“In the Middle of a Revolution”

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Conference on Strongly Correlated Topological Insulators: SmB6 and Beyond

June 2, 2015 to June 5, 2015

Ann Arbor, MI

Meeting host:

Institute for Complex Adaptive Matter and the University of Michigan

By:

Pavel Shibayev

SPS Chapter:

Slide depicting the interplay between strongly correlated electron systems and topology on a relative timeline of emergent physics, as seen by Prof. Piers Coleman (Rutgers University), whom I have had a chance to interview after his concluding talk at the conference. Photo courtesy of Pavel Shibayev.

“Let me make a bet,” suddenly announced the chair of a discussion session at a unique conference on research in the field of strongly correlated topological insulators. He continued, “if we make it 1:1, it wouldn’t be a fair bet, so let us do 5:1,” to cheers and applause from the majority of the sixty-people audience that included a number of theorists and experimentalists known to be leaders of their subfields. Were someone to walk in the room at this very moment, the lighthearted manner with which these words were uttered could have left them astonished and wondering whether they were in the right place, but only momentarily, as the discussion immediately resumed its natural academic style. At the same time, the continued exchange among several participants of the discussion session, articulated in exceptionally civil but almost impatient tones, provided further evidence that the topological nature of the compound under investigation is at the core of an ongoing debate in the condensed matter community around the globe.

Objective

To backtrack by one day, on June 2, 2015 I graduated from Princeton University and theoretically had the freedom to spend the rest of the afternoon any way I found worthwhile. But instead of wasting time on celebrating this memorable life event, immediately upon the conclusion of our commencement ceremony I took a train to the airport and flew across several states to Michigan, the site of this year’s conference on the recent developments in research conducted on the SmB6 compound and related hexaborides.

My objective for attending this thematic conference was to gain additional familiarity with the findings of many prominent experimental and theoretical groups in the subfield of strongly correlated systems due to its many similarities with elements of my own senior thesis research that I have pursued in the past year. Upon arrival at West Hall of the University of Michigan, where all the talks took place over a period of four days, I was immediately immersed in the atmosphere of heated discussions among researchers of all ages. While I am quite confident that at this conference I have been the only undergraduate student in attendance, or rather, someone who had just completed their undergraduate education the day before, I was pleasantly surprised to observe a diverse group of researchers ranging from PIs to graduate students at each talk and discussion session.

Notable talks, Interviews

Prof. Peter Armitage (Institute of Quantum Matter, Johns Hopkins University, MD) gave an interesting talk on the electrodynamics of correlated topological states of matter, concentrating on recent work utilizing time domain terahertz spectroscopy (TDTS) to probe surface states of SmB6 and Dirac semi-metals. Besides an expected saturation of THz transmission, associated with a plateau in resistivity due to the insulating behavior in the DC limit, a peculiar finding of this group included a secondary bulk conduction mechanism for the AC frequencies down to as low as 1.6 K. This research has been carried out in close collaboration with the group of Prof. Sean Oh (Rutgers University, NJ), which utilizes a highly unique molecular beam epitaxy system for the growth of single crystals and thin films.

The following talk, by Dr. Filip Ronning (Los Alamos National Laboratory, NM), dealt with transport properties of possible topological Kondo insulators (TKIs). The group, which includes Dr. Madhab Neupane, my colleague and mentor from Prof. Zahid Hasan's research group at Princeton, has measured the resistivity of SmB6 as a function of ion irradiation and found that the topologically protected surface state may survive in parallel with the metallic layer formed by irradiation. The group also estimated an effective mass of the topologically protected state by analyzing their electrical and thermal transport data. What I found remarkable, both with respect to this talk and others, was the sudden increase in the level of excitement over certain new specific details in the data presented, evident from the prompt reaction of a large swath of the audience as soon as a corresponding slide was shown. As an example, in this LANL group's presentation a very small, previously undetected dip in the graph depicting two-channel conductivity in SmB6 attracted much attention on the part of the audience members, leading to a long series of questions and sparking further interest in the work.

