Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T12: Atomic Structures and Mechanical Properties in Semiconductors 
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Sponsoring Units: FIAP Room: D223/224 
Wednesday, March 23, 2011 2:30PM  2:42PM 
T12.00001: A dual approach to quantum fluctuations in frustrated lattice spin models Anirban Gangopadhyay, Victor Galitski We develop a dual approach to describe quantum dynamics in lattice spin models, which allows us to describe nonperturbative \emph{quantum} trajectories in the spin path integral. The spin path integral takes the form of a combination of traces of ``dynamic density matrices'', which belong to either special linear group $SL(2,C)$ for a quantum ferromagnet or $SU(2)$ for an antiferromagnet. We analyze the latter model of a highly frustrated quantum antiferromagnet and find a class of nonperturbative trajectories that contribute to thermodynamics of the model at low temperatures. [Preview Abstract] 
Wednesday, March 23, 2011 2:42PM  2:54PM 
T12.00002: Potassiuminduced semiconducting to metallic transition on the $\beta$SiC(100) c(4x2) surface Barry Haycock We present new data on the potassiuminduced semiconducting to metallic transition of the siliconterminated $\beta$SiC(100) c(4x2) surface, resulting from density functional theory simulations. In our calculations we have analysed many different KSiC(100) structures, corresponding to K coverages ranging from 0.08 to 1.25 monolayers (ML), paying special attention to the 2/3 ML and 1 ML cases where the transition has been reported. We find that the KSiC(100) surface is metallic in all the cases. In spite of that, the K layer shows a semiconducting density of states (DOS) up to K coverages of $\sim$1ML, beyond which the potassium layer undergoes a transition to metallic behaviour, explaining the experimental observation. We propose a new atomic model for the surface reconstruction of the 1ML case which has far lower total energy than the previously suggested model based on linear K chains after simulated annealing studies. [Preview Abstract] 
Wednesday, March 23, 2011 2:54PM  3:06PM 
T12.00003: The role of structural ordering in the semiconducting behavior of Cr$_3$Al Zoe Boekelheide, D.A. Stewart, F.J. Wong, Y. Suzuki, F. Hellman Cr$_{1x}$Al$_x$ displays unexpected semiconducting behavior for x $\sim$ 0.25; an ordered Cr$_3$Al structure has been proposed to explain it.[1, 2] In this work, density functional theory calculations and nonequilibrium thin film growth were used to study the role of ordering on the transport properties. The atoms in Cr$_3$Al occupy the sites of a bcc lattice, like Cr. Calculations comparing possible structures show that the proposed chemically ordered, rhombohedrally distorted Cr$_3$Al structure, with ordering along the $<$111$>$ direction, is the lowest energy of those considered. In addition, the band structure shows a pseudogap, consistent with experimentally observed transport properties. Thin films of Cr$_{1x}$Al$_x$ were grown with various growth and annealing temperatures to vary the properties. Samples with the most rhombohedral ordering are semiconducting. Decreased rhombohedral ordering leads to lower resistivity. Samples with a tetragonal distortion due to the C11$_b$ (Cr$_2$Al) structure have metallic resistivity. References: [1] D. J. Chakrabarti and P. A. Beck, J. Phys. Chem. Solids 32, 1609 (1971) [2] F. J. A. den Broeder et al, Phys. Status Solidi A, 67, 233 (1981) [Preview Abstract] 
Wednesday, March 23, 2011 3:06PM  3:18PM 
T12.00004: Raman Investigation of Ru$_{2}$Si$_{3}$ Single Crystals and Thin Films A. Glen Birdwell, Daniel Lenssen, Conin B. Vining, Frank Crowne, Robert Glosser Raman spectra were obtained from Ru$_{2}$Si$_{3}$ single crystals and thin films. The spectra were taken with the incident light polarization parallel and perpendicular to each axis of the single crystal. When inspecting all the spectra of multiple geometric configurations, we were able to observe a total of thirteen phonon modes. To the best of our knowledge, there are very few published results with which to compare. However, the encouraging point of this work is that when spectra were obtained on thin film material composed of multiple crystal orientations, we have a very close correspondence to the modes found from the experiments on the single crystal. [Preview Abstract] 
Wednesday, March 23, 2011 3:18PM  3:30PM 
