Thursday, March 19, 2015

JMU Students at a National Conference! - The APS March Meeting

The week before Spring Break, eight JMU students had the opportunity to present their research in San Antonio, TX at the APS March meeting. Coming from the labs of several professors with a variety of research interests, these students either gave talks or presented posters of their research projects. From the lab of Klebert Feitosa there was Keely Criddle (yours truly) and Seth Heerschap. From the lab of Jason Haraldsen there was Nikki Creange, Brock Crook, Greg Houchins, and Galen Richard. From the lab of Chris Hughes there was Kathleen Krist. And from the lab of Brian Utter there was George Wilkes. Below are pictures of these students (in the order the are listed above) as well as the title and abstracts of the projects they presented. Unfortunately there is no picture of George Wilkes, so beside his abstract you will find a picture of the exhibit hall in which his poster was hung.

Mechanical Properties of Hydrogel Beads

Fragile solids made of dense disordered packing of bubbles, droplets, and grains are able to withstand small stresses by virtue of system-wide force chains that lock the system into a jammed state.  The nature of the jamming transition in such soft materials has been the subject of intense research, but despite much effort, a deep understanding remains elusive. In this experiment we study the mechanical properties of hydrogel beads to exploit them as force transducers in densely packed systems. The experiment consists of applying uniaxial planar compressions on the beads and correlating the force to the bead’s strain and contact area.  The results show that while the strain scales linearly with the radius of the contact area, the force and strain are found to obey a  power law relation with a range of exponents from 1.9 to 2.7. This result leads to a power law dependence of the force on the contact area radius of the compressed beads of similar size. 
 Aqueous Foam Stabilized by Tricationic Amphiphilic Surfactants 

The unique surface properties of amphiphilic molecules have made them widely used in applications where foaming, emulsifying or coating processes are needed. The development of novel architectures with multi-cephalic/tailed molecules have enhanced their anti-bacterial activity in connection with tail length and the nature of the head group. Here we report on the foamability of two triple head double, tail cationic surfactants (M-1,14,14, M-P, 14,14) and a triple head single tail cationic surfactant (M-1,1,14) and compare them with commercially available single headed, single tailed anionic and cationic surfactants (SDS,CTAB and DTAB). The results show that bubble rupture rate decrease with the length of the carbon chain irrespective of head structure. The growth rate of bubbles with short tailed surfactants (SDS) and longer, single tailed tricationic surfactants (M-1,1,14) was shown to be twice as high as those with longer tailed surfactants (CTAB, M-P,14,14, M-1,14,14). This fact was related to the size variation of bubbles, where the foams made with short tail surfactants exhibited higher polydispersivity than those with short tails. This suggests that foams with tricationic amphiphilics are closed linked to their tail length and generally insensitive to their head structure.

Understanding the optical and electronic properties of Ga-doped graphene

We simulate the optical and electrical responses in gallium-doped graphene, using density functional theory with a local density approximation. We show the effects of impurity doping (0-3.91\%) in the graphene sheet and for each doping percentage the change in electron density, refractive index, and optical conductivity are reported. Here, gallium atoms are placed randomly (using a 5-point average) throughout a 128-atom sheet of graphene. These calculations demonstrate the effects of hole doping due to direct atomic substitution, where we find a disruption in the electron density for small doping levels, which is due to impurity scattering of the electrons. However, there seems to be a doping percentage, above which we have calculated, at which the system transitions to produce metallic or semi-metallic behavior. These calculations are compared to a purely theoretical 100\% Ga sheet for comparison of conductivity. Furthermore, we examine the change in the electronic band structure and density of states, where the introduction of gallium electronic bands produces a shift in the electron bands and dissolves the characteristic Dirac cone within graphene.

