Colloquium Spring 2007



January 22
Exploring new states of matter by NMR
Raivo Stern
National Institute of Chemical Physics and Biophysics (NICPB), Estonia


BaCuSi_2O_6 (Han Purple), recently acclaimed as a perspective realization for Bose-Einstein Condensate of triplons is a quasi-2D oxide where the Cu2+ ions are arranged in well-separated dimers perpendicular to 2D layers. Consistent with its dimerized structure, the material is observed to have a singlet ground state in zero magnetic field, with a large gap to the lowest excited triplet states. Magnetic fields in excess of H_cl = 23.5 T close the spin gap, such that cooling in even larger applied field results in a state characterized by long-range magnetic order, the nature of which has not been determined yet. I will present nuclear magnetic resonance (NMR) measurements on single crystals of Han Purple below as well as above H_c1. Our results confirm and complete the previous data, proving that the system is less symmetric and more complicated than initially supposed.

January 29
     -Special event combining theoretical physics and biology, co-hosted with the Neuroscience Research Institute.
The Scalable Architecture of the Brain and an Underlying Universal Scaling Law
Charles F. Stevens
The Salk Institute , SAL 101 - 4:00pm - Coffee 15 minutes before talk, full reception following the talk.


Physics-style theory - for example, variational principles and symmetry arguments - is not frequently used in biology, but can be a very powerful way to understand biological systems. I describe the unique characteristics of physics-style theory in the biological context, and illustrate this approach to determine the value of an exponent in a scaling law that governs aspects of brain architecture. All vertebrate brains conform to one basic design - with elaborations - and this design has the property that the brain's computing power can be increased by simply making the brain, or parts of it, larger. I am interested in the design principles that endow vertebrate neural circuits with this scalable architecture. Information is spread over regions of the brain by axon arbors, and this information is sampled by dendritic arbors. These arbors are statistically self-similar and follow a universal scaling law that relates arbor length to the size of the region over which information is distributed or sampled (the arbor's territory). The exponent in this scaling law is the result of the principle that the potential for constructing a particular neural circuit is independent of scale.

February 5
The Cryogenic World of Triton
Gary Peterson
Department of Geological Sciences, San Diego State University


Triton is a large satellite in retrograde orbit around Neptune, the most distant (40AU) of the giant gaseous planets. Surface temperatures average about 40K and are cold enough to condense all of the heavier gases, including nitrogen. A pronounced tilting of Triton's spin axis gives rise to a strongly seasonal climate and the available imagery indicates a deteriorating south polar cap made of nitrogen. Evidently the released nitrogen is being transferred to the north pole and condensing there during the long winter season. Distant though the sun is, it evidently supplies enough energy to provide seasonal transfer of nitrogen much in the same manner that water alternately collects and melts at the earth's poles with the seasons. Triton has been resurfaced and shows little evidence of cratering. Probably the satellite has been heated, evaporating the surface ices and then the released gases have recondensed to provide a new surface. The energy source for the heating would appear to be tidal friction following capture by Neptune. The retrograde orbit of Triton would indicate capture and the lack of surface craters would suggest that the capture was a fairly recent event. Numerous irregular surface textures indicate repeated expansion and contraction and are compared with somewhat similar features on earth. Although the Earth/Triton surface features resemble one another, they would be composed of totally different material.

February 12
Approaching the Fermi surface: Correlated fermions viewed by the functional renormalization group
Carsten Honerkamp
Institute for Theoretical Physics and Astrophysics, University of Würzburg


While the renormalization group has always been an appealing theoretical concept with applications in a broad range of physical problems, only in recent years powerful functional renormalization group (fRG) methods have been introduced for the analysis of interacting many-fermion systems in two and more spatial dimensions. Here we describe how these methods can be used to investigate competing ordering tendencies and the interplay of different degrees of freedom in models for correlated fermions. The applications range from high-$T_c$ superconductivity, and other correlated systems such as graphene, to ultracold fermions in optical lattices. We furthermore explain the ongoing efforts to advance the current method for an unbiased comparison of ordering tendencies to a quantitative tool for the description of the low-temperature phases of correlated fermion systems.

