Physics Colloquium Series
(Refreshments at 3:15 p.m. in 245 Fronczak)
Contact: Prof. William H. Kinney , (716) 645-2017 x111;
Prof. Sambandamurthy Ganapathy, (716) 645-2017 x244, 129;
Prof. Peihong Zhang, (716) 645-2017 x144
January Go to Feb. '09 Mar. '09 Apr. '09 May. '09
01/22/2009, Thursday, 3:30 p.m., 205 NSC
Semiconductor nanostructures for efficient solar-energy conversion
Dr. Alberto Franceschetti
NRELThe future of solar energy as a cost-effective alternative to conventional energy sources depends on the ability to increase the efficiency of solar cells beyond the Shockley-Queisser limit (31%). Semiconductor nanostructures have been recently proposed as a possible route to exceed the Shockley-Quiesser limit, because of their ability to (i) generate multiple electron-hole pairs from a single high-energy photon, and (ii) harvest sub-band-gap photons via creation of an intermediate band in the band gap of a bulk semiconductor host. I will present a discussion of these novel concepts based on atomistic pseudopotential calculations of the electronic and optical properties of semiconductor nanocrystals.
01/29/2009, Thursday, 3:30 p.m., 205 NSC
Unconventional Superconductivity induced by Interfaces and Surfaces
Dr. Matthias Eschrig
Univ. KarlsruheOrdered many-body states in solids are often characterized by an order parameter that breaks one or more of the symmetries of the crystal. Unconventional superconductors are examples where, in addition to the gauge symmetry, the crystal symmetry is broken. The resulting superconducting states lead to interesting new physics associated with the spontaneously broken symmetries. However, many unconventional superconducting phases have never been found experimentally in bulk materials as they are energetically not favored. Interfaces between materials with different ordered phases present unique opportunities to study such evasive superconducting states. We show that the induced symmetry breaking that results from interfaces with magnetic materials in heterostructures allows for unconventional superconducting phases with non-trivial spin structure to be present locally near the interface. This leads to new, unexpected, phenomena with applications in nanoelectronics and opens a path to study pairing states that otherwise would not be accessible.
February Go to Jan. '09 Mar. '09 Apr. '09 May '09
02/05/2009, Thursday, 3:30 p.m., 205 NSC
Can a superfluid be used as a powerful light source ?
Dr. Jinwu Ye
Pennsylvania State UniversityWe study quantum nature of photons emitted from the excitonic superfluid phase in semiconductor electron-hole bilayer systems. We find that the light emitted from the excitonic superfluid has unique and unusual features not shared by any other atomic or condensed matter systems. We show that the emitted photons along the direction perpendicular to the layer are in a coherent state which possesses several remarkable properties, while those along all tilted directions are in a two-modes squeezed state. We determine the two mode squeezing spectrum and the angle resolved power spectrum. We also evaluate the line shapes of both the momentum distribution curve and the energy distribution curve. We comment on available experimental data on the energy distribution curve and also suggest that all the theoretical predictions can be measured by possible future angle resolved power spectrum, phase sensitive homodyne and HanburyBrown-Twiss type of experiments. We demonstrate that the photoluminescence from the excitons is a very natural, feasible and unambiguous internal probe of the nature of quantum phases of excitons in semiconductor electron-hole bilayer systems.
02/12/2009, Thursday, 3:30 p.m., 205 NSC
Optical control of quantum dots for quantum information
Tom Reinecke
Naval Research LaboratoryThere is currently much interest in finding physical implementations for quantum information technologies (quantum communications, quantum computing). Spins in quantum dots with fast optical control provide attractive candidates for the carriers of quantum information (qubits). Recent theoretical and experimental work in this area will be described, including spin dephasing and initialization, single spin rotations, coupling between spins, and conditional control of two spin qubits for gates.
