Colloquium Spring 2008 - Department of Physics and Astronomy

January 21
Martin Luther King, Jr. Birthday, University Holiday

January 28
Single-molecule studies of entangled DNA dynamics, protein-mediated DNA looping, and viral DNA packaging
Prof Doug Smith
Department of Physics, UCSD

Abstract

My group uses optical tweezers and fluorescence microscopy to manipulate and visualize single DNA molecules, with applications in polymer physics and molecular biophysics research. When long polymers such as DNA are in a highly concentrated state they become entangled, leading to restricted mobility. Using optical tweezers we directly measure the intermolecular forces, permitting us to quantify key assumptions in the reptation model. DNA is naturally found in both linear and circular forms. Using single-molecule tracking we show that diffusion depends strongly on the topologies of both the diffusing molecule and surrounding molecules. Protein complexes interacting at multiple binding sites via DNA looping play an important role in many fundamental biochemical processes, including regulation of gene expression. We directly measure single DNA looping using optical tweezers. We characterize looping rates and distributions of loop sizes and unbinding forces for many different enzymes and compare our findings with recent theoretical models. A key step in the assembly of many viruses is the packaging of DNA into a pre-assembled procapsid by an ATP-powered molecular motor. We measure the packaging of single DNA molecules in real time using optical tweezers. We study ionic effects on the forces resisting DNA confinement in bacteriophage phi29, confirming the importance of electrostatic repulsion. We have further studied packaging in E. coli bacteriophages lambda and T4, which are key model systems in molecular biology. We compare and contrast these three different viral motor complexes.

February 4
Quantum spin Hall effect
Prof Shoucheng Zhang
Physics, Stanford University

Abstract

Search for topologically non-trivial states of matter has become a important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. We show that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells. By varying the thickness of the quantum well, the electronic state changes at a critical thickness. This is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. This theoretical proposal has been tested in a recent experiment carried out at University of Wuerzburg, and the distinct signatures of the QSH state have been experimentally observed. [1] Bernevig, Hughes and Zhang, Science, 314, 1757, (2006) [2] Koenig et al, Science Online, Sept 21, 2007

February 11
“Giant” strengthening of the pair correlation in metallic nanoclusters: high Tc state and potential for room temperature superconductivity
Prof Vladimir Kresin
LBNL

Abstract

The pair correlation leads to a peculiar state of matter which is manifested in such diverse fields as solid state physics (superconductivity), nuclear physics, astrophysics, biology, etc. We focus on small metallic nanoclusters (N ≈ 102-103 ; N is a number of free carriers). Delocalized electrons form energy shells similar to those in atoms or nuclei. It turns out that under special, but perfectly realistic conditions the superconducting pairing is very strong, and the nanoclusters form a new family of high temperature superconductors. For realistic sets of parameters one can observe high value of Tc (Tc ≈ 150 K ) as well as a strong modification of the energy spectrum. In principle, it is possible to raise Tc up to room temperature. One can propose specific experiments aimed at detecting the phenomenon (spectroscopy, magnetic and thermodynamic properties). * Jointly with Y. Ovchinnikov, L.Landau Institute for Theoretical Physics

February 18
President’s Day, University Holiday

February 25
Angle-Resolved Photoemission Study of High-Tc Superconductors
Prof Zhi-Xun Shen
Stanford University

Abstract

March 3
Nanomechanics: From Basic Science to Applications
Prof Andrew Cleland
UCSB

Abstract

Nanomechanical resonant devices have been under study for about the past 10 years, due to their ability to achieve very high operating frequencies (approaching 10 GHz), their extremely small mass (approaching tens of femtograms), and their very high sensitivity to changes in parameters, local forces, and their potential for achieving quantum limited motion. There are efforts underway to develop these devices as mass spectrometers, biomolecule detectors (both in fluid and in vacuum), electrometers, and there are attempts to integrate them with displacement sensors in order to achieve ultrasmall force detection and possibly achieve measurements at the quantum limit of single phonons, the quanta of mechanical vibrations. In this talk, I will attempt to outline the basic properties of nanomechanical resonators, point out some interesting applications, and outline how their quantum behavior can perhaps be measured.

March 10
SIZE MATTERS: Nano-scale mechanical properties of crystals
Prof Julie Greer
Caltech

