Quantum Theory of Advanced Materials

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (T-c) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above T-c and the theoretical calculations, we found that the T-c oscillation correlates directly with the density of states oscillation at E-F. The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.

We present a first-principles investigation of the lattice dynamics and thermodynamical properties of a complex hydride NaAlH(4), a promising material for hydrogen storage. The calculations are performed within the density-functional-theory framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z(*), and dielectric constants in high and low frequency limits are reported. The mode characters of the zone-center phonons, including the LO-TO splitting, are identified and compared to the experiment. The quasiharmonic approach is used to study thermal expansion as well as the mean square displacement of each atom as a function of temperature. A connection is established between the latter and the melting point. The inclusion of the zero-point motion leads to an expanded lattice compared to the static lattice, while the low frequency oscillations are found to play an important role in the melting and decomposition of NaAlH(4).

We have used the combination of the coupling-constant integration procedure and the variational quantum Monte Carlo method to study the exchange-correlation (XC) interaction in small molecules: Si-2, C2H2, C2H4, and C2H6. In this paper we report the calculated XC energy density, a central quantity in density functional theory, as deduced from the interaction between the electron and its XC hole integrated over the interaction strength. Comparing these "exact" XC energy densities with results using the local-density approximation (LDA), one can analyze the errors in this widely used approximation. Since the XC energy is an integrated quantity, error cancellation among the XC energy density in different regions is possible. Indeed we find a general error cancellation between the high-density and low-density regions. Moreover, the error distribution of the exchange contribution is out of phase with the error distribution of the correlation contribution. Similar to what is found for bulk silicon and an isolated silicon atom, the spatial variation of the errors of the LDA XC energy density in these molecules largely follows the sign and shape of the Laplacian of the electron density. Some noticeable deviations are found in Si-2 in which the Laplacian peaks between the atoms, while the LDA error peaks in the regions "behind" atoms where a good portion of the charge density originates from an occupied 1 sigma(u) antibonding orbital. Our results indicate that, although the functional form could be quite complex, an XC energy functional containing the Laplacian of the energy is a promising possibility for improving LDA.

We show that, if a variational parameter in the Fock-Darwin states is optimized, the lowest Landau level (LLL) approximation agrees well with configuration interaction results using several Landau levels for a wide range of confining and Coulomb interaction strengths. Within the optimized LLL approximation, we study several phase transitions beyond the maximum density droplet for four to nine electrons and find similar patterns in the phase-space diagrams for angular momenta up to N(N+1)/2. Calculations for larger angular momentum reveal unpolarized phases for filling factors up to nu=1/3.

Recently it was discovered that a total of 5.6 wt. % H-2 could be released from the 1:2 mixture of lithium amide and magnesium hydride at temperatures as low as 150 degrees C. With a reaction enthalpy of 44 KJ/mol H-2, this system has high potential for on-board hydrogen storage applications. The fully desorbed product is believed to be a mixed lithium and magnesium imide Li2Mg(NH)(2). In this work, the crystal structure of this mixed imide is studied from total-energy density-functional calculations. Based on a recent experimentally established space group, possible ordered configurations are examined. Important local orderings are identified for the experimentally observed disordered phase at room temperature. These unique local arrangements are also connected with the observed structural transitions above room temperature. In addition, the local ordering in Mg(NH2)(2) is analyzed. The similarity and difference of local arrangements among hydrogen, cations, and vacancies are discussed for the three amide (imide) systems: LiNH2, Mg(NH2)(2), and Li2Mg(NH)(2). The identification of the cation and hydrogen local orderings are expected to facilitate the design of new mixed imides and amides as hydrogen storage materials with desired physical properties.

We present a first-principles study of the correlated electron-hole states in a silicon nanowire of a diameter of 1.2 nm and their influence on the optical absorption spectrum. The quasiparticle states are calculated employing a many-body Green's function approach within the GW approximation to the electron self-energy, and the effects of the electron-hole interaction to optical excitations are evaluated by solving the Bethe-Salpeter equation. The enhanced Coulomb interaction in this confined geometry results in an unusually large binding energy (1-1.5 eV) for the excitons, which dominate the optical absorption spectrum.

We report first-principles calculations of Pb (100) films up to 22 monolayers to study variations in the surface energy and work function as a function of film thickness. An even-odd oscillation is found in these two quantities, while a jelliumlike model for this s-p metal predicts a periodicity of about three monolayers. This unexpected result is explained by considering a coherent superposition of contributions from quantum-well states centered at both the Gamma and M points in the two-dimensional Brillouin zone, demonstrating the importance of crystal band structure in studying the quantum size effect in metal thin films.

