Bulletin of the American Physical Society
81st Annual Meeting of the APS Southeastern Section
Volume 59, Number 18
Wednesday–Saturday, November 12–15, 2014; Columbia, South Carolina
Session CB: Condensed Matter Physics II |
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Chair: Michel Pleimling Room: Richland I |
Thursday, November 13, 2014 11:00AM - 11:12AM |
CB.00001: Bound exciton model for ``shallow'' impurities in semiconductors Yong Zhang, Jianwei Wang Most impurities can be described by a unified bound exciton model [1] that includes impurity binding energy E$_{\mathrm{I}}$, exciton binding energy E$_{\mathrm{XB}}$, and excitonic transition energy E$_{\mathrm{X}}=$E$_{\mathrm{I}}$-E$_{\mathrm{XB}}$. E$_{\mathrm{XB}}$ corresponds to the commonly known acceptor binding energy E$_{\mathrm{A}}$ or donor binding E$_{\mathrm{D}}$ for the respective case. Analogous to the free exciton problem, E$_{\mathrm{g}}$ or E$_{\mathrm{I}}$ is the single particle transition energy, E$_{\mathrm{XB}}$ is due to many-body effect that can sometimes be simplified as an effective mass (EM) equation with a screened Coulomb interaction between the electron and hole. Although E$_{\mathrm{XB}}$ typically represents a small modification to the inter-band transition energy (e.g., E$_{\mathrm{g}} = $ 1.519 eV and E$_{\mathrm{BX}} = $ 4 meV for GaAs), the excitonic effect is responsible for the strong absorption at E$_{\mathrm{X}}$ and other discrete excitonic transition peaks. The same phenomenon occurs in impurities either known as ``deep'' or ``shallow.'' The standard theory for ``shallow'' impurities overlooks an important aspect of the problem, the E$_{\mathrm{I}}$ part associated with a short-range potential. The attempt to consolidate the discrepancy between the EM model and experimental data by introducing a ``core correction'' into the EM equation is conceptually problematic, equivalent to ``correcting'' E$_{\mathrm{BX}}$ to match the E$_{\mathrm{g}}$ value for a free exciton.\\[4pt] [1] Zhang and Wang, PRB (in press). [Preview Abstract] |
Thursday, November 13, 2014 11:12AM - 11:24AM |
CB.00002: Irradiation of commercial, high-Tc superconducting tape for potential fusion applications: electromagnetic transport properties A.A. Gapud, N.T. Greenwood, J.A. Alexander, A. Khan, K.J. Leonard, T. Aytug, F.A. List III, M.W. Rupich, Y. Zhang Effects of low dose ion irradiation on the electrical transport current properties of commercially available high-temperature superconducting, coated-conductor tapes were investigated, in view of potential applications in the irradiative environment of fusion reactors. Three different tapes, each with unique as-grown flux-pinning structures, were irradiated with Au and Ni ions at energies that provide a range of damage effects, with accumulated damage levels near that expected for conductors in a fusion reactor environment. Measurements using transport current determined pre- and post-irradiation resistivity, critical current density, and pinning force density, yielding critical temperatures, irreversibility lines, and inferred vortex creep rates. Results show that, at the irradiation damage levels tested, any detriment to as-grown pre-irradiation properties is modest; indeed in one case already-superior pinning forces are enhanced, leading to higher critical currents. [Preview Abstract] |
Thursday, November 13, 2014 11:24AM - 11:36AM |
CB.00003: Local Heterogeneity in a Polymer Melt Demonstrated by Polarized Two-Dimensional Spectroscopy Haorui Wu, Mark Berg The rotation of a solute in a small-molecule solvent is exponential, but it becomes nonexponential in a polymer melt. This nonexponential behavior may be explained by either variations in the local viscosity of the polymer---a heterogeneous model---or local anisotropy of the polymer structure---a homogeneous model. Two-dimensional spectroscopies using six optical pulses called MUPPETS (multiple population-period transient spectroscopy) can distinguish homogeneous and heterogeneous causes of nonexponential decay, but have never been applied to molecular rotation. We developed a new, polarized version of MUPPETS and measured the anisotropy decay of Pyrromethene 597 in poly(dimethylsiloxane) (PDMS). The results show strong molecule-to-molecule variation in the rotation rate. They are consistent with local, short-length scale variations in viscosity within the polymer. No evidence for local anisotropy is found. [Preview Abstract] |
