Physics Seminar Series
Contact: Prof. Hao Zeng, haozeng@buffalo.edu, (716) 645-2017 x1243
Tuesday, September 6, 2005
Computer Simulation of Enzyme Reactions
Dr. Yingkai Zhang
Department of Chemistry, New York University
Enzymes are remarkable catalysts and play an essential role in most biological processes. They catalyze a variety of chemical reactions with great efficiency and specificity. To understand the structural origin of enzyme catalysis is a primary goal of molecular biology, and is essential for rational design of new inhibitors and novel enzymes. The primary difficulties encountered in computational studies of enzyme reactions stem from the need to describe chemical bond breaking and forming, and the large size of the enzyme systems. In this talk, I will first describe a pseudobond ab initio QM/MM approach to studying enzyme reactions, which allows for the use of the state-of-the-art quantum mechanical method coupled with molecular mechanical method to determine the catalytic reaction pathways with a realistic enzyme environment. Then the applications to study several important enzymes will be discussed, including peptide deformylase and histone lysine methyltransferase.
Tuesday, September 13, 2005
Mid-infrared Hall effect in ferromagnetic oxides and semiconductors
Dr. John Cerne
Department of Physics, University at Buffalo
Strongly correlated materials ranging from diluted magnetic semiconductors (DMS) to transition-metal oxides, such as ruthenate perovskite (RP) compounds and high temperature superconductor cuprates, are revolutionizing fundamental concepts in condensed matter physics and show great potential for applications to spin-based electronics and multifunctional devices. These materials exhibit unusual properties, such as carrier-mediated magnetism, metamagnetism, quantum criticality and non-Fermi liquid behavior that continue to challenge the condensed matter community. Despite the wide range of properties exhibited by these materials, they all exhibit anomalous behavior in dc Hall effect measurements. In this talk I will discuss why one should and how one can explore the Hall effect as a function of frequency using magneto-polarimetry. Recent results on RP and DMS materials will be presented and compared with theoretical predictions.
Tuesday, September 20, 2005
Simulating Electron Transport with Quantum Many-body Interaction
Dr. Jong Han
Department of Physics, University at Buffalo
I present a method of simulating quantum steady-state non-equilibrium. Today, non-equilibrium theories are mostly based on Green function technique, known as Keldysh formalism. Through a mapping of quantum steady-state non-equilibrium to an effective equilibrium, I introduce a new perspective of non-equilibrium steady-state problem. A systematic implementation of the non-equilibrium boundary condition in steady-state is discussed in the electronic transport on quantum dot (QD) structures. This formulation enables the use of existing powerful numerical quantum many-body techniques such as quantum Monte Carlo technique. The algorithm coherently demonstrates various transport behaviors from phonon-dephasing to $I-V$ staircase and phonon-assisted tunneling in QD with electron-phonon coupling. We briefly discuss how strong correlation physics can be handled within the formulation.
Tuesday, September 27, 2005
Quantum Phases of the Extended Bose-Hubbard Hamiltonian: Possibility of a Supersolid State of Cold Atoms in Optical Lattices
Dr. Vito Scarola
Department of Physics, University of Maryland
Cold atoms confined to optical lattices offer the unique opportunity for direct simulation of strongly correlated Bose-Hubbard models in a pristine environment. Recent experimental progress in simulating zero-range Bose-Hubbard models has provided direct experimental evidence for the superfluid and Mott states. The extended Bose-Hubbard model yields a rich phase diagram including supersolid and density wave order. I will discuss a specific proposal designed to extend the range of interaction in these, otherwise zero-range, systems to effectively simulate an extended Bose-Hubbard model in a cold atom optical lattice. I will also discuss potential issues affecting experimental detection of supersolid and density wave order.
Tuesday, October 11, 2005
Higher-Order Calculations for Precision and New Physics Studies at the CERN LHC
Dr. Doreen Wackeroth
Department of Physics, University at Buffalo
The CERN Large Hadron Collider (LHC) is expected to provide answers to some of the most fundamental questions in particle physics. For instance, we hope to unravel the origin of spontaneous symmetry breaking, the mechanism that is thought to be responsible for fermion and gauge boson masses. Moreover, the anticipated detection of signals of new physics, i.e. physics not described by the existing, well-tested theoretical framework of electroweak and strong interactions among fundamental particles, dubbed the Standard Model (SM), will hopefully resolve some of the many shortcomings of the SM.
