Bulletin of the American Physical Society
Joint Fall 2012 Meeting of the Texas Sections of the APS, AAPT, and Zone 13 of the SPS
Volume 57, Number 10
Thursday–Saturday, October 25–27, 2012; Lubbock, Texas
Session B8: Condensed Matter and Materials Physics I |
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Chair: Suresh Sharma, University of Texas at Arlington Room: Holiday Inn Towers Petroleum |
Friday, October 26, 2012 10:30AM - 10:42AM |
B8.00001: Thermal conductivity of Si nanowires: the impact of the surface Byungkyun Kang The thermal conductivity of Si nanowires is calculated from first principles at T=125K using the theoretical ``laser-flash'' method. The nanowires are represented by the 1-D periodic supercells Si$_{200}$X$_{32}$ and si$_{296}$X$_{112}$, where X is H or D or an OH group. The present focus is on the impact of the surface of the nanowire on the thermal conductivity. The bulk phonons cannot ``scatter'' off the surface of the nanowire as this would require exciting modes with frequencies higher than bulk-mode frequencies, a low probability event. Instead, the high-frequency wag modes of the H surface atoms couple resonantly to each faster than they decay into linear combinations of bulk modes. Thus, the surface reduces the thermal conductivity of the nanowire because heat propagates at the surface much slower than in the bulk. [Preview Abstract] |
Friday, October 26, 2012 10:42AM - 10:54AM |
B8.00002: Isotope effect in the vibrational lifetime of the CH$_{2}^{\ast}$ defect in Si Michael Gibbons, Stefan Estreicher, Michael Stavola The CH$_{2}^{\ast}$ defect in Si has two metastable configurations with H bound at bond-centered (BC) or antibonding (AB) sites of Si and C: Si-H$_{BC}$...C-H$_{AB}$ or H$_{AB}$-Si...H$_{BC}$-C. Th IR absorption signature of this defect should consist of four sharp lines associated with the two Si-H and two C-H stretch modes, respectively. Yet, only three modes have seen by FTIR, and the fourth, very broad, line has only recently been reported by the highly sensitive multiple-internal-reflection FTIR. Further, the ``missing'' mode produces a very sharp line only for the deuterium substitution Si-H$_{BC}$...C-D$_{AB}$. Our calculations show that this is due to the isotope-dependence of the vibrational lifetime of this mode. The C-H mode decays very quickly into two phonons and the very short lifetime causes the IR line to be extremely broad. On the other hand, the C-D mode has a much longer lifetime and decays into at least three phonons, resulting in a much sharper IR line. We will report the calculations of these vibrational lifetimes. [Preview Abstract] |
Friday, October 26, 2012 10:54AM - 11:06AM |
B8.00003: Thermal conductivity and specific heat of metallic micro and nanowires Denis Myasishchev, Josef Cepak, Mark Holtz, Jordan Berg Thermal conductivity drops dramatically at the nanoscale. Effective power dissipation is crucial for solid state devices, but thermal conductivity decreasing with size complicates miniaturization efforts. There are few direct measurements of thermal conductivity of nanoscale structures. We report fabrication and characterization of nickel nanowires. The data analysis used by previous authors neglects time-varying and higher-order terms in a series expansion of the one-dimensional transient heat equation. This approximation is inaccurate at ``high'' currents, restricting the attainable signal-to-noise ratio. We remove this source of estimation error with a transient electrothermal finite element model. The approach has been validated on a 25$\mu $m diameter platinum wire over a broad temperature range and extension to the nickel nanowires will be discussed. [Preview Abstract] |
Friday, October 26, 2012 11:06AM - 11:18AM |
B8.00004: Majorana Fermions Under Stress Li Mao, Ming Gong, Sumanta Tewari, Chuanwei Zhang Spin-orbit coupled semiconductor nanowires with Zeeman splitting in proximity contact with bulk s-wave superconductivity have recently been proposed as a promising platform for realizing Majorana fermions. However, in this setup the chemical potential of the nanowire is generally pinned by the Fermi surface of the superconductor. This makes the tuning of the chemical potential by external electrical gates, a crucial requirement for unambiguous detection of Majorana fermions, very challenging in experiments. Here we show that tunable topological superconducting regime supporting Majorana fermions can be realized in semiconductor nanowires using uniaxial stress. For n-type nanowires the uniaxial stress tunes the effective chemical potential, while for p-type systems the effective pairing may also be modified by stress, thus significantly enhancing the topological minigap. We show that the required stress, of the order of 0.1\%, is within current experimental reach using conventional piezo crystals. [Preview Abstract] |
Friday, October 26, 2012 11:18AM - 11:30AM |