02/19/2009, Thursday, 3:30 p.m., 205 NSC
Dark Energy and the Accelerating Universe
Dragan Huterer
Univ. of MichiganOne of the great mysteries of modern cosmology is the origin and nature of dark energy - a smooth component that contributes about 75% of the total energy density in the universe and causes its accelerated expansion. Although discovered just a decade ago, dark energy has recently been confirmed via several independent cosmological probes; nevertheless, there are no good theoretical leads as to its physical provenance. In this talk I examine critically various approaches to measure the macroscopic properties of dark energy, and describe methods to model the expansion history of the universe in the presence of dark energy.
02/24/2009, Tuesday, 10:00 a.m., 219 Fronczak Hall
Research in UB's High-Pressure Semiconductor Physics Laboratory
Prof. B. A. Weinstein, Department of Physics, State University of New York at BuffaloResearch on solids under extreme pressure offers powerful ways to probe atomic bonding and electronic states. Using modern Diamond-Anvil methods (developed in part by the speaker), megabar pressures (~15 million lbs/sq in) can be applied at cryogenic and elevated temperatures. This is the pressure at 1/3 the distance to the Earth's center - and is able to change bond-overlaps by 1-5% and electron energies by several electron volts. These are large effects that can not be treated as perturbations, and often cause fascinating crystal and electronic phase transitions. This talk will focus on the opportunities for graduate students to conduct high-pressure research on important semiconductor and oxide systems in Dr. Weinstein's laboratory. Recent results from Raman scattering experiments under pressure on the vibrational modes in InP/CdS nanocrystals, and on anharmonic phonon decays in ZnS will be presented as examples. I will also describe new and future work directed at studying the effects of strain on the properties of metal oxides, and glassy Se - both materials of extreme importance for advanced devices.
02/26/2009, Thursday, 3:30 p.m., 205 NSC
Cosmological Constraints from Galaxy Cluster Peculiar Velocities
Arthur Kosowsky, Universit of PittsburghUpcoming high-resolution measurements of the microwave background radiation have the prospect of measuring peculiar velocities of galaxy clusters via the kinematic Sunyaev-Zeldovich effect. This will be a direct measurement of cluster velocities with respect to the cosmic rest frame, independent of cluster distance. The resulting cluster velocity catalogs can be used to constrain the evolution of structure in the universe. I will describe recent work to quantify the information contained in cluster velocities. A velocity catalog of a thousand clusters has the potential to constrain dark energy parameters with a precision comparable to other commonly considered techniques. Cluster velocities can also be used to place interesting constraints on modifications of gravity on cosmological scales. Finally, I will give a brief overview of the current observational status of Sunyaev-Zeldovich measurements.
March Go to Jan. '09 Feb. '09 Apr. '09 May '09
03/26/2009, Thursday, 3:30 p.m., 205 NSC
First-principles design of functional materials for energy applications
Su-Huai Wei, Theoretical Materials Science Group, National Renewable Energy Laboratory, Golden, Colorado, USAMaterials design using first-principles techniques is one the ultimate goals in computational materials science. Due to the recent advancement in first-principles electronic structure theory and computing power, it is now possible to perform knowledge-based computational design of materials with unique optical, electrical, or magnetic properties that are tuned to specific energy related applications. This vital tool, therefore, has the great potential to accelerate scientific discovery. In this talk, selective recent works from my group will be discussed to illustrate how computational methods can be used to design functional materials. These include (i) Absorber materials through cation mutation for solar cells, (ii) n-type, p-type or bipolarly dopable transparent conducting oxides (TCO) for optoelectronic devices. (iii) Defect complexes that has shallow donor or acceptor levels. (iv) Filled tetrahedron nitride compounds for solid state lighting applications. (v) Low band gap oxides for photoelectrochemical hydrogen production through water splitting. Other materials may also be discussed.
