UCSC Physics Department
Condensed Matter Seminar Schedule - Winter 2004
January 9, 2004
Department of Electrical Engineering
Magnetization switching and imaging of single-domain magnets on the nanoscale
The micro- and nanofabrication technology that enabled miniaturization of electronic devices also allows us to shrink magnetic elements. As the dimensions of a magnet are reduced to the sub-micron scale, it undergoes a qualitative change to the single-domain regime. Such nanomagnets have a variety of potential applications, most notably in magnetic storage.
In this talk, I will focus on two aspects of nanomagnetic structures. First, I will discuss the internal and external parameters that determine changes in the magnetization orientation of the domain. Secondly, I will describe the development of an optical imaging technique to study the magnetization dynamics of individual nanomagnets.
January 16, 2004
University of California, Santa Barbara
Quantum information processing with semiconductor quantum bits and Terahertz cavity QED: a progress report.
I will describe an ongoing research program aimed at all-optical quantum information processing in semiconductors. As quantum bits, the orbital states of electrons bound to Hydrogenic impurities in GaAs, and of electrons in coupled GaAs/InGaAs self-assembled quantum dots are being investigated. The (distant) goal is to insert these quantum bits into a high-Q cavity for THz radiation, to controllably entangle quantum bits using the cavity as a bus, and to manipulate and read out these quantum bits using tightly-focused interband radiation. 2-D THz photonic crystals made from Silicon have been fabricated and tested. Defects in such crystals are expected to form the required high-Q microcavities. Intense ps pulses of THz radiation from UCSB's Free-Electron Lasers have been used to induce Rabi oscillations between the 1s and 2p states of Hydrogenic donors in a magnetic field, and also to investigate the polarization selection rules associated with these transitions. Gate voltages and pulses of light have been used to persistently load electrons or holes into self-assembled quantum dots (preliminary). The advantages and disadvantages of operating at THz frequencies will be discussed.
January 23, 2004
Hitachi Global Storage Technologies
The Computer Hard Disk Drive--Nanotechnology on a $100 Budget
Nanotechnology is expected to change and shape our lives in the 21st century. It is, however, not just a far out vision, but actually part of our daily lives already. The computer Hard Disk Drive (HDD), which is found in all personal computers and increasingly in consumer electronics applications, is one of the technologies that require and utilize nm-scale precision already today.
In my talk, I will give a basic introduction into HDD technology and outline some of today's challenges related to the nanometer scale of its components. In detail, I will discuss modern magnetic disk media, in particular the newly introduced Anti-Ferromagnetically Coupled (AFC) media. The physics of these AFC-media is presently the focus of intense research efforts with the goal to enable storage densities of up to 300 Gbit/inch2. Besides their commercial applications, such complex nanostructures can also be utilized as nano-laboratory allowing the study of various fundamental magnetic properties on the nm-length scale.
January 30, 2004
Department of Physics
Tohoku University, Japan
Dynamics of one-dimensional electrons and fractional statistics
Interactions among one-dimensional electrons cause nonperturbative effects such as instability of the Fermi liquid and the spin-charge separation. The Tomonaga-Luttinger (TL) theory has been very successful in describing the the low-energy dynamics in terms of bosonic excitations. However, understanding the dynamics probed by neutron scattering and photoemission requires global features beyond the scope of the TL theory. We take the supersymmetric t-J model with 1/r^2 interaction, and present exact analytical results for the dynamical structure factor and the one-particle spectral function. It is shown that spin and charge are carried by different quasi-particles obeying fractional statistics, but that they seem to recombine in the particular momentum range at high energy.
February 6, 2004
University of Washington
How magnets remember/forget: Truth from speckle
What is the microscopic origin of magnetic hysteresis, and how can we study it experimentally? In this talk, I will explain how we have been able to use coherent x-rays to generate magnetic speckle patterns, and how we have been able to use these speckle patterns to study the detailed evolution of the magnetic domains versus their applied magnetic field history. I will explain the failings of the best current microscopic theories, and I will suggest how they can be repaired.