February 19
President's Day, University Holiday

February 26
No Colloquium
No Colloquium
No Colloquium

March 5
Viewing dark matter with weak gravitational lensing
Richard Massey
Astronomy, California Institute of Technology


Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious "dark matter" component, which does not interact via the electromagnetic force and thus neither emits nor reflects light. However, it has the usual gravitational signature and gravitational evidence for it is mounting. The past seven years have seen dramatic progress in measurements of weak gravitational lensing, the slight deflection of light from distant sources due to the curvature of intervening space. Recent observations from the Hubble Space Telescope have provided direct proof for, and large-scale maps of dark matter in the Universe. I review the current state of the art and prospects/challenges for the future of the field. Indeed, gravitational lensing provides one of the most promising routes to fulfil the astrophysical end of the deal in a larger quest to determine the nature of dark matter.

March 12
Spring Recess

March 19
Quantum Magnetism and possible BEC in an organic Nickel compound
Vivien Zapf
National High Magnetic Field Laboratory, Los Alamos National Laboratory


I will review recent experimental and theoretical work on the S=1 quantum magnet, NiCl2-4SC(NH2)2. This compound exhibits field-induced XY antiferromagnetism for magnetic fields along the tetragonal c-axis between Hc1 = 2.1 T and Hc2 = 12.6 T. The axial symmetry of the spin environment allows us to understand the quantum phase transitions at Hc1 and Hc2 in terms of Bose-Einstein condensation (BEC) of spin degrees of freedom. Here the tuning parameter for BEC transition is the magnetic field and not the temperature. Specific heat, magnetocaloric effect, magnetization and ESR data at low temperatures confirm the predicted behavior for a BEC: Hc-Hc1 ~ Tα and M(Hc1) ~ Tαwhere α = 3/2. I will also present magnetostriction data taken at dilution refrigerator temperatures that show significant magnetoelastic coupling and magnetic-order-induced modifications of the lattice parameters in this soft organic compound. The magnetostriction is proportional to the spin-spin correlation function, allowing us to make a quantitative determination of the magnetoelastic coupling, and also extract the spatial dependence of the exchange coupling.

March 26
What Is the "Dark Matter"?
Stefano Profumo
Theoretical Astrophysics, CalTech


The matter content of the Universe appears to be dominated by a form of matter whose existence is inferred on the basis of its gravitational effects, and whose fundamental microscopic nature is at present unknown. This form of matter must be qualitatively different from the ordinary matter (neutrons, protons, electrons) that accounts for planets, stars and galaxies, and must couple very weakly to the known particles of the Standard Model. In this colloqium, I will review and outline the wide ensemble of evidence for dark matter, the best motivated dark matter candidates, and the ongoing experimental and theoretical effort in the quest for the identification of this elusive, and yet fundamental, constituent of the Universe.

April 2
The Art and Science of Magnetism at Nanoscale.
Igor V. Roshchin, Department of Physics, University of California San Diego


Magnetism at nanoscale, when the size of the structures is comparable to or smaller than the ferromagnetic domain size, offers a great potential for new physics. Recent advances of fabrication and characterization techniques and miniaturization of electronic devices and their elements resulted in a wide interest to nanometer-sized structures. Unique properties of nanostructured materials provide high potential for their biochemical and biomedical applications. I will give a general overview of research in this field from fabrication to characterization and physical properties. Among others, I will describe a new method of fabrication of magnetic sub-100 nm nanodot arrays covering a macroscopic, over 1 cm2, area, using self-assembled alumina nanopores shadow masks. This method provides a good control of the dot size and separation. I will demonstrate a few recent results showing the effect of confinement in magnetic nanodots on their magnetic properties. Quantitative studies of magnetic vortices in nanodots, including the world's first polarized neutron scattering measurements of sub-100 nm nanodots, will be presented.

