The David and Edith Harris Physics Colloquium Archives

Fall 2011 Schedule

September 8, 2011
University of California - Santa Barbara
Hosted by Senthil Todadri

"Spin Liquids: Time to Slake Our Thirst"

Envisioned many decades ago, a spin liquid is a seemingly paradoxical "magnet without magnetism", in which local magnetic moments are present and highly correlated with each other, yet remain strong fluctuation even at low temperature.   Spin liquids may be classical or quantum, and according to theory, display remarkable collective phenomena such as emergent gauge fields and fractional particle excitations.   Experiments are increasingly uncovering this exotic behavior in the laboratory, both supporting the basic ideas of spin liquid physics, and exposing new challenges to theory.

September 15, 2011
Rockefeller University and Institute for Advanced Study
Hosted by Mehran Kardar

“Physical Biology: is there an elephant in the room?”

In physics we are attracted by simple and universal phenomena. In biology problems turn out to be often more complicated than anticipated and universality is approximate and rare. One possible reason for this is that biology is essentially a historical science and contingency plays a crucial role. Physical biology tends to ignore this “elephant in the room”, mainly by limiting itself to molecular investigations. On the other hand, it could be interesting if one “replayed the tape” of biology multiple times and then performed statistical analysis of evolutionary histories.  Maybe simple laws of living matter and universality classes of its behavior could then emerge?

In the meantime, I will present some first results of long-term laboratory experiments in closed microbial ecosystems. Each system is a small world in itself, with its idiosyncrasies and (hi)story to tell. Local population dynamics is monitored in dozens of replicates. Surprisingly, statistical laws governing the population fluctuations seem to be quite simple.

September 22, 2011
Hosted by The Society of Physics Students

"Doing Science in the Open"

I'll start this talk by describing the Polymath Project, an ongoing experiment in "massively collaborative" mathematical problem solving. The idea is to use online tools -- things like blogs and wikis -- to collaboratively attack difficult mathematical problems.  By combining the best ideas of many minds from all over the world, the Polymath Project has made breakthroughs on important mathematical problems.

What makes this an exciting story is that it's about much more than just solving some mathematical problems.  Rather, the story suggests that online tools can be used to transform the way we humans work together to make scientific discoveries.  We can use online tools to amplify our collective intelligence, in much the same way as for millennia we've used physical tools to amplify our strength.  I'll describe examples suggesting that this has the potential to accelerate scientific discovery across all disciplines.

This is an optimistic story, but there's a major catch.  Scientists have for the most part been extremely conservative in how they use the net, often using it for little more than email and passive web browsing.  Projects like Polymath are the exception not the rule.  I'll discuss why this conservatism is so common, why it's so damaging, and how we can move to a more open scientific culture.

September 29, 2011
Hosted by Patrick Lee

"Magnetic Reconnection in Plasmas; a Celestial Phenomenon in the Laboratory"

Coronal mass ejections from the sun are the most explosive events that occur in our solar system. Closer to home, the aurora borealis is one of the most spectacular, naturally occurring, light show at high latitudes on the earth. Both of these large scale events are driven by magnetic reconnection in plasmas. The spontaneous rearrangement of magnetic field topology provides the enormous energy needed for these celestially magnificent and diverse phenomena.

For more than fifty years, magnetic reconnection has been a fascinating topic of research in plasma physics. While we do not fully understand the process of reconnection, significant progress has been made in the past decade through detailed analysis of laboratory experiments, and computer simulations. The Versatile Toroidal Facility at MIT is one such experiment dedicated to the study of magnetic reconnection. In this talk I will describe the recent experimental observations which have led to a new theoretical paradigm for magnetic reconnection. Large scale computer simulations support the theoretical and experimental results. The analysis of experimental observations in a laboratory device has led to a comprehensive understanding of data from spacecraft observing celestial reconnection events in the earth’s magnetosphere.

October 6, 2011
Hosted by Peter Fisher

"Weighing Neutrinos"

Neutrino oscillation experiments performed throughout the latter half of the twentieth century have yielded valuable information on the nature of neutrino masses and mixings. The evidence gathered represent the first significant challenge to the Standard Model of particle physics in many years of searching. As the next century begins, a new suite of precision experiments will come online to provide greater insight into the physics and significance of neutrino mass. This talk will review our current state of knowledge on neutrino masses, their connection to cosmology, and how new experiments will complement the knowledge of those two disciplines for years to come.

October 13, 2011
Harvard University
Hosted by Martin Zwierlein

"Quantum Magnetism with Ultracold Atoms – A Microscopic View on Artificial Quantum Matter"

Understanding the behaviors of strongly-interacting spin systems is one of the central objectives of modern manybody quantum physics. I will present experiments in which we have realized quantum magnetism with ultracold atoms in an optical lattice. We carry out a quantum simulation of an Ising spin chain and demonstrate a quantum phase-transition from a paramagnetic phase to an anti-ferromagnetic phase. The magnetic phases are detected in situ through our quantum gas microscope.  This work opens a wide range of new possibilities for studying quantum magnetism. Exotic states of matter and frustrated spin physics in optical lattices are now within experimental reach.

October 20, 2011
Distinguished Pappalardo Lecture
Johns Hopkins University and Space Telescope Science Institute
Hosted by Edmund Bertschinger

"Expansion and Dark Energy"

The expansion rate and its evolution must be empirically determined for our Universe to reveal its composition, scale, age, and fate. The Hubble Space Telescope is unique in its ability to measure the keystones of cosmic expansion, distant Type Ia supernovae and Cepheid variables in their hosts.  

