# Seminar Tuesday May 19 – Neil Johnson

Speaker: Neil Johnson
Location: Physics 127
Time: 3:30 May 19 2015

Abstract: Graphene is one of the most celebrated and intensively studied materials of the last decade. Its potential uses include transparent conductors, filtration, optoelectronics, photovoltaics and as a semiconductor for electronic devices. However, its implementation as a semiconductor is hampered by its utter refusal to stop conducting, even in the absence of an external field or doping. This has a deleterious effect on the ON/OFF ratios of graphene-based devices, and as such none have ever been made that are able to outperform current bulk silicon devices.

Silicene, the silicon-based analogue to graphene, is expected to have a number of significant advantages over its carbon-based cousin. It should have a tuneable bandgap, an ambipolar field effect and possibly even spin-polarization all from the application of an external gating voltage. Clearly, silicene is much, much better than graphene could ever hope to be. However, one slight, minor stumbling block in the implementation of freestanding monolayers of silicene is that nobody has been able to make them yet, and it’s been questioned whether or not they can actually exist. Instead, silicene has only been observed while stuck to growth substrates, and until recently it was unclear how this would affect the properties of the material. In this seminar I will briefly outline the history of 2D materials, the development of silicene and my research into silicene grown on the Ag(111) surface.

# Seminar Thursday Aug 14 – Stephanie Goertzen

Speaker: Stephanie Goertzen
Location: Physics 175
Time: 3:30, August 14 2014

Abstract: Recently, the discovery of a new class of materials called topological insulators has stimulated much excitement in the field of condensed matter physics. Their discovery has also begun the search for the exotic topological superconductor – a superconductor whose properties are protected by topology. A novel feature of such topological superconductors is the possible existence of the Majorana fermion as an elementary excitation. Traditionally, the Hamiltonian of the system is directly diagonalized in order to calculate the mean-field potentials. However this method is very computationally expensive and is widely regarded as impractical. As such, we make use of two efficient numerical algorithms that have recently been developed in order to self-consistently solve the Bogoliubov-de Gennes equations. The first is Chebyshev polynomial expansion which allows us to bypass directly diagonalizing the Hamiltonian and the second is the Sakurai-Sugiura (SS) method for efficient computation of eigenvalues and eigenvectors. Armed with this combination of techniques, we aim to investigate two-dimensional topological superconductivity.

# Seminar Thursday Aug 07 – Jason Ho

Speaker: Jason Ho
Location: Physics 175
Time: 3:30pm, August 07 2014

Title: QCD sum-rules analysis of open-charm hybrid mesons

Abstract: We briefly discuss the development of quantum chromodynamics (QCD) and introduce the concept of hadronic structures outside the quark model. Within this group of unconventional hadrons, of interest to us are hybrid mesons consisting of a quark-antiquark pair and a constituent gluon.
Through the use of QCD sum-rules, we intend to calculate the ground state mass of the $\dpi{300}\inline D$ (charm quantum number $\dpi{300}\inline c = \pm 1$) and $\dpi{300}\inline D_s$ (charm/strange quantum numbers $\dpi{300}\inline c = s = \pm 1$) hybrid mesons with exotic $\dpi{300}\inline J^{PC} = 1^{-+}$ (spin $\dpi{300}\inline J$, parity $\dpi{300}\inline P$, and charge conjugation $\dpi{300}\inline C$), and present preliminary results.

# Seminar Thursday July 17, 2014

## Seminar Thursday July 17, 2014

Speaker: Evan Smith
Location: Physics 175
Time: 3:30, July 17 2014

Abstract: Over one hundred years ago, Dutch physicist Heike Kamerlingh Onnes discovered the phenomena of superconductivity, a property wherein some materials exhibit exactly zero electrical resistance under specific conditions. The pursuit of broadening our knowledge on this peculiar phenomena has lead the scientific community to discovering new classes of materials. Some of these materials exhibit a property known as topological superconductivity. This theoretically implies the existence of Majorana fermions, which if found to exist, hold significant promise to several areas of condensed matter and material science research. Majorana fermions allow for the most theoretically stable form of quantum computation – their existence truly has the potential to usher in a new paradigm of computation and catapult society into a new era of technology.

The goal of my research is two-fold: examining the effects of various lattice inhomogeneities on crystal structure, and gaining insight into the properties of Majorana fermions and how they might exist within topological superconductors. It is necessary to understand how the creation, stabilization, and manipulation of Majorana fermions is achieved in order to evolve into this new era. At the same time, it is fundamental that we understand how a set of inhomogeneous regions influences the properties of the elusive Majorna fermion, and how we might use that knowledge to our benefit.

We make use of the Bogoliubov de-Gennes theory, a generalization of the more conventional and well-known BCS theory, and the extended Hubbard model to obtain a low-energy effective Hamiltonian which is then used to investigate the theoretical zero-energy Andreev bound states of Majorana fermions and inhomogeneities. We utilize a set of recently adapted and developed computationally efficient algorithms along with highly parallel programming to achieve our goals.

# Predicted novel crystal structures of Xe-Clx (x=1, 2 and 4) by ASAP and CALYPSO

Speaker: Niloofar Zarifi
Location: Physics 175
Time: 3:30,  June 4th, 2014

The presentation can be viewed here.

Abstract: Noble gases are elements with close shell configurations and not expected to form chemical bonding easily except with electronegative halogen atoms. Therefore, these halide compounds have attracted great attentions due to the importance for scientific interest. Here, we performed extensive structure searches on XeClx (x=1, 2 and 4) under high pressure using the GA and PSO methodology. Several novel crystal structures were uncovered for the first time at high pressures. Our calculations confirm existence of molecular Cl2 and Xe atoms in crystalline structures for XeCl. This compound remains an insulator up to 60GPa. The high pressure phase transition has been widely studied for XeCl2. Electronic calculations predicted their metallic property up to 80 GPa. For XeCl4 the metallic state exists at lower pressure.

Our results provide new understanding of novel crystalline structure of Xe compounds at high pressure.

# Crystal structure prediction via evolutionary algorithm

Who (supervisor): Niloofar Zarifi (John Tse)
Venue: PEGASUS Student Summer Seminar, Aug 23, 2012.
Abstract: The ability to predict the structure of a solid presents a challenge to the chemistry, physics, and material science communities. The difficulty of crystal prediction comes from the complexity of the potential energy landscape of a solid, which depends on many variables like unit cell parameters. Moreover, it may not be known how many atoms comprise the primitive unit cell. Particle swarm optimization and genetic algorithm are two powerful techniques to structure determination. These two methods are extremely successful to predict stable or metastable structures at given extreme condition.

# Computational Studies on Alternative Materials for Lithium-Ion Batteries

Who (supervisor): Jianjun Yang (John Tse)
Venue: PEGASUS Student Summer Seminar, Aug 16, 2012.
Abstract: Lithium ion batteries are widely used in portable electronics because of their high energy density and low weight, and they are currently intensively investigated for more demanding applications such as electrical vehicles and for the stationary power backup systems. To improve its performance, computational modeling has become an essential component in design of new battery materials in addition to the physical work in laboratories. This talk will outline the major challenges of the current Li-ion batteries and summarize several successful computations on the alternative materials.  My own research work on the Li-ion battery materials will be introduced at the end.

# Band gap investigation of photocatalytic energy materials

Who (supervisor): Eamon McDermott (Alex Moewes)
Venue: PEGASUS Student Summer Seminar, June 14, 2012.
Abstract: Eamon will give a brief overview of his research into the near-band gap electronic structure of energy materials, including semiconductor photocatalysts and lithium organics for use in alloying battery electrodes