Hydrogenation of the inten-netallic compound LaMg2Pd at 200 degrees C and 10 bar leads to a complex metal hydride of composition LaMg2PdH7. Its structure has orthorhombic symmetry and displays tetrahedral [PdH4](4-) anions. The Pd-H bond distances as measured on the deuteride range from 1.71 to 1.78 angstrom and the H-Pd-H bond angles from 95 degrees to 122 degrees. Three additional hydride anions H- occupy La2Mg2-type interstices having tetrahedral metal configurations. Band structure calculations suggest the hydride to be non-metallic and to have a band gap of similar to 1.0ev. The compound desorbs hydrogen at 125 degrees C yielding a pressure of more than I bar absolute. (C) 2006 Elsevier B.V. All rights reserved.

By using first-principles pseudopotential methods, we have studied the electronic properties of hydrogen-passivated silicon nanowires along the [100], [110], and [111] directions with diameter up to 3.4 nm. It is found that as the diameter decreases, the energy band gaps are distinctly enlarged due to the confinement effect. The valence-band maximum moves down while the conduction-band minimum moves up compared with the bulk. By using the many-body perturbation theory within the GW approximation, we have also investigated the self-energy correction to the energy band gaps. Our calculational results show that, although the band gap values strongly depend on both the diameter and orientation, the GW corrections are mainly dependent on diameter and less sensitive to the growth orientation. The effective mass as a function of diameter is also discussed.

Aluminum hydride (alane) AlH(3) is an important material in hydrogen storage applications. It is known that AlH(3) exists in multiply forms of polymorphs, where alpha-AlH(3) is found to be the most stable with a hexagonal structure. Recent experimental studies on gamma-AlH(3) reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in alpha-AlH(3) or other polymorphs. Using density functional theory, we have investigated the energetics, and the structural, electronic, and phonon vibrational properties for the newly reported gamma-AlH(3) structure. The current calculation concludes that gamma-AlH(3) is less stable than alpha-AlH(3) by 1.2 KJ/mol, with the zero-point energy included. Interesting binding features associated with the unique geometry of gamma-AlH(3) are discussed from the calculated electronic properties and phonon vibrational modes. The binding of H-s with higher energy Al-p,d orbitals is enhanced within the double-bridge arrangement, giving rise to a higher electronic energy for the system. Distinguishable new features in the vibrational spectrum of gamma-AlH(3) were attributed to the double-bridge and hexagonal-ring structures.

The phonon dispersions of monolayer and few-layer graphene (AB bilayer, and ABA and ABC trilayers) are investigated using the density-functional perturbation theory. Compared with the monolayer, the optical phonon E(2g) mode at Gamma splits into two and three doubly degenerate branches for bilayer and trilayer graphene, respectively, due to the weak interlayer coupling. These modes are of various symmetries and exhibit different sensitivities to either Raman or infrared measurements (or both). The splitting is found to be 5 cm(-1) for bilayer and 2-5 cm(-1) for trilayer graphene. The interlayer coupling is estimated to be about 2 cm(-1). We found that the highest optical modes at K move up by about 12 cm(-1) for bilayer and 18 cm(-1) for trilayer relative to monolayer graphene. The atomic displacements of these optical eigenmodes are analyzed.

The electronic structure of Si/Ge core-shell nanowires along the [110] and [111] directions are studied with first-principles calculations. We identify the near-gap electronic states that are spatially separated within the core or the shell region, making it possible for a dopant to generate carriers in a different region. The confinement energies of these core and shell states provide an operational definition of the "band offset," which is not only size dependent but also component dependent. The optimal doping strategy in Si/Ge core-shell nanowires is proposed based on these energy results.

The Li-Mg-N-H system has been identified as a promising hydrogen storage material due to its moderate operation conditions as well as the high capacity and reversibility. Recently Rijssenbeek et al. [J. Alloys Compd. 454, 233 (2008)] reported that Li(2)Mg(NH)(2) has disordered cation and vacancy arrangements at room temperature and above. We present our first-principles calculations to investigate a series of ordered low-energy configurations for this compound. Specific local orderings are found in the cation-vacancy arrangement, shedding light on the experimental disordered structure models. A possible ordered phase at low temperature is proposed based on these local orderings. Reaction energetics and phase stability are further discussed. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3003067]

Using first-principles calculations within density functional theory, we have investigated the electronic properties of H-passivated Si nanowires (SiNWs) oriented along the 112 direction, with the atomic geometries retrieved via global search using genetic algorithm. We show that [112] SiNWs have an indirect band gap in the ultrathin diameter regime, whereas the energy difference between the direct and indirect fundamental band gaps progressively decreases as the wire size increases, indicating that larger [112] SiNWs could have a quasi-direct band gap. We further show that this quasi-direct gap feature can be enhanced when applying uniaxial compressive stress along the wire axis. Moreover, our calculated results also reveal that the electronic band structure is sensitive to the change of the aspect ratio of the cross sections.

