2025 Nobel Prize in Physics

It's unusual that a Nobel Prize in Physics is given to physicists working in the field that was the same as my PhD research work. It finally happened this year.

I did research work in tunneling spectroscopy in cuprate superconductors, and we did both superconductor-insulator-normal metal and superconductor-insulator-superconductor tunnel junctions, the latter of which is where we observe the Josephson tunneling current. Therefore, the work cited here is something that I'm quite familiar with. I just never realized till now that it was such a major discovery to be awarded a Nobel Prize. I know that one of my colleagues had John Clarke as his PhD advisor at Berkeley.

Interesting that this is such an old and well-established phenomenon and technique that is only now being recognized.

Zz. 

A Century of Bose-Einstein Condensation

Nature has published a wonderful review of the discovery and progress that we have made in understanding BE condensation since its discovery. It is an open access article and you can download the full article. I definitely like the figure that shows the major milestone in its development, but it would be nice if that is expanded even more to include references, or at least citation numbers so that I don't have to go hunting for them. 

Scanning through the article, I actually did a quick headcount on how many of the names mentioned in the article that I had met personally: Schrieffer, Leggett, Anderson, and Abrikosov. I believe Leggett is the only one still around as of this writing.

I didn't get too much into BE condensation even though I was working in superconductivity at that time. I was transitioning out of that field of study when the big BEC-BCS connection was experimentally established. Still, it was, and still, an exciting field to follow even on the peripheral.

Zz. 

Room-Temperature Superconductor?

Here we go again!

Big news with the new publication out of Nature this week. A report on an observation of room-temperature superconductivity on a sample that is under pressure at only 1 GPa. That pressure is exceedingly low considering that most of the other superconductors that that has a high transition temperatures tend to be under hundreds of GPa.

Superconductivity has been observed at 20 °C (294 K) in a nitrogen-doped lutetium hydride under a pressure of 1 GPa (10 kbar). The material was made and studied by Ranga Dias and colleagues at the University of Rochester in the US, who claim that the finding raises hopes that a material that superconducts at ambient conditions may soon be found.

Not only that, this thing changes color as pressure is increased, with it turning from blue to pink at the onset of superconductivity. I'm sure doing a reflectivity measurement such as UV-VIS to look at the phonon modes would be very interesting here. 

But as with anything here, this needs to be independently verified, meaning that another group must be able to replicate the recipe and observe the same thing, before this is widely accepted. We will just have to wait.

Z.

Signature of Tc Inside the ARPES Pseudogap?

The physics of high-Tc superconductors (or the cuprate superconductors) continues to be elusive. After its first discovery in mid 1980's, a coherent and consistent theory on why this family of material becomes superconducting is still up for debate. There are candidate theories, but we do not have an accepted consensus as of yet.

One of the main reason for this is that this is such a rich and complex material, exhibiting so many different characteristics and puzzles. As a result, different versions of theories are competing to describe as many of the experimental results as possible. But the target is also moving. As our instrumentation improves, we are discovering new, more subtle, and more refined behavior of these material that we haven't seen before.

The existence of the so-called pseudogap in the cuprates is well-known. I've posted several articles on them. This is the gap in the single-particle spectral function that opens up well above the transition temperature Tc. In conventional superconductors, the formation of this gap coincides with Tc, below which the material becomes superconducting. However, in the cuprates, and especially in the underdoped cuprates (less oxygen doping than the optimally-doped), a gap opens up well above the Tc. The material doesn't become superconducting yet even as you lower the temperature even more. It is only when the temperature gets to Tc will the material becomes superconducting.

The origin of this pseudogap has long been debated. The posts that I had made discussed all this. However, in this new paper published in Nature (the article I linked too erroneously wrote "Science" at the time of this citation), the Z-X Shen group out of Stanford has detected the signature of Tc in the pseudogap region from ARPES measurement. But what is interesting here is that it was detected in the overdoped cuprate Bi2212.

Typically, the overdoped regime of the cuprates does not exhibit clear pseudogap signatures. When I studied a highly-overdopped Bi2212 using ARPES a long time ago, we did not detect any pseudogap at all since we saw the opening of the gap only at the bulk Tc value. Of course, this does not mean it wasn't there because it depends on the temperature resolution of our experiment. So it is rather interesting that this study decided to focus on the overdoped region where the pseudogap is more difficult to detect, as opposed to the optimally-doped or underdoped region where the pseudogap is much more obvious.

In any case, they apparently saw spectroscopic signatures of Tc within the pseudogap as the material cools down through Tc. According to them, this seems to be a strong evidence in support of a phase fluctuation (spin fluctuation?) model as the driving mechanism for superconductivity in these materials.

I tell ya, almost 40 years since its discovery, the cuprates continue to amaze and surprise us!

Zz.

