Physics Colloquium

Spring 2015

Thursdays 4:00 p.m., Room 104 Physics
Refreshments served at 3:40 p.m.
  Colloquium organizer: Julia Medvedeva
(Link to main colloquium page)

Green - open date
Yellow - tentative (reserved)
Red - firm commitment

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Date Host Speaker Title of the talk Abstract
Jan. 29

Thomas Vojta
Filling the world with new light:  Physics Nobel Prize 2014

Feb. 5
Waddill
Alexander Chernatynskiy,
U. of Florida
Thermal Transport at Extreme Environments from Atomistic Simulation
Most thermal transport studies have explored rather conventional conditions of temperature, pressure and irradiation. Extreme environments place materials under conditions not usually encountered; the thermal response under these conditions allows the better elucidation of the physics of the various phenomena. While the conditions of high temperature, high pressure and irradiation are interesting from a fundamental perspective, they are also relevant technologically. For example, irradiation effects are crucial for the understanding thermal transport and therefore performance characteristics of nuclear fuels. As another example, the high pressure and temperature conditions present in the Earth’s interior greatly affect thermal conductivity of the mantle and influence its heat balance and consequently evolution. In this presentation, we will discuss how these conditions influence thermal conductivity and demonstrate insights obtained with the combination of the atomistic simulations techniques such as molecular dynamics and lattice dynamics on the basis of classical potentials and first principles methods.
Feb. 12 Waddill
Handan Yildirim, Purdue U.
Atomistic Scale Modeling of Materials for Applications in Electrochemical Energy Storage
Large-scale electrochemical energy storage that would allow wider use of renewable electricity requires new and advanced functional electrode materials to be developed for Li ion batteries, as well as those beyond Li ion technologies. This grand challenge places an ever-growing demand for the design of efficient electrode materials, providing suitable energetics and fast diffusion kinetics to enable far more efficient and longer lifetimes for electrical energy storage devices. Therefore, there have been substantial research efforts for developing advanced materials with capabilities exceeding those of the current electrode materials.
Accelerating the speed of discovery and the deployment of advanced functional materials will benefit from fundamental understanding of materials properties and behavior at long time and length scales, utilizing hierarchical modeling across the scales. Complementing the advanced characterization techniques, such predictive modeling and simulation offers an exciting possibility to provide fundamental atomistic level understanding, and accelerate the discovery of advanced materials for energy storage applications.
In this talk, I will present some examples of the computational studies of materials for electrochemical energy storage applications, in particular for Li ion batteries. Broad focus of these studies is to obtain atomistic level understanding of the processes and materials performances in Li ion battery environment. In particular, I will discuss the lithiation characteristics of negative electrode materials, promising role of nanostructured materials, the mechanism leading to the stability of negative electrode material under extreme condition, the feasibility of an engineered electrode material to store more Li, and ion diffusion characteristics in electrode materials. Finally, enabling computational modeling approaches and basic research needs for future Li ion battery design will be presented.
Feb. 19
Waddill
Liping Yu,
U. of Colorado Boulder
Computational Design of New Materials for Energy Applications from First Principles
Discovering new functional material is a crucial scientific grand challenge. Most currently used technology-critical materials were discovered by luck or trial-and-error experiment, and then subsequently improved incrementally over tens of years, at significant R&D cost. In this talk, I will present how to design new materials for energy applications by a more effective approach of “inverse design”: given a set of target properties, predict the material that has them. This approach, powered by theory that guides experiment, places functionality first, and uses search and optimization strategies based on first principles calculations. As an example, this talk will focus on the inverse design of new earth-abundant thin-film photovoltaic absorber materials, which are critical in realizing the promise of thin-film solar cells for reducing the cost of sunlight-to-electricity compared to conventional crystalline silicon. I will present our recently developed novel selection metric of “Spectroscopic Limited Maximum Efficiency”. This metric takes into account the leading physics related to solar cell efficiency and goes beyond the commonly adopted “Shockley-Queisser Limit Efficiency” that depends solely on material band gap. Applying this metric to ternary chalcogenide (e.g., I-III-VI, I-VI-VI) materials has identified a set of promising thin-film PV absorber materials (e.g., CuSbS2 and Cu3SbS4).  At last, I will also discuss the research challenges and opportunities in materials design for various applications.
Feb. 26 Waddill
Department meeting


