Bulletin of the American Physical Society
2013 Annual Fall Meeting of the APS Prairie Section
Volume 58, Number 15
Thursday–Saturday, November 7–9, 2013; Columbia, Missouri
Session H2: Condensed Matter Physics IV |
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Chair: Carseten Ullrich, University of Missouri Room: Physics Building 126 |
Saturday, November 9, 2013 8:30AM - 9:06AM |
H2.00001: A look at graphene's atomistic geometry and electronic properties from the perspective of discrete differential geometry Invited Speaker: Salvador Barraza-Lopez A host of amazing properties of graphene originate from \textit{geometry}. A prime example being actively pursued nowadays is the creation of gauge fields on graphene's conduction electrons, solely from mechanical strain [1-5]. This perspective is remarkable: Indeed, from an applied point of view, and just as an example, strain can help furnish large (pseudo-)magnetic fields, in excess of the $\sim$100 Tesla limit reached so far in state-of-the-art facilities [4]. From a fundamental perspective, graphene is a medium for discussion of (effective) relativistic Dirac-fermion Physics, so curved membranes make it necessary to uncover and revise our understanding of the Physics of Dirac fermions on curved spaces [2]. The theory (References [1-3] and a larger host of work) has been expressed in terms of an effective continuum media. Since graphene is an atomic membrane, our group is realizing a complementary and unique route [6-9] to study the relations among Dirac electrons and graphene's geometry, by applying concepts of Discrete Differential Geometry [10] to graphene. Essentially, the idea is to build the theory for electronic properties up from unit cells and atoms, so that the atomistic conformation is never lost, and no continuum limit is to be applied. The insight gained from this new perspective enters into basic checks of theory [1-3], the furnishing of electronic `mass,' [7] and other geometrical aspects [9]. An extensive discussion of this approach and salient results will be given on this talk. \\[4pt] [1] H. Suzuura and T. Ando, Phys. Rev. B 65, 235412 (2002); V. M. Pereira and A. H. Castro-Neto, Phys. Rev. Lett. 103, 046801 (2009). \\[0pt] [2] M. A. H. Vozmediano, M. I. Katsnelson, and F. Guinea, Phys. Rep. 496, 109 (2010). \\[0pt] [3] F. Guinea, M. I. Katsnelson, and A. K. Geim, Nature Physics 6, 30 (2010). \\[0pt] [4] N. Levy et al. Science 329, 544 (2010) \\[0pt] [5] K. K. Gomes, W. Mar, W. Ko, F. Guinea, and H. C. Manoharan, Nature 483, 306 (2012). \\[0pt] [6] J. V. Sloan et al., Phys. Rev. B 87, 155436 (2013). \\[0pt] [7] S. Barraza-Lopez et al., Solid State Comm 166, 70 (2013). \\[0pt] [8] J. V. Sloan, A. A. Pacheco Sanjuan, Z. Wang, C. M. Horvath, and SBL. MRS Online Proceedings Library. Volume 1549 (2013). doi:10.1557/opl.2013.1030. \\[0pt] [9] A. A. Pacheco-Sanjuan et al, Submitted on July 11, 2013. \\[0pt] [10] A. I. Bobenko, P. Schroder, J. M. Sullivan, and G. M. Ziegler, eds., ``Discrete Differential Geometry.'' vol. 38 of Oberwolfach Seminars (Springer, 2008), 1st ed. [Preview Abstract] |
Saturday, November 9, 2013 9:06AM - 9:18AM |
H2.00002: Electrical Transport Properties In Large Area Boron-Nitrogen-Carbon Layers Baleeswaraiah Muchharla, Arjun Pathak, Zheng Liu, Li Song, Thushari Jayasekera, Swastik Kar, Robert Vajtai, Luis Balicas, Pulickel M. Ajayan, Saikat Talapatra, Naushad Ali In this work, we present a detailed investigation of the temperature dependence of transport in thin layers of Boron Nitrogen and Carbon (BNC) and compare it with electrical transport in large area graphene. We find that the temperature dependence of resistance (5K \textless\ T \textless\ 400 K) of pure graphene shows a metallic behavior, whereas the BNC samples display an increasingly semiconducting behavior with increasing B and N concentrations. Density Functional Theory (DFT) calculations performed on pure graphene and BNC structures were in good agreement with this experimental observations. The observed temperature dependence of the electrical resistivity of BNC samples can be classified into two regimes. At higher temperatures (50K \textless\ T \textless\ 400 K), the BNC samples showed a band-gap dominated Arrhenius-like temperature dependence. At the lowest temperatures (5K \textless\ T \textless\ 50 K), the temperature dependence deviates from an activated behavior, and presents evidence for a conduction mechanism that is consistent with Mott's 2D-Variable Range Hopping (2D-VRH). [Preview Abstract] |
Saturday, November 9, 2013 9:18AM - 9:30AM |
