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Department of Physics

Research Highlights

The reputation of our department continues to grow, in large part due to the research conducted here and the impact it has on other scholars around the world.  Here we collect a few highlights of the research results achieved in the past year, representing a small fraction of our total research productivity.  

Condensed Matter Physics

1) The condensed matter nuclear magnetic resonance group led by Nicholas Curro recently published an article in the Proceedings of the National Academy of Sciences.  (http://arxiv.org/abs/1206.1879).  Using detailed temperature and orientational dependent studies of the NMR Knight shift of single crystals, the group was able to investigate the interplay between the local and itinerant degrees of freedom of several different heavy fermion compounds.  The paper was published side-by-side with a theoretical paper led by David Pines (http://arxiv.org/abs/1206.1115 ), and in collaboration with former UC Davis postdoc Yifeng Yang (now at the Institute of Physics in Beijing.)  Together these two papers advance our general understanding of the Kondo lattice phase diagram, and shed light on the relationship between long range ordered states and the emergence of a heavy electron fluid.  Davd Pines writes regarding the second paper, "we introduce the concept of hybridization effectiveness and show it is the organizing principle that determines whether the ground state is a local moment antiferromagnet, an unconventional superconductor, or, in some cases, both.

 2) Warren Pickett, together with graduate student Yundi Quan and his former postdoc Victor Pardo made a discovery that raises fundamental questions about phase transitions in the movement of electrons in solids, as reported in Phys. Rev. Lett. 109, 216401 (2012), and also available here. Using first principles (i.e. parameter-free, model-independent) calculations of the quantum mechanical states, this three-person team has discovered that in several metal-insulator transitions that have been understood as charge-order transitions, there is (to high accuracy) no charge order; i.e., the charges are distributed evenly among the atoms. Substantial changes in the structure and various microscopic characteristics of the materials do change, but without the variation of charge that has been believed to cause these transitions.  Their work begins the re-examination of the underlying basic quantum processes that will reveal the actual mechanism of these striking changes between quantum phases in solids, which will also bear directly on the basic chemical concepts of "formal valence" and "oxidation state."

Cosmology

1) Three different groups of UC Davis researchers had remarkable findings in the past year about the nature of dark matter as revealed by galaxy cluster collisions.  Graduate student Will Dawson, his adviser David Wittman, and a cast of others discovered a merging cluster of galaxies in which the collisionless dark matter and galaxies have separated from the collisional gas which constitutes most of the baryonic mass of any cluster.  This "dissociation" allowed the group to put an upper limit on the self-interaction cross-section of dark matter, which they hope to tighten with further data.  For more, visit this post on Will Dawson's blog.   

Meanwhile, postdoc James Jee first-authored a surprising paper this spring that hints at non-gravitational self interactions for dark matter.  See NPR coverage of this work here, or read the paper here.  

Professor Bradac also had something to say on the subject, writing, "with new and better data for the cluster A520 we have once again measured the properties of dark matter and have confirmed that (excluding gravity) dark matter interacts very little (if at all) with itself."  You can read about her work in "On Dark Peaks and Missing Mass: A Weak Lensing Mass Reconstruction of the Merging Cluster System Abell 520."

2) UC Davis postdoc Zhen Hou, graduate students Brent Follin and Marius Millea and Professor Lloyd Knox, together with their South Pole Telescope teammates, recently sharpened up constraints on models of early Universe inflation, as covered in this Cosmic Variance blog post.

3) Professor Tyson is the director of the planned Large Synoptic Survey Telescope (LSST) which was given top ranking in last year's Decadal Review, as described in last year's Newsletter article.  This July the National Science Board gave approval for the National Science Foundation to advance LSST to the final design stage.  Read more about this critical project milestone here.  

4) Professor Bradac and collaborators observed a source so distant that they are seeing it as it was when the Universe was less than 10% of its current age. To date, this is the faintest galaxy with a successfully measured spectrum. The discovery is important because other similar observations of similarly distant galaxies use unusual objects that are not representative. These observations are helping us answer questions of how the first galaxies formed and how they transformed the Universe from a highly opaque state to transperent by clearing out the cosmic fog and starting the cosmic dawn.  This work was reported in Spectroscopic Confirmation of a z = 6.740 Galaxy behind the Bullet Cluster.  

Environmental Physics

Professor Cahill completed his 14-year effort to understand the sharply enhanced death rate in the southern San Joaquin Valley with work published as two articles in a special EPA-sponsored ten-article issue of Aerosol Science and Technology.  He used the synchrotron X-ray Fluorescence capabilities of the LBNL Advanced Light source to establish the hard-to-measure ambient air content of very fine (0.26 um > Dp > 0.09 um) and ultra fine (< 0.09 micrometer diameter) particles in the air of the California Central Valley, Redding to Bakersfield. When these data were merged with the excellent California data on mortality, the long-suspected link between ultra fine insoluble particles and ischemic heart disease deaths was established, showing a 60% enhancement in the southern San Joaquin Valley. Through the compositional analyses, the sources were identified - debris from brake drums and pads, and zinc in lubricating oil, especially from  heavy trucks. 

Heavy Ion Physics

The Relativistic Heavy Ion group led by Manuel Calderón de la Barca Sánchez and Daniel Cebra has been making news in the study of bottom quarks in a hot Quark Gluon Plasma. Graduate student Guillermo Breto Rangel was one of the lead authors in a paper published in Physical Review Letters by the CMS collaboration on the study of Upsilon mesons, a bound state of a bottom quark and a bottom anti-quark, in Pb-Pb collisions at the LHC (http://prl.aps.org/abstract/PRL/v109/i22/e222301).  The paper discusses the first observation of sequential suppression of the Upsilon states, in order of their binding energy. This observation was predicted over a decade ago and the effect has now been confirmed experimentally.  This observation is important in establishing that the system produced in heavy ion collisions is made of deconfined quarks and gluons, as expected for a Quark Gluon Plasma. The details of the suppression pattern can be used as a way to infer the temperature of the Quark Gluon Plasma, which is estimated to be about 500 MeV (over 5 orders of magnitude hotter than the sun's core). An accompanying article discussing the importance of this result written by Ramona Vogt, an adjunct professor in the department, appeared in APS "Physics" (http://physics.aps.org/articles/v5/132).

High Energy Physics

Of course there was big news in the high energy physics community with the July 4, 2012 discovery of the Higgs.  See the article by Professor Mulhearn elsewhere in this Newsletter about the significant UC Davis role in that discovery.  As a sample of other work by the high energy group, we note that Professors Chertok and Gunion published in Physical Review Letters their results on what the CERN data are telling us about the Higgs sector and super symmetry.  The next-to-minimal supersymmetric model in particle physics predicts light pseudoscalar Higgs bosons.  Chertok and Gunion led a search using data taken with the CMS detector at the LHC for such a Higgs with decays to muon pairs. Although the analysis clearly showed the production of the Upsilon mesons, no Higgs signal was observed, and world's-best constraints on this model were extracted.

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