T12.00005: Longrangeorder and Shortrangeorder Structures in Sndoped Ge Thin Films YunLiang Soo, S.L. Chang, J.F. Lee, H.H. Cheng The Sndoped Ge semiconductor thin films have shown direct bandgap and many other interesting physical properties with great potential for technological applications. To understand the underlying mechanism for the unique properties of these materials, information on the locations of Sn atoms in the matrix and the effects of Sn doping on the crystal structure of Ge host is an important prerequisite. Samples of Sndoped Ge thin films of thickness around 300{\AA} and Sn concentration 4 at.{\%} to 28 at.{\%} have been prepared by molecular beam epitaxy (MBE) method. Longrangeorder and shortrangeorder structures in these films have been probed by using xray diffraction (XRD) and extended xray absorption fine structure (EXAFS) techniques, respectively. Our xray results demonstrate that Sn impurity atoms are located on the substitutional sites in the Ge films with Sn concentration up to 20 at.{\%}. Variation of lattice constant as a result of Sn doping in the Ge host will also be presented. [Preview Abstract] 
Wednesday, March 23, 2011 3:30PM  3:42PM 
T12.00006: Wurtzitederived polytypes of kesterite and stannite quaternary chalcogenide semiconductors Shiyou Chen, Ye Luo, XinGao Gong, Aron Walsh, SuHuai Wei The I$_2$II$$IV$$VI$_4$ quaternary chalcogenide semiconductors (\textit{e.g.}, Cu$_2$ZnSnS$_4$, Cu$_2$ZnGeSe$_4$) have been studied for more than 40 years, but the nature of their crystal structures has proved contentious. Literature reports exist for the stannite and kesterite structures, which are zincblendederived structures, and wurtzitestannite, which is a wurtzitederived structure. In this talk we report a new wurtzitederived structure, wurtzitekesterite (space group $Pc$), which is the ground state for some I$_2$II$$IV$$VI$_4$ compounds, but is easily confused with the wurtzitestannite structure. We show that there is a clear relationship between the properties of the wurtzitekesterite and zincblendederived kesterite structures,, as well as between wurtzitestannite and stannite. The energy stability of different structures are studied according to the strain and Coulomb energy contributions, showing a dependence on the size and ionicity of the component atoms. Electronic structure of the wurtzitederived structures will also be discussed. [Preview Abstract] 
Wednesday, March 23, 2011 3:42PM  3:54PM 
T12.00007: Spontaneous symmetry breaking in two coupled nanomechanical electron shuttles Chulki Kim, Jonghoo Park, Robert Blick We present spontaneous symmetry breaking in two coupled nanomechanical electron shuttles. The electron shuttles are realized as silicon nanopillars and placed between two capacitor plates in a homogeneous electric field. Instead of being mechanically coupled through a spring they exchange electrons, i.e., they shuttle electrons from the source to the drain capacitor plate. The nonzero dc current through this system by external ac excitation is caused via dynamical symmetry breaking. The oscillation frequencies of the shuttling system are mode locked to the applied voltage frequency. [Preview Abstract] 

T12.00008: ABSTRACT WITHDRAWN 
Wednesday, March 23, 2011 4:06PM  4:18PM 
T12.00009: Vibrational Modes of thin Silicon Membranes Reimar Waitz, Olivier Schecker, Elke Scheer Membranes with thicknesses in the range of hundred nanometers and macroscopic lateral size are interesting systems to study the mechanical properties of solids on various length scales. In our experiment a piezo is used to couple in vibrations, which can be observed with a phaseshift interferometer using stroboscopic light. With this technique we image transverse modes of frequencies up to 12 MHz. In general, the observed wave pattern of the membrane deflection will be a superposition of the mode corresponding to the excitation frequency and several higher harmonics. Using a Fourier transformation in time, it is possible to separate these contributions. This way eigenmodes up to the 8th harmonic of the excitation frequency can be imaged. The influence of strain on the dispersion relation is investigated by applying a pressure difference between both sides of the membrane. The results are compared to finiteelements simulations. [Preview Abstract] 
Wednesday, March 23, 2011 4:18PM  4:30PM 