Charge Fluctuation at a Quantum Critical Point.

This is a fascinating paper[1] (which I'll be reading more of in the next several weeks). But for now, I'll just highlight it here.

The authors found that charge fluctuation in a "strange metal" antiferromagnetic compound exhibit a scaling of f/T (frequency over temperature) in the optical conductivity, which often indicates the presence of a quantum critical point.

If anyone has done MBE before, you'll know how tedious and difficult it is to synthesize a material such as this, and have it be pristine enough to produce these effects that can be measured, at a THz level, no less!

There are many implications here, not the least of which is that the cuprate high-Tc superconductors share the same "parent" or undoped state, being antiferromagnetic perovskites themselves. There have been experiments indicating that the cuprates superconductors are also influenced by their proximity to a quantum critical point.

This is another example where some of the most fundamental aspects of quantum mechanics, in this case the concept of quantum criticality, can often be clearly manifested in a condensed matter system, not in elementary particle physics experiment.

Zz.

[1] L. Prochaska et al., "Singular charge fluctuations at a magnetic quantum critical point." Science v.367, p.285 (2020). ArXiv version of the paper can be found here.

RIP J. Robert Shrieffer

I'm sad to hear the passing of a giant in our field, and certainly in the field of Condensed Matter Physics. Nobel Laureate J. Robert Schrieffer has passed away at the age of 88. He is the "S" in BCS theory of superconductivity, one of the most monumental theories of the last century, and one of the most cited. So "complete" was the theory that, by early 1986, many people thought that the field of superconductivity has been fully "solved", and that nothing new can come out of it. Of course, that got completely changed after that.

Unfortunately, I wasn't aware of his predicament during the last years of Schrieffer's life. I certainly was not aware that he was incarcerated for a while.

Late in life, Dr. Schrieffer’s love of fast cars ended in tragedy. In September 2004, he was driving from San Francisco to Santa Barbara, Calif., when his car, traveling at more than 100 miles per hour, slammed into a van, killing a man and injuring seven other people.

Dr. Schrieffer, whose Florida driver’s license was suspended, pleaded no contest to felony vehicular manslaughter and apologized to the victims and their families. He was sentenced to two years in prison and released after serving one year.

Florida State placed Dr. Schrieffer on leave after the incident, and he retired in 2006.

I've met him only once while I was a graduate student, and he was already at Florida State/NHML at that time. His book and Michael Tinkham's were the two that I used when I decided to go into superconductivity.

Leon Cooper is the only surviving members left of the BCS trio.

Zz.

New Family of High Tc Superconductors?

We interrupt your year-end holiday to bring you this news.

It seems that there are two groups reporting the discovery of possible high-Tc superconductors in a new family of material, the hydrides. The Tc's are well above 200 K. The caveat? So far, they become superconducting at high pressures.

Researchers at the Max Planck Institute for Chemistry in Mainz, Germany say that lanthanum hydride (LaH₁₀) could be superconducting at the remarkably high temperature of 250 K (-23°C), albeit at extreme pressures of around 170 GPa. Meanwhile, another team from George Washington University in the US says that it has found evidence of superconductivity in the same material at even higher temperatures of 280 K (7°C) under 202 GPa pressures. If confirmed, the findings could be a major step towards finding room-temperature superconductors.

You may read the preprint of one of the reports here.

As usual, we need to sit back, take a deep breath, and let the process runs through. These needs to be published first, and then independent groups will have to verify the results. Only THEN can we get excited about this news. So stay tune, a lot more will be coming.

Zz.

Bismuthates Superconductors Appear To Be Conventional

A lot of people overlooked the fact that during the early days of the discovery of high-Tc superconductors, there was another "family" of superconductors beyond just the cuprates (i.e. those compounds having copper-oxide layers). These compounds are called bismuthates, where instead of having copper-oxide layers, they have bismuth-oxide layers. Otherwise, their crystal structures are similar to the cuprates.

They didn't make that much of a noise at that time because Tc for this family of material tends to be lower than the cuprates. And, even back then, there were already evidence that the bismuthates superconductors might be "boring", i.e. the results that they have produced looked like they might be a conventional superconductor. This is supported by several experiments, including a tunneling experiment[1] that showed that the phonon density of states obtained from tunneling data matches that of the density of states obtained from neutron scattering.

Now it seems that there is more evidence that the bismuthates are conventional BCS superconductors, and it comes from ARPES experiment[2]. There have been no ARPES measurement done on bismuthates before this because it had been a serious challenge to get a single-crystal of this compound large enough to perform such an experiment. But obviously, large-enough single-crystals have been synthesized.

In this latest experiment, they look at the band structure of this compound, and extract, among others, the strong electron-phonon coupling that matches the superconducting gap. This strongly indicates that phonons are the "glue" in the superconducting mechanism for this compound.

