Glut of Iron-Arsenide Superconductor Papers

Whoa! I looked at the ArXiv this morning in the Condensed Matter section and saw a glut of preprints on this iron-arsenide-based superconductors. The first 4 preprints, and 8 of the first 20 listed, are all on it.

This is as good as any of an indicator that this compound is the hottest thing to hit this field since MgB2 revealed itself to be a superconductor several years ago.

Zz.

High-Tc Superconductors Are Very Kinky - Update 2

A new update to my first essay on the kink feature in the ARPES spectra of high-Tc superconductors. This time, it could throw a major wrench into the analysis done previously on this high-energy kink feature that has been seen around 500 meV. The new paper[1] disputes the idea that this high energy kink is intrinsic to the band dispersion of the material. Rather, they argued that it is an artifact of the momentum distribution curve (MDC) method. Their analysis of the energy distribution cureve (EDC) does not show the same effect for that energy range.

It would be interesting to see if the previous authors who have done the analysis on this high energy kink would respond to this paper.

Zz.

[1] W. Zhang et al., Phys. Rev. lett. v.101, p.017002 (2008).

High-Tc Superconductors Are Very Kinky - Update 1

Since I last completed the essay on the "kink" that is observed in ARPES spectrum of high-Tc superconductors, I've made 2 updates on the list of references. There have been 2 preprints appearing on arXiv that argued for the phonon origin of this kink. It there does not seem to be any end to this issue in sight, at least for now.

I wonder how the ARPES spectrum for the FeAs-based superconductors are going to look at. I bet many people are scurrying to be the first to report on that, assuming of course that one has a sizable single-crystal sample that can be easily cleaved in vacuum.

Zz.

A BCS-Like Gap for the New Iron-Arsenic Superconductor

This ought to throw a large wrench into any similarities between the copper-oxide superconductors and the newly-found iron-arsenic superconductors. A new measurement of the superconducting gap of the latter found that they behave very much like that predicted by the conventional, good-old BCS theory[1]. This means that, even though the crystal structure has similarities with the copper-oxide superconductor (and the notion that maybe the same theory might be applicable as well), the behavior so far rules out many of the exotic theories that have been set up for the copper-oxide superconductors. The authors argue that in light of this, new theories may be needed to describe the iron-arsenic superconductors.

Fasten your seatbelts, folks. It's going to be a long and bumpy ride here as more experimental data pour in.

Zz.

[1] T.Y. Chen et al., Nature v.453, p.761 (2008).

Iron-Based High-Tc Superconductors - Follow-up

Actually, this is more of an in-depth review of what we know so far about this iron-based superconductors. Physics Today has quite a good article covering what has now become a rather "hot" material in condensed matter physics. It definitely appears that the spin-density wave has a major role in this family of material.

Zz.

Death For Phonons In High-Tc Superconductors?

This is a highly interesting and certainly provocative result.

Remember I posted a while back on the "kink" observed in the angle-resolved photoemission spectra (ARPES) on high-Tc superconductors? There have been a continuing debate since the kink was observed on the origin of this observation. Two leading candidates are the coupling of the charge carrier to a spin fluctuation mode, and a coupling to the phonons.

Now two separate theoretical papers have calculated the coupling to the phonon modes and have arrived at the conclusion that such coupling cannot account for the strength of the kink observed in ARPES spectra.

In recent years, despite mounting experimental evidence against it, some physicists have clung on to this interpretation. But now teams from Germany and the US have performed calculations to suggest that lattice vibrations in cuprates can at best account for just a small fraction of the materials’ superconducting behaviour.
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Manske’s team found that the theoretical energy–momentum relationship produced by these calculations did contain a kink — but about a three to five times smaller than the 2001 observations (Phys. Rev. Lett. 100 137001). This is bad news for physicists who have been hoping phonons can account for all of the behaviour of high-temperature superconductors. “It is embarrassing for people to admit they have worked on something for 20 years if it is not true,” jokes Manske.

