Fall 2006

September 15, 2006
Dr. P Jena
Physics Department, Virginia Commonwealth University

Title: Materials Issues in a New Hydrogen Economy

Abstract: The limited supply of fossil fuels, its adverse effect on the environment, and growing worldwide demand for energy has necessitated the search for new and clean sources of energy. Hydrogen is not only among the most abundant element on earth, but also it is renewable and produces only water when it burns. However, hydrogen is not an energy source, but an energy carrier. The possibility of using hydrogen to meet the growing energy need has rekindled interest in the study of safe, efficient, and economical means for its production, storage and use. Among the many challenges, finding efficient methods for hydrogen storage is considered to be the key for a successful hydrogen economy. The current methods for storing hydrogen as a compressed gas or liquid are not suitable for practical applications. An alternate method for hydrogen storage involves metal hydrides and nanostructured materials. Although conventional intermetallic hydrides can store hydrogen reversibly at around room temperature, the relative weight of stored hydrogen in these materials is rather low (1 - 3 wt %) and do not meet the requirements of the transportation industry (~9 wt %). For this the host materials have to consist of light elements such as Li, Be, B, C, Na, Mg, and Al. Unfortunately, the bonding of hydrogen in these materials is rather strong (covalent or ionic) and the thermodynamics and kinetics are poor. Ways must, therefore, be found to weaken the hydrogen bond strength so that hosts consisting of light elements can be used as effective hydrogen storage materials.

This talk will focus on the issues and challenges in storing hydrogen with high gravimetric and volumetric density and discuss the role of nanostructuring and catalysts that can improve the thermodynamics and kinetics of hydrogen. In particular, I will discuss storage of hydrogen in organic complexes, Boron Nitride and Carbon nanocages and demonstrate that metallization of these nanostructures is necessary to store hydrogen with large gravimetric density. I will also discuss the properties of alanates which have the chemical composition [Mn+(AlH4)n-, M= Li, Na, K, Mg] and can store up to 18 wt % hydrogen, although the temperature where hydrogen desorbs is rather high. It was recently discovered that doping of Ti-based catalysts in NaAlH4 can significantly lower the hydrogen desorption temperature, but why and how Ti accomplishes this task remains a mystery. Using first principles calculations, I will provide a fundamental understanding of the electronic structure and stability of sodium alanates and how it is affected due to Ti doping. The role of Ti based precursors in introducing vacancy like defects and their influence on hydrogen desorption will be highlighted. It is hoped that the understanding gained here can be useful in designing better catalysts as well as hosts for hydrogen storage.



September 22, 2006
B. Dasgupta
IGPP, UC Riverside, Riverside, CA 92521

Title: Solar Arcade Structures as Minimum Dissipative Relaxed States

Abstract: Arcade-type magnetic field structures originating from the photosphere are relevant to the understanding of different types of solar prominences, coronal loops and coronal heating. In most of the existing literature, these loop-like magnetic structures are modelled as force-free fields (FFF) without any plasma flow plasma pressure and gravity. The system is assumed to be isolated and non-dissipative. In reality the photospheric plasma is hardly isolated in nature and the existence of an external drive in the form of a dynamo field is always possible. Solar prominences are not ideal either since dissipative effects are believed to play a major role in coronal heating. The above observations indicate that a non force-free model based on driven dissipative plasma may be a more suitable candidate to replicate the arcade structures and further investigations in this direction are required. In this work, arcade structures have been proposed as minimum dissipative relaxed states (including both the viscous and resistive channels) using a two-fluid description of the plasma. The obtained relaxed state is non force-free in nature and appropriate to an open system with external drives. The Euler-Lagrange equations are solved in Cartesian coordinates subject to the appropriate photospheric boundary conditions. The solutions are seen to support flow-containing arcade-like magnetic field configurations with inherent dissipative properties that may play an important role in coronal heating. An interesting feature observed is the generation of different types of arcades with the variation of a single parameter characterizing the relaxed state. Also, observations with the LASCO coronagraph on board the SOHO spacecraft suggest that helmet streamers originating from the sun may have an internal triple-arcade structure. The two-fluid relaxed state obtained here is also seen to support such structures. A preliminary test case study of an analytical non-force free field model is presented to illustrate the method.