To fully exploit the potential of the LHC for finding the Higgs boson and search for and disentangle signals of new physics, it is crucial that the underlying processes are predicted at a level of precision that at least matches, better exceeds, the experimental accuracy. This requires the calculation of cross sections beyond the lowest order in perturbation theory. I will discuss both the challenges and the power of precision physics on the example of Higgs production in association with heavy quarks, weak gauge boson production and top pair production in the SM and a supersymmetric extension of the SM.
Tuesday, October 18, 2005
NMR, Optical Activity, and other properties; with consideration of Relativistic Effects, Solvent Effects, and Vibrational Corrections
Dr. Jochen Autschbach
Department of Chemistry, University at Buffalo
I will give an overview of the approach to calculate magnetic and mixed electric-magnetic properties of closed-shell molecules in the framework of density-functional response theory. Examples for such properties that will be discussed are NMR parameters (nuclear magnetic shielding, nuclear spin-spin coupling), optical activity (optical rotation, circular dichroism (CD)), magnetizability of molecules and magneto-optical rotation. Despite the application of these properties in a wide variety of research fields they are closely related theoretically. For NMR parameters, emphasis will be put on relativistic effects in heavy-atomic systems. E.g. for spin-spin coupling, the magnitude of relativistic effects can easily be of the order of 100 to 1000% of the nonrelativistic values if one or two heavy nuclei are involved. Solvent effects can be of high importance, too, which will be demonstrated for several transition metal complexes. Spin-orbit coupling is highly important for coupling between p-block atoms. Other computational NMR studies that will be briefly presented have focused on vibrational effects and the temperature dependence of spin-spin coupling as well as the first prediction of the chemical shift range for single-walled carbon nanotubes. In the optical activity field, I will outline how calculations of CD spectra of tris-bidentate metal complexes such as Ru(bipy)3(2+) or Fe(phen)3(2+) can help to study their excited states in detail. Further, I will demonstrate the importance of vibrational corrections in calculations of optical rotation as well as circular dichroism spectra. Recently, we have implemented using an empirical finite-lifetime damping parameter in time-dependent DFT linear response calculations which allows to obtain realistic optical rotatory disperion (ORD) curves. Such calculations might therefore be used to unambiguoulsy assign the absolute configuration of a chiral molecule if ORD is available experimentally. Calculated and experimental ORD will be compared for several organic molecules. If time permits, I will present some recent results for optical rotation, magnetizability, and magneto-optical rotation calculated with a "gauge-including atomic orbital" (GIAO) basis which cures the origin-dependence of magnetic properties calculated with a finite basis set.
Tuesday, October 25, 2005
Spintronics: Challenges and Opportunities
Dr. Igor Zutic
Department of Physics, University at Buffalo
Spintronics is an interdisciplinary field in which the central idea is the manipulation of spin degrees of freedom in solid state systems [1]. The motivation to examine spintronics ranges from fundamental studies, where the changes of the spin degrees of freedom can be a sensitive probe for basic physical phenomena [1,2], to applications that are neither feasible nor effective with conventional electronics. We first discuss spin-polarized transport in junctions with superconductors which were recently used for the first direct measurement of the spin polarization in a novel class of ferromagnetic semiconductors [1]. We next develop a theory of spin transport in inhomogeneously doped magnetic semiconductors. Using this theory we predict that a nonequilibrium spin leads to the spin-voltaic effect [3], a spin-analog of the photo-voltaic effect. The direction of the charge current, which can even flow at no applied bias, can be switched by reversal of the equilibrium magnetization or by reversal of the polarization of the injected spin.
[1] I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004).
[2] S. Garzon, I. Zutic, and R. A. Webb, Phys. Rev. Lett. 94, 176601 (2005).
[3] I. Zutic, J. Fabian, and S. Das Sarma, Phys. Rev. Lett. 88, 066603 (2002).