H2.00003: Structural and electronic properties of SrTiO$_{3}$/LaNIO$_{3}$ slabs with and without oxygen vacancies Pablo Rivero, Salvador Barraza-Lopez, Jak Tchakalian, Srimanta Smiddey Perovskite oxide heterostructures with transition metal ions exhibit interfacial electronic states completely different to the bulk [1]. Theoretical and experimental researchers are reaching the conclusion that this new behavior emerge as a result of charge redistribution at the interface as a consequence of the polar discontinuity between materials. The creation of Oxygen vacancies [2] is one of the mechanisms known to avoid the electric field divergence, resulting in some cases in high mobility carriers at interfaces. Studying the structural and electronic properties of these compounds with and without Oxygen vacancies could shed more light on the interesting physical phenomena originating at the interface. We use first principle DFT calculations with the LDA exchange-correlation functional to study the formation energy of oxygen vacancies of (5,n) SrTiO$_{3}$/LaNiO$_{3}$ multilayer slab systems, containing n $=$ 1, 2 and 5 layers of LNO. Systems slabs were oriented in the strongly polar (111) direction. We also studied the electronic properties through the electronic density of states (DOS) and the projected full band structure onto the two-oxide interface. We acknowledge computer support from NSF-XSEDE (Project TG-PHY090002. Stampede Supercomputer at TACC). [1] J. Chakhalian, A. J. Millis and J. Rondinelli, Nature Mater. \textbf{11}, 92 (2012). [2] G. Herranz, M. Basleti, M. Bibes, C. Carr\'{e}t\'{e}ro, E. Tafra, E. Jacquet, K. Bouzehouane, C. Deranlot, A. Hamzi, J.-M. Broto, A. Barth\'{e}l\'{e}my, and A. Fert, Phys. Rev. Lett. \textbf{98}, 216803 (2007). [Preview Abstract] |
Saturday, November 9, 2013 9:30AM - 9:42AM |
H2.00004: Electric Field Tuning of the Rashba Effect Shanavas Veedu, Sashi Satpathy The Rashba effect describes the momentum-dependent spin splitting of the electron states at a surface or interface. The control of the Rashba effect by an applied electric field is at the heart of the proposed Rashba-effect-based spintronics devices for manipulating the electron spin. We studied how the Rashba SOI at the polar perovskite surfaces and interfaces can be tuned by manipulating the two-dimensional electron gas (2DEG) by an applied electric field. We saw that the Rashba SOI originates from the first few layers near the surface and it therefore can be altered by drawing the 2DEG to the surface or by pushing the 2DEG deeper into the bulk with an applied electric field. We carried out a comprehensive density-functional study of the recently-discovered polar KTaO$_3$ surface both with and without an applied electric field. y-discovered polar KTaO$_3$ surface both with and without an applied electric field. The strength of the Rashba effect depends intricately on the surface induced asymmetry of the Ta(d) states as well as the strength of the spin-orbit interaction, which is unraveled from the study of a tight-binding model Hamiltonian to describe the Rashba effect. [Preview Abstract] |
Saturday, November 9, 2013 9:42AM - 9:54AM |
H2.00005: Theoretical studies of terahertz spectra of crystalline energetic materials using molecular dynamics Andrey Pereverzev, Thomas Sewell, Donald Thompson Terahertz infrared (THz IR) absorption spectra of crystalline pentaerythritol tetranitrate (PETN) and the $\alpha $ and $\gamma $ polymorphs of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) were calculated at 300 K for pressures between 0 and 7 GPa using two different theoretical approaches based on molecular dynamics (MD). Beyond the choice of force field, neither method entails the specification of adjustable parameters. In the first approach spectral line frequencies and intensities were calculated using normal mode analysis of three-dimensionally periodic crystal supercells, while spectral line widths were calculated using relaxation times obtained from MD simulations of energy loss from selectively excited THz-region IR-active modes. The IR spectrum was then generated as a superposition of Lorentzian functions completely specified by the calculated frequencies, intensities, and line widths. In the second approach spectra were calculated from equilibrium MD simulations using the Fourier transform of the dipole autocorrelation function of the crystal. Spectra obtained using the two methods are similar and agree reasonably well with experimental results. [Preview Abstract] |
Saturday, November 9, 2013 9:54AM - 10:06AM |
H2.00006: Time-dependent transition density matrix for visualizing charge-transfer excitations in photoexcited organic donor-acceptor systems Yonghui Li, Carsten Ullrich The time-dependent transition density matrix (TDM) is a useful tool to visualize and interpret the induced charges and electron-hole coherences of excitonic processes in large molecules. Combined with time-dependent density functional theory on a real-space grid (as implemented in the octopus code), the TDM is a computationally viable visualization tool for optical excitation processes in molecules. It provides real-time maps of particles and holes which gives information on excitations, in particular those that have charge-transfer character, that cannot be obtained from the density alone. Some illustration of the TDM and comparison with standard density difference plots will be shown for photoexcited organic donor-acceptor molecules. [Preview Abstract] |
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