Colloquium Fall 2003
Labor Day, University Holiday
Low-dimensional magnets are an invaluable source of non-trivial behavior and surprises. Usually low dimensionality is associated with enhanced fluctuations and therefore disordered behavior. In this colloquium I will instead address the less investigated topic of ordering in low-dimensional magnets (quasi-1D and quasi-2D) under realistic conditions, and I will stress the peculiarity of the D=2 case, where ordering can occur through condensation of non-linear excitations (vortices). I will report on recent Quantum Monte Carlo results that support the perspective of experimentally observing and tuning such unconventional ordering behavior in real quasi-2D antiferromagnets.
Origin of the Solar Wind
Jet Propulsion Laboratory
The origin of the solar wind, coronal magnetic field topology, and turbulence in the solar wind, are subjects of fundamental interest in research on the Sun and solar wind. Major changes in their understanding have taken place as a consequence of advances by radio occultation measurements of the solar corona. These unique measurements have also unified a wide range of diversified remote sensing and in situ measurements of the solar wind. The purpose of this talk is to summarize the new views of the Sun's atmosphere that have emerged solely from observations.
Glass breaks like metals, but at the nanometer scale
Physics and Chemistry of Surfaces and Interfaces Division, CEA-SACLAY , France
Glasses are the most common example of brittle materials, which break abruptly, without first deforming irreversibly as metals do. In the latter case of ductile fracture, a macro-crack progresses through the growth and coalescence of damage cavities nucleated ahead of the crack tip. This crack propagation mode leads to very rough fracture surfaces, which have been extensively studied. For slow propagation, the fracture surfaces of glass appear to be very flat if observed within the same range of length scales. But when examined at length scales one thousand times smaller (typically 1-100 nm), they exhibit a rough morphology quantitatively similar to the morphology of metallic fracture surfaces. As a matter of fact, fracture surfaces exhibit two self-affine regimes characterized by universal roughness exponents, independent of the material. On the contrary, relevant length scales strongly depend on the microstructure, and scaling was shown to extend over five decades of length scales (a few nanometers to a few tenths of millimetre) for various metallic alloys. Could the similarity between the morphology of fracture surfaces originate in a similarity of fracture modes, i.e. does the fracture mode of glass proceed also by the growth and coalescence of cavities, but at the nanoscale? Our in situ Atomic Force Microscopy experiments, performed on various glasses, clearly show the existence of such nanometric damage cavities, i.e. that glass breaks like metals, but at the nanometer scale. The extension of the damaged zone, which can reach a few hundreds of nanometers in pure silica, depends on the average crack speed, which is in agreement with the results obtained on the morphology of fracture surfaces. A model based on the emission of crack front waves during crack propagation is also evoked. This phenomenon might explain the observed roughness at small scales of observation.
Feature sizes of conventional electronic devices are already on the order of the acoustic phonon mean free path in silicon. The lateral dimensions of nanowires and sizes of quantum dots are approaching the dominant phonon wavelength at room temperature. At this length scale one can expect modification of acoustic phonon spectra in nanostructures formed by elastically dissimilar materials. The phonon spectrum modification due to phonon confinement in nanostructures manifests itself via phonon zone folding, flattening of dispersion branches, formation of mini-bands and, in some cases, phonon band gaps [1-2]. Phonon confinement may affect the lattice thermal conductivity via enhanced phonon relaxation rates or reduction of the phonon density of states in the relevant frequency range. In this talk I will present results of our investigation of thermal conductivity in semiconductor nanostructures such as quantum wells, quantum wires and quantum dot superlattices. It will be shown that phonon confinement may lead to significant decrease of the thermal conductivity, thus deteriorating thermal management of nanoscale devices. At the same time, a proper selection of the nanostructure size, geometry and cladding (barrier) material can open up a way for tuning phonon spectrum in a desirable way, thus enhancing device operation. I will also discuss relevant experimental data on thermal conductivity in nanostructures and high-resolution micro-Raman spectroscopy as a model validation tool for phonon spectrum calculations. . A.A. Balandin and K.L. Wang, Phys. Rev. B, 58, 1544 (1998). . O.L. Lazarenkova and A.A. Balandin, Phys. Rev. B, 66, 245319 (2002). This work was supported in part by NSF, AFOSR, ONR and CRDF.
Photonic Crystals, the electromagnetic analog of semiconductor crystals, have stirred the imagination toward photonic integrated circuits. Integration at the tiniest scale of photonic crystals allows the largest number of components to be produced from a single wafer, reducing cost, and allowing considerable optical complexity. There have been a series of practical difficulties standing in the way of building practical nano-photonic circuits, that are gradually being solved; including, the input/output coupling efficiency problem, the nano-fabrication accuracy problem, the active device issues, electrical modulation schemes, device design software and simulation. Some of these problems are already solved, and we can project solutions to the others over the next few years.
