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Intense work studying the ballistic regime of electron transport in two dimensional systems based on semiconductors and graphene had been thought to have established most of the key experimental facts of the field. In recent years, however, new forms of ballistic transport have become accessible in the quasi-two-dimensional delafossite metals, whose Fermi wavelength is a factor of 100 shorter than those typically studied in the previous work, and whose Fermi surfaces are nearly hexagonal in shape, and therefore strongly faceted. This has some profound consequences for results obtained from the classic ballistic transport experiment of studying bend and Hall resistances in mesoscopic squares fabricated from delafossite single crystals. We observe pronounced anisotropies in bend resistances and even a Hall voltage that is strongly asymmetric in magnetic field. Although some of our observations are non-intuitive at first sight, we show that they can be understood within a non-local Landauer-B\"uttiker analysis tailored to the symmetries of the square/hexagonal geometries of our combined device/Fermi surface system. Signatures of non-local transport can be resolved for squares of linear dimension of nearly 100 $\mu$m, approximately a factor of 15 larger than the bulk mean free path of the crystal from which the device was fabricated., Comment: 23 pages, 9 figures (including supplementary information)

Funding: This project was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant no. 715730, MiTopMat) and also was supported by the Max Planck Society. A.P.M. and R.M. acknowledge support from the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (EXC 2147). M.D.B., P.M., and V.S. acknowledge studentship funding from the EPSRC under grant no. EP/L015110/1. A.S. was supported by the Israel Science Foundation, the European Research Council (Project LEGOTOP), and the DFG through projectno.CRC-183. M.K. acknowledges support from the SIRIUS irradiation facility through project no. EMIR 2019 18-7099. Microstructures can be carefully designed to reveal the quantum phase of the wave-like nature of electrons in a metal. Here, we report phase-coherent oscillations of out-of-plane magnetoresistance in the layered delafossites PdCoO2 and PtCoO2 The oscillation period is equivalent to that determined by the magnetic flux quantum, h/e, threading an area defined by the atomic interlayer separation and the sample width, where h is Planck's constant and e is the charge of an electron. The phase of the electron wave function appears robust over length scales exceeding 10 micrometers and persisting up to temperatures of T > 50 kelvin. We show that the experimental signal stems from a periodic field modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in delafossites. Postprint

The delafossite metals PdCoO2, PtCoO2, and PdCrO2 are among the highest conductivity materials known, with low-temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high-energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first-principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately 0.001%. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths and highlights how unusual these delafossite metals are in comparison with the vast majority of other multicomponent oxides and alloys. We discuss the implications of our findings for future materials research.

A n I nterview with P rofessor A lexandra von M eier on an E fficient E lectric G rid : I mproving V isibility and I ntegrating R enewable S ources B S J By: Manraj Gill, Saavan Patel, Harshika Chowdhary, Daniel Miller, Philippa McGuinness Dr. Alexandra von Meier is an adjunct associate professor in the department of Electrical Engineering and Computer Science at the University of California at Berkeley and is co-director of the electric grid program of the California Institute for Energy and Environment (CIEE). Her interest in electric energy spans from making distribution systems more efficient to improving the integration of renewable energy sources into the electric grid. We got the opportunity to talk with Dr. von Meier about her passion for energy, the current and future power distribution grids, micro-synchrophasor technology and the challenges with optimizing the incorporation of renewable energy into the pre-existing system. about essentially how the system works. Not dumbing it down, but not starting with a bunch of phasor diagrams that would be immediately unintelligible. So, the goal was to write something that would qualitatively explain what’s happening and what are the constraints when you’re operating the electric grid. That was a labor of love, and I started it during my postdoc. Figure 1. Professor Alexandra von Meier teaches courses in electric power systems at UC Berkeley. Berkeley Scientific Journal: How do you find yourself here, in this research field? Professor Alexandra von Meier: I was a physics major as an undergraduate at Berkeley. Then I taught high school physics and chemistry for two years. I came back here and as a master’s and PhD student in energy and resources here at UC Berkeley. Then I did a postdoc in electrical engineering here with Felix Wu, during which time I started writing a textbook, which has done pretty well. The book is called “Electric Power Systems.” It’s a little atypical as an engineering text. The idea [with the book] was to teach people who are interested in energy and electricity but who don’t have an electrical engineering background Then I did another postdoc in nuclear engineering, also at Berkeley. I studied plutonium, in the context of nuclear materials management: what to do with spent fuel from nuclear reactors and what to do with plutonium that comes out of dismantled nuclear warheads. Long story, but then I got a job as a professor at Sonoma State University where I taught in the Environmental Studies and Planning Department. I taught energy management and design. This is a wonderful program, where I was for about twelve years, and taught a curriculum that’s really pretty unique. It spans renewable energy, energy efficiency, green Figure 2. Professor von Meier’s building. It’s not really book, “Electric Power Systems: an engineering program, A Conceptual Introduction,” but it’s quite technical is used in her ‘Introduction and quantitative so it to Electric Power Systems’ at 44 • B erkeley S cientific J ournal • W aste • S pring 2015 • V olume 19 • I ssue 2 Berkeley.