Meanwhile, Steven Louie and colleagues at the Univerisity of California in Berkeley have come to a similar conclusion with the cuprate LaSrCuO4. From their calculations, the phonon contribution is almost an order of magnitude too small for the observed kink (Nature 452 975).

This could be rather devastating to the phonon picture. If this is true, the two new results still cannot account for the origin of superconductivity, but at least they have eliminated a red herring. Still, all this could be moot if an earlier report is true about the absence of any kind of "glue" in the mechanism for high-Tc superconductors.

So stay tune. The story is by no means over, and the fat lady hasn't even warmed her vocal cords yet.

Phenomenology of the Normal State In-Plane Transport Properties of High-Tc Cuprates

I always like it when authors upload their published papers. It gives those who do not have access to the various journals a chance to get a copy of a peer-reviewed, published papers, especially if they're in respectable journals.

This paper is more of a review of the current understanding of the anisotropic nature of the in-plane transport of the cuprate superconductors. The cuprate superconductors are well-known to have highly 2D nature in its charge transport, most probably due to the layered nature of the compound. It is believed that the transport occurs in the copper oxide planes, especially since the resistivity perpendicular to the planes is roughly 3 to 4 orders of magnitude higher. However, even within the copper oxide plane itself, there is a considerable anisotropy in the transport direction, as seen in various measurements.

The paper has a very good review of our state of knowledge on this issue so far, and also a good background info regarding the cuprates superconductor for those who need to get up to speed.

Zz.

Who or What is RVB?

In an earlier entry related to the "No glue for cuprate superconductors", I mentioned that Phil Anderson might be right after all since this could be consistent with what he has been pushing all along. Of course, what I had in mind was his Resonating Valence Bonds (RVB) theory as applied to the cuprate superconductors. I believe he has argued for the non-conventional (as in no boson coupling) origin of the superconductivity in the cuprates. So this result is certainly in favor of that picture.

In any case, not sure if it was deliberate or terrific timing, there is a start of at least a couple of columns on RVB in Physics Today. The first article appeared in the April 2008 issue, which you can read for free even if you're not an APS member. In this article, he describes basically the history of RVB theory. But I think I am more interested in the teaser that he gave at the very end, which is supposed to describe "... the rise and fall and rise again of RVB's relevance to high Tc and other superconductors..."

Now THAT, is a physics cliff-hanger!

:)

Zz.

No "Glue" For Cuprate Superconductors?

There are finally some startling evidence that points to the very unconventional nature of the cuprate superconductors. A paper just published in Science[1], heading by Ali Yazdani, has found that while there are coupling between the electrons and a bosonic mode, this coupling may not be responsible at all for superconductivity.

You can read a review of this at the Science Daily website. This discovery certainly is consistent with what Phil Anderson has been trying to push. It also could mean that the "kink" in the ARPES spectrum that I've mentioned may be another red herring that isn't directly connected to the superconducting mechanism.

But as always, this isn't the first time something of this nature has occurred in the study of high-Tc superconductors, to later turn out to be insufficient in formulating a theory of these material. So to say that this is still a controversial (in terms of interpreting what it means theoretically) result is to put it mildly.

Zz.

[1] A.N. Pasupathy et al., Science v.320, p.196 (2008).

High-Tc Superconductors Are Very Kinky

In trying to decipher the mysteries of the mechanism that causes superconductivity in high-Tc superconductors (HTS), we have to characterize and understand the many-body interactions that influence the behavior of the charge carriers in these material. In a standard Fermi Liquid theory, these charge carriers are called quasiparticles, and their behavior are described by what is known as the spectral function A(k,E) (i.e. the imaginary part of the single-particle Green's function). The interactions that influence the behavior of these quasiparticles are quantified in the spectral function via the complex self-energy term Σ. The real part of the Σ shows how these collective interactions influences the dispersion relations/band structure of the material (i.e. the E vs. k curves), while the imaginary part of Σ indicates the scattering rate or lifetime of the quasiparticles. For an idealized metal having a non-interacting free-electron gas, the self-energy term is zero. This means that there's no deviation from the non-interacting electronic band structure/dispersion (ReΣ=0) and it has zero scattering rate/infinite lifetime (ImΣ=0).