Mar. 5
No colloqium
March Meeting


Mar. 19
Waddill Ian Ferguson,
Dean of Engineering & Computing, Missouri S&T
Beyond Nano: Spinning; Dazed and Confused!
Recent predictions of room temperature ferromagnetism in transition metal doped wide bandgap semiconductors such as GaN have spawned a great interest for their use in the field of semiconductor-based spintronics. Spintronic devices differ from traditional electronics in that they are based on the electron spin instead of its charge. Both improved processing and efficiency within existing devices, as well as novel functionalities such as reconfigurable logic, nonvolatile chip-based memory, and a solid state quantum computing may be possible within magnetic semiconductor systems. If these devices can be implemented at room temperature, they will advance the state of the art in spintronics and create a new technological revolution similar to the invention of the transistor.
Dilute magnetic semiconductors (DMS), which consist of semiconductors doped with rare earth or transition metals to provide magnetic functionality, have been suggested to be a suitable platform for spintronics due to their inherent similarity and compatibility with the existing III-V material technological base. A number of novel features have been demonstrated in Ga1-xMnxAs, such as spin-polarized luminescence from light emitting diodes. Unfortunately the Curie temperature of Ga1-xMnxAs is much less that room temperature, making it unsuitable for practical room temperature spintronics.
Attention for room temperature spintronics has thus turned to the wide bandgap nitrides and oxides based on theoretical predictions of room temperature ferromagnetism in this system using a mean field approximation to the Zener model for carrier-mediated ferromagnetism. Reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. It is not well understood whether the ferromagnetism derives from an intrinsic material property or from nano-scaled cluster distributions in the system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material.
In this presentation, we will review the current theoretical and experimental status of the transition metal doped nitrides and discuss their suitability for future spintronic applications. A series of GaN samples grown by metalorganic chemical vapor deposition doped with Mn, Fe, and Cr have been investigated. A comparison of the predominant theoretical models and predictions for ferromagnetism in the nitrides will be compared with the available literature for Ga1-xMnxN and GaN doped with other transition metals produced by a variety of techniques, including molecular beam epitaxy and ion implantation. In particular, the correlation of the structural, optical, and magnetic behavior will be analyzed in relation to theories of ferromagnetism in these materials. Recent results obtained on ferromagnetic nanostructures and Gd-doped GaN will also be reviewed. A spin-polarized LED with evidence for spin injection has been produced using GaGdN and the emission could be manipulated an external magnetic field.
Apr. 2
Medvedeva
David J. Singh, Oak Ridge National Laboratory
Thermoelectric Materials
Thermoelectrics are solid state energy conversion materials. They can be used to produce electrical power from temperature differences and can also be used for solid state refrigerators. They have been widely used in for spacecraft power as well as a number of niche applications. There is increasing interest in thermoelectric materials motivated in part by recent progress and in part by the potential of these materials in various energy technologies. Thermoelectric performance is a multiply contra-indicated property of matter. For example, it requires (1) high thermopower and high electrical conductivity, (2) high electrical conductivity and low thermal conductivity and (3) low thermal conductivity and high melting point. The keys to progress are finding an optimal balance and finding ways of using complex electronic and phononic structures to avoid the counter-indications mentioned above. In this talk, I discuss some of the issues involved in the context of recent results. One key aspect is optimization of the doping level in a given thermoelectric material. While this has long been understood in terms of standard semiconductor parabolic band models, we find surprisingly different results for many thermoelectric materials when the actual first principles band structures are used. This has led to prediction of a number of useful thermoelectrics, some that are new, and surprisingly some that are old. A key theme is the connection of high thermoelectric performance with complex band structures. This leads to the connection between thermoelectrics and topological insulators.
Apr. 9
Medvedeva
Mark Lusk, Colorado School of Mines
Energy Pooling Upconversion in Organic Molecular Systems
The frequency of available light is often lower than desired for a given task. For instance, low-energy light can take advantage of an optical transparency window of biological tissue and penetrate to ingested theranostic nanoparticles, but the photoactivated release of cancer drugs requires energy in the UV range. Motivated by such disconnects between the desired light and that available, a number of strategies have been developed to upconvert light to higher frequencies. From the perspectives of biocompatibility, environmental safety, expense, and resource abundance, it would be advantageous to use organic materials to carry out such processes.

