Inter Actions Department of Physics 2017

INTER ACTIONS DEPARTMENT OF PHYSICS 2017 Scott Dodelson Appointed Head of Department of Physics By Jocelyn Duffy Renowned physicist Scott Dodelson has been named the head of the collaborate with colleagues in Statistics, Computer Science and Department of Physics in Carnegie Mellon University’s Mellon College Engineering. Dodelson also hopes to increase the department’s of Science. partnerships with other universities and research initiatives worldwide and bring physics to the community through outreach programs. Dodelson conducts research at the interface between particle physics and cosmology, examining the phenomena of dark energy, dark “I was drawn by the university’s enthusiasm for foundational research,” matter, inflation and cosmological neutrinos. Dodelson said. “The Physics Department will strive to bring this excitement to students, alumni and the broader community.” He is the co-chair of the Science Committee for the Dark Energy Survey (DES), an international collaboration that aims to map Dodelson comes to Carnegie Mellon from the Fermi National hundreds of millions of galaxies, detect thousands of supernovae and Accelerator Laboratory (Fermilab), where he was a distinguished find patterns of cosmic structure in an attempt to reveal the nature of scientist, and the University of Chicago where he was a professor in dark energy. On Aug. 3, the DES released results that measured the the Department of Astronomy and Astrophysics and Kavli Institute structure of the universe to the highest level of precision yet. for Cosmological Physics. While at Fermilab, Dodelson served as head of the Theoretical Astrophysics Group and co-founder and interim Dodelson also works with the South Pole Telescope and the Large director of the Center for Particle Astrophysics. Synoptic Survey Telescope (LSST). The South Pole Telescope studies the Cosmic Microwave Background to gain a better understanding Dodelson earned a joint B.A./B.S. degree in applied physics and a of inflation, dark energy and neutrinos. The LSST, which is currently Ph.D. in theoretical physics from Columbia University. He completed a being built in Chile, will survey the sky for a decade, creating an post-doctoral fellowship at Harvard University. enormous data set that will help scientists determine the properties Dodelson assumed the position of department head from Stephen of dark energy and dark matter and the composition and history of Garoff who has served as head since 2013. our solar system. Dodelson was attracted to CMU in part by the Physics Filled: DES Constraints on two Department’s varied areas of strength and the leadership role 0.96 Open:Planck cosmological parameters the department’s McWilliams Center for Cosmology and its (the density of matter and faculty play in a number of large, international cosmological amplitude of fluctuations) from the first year of data surveys, including LSST and the Sloan Digital Sky Survey. 0.90 from the Dark Energy Survey “Within the McWilliams Center, I found kindred spirits in (DES-Y1) and from the Planck the faculty who are leading scientific projects aimed at satellite. Planck probes the 0.84 understanding the universe, but I was equally attracted to the luctuations universe when it was only SF 400,000 years old, while DES department’s strong groups in biological physics, condensed RM probes the universe billions matter and nuclear and particle physics,” said Dodelson. “I’m of years later. Remarkably, 0.78 excited to learn about these diverse fields and connect with the two very different other departments throughout the university.” experiments produce similar constraints, although there Under Dodelson’s leadership, the Physics Department will 0.72 is a hint of tension. Dodelson partner with other departments within the Mellon College serves as co-chair of the DES Science Committee and of Science through a new theory center and continue to 0.24 0.30 0.36 0.42 MatterDensity coordinated the analysis. Figure 1 Quantum Electronics by Randall Feenstra The Condensed also arise from their motion, often referred magnetic materials, historically somewhat Matter group to as “orbits” for the case of motion around separate, nicely come together in the new within the Physics individual atoms, with the associated classes of materials and phenomena that Department has, orbital angular momentum then providing form the core of quantum electronics. Randall Feenstra over the past a measure of these additional magnetic Di Xiao, a theorist hired in 2012, is an decade, built up considerable expertise in moments. Spin-orbit coupling is familiar to expert (and co-inventor) of the area of the area of quantum electronics. This article all physics students, at least for the case “valleytronics” as well as other emergent discusses what is encompassed by this field of isolated atoms. In solids the trajectories type of electronic phenomena in solids of research and what’s happening in the of the electrons are much more complex (often utilizing Berry phase). Benjamin department to further this activity. than in atoms, and the spin-orbit interaction Hunt, an