So if one can actually measure this A(k,E), one can gain a lot of insight into the interactions that influence the behavior of the quasiparticles that are responsible for superconductivity in high-Tc superconductors. One of the ways to make a direct measurement of the spectral function (at least the occupied side of the band) is by using the angle-resolved photoemission spectroscopy (ARPES) technique. In an earlier post, I have highlighted a review of this powerful technique and how it can directly measure A(k,E). This technique has produced very clear signature of the various interactions in a typical metal such as Be[1] and Mo[2], showing very clearly the electron-electron interaction, the electron-phonon interaction, and electron-impurity interaction, all based on the self-energy obtained from ARPES measurement. The parameters obtained from the results, such as the electron-phonon coupling strength, all agree with existing theoretical predictions and previous measurements. So we know that such a technique works.

Because of that, ARPES has been extensively used in the study of HTS. This family of material is high suitable for this technique because of its layered, 2D structure. Furthermore, HTS compounds such as the BSCCO family are easily cleaved in situ along these 2D planes, exposing a pristine surface perfect for photoemission studies. A major progress in ARPES technique came in 1999 whereby not only the energy distribution curve (EDC) at a particular momentum are collected, but also the momentum distribution curve (MDC) at a particular energy can also be obtained simultaneously! This resulted in the collection of a 2D raw data of energy distribution and momentum distribution of the photoelectrons within an energy and momentum window[3]. What is essentially obtained is the dispersion curve. When this technique was done on the BSCCO family, a distinctive feature of a "kink" in the dispersion with an energy scale of around 50 meV was clearly observed[4]. This kink is the deviation of dispersion curve from the non-interacting dispersion, which signifies a coupling to some bosonic mode. By 2001, the race was on to analyze the nature of this kink to see if one can identify this "bosonic mode" that affects the electronic dispersion. This bosonic mode might be the "glue" that holds the Cooper pairs together in the formation of superconductivity.


[Figure shows the "kink", i.e. the deviation from the non-interacting dispersion (Campuzano et al. in The Physics of Superconductors, Vol. II, ed. K. H Bennemann and J. B. Ketterson (Springer, New York, 2004), p. 167-273., or at http://arxiv.org/abs/cond-mat/0209476]


There were three important papers that were published in 2001 that were the first to analyze the origin and nature of this kink[5,6,7]. Two different groups arrive at roughly the same conclusion - that the kink seems to indicate a coupling to a magnetic (spin) bosonic mode[5,6], while the Stanford group[7] proposed the phonon as the origin of the kink. There have been many publications since then arguing for various scenario for the origin of this kink and until today, there is no overwhelming consensus for either the magnetic, although my quick review of this matter seems to indicate that there are more publications related to ARPES experiment in favor of the magnetic channel [8,9]. Still, the phonon picture has many strong advocates and the issue isn't settled even to this date.

But that is not the end of the story. It appears that the high-Tc cuprate familyis even kinkier than first thought. There seems to be, in addition to the low energy kink (now regarded to be between 50 to 70 meV scale), a higher energy kink in the dispersion has been discovered. It was reported, in succession within the span of a couple of weeks, by 2 papers in Phys. Rev. Lett.[10,11]. This new kink occurs at an energy scale of around 340 meV, so it is considerably larger than the low energy kink. The latter concluded that this new high energy kink is consistent with coupling to the magnetic (spin excitation) mode.

Since then, there have been even more reports on high energy kink[12], and this one is even at a different energy scale (115 and 150 meV) and the authors are suggesting that these may be due to neither spin fluctuation nor phonon modes.