Determination of superexchange correlations in magnetically substituted graphene

 We investigate the electronic and magnetic properties between two homogeneous magnetic impurities (vanadium, chromium, or manganese) in a 128-atom graphene superlattice. With varying the impurity distance, we calculate these properties using a first principles approach. For each configuration, we determine the electronic bandstructure and density of states, along with the Mullikan populations for each atom. Furthermore, we calculate the exchange parameter between the two magnetic ions through the analysis of the change in total energy for different magnetic configurations. We found that the magnetic impurities induce a mangetic moment in the graphene superlattice, helping to meditate the superexchange between the impurities. Depending on the choice of ion used, the interactions between the two ions can exhibit either a ferromagnetic or an antiferromagnetic behavior. These correlations indicate an RKKY-like behavior in the system.

Generalization of Magnetic Dimer Excitations

Magnetic dimers commonly appear in the study of molecular magnets and quantum dots. Here, we discuss analytical representations for the inelastic neutron scattering excitation cross sections and static structure factor for the general S1S2 dimeric system. Employing generalized Pauli matrices and the Kronecker tensor product to construct the matrix representation of the spin Heisenberg spin-spin Hamiltonian. After using exact diagonalization to determine the eigenstates of the spin Hamiltonian, we formulated an analytical solution to find the structure factor coefficients used in determining the inelastic neutron scattering excitation cross section from both the ground state and first excited state. We also detail a method for finding the Sz polarization constant within an applied field that may represent the presence of an external magnetic field. Furthermore, we provide a sample set of data and intensity plot generated from our results to illustrate experimental representations for split energy levels.

Variational calculations for spin canting at ferromagnetic/antiferromagnetic 


Understanding the complex interaction between materials is critical for the development of spintronic and electronic devices in the technology industry. In this report, we examine the canting of local moments throughout a ferromagnetic/antiferromagnetic heterostructure, where a combination of interlayer mixing and orbital reconstruction can be described as a local exchange field at the interface. Using a variational method and semi-classical approach, we examine the canting of spins throughout the full multilayer heterostructure. We approximate the interlayer interactions as an effective field throughout the interface and apply a standard spin Hamiltonian with spin anisotropy for the intralayer interactions of the ferromagnetic and antiferromagnetic layers. Overall, we show that observed finite magnetization and rotation of the local moment observed in LSMO/BFO is due to the interface interactions. Furthermore, we predict a size limit for this effect in the antiferromagnetic (BFO) layer.

The Utilization of Chloroform Post-Treatment to Improve the Adhesion of Au Thin Films onto PMMA

The metallization of Au onto plastics is an important processing step in the fabrication of microfluidic devices. While its corrosion resistance and excellent electrical and thermal conductivity make Au a good choice, its inertness results in poor adhesion to polymer surfaces. Previous studies have indicated that exposing commercially available Poly(methyl methacrylate) (PMMA) sheets to chloroform vapor following Au deposition significantly improves adhesion. In this study, we deposited 6 nm of Au onto 1.50 mm thick PMMA and exposed the samples to vapor released from chloroform heated on a hot plate set at 70 C. The force required to remove the Au thin films was determined by placing samples on a polisher spinning at 150 rpm and utilizing UV-VIS spectroscopy to measure the transmittance of 700 nm light through the films to quantify their removal as a function of applied polishing force. The Au thin films were also characterized using AFM. AFM images demonstrated a progressive roughening of the surface corresponding to an increase in applied force. Additionally, these images support a model in which the chloroform treatment softens the PMMA surface, producing a softened layer that the polisher removes simultaneously with the Au thin film.

Granular gas mediated attraction of intruders in a granular Casimir effect

When two objects are submerged in a granular gas, entropic effects due to inelastic collisions lead to attractions between the objects. This has been referred to as an analog to the Casimir effect, though arises via a different mechanism. In this experiment, we place two objects (such as vertical plates or spheres) in either a strongly driven granular gas or dense fluid. We find that when the plates are closely spaced, there is a net attractive force. By analyzing high-speed video, we track the distance between these plates and characterize the effective force versus distance with changes in the vibration parameters and initial separation. A 2D simulation is also used to further explore parameter space.

-Keely Criddle
JMU Physics & Astronomy Blogger