We have systematically investigated the effect of oxidation on the structural and electronic properties of graphene based on first-principles calculations. Energetically favorable atomic configurations and building blocks are identified, which contain epoxide and hydroxyl groups in close proximity with each other. Different arrangements of these units yield a local-density approximation band gap over a range of a few eV. These results suggest the possibility of creating and tuning the band gap in graphene by varying the oxidation level and the relative amount of epoxide and hydroxyl functional groups on the surface.

We present a first-principles investigation to study the possible alloy phases of sodium and lithium alanates. Structural and energetics properties of alloy systems Na(1-x)Li(x)AlH(4) and Na(3(1-x))Li(3x)AlH(6) are studied via phase interpolation. Alloy system Na(1-x)Li(x)AlH(4) is found to have a small mixing energy (<5 kj/mol). The equilibrium structure undergoes a transition from a tetragonal structure to a monoclinic structure between x = 0.25 and 0.5. Within each structure the cell volume decreases with increasing x, which can be explained by Li having a smaller ion size than Na. Alloy system Na(3(1-x))Li(3x)AlH(6) is also studied, and one intermediate composition Na(2)LiAlH(6) is found to be stable in agreement with experimental findings. (C) 2009 Elsevier B.V. All rights reserved.

Accurate multideterminant ground-state energies of circular quantum dots containing N <= 13 electrons as a function of interaction strength have been evaluated by the diffusion quantum Monte Carlo method. Two unique features are found for these confined two-dimensional systems: (1) as the electron density decreases, the quantum dots favor states with zero orbital angular momentum (L = 0); and (2) for some values of N, the ground state cannot be fully spin-polarized because of a symmetry constraint.

The physical and chemical properties of thin metal films show damped oscillations as a function of film thickness (one-dimensional shell effects). While the oscillation period, determined by subband crossings of the Fermi level, is the same for all properties, the phases can be different. Specifically, oscillations in the work function and surface energy are offset by 1/4 of a period. For Pb(111) films, this offset is similar to 0.18 monolayers, a seemingly very small effect. However, aliasing caused by the discrete atomic layer structure leads to striking out-of-phase beating patterns displayed by these two quantities.

We present a first-principles study of the electron-phonon (e-ph) interactions and their contributions to the linewidths for the optical-phonon modes at Gamma and K in one-layer to three-layer graphene. It is found that, due to the interlayer coupling and the stacking geometry, the high-frequency optical-phonon modes in few-layer graphene couple with different valence and conduction bands, giving rise to different e-ph interaction strengths for these modes. Some of the multilayer optical modes derived from the Gamma-E(2g) mode of monolayer graphene exhibit slightly higher frequencies and much reduced linewidths. In addition, the linewidths of K-A(1)(') related modes in multilayers depend on the stacking pattern and decrease with increasing layer numbers.

We report on a first-principles study of the conductance through graphene suspended between Al contacts as a function of junction length, width, and orientation. The charge transfer at the leads and into the freestanding section gives rise to an electron-hole asymmetry in the conductance and in sufficiently long junctions induces two conductance minima at the energies of the Dirac points for suspended and clamped regions, respectively. We obtain the potential profile along a junction caused by doping and provide parameters for effective model calculations of the junction conductance with weakly interacting metallic leads.

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant kappa, resulting in a very rich phenomenon of "quantum oscillations" in kappa as a function of thickness, L, and bias voltage, V(s). More specifically, we find that the phase of the quantum oscillations in kappa vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (vertical bar V(s)vertical bar < 0.03 V) the measurement of kappa vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of kappa vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

Both the size and the magnetic-field dependences of low-lying spectra of two-dimensional (2D) graphene based magnetic dot and ring in perpendicular inhomogeneous magnetic fields, where the magnetic field is zero inside the dot and ring, and constant elsewhere, are studied by the massless Dirac-Weyl equation. Numerical results obtained from direct diagonalization with 2D harmonic basis show that, under nonuniform magnetic fields, the higher Landau levels (N >= 1) for such massless Dirac electron interacting system in general become nondegenerate and split into discrete angular momentum states with level crossings with the lowest one (N=0) being an exception. (C) 2010 American Institute of Physics. [doi:10.1063/1.3435478]