College of Arts and Sciences

Graphene is a two-dimensional material with conical bands that touch at the Dirac or Charge-Neutrality point. Its zero bandgap and atomically thin body allow it to switch between n-type and p-type conduction when assembled into a field-effect transistor geometry. The current modulation however is limited due to a finite minimum conductivity at the Charge Neutrality point, which prevents us from using graphene for digital electronic applications. We therefore investigate graphene as an optical and analog electronic material, where the low on-off ratio is less of a problem. Especially the high frequency (RF) electronic applications are promising since graphene can be gated efficiently and has high carrier mobility.

Continue reading… Graphene Optics and Electronics – Marcus Freitag

In the first part, I will present a novel strategy for processing of colloidally stable semiconductor nanoparticles (also known as nanocrystals or quantum dots) into all-inorganic solid films, deployable for photovoltaic applications. The method relies on encapsulation of semiconductor nanocrystal arrays within a matrix of a wide-band gap inorganic material, which preserves optoelectronic properties of individual nanoparticles, yet, renders the nanocrystal film photoconductive. The photovoltaic performance of fabricated nanocrystal solids is demonstrated through the development of prototype solar cells exhibiting stable and efficient operation in ambient conditions. The second part of the presentation will focus on ultrafast electron processes taking place in heterostructured nanocrystals comprising metal (Au) and semiconductor (CdS) material domains.

Continue reading… Charge carrier dynamics in heterostructured semiconductor nanocrystals and nanocrystal solids – Michail Zamkov

I present two recent projects in theoretical ecology and point out the connections to math loved by physicists. The first concerns life in a variable environment: when should an organism buffer itself against environmental variation and when should it try to take advantage of environmental variation? The second concerns the viability of mussel populations in a marine reserve network when shifting ocean currents cause fluctuating dispersal between reserves.

Continue reading… A physicist walks into a biology department… – Robin Snyder

The exploration of topological phases of matter is one of the main challenges in condensed matter physics. Among the exciting recent developments in this direction are the discoveries of the new phases of matter with many intriguing properties such as topological insulators and superconductors. In my talk, I will focus on topological superconductors and discuss how to realize spinless p-wave superconductivity in semiconductor/superconductor heterostructures. I will show that such a non-trivial topological state emerging at the interface supports zero-energy modes that can be occupied by Majorana fermions. These quasi-particles, which are exotic in the sense that they are at the same time their own antiparticles,

Continue reading… The search for Majorana Fermions in semiconductor nanowires – Roman Lutchyn

Effective configurational and spin Hamiltonians are commonly used to study magnetic and structural thermodynamics. For some purposes, such as the description of phase transitions in substitutional alloys, they can be routinely constructed by high-throughput first-principles calculations. As an illustration of this standard approach, I will describe the calculation of the phase diagrams of Gd-doped EuO and EuS using the cluster expansion technique [1]. Many problems, however, require physical insight for the selection of the relevant degrees of freedom and for an adequate representation of their interactions. The main focus of this talk will be on such problems, including the structural phase transitions at the Cr2O3 (0001) surface and the magnetic thermodynamics of the parent compounds of ferropnictide superconductors [2].

Continue reading… Theoretical studies of magnetic and structural thermodynamics using effective Hamiltonians – Kirill Belashchenko

Holography is a technique commonly used to display objects in three-dimensions, as it has the potential to accurately reproduce all features of the light from a real object. Holographic telepresence has been a compelling fantasy for decades, but modern science has failed to deliver such a system, primarily due to the computational power required and the lack of a suitable recording material. I will discuss my graduate work at the University of Arizona on the use of organic photorefractive polymers as a medium for updatable 3D holographic displays. These exhibit a reversible index change in response to light, and the wavelength sensitivity can be modified using different chromophores,

Continue reading… Photorefractive Polymers for an Updatable Holographic Display – Cory Christenson

Because of the crucial roles of point defects in the physical properties of pristine and doped oxide semiconductors, a fair amount of experimental research has been devoted to their characterization in previous decades. However, the understanding of the defects is limited, particularly at the atomistic and electronic level. A density functional approach is useful for the study of the defects and has provided various insights into their characteristics. In this talk, I will present our recent results on the defects in several oxide semiconductors, ZnO [1], SrTiO3 [2], BaTiO3 [3], and SnOx [4, 5], obtained using semilocal and hybrid density functional calculations.