Tuesday, November 01, 2005
Correlations in Low Dimensional Electron Systems: Quantum Dot Molecules
Dr. Ramin Abolfath
National Research Council of Canada
In this talk I present a theory of laterally coupled quantum Hall droplets with electron numbers (N1,N2) at filling factor 2. The edge states of each droplet are tunnel coupled and form a two-level artificial molecule. By populating the edge states with one electron each, a two electron molecule is formed. We predict the singlet-triplet transitions of the effective two-electron molecule as a function of the magnetic field, the number of electrons, and confining potential using the configuration interaction method coupled with the unrestricted Hartree-Fock (URHF) basis, optimized for two-dimensional quantum dot molecules with complex potentials and large number of electrons [1]. Experimental charge detection with controllable charge states, tunable coupling, and the equilibrium state of a single electron trapped in artificial Hydrogen molecule is pointed out [2].
[1] Ramin M. Abolfath, W. Dybalski, Pawel Hawrylak, cond-mat/0509585.
[2] M. Pioro-Ladrire, M. R. Abolfath, P. Zawadzki, J. Lapointe, S. A. Studenikin, A. S. Sachrajda, and P. Hawrylak, Phys. Rev. B 72, 125307 (2005).
Tuesday, November 22, 2005
Spin quantum computing in solids: Electron spin decoherence in semiconductor quantum dots
Dr. Xuedong Hu
University at Buffalo
Spins in semiconductor nanostructures are promising qubit candidates for a solid state quantum computer, and have seen some truly impressive experimental progresses in the past two years. A central issue in spin-based quantum information processing is quantum coherence. In this talk I will discuss a recent theoretical study where we analyze electron spin decoherence through interaction with the surrounding nuclear spins. Specifically, we find that virtual nuclear spin flip-flops mediated by the electron contribute significantly to a complete decoherence of the transverse electron spin correlation function on a time scale from tens of nanosecond to a few microsecond. We also find a long-time asymptotic behavior for the electron spin that is non-Markovian (non-exponential).
Wed., Dec. 14, 2005, 2:00pm, 306 NSC
Multiscale approach to modeling cell-cell interaction and organization
Dr. Yi Jiang
LANL
Modeling and simulation have become central research tools in biology. The most advanced of these efforts have focused on single levels or scales, e.g. genomics, molecular, cellular, tissue, whole body, behavioral, and population. The integration of models from microscales to macroscales in a seamless fashion is increasingly important to capture the complexity of biological problems. Developing the abstracts to integrate between scales will also lead to a much deeper understand of the universal or generic features of biological phenomena. I will introduce a multiscale modeling framework and propose to apply this framework to study cell-cell interaction and organization. In particular, I will apply this approach to model cancer development, immune response, and biofilm formation.
Friday, Dec. 16, 2005, 2:00pm, 245 Fronczak
Self-organization of long sparse neural sequences: a network model of neural trajectory generation in the songbird.
Dr. Ila Fiete
Kavli Institute for Theoretical Physics, UC Santa Barbara
How does the brain wire-up to robustly produce long chains of sequential activity or long bouts of stable activity necessary for driving all forms of motor activity, given that individual neurons can only fire for short periods without external inputs? In the neural network literature, networks with synapses connected to form "synfire chains" can accomplish the task of sequence production, but theoretical attempts to understand how such patterns of synaptic connectivity may arise from natural learning rules have met with scant success. We study the emergence of long neural sequences in a system known to produce such sequences, vocal control area HVC in the songbird. Using known properties of HVC neurons and inputs to HVC, we show through simulation and analysis that a combination of antisymmetric spike-time dependent plasticity and competitive weight decay at all pre- and post-synaptic junctions is sufficient to self-organize stereotyped, sparse, and long sequences from random, non-stereotyped inputs.
Tantalizingly, the resulting distribution of sequence lengths roughly matches syllable length distributions in blackbird songs. Thus, our work shows how a simple activity-dependent learning rule, driven by random inputs, can lead to the self-organization of long, stable chains of neural activity, and may also help to understand bottom-up (neural) constraints on song syllable durations in songbirds.