T12.00010: Phononimaging measurements of CaWO$_{4}$ Elastic Constants Timothy Head, Madeleine Msall Recent use of CaWO$_{4}$ in phononmediated detectors for a dark matter interactions by the Cryogenic Rare Event Search using Superconducting Thermometers (CRESST) collaboration have increased interest in precise measurements of CaWO$_{4}$ elastic constants. Phononimaging simulations based on continuum elasticity theory show that position and shape of phonon caustics depend sensitively on the elastic constants. Spatial and Temporal phonon flux distributions arising after point heatpulse excitation have been measured for [001] and [010] oriented CaWO$_{4}$ crystals. We report elastic constants derived from time of flight measurements along symmetry directions, and calculated by matching experimental phonon images to simulations based on continuum elasticity theory. [Preview Abstract] 
Wednesday, March 23, 2011 4:30PM  4:42PM 
T12.00011: Kinetic Monte Carlo simulation of GaAs homoepitaxy and droplet epitaxy Kris Reyes, Denis Nothern, Joanna Millunchick, Peter Smereka We present a new model for atomistic simulation of IIIV semiconductors that is based on the solidonsolid model and allows for multiple species, atom exchanges, and local considerations for atom bonding energies. The model is validated by comparison with experimental observations of GaAs homoeptiaxial growth. In particular the simulated surface concentration and growth modes agree with experiments over a wide range of growth conditions. An important feature of this model is that Ga and As atoms are treated explicitly, resulting in the ability to realistically model Ga droplet formation under low As overpressure and their recrystallization upon exposure to As. [Preview Abstract] 
Wednesday, March 23, 2011 4:42PM  4:54PM 
T12.00012: First principles determination of vibrational and elastic properties of quaternary compounds Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ Tanju Gurel, Cem Sevik, Tahir Cagin Recently, the quaternary compounds, Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$, have been attracted pretty much attention because of their potential use in the field of energy harvesting applications. Several theoretical calculations have been reported about their first principles electronic, optic and transport properties. However, no lattice dynamic calculations have been published yet despite the discussions about their possible ground state crystal structures and measured low thermal conductivity values. In this systematical study, we examined the vibrational and elastic properties of the two different crystal phases, kesterite (KS) with space group $I\overline{4}$ and stannite (ST) with space group $I\overline{4}2m$, of this quaternary compounds by using density functional perturbation theory. In addition, we predicted the relaxation time dependent lattice thermal conductivity within the solution of phonon Boltzmann transport equation by making use of the acquired vibrational frequency data. [Preview Abstract] 
Wednesday, March 23, 2011 4:54PM  5:06PM 
T12.00013: Measuring graphene's thermal coefficient of expansion with bimetallic cantilevers Hiram Conley, Nickolay Lavrik, Dhiraj Prasai, Kirill I. Bolotin We developed a method for probing the thermal coefficient of expansion of 2D materials by measuring the deflection of a bimetallic cantilever. We fabricate suspended bimetallic cantilevers composed of singlelayer or multilayer graphene and either gold or silicon nitride. Because of the mismatch of the thermal expansion coefficients, these cantilevers bend when heated. We employ laser interferometry to measure the bending and to extract the thermal expansion coefficient from 170 C to 250 C. We find that this technique provides a reliable measurement of the thermal expansion coefficient for graphene. Through comparison of the coefficient of expansion obtained from our bimetallic cantilevers and that of suspended graphene we demonstrate how graphene's dimensionality is perturbed by contact with other materials. [Preview Abstract] 
Wednesday, March 23, 2011 5:06PM  5:18PM 
T12.00014: Temperature Dependence of Cyclotron Decoherance Time in a High Mobility TwoDimensional Electron Gas J.A. Curtis, J.D. Moore, T.T. Tokumoto, J.G. Cherian, X. Wang, J.L. Reno, A. Belyanin, J. Kono, S.A. McGill, D.J. Hilton Using timedomain THz magnetospectroscopy, we studied the dynamics of a 2DEG ($\mu =3.4\times 10^{6}\mathrm{cm}^{2}\,\mathrm{V}^{1}\,\mathrm{s}^{1}$) as a function of temperature ($0.4$K$100$K).The decoherence lifetimes increase monotonically as temperature decreases below 1 K, which we have fit to a power law ($\tau \sim T^{0.29}$). We will discuss the mechanisms that contribute to the lifetimes. The transmitted pulse amplitude increases from 0.4K1.2K, saturates from 1.5K25K, and decreases from 50K100K. J. A. Curtis is supported by a US Dept. Education GAANN Fellowship (P200A090143). [Preview Abstract] 
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