Abstract

While “super-sizing” seems to be the driving force of our food industry, the direction of materials research has been quite the opposite: the dimensions of most technological devices are getting ever smaller. These advances in nanotechnology have a tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. Some of the examples include data storage at densities greater than one terabit per square inch, high-efficiency solid-state engines, single-cell diagnostics of complex diseases (e.g. cancer), and the development of ultra-light yet super-strong materials for vehicles, with the component sizes comprising these technological devices reduced to the sub-micron scale. The functionality of these devices directly depends on their structural integrity and mechanical stability, driving the necessity to understand and to predict mechanical properties of materials at reduced dimensions. Yield and fracture strengths, for example, have been found to deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature. Unfortunately, the few existing experimental techniques for assessing mechanical properties at that scale are insufficient, not easily accessible, and are generally limited to thin films. In order to design reliable devices, a fundamental understanding of mechanical properties as a function of feature size is desperately needed; with the key remaining question whether materials really are stronger when the instrumental artifacts are removed, and if so then why and how. A key focus in Professor J.R.Greer’s research is the development of innovative experimental approaches to assess strengths of specimens whose dimensions have been reduced to nanoscale not only vertically but also laterally. One such approach involves the fabrication of single crystal nanopillars ranging in diameter from 100 nm to 800nm by using Focused Ion Beam (FIB). Their strengths in uniaxial compression are subsequently measured by modifying a standard nanoindentation setup to remove the strain gradient effect from the observed mechanical response. Some representative images of nano-pillars before and after compression are shown in the Figure. In this seminar we will discuss the differences observed between mechanical behavior in gold and molybdenum single crystals, which represent two fundamental types of crystals: face-centered cubic, fcc, and body-centered cubic, bcc, with nano-scale dimensions. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal size is reduced to 100nm; however in gold crystals (fcc) the highest strength achieved represents 44% of its theoretical strength while in molybdenum crystals (bcc) it is only 7%. Moreover, unlike in bulk where plasticity commences in a smooth fashion, both nano-crystals exhibit numerous discrete strain bursts during plastic deformation. These remarkable differences in mechanical response of fcc and bcc crystals to uniaxial micro-compression challenge the applicability of conventional strain-hardening to nano-scale crystals. We postulate that they arise from significant differences in dislocation behavior between fcc and bcc crystals at nanoscale and serve as the fundamental reason for the observed differences in their plastic deformation. Namely, dislocation starvation is the predominant mechanism of plasticity in nano-scale fcc crystals while junction formation and subsequent hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed for both crystals as a function of size and compared with stochastic models.

March 17
Spring Recess, University Holiday

March 24
Inelastic Electron Scattering off Magnetic Impurities
Prof Leonid Glazman
Yale

Abstract

This talk reviews the theory and experiments investigating the electron relaxation in metals with magnetic impurities. Such impurities enhance the energy exchange between electrons and make the electron energy relaxation sensitive to the magnetic field. The developed theory explains fast electron relaxation discovered about a decade ago in experiments with tiny metallic wires. It also suggests a direction for new experiments.

March 31
Gossamer superconductivity, a new paradigm for high Tc cuprate superconductivity
Prof Kazumi Maki
USC

Abstract

Since the discovery of high Tc cuprate superconductivity by Bednorz and Mueller in 1986,even now the origin and the nature of high Tc cuprate superconductivity is hotly debated. In particular the remarkable phase diagram in this class of materials is one of the outstanding problems in the condensed matter physics. However,the followings are clear; a)the normal state is a Fermi liquid a la Landau, b)the pseudogap phase is dSDW, c)the superconductivity is d-wave BCS one, d)the superconductivity in the under-doped region is gossamer (ie. d-wave SC in the presence of dSDW). We shall illustrate this with a few examples.

April 7
Beating the Diffraction Limit: Atomic Scale Optical Phenomena
Wilson Ho
Department of Physics, University of California, Irvine

Abstract

Optical microscopy has played an enabling role in experimental observation, particularly the biological sciences. The desire for observing increasingly finer details has pushed instrument development toward the limit of spatial resolution. By using a low temperature scanning tunneling microscope (STM) and coupling light to the nano-junction, it has become possible to probe optical phenomena with sub-atomic resolution. Such capability provides a new window for viewing molecular properties. In “molecular acupuncture”, the way the molecule behaves can be controlled by pinpointing the specific part of the molecule that is initially perturbed. Specific examples of such control include the spatial dependence of single molecule fluorescence and the primary step of electron transfer to a single molecule.

April 14
colloquium break

April 21
Magnetic Rings for Memory and Logic
Prof Caroline Ross
Materials Science and Engineering, MIT

Abstract

Magnetic data storage devices, including magnetic random access memories and patterned media, are based on thin film magnetic nanostructures. Magnetic multilayer thin film rings present a particularly interesting geometry, and their rich behavior offers opportunities for development of multibit magnetic memories and programmable, non volatile logic devices. A single layer magnetic ring can adopt a variety of stable and metastable magnetic states characterized by different numbers of domain walls, and the behavior of a multilayer ring is further complicated by magnetostatic and exchange interactions between the individual magnetic layers. In this study, rings with nanoscale to micron scale dimensions are made using electron beam lithography and self-assembled block copolymer lithography. We will describe the behavior of single layer, multilayer and exchange-biased magnetic rings, including control of the chirality of the magnetization direction, and magnetotransport measurements made on electrically contacted rings that show voltage signals of 1000s of percent, and we will discuss how these structures may be used in multibit memory cells and logic devices. Caroline Ross is Professor of Materials Science and Engineering at Massachusetts Institute of Technology. She joined MIT in 1997, after spending six years in research and development at Komag, a hard disk manufacturer in San Jose, California. Her background includes a BA and PhD (1988) in materials science from Cambridge University, UK, and a postdoctoral fellowship at Harvard University. Her areas of research are focussed on magnetic materials, especially for data storage applications in hard disks, patterned media, magnetic random access memories and magnetic logic; materials for magnetooptical applications; and templated self-assembly processes such as the formation of ordered structures in block copolymers for nanolithography applications, and templated nanostructure formation.

April 28
The warped side of the universe
Prof Kip Thorne
Physics, Caltech

Abstract

There is a “warped side” to our universe, consisting of objects and phenomena that are made solely or largely from warped spacetime. Examples are the big-bang singularity, black holes, and cosmic strings. Supercomputer simulations (numerical relativity) are beginning to revolutionize our understanding of what COULD exist on our universe’s warped side; and gravitational-wave observations (LIGO, LISA, …) will reveal what phenomena actually DO exist there, and how they behave.