College of Arts and Sciences

The optical properties and electronic structure of materials are critical to the development of new optical materials,(1) novel processes of nanoscale assembly, and the viability of advanced energy technologies. They are the origin of the electrodynamic van der Waals-London dispersion (vdW-Ld) interactions (2) which play a universal role in wetting, interfacial energies, and nanoscale assembly.(3) The challenge of nanotechnology is for science to span more than nine orders of magnitude in dimension. Advanced energy technologies, with their 25 or 50 year capital lifetimes, challenge us to span 24 orders of magnitude in time so as to control degradation processes, damage accumulation,

Continue reading… Optical Material Science: Electrodynamics of Nanoscale Assembly, and Lifetime and Degradation Science for Photovoltaics – Roger H. French

We present measurements of the magnetoresistivity of a weakly interacting 2D electron liquid in an unexplored region near the boundary of the 2D electron gas supported by a liquid helium surface. The magnetoresistivity is calculated by Dykman in the self-consistent Born approximation. For fields greater than a field B0, the magnetoresistivity is proportional to (muB)^3/2, where mu is the mobility. Electron-electron interactions cause a crossover to the Drude behavior as the density is increased. All of our data scale with the density-dependent parameter B0 as B/B0, with the magnitude of the magnetoresistivity scaled as 1/n. For low electron densities and fields less that B0,

Continue reading… Magnetoresistance in Two Dimensions – Arnold J. Dahm

Understanding the physics of structurally disordered materials is a challenge to experimentalists and theorists alike. In this talk, I discuss the character of electronic states in disordered materials and emphasize the interplay between structure and electronic properties. I begin by discussing the consequences of atomic structural disorder on electron states. As shown long ago by Anderson, disorder in atomic coordinates creates spatially confined or “localized” electron eigenstates near the Fermi level. I explore these states with large and realistic structural models and suitable electronic structure techniques. I begin with the structure of electron states in large and realistic models of a-Si,

Continue reading… Electronic structure of disordered solids – David A. Drabold

Thirty years after its initial discovery, the fractional quantum Hall effect continues to challenge our understanding of electronic correlations in low dimensions. Throughout this history advances in molecular beam epitaxy (MBE) have played an important role. Presently, the fragile v=5/2 fractional quantum Hall state is the subject of intense scrutiny. It is theoretically conjectured that the v=5/2 state is described by the Moore-Read Pfaffian wavefunction, possessing excitations obeying non-Abelian braiding statistics. If experimentally confirmed, excitations with non-Abelian braiding statistics may provide a platform for proposed schemes of topologically-protected quantum computing. While there are many aspects to the physics at v=5/2,

Continue reading… The role of molecular beam epitaxy in fundamental physics through an example: assessing the impact of disorder on the v=5/2 fractional quantum Hall effect – Mike Manfra

This seminar will give an overview of micro and nano technologies at the University of Michigan Lurie Nanofabrication Facility (LNF). In addition, we will present examples of research accomplishments and applications of these technologies in diverse fields including but not limited to Electrical Engineering, Physics, Life Sciences, Biomedical Engineering and Chemical Engineering. Operated by the University of Michigan Solid-State Electronics Laboratory (SSEL), the LNF has extensive experience in microelectronics, micromechanics, optoelectronics, and micro and nano technologies based on silicon, compound semiconductor, and organic materials. It offers a complete laboratory for the fabrication of nanofabricated semiconductor and polymer electronic and optoelectronic devices and circuits,

Continue reading… Micro and Nano Technology at the Lurie Nanofabrication Facility – Robert Hower

Since the mid 1980’s there have been predictions of protein structural vibrations with ~ 1meV energies, which corresponds to the terahertz frequency range. These large scale motions involve the correlated movement of many atoms and are associated with the conformational motions involved in protein function. There have been many attempts to measure these modes, but the energy range overlaps with that of local librational motions of the surface side chains and the solvent, and these contributions give rise to a strong glass-like response. In this talk I will discuss our development of a technique to isolate the large scale structural contribution from a glassy background called Modulated Orientation Sensitive Terahertz Spectroscopy (MOSTS).