This talk will consider one such upconversion process known as energy pooling in which the energy of two electrons in excited states is transferred to the third  electron. The associated dynamics access virtual states to mediate upconversion; therefore, there is no residence time during which an excited state has the chance to lose energy to phonons.

A combination of molecular quantum electrodynamics, perturbation theory, and ab initio calculations was used to create a computational methodology capable of estimating the rate of energy pooling in organic molecular assemblies. The approach was applied to quantify the conditions under which such relaxation rates become meaningful for two test systems, stilbene−fluorescein and hexabenzocoronene−oligothiophene. Both exhibit low intramolecular conversion, but intermolecular configurations exist in which pooling efficiency is at least 90% when placed in competition with more conventional relaxation pathways. A set of design rules for the optimization of energy pooling will be discussed.
Apr. 16
Kurter
Claudia Ojeda-Aristizabal, UC Berkeley
Graphene : Quantum phenomena and layered heterostructures
Graphene, the one atom thick layer of carbon, has opened a fruitful field of research in condensed matter physics. Even today, ten years after its discovery, graphene produces approximately ten thousand papers per year. Having reached a suitable understanding of graphene’s fundamental properties and applications, scientists are now turning their attention to structures composed of different 2D crystals. Significant effort has been put into the production of 2D crystals other than graphene, ranging from thin insulators (like hexagonal boron nitride) to thin semiconductors (like molybdenum disulfide). Combination of these materials into structures named van der Waals heterostructures could allow for the engineering of tunable phenomena at the interface.
In this talk, I will give an introduction to the Physics of graphene and I will describe two different quantum phenomena that can be observed in this two dimensional crystal: universal conductance fluctuations and superconducting proximity effect. I will present tunable phenomena that can be found at the interface between graphene and other materials like pentacene, a well-known organic semiconductor, and C₆₀, the 0 dimensional version of graphene. To conclude, I will expose layered materials that rest unexplored and layered heterostructures that can lead us to exciting Physics.
Apr. 23
Kurter
Heng Pan, Mechanical Engineering
Electronics Manufacturing with Laser Processing of Nanoscale Materials and Fundamentals
Laser processing (heating, sintering, melting, crystallization and ablation) of nanoscale materials has been extensively employed for electronics manufacturing including both integrated circuit and emerging printable electronics. This presentation will firstly review some recent developments in this topic. It will then focus on fundamental transport phenomena in laser annealing/crystallization. Many applications in semiconductor devices require annealing step to fabricate high quality crystalline domains on substrates that may not intrinsically promote the growth of high crystalline films. Applications include thin film transistors for advanced displays, high performance thin film solar cells, 3D electronic devices, and memory devices, etc. Various energy sources including scanning continuous wave (CW) lasers, electron beam sources, graphite strip heaters, and pulsed lasers, have been utilized to crystallize amorphous materials in thin film configurations. Recently, the emergence of FinFETs (Fin-shaped Field Effect Transistor) and 3D Integrated Circuits (3D-IC) has inspired the study of crystallization of amorphous materials in nano/micro confined domains. Using Molecular Dynamics (MD) simulation, we study the characteristics of unseeded crystallization within nano/microscale confining domains. It is demonstrated that unseeded crystallization can yield single crystal domains facilitated by the confinement effects. The stochastic nature of this process and the mechanisms leading to single crystal formation are revealed. A phenomenological model has been developed and tailored by MD simulations, which was applied to quantitatively evaluate the effects of domain size and processing laser pulse width on single crystal formation.
Apr. 30
Waddill
44nd Annual Harold Q. Fuller Prize Colloquium


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