This work is carried out with R. Bhattacharya., M. S. Janaki (Saha Institute of Nuclear Physics, Calcutta, India) and Qiang Hu and G. P. Zank (IGPP, UC Riverside, Riverside, CA 92521)



October 4, 2006
Dr. Henk W. Ch. Postma
Dr. Yohannes Shiferaw
Dr. Deqing Ren

Each new faculty member will give a 15 minutes short presentation on their current research interests.


September 11, 2006
Dr. Kunal K. Das
Fordham University

Title: Interaction-induced adiabatic quantum pumping of spin-singlets

Abstract: After a brief introduction to the Landauer description of carrier transport in mesoscopic physics, I will review the mechanism of adiabatic quantum pumping of electrons through nanoscale structures and discuss its analogies with geometric phase. I will then apply the idea of quantum pumping to dynamically generate and control the flow of spin-entangled electrons, each electron belonging to a spin singlet, by means of periodic time variation of localized two-body interactions. This requires a generalization of the theory of quantum pumping since traditional methods based on 1D scattering matrices cannot be applied here.

I will also discuss possible implementation in a Hubbard-like model, present designs using realistic quantum dots, and provide numerical estimates that show that the relevant parameters are experimentally accessible currently.

Ref: K.K. Das, S. Kim and A. Mizel, Phys. Rev. Lett. 97, 096602



September 18, 2006
Paul M. Bellan
Caltech

Title: Simulating Solar Coronal Loops and Astrophysical Jets in Laboratory Experiments

Abstract: Novel laboratory plasma configurations have been used to simulate both solar coronal loops and astrophysical jets. While having similar parameters and involving similar physics, the two configurations employ different boundary condition symmetries. The experiments use pulsed power of ~10E8 watts, currents of ~10E5 amps and an experimental time scale of ~1E-10 x 10E-6 seconds. The experiments are diagnosed using high-speed photography, magnetic probes, current/voltage measurements, spectroscopy, and x-ray detectors. The solar coronal loop simulation experiment exhibits twisting and kinking, expansion of the major radius, and strong interaction between adjacent loops having parallel currents. The simulated astrophysical jet is observed to evolve through a distinct sequence of steps consisting of jet formation, collimation, kink instability, and for appropriate parameters, detachment. Interpretation of the observed coronal loop physics provides insight into astrophysical jet physics and vice versa. Plasmas in these experiments are highly collimated (filamentary) and this has motivated an analytic model showing why coronal loops have a strong tendency to be collimated.



September 25, 2006
B. Anandarao, PRL
Ahmedabad, India

Title: The Post-Mainsequence Evolution of Sun-like Stars

Abstract: Stars of mass ~ 1-8 Msun undergo dramatic changes during their post-mainsequence evolutionary stages. During these stages the stars lose substantial mass to the interstellar medium (ISM) as a result of increase in their sizes first after the exhaustion of core hydrogen (Red giant branch stage) and a second time after the end of core helium burning (asymptotic giant branch stage). Freshly processed elements are brought into the surface layers by a process called dredge-up. During the second giant phase these stars suffer a heavy mass loss that manifests as a planetary nebula. The hot cores eventually cool down to become white dwarves. The planetary nebulae (PNe) are ionized and excited by the UV radiation from the central hot cores and exhibit spectacular emission lines of allowed and forbidden transitions from various atomic and ionic species. The PNe however are hardly spherically symmetric as one expects but appear to be axial- or point-symmetric objects. One of the most challenging and yet unsolved problems in stellar astrophysics has been to understand the non-spherical nature of PNe and the nature of the forces that bring upon the variety in their morphology. In this colloquium we shall discuss some of these phenomena and the current understanding of these fascinating objects.



