Search Results (118 total)

We present results of density functional theory (DFT) calculation of the structural supermodulation in Bi₂Sr₂CaCu₂O_8+x structure, and show that the supermodulation is indeed a spontaneous symmetry breaking of the nominal crystal symmetry, rather than a phenomenon driven by interstitial O dopants. The structure obtained is in excellent quantitative agreement with recent x-ray studies, and reproduces several qualitative aspects of scanning tunneling microscopy (STM) experiments as well. The primary structural modulation affecting the CuO₂ plane is found to be a buckling wave of tilted CuO₅ half-octahedra, with maximum tilt angle near the phase of the supermodulation where recent STM experiments have discovered an enhancement of the superconducting gap. We argue that the tilting of the half-octahedra and concomitant planar buckling are directly modulating the superconducting pair interaction.

In this Letter, we demonstrate that nonadiabatic dynamics of molecular scattering from metal surfaces can be efficiently simulated by semiclassical Gaussian wave packet propagation on a local complex potential. The method relies on the wideband limit decoupling of the nuclear equations of motion on different electronic states. If the continuum diabatic potential surfaces are assumed to be parallel, the number of Gaussian wave packets spawned scales at most linearly with propagation time, allowing efficient propagation of nuclear dynamics.

When an electron in a localized orbital approaches a continuum, resonant charge transfer can occur. For instance, when a beam of ions is scattered from a metal surface, neutralization can take place through the transfer of an electron either from ion to metal—in the case of a negative ion—or from metal to ion—in the case of a positive ion. Similarly, a neutral atom may be ionized by removing an electron from the metal surface. In this paper, we introduce the wide-band diabatic dynamics (WBDD) method for simulating the quantum dynamics of such systems by decoupling the equations of motion for the ionic and neutral diabats. We show numerically that our approximation accurately describes the quantum nuclear dynamics near the metal surface. Although we treat the case of ion neutralization, our method is general, and can be applied to any quantum state interacting with a continuum via a position-dependent interaction, provided that the wide-band approximation holds.

We present first principles studies using density functional theory on adsorption of fullerene molecules C_n (n=32,36,40,44,48,60) on the c(4×4) reconstructed GaAs(001) surface. Adsorption at various surface sites coupled with numerous fullerene orientations were systematically sampled to obtain energetically most stable structures. With the orientation of a fullerene with one of the hexagons facing down the substrate, the surface trenches were identified as the sites that give the strongest adsorption. The orientational preference for C₃₂ adsorption is an exception due to its unique stability in its electronic structure. It was found that the As atoms with a dangling bond in the second surface layer play a most critical role in determining the adsorption structure and strength, while the top layer As dimers are only capable of forming weak bonding with fullerenes, which differs significantly from adsorption of fullerenes and small unsaturated organic molecules on silicon surfaces. Strong covalent bonds between fullerenes and the substrate are formed and considerable deformation of fullerenes near the adsorption sites is observed. The calculated adsorption energy decreases as the size of fullerenes increases. The calculation yields useful physical insight into the adsorption mechanism and the physicochemical properties of the materials.

The cuprate material Bi₂Sr₂CaCu₂O₈ (BSCCO-2212) is believed to be doped by a combination of cation switching and excess oxygen. The interstitial oxygen dopants are of particular interest because scanning tunneling microscopy (STM) experiments have shown that they are positively correlated with the local value of the superconducting gap, and calculations suggest that the fundamental attraction between electrons is modulated locally. In this work, we use density functional theory to try to ascertain which locations in the crystal are energetically most favorable for the O dopant atoms, and how the surrounding cage of atoms deforms. Our results provide support for the identification of STM resonances at -1  eV with dopant interstitial O atoms, and show how the local electronic structure is modified nearby.

Transition state theory is used to estimate rate constants for dissociative chemisorption of H₂ on copper clusters. Activation energies and transition state partition functions are obtained from density functional theory for small clusters of less than 10 atoms. The violation of the Bronsted-Evans-Polanyi relation, which was previously observed for these clusters, is explained in terms of structural relaxation due to the chemisorption process. For large clusters, the impact of chemisorption on the global structure of the clusters is reduced. This restores the validity of the Bronsted-Evans-Polanyi relation and allows an extrapolation scheme for nano-size clusters to be developed.

The sequential growth of small copper clusters up to 15 atoms and the dissociative chemisorption of H₂ on the minimum energy clusters are studied systematically using density functional theory under the generalized gradient approximation. We found that small Cu_n clusters grow by adopting a triangular growth pathway. The pentagon bipyramid structural arrangements are strongly favored energetically in the growth and the new addition in the cluster occurs preferably at a site where the atom is capable of interacting with more adjacent atoms. To understand the evolution of small copper clusters, we also performed calculations on selected icosahedral clusters (for n=13,19,25,55) and fcc-like clusters (n=14,23,32,41). By extrapolating/interpolating the binding energies of triangular clusters, icosahedral clusters, and bulk-like clusters, we found that structural transitions from the triangular growth clusters to the icosahedral and fcc-like clusters occur at approximately n=16 and n=32, respectively. Subsequently, we performed extensive calculations on the dissociative chemisorption of H₂ on the minimum energy clusters. The chemisorption likely occurs near the most acute metal site with the two H atoms residing on the edges, which differs significantly from the chemisorption on Cu surfaces that usually takes place at the hollow sites.

