### Session DB: Condensed Matter II: Computational Materials Science

Chair: Chris Stanton, University of Florida
Room: Hilton Hawthorne

 Thursday, November 10, 2005 2:00PM - 2:12PM DB.00001: Quantum Nature of Hydrogen on Metals: Ground-State Geometry of Vacancies Sungho Kim , Seong-Gon Kim , Steven C. Erwin When hydrogen is adsorbed on a Pd(111) surface, very simple vacancy defects -- which form quite commonly -- exhibit fundamentally quantum wavelike behavior. Conventional wisdom has it that the quantum nature of hydrogen and other light atoms is only rarely manifested, typically in specially designed experimental protocols. A great many studies have proceeded under the assumption that, for most purposes, as long as the electronic motion is handled quantum mechanically, the nuclear motion of hydrogen can be treated classically. We show that this approximation fails badly in a very simple system. Moreover, by treating the problem fully quantum mechanically, several recent experimental finding [Mitsui et al, Nature 422, 705 (2003)] can be plausibly interpreted as having a fundamentally quantum origin. In particular, we present new ground-state geometry of hydrogen vacancies on Pd(111) surface that are obtained from a full quantum treatment of the hydrogen nuclear motion. This new structures enable us to predict that all vacancy defects to have an unexpected triangular appearance in scanning tunneling microscopy; recent experiments have found precisely this behavior. Thursday, November 10, 2005 2:12PM - 2:24PM DB.00002: Compact Wavefunctions for He-like Systems Frank E. Harris Wavefunctions which are compact, but still quite accurate, are extremely valuable as tools for gaining understanding of quantum systems. Here we study the ground electronic states of the three-body systems comprising the He isoelectronic series, using spatial wavefunctions that depend exponentially on all the interparticle distances, i.~e. of the form $(1+{\sf P}_{12})\exp(-\alpha r_1 -\beta r_2 -\gamma r_{12})$, where $r_1$ and $r_2$ are the electron nuclear distances, $r_{12}$ is the electron-electron separation, and ${\sf P}_{12}$ permutes the electron coordinates. When the nonlinear parameters are carefully optimized (a nontrivial task), this type of basis is found to be extraordinarily efficient; using as few as four basis functions, it is found that nonrelativistic energies are reproduced to within 38 microhartrees of the exact values, an error far less than for compact wavefunctions previously proposed by others. Other properties, including those totally dependent upon the electron correlation, are also well represented. Thursday, November 10, 2005 2:24PM - 2:36PM DB.00003: Quantum Nature of Hydrogen on Metals: Dissociative Adsorption Seong-Gon Kim , Sungho Kim , Steven C. Erwin When hydrogen is adsorbed on a Pd(111) surface, very simple vacancy defects -- which form quite commonly -- exhibit fundamentally quantum wavelike behavior. We show that a full quantum treatment of the hydrogen nuclear motion, in addition to electron motion, predicts that the dissociative adsorption of H2 will be strongly suppressed at divacancies---in violation of the standard description from classical Langmuir adsorption kinetics---but will easily occur at trivacancies. Recent experiments confirm that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation, while divacancies are inactive [Mitsui et al, Nature 422, 705 (2003)]. Our findings indicate that traditional classical explanations for such observations require revisiting, and suggestthat the quantum nature of hydrogen may play a surprisingly prominent role in future hydrogen technologies. Thursday, November 10, 2005 2:36PM - 2:48PM DB.00004: GaSb(001) Surface Reconstruction: A First-principles Study Jeffery L. Houze , Sungho Kim , Seong-Gon Kim , Steven C. Erwin We use total-energy and electronic structure calculations based on density functional theory to study the structure of GaSb(001) growth surfaces. We consider different reconstruction models proposed in the literature on the basis of experimental observations under typical GaSb growth conditions and present the $T=0$ surface stability diagram. We found that all $(n\times 5)$-like reconstructions proposed in various literature have too high surface formation energies to adequately model the structures observed experimentally under typical Sb-rich growth conditions. Our results indicate that the existing reconstruction models for GaSb(001) surface require revisiting and the need to search for a better reconstruction model. Thursday, November 10, 2005 2:48PM - 3:00PM DB.00005: MEAM Potentials for Al and Mg Alloys and Interfaces Bohumir Jelinek , Seong-Gon Kim , Mark F. Horstemeyer , M.I. Baskes The ab-initio calculations based on density functional theory (DFT) are performed for the Al and Mg crystals and their alloy in reference structures, such as NaCl structure. The lattice constant (volume), bulk modulus and shear moduli for each element and the alloy are determined from the total energy calculations. These material parameters are then used to determine the Modified Embedded Atom Method (MEAM) potentials for these elements and their alloys. The transferability of these parameters are tested by obtaining relevant physical quantities on structures different than the reference structures and compare them with the results from ab-initio calculations. MEAM potentials determined for these materials are also used to study the properties of alloys and interfaces. Thursday, November 10, 2005 3:00PM - 3:12PM DB.00006: A new method to solve the Nd breakup scattering problem in configuration space Vladimir Suslov , Branislav Vlahovic A new computational method for solving the configuration-space Faddeev equations for three nucleon system has been developed. This method is based on the spline-decomposition in the angular variable and a generalization of the Numerov method for the hyperradius. The $s$-wave calculations of the inelasticity and phase-shift, as well as breakup amplitudes for \textbf{nd} and \textbf{pd }breakup scattering for lab energies 14.1 and 42.0 MeV were performed with the Malfliet -Tjon MT I-III potential. In the case of \textbf{nd} breakup scattering the results are in good agreement with those of the benchmark solution [1],[2]. In the case of \textbf{pd }quartet breakup scattering disagreement for the inelasticities reaches up to 6{\%} as compared with those of the Pisa group [3]. The calculated \textbf{pd }amplitudes fulfill the optical theorem with a good precision. 