October 4, 2006
Markus J. Buehler
Atomistic Mechanics Modeling Laboratory
Civil and Environmental Engineering, Massachusetts Institute of Technology
77 Mass. Ave, Room 1-272, Cambridge, MA, 02139
Email: mbuehler@MIT.EDU
Website: http://web.mit.edu/mbuehler/www/

Title: Fracture and deformation of hierarchical protein materials

Abstract: Structural biological materials such as bone, dentin or nacre show intriguing hierarchical features, ranging from molecular to macroscopic scales, forming stiff and tough materials. Mineralized collagen fibrils, a nanocomposite of tiny hydroxyapatite crystals embedded in a collagen matrix represent the most fundamental building block of the seven hierarchies of bone. Past experiments helped to elucidate the composition of mineralized fibrils, and revealed how mineralized fibrils and the interfibrillar matrix participate in deformation. However, the precise role of the highly conserved nanostructural design of mineralized collagen fibrils, participation of tropocollagen molecules and hydroxyapatite crystals in deformation, the structure-function relationship of its material design, and whether mineralization of collagen fibrils contribute to the stiffness and toughness of bone remains unclear. Here we report molecular modeling that reveals that it is due to the characteristic nanostructure of mineralized collagen fibrils that leads to its high strength and ability to sustain large deformation, as relevant to its physiological role, creating a strong and tough material. Our studies reveal why collagen isolated from different sources of tissues universally displays a design that consists of TC molecules with lengths of approximately 300 nanometers, forming the basis for efficient mineralization of collagen fibrils (M.J. Buehler, P. Natl. Acad. Sci. USA, 2006). We find that adhesion between minerals and tropocollagen molecules must be on the order of 0.1 J/m2, suggesting that ionic interactions dominate the mineral-protein interactions. We report a detailed analysis of the participation of protein and mineral phases in deformation, and describe the molecular mechanisms of the elastic, plastic and fracture regime, finding that component strain is always smaller than tissue strain. Our modeling helps to interpret recent experimental results, and reveals that the largest strains are carried by the tropocollagen molecules, while stresses in mineral platelets can reach 1 GPa and more. We discuss the significance of the change of the elastic environment due to mineralization on directing cell differentiation towards an osteogenic lineage. Our studies exemplify how hierarchical multi-scale modeling can be used to develop quantitative models of chemically complex hierarchical biological materials.

October 11, 2006
Evgeny Tsymbal
Department of Physics and Astronomy
University of Nebraska-Lincoln

Title: Electron tunneling: from magnetic to ferroelectric tunnel junctions

Abstract: In recent years spin-dependent tunneling in magnetic tunnel junctions has aroused enormous interest and developed into a vigorous field of research due to possible applications in non-volatile random access memories and next-generation magnetic field sensors [1]. Extensive efforts, both in theory and in experiment, have been made to elucidate the mechanisms of spin-dependent tunneling. This talk will overview major factors controlling the spin polarization of the tunneling current in epitaxial magnetic tunnel junctions. In particular, we will discuss the decisive role of evanescent states in the insulating barrier layer and the electronic structure of the ferromagnet-insulator interface. Stimulated by experimental observations of ferroelectricity in thin films of a nanometer thickness, we will consider a new class of tunnel junctions which utilize a ferroelectric material as a barrier layer [2]. In such ferroelectric tunnel junctions (FTJs) the conductance may depend strongly on the direction of the electric polarization. This property makes FTJs appealing for application as resistive switches and non-volatile memory cells. Using a ferroelectric barrier in a magnetic tunnel junction makes it multiferroic, where ferromagnetic electrodes are separated by a ferroelectric barrier. Multiferroic tunnel junctions (MFTJs) have the potential to provide an additional degree of freedom in controlling the conductance. We will discuss possible implications following from the interplay between ferroelectric and ferromagnetic properties of the two ferroic constituents in these junctions.

1. E. Y. Tsymbal, O. N. Mryasov, and P. R. LeClair, Topical Review: "Spin-dependent tunneling in magnetic tunnel junctions", J. Phys.: Condensed Matter 15, R109 (2003).
2. E. Y. Tsymbal and H. Kohlstedt, "Tunneling across a ferroelectric", Science 313, 181 (2006).