The subsequent discussion led by one of the conference organizers, Prof. Çagliyan Kurdak (University of Michigan), was extremely intense and fervent, nothing like I have ever heard at prior conferences attended, such as this year's APS March Meeting in San Antonio, TX. Several notable theorists and experimentalists underwent a heated mutual question-answer exchange challenging each other on the broad understanding of the topological nature of the compound under consideration. Among those who asked some of the most direct questions, primarily on the debate concerning surface conductance, was Prof. Sankar Das Sarma, the Director of the Condensed Matter Theory Center at the University of Maryland. I had the opportunity to interview Prof. Das Sarma the following day with the intention of learning more about his views of both the conference and the current state of research on the hexaborides discussed.

While conducting my interview, I asked Prof. Das Sarma, who had come to the conference on invitation by his former postdoc, Prof. Kai Sun, and had a very busy travel schedule that week, to share his opinion on the organization of the talks. The conference “far surpassed my expectations,” Prof. Das Sarma stated, citing several reasons. One reason was that “it is a narrowly focused conference on ostensibly just one material,” and at the same time, according to Prof. Das Sarma, the organizers “were very clever: on the first and the last day they scheduled some of the talks which take us a little bit outside of the material,” covering some of the “really exciting and interesting things that can be done” with it. The second reason was that the organizers chose the speakers very carefully, so all the topics that are currently being discussed were represented and different sides of these topics were introduced. As an example, Prof. Das Sarma cited two talks from the previous day: one dealing with a very difficult spin-ARPES measurement as part of an experiment done by an ARPES team in Switzerland, where the main group is based in China, and the other done by a Berlin group. Prof. Das Sarma found the first group’s claim that “there is a 2D-surface layer and this 2D-surface layer has the spin helix, and it has an odd number of band crossings” very promising, but also found compelling the case made by the second group, who see an even number of band crossings and no sign of spin polarization. At the same time, Prof. Das Sarma clarified that he has no viewpoint on this subject and is more interested in the experimental evidence presented to argue for the corresponding cases. Consequently, this example alone, the two talks with contradictory findings, brought to light a very important issue at the core of the conference: “is SmB6 a topological insulator or is it not?” Finally, according to Prof. Das Sarma, the third reason for his highly positive view of the conference was its format, which was very conducive to discussion. In fact, Prof. Das Sarma believes that at mostly every conference it may be beneficial to eliminate one talk for every four talks and instead to divide that time into discussion sessions, which at this particular conference was already taken care of very carefully. This format yielded much opportunity for question-answer exchanges described above, something I personally also found very valuable for young researchers like me. Prof. Das Sarma added that the he is by nature very skeptical because in physics “by definition a new frontier should be broken very rarely,” so opportunity for substantial interaction of the speaker with those in the audience who have questions or concerns about the work is crucial, and given a size of 60-70 people, of which 30-40 are active leaders in the field (according to Prof. Das Sarma, “if it were much larger, one cannot have a meaningful discussion, and if it were much smaller, the conference would resemble a group meeting”), such opportunity was made available to everyone this time, thereby solidifying the highly positive impression left on him by the organization of the conference.

On the last day of the conference I also had the opportunity to interview two other researchers: one of its main organizers and discussion chairmen, Prof. Jim Allen (University of Michigan), and the renowned condensed matter theorist, Prof. Piers Coleman (Rutgers University, NJ).

In his interview, Prof. Allen expressed enthusiasm with regard to the multitude of works that have been discussed at the conference. He stated, “I am pleased that we have had such a range of people who were willing to come and were interested to come from many different techniques that have been used to study this particular material.” He added that there has been “almost every technique that you could use to study a solid on display here,” including many different kinds of spectroscopy and transport, in addition to the theoretical community, remarking: “so it’s a course in solid state physics, almost.” In fact, the organizers “had in mind that it would be a good opportunity for graduate students and postdocs to broaden their understanding of condensed matter physics,” which I can attest has been advantageous for me as well. Prof. Allen also commented on a topic I have raised earlier in this report, observing: “we’ve had vigorous debate, which at the same time has been very collegial and cordial, and I think that, too, is a nice sign of how physics ought to be done.”

Prof. Coleman, whom Prof. Allen had introduced to the audience as “the intellectual leader of this field at this moment,” gave the concluding overarching talk at the conference, declaring: “we are in the middle of a revolution, right now, right here.” Prof. Coleman then went on to reconstruct a conceptual timeline of emergent physics continuously branching out into two directions: strongly correlated electron systems and topology, gradually converging in a field described by a recently introduced term, “topotronics.” He then highlighted some of the most promising directions for further research of SmB6 and related questions to ponder on, such as: the relationship of SmB6 to magnetism, the effects of correlation on the topological Fermi surface (if SmB6 is topological), and others. The talk concluded with further questions to explore, including the need to identify materials in which topology plays a crucial role even if they are not topological insulators and, independently, to investigate and pursue new strongly correlated TI materials.

