missing parts on the arcs are the opening of the pseudogap in certain parts of the zone.

Gaps in parts of the Fermi surface need not have to do with SC. An SDW
or CDW instability might gap part of the zone leaving arcs, pieces of Fermi surface or lines, in the rest. That's not what's going on in the cuprates but it does happen in other materials

Still SdH oscillations point toward coherent electronic quasiparticles. This would certainly be surprising and thus important if true as the Mott side should be dominated by more
particle like rather than wave properties. I might be blowing a lot of wind because I don't know the doping of the material.

Well, well, well. You are worried about inhomogeneities and disorder. First, yes, you're right disorder is a bitch. But, it is important and seems to be especially important in these materials.

We might be like proverbial drunks, but who says that this nanoscale disorder are the keys? I don't mean to be disparaging. This are wonderful discoveries and given that high T_c has been such a stubborn problem, all information needs to be found out in order to fish fro clues and prioritize it. I just think it is somewhat early to
assess how relevant these inhomogeneities are. They might hold the key to high T_c but they might be just interesting epiphenomena.

As far as critical phenomena you might get very bad scaling as you mention because disorder rounds a first order transition. There is also a more devilish possibility. Disorder might even produce extremely beautiful scaling by taking you to a new disorder dominated critical point. These are
a lot less secure theoretical footing. Disorder is such a bitch because almost always is a relevant perturbation.

I agree completely that since photoemission and STM are surface probes even when the materials cleave beautifully, it's hard to know if you are just measuring surface properties or the truly relevant bulk properties.

So I would bet their lives but not mine.

Doug Natelson said...

IP - your concerns about surface vs. bulk are very reasonable, at least to me. Someone asked Pasupathy whether the local variations in gap were equivalent to local variation in doping. Interestingly, he said that they could see where the dopants are (ahh, the beauty of STM), and implied that the variations in gap didn't particularly correlate with such a simple idea.

As far as Fermi arcs and S-deH oscillations are concerned.... You need closed orbits in k-space to get S-deH oscillations, and true arc segments can't do that for you. Hole pockets that only look like arcs in ARPES could do that, and Taillefer was careful to point out that many ideas about high-Tc could be consistent with such pockets.

David, they're killing SC with a whopping B-field, and the normal state is (allegedly) not a simple FL (as seen by, e.g., resistivity that is not T^2 in the low T limit.

N. Peter Armitage said...

As a former ARPES guy and a current (more) bulk probe guy, I'd say that one is correct to be concerned about bulk vs. surface. There is undoubtedly some connection between surface and bulk in these materials, the community in general should look with a critical eye at parameters like measured ARPES peak widths. For instance one can see that much smaller peak widths are measured with low energy laser photoemission (Dessau's group) which is more bulk sensitive.

As far as the inhomogeneous superconductivity seen by Davis, Kapitulnik, and Yazdani groups go one needs to ask if it is representative of the bulk, a property of the surface, or only relevant in BSCCO? For instance, it seems very unlikely that the extremely small scattering of quasiparticles found in YBCO by Bonn/Hardy is consistent with the degree of inhomogeneity found by the STM crowd.