B8.00005: Optical limiting by absorption bleaching in carbon nanotube devices: comparison of field induced and electrochemically induced charge injection W. Joshua Kennedy, Z. Valy Vardeny We studied direct charge injection in a heterogeneous film of single-wall carbon nanotubes using the technique of charge-induced absorption. We found that the injected charges screen the excitons in the semiconducting tubes, reducing their binding energy and transferring oscillator strength from the exciton transitions to free carriers. These effects parallel those of the electrochemical doping in the same samples. Furthermore, we interpret the bleaching bias in the electroabsorption (a $\chi_3$ process) in isolated SWNT as being due to injected charges, which has implications for a variety of SWNT based optoelectronic devices, including nanoscale optoelectronic switches. [Preview Abstract] |
Friday, October 26, 2012 11:30AM - 11:42AM |
B8.00006: Many-body Landau-Zener Transition in Cold Atom Double Well Optical Lattices Yinyin Qian, Ming Gong, Chuanwei Zhang Ultra-cold atoms in optical lattices provide an ideal platform for exploring many-body physics of a large system arising from the coupling among a series of small identical systems whose few-body dynamics are exactly solvable. Using Landau-Zener (LZ) transition of bosonic atoms in double well optical lattices as an experimentally realizable model, we investigate such few to many body route by exploring the relation and difference between the small few-body (in one double well) and the large many-body (in double well lattice) non-equilibrium dynamics of cold atoms in optical lattices. We find the many-body coupling between double wells greatly enhances the LZ transition probability, while keeping the main features of the few-body dynamics. Various experimental signatures of the many-body LZ transition, including atom density, momentum distribution, and density-density correlation, are obtained. [Preview Abstract] |
Friday, October 26, 2012 11:42AM - 11:54AM |
B8.00007: Single-Photon X-ray Detector Using AlGaN/GaN Heterostructure Iman Rezanezhad Gatabi, Joel Sander, Rupak Mahapatra, Harlan Rusty Harris Rejection of external photons is critical in eliminating background signals for low background detectors including dark matter detectors. Experiments typically employ expensive shielding including ancient Pb. Herein we describe the design of a single-photon detector in the MeV range using an AlGaN/GaN heterostructure. Thin, electrically isolated GaN is used as the bulk material for electron-hole pair generation, and a device is formed with the 2D electron gas (2DEG) at AlGaN/GaN interface due to GaN spontaneous polarization. Numerical simulations of the distribution of carriers with MeV excitation are used to estimate device response of the structure. Applications to photon veto in dark matter and double beta decay research is discussed, and an integration scheme is outlined that will allow nearly complete photon veto of cosmic EM background. [Preview Abstract] |
Friday, October 26, 2012 11:54AM - 12:06PM |
B8.00008: Using phonon imaging to measure elastic constants in crystals of low symmetry Adam Simpson, Elizabeth Carlisle, Tim Head We attempt to use spatial data from phonon-imaging to identify elastic constants of materials. The current analysis compares continuum limit Monte-Carlo simulations of phonon images to data and attempts to use a minimization process to find the appropriate elastic constants for a material. We are currently working on a proof of concept for Si which has 3 independent elastic constants, but hope to scale up the process to be able to identify the 7 independent elastic constants in CaWO$_4$. [Preview Abstract] |
Friday, October 26, 2012 12:06PM - 12:18PM |
B8.00009: QMSA Measurements of III-V Heterostructures on Silicon Thiess Cunningham, Ravi Droopad, Richard Hill, Man Hoi Wong There is widespread consensus that high mobility III-V channel materials will enable increased performance and reduced power consumption at scaled geometries. The industry is currently targeting the 11 nm technology node for their introduction. A most significant challenges is the heterointegration of III-V channel materials on Si substrates. Carrier transport of MBE grown InGaAs/InAlAs HEMTs on InP and Si substrates using Quantitative Mobility Spectrum Analysis (QMSA). Measurements taken determine effect of epitaxial defects on channel transport and buffer leakage. The continued scaling of Si CMOS devices has reached a point of, alternative solutions to conventional MOSFETs are needed. A solution considered is use of III-V compound semiconds as channel materials. However, requirements are that materials need be epitaxially integrated to silicon, be able to withstand the thermal budget in various CMOS processing modules. This presentation, will present the electrical characterization of MBE grown III-V InGaAs/InAlAs heterostructures on silicon. Transport measurements at various temperatures ranging from 10k-room temperature in magnetic fields from 0-10T. From these measurements, QMSA of the data is carried out to the densities and mobilities of the conducting and buffer layers. [Preview Abstract] |
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