April Go to Jan. '09 Feb. '09 Mar. '09 May '09
04/02/2009, Thursday, 3:30 p.m., 205 NSC
Relativistic Quantum Mechanics and the Pion-Nucleon System
Dr. Michael G. Fuda
Department of Physics, SUNY University at BuffaloA brief review of the Bakamjian-Thomas approach to relativistic quantum mechanics will be given. The application of this approach to the pion-nucleon system will be explained, and a specific model of the system will be presented. The interactions in the model include nucleon exchange, sigma and rho meson exchange, and a number of resonant processes. The model allows for single-baryon, meson-baryon, and pion-pion-nucleon states. It will be shown that even though three-particle states are treated exactly, the model can be reduced to solving effective two-particle equations. A comparison with the latest experimental information will be given.
04/09/2009, Thursday, 3:30 p.m., 205 NSC
Cancelled
04/16/2009, Thursday, 3:30 p.m., 205 NSC
Single molecule imaging of the dynamic organization of cellular membranes
Dr. Ken Richie
Department of Physics, Purdue UniversityAll living cells are encapsulated by an outer envelope that contains a fluid phospholipid-based membrane. Embedded within this membrane is the machinery (proteins) required for the cell to sense and interact with its environment. As such, one postulates that there may be some mechanisms in place to control the location, mobility and interactions of the membrane proteins in the fluid membrane in order to perform these sometimes complex and critical tasks. In this talk, I will present our recent studies into the (dynamic) organization of cellular membranes including single molecule observations of the diffusion of transmembrane proteins 1) in mammalian tissue cells, 2) in normal and pathogenic human red blood cells and 3) in the membranes of E. coli bacteria.
04/23/2009, Thursday, 3:30 p.m., 205 NSC
OCTONIONIC GEOMETRY
Dr. Merab GOGBERASHVILI
Andronikashvili Institute of PhysicsThe colloquium will be devoted to the applications of hyper-complex numbers in physics. In details will be considered the model where physical signals are described by split octonions. The eight real parameters of octonions are interpreted as space-time coordinates, energy and momentum. In this model both the velocity of light and Planck's constants have a similar geometrical meaning and arise from the condition of positive definiteness of the norm. Generalized Lorentz factor contains extra 'quantum' terms and in the limit when Planck's constant goes to zero transforms to the standard one. It will be shown how the Maxwell and Dirac equations can be written as the simple octonionic continuity conditions.
May Go to Jan. '09 Feb. '09 Mar. '09 Apr. '09
05/28/2009, Thursday, 3:30 p.m., 245 Fronczak
Generalized Bose-Einstein condensation in superconductivity
Prof. M. De Llano
Department of Physics, University of Connecticut, USA and Instituto de Investigaciones en Materiales, Universidad Nacional, MexicoThe Bardeen, Cooper and Schrie¤er (BCS) and the Bose-Einstein condensation( BEC) theories are united via a generalized BEC (GBEC) formalism describing a ternary boson-fermion gas mixture consisting of fermion-particle-as well as fermion-hole- Cooper-pairs (CPs) in thermal and chemical equilibrium with unpaired fermion particles. One then switches on an interaction Hamiltonian that is reminiscent of the Frohlich "two-fermion/one-boson" interaction vertex but in which the bosons are both kinds of CPs instead of phonons. In contrast with the well-known BCS "four-fermion" Hamiltonian, the full GBEC Hamiltonian is exactly diagonalized with a Bogolyubov-Valatin transformation provided one ignores nonzero-total-momenta CPs in the interaction Hamiltonian but not in the unperturbed Hamiltonian that describes the ideal ternary gas. This is possible since the resulting Hamiltonian becomes bilinear in the fermion creation/annihilation operators on applying the Bogolyubov "recipe" of replacing the remaining zero-total-momenta boson hole-and particle-CP operators by their temperature-and coupling-dependent boson c-numbers. If the fermions are electrons, the result subsumes all...ve statistical theories of superconductors and yields hundredfold enhancements in predicted Tcs when compared with BCS predictions with the same two-electron model interaction.