When I interviewed Prof. Coleman for SPS after this talk, he acknowledged being very pleased with the flow of the conference, noting: “it’s science in the making: there’s controversy, there’s disagreement, there are new ideas, there are experiments that don’t entirely match up to each other, but that’s how active science is.” On the topic of the nature of samarium hexaboride, he immediately remarked, “I think there is a growing consensus that it is a topological insulator.” One of the most compelling talks at this conference backing the topological claim, for Prof. Coleman, was on a study which showed that the reason the conductivity of the system increases when one scratches the surface of the compound “is because you form cracks, and what’s remarkable is that the cracks are conducting.” The only way to understand conducting cracks, in turn, is if and only if the material is topological, in which case every time one breaks it there is a new conducting surface formed. Prof. Coleman noted that some of the ARPES data at the conference cast some doubt on whether the surface states were topological, such as data from the Helmholtz center in Germany (in fact, this was the same study as the second talk cited by Prof. Das Sarma in his interview, as reported above), where a group saw an even number of states that they interpreted as Rashba surface states, which would mean, if they are correct, that the system is not topological. Thus, Prof. Coleman concluded, “it will take more experiments to fully clarify the situation,” echoing the sentiment of Prof. Das Sarma.

Conclusion

In a competitive environment that thematic conferences like this one are naturally expected to cultivate and preserve, it was quite unusual to witness several instances of very frank public exchanges between distinguished contributors to their respective subfields of research. As an example, in the midst of a talk during one afternoon session of the conference, the speaker confessed to a lack of complete comprehension of all the crucial details of a theory and publicly remarked to one of the conference organizers that despite a helpful one-on-one exchange earlier in the day “the halo of understanding went away with you.” This openness and sincerity on the part of a research group leader, among numerous other subtle details observed throughout the event, is what made the overall conference environment warm and welcoming to participants of any academic status. For this reason, and also because I have met my objective of absorbing as much new theory as possible at the conference, my three days of learning more about the other groups’ groundbreaking research performed in the subfield were highly valuable to me as a developing researcher.

While the question that had been posed in the form of a bet officially remained unresolved, the conference has certainly shifted the participants’ understanding of topology in the series of compounds investigated by shining light on new details that could help explain the hexaborides’ topological behavior from a variety of new angles, and as such, this conference has been a resounding success!

About the Author

Pavel Shibayev.I have spent my undergraduate career at Princeton University, having just graduated with an A.B. in physics and a certificate in Materials Science and Engineering. Throughout my experimental work in Professor Zahid Hasan’s research group at Princeton I have studied several classes of new materials from the perspective of topology and correlation via a number of techniques, including angle-resolved photoemission spectroscopy (ARPES). In particular, last year I have attended beam times and gained ARPES experience at the Stanford Synchrotron and Radiation Lightsource (SSRL) near Stanford and the Advanced Light Source (ALS) in Berkeley, CA. This year I had the opportunity to attend my first international beam time, conducting research at the Diamond Light Source in Didcot, Oxfordshire, UK. Besides research, I have been active in organizing many events for our SPS chapter at Princeton, leading the group throughout 2014-2015 and directing the first Princeton University Physics Competition (PUPC) for high school students concurrently. Further information on our research group at Princeton [1], my work [2], and the PUPC [3], can be found in corresponding references.

References

1. http://physics.princeton.edu/zahidhasangroup

2. http://physics.princeton.edu/~shibayev

3. http://physics.princeton.edu/pupc

Areas of Alignment: Career Resources: Scientific Categories:

Part of the lecture room in West Hall, where all talks took place, at the conclusion of the conference, with Prof. Piers Coleman standing to answer participants’ questions. Photo courtesy of Pavel Shibayev.

Two of the main organizers of the event: Prof. Çagliyan Kurdak (left) and Prof. Jim Allen (right) after the conclusion of the last talk at the conference. Photo courtesy of Pavel Shibayev.