Moral of the story: the high-Tc family of material is very kinky, and that the study on the origin of these kinks could hold a vital clue on the mechanism of superconductivity in these materials. However, the issue is highly complicated, especially when new discoveries are continually being made as one tries to solve the old ones. Whether these kinks are due to coupling to the magnetic mode, phonon modes, or neither, remains to be seen. A lot more work is still to be done.

Zz.

Edit (04/10/08): A new preprint on arXiv[13] has appeared that studied the phonon mode in La doped Bi-cuprate compound. The result supports the electron-phonon coupling as the source of the kink in the cuprate superconductor.

Edit (06/10/08): A new preprint on arXiv[14] has studied the phonon dispersion in on of the Bi cuprate family using inelastic X-ray scattering. They found a significant phonon softening that's consistent with the same energy and momentum scale as the kink observed in the ARPES result. So this argues for the phonon origin of the kink.

Edit (07/02/08): This is getting to be quite interesting. A new PRL paper[15] has just been published, arguing that the high energy kink is more of an artifact of the MDC analysis, and that the EDC spectra provides a more accurate information about the band dispersion in high-Tc superconductors. So this certainly calls into question the conclusion made in previous papers on this high-energy kink.

Zz.

[1] S. LaShell et al. Phys. Rev. Lett. v.61, p.2371 (2000).
[2] T. Valla et al., Phys. Rev. Lett. v.83,p.2085 (1999).
[3] T. Valla et al., Science v.285, p.2110 (1999).
[4] P.V. Bogdanov et al., Phys. Rev. Lett. v.85, p.2581 (2000).
[5] P.D. Johnson et al., Phys. Rev. Lett. v.87, p.177007 (2001).
[6] A. Kaminski et al., Phys. Rev. Lett. v.86, p.1070 (2001).
[7] A. Lanzara et al., Nature v.412, p.510 (2001).
[8] A.A. Kordyuk et al., Phys. Rev. Lett. v.92, p.257006 (2004); A.A. Kordyuk et al., Phys. Rev. Lett. v.97, p.017002 (2006).
[9] A. Macridin et al., Phys. Rev. Lett. v.99, p.237001 (2007).
[10] B.P. Xie et al., Phys. Rev. Lett. v.98, p.147001 (2007).
[11] T. Valla et al., Phys. Rev. Lett. v.98, p.167003 (2007).
[12] http://arxiv.org/abs/0711.1706.
[13] http://arxiv.org/abs/0804.1372
[14] http://arxiv.org/abs/0804.1372
[15] W. Zhang et al., Phys. Rev. lett. v.101, p.017002 (2008).

Iron-Based High-Tc Superconductors

This certainly came out of nowhere.

There's another "high-Tc" superconductor joining the fun. This time, unlike the cuprates, it consist of iron-arsenic planes, and is doped with fluoride atoms. Currently, it has a Tc of 26K, but that would probably change.

From preliminary report, it seems that it is not phonon-mediated, but rather via spin-fluctuation. It would be interesting to see if it has the same phase diagram as the cuprates and if it also has a pseudogap state.

Update 04/18/08 : There's a rather good summary on this in a Science daily news.

Zz.

"This Coincidence Cannot Be Accidental"

That's the most dramatic statement coming out of a newly-published paper in Science, and that was the title that Doug Scalapino used in his Perspective article on this paper in the same issue.

Just when we think that we know everything there is to know about conventional, metallic superconductors, Mother Nature throws a wrench at that fallacy. Reported today by Ayanajian et al.[1], metallic superconductors such as Pb and Nb still holds a mystery. This time, it appears that the exact phonon energies of the Kohn anomalies happen to coincide with the superconducting binding energies of the cooper pairs of that particular metal. This is something that was not expected, and current formalism of conventional superconductivity says nothing about such a thing. As Scalapino pointed out his is Perspective article[2],

... although the polarization created by the conduction electrons and the response to the ionic lattice contribute to determining both the Kohn anomaly and the pair binding energy, there must be something else at work that locks (the superconducting gap) to (Kohn anomaly energy). As noted, this could mean that there is some new physics that is not captured by the Eliashberg formulation.