October 18, 2006
Dr. Ian J. O'Neill
web: www.astroengine.com
email: ian.oneill@gmail.com

Title: Turbulence and Alfven waves: A possible solution to the coronal heating problem?

Abstract: The solar corona continues to cause debate half a century after it was observed the solar atmosphere was hotter than the Sun below. This violation of basic thermodynamic law has led to many theories as to how the tenuous coronal plasma can be energized to such temperatures. The magnetic field of the Sun has a significant part to play in this phenomenon, channelling waves into the corona where resonance occurs with solar plasma. In research published (O'Neill & Li, A&A, 435, p1159, 2005) the basic coronal structures, coronal loops, are investigated as a possible source of heating for the inner corona. Plasma waves are readily transmitted, which in turn drive the flow of multi-million degree, highly radiating solar plasma around these magnetic loops. Turbulence is assumed to be generated, amplifying the heating effect within these structures.

The results of a four year study will be presented, exploring the unique way in which turbulence and Alfven waves may reproduce the observed coronal loop densities and temperatures as observed by TRACE, Yohkoh and SOHO. Implications for the recent launch of Hinode (Solar-B) and STEREO will also be discussed."



October 25, 2006
Virginia Trimble
Prof. of Physics & Astronomy
University of California, Irvine
Las Cumbres Observatory, Goleta CA

Title: Astrophysics Faces the Millenium

Abstract: As far back as records go, people have been asking many of the same questions: How big is the cosmos? How old? Are there other suns? Other earths and moons? Are the physical processes and substances out there the same as the ones here or different? Are cosmic processes cyclic like the year or linear like our lives? And what sorts of data and arguments ought to be allowed when we try to answer such questions? The talk will explore how our modern answers to some of these have developed and on where we might go next with some others.



November 1, 2006
Dr. Elizabeth Nagy-Shadman
Department of Geological Sciences
California State University Northridge

Title: From Continental Drift to the Theory of Plate Tectonics

Abstract: As world maps improved during the 18th and 19th centuries, people noticed that some of the continents seemed to fit together like jigsaw puzzle pieces. It was not until the German meteorologist and geophysicist Alfred Wegener published his book The Origin of Continents and Oceans in 1915 that the idea received any serious attention. In his book Wegener proposed his radical hypothesis of continental drift, which suggested that all continents were once assembled together in a great supercontinent that eventually broke into smaller continents and "drifted" to their present positions. Evidence that Wegener used to support his hypothesis, in addition to the jigsaw puzzle fits, included fossils, rock types, and geologic structures that match across present-day ocean basins, and the geographic distribution of ancient climates and ice sheets. Because Wegener did not have an acceptable mechanism for the movement of the continents, his hypothesis was treated with hostility and ridicule by many respected geologists, especially after it was translated into English in 1924. He continued to defend his hypothesis until his death in 1930.

The theory of plate tectonics is the modern version of continental drift. Among other things it provides the crucial mechanism for plate movement that Wegener needed to satisfy his critics. Whereas Wegener thought that continents moved through ocean basins, much like an ice breaker through ice, we now know that the earth's surface is broken into large, rigid plates that contain both continental and oceanic crust. These plates are in motion with respect to one another. Volcanoes, earthquakes, and uplift (mountain building) occur almost exclusively at active plate margins where plates either move together or apart, or slide past each other. Convection in earth's
mantle is thought to drive plate motion. Although the theory of plate tectonics is strongly supported by abundant and varied data from the oceans
and continents, there remain some controversies such as the source of hot spot volcanism and the nature of the convecting mantle.

