Bulletin of the American Physical Society
Fall 2012 Meeting of the APS Ohio-Region Section
Volume 57, Number 6
Friday–Saturday, October 5–6, 2012; Detroit, Michigan
Session EB: Condensed Matter and Materials Physics |
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Chair: Zhixian Zhou, Wayne State University Room: McGregor Conference Center FG |
Saturday, October 6, 2012 8:00AM - 8:12AM |
EB.00001: Strained Islands Evolution during Thin Film Heteroepitaxy with Planar and Pre-patterned Substrates Champika Gigiriwala Gamage, Zhi-Feng Huang Evolution of strained islands or quantum dots is analyzed via constructing a continuum elasticity model based on the second order perturbation theory. The resulting nonlinear dynamic equation for film morphology also incorporates the film-substrate wetting effect. In order to study the formation and evolution of strained islands on a planar substrate for different film-substrate misfit strain, we analyze the time dependent behavior of the structure factor for surface heights, its moments and the surface roughness, and obtain the detailed dynamic process of island coarsening and stabilization. We also study the formation of these nano structures on a nonplanar patterned substrate. The properties of islands formed are highly affected and controlled by the periodicity and amplitude of the pre-defined substrate patterns, as shown in our analytic and numerical results. [Preview Abstract] |
Saturday, October 6, 2012 8:12AM - 8:24AM |
EB.00002: Effects of Electric Fields on Lamellar Structures of Liquid-Crystalline Block Copolymers Simiso Mkhonta, Zhi-Feng Huang, Ken Elder, Martin Grant We investigate the electrically induced lamellar contraction in microphase separated liquid-crystalline diblock copolymer using the phase field-crystal model. We demonstrate that collective rotations of the constituents liquid-crystal molecules relative to the layer normal can lead to unusually large changes of the lamellar spacing. We also demonstrate that the orientational order of the constituent liquid-crystals can accelerate microphase separation and can also lead to fast dislocation motion. Our results are directly relevant to the studies of electrically tunable block copolymer photonic crystals with a full color display. [Preview Abstract] |
Saturday, October 6, 2012 8:24AM - 8:36AM |
EB.00003: Characterization and magnetic properties of pure and transition metal doped ZnO thin films Ehab Abdelhamid, Debabrata Mishra, Wissam Fawaz, Boris Nadgorny, Gavin Lawes Zinc oxide (ZnO), a hexagonal n-type semiconductor, has been widely studied because of the ease of changing its properties by doping with selected elements. Although pure ZnO is paramagnetic, many studies confirm that doped ZnO thin films show room temperature ferromagnetism. This allows the potential to control the electrical properties of the semiconductor using magnetic fields. In this study, pure and Ti, V, and Co doped ZnO thin films were prepared on silicon substrates by spin coating. X-ray diffraction and Raman spectroscopy both confirm the formation of a single phase ZnO wurtzite structure. Magnetic measurements on the samples show magnetization for the as-prepared samples. The magnetization increases dramatically on vacuum annealing, which can be attributed to the incorporation of oxygen vacancy defects. Moreover, doping with Ti increases the saturation magnetization around five times as compared to pure ZnO, while the magnetization in the Co doped samples increases by 12 times. Finally, fitting the magnetization curves shows that the approach to saturation follows a 1/H dependence, suggesting the governance of defects rather than magnetocrystalline anisotropy. [Preview Abstract] |
Saturday, October 6, 2012 8:36AM - 8:48AM |
EB.00004: Pattering of Heteroepitaxial Overlayers from Nano to Micron Scales Fabio Iunes Sanches, Ken Elder, Giulia Rossi, Pekka Kanerva, See-Chenn Ying, Enzo Granato, Cristian V. Achim, Tapio Ala-Nissila Thin heteroepitaxial overlayers have been proposed as templates to generate stable, self-organized nanostructures at large length scales, with a variety of important technological applications. However, modeling strain-driven self-organization is a formidable challenge due to different length scales involved. In this talk a method for predicting the patterning of ultra thin films on micron length scales with atomic resolution will be presented. The model is used to make quantitative predictions for the type of superstructures (stripes, honeycomb, triangular) and length scale of pattern formation of two metal/metal systems, Cu on Ru(0001) and Cu on Pd(111). The findings are in excellent agreement with existing experiments and call for future experimental investigations of such systems. [Preview Abstract] |