Continue reading… Energetics and Electronic Structure of Point Defects in Oxide Semiconductors: A Density Functional Approach – Fumiyasu Oba

The two dimensional kagome lattice is a highly frustrated spin system. When spins are placed on the vertices of the lattice with an antiferromagnetic interaction, there is no unique classical ground state. The large degeneracy of classical configurations with the same energy appear to give rise to an unusual quantum ground state. In this talk, I will discuss theoretical attempts to understand the ground state of the antiferromagnetic Heisenberg model on the kagome lattice. In the first portion of the talk, I will review several theoretical proposals for the ground state put forth over the past two decades. These fall into two basic categories: spin liquids and valence bond crystals.

Continue reading… Variational Studies on the Kagome Lattice – Jesse Kinder

There has been intense interest in recent years to control the electronic structure in quasi one-dimensional nanowires through the fabrication of novel axial and radial heterostructures. Unlike materials in higher dimensions, nanowires have the unique ability to grow axial or radial heterostructures between almost any two materials regardless of lattice mismatch or strain. Understanding exactly how the electronic properties of the nanowire are changed through this control is extremely important and requires spectroscopies with high spatial, temporal and spectral resolution. I will discuss a number of examples in which the electronic structure in nanowire heterostructures can be modified either through strain,

Continue reading… Measuring the electronic properties of single semiconductor nanowire heterostructures using advanced optical spectroscopies – Leigh M. Smith

Many proposed spin-based devices require transfer of spin into non-magnetic materials, which is usually accomplished by driving a charge current from a ferromagnet into a non-magnetic material. Heat can also be used to transfer spins into non-magnetic material using the spin-Seebeck effect, as demonstrated by Uchida et al. in permalloy[1]. We also observed this in GaMnAs [2], a ferromagnetic semiconductor. A different orientation of spin is injected into platinum bars on the hot side of the sample as compared to the cold side, and this spatial distribution of spin currents is unaffected by electrical breaks highlighting that the effect is driven by phonons in the substrate.

Continue reading… Moving spins with heat: spin-Seebeck effect in a ferromagnetic semiconductor and Polarization-induced pn-junctions in wide band gap semiconductor nanowires – Roberto Myers

As the world strives to reduce its reliance on fossil fuels, materials innovations can help catalyze the switch to renewable energy and the engineering of energy-efficient devices. Powered by modern high-performance computers, s first-principles methods can provide an understanding of fundamental materials processes at the microscopic level and play an important role in the development of novel energy materials and devices. In this talk, I will present insights garnered from first-principles calculations for the study of the performance of optoelectronic devices for energy. I will discuss the loss by non-radiative recombination in nitride LED light bulbs, the internal reabsorption of light and loss in nitride green lasers and transparent conductors,

Continue reading… First-principles electronic structure calculations in energy research – Emmanouil (Manos) Kioupakis

The origin of the abrupt metal-insulator transition (MIT) in VO2 has been a subject of debate for several decades and remains an important problem for condensed matter physics. The change from high temperature metallic rutile phase to low temperature insulating monoclinic occurs abruptly at 360 K for bulk VO2. The origin of the MIT, whether structural (i.e. Peierls-like instability due to V-V dimerizing and tilting along the cR axis) or electronic (i.e. Mott-Hubbard transition due to strong electron correlation effects) or some combination of the two still remains a matter of debate. Recent advances in the growth of VO2 compounds have provided an opportunity to really examine this system.

Continue reading… The metal insulator transition of VO2: Shining new (synchrotron-based) light on an old problem – Louis Piper