A J oint I nterview with P rofessor J oonhong A hn and P rofessor C athryn C arson on N uclear W aste M anagement : a T echnical and S ocial P roblem By: Manraj Gill, Juwon Kim, Daniel Miller, Harshika Chowdhary, Kevin Nuckolls, Philippa McGuinness Dr. Joonhong Ahn is a professor in the department of Nuclear Engineering and Dr. Cathryn Carson is an associate professor in the department of History at the University of California, Berkeley. We got the opportunity to talk with both of them in a joint interview discussing topics ranging from technical aspects of nuclear waste and energy to the challenges surrounding the lack of communication between society and engineers. Berkeley Scientific Journal: How did you involved in beginning of your career. How were you exposed to this field? Professor Joonhong Ahn: I became interested in the JA: I read a book for the general public about nuclear power your field of research? utilization when I was in high school. I was fascinated by field of radioactive waste management at the beginning the fact that the nuclear waste issue was not of my career. I thought that nuclear solved or even considered at that time, so I power utilization was a crucial issue that knew that was what I wanted to pursue. needed further research in any country. Just like nuclear fusion, in the 1970s and Professor Cathryn Carson: I didn’t the 1980s, people said that at the turn of really get to know Professor Ahn until the century we would solve this issue. 2007-08. I was given a year off by the I started my career around the 1980s, history department to sit in on courses in so I thought that would be the best nuclear engineering as I had a long term topic for me to tackle because around history research project on the specialty the middle of my career I expected of nuclear engineering waste management to see major accomplishments in this that Professor Ahn was working on. I was field! The issue of nuclear energy is in a lot of his classes asking lots of non- socially very important and technically technical questions because, for me, nuclear very challenging, so I thought, “Why engineering was a perfect example of a not?” The turn of the century actually problem that was both technical and social [happened to be the beginning] of [a at the same time. lot of advances in the field] which was Figure 1. Professor Joonhong Ahn good for me as well! teaches courses in radioactive waste CC: I also found it wonderful to have management and the nuclear fuel cycle a professor of nuclear engineering who BSJ: You mentioned that you were at Berkeley. understood where those questions were interested in nuclear energy at the B erkeley S cientific J ournal • W aste • S pring 2015 • V olume 19 • I ssue 2 • 29 B S J Professor Carson’s interests in nuclear history and the relationship between scientists and non-scientists developed into a collaboration with Professor Ahn that led to the organization of an advanced summer school in social-scientific literacy. They have been working together to bridge the gap between society and engineers, a gap that was evident during the 2011 Fukushima-Daiichi nuclear disaster. In 2015, they published together, along with other experts of nuclear engineering and education, “ Reflections on the Fukushima Daiichi Nuclear Accident: Toward social-scientific literacy and engineering resilience ” depicting various accounts of what transpired in leading up to and in response to the event.

We present the results of an experimental investigation into the effects of a sinusoidal modulation of the rotation rate on the segregation patterns formed in thin drum of granular material. The modulation transforms the base pattern formed under steady conditions by splitting or merging the initial streaks. Specifically, the relation between the frequency of modulation and the rotation rate determines the number of streaks which develop from the base state. The results are in accord with those of Fiedor and Ottino [J. Fluid. Mech. 533, 223 (2005)10.1017/S0022112005003952], and we show that their ideas apply over a wide range of parameter space. Furthermore, we provide evidence that the observed relationship is maintained for filling fractions far from 50% and generalize the result in terms of the geometry of the granular deposit.