In 1998, high-redshift Type Ia supernovae provided the first and only direct evidence for an accelerating Universe and the existence of dark energy.   Presently, refinements in these measurements and new techniques have begun to narrow the range of properties and explanations for the nature of dark energy.  I will review a number of these recent experiments including improvements in the determination of the Hubble constant  using a new infrared array on the Hubble Space Telescope and a 3 year search for the most distant exploding stars, perhaps among the very first supernovae in the Universe.

October 27, 2011
Hosted by Sara Seager

"Spin-Orbit Interactions for Exoplanetary Systems"

In the Solar system, the planets follow orbits that are aligned with the Sun's equatorial plane to within about 7 degrees.  But what about planets around other stars?  Recently we have measured the spin-orbit angles of about 40 exoplanet-hosting stars, using a technique first theorized in the 19th century, as well as a new technique based on data from the NASA Kepler spacecraft.  Many exoplanetary systems have good alignment, as in the Solar system -- but there are also many surprises.  I will discuss these results and their implications for theories of planet formation, and tidal spin-orbit interactions.

November 3, 2011
IBM Fellow Emeritus
Hosted by Aron Bernstein

"Physics in the Interest of Society"

I discuss experience and colleagues in the nuclear weapon program at Los Alamos, as a consultant to and member of the President's Science Advisory Committee, and since then as a JASON member and independent contributor to science and technology in the interest of national security and national benefit.  Topics include the first hydrogen bomb, the Global Positioning System, unmanned aircraft, Air Traffic Control, Ballistic Missile Defense, and also some innovations that have application far beyond the problem for which they were first proposed.

November 10, 2011
Stanford University and SLAC National Accelerator Laboratory
Hosted by Patrick Lee

"Emergent Phenomena at Oxide Interfaces"

Complex oxides are fascinating systems which host a vast array of unique phenomena, such as high temperature (and unconventional) superconductivity, ‘colossal’ magnetoresistance, all forms of magnetism and ferroelectricity, as well as (quantum) phase transitions and couplings between these states. In recent years, there has been a mini-revolution in our ability to grow thin film heterostructures of these materials with atomic precision. With this level of control, a number of new electronic phases have been discovered at their interfaces. Between two insulators, for example, metallic, superconducting, and magnetic states can be induced. In analogy to the rich science and technology that emerged from the development of semiconductor heterostructures, we are using these techniques to create novel low-dimensional states inaccessible in bulk oxides. After a general overview, I will focus on recent results on two-dimensional superconductivity in oxide heterostructures, contrasting two systems based on SrTiO3 which approach the ‘dirty’ and ‘clean’ limit.

November 17, 2011
Stanford University
Hosted by Vladan Vuletic

"Quantum Dots in Optical Nanocavities: from Cavity QED to Device Applications"

Quantum dots in optical nanocavities are interesting both as a test-bed for fundamental studies of light-matter interaction (cavity quantum electrodynamics - QED), as well as a platform for classical and quantum information processing. As a result of the strong field localization inside of sub-cubic wavelength volumes, they enable very large emitter-field interaction strengths (vacuum Rabi frequencies in the range of 10's of GHz - several orders of magnitude larger than in atomic cavity QED). In addition to the study of the new regimes of cavity QED, this can also be employed to build novel devices, such all-optical switches operating at the single photon level, electro-optic modulators controlled with sub-fJ energy and operating at GHz speed, and lasers with threshold current of 100nA.

December 1, 2011
Stanford University
Hosted by The Physics Graduate Student Council

"Bridging the Gap in High Temperature Superconductor"

It is now exactly 100 years since superconductivity was discovered and it took 45 years before a complete theory was formulated by Bardeen-Cooper-Schrieffer. Once understood, the impact has been felt far behind superconductivity itself, and superconductivity became a prime example of emerging properties in quantum system. High-Tc superconductivity in cuprate oxides was discovered 25 years ago and it remains a major unsolved physics problem today. The challenge of the cuprate research is symbolized by its complex phase diagram consists of states with extreme and unconventional properties in addition to unconventional superconductivity – such as Mott Hubbard insulating state, the peculiar pseudogap state, and so-called strange metal state. None of them are understood by conventional theory, thus compounding the difficulty to understand high-Tc superconductivity itself as these states are different manifestations of the same underlying physical system, making an integrated understanding a necessity.

Angle-resolved photoemission spectroscopy (ARPES) has emerged as a leading experimental tool to address this problem. Over the last two decades, substantial progress towards understanding the cuprate problem has been made in concert with breathtaking progresses in ARPES technique. In this talk, I will use ARPES derived energy gap as a bridge to link the relationship between the different parts of the phase diagram, with focus on the complex relationship between pseudogap state and superconductivity. The result points to a trisected superconducting dome with interweaving states. In particular, our data is consistent with the presence of a quantum critical point, accompanied by strong dynamic competition between superconductivity and pseudogap state nearby. Such phase competition will likely emerge as a key signature of high-Tc physics, and suggests a revised phase diagram for cuprates that reconciles two conflicting versions currently used in the field.

December 8, 2011
Hosted by Christoph Paus

"Where art thou, O Higgs?"

The world's most powerful particle accelerator, the Large Hadron Collider (LHC),  has just finished its second year of proton proton collisions at center-of-mass energy of 7 TeV, increasing the data sample by a  factor of ~125.  So, the world is waiting (and starting to get impatient) for the answer to one of the flagship questions addressed by the LHC: what is the mechanism by which fundamental fermions and bosons are endowed with mass?    There is a simple hypothesis, embodied by the so called "Higgs Mechanism" embedded into the Standard Model of particle physics, which requires one new particle, giving experimenters at the LHC a quest: The hunt for the Higgs.   This talk will elaborate on  why this is an interesting question for the LHC, demonstrate a bit of what it takes in terms of experimentation to answer the question, and discuss the current state of the hunt.