Space Weather: Geomagnetic Quiet, HILDCAAs, Magnetic Storms and Extreme Storms
Jet Propulsion Laboratory
Various aspects of space weather will be discussed. The types of space weather naturally divide themselves into those that occur during solar maximum and those that occur during solar minimum. During solar maximum, Coronal Mass Ejections (CMEs) and their interplanetary counterparts (ICMEs) dominate geomagnetic activity at Earth. The various phases of magnetic storms (initial, main and recovery) will be explained in terms of a simple picture of an ICME interaction with the magnetosphere. It will also be shown that ICME shocks can cause major changes to the dayside equatorial ionosphere. During solar minimum, coronal holes dominate solar activity. High speed corotating streams emanating from coronal holes cause recurring magnetic storms. Following these storms, High-Intensity Long-Duration Continuous AE Activity (HILDCAAs) can last for days to weeks. HILDCAAs will be shown to be caused by interplanetary Alfvén waves. Relativistic (killer) electrons are accelerated in the magnetosphere during these high speed stream/HILDCAA events. Acceleration mechanisms will be briefly reviewed. Finally, the largest magnetic storm on record (September 1-2, 1859) will be discussed. This Carrington flare event caused the largest >30 MeV proton event since 1561. During this storm, auroras were sighted from Hawaii and Santiago, Chile (23� magnetic latitude). We will discuss the possibility/probability of such an event happening again and what some of the consequences may be.
Bruce Tsurutani is currently on Sabbatical at USC (SHS 362, ph. 740-6343).
The atmospheres (including water) of the terrestrial planets have two sources: outgassing from the interior and impacting of comets. The bulk of the atmospheres and oceans from these sources collected early in solar system history. What happened to the original constituents is a function of planetary mass and distance from the Sun. The small planets such as Mercury and the Moon lost all original gases whereas Venus and the Earth lost only hydrogen and helium. Mars, intermediate in size and farther from the Sun, had an ocean and much denser atmosphere early in solar system history but gradually lost gases and is now cold, barren and with a sparse residual atmosphere. The water of Venus underwent photochemical dissociation and resulted in a dense carbon dioxide atmosphere whereas the Earth stored significant gaseous compounds as rocks and liquid water. Dissociation of water on the Earth is going on at a much slower rate, but will ultimately transform the planet into a milder version of Venus.
The scope of nanotechnology is greatly expanded if molecular motors, which can actively transport molecules and supramolecular assemblies, are integrated into nanodevices. However, the development of synthetic molecular motors is still in its infancy. Our approach is to utilize biomolecular motors, in particular motor proteins, to assemble hybrid devices with micro- and nanoscale dimensions. In this effort we are inspired and guided by the diverse applications nature has found for molecular motors, ranging from intracellular transport to the actuation of muscles. We will present prototypes of molecular shuttles (a nanoscale transport system) (1-3), and a piconewton forcemeter (4), and introduced a novel surface imaging method based on self-propelled probes (5). In these systems, motor proteins are immobilized on microfabricated surfaces and transport functionalized microtubules (hollow protein filaments with a diameter of 30 nm and a length of several micrometers). Currently, we are exploring the application of the motor protein-based molecular shuttles as transport elements in biosensors with the goal to increase sensitivity and speed of detection. We will discuss our first proof-of-principle experiments, which take advantage of our improved understanding of the critical elements in the design of such hybrid devices (e.g. described in (6)).
(1) Hess, H.; Clemmens, J.; Qin, D.; Howard, J.; Vogel, V. Light-controlled molecular shuttles made from motor proteins carrying cargo on engineered surfaces. Nano Letters 2001, 1(5), 235-239.
(2) Hess, H.; Vogel, V. Molecular shuttles based on motor proteins: Active transport in synthetic environments. Reviews in Molecular Biotechnology 2001, 82, 67-85.
(3) Hess, H.; Clemmens, J.; Matzke, C.M.; Bachand, G.D.; Bunker, B.C.; Vogel, V. Ratchet patterns sort molecular shuttles. Appl. Phys. A 2002, 75, 309-313.
(4) Hess, H.; Howard, J.; Vogel, V. A piconewton forcemeter assembled from microtubules and kinesins. Nano Letters 2002, 2(10), 1113-1115.
(5) Hess, H.; Clemmens, J.; Howard, J.; Vogel, V. Surface imaging by self-propelled nanoscale probes. Nano Letters 2002, 2(2), 113-116.
(6) Clemmens, J.; Hess, H.; Howard, J.; Vogel, V. Analysis of microtubule guidance by microfabricated channels coated with kinesin. Langmuir 2003, 19(5), 1738-1744.