Saturday, October 6, 2012 8:48AM - 9:00AM |
EB.00005: Percolation behavior in metallic-insulator composite systems and the filling factor near the percolation threshold Rupam Mukherjee, Debabrata Mishra, ZhiFeng Huang, Boris Nadgorny We investigate the percolation behavior in various composite metal -- insulator systems including LiCoO$_{2}$/ CrO$_{2}$, MgB$_{2}$/Al$_{2}$O$_{3}$, CrO$_{2}$/Al$_{2}$O$_{3,}$ CrO$_{2}$/ CaCO$_{3}$. The effect of particle size and shapes in these systems has been studied to better understand the geometrical phase transitions. The power law exponent around the percolation threshold has been found to be 2.0$\pm $0.04 in all the cases, which agrees well with the theoretical result. Interestingly, the filling factor of these composite systems also exhibits the power law dependence near the percolation threshold with the value found to be dependent on the shape of the insulating particle. The exponent ranges from 0.2 to 0.4 depending on size of particles of a given shape in the composite system. [Preview Abstract] |
Saturday, October 6, 2012 9:00AM - 9:12AM |
EB.00006: Magnetoresistance in ferromagnetic metal/ferromagnetic semiconductor micro-structures Taylor Reid, Andrei Sokolov, Robert Tolley, Justin Guenther, Xinyu Liu, Jacek Furdyna, Khalid Eid We use SQUID magnetometry and circular contacts to study the magnetoresistance in GaMnAs/Py bilayer structures. Our magnetization hysteresis loops show that there is no measurable coupling between the two ferromagnetic layers, even though they are not separated by any nano-magnetic spacer layers. Furthermore, the field-dependent magnetization shows a rich behavior that depends on the width of the GaMnAs in the circular structures. Samples with the narrowest gaps show a magnetoresistance effect that seems to be due to the tunneling magnetoresistance at the interface between py and GaMnAs. [Preview Abstract] |
Saturday, October 6, 2012 9:12AM - 9:24AM |
EB.00007: Andreev effect in GaMnAs/Nb microstructures for improved extraction of spin polarization Justin Guenther, Diana Dahliah, Taylor Reid, Robert Tolley, Chris Little, Xinyu Liu, Jacek Furdyna, Khalid Eid Point contact Andreev reflection is a powerful technique for extracting the spin polarization in a large variety of ferromagnetic materials. Yet, it produced conflicting data that proved difficult to model when studying spin polarization in GaMnAs, due in part to two main problems: the high resistivity of GaMnAs makes it difficult to isolate the interfacial conductance and characterize it properly and there can be a Schottky barrier at the GaMnAs/superconductor interface. We use photolithography to fabricate GaMnAs/Nb micro-structures that offer a direct way to extract the interface conductance and the spin polarization at the interface. Furthermore, our results show that the Schottky barrier can play a crucial role in determining the behavior of the interface conductance when varying the applied voltage. [Preview Abstract] |
Saturday, October 6, 2012 9:24AM - 9:36AM |
EB.00008: Combustion Synthesis of Energy Storage Materials W. Ethan Eagle, Margaret Wooldridge Advancement in the understanding of state of charge and efficiency requires better coupling of battery level properties with the micro-structure of the constituents. The composition of the target synthesis material, lithium manganese oxide ($Li Mn_2 O_4$, or LMO for short) is known to impact lithium ion battery properties. Following this motivation, our aim is to demonstrate control over the microstructure and compositional properties of LMO using parameters of the combustion synthesis environment. In this experiment, one or both solid phase precursors, lithium acetate-hydrate and manganse acetate-hydrate, were aerosolized and delivered to a hydrogen-oxygen Henken burner at atmospheric pressure. The characteristic time scales for reaction and flow control the synthesis process. Controlling reactant concentrations targets changes in nanoparticle composition and flow rate controls residence times and synthesis temperatures. To explore the effects of composition, first lithium oxide ($Li_2 O$) and manganese oxide ($Mn_2 O_2$) powders are generated independently from the corresponding acetate precursors. Following that, several mixtures of lithium and manganese acetate precursor trials were conducted and the resulting material properties were investigated using TEM and XRD. [Preview Abstract] |