April 9
Quark Soup al dente: Applied String Theory
Robert C Myers
Perimeter Institute for Theoretical Physics


In recent years, experiments at the Relativistic Heavy Ion Collider have discovered an exotic new state of matter known as the quark-gluon plasma. Simple theoretical considerations suggested that this plasma would behave like an ideal gas, however, the experiments show that it actually behaves very much like an ideal liquid. Thus the standard theoretical tools, such as perturbation theory and lattice gauge theory, are poorly suited to understand this new phase. However, recent progress in superstring theory has provided us with a theoretical laboratory for studying very similar systems of strongly interacting hot non-abelian plasmas. This surprising new perspective extracts the fluid properties of the plasma from physical processes in a black hole spacetime. At present, this approach seems to provide some of the best tools which theoretical physicists have to understand the heavy ion collisions at RHIC.

April 16
Soft Materials: Controlling and Probing Structure Formation at the Meso Scale
Hans Wyss
Experimental Soft Condensed Matter Group, Department of Physics, Harvard University


Soft materials such as polymers, colloids or surfactant systems exhibit a wide range of fascinating structural, dynamic and mechanical behaviors. They contain structural elements at the meso scale, larger than molecules, but smaller than macroscopic objects. Examples of soft materials are ubiquitous; they include for instance paints, food systems, ceramic precursors, cosmetics, and many other industrial products. I will present a very general mechanism for the control of structure formation in these systems. Based on our experimental data on a colloidal model system, we develop a simple physical picture that bridges the gap between colloidal gel networks and colloidal glasses, two distinctly different out-of-equilibrium states of soft matter. Surprisingly, even though the formed structures consist of a network of strands, their macroscopic mechanical response is governed by the glass-like behavior and slow structural relaxation processes within those strands. Studying these macroscopic properties on a variety of soft materials, we find remarkable similarities in their rheological behavior both in linear and nonlinear viscelastic measurements. Our experiments show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, Strain-Rate Frequency Superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.

April 23
MnAs/GaAs - A Model System for Spin-Based Physics
Thorsten Hesjedal
Electrical and Computer Engineering, University of Waterloo


Spin-based phenomena are of fundamental research interest not only for basic solid state physics, but also for novel device concepts. Several attempts have been reported in the research fields of spintronics, spin-based logic and nanomagnetism to overcome the traditional barrier separating logic operations and storage functionality. At the basis to integrate logic and storage capabilities is the simultaneous utilization of spin and charge of the electrons. For mastering this challenging endeavor, phenomena related to spin injection, manipulation, transport and spin detection have to be explored. Epitaxial ferromagnet-semiconductor hybrid systems with perfect interfaces are one way to combine magnetic and semiconducting properties. Manganese arsenide has special importance for spin-based physics since it is ferromagnetic above room temperature and it can be grown with high epitaxial quality and sharp crystal interfaces by molecular beam epitaxy on common semiconductors. I will report on the synthesis, structural properties and nanofabrication of MnAs films on GaAs. The intrinsic properties of MnAs are being altered combining it heteroepitaxially with GaAs due to the involved strain at the interface. This drives a self-organization of magnetic nanostructures depending on substrate orientation and temperature. In the second part, I will focus on the collective magnetic behavior of the magnetically coupled nanostructures and I will summarize the micromagnetic properties of MnAs obtained by combining surface-sensitive X-ray magnetic circular dichroism photoemission electron microscopy, magnetic force microscopy and micromagnetic simulations. Based on these findings I will give examples of magnetologic device concepts in MnAs/GaAs. I will finish my talk with an outlook on novel diluted magnetic semiconductor materials -- a different route to combine magnetic and semiconducting properties in a single materials system with above room-temperature ferromagnetic order.

  • Department of Physics and Astronomy
  • University of Southern California
  • 825 Bloom Walk
  • ACB 439
  • Los Angeles, CA 90089-0484