Physics Colloquia

Colloquia are held on Mondays at 4:00pm (Refreshments at 3:30pm) in Barus & Holley 168.  Past colloquia from the current and previous years, including many videos, can be found on the Past Physics Colloquia page.

Spring 2015

January 26, 2015 -- Carlton Caves (University of New Mexico)

"Quantum-limited measurements: One physicist's crooked path from quantum optics to quantum information"
Quantum information science has changed our view of quantum mechanics. Originally viewed as a nag, whose uncertainty principles restrict what we can do, quantum mechanics is now seen as a liberator, allowing us to do things, such as secure key distribution and efficient computations, that could not be done in the realistic world of classical physics. Yet there is one area, that of quantum limits on high-precision measurements, where the two faces of quantum mechanics remain locked in battle. I will trace the history of quantum-limited measurements, from the use of nonclassical light to improve the phase sensitivity of an interferometer, to the modern perspective on how quantum entanglement can be used to improve measurement precision, and finally to current ideas on quantum limits on detecting a force.

February 2, 2015 -- Vidya Madhavan (University of Illinois at Urbana-Champaign)

"Symmetry protected Dirac Fermions in Topological Crystalline Insulators"
In Dirac materials like graphene and topological insulators, electrons behave like relativistic Dirac particles with zero mass. This is a direct consequence of the form of the low energy effective Hamiltonian describing these electrons and has important implications for realizing physical properties predicted for high-energy particles, now in the laboratory setting.  Topological crystalline insulators are recently discovered topological materials[1,2] where topology and crystal symmetry intertwine to create relativistic massless electrons. Among the theoretical predictions for topological crystalline insulators is the possibility of imparting mass to these massless Dirac fermions by breaking crystal symmetry. In this talk I will discuss our recent experimental and theoretical investigations of a topological crystalline insulators, Pb1-xSnxSe[3,4,5]. We performed scanning tunneling microscopy (STM) studies at low temperatures and as a function of magnetic field. We find that the STM images and spectra reveal the coexistence of zero mass Dirac fermions protected by crystal symmetry with massive Dirac fermions resulting from crystal symmetry breaking[3,4]. We further track the evolution of the resultant mass as well as the Dirac surface states as we go through a quantum phase transition from the topological to trivial regime [5] and find that the mass is controlled by the surface state penetration depth in these systems.
[1] L. Fu, Topological Crystalline Insulators. Phys. Rev. Lett. 106, 106802 (2011).
[2] T. H. Hsieh et al., Topological crystalline insulators in the SnTe material class. Nat.Commun. 3, 982 (2012).
[3] Y. Okada, et al., Observation of Dirac node formation and mass acquisition in a topological crystalline insulator, Science 341, 1496-1499 (2013).
[4] Ilija Zeljkovic, et al., Mapping the unconventional orbital texture in topological crystalline insulators, Nature Physics 10, 572–577 (2014).
[5] Ilija Zeljkovic, et al., Dirac mass generation from crystal symmetry breaking on the surfaces of topological crystalline insulators,arXiv:1403.4906, to be published in Nat. materials (2015).

February 9, 2015 -- David Weitz (Harvard University)

February 23, 2015 -- Jané Kondev (Brandeis University)

"The Physical Genome"
Every day there seems to be a story in the news about DNA and some gene that it encodes. While this abstraction of DNA as an information storage device is useful, in this talk I will consider the physical nature of DNA, namely the fact that it is a long, flexible, and charged molecule. This is important because the physical properties of DNA affect a number of critical functions that it performs in the cell. One such function is to turn on and off the production of proteins, which are the building blocks of the cell,  in response to different physical and chemical ques. Another is to repair itself when breaks occur. In this talk I will describe how simple physics models combined with experiments on cells and single molecules are being used to develop a quantitative understanding of the physical genome.

March 2, 2015 -- Julio Navarro (University of Victoria)

"Dwarf Galaxies as Cosmological Probes"
A prime challenge to our understanding of galaxy formation concerns the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in the current ΛCDM paradigm. This is usually accounted for by assuming that energetic feedback from evolving stars confines dwarf galaxy formation to relatively massive halos spanning a narrow mass range. I will highlight a number of observations that may be used to test this assumption and discuss the puzzles and challenges that arise from this analysis. I will also discuss a number of challenges that ΛCDM faces on the scale of dwarf galaxies and their possible resolutions.

March 9, 2015 -- Benjamin Wandelt (Sorbonne University)

March 30, 2015 -- Gary Horowitz (UC Santa Barbara)

April 6, 2015 -- William Irvine (University of Chicago)

April 13, 2015 -- Vicki Colvin (Provost, Brown University)

Arthur O. Williams Lecture:

April 20, 2015 --  Rolf Heuer (Director-General,CERN)



Past Colloquia

Video Lectures