Saturday, October 6, 2012 9:36AM - 9:48AM |
EB.00009: Photoluminescence Enhancement of Embedded Ga:GaAs Nanocomposites Sunyeol Jeon, Myungkoo Kang, Jia-Hung Wu, Jieun Lee, Vanessa Sih, Rachel Goldman When electromagnetic radiation is incident upon metallic nanoparticles(NPs), surface plasmon resonance(SPR) is generated. Metallic NPs on semiconductors have shown significant promise for various applications, such as enhanced light emission, and negative refractive index metamaterials. Metallic NP-induced photoluminescence(PL) enhancement has been demonstrated and attributed to the matching of the NP SPR energy with the semiconductor band gap energy. To date, plasmonics research has focused exclusively on Ag and Au NPs; however, their optical response is limited to low SPR energies(3.5eV). It was recently shown that Ga NPs produce size-dependent SPR, ranging from near-IR to visible wavelengths. Furthermore, 2D and 1D Ga NP arrays with SPR quality factors comparable to Ag and Au were reported. Here, we fabricate embedded Ga:GaAs nanocomposites utilizing off-normal focused-ion-beam(FIB) irradiation followed by molecular beam epitaxy overgrowth. The density(diameter) of the close-packed Ga NPs increases(decreases) with increasing the off-normal irradiation angle. We discuss the relative influences of Ga NP diameters and GaAs overgrowth thickness on the enhancement of the GaAs near-band edge emission, including the donor-acceptor pair and band-to-band emission. [Preview Abstract] |
Saturday, October 6, 2012 9:48AM - 10:00AM |
EB.00010: Nanophotonic Finite Difference Time Domain Simulations for Plasmon Enhanced Chemical Sensors Andrew Makepeace, Parveen Kumar, Jan Yarrison-Rice, Howard Jackson, Leigh Smith, J.-G. Park, K.-J. Choi We propose the fabrication of a plasmon-enhanced semiconductor nanosheet chemical sensor which is small in size and of high sensitivity and selectivity. Gold nanoparticles are photo-excited to produce strong surface plasmons, which amplify their local electric field emission. This enhancement can be several orders of magnitude. Semiconductor CdS nanosheets with a 40-50 nanometer thickness and micron length area are used to produce a two-photon-excited photocurrent when illuminated at energies just above the two photon absorption band. We report on the predicted CdS photocurrent enhancement in the presence of gold nanoparticles of various morphologies. The gold nanoparticle plasmon-enhancement and the enhanced photoexcited current in the CdS nanosheets are modeled using finite difference time domain calculations. Simulations determine the resulting fields produced by nanoparticles, the resulting fields at the nanosheet surface, and the total absorption within the nanosheet. The modeling results provide a predictive strategy for fabricating highly accurate chemical sensors. [Preview Abstract] |
Saturday, October 6, 2012 10:00AM - 10:12AM |
EB.00011: High-Efficiency Magnetization Measurements of Variable Composition Co-Fe-Ni Alloys with a Scanning Hall Probe Microscope Girfan Shamsutdinov, Debabrata Mishra, Boris Nadgorny, Zhao Peng, Ji-Cheng Zhao A Scanning Hall Probe Microscope with a submicrone scale Hall probe (HP) was used for high efficiency measurements of magnetic properties of a Co-Fe diffusion couples. This Co-Fe couple is an assembly of two metal blocks, which were placed in intimate contact and then exposed to high temperature to allow thermal interdiffusion to create solid-solution with a composition varying gradually from pure Fe to pure Co. The change in the magnetic field in the vicinity of this variable composition Fe-Co alloy with the width of approximately 400 microns was measured continuously as the HP was scanned across the interdiffusion region. Using a simple model we then determined the corresponding saturation magnetizations of Co-Fe alloy. The values of the saturation magnetization were found to be in good agreement with the known values for pure Fe and Co. The composition variation along the scan line was measured independently using Energy Dispersive X-ray Spectroscopy (EDS). Similar measurements were performed for the Fe-Ni and Co-Ni interfaces. This study demonstrates that Scanning Hall microscopy can be used for high efficiency and high accuracy measurements of saturation magnetization in variable composition alloys. [Preview Abstract] |
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