Description:

The Department of Materials Science and Engineering welcomes you to its 2014 Fall Colloquium Series.

Location: Tech L361, 4:00pm

Chris Van de Walle
Professor, Materials Department
University of California Santa Barbara

Effects of High Doping in Transparent Conductors
Basic information about transparent conductors, particularly about doping and how it affects electronic and optical properties, is often lacking. First-principles calculations are now capable of accurately predicting quantities that are directly relevant for applications. In oxides that can be highly doped, the large carrier concentrations significantly affect optical transparency. While direct absorption (either across the gap or to higher-lying conduction bands) is usually not a problem, indirect processes assisted by electron-phonon scattering create absorption, sometimes with unexpected wavelength dependence. First-principles evaluations of free-carrier absorption provide insight into the factors that limit this key criterion for transparent conducting oxides. The presence of large concentrations of electrons in the conduction band also affects the absorption edge, not only because of conduction-band filling but also through band-gap renormalization. I will discuss these processes and how we can treat them consistently and in quantitative detail.
Biography:
Chris Van de Walle is the inaugural recipient of the Herbert Kroemer Endowed Chair in Materials Science at the University of California, Santa Barbara. Van de Walle develops and employs first-principles computational techniques to model the structure and behavior of materials. He has performed extensive studies of semiconductor interfaces (including the development of a widely used model for band offsets) and of defects and impurities in semiconductors, with particular emphasis on doping problems. In recent years he has been focusing his attention on wide-band-gap semiconductors, nitrides, oxides, on the behavior of hydrogen in materials, and on spin centers for quantum computing. He co-leads IRG-2, “Correlated Electronics”, in the UCSB MRSEC, and his group is actively engaged in studies of efficiency limits in light emitters, novel channel materials for CMOS, transparent conducting oxides, and hydrogen storage materials. He has published over 350 research papers, holds 23 patents, has given 150 invited and plenary talks at international conferences, and is included in the 2014 “Highly Cited Researchers” list (www.highlycited.com). Professor Van de Walle has chaired three conferences, and was Program Chair for the 27th International Conference on the Physics of Semiconductors in 2004. He is a Fellow of the APS, AVS, AAAS, MRS, and IEEE, as well as the recipient of a Humboldt Award for Senior US Scientist, the David Adler Award from the APS, and the Medard W. Welch Award from the AVS.

Before joining the UCSB Materials Department in 2004, Professor Van de Walle was a Principal Scientist in the Electronic Materials Laboratory at the Xerox Palo Alto Research Center (PARC). He received his Ph.D. in Electrical Engineering from Stanford University in 1986. He was a postdoctoral scientist at the IBM T. J. Watson Research Center in Yorktown Heights, New York (1986-1988), a Senior Member of Research Staff at Philips Laboratories in Briarcliff Manor, New York (1988-1991), and an Adjunct Professor of Materials Science at Columbia University (1991).

Description:

The Department of Materials Science and Engineering welcomes you to its 2014 Fall Colloquium Series.

Location: Tech L361, 4:00pm

Geoffrey Beach
Professor
Massachusetts Institute of Technology

Spin-Orbitronics: Interfacial Design of Spintronic Materials and Devices

There is great interest in electrically manipulating the magnetization in nanoscale materials for high-performance memory and logic device applications. In this talk I will describe recently-discovered mechanisms, based on symmetry breaking and spin-orbit coupling at interfaces, whereby the magnetization can be controlled using very low currents1,2 or by a gate voltage alone.3-5 I will focus on ultrathin transition metal ferromagnets sandwiched between an oxide and a nonmagnetic heavy metal, in which magnetic, electronic and ionic effects at the interface can be exploited in new and unexpected ways.
I first focus on the heavy-metal/ferromagnetic interface, where spin-orbit coupling influences not only spin transport, but the nature of magnetism itself in the ferromagnet. In nonmagnetic heavy metals, spin-orbit coupling leads to a left-right scattering asymmetry such that spin “up” and spin “down” electrons pile up on either side of a material when a charge current flows through it. I will show how this spin Hall effect can be used to create pure spin currents at the interface that can drive magnetization switching and domain wall motion in an adjacent ferromagnetic film.1,2 In these same materials, broken inversion symmetry can lift the chiral degeneracy and generate new topological spin textures such as spin-spirals and skyrmions. We show for the first time that chiral ferromagnetism can persist at room temperature and can be engineered simply by appropriately designing interfaces between magnetic and nonmagnetic materials.2
I will then turn to the ferromagnet/oxide interface3-6 and describe our discovery of a new class of “magneto-ionic” materials,5,6 in which a gate voltage can be used to electrochemically switch the interfacial oxidation state to realize unprecedented control over magnetic properties. Here we use Pt/Co/Gd2O3- ultrathin film stacks, where Gd2O3- serves as an efficient oxygen ion conductor. I show that the magnetization in the thin Co layer can be switched between perpendicular and in-plane orientations, or quenched entirely, by driving O2- towards or away from the Co/GdOx interface with a small gate voltage.6 I then show that magneto-ionic gates can be used to locally tune magnetic properties and create a magnetic analog of field-effect transistors.5
Finally, I will describe emerging nanomagnetic devices that utilize these and other effects for memory,3,4 logic,5 and biosensing,7 and assess the progress and outlook for a variety of applications.

Biography: Geoffrey Beach is an Associate Professor of Materials Science and Engineering at MIT. He received a B.S. in Physics from Caltech, a Ph.D. in Physics from the University of California, San Diego, and conducted postdoctoral work at the University of Texas at Austin. At MIT, Prof. Beach has established the Laboratory for Nanomagnetism and Spin Dynamics, which designs advanced materials for spin-based memory, logic, and biomedical applications. His work has been recognized with numerous awards including most recently a Deshpande Center Award for Technological Innovation, the MIT Junior Bose Award for Excellence in Teaching, the MIT Class of 1958 Institute Chaired Professorship, and the Department of Energy (DoE) Early Career Award.

1. S. Emori, D. Bono, and G. S. D. Beach, Appl. Phys. Lett. 101, 042405 (2012).
2. S. Emori, U. Bauer, S.-M. Ahn, E. Martinez, and G. S. D. Beach, Nature Materials 12, 611 (2013).
3. U. Bauer, M. Przybylski, J. Kirschner, and G. S. D. Beach, Nano Lett. 12, 1437 (2012).
4. U. Bauer, S. Emori, and G. S. D. Beach, Appl. Phys. Lett. 100, 192408; ibid 101, 172403 (2012).
5. U. Bauer, S. Emori, and G. S. D. Beach, Nature Nanotechnology 8, 411 (2013).
6. U. Bauer, L. Yao, S. Emori, H. L. Tuller, S. van Dijken, and G. S. D. Beach, under review; arXiv:1409.1843 (2014).
7. E. Rapoport, D. Montana, and G.S.D. Beach, Lab Chip. 12, 4433-4440 (2012).

Description:

The Department of Materials Science and Engineering welcomes you to its 2014 Fall Colloquium Series.

Location: Tech L361, 4:00pm

Carelyn Campbell
National Institute of Standards and Technology (NIST)

Description:

The Department of Materials Science and Engineering welcomes you to its 2014 Fall Colloquium Series.

Location: Tech L361, 4:00pm

Ibrahim Karaman
Texas A&M University

Description:

The Department of Materials Science and Engineering welcomes you to its 2014 Fall Colloquium Series.

Location: Tech L361, 4:00pm

Marcus Young
University of North Texas