So this adds another piece of the puzzle for the whole mystery of the origin of superconductivity in the cuprates. Certainly, having similar layered crystal structure does not discount being a conventional superconductor. Yet, the cuprates have very different behavior when we perform tunneling and ARPES experiments, and they certainly have higher Tc's.

The mystery continues.

Zz.

[1] Q. Huang et al. Nature v347, p369 (1990).
[2] CHP. Wen et al. PRL  121, 117002 (2018). https://arxiv.org/abs/1802.10507

Another Superconductor Scandal Brewing?

I heard about this preprint and the reported result towards the end of July, and my reaction to this type of "discovery" is "wait-and-see". In the history of superconductivity, we have had MANY of such similar claims, and many of them amounted to nothing.

However, this one seems to have taken a life and a drama of its own. SciAm has a report on what has transpired so far.

I heard about the identical background noise in the data more than a week ago when Brian Skinner posted his ArXiv comment. The first thing that came to my mind was "Oh no, this is Hendrik Schon all over again!" Turns out, I'm not the only one based on what was written in the SciAm article.

The only way this will be determined is an independent verification. That is how science works, and this is how experimental discovery works. We simply do not accept something just because someone says so.

Zz.

RIP David Pines

This is another one of the physicist who is a giant in his field, but relatively unknown to the general public.

Renowned condensed matter theorist David Pines passed away on May 3, 2018 at the age of 93. I practically read his text (co-authored by Nozieres) on Fermi Liquid from cover to cover while I was a graduate student. In fact, he was on the cusp of a Nobel Prize when he was working with John Bardeen at UIUC. They published a paper on the electron-phonon interaction in superconductors in 1955, a paper that many thought was the precursor to the subsequent BCS Theory paper in 1957. Unfortunately, he left UIUC, and Bob Schrieffer took over his work on this, which ultimately led to the BCS theory and the Nobel prize.

This did not diminished his body of work throughout his life. He certainly was a main figure during the High-Tc superconductivity craze of the late 80's and 90's. His 1991 PRL paper with Monthoux and Balatsky and the 1992 PRL paper with Monthoux, both on the spin-fluctuation effect as the possible "glue" in the cuprate superconductors, where ground-breaking and highly cited.

His contribution to this body of knowledge will have a lasting impact.

Zz.

Another "Unconventional" Superconductor?

This is definitely exciting news, because if verified, this will truly open up a whole new phase space for superconductivity.

An advanced publication has appeared reporting the discovery of high-spin state quasiparticles that are involved in superconducitivty.[1] This occurs in a topological semimetal YPtBi.

Previously, superconductivity occurs due to quasiparticles of spin 1/2 forming pairs called Cooper pairs. Now these Cooper pairs can have a total spin of either 0 (singlet state), or 1 (triplet state). This new superconductor seems to be formed by quasiparticles having spin 3/2! The resulting Cooper pairs may have total spin of 3 or 2.

It turns out that based on their measurements, the pairing symmetry appears to be predominantly in the spin state of 3, with a sub-dominant component having 0 (the singlet) state.

If you want to know how a quasiparticle here could have a spin 3/2 state, then you need to learn about spin-orbit coupling that we all learned in intro QM classes, and read the article.

This is utterly fascinating. Just when you think you can't be surprised anymore by the phenomenon of superconductivity, along comes one!

Zz.

[1] H. Kim et al., Sci. Adv.2018;4

SQUID: History and Applications

No, this is not the squid that you eat. It is the Superconducting Quantum Interference Device, which is really a very clear application of quantum mechanics via the use of superconductors.

This is a lecture presented by UC-Berkeley's John Clarke at the 2018 APS March Meeting.



Zz.

SRF Technology For Particle Accelerator

Here's a brief video on the superconducting radiofrequency (SRF) cavity for particle accelerators.



I wouldn't call it "better particle accelerator" as in the video, because SRF cavity with Nb currently have a limit of 20-30 MV/m gradient, whereas normal conducting cavity can reach 100 MV/m or even higher at 1.3 GHz.

Still, these SRF cavities have properties that are "better" in other characteristics, especially in the Q-value. And in a number of applications, these cavities are the most efficient accelerating structures.

The technology for SRF is still evolving, especially in whether there is a need for superconducting photocathode sources for SRF guns. So there's a lot more to do in this field of study, both in terms of the physics, and in engineering.

Zz.

Stnky Superconductor Breaks Record

No, I wasn't being deragoratory by calling it "stinky".

It turns out that hydrogen sulfide, the same compound that smells like rotten eggs, becomes a superconductor when solidified under pressure. And not only that, but it has recently be shown that it becomes a superconductor at a record highest transition temperature of 203.5 K.