I love it! I love unexpected surprises like this!

Zz.

[1] P. Ayanajian et al., Science v.319, p.1509 (2008).
[2] D.J. Scalapino, Science v.319, p.1492 (2008).

From BCS to the LHC

READ THIS ARTICLE!

Now, was that clear enough? :)

This is the text of the speech given by Steven Weinberg at the recent celebration marking the 50th anniversary of the BCS Theory of superconductivity. Yup! You read it right. A reductionist, elementary particle physicists speaking at a condensed matter, anti-reductionism event! :)

In it, he mentioned everything that I had mentioned earlier regarding emergent behavior and the fundamental importance of condensed matter physics that transcends into other areas of physics, especially high energy/elementary particles.

Just go read that article, why don't you? :)

Zz.

When Superconductivity Became Clear (to Some)

I mentioned last October (2007) of the conference being held at UIUC to commemorate the 50th Anniversary of the BCS Theory of Superconductivity. Well now the NY Times has an article related to that, and includes a delightful anecdotal history on how Bardeen recruited Cooper to come to UIUC to work on the superconductivity problem. I think most people are not aware that some of the giants in physics at that time had tried to solve the superconductivity problem and failed!

After wrapping up special and general relativity, Albert Einstein tried, and failed, to devise a theory of superconductivity. Werner Heisenberg, the physicist who came up with the Heisenberg uncertainty principle, struggled with the problem, as did other pioneers of quantum mechanics like Niels Bohr and Wolfgang Pauli. Felix Bloch, another thwarted theorist, jokingly concluded: Every theory of superconductivity can be disproved.

You seldom hear in any of Einstein biography of him dabbling in trying to solve this.

This is a terrific story. Don't miss it.

Zz.

From High Temperature Superconductivity to Quantum Spin Liquid: Progress in Strong Correlation Physics

This is a fascinating review article by Patrick Lee of MIT[1], even if you don't agree with his take on the mechanism of cuprate superconductivity. He covers the outstanding issue in strongly-correlated electron system, which is the main area of study in condensed matter physics since it covers essentially that whole field of study.

What is interesting is that at the end of the article, he has a question-answer section that addresses specific issues and his take on the answer. This is always something I like to read because even if you disagree with him, at least you know clearly where he stands and why he disagrees. In many instances, it can be vague on what exactly people are disagreeing on. Here, it is rather clear.

In any case, I would think anyone in this field of study would want to read this article. At the very least, it'll get you up to speed on the theoretical progress in this area.

Zz.

[1] P.A. Lee, Rep. Prog. Phys. v.71, p.012501 (2008).

'Cooper Pairs' Can Be Found In Insulators As Well Superconductors

This is a rather fascinating report. A team from Brown University has found evidence that Cooper Pairs can exist not just in a superconductor, but also in an insulator.

“Our finding is quite counterintuitive,” said James Valles, a Brown professor of physics who led the research. “Cooper pairing is not only responsible for conducting electricity with zero resis-tance, but it can also be responsible for blocking the flow of electricity altogether.”

However, unlike a superconductor, these Cooper pairs do not condense into a coherent state and, therefore, do not conductor electricity. It is interesting to note that this is similar to the behavior of the high-Tc cuprates in the pseudogap state. This is where one obtains a paring of the charge carriers ABOVE Tc. Here, no long-range coherence occurs even though pairing has occurred. Whether these paired carriers are precursor to superconductivity as the material goes below Tc, or are competing with superconductivity, is still a question yet to be answered.

Also interesting to note that this isn't the only situation where paring, and even superconductivity, can occur in an insulator. When an insulator shares a common surface with a superconductor, there is something call the proximity effect, whereby the superconducting wavefunction "leaks" into the insulator over some length (of the order of the the coherence length). What is different in this new work is that this pairing occurs on stand-alone insulator.