1. J. L. Friar, B. F. Gibson, G. Berthold, W. Gloeckle, Th. Cornelius, H. Witala, J. Haidenbauer, Y. Koike, G. L. Payne, J. A. Tjon, and W. M. Kloet,: \href{http://link.aip.org/link/?\&l\_creator=getabs-normal\&l\_dir=FWD\&l\_rel=CITES\&from\_key=PRVCAN000069000004044003000001\&from\_keyType=CVIPS\&from\_loc=AIP\&to\_j=PRVCAN\&to\_v=42\&to\_p=1838\&to\_loc=APS\&to\_url=http\%3A\%2F\%2Flink.aps.org\%2Fabstract\%2FPRC\%2Fv42\%2F}{Phys. Rev. C 42, 1838 (1990)}. 2. Frair J.L, Payne G.L., Gl\"{o}ckle W., Hueber D., Witala H.: Phys. Rev. C 51, 2356 (1995) 3. Kievsky A., Viviani M., and Rosati S.: Phys. Rev. C 64, 024002 (2001) Thursday, November 10, 2005 3:12PM - 3:24PM DB.00007: H$_{2}$N: Part 1. Hyperfine energies Arthur S. Brill H$_{2}$N, (from frozen, irradiated ammonia), is the smallest of the large group of $\pi$ (or p)-electron free radicals. With $^{1}$H, $^{2}$H, $^{14}$N and $^{15}$N there are 4 H$_{2}$N isotopes, with corresponding sets of hyperfine interactions, available for measurements. In a simple model of H$_{2}$N, 1.0 electron spin is in a Slater N2p-wave perpendicular to the molecular plane and -0.033 electron spin density in 1s waves on each H; the small effects of 0.066 electron spin (in other waves) required for net unit electron spin can be added. In a more complex model, the electronic structure is expressed with the 19 function 6-31G$^{\ast }$ basis. Nuclear spin-state mixing arises from linear combinations of dipolar off-diagonal matrix elements, e.g. M$_{xx} \quad \equiv \quad \sigma \kappa <\Psi \vert \Sigma$(S$_{kz}$x$^{2}_{kn}$/r$^{5}_{kn }$+ S$_{k\mbox{'}z}$x$^{2}_{k\mbox{'}n}$/r$^{5}_{k\mbox{'}n})\vert \Psi >$ (Airne and Brill, Phys. Rev.A \textbf{63} 052511). The M's are calculated in a molecular coordinate system with formulas applicable to any basis. Euler angles transform from molecular to lab spherical polar angles giving \textbf{B} with respect to the principle hyperfine axes at each nucleus. It is now shown that the principle hyperfine A-values can be expressed in terms of the M's, e.g. A$_{zz}$ = A$_{Fermi}$- (4/3$\sigma )$( M$_{xx}$ + M$_{yy}$ - 2 M$_{zz})$, thereby simplifying the energy matrices. Thursday, November 10, 2005 3:24PM - 3:36PM DB.00008: H$_{2}$N: Part 2. Mixed spin-states and magnetic resonance transition probabilities Arthur S. Brill In the absence of nuclear spin-state mixing (i.e. each state pure m$_{I})$ there are, e.g. 10 epr transitions in D$_{2}^{15}$N and 15 in D$_{2}^{14}$N, all $\Delta$m$_{I}$ = 0 fully allowed. In the presence of mixing there are 243 in D$_{2}^{15}$N and 729 in D$_{2}^{14}$N, with large differences in probability among transitions, many 0 or small. Because of numerous, at least partially allowed, overlapping transitions, useful information can be obscured in EPR spectra; Part 3 deals with experimental conditions to aid in extracting this information. In the literature there is quantitative disagreement among measured hyperfine splittings in H$_{2}$N, and spectral features have the appearance of little nuclear spin-state mixing (L. G. DeMarco, A. S. Brill and D. G. Crabb, J. Chem. Phys. \textbf{108} 1423 (1998) and references cited therein). With substantial spin-state mixing present, the latter behavior can be simulated over small ranges of a few parameters. Among these parameters is the HNH bond angle which affects both the M matrix elements discussed in Part 1 and how the contributions from the two H superpose. This bond angle is 104.4$^{o}$ in the 6-31G* model, but is probably along a soft scissors mode. Thursday, November 10, 2005 3:36PM - 3:48PM DB.00009: H$_{2}$N: Part 3. Experimental considerations Arthur S. Brill When nuclear spin-state mixing is present, a wide range of transition probabilities occurs and resonances are subject to a corresponding range of microwave saturation behavior. Simulations exhibit useful effects of experimentally-controlled power and spin-lattice relaxation time (dependent upon sample temperature) to intensify resonances of lower transition probability relative to those with higher. The nuclear Zeeman interaction (proportional to B) perturbs both the energy and state mixing of nuclear levels, thereby affecting the separation and probability of resonances. Of special interest are the fields B$_{cross }$at which pairs of hyperfine levels (of the ground or the excited electronic state, but not both) draw closest together (A. R. Airne and A. S. Brill, Phys. Rev.A \textbf{63} 052511). If B is scanned, one chooses its central value, and hence the microwave frequency, to produce a desired effect upon hyperfine splittings or transition probabilities. For pulsed operation, the same factors affect the choice of fixed B. A spectrometer with microwave frequency scanning at fixed B would be useful for centers like H$_{2}$N in which on-diagonal hyperfine energy matrix elements depend significantly upon B.