Still, there are two points here that may make this not as "exciting" as one wold hope for. Earlier theoretical studies have predicted this to occur, and this material is expected to be a conventional superconductor mediated by phonons.

But the other issue, as in the practical aspect of this, may be even less enticing. This is because this material becomes a superconductor only under very high pressures.

The result may revive visions of superconductors that work at room temperature and magnetically levitated trains. But there's a catch: Hydrogen sulfide works its magic only when squeezed to more than 100 million times atmospheric pressure, roughly one-third as high as the pressure in Earth’s core. This condition makes it impractical for most applications. “Where does it go from here?” asks Igor Mazin, a theorist at the U.S. Naval Research Laboratory in Washington, D.C. “Probably nowhere.” Even so, the discovery is already altering the course of research in superconductivity.

So, while I think this is an exciting discovery, I'm not sure how much it will add to the physics and to applications..... yet.

Zz.

Electron Pairing Without Superconductivity

The interesting news from last week is the publication in Nature of the confirmation of the presence of electron pairs in STO, but without superconductivity.

This is significant because this has always been a possibility, i.e. where the electrons pair up but do not form any long range order or become a condensate. This phenomenon was hinted at in the cuprate superconductors especially in the underdoped regime where experiments such as tunneling and ARPES have shown the presence of a gap, called the pseudogap, above the critical temperature Tc. Whether this pseudogap is the precursor to the electrons having long-range order and condenses below Tc, or whether these electrons are actually competing with those that do, is still a highly debated question.

My guess is that this paper will be a significant piece of information to that puzzle.

Zz.

Superconductivity And The Environment - A Roadmap

A rather interesting and unusual review article. It describes how superconductivity and superconductors can actually help in improving our environment. As stated in this PhysicsWorld blog, it is an unusual assertion because "... superconductors only work at very low temperatures and lots of energy is needed to cool them.. "

Still, you might want to check out the list of things that have been argued that superconductors can do to help with the environment. The review paper is available for free.

Zz.

Paper On Possible Sign Of Majorana Fermions In Solid-State System Published

I mentioned this a while back on the possible discovery of Majorna fermions in a solid state system. While the paper has appeared earlier, it has now officially been published by Science.

V. Mourik et al., Science v.336, p.1003 (2012).

Abstract: Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. Here, we report electrical measurements on indium antimonide nanowires contacted with one normal (gold) and one superconducting (niobium titanium nitride) electrode. Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields on the order of 100 millitesla, we observe bound, midgap states at zero bias voltage. These bound states remain fixed to zero bias, even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors. 

I'm sure they'll continue to have a better experiment to nail this down even more.

Zz.

More Evidence Against Phonon-Origin As The Glue In Cuprate Superconductors

This latest result will not settle it, but it is another evidence against phonons as the dominant "glue" for the origin of superconductivity in the cuprate family of high-Tc superconductors. This latest work comes from fast optical measurements on the Bismuth-based cuprate superconductors[1].

The researchers measured the part-in-10-thousand changes over a few thousand femtoseconds and then plugged the numbers into a computer model to gauge which processes were most important in carrying energy through the lattice. Electron-electron interactions such as spin fluctuation should carry energy away much faster than phonons do, the researchers argued, making it possible to separate the different contributions.

The ability to study the reflectivity at different wavelengths was key, Giannetti says. That's because the ultrafast electron-electron processes were too fast to observe in the time traces. However, those processes affect the reflectivity at different wavelengths in different ways-100 femtoseconds after the pulse the material was less reflective at longer wavelengths and more reflective at shorter wavelengths. Taken all together, the data show that phonons aren't needed to explain BSCCO's superconductivity, Giannetti says. Electron-electron interactions are strong enough to do the job all by themselves.

As you can read from the article itself, while this experiment convinces people who are already in the spin-fluctuation camp, those in the phonon camps are not convinced at all due to possible issues in the analysis of the data.

In other words, this is still not the smoking gun, and the debate continues.

Zz.

 [1] S. Dal Conte et al., Science v.335, p.1600 (2012).

Fermi Gasses And Superfluids

This is a concise review of the physics of Fermi gasses and BE condensate, and how they are related to each other in light of the BE-BCS crossover discovery from a few years ago. Note that this review is highly technical and probably meant for condensed matter physicists. But it still provides ample overview of the theory and experiments that we have so far.

Zz.

Intro To Metal-Insulator Transition

This is a terrific review of the phenomenon surrounding the metal-insulator transition. I highly recommend it.

And if you think this is a boring topic (who cares about a metal-insulator transition?), read this review. In one swoop, this topic covers everything from high-Tc superconductivity to quantum phase transition. It also has a concise coverage of band structure description, why it works, where it doesn't, and why it doesn't. In the process, you get to learn about charge transport, charge localization, and a bunch of other fascinating stuff in the world of strongly-correlated systems.

Zz.