The exact citation to this work is:

M. D. Stewart, Jr. et al., Science v.318, p.1273 (2007).

Zz.

BCS 50th Anniversary Conference

Today is the start of the conference marking the 50th anniversary of the BCS theory of superconductivity at University of Illinois Urbana-Champaign. I wrote about the news of this conference in an earlier blog entry.

Several news organizations are reporting on this event such as this one. It is important to note the significance of the BCS theory of superconductivity and its impact on physics as a whole.

Not just a milestone in condensed-matter physics or physics in general, something most physicists would agree upon, but "one of the high-water marks of human thought" is how Goldbart characterized the so-called BCS Theory recently.
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"I really feel that," he said.

"This is a theory that had an enormous impact on physics in all areas, I would say," added Goldbart's colleague in the UI Physics Department, Professor Gordon Baym.

So, if you're at UIUC and attending this conference, I'd appreciate a first-hand report from you.

Zz.

Interface Between 2 Insulators Becomes Superconducting

As Alice in Wonderland says "things are becoming curioser and curioser".

First there was a report that the 2D interface between 2 insulators can become metallic[1]. Now a new report in Nature has indicated that such interface can in fact become superconducting, albeit at 200 mK[2]. Fantastic!

This interface is a 2D thin region (~10 nm) and evokes exotic transport mechanism, I would assume. There's still considerable work in coming up the theoretical explanation for this phenomenon. So there's plenty of work left to be done.

Zz.

[1] A Ohtomo, H.Y. Hwaing, Nature v.427, p.423 (2004).
[2] N. Reyren et al., Science v.317, p.1196 (2007).

String Theory Might Provide Insight Into High-Tc Superconductors and Quark-Gluon Plasma?

Say it isn't so!!

:)

Jan Zaanen's article in this week's Nature (Nature v.448, p.1000; 30 August 2007) highlights an ArXiv preprint by Hartnoll et al. that seems to have made a connection between one aspect of String Theory called the anti-de-Sitter/conformal field theory correspondence(AdS/CFT), and the heat and charge transport in high-Tc superconductor called the Nernst effect.

Hartnoll et al. push what one might term the 'AdS-to-high-Tc correspondence' to its logical conclusion. They study its application to a particular, rather recondite transport phenomenon known as the Nernst effect — the crosswise flow of heat and charge currents in the presence of a magnetic field7 — in the nearly quantum-critical matter of a two-dimensional cuprate system. In a theoretical tour de force, they use the physics of a black hole in a three-dimensional anti-de-Sitter space that carries both electrical and magnetic charge to guide them in the very complex derivation of the relevant transport equations directly from quantum field theory. They show that these theoretical results are seemingly consistent with a number of hitherto unexplained features of the Nernst effect in a high-temperature superconductor7.

Man, that takes a lot of balls to do that! :) And what about the quark-gluon plasma that has been studied at RHIC?

Here, the AdS/CFT correspondence comes to the aid of the experimentalists in a similar way. The background is the observation that quark–gluon fireballs, as have been created in the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory on Long Island, behave in a remarkably simple way, but one that current theories find difficult to explain — they are governed by normal hydrodynamics, but have extremely low viscosity. Quite simply, the AdS/CFT correspondence tells us that when the quantum physics is scale invariant, the viscosity of such a system can be as small as it is. This result is far from obvious given our current understanding of quantum chromodynamics, the standard-model quantum-field theory of the strong nuclear force that governs interactions in the quark–gluon plasma.

I'm not going to jump onto the String Theory bandwagon and proclaiming it as the next best thing since sliced bread, but this is getting to be rather interesting.

Zz.

Vitaly Ginzburg and I

Not exactly The King and I, but close enough in the Physics world. :)

This is a rather fascinating personal recollection of I.I. Mazin of the great Vitaly Ginzburg, who won the Nobel Prize a few years ago for his work in superconductivity. What is also interesting is the background information on the educational system and policy of the Soviet Union at that time.

Zz.