February 13, 2004
Messy Materials and Abstruse Algorithms
The low temperature phases of disorder-dominated materials, such as random magnets and type-II superconductors, are often studied using optimization algorithms that determine the ground state configuration. This talk will review some results from these calculations for the random-field Ising model and more recent results for flux lines in superconductors. I will note some useful links between zero-temperature phase transitions and the dynamics of the algorithms used to study models of disordered materials. These results advance both our understanding of the model systems and the search for improved optimization algorithms.
M. Zahid Hasan
February 20, 2004
Low-energy electronic structure of a new class of low-dimensional Mott system (NaxCoO2): perspective from momentum-resolved spectroscopies
Doped Mott insulators continue to surprise us. Recently, attention has focused on the triangular-lattice layered cobaltate NaxCoO2. This system exhibits an unusually large thermopower, T-linear resistivity, magnetic frustration, charge-order and unconventional Hall behavior at high dopings. At low doping when intercalated with water, the system becomes superconducting below 5 K with a cuprate-like dome-shaped phase diagram. We have recently studied the low-energy electronic structure of the host compound of this series using high resolution momentum-resolved photoemission spectroscopy. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band and a small but anisotropic Fermi velocity. Temperature dependence of low-energy spectral weights suggests that the quasiparticles are well defined only in the T-linear resistivity regime. Small single particle hopping and unconventional quasiparticle dynamics may have implications for understanding a host of unusual behavior exhibited by this new class of strongly correlated and strongly frustrated system.
February 27, 2004
NASA Ames Research Center
Simple Math Is Enough: Two Examples of Inferring Functional Associations from Genomic Data
High-throughput experiments such as genomewide monitoring of mRNA expressions and protein-protein interactions are expected to be fertile ground for deriving protein functions. These data usually contains a high rate of false positives. This talk discusses how to extract reliable biological associations from genome-wide data. Using two examples, we emphasize the importance of p-value often derivable using simple mathematics. The first example is based on our recently published work (PNAS 100, 12579) on predicting reliably protein functions from certain non-random features in the large-scale 2-hybrid data. Our method assumes that if two proteins share significantly larger number of common interaction partners than random, they have close functional associations. Base on an analysis on yeast protein-protein interaction data, we have derived more than 2800 functional associations. We derived tentative functions for 81 unannotated proteins. Since the completion of the work, 23 of them have been annotated in Saccharomyces Genome Database, all but one of our predictions proved to be correct. In the second example, we will discuss our improvement to the REDUCER algorithm of Bussemaker et al.(Nat. Genet. 27 167) that extract protein-binding motif from microarray experiments. In this method, statistical significance is derived from linear regression of the expression values to the copy number of motifs in gene's promoters.
March 5, 2004
University of Illinois, Urbana-Champaign
Mapping Electronic States in High Temperature Superconductors
High temperature superconductivity in the cuprates continues to be one of the main challenged facing condensed matter physics today. After many years of effort in both experiment and theory there are still a number of unanswered questions about the nature of electronic states in these materials. Using the scanning tunneling microscope, we have the capability to directly map the electronic states in these compounds and address some of these questions. At the center of the debate is the possibility that in addition to superconductivity and antiferromagnetism, these compounds may exhibit other types of ordering phenomena, such as those associated with spin, charge density waves, or orbital currents. In this talk I will focus on recent experimental evidence from our lab that strongly suggests that electronic state can indeed spatially organize in these compounds. I will describe these experiments as well as the experimental technology that we have developed to make them possible.
March 12, 2004
Department of Biochemistry and Biophysics
University of California at San Francisco
Genomic Reconstruction of Yeast Transcription Networks
In this talk, I will describe a systematic approach to reconstructing the transcription networks of a cell, using yeast as a model organism. The approach integrates information from the complete genome sequence and large scale functional data. It consists of three steps: 1) accurately constructing the transcription modules (defined by a transcription factor, its DNA binding site and target genes) using functional genomics data; 2) identifying environmental or genetic perturbations which activate/deactivate each transcription module; 3) analyzing combinatorial regulation by examining co-activated modules and the overlaps between them.