Continue reading… Anisotropic response in molecular crystals and the development of Modulated Orientation Sensitive Terahertz Spectroscopy (MOSTS) – Andrea Markelz

Energy efficiency and renewable energy technologies have significant importance for achieving sustainable energy systems in modern society. Lighting accounts for more than 22% of the total electrical energy usage in US, and technologies based on solid state lighting (SSL) utilizing semiconductor-based material has tremendous promise to replace the existing lighting devices. As compared to traditional incandescent and fluorescent lamps, SSL is more energy-efficient, reliable, and environmentally-friendly. Once widely used, SSL could lead to the decrease of worldwide electricity consumption for lighting by >50% and reduces total electricity consumption by >10%. The U.S. Department of Energy describes SSL as a pivotal emerging technology that promises to fundamentally alter lighting in the future.

Continue reading… III-Nitride Light-Emitting Diodes for Solid-State Lighting – Hongping Zhao

HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating. Our transport data provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator.

Continue reading… HgTe as a Topological Insulator – Laurens Molenkamp

25 years after their discovery, the microscopic problem of high Tc superconductivity in cuprates is still not “solved”. I will focus on summarizing the experiments that show us that the observed phases, with varying carrier concentration, challenge three paradigms of 20th century condensed matter physics. (i) The parent Mott insulator cannot be understood within band theory; (ii) the superconducting state and phase transition force us to go beyond a BCS mean-field description; and (iii) the “normal” metallic state cannot be described within Landau Fermi liquid theory. I will then briefly describe some of the success in theoretically understanding the superconducting state and indicate open questions about the normal state.

Continue reading… High Tc superconductivity in cuprates: A status report – Mohit Randeria

Optomechanical systems couple light stored in an optical resonant cavity to the motion of a mechanical motion of the cavity walls. Single optomechanical cells have been successfully fabricated in a wide variety of systems. Recent experiments have further demonstrated setups, such as photonic crystal structures, that in principle allow to confine several optical and vibrational modes on a single chip. In the first part of my presentation I will demonstrate the emergence of a robust, long-living and highly non-classical mechanical state in a standard single cell optomechanical setup. I will show that under some parameters, the longtime steady state of the mechanical degrees of freedom has significantly negative Wigner density.

Continue reading… Quantum Signatures of Optomechanical Instability and Synchronization in Optomechanical Arrays – Jiang Qian

The 2008 discovery of high temperature superconductivity in doped LaFeAsO by Kamihara and co-workers provided the second class of high Tc materials, the other being the cuprate family discovered in 1986 by Bednorz and Mueller. This discovery was revolutionary in that many of the properties of the iron based superconductors are radically different from those of the cuprates, apparently requiring a new and broader understanding of the physics of high temperature superconductivity. The purpose of this talk is to discuss the chemistry and physics of the new superconductors in relation to cuprates. So far, many puzzles remain. The materials appear to be much more band-like and show much stronger signatures of metallic (Fermi surface related) physics than cuprates,

Continue reading… Fe pnictide superconductors – David Singh

Nanoscience today enables many fascinating low-dimensional structures and new materials with previously inaccessible properties. Nanostructures with mechanical degrees of freedom offer compelling characteristics that make them interesting for both fundamental studies and technological applications. This talk will describe my collaborative research efforts in exploring vibrating nanowires, and in engineering these very thin nanowires into functional and high-performance nanoscale electromechanical systems (NEMS). I will show NEMS resonators operating in the very-high and ultra-high frequency (VHF/UHF, 30MHz – 3GHz) ranges, based on silicon nanowires enabled by a hybrid bottom-up/top-down process. Exploiting the interesting properties of thin silicon nanowires, we have developed an all-electronic,

Continue reading… The Incredible Shrinking Tuning Forks – Nanowire Electromechanical Systems at Radio and Microwave Frequencies – Philp Feng