Thursday, November 13, 2014 11:36AM - 11:48AM |
CB.00004: Multidimensional Correlation-Function Analysis of Single-Molecule Data: Heterogeneity in a Polymer near the Glass Transition Sachin Dev Verma, David Vanden Bout, Mark Berg Nonexponential decays, i.e. rate dispersion, are common in the relaxation of complex materials. Single-molecule measurements should determine whether such rate dispersion is due to heterogeneity. Current methods of analysis are based on averaging molecule-by-molecule. They are inconsistent with ergodicity and are ambiguous in practice. A new definition of heterogeneity based on multidimensional correlation-functions is proposed. Averages are taken over both time and the entire ensemble of molecules, ensuring consistency with ergodic behavior. An efficient algorithm has been developed to calculate multidimensional correlation-functions. The effectiveness of the new method is demonstrated on linear-dichroism data in poly(cyclohexyl acrylate) near its glass transition ($T_{g} +2K)$ [J. Chem. Phys. 125, 124701 (2006)]. Molecule-by-molecule averaging of this data fails to provide useful conclusions. Multidimensional analysis gives clear and quantitative results: 95$\pm $5$\% $ of rotational-rate dispersion is due to heterogeneous local environments, and the rate exchange is at least 11 times longer than the mean rotation time. [Preview Abstract] |
Thursday, November 13, 2014 11:48AM - 12:00PM |
CB.00005: Measuring potential energy dissipation effects via below-surface electronic excitations in solids using MOS devices Dhruva Kulkarni, Radhey Shyam, Daniel Field, Jim Harriss, Daniel Cutshall, William Harrell, Chad Sosolik We have conducted measurements on oxides irradiated by multicharged ions ($Q>1$) to explore the potential energy/charge state effect on subsurface damage caused in the target. Our goal was to determine charge state dependence of stopping power, or energy lost per unit distance, relative to the singly charged ($Q=1$) ions usually encountered in ion-solid interactions. Specifically, we have irradiated a 170 nm thick SiO$_2$-on-Si sample with Ar$^{Q+}$ ions ($Q=1,4,8,11$) at a fixed K.E. of 1 keV and tracked the electronic excitations in the oxide by capacitance-voltage (C-V) characterization post irradiation. Energy dissipation within the oxide generates electron-hole pairs which leads to mobile holes that can transport across the oxide and have a finite probability of getting trapped within the oxide, which causes a flatband voltage shift in its C-V curve. To obtain C-V characteristics, we deposited top metal contacts on our oxides to create MOS devices (Al-SiO$_2$-Si). These measurements showed increased flatband voltage shifts dependent on impact position, ion dose and the ion charge state. the ion charge state. Preliminary analysis reveals a power law ($Q^{2.2}$) dependence on the charge state which is compared to theoretical predictions and other experimental studies. [Preview Abstract] |
Thursday, November 13, 2014 12:00PM - 12:12PM |
CB.00006: Understanding ultrafast relaxation dynamics of hot carriers and phonons at the graphene-diamond interface Zeynab Jarrahi, Jimmy L. Davidson, Norman Tolk In the past few years, there has been increasing interest in single and poly crystalline diamond as the substrate of choice for graphene FET and interconnects. Yu et al have observed a substantial increase in the current carrying capacity of graphene-diamond FETs compared to conventional SiO2-Si [1]. The performance of such devices is ultimately dependent on the relaxation dynamics of hot carriers and phonons at the Gr /diamond interface which is the motivation behind this research. Using ultrafast degenerate pump probe spectroscopy, our main goal is to understand the vastly diverse hot carrier induced dynamics of CVD single layer graphene on different substrates. The transient differential reflection and transmission spectra reveal the time scales associated with the cooling cascade of non equilibrium photo generated species and the band filling dynamics. Understanding how the relaxation dynamics of graphene is altered by coupling to different substrates such as diamond and deciphering the effects of electron and optical and acoustic phonons scattering mechanisms involved, will pave the way towards realization of higher performance carbon sp2/sp3 technology. \\[4pt] [1] Yu et al. Nano Letters 2012 12 (3), 1603-1608 [Preview Abstract] |
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