College of Arts and Sciences

New Views of the Universe

I will discuss how progress in cosmology over the past decade has improved our understanding of dark matter, dark energy, and the physics of the early universe. I will particularly concentrate on the developments in mapping out the expansion rate of the universe and the growth of density fluctuations in order to better understand dark energy and, eventually, identify the physics responsible for universe’s accelerated expansion.  The talk will provide basic background and discuss exciting new developments at a level accessible to graduate students.

Continue reading… Dragan Huterer (Univ Michigan)

Mechanism of Aluminum-Oxide Scale Formation on some High-Temperature Structural Alloys

The formation of Al2O3 scale on high-temperature structural alloys is a subject of immense technological importance, as well as of considerable scientific interest. Contrary to much current thinking in the field, the kinetics of scale growth appear to be controlled by the electrical conductivity of the scales, rather than solely by the diffusion of aluminum and oxygen at grain boundaries. Considerations of band structure thus become of major importance. The atomic structures and the electronic density of states were computed for a group of bi-crystal boundaries using density-functional theory.

Continue reading… A.H. Heuer (CWRU Materials Science and Engineering)

2D Materials: from Semiconductors to Topological Insulators

Abstract: Since the first isolation of one-atom thick graphene, research on two-dimensional (2D) materials with layered crystal structure has exploded over the past decade. One focal point in the recent studies of 2D materials beyond graphene is the development of metal chalcogenides (e.g. MoS2) as 2D semiconductors. In this talk, I will first highlight our exploration of non-transition metal chalcogenides InSe and SnS for future 2D semiconductor applications. While multilayer InSe is demonstrated to be a promising new 2D semiconductor for high performance n-type transistor devices, SnS’s intrinsic p-type nature leads to both opportunities and challenges in p-type semiconductor device applications.

Continue reading… Xuan Gao (CWRU Physics)

Gelation of Particles with Short-ranged Attraction

Nanoscale or colloidal particles are exceptionally important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behavior to a wide variety of complex fluids, from yoghurt to cast ceramics. This behavior arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA), long-understood as a purely kinetic phenomenon,

Continue reading… Peter Lu (Harvard University)

Lecture co-sponsored by the departments of Physics and Art History, the Baker-Nord Center for the Humanities, and the Cleveland Museum of Art. Note unusual time and venue.

The conventional view holds that geometric star-and-polygon patterns in medieval Islamic architecture were designed using a straightedge and a compass. Peter Lu, a research associate at Harvard University, will present his findings that, instead, a wide variety of patterns with five- and ten-fold symmetry were conceived as tessellations of specific decorated puzzles pieces, called girih tiles, that appear in medieval Islamic architectural scrolls. Beginning in the 12th century, patterns designed with these girih tiles appeared throughout the Islamic world,

Continue reading… Peter Lu (Harvard University) (Not a Colloquium but of related interest)

Majorana Materializes 

In 1937 Ettore Majorana introduced the concept of what are now fittingly called Majorana fermions — fermionic particles that are their own antiparticles. Nowadays an active search for condensed-matter analogues of these elusive objects is well underway, motivated by both the prospect of revealing new facets of quantum mechanics and longer-term quantum computing applications. This talk will survey recent advances in this pursuit. In particular, I will describe strategies for “engineering” Majorana platforms from simple building blocks, preliminary experimental successes, and future milestones that reveal foundational aspects of Majorana physics directly relevant for quantum computation.

Continue reading… Jason Alicea (Caltech)

Continue reading… No colloquium this week

Pure & Poetic: Butterfly in the Quantum World

The Hofstadter butterfly is a fascinating two-dimensional spectral landscape – a graph of the allowed energies of an electron in a two-dimensional crystal in a magnetic field. It is a quantum fractal made up of integers, describing topological states of matter known as the integer quantum Hall states. My butterfly story tells the tale of its discovery by a graduate student named Douglas Hofstadter and discusses its number theoretical, geometrical and topological aspects [1]. I will describe how the integers of the butterfly are convoluted in the Pythagorean triplets and the integer curvature of Apollonian gaskets,

Continue reading… Indu Satija (George Mason University)

Galaxy Clustering and the Galaxy-Halo Connection

In the contemporary view of the Universe, galaxies form and evolve in dark matter halos.  Modern galaxy surveys, most notably the Sloan Digital Sky Survey, have transformed the study of large-scale structure enabling detailed measurements of the spatial distribution of galaxies. I will discuss how we interpret these measurements using contemporary models of galaxy clustering which elucidate the relation between galaxies and dark matter halos. I will further describe one of the main challenges currently facing such analyses and present new results for the dependence of the galaxy content of halos on the assembly history of their host halos.

Continue reading… Idit Zehavi (CWRU, Astronomy)

Correlated Nanoelectronics 

The study of strongly correlated electronic systems and the development of quantum transport in nanoelectronic devices have followed distinct, mostly non-overlapping paths.  Electronic correlations of complex materials lead to emergent properties such as superconductivity, magnetism, and Mott insulator phases.  Nanoelectronics generally starts with far simpler materials (e.g., carbon-based or semiconductors) and derives functionality from doping and spatial confinement to two or fewer spatial dimensions.  In the last decade, these two fields have begun to overlap.  The development of new growth techniques for complex oxides have enabled new families of heterostructures which can be electrostatically gated between insulating,

Continue reading… Jeremy Levy (Univ Pittsburgh)

Quantized electro-dynamical responses in topological materials

Although solid-state systems are usually considered “dirty” with impurities and imperfections, it is still the case that macroscopic, quantized phenomena can be observed in the form of the Josephson effect in superconductors and the quantum Hall effect in 2DEG. Combinations of these measurements allow you to determine Planck’s constant and the fundamental charge in a solid-state setting. In my talk, I will show you the observation of a new quantized response in units of the fine structure constant in a new class of material so called “topological insulators” (Tis). First,

Continue reading… Liang Wu (Berkeley); Michelson Postdoctoral Prize Lecture

History, Geometry and the Future of Mobility

For over a century the personal automobile has served as a highly adaptable transportation tool and an aspirational symbol of wealth and freedom.  However, two megatrends would appear to spell its doom:  climate change with the recognition of the need to reduce CO2 emissions, and urbanization with the unprecedented size and density of new emerging megacities where significant vehicle ownership would result in ‘total gridlock’.  Surprisingly, both of these are actually questions of geometry – and a little physics – informed by the history of cities in the developed world. 

Continue reading… Mike Tamor (Ford Research)

Quantum valley Hall kink states and valleytronics in bilayer graphene

Conventional field effect transistors control current transmission by controlling the charge of carriers. The advent of two-dimensional materials with hexagonal crystal symmetry offers a new electronic degree of freedom, namely valley, the manipulation and detection of which could potentially be exploited to form new many-body ground states as well as new paradigms of electronic applications. In this talk, I will describe our work in creating valley-momentum locked quantum wires, namely quantum valley Hall kink states, in Bernal stacked bilayer graphene and show the operations of a waveguide,

Continue reading… Jun Zhu (Penn State)