experimentalist hired in 2015, is plays a correspondingly less obvious role. It an expert in the study of atomically thin The area of quantum electronics concerns is critical, however, in many of the modern, two-dimensional (2-D) materials, which the electronic and magnetic properties of novel phenomena associated with quantum he studies using electronic transport of materials, which we view in terms of the transport in solids. low (milliKelvin) temperatures and high various degrees of freedom of the electrons. magnetic fields. Michael Widom, a theorist The familiar properties of the electrons are, The Physics Department now has a group who has been active in many research areas of course, charge and spin. The latter refers of five faculty members active in this area including quasicrystalline materials and to the fact that all electrons carry a magnetic of quantum electronics. My own expertise aspects of biological physics, is now also dipole moment – they are tiny magnets – is in the area of semiconductor materials working in the quantum electronics area. with the “spin” being a way of characterizing and devices, and that of Sara Majetich is in Additionally, a search is presently underway the strength and direction of those magnetic magnetic phenomena and nanostructures. for a further experimentalist in this area, dipoles. Magnetic moments from electrons These areas of semiconductors and with expertise in optical phenomena. 2 Experimentally, many of the existing different ways that electrons might trivial”), but rather, they involve an inversion facilities in the Condensed Matter group are circulate around the edges of a piece of in symmetry between two bands in the solid now devoted to research in this Quantum material. Picture a square of material, with — the valence and conduction bands — Electronic area, including a facility we micrometer size on each side and being which occurs due to spin-orbit interaction. constructed several years ago (funded by very thin in the third dimension such This same inversion in symmetry does the deans of MCS and Engineering) for that only a single quantum state for the not exist in the vacuum, outside of the preparing atomically thin 2-D materials. electrons exists in that dimension, i.e., a material, and hence the special edges Additionally, in 2016 we successfully 2-D electron system. It has been known states necessarily form between the inner obtained a Major Research Instrumentation for decades that such materials could part of the material and the vacuum that is grant from the National Science Foundation be made of depositing a thin layer of a outside. Developing and understanding new to purchase a low-temperature scanning semiconductor such as gallium arsenide materials and nanostructures that may host tunneling microscope. This instrument, between thicker layers of a material with such states is a research topic of current pictured in Figure 1 (at the factory in larger band gap, such as aluminum gallium interest for our quantum electronics group. Germany where it was being commissioned arsenide. Electrons were thus constrained Of course, one goal of the “quantum during July of this year), allows atomic- to reside in the “quantum well” formed in electronic” phenomena that we study scale imaging and electronic studies of the gallium arsenide. If one then applies is for applications. Instructive examples surfaces of materials. The low temperature a large magnetic field perpendicular to can be derived from Figure 2. For the (liquid helium, 4.2 Kelvin) operation of this thin quantum well, the electrons will integer Quantum Hall effect, Figure 2(a), the instrument permits the study of move in circular, cyclotron orbits. However, the necessity for very high fields and low phenomena that occur only at those some electrons near the edges of the temperatures clearly limits its application temperatures, e.g., superconductivity and material move in a “skipping” type of orbit, (although it is used for a number of charge density waves. It also provides the pictured in Figure 2(a). These particular instruments throughout the world devoted instrument with an extraordinary degree electrons then give rise to measureable to precise definition of “standards,” i.e., of stability, so that sensitive and time- conductivity that is quantized in units of for the values of e and h). However, most consuming measurements as a function of 2e2/h, the quantum of conductance (e is the significantly, the very same type of gallium spatial location and of energy can be made magnitude of the electron charge, and h is arsenide quantum wells used there, formed of the quantum states in materials. At the Planck’s constant). This discovery led to a with unprecedented levels of purity and time of writing, this instrument is being air Nobel Prize for the integer Quantum Hall control, were utilized in “high-electron- shipped to Pittsburgh. A laboratory has effect in 1985, with a further prize for the mobility modulation-doped field effect been prepared on the first floor of Wean fractional Quantum Hall effect in 1998.

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