November 8, 2006
Emily A. Carter
Department of Mechanical and Aerospace Engineering and Program in Applied and Computational Mathematics
Princeton University
Princeton, NJ 08544-5263

Title: Nanoscale Kondo Physics and Nanomechanics of Metallic Systems from First Principles

Abstract: In the past decade, we have been developing two classes of first principles electronic structure approaches for condensed matter that go beyond the capabilities of standard implementations of Kohn-Sham density functional theory (DFT). The first, an embedding theory, addresses the inability of DFT to handle strong, many-body electron correlation effects. We embed a correlated quantum chemistry description into surroundings described by periodic density functional theory (DFT).1 Recent technical advances in the theory include: (i) implementation of ultrasoft pseudopotentials (USPPs) in a consistent manner across all levels of theory (periodic DFT, CASSCF, and CI), (ii) self-consistent updates of the density of the total system, thereby allowing a fully-self-consistent embedding operator, and (iii) a multi-reference singles and double excitation CI (MRSDCI) treatment of electron correlation in the embedded region.2 This embedded configuration interaction (ECI) theory is used to study a variety of systems/phenomena where DFT is known to fail, due to either neglect of many-body effects or self-interaction artifacts. We will illustrate how the embedding theory is able to give a qualitatively (as well as quantitatively) different view of these systems/phenomena. We will focus on the Kondo effect, a long standing problem in condensed matter physics. The Kondo effect refers to the observation of an anomalous resistivity minimum at low temperatures for materials containing magnetic transition metal impurities in nonmagnetic host metals. We will show that the ECI theory is able to capture the physics and offer a new view of this phenomenon, while periodic DFT and finite cluster quantum chemistry calculations do not.
The second class of techniques concerns orbital-free density functional theory (OFDFT), which addresses speed limitations of Kohn-Sham DFT. OFDFT is a powerful, O(N) scaling method for electronic structure calculations. Unlike Kohn-Sham DFT, OFDFT goes back to the original Hohenberg-Kohn idea of directly optimizing an energy functional which is an explicit functional of the density, without invoking an orbital description. The speed of OFDFT allows direct electronic structure calculations on large systems on the order of tens of thousands of atoms, an expensive feat within Kohn-Sham. Due to our incomplete knowledge of the exact, universal energy density functional, this speedup comes at the cost of accuracy with respect to Kohn-Sham methods, due to necessary approximations to the kinetic and ion-electron energies. However, OFDFT has been shown to be remarkably accurate with respect to Kohn-Sham when used in the study of nearly-free-electron-like metals, e.g., Al, for which good density functionals have been derived. We will review our methodological contributions, as well as recent applications of OFDFT to nanomechanics of metallic systems.4

1) N. Govind, Y. A. Wang, A. J. R. da Silva, and E. A. Carter, Chem. Phys. Lett., 295, 129 (1998); N. Govind, Y. A. Wang, and E. A. Carter, J. Chem. Phys., 110, 7677 (1999); T. Klüner, N. Govind, Y. A. Wang, and E. A. Carter, Phys. Rev. Lett., 86, 5954 (2001); T. Klüner, N. Govind, Y. A. Wang, and E. A. Carter, J. Chem. Phys. 116, 42 (2002); T. Klüner, N. Govind, Y. A. Wang, and E. A. Carter, Phys. Rev. Lett., 88, 209702 (2002).
2) P. Huang and E. A. Carter, Nano Letters, June cover article, 6, 1146 (2006); P. Huang and E. A. Carter, J. Chem. Phys., 125, 084102 (2006).
3) Y. A. Wang and E. A. Carter, in "Theoretical Methods in Condensed Phase Chemistry," S. D. Schwartz, Ed., within the series "Progress in Theoretical Chemistry and Physics," Kluwer, 117-84 (2000); S. C. Watson and E. A. Carter, Comp. Phys. Comm. 128, 67 (2000).
4) M. Fago, R. L. Hayes, E. A. Carter, and M. Ortiz, Phys. Rev. B 70, 100102(R) (2004); R. L. Hayes, M. Fago, M. Ortiz, and E. A. Carter, Multiscale Mod. Sim. 4 359 (2005); R. L. Hayes, G. S. Ho, M. Ortiz, and E. A. Carter, Phil. Mag. 86, 2343 (2006); G. S. Ho, V. Ligneres, and E. A. Carter, to be submitted (2007).