We describe a method for calculating, within density functional theory, the electronic structure associated with typical defects which substitute for Cu in the CuO₂ planes of high-T_c superconducting materials. The focus is primarily on Bi₂Sr₂CaCu₂O₈, the material on which most scanning tunnel microscopy (STM) measurements of impurity resonances in the superconducting state have been performed. The magnitudes of the effective potentials found for Zn, Ni, and vacancies on the in-plane Cu sites in this host material are remarkably consistent with phenomenological fits of potential scattering models to STM resonance energies. The effective potential ranges are quite short, of order 1 Å with weak long-range tails, in contrast to some current models of extended potentials which attempt to fit STM data. For the case of Zn and Cu vacancies, the effective potentials are strongly repulsive, and states on the impurity site near the Fermi level are simply removed. The local density of states (LDOS) just above the impurity is nevertheless found to be a maximum in the case of Zn and a local minimum in the case of the vacancy, in agreement with experiment. The Zn and Cu vacancy patterns are explained as due to the long-range tails of the effective impurity potential at the sample surface. The case of Ni is richer due to the Ni atom’s strong hybridization with states near the Fermi level; in particular, the short-range part of the potential is attractive, and the LDOS is found to vary rapidly with distance from the surface and from the impurity site. We propose that the current controversy surrounding the observed STM patterns can be resolved by properly accounting for the effective impurity potentials and wave functions near the cuprate surface. Other aspects of the impurity states for all three species are discussed.

Chemisorption of fullerenes on semiconductor surfaces is of current technological interest. Using ab initio density functional theory under the generalized gradient approximation, we performed extensive theoretical studies on the chemisorption of a small fullerene molecule, C₂₈, on the c(4×4) reconstructed GaAs(001) surface. The chemisorption structures and energetics at various adsorption sites, in combination of possible fullerene configurations, were carefully examined and the adsorption at the trench site with one hexagon of C₂₈ facing down was found to be energetically most favorable. In all cases, we found that upon C₂₈ adsorption the c(4×4) reconstructed GaAs(001) surface undergoes considerable lattice relaxation and structural deformation of fullerene molecule also occurs. The chemisorption is dictated by the [2+2] cycloaddition reaction and/or by simple electron lone pair mediated charge transfer reaction from the substrate to the fullerene molecule. It was found that the monolayer formed by the C₂₈ molecules on the surface is stable and naturally porous.

The structural evolution of small copper clusters of up to 15 atoms and the dissociative chemisorption of H₂ on the minimum energy clusters are studied systematically using density functional theory. The preferred copper sites for chemisorption are identified and the transition state structures and activation barriers for clusters four to nine atoms are determined and found to be inconsistent with the empirical Brønsted-Evans-Polanyi relationship. The physicochemical properties of the clusters are computed and compared with the bulk and surface values. The results indicate that a phase transition must occur in the going from cluster to bulk.

We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.

The g factor of the first excited 2⁺ state of ¹⁶⁴Yb was measured by perturbed γ-γ angular correlation in an external static magnetic field of 5.55  T. The result, g(2₁⁺)=0.32(5), extends the systematics of g(2₁⁺) for Yb isotopes down to N=94. The behavior of the known experimental values of g(2₁⁺) vs neutron number N for all isotopic chains from Ba to Pt is discussed. Several trends are observed: for some chains, especially in transitional regions, a rather strong N dependence of the g factors is observed; in regions of stable structure this dependence is much weaker, and for the Yb and Pt chains it is almost flat. While the different behaviors can be separately explained by valence or collective models, a unified interpretation of these systematic trends remains a challenge to microscopic theories.

A general force field methodology is developed for description of molecular interactions in carbon-based materials. The method makes use of existing parameters of potential functions developed for sp² and sp³ carbons and allows accurate representation of molecular forces in curved carbon environment. The potential parameters are explicitly curvature and site dependent. The proposed force field approach was used in molecular dynamics (MD) simulations for hydrogen adsorption in single-walled carbon nanotubes (SWNTs). The results reveal significant nanotube deformations and the calculated energies of adsorption are comparable to the reported experimental heat of adsorption for H₂ in SWNTs.

An approach based on parametric equations of motion for the transition operator and the Green’s operator is presented. The formulation applies generally to discrete and continuous states of quantum-mechanical many-body systems. The method provides an alternative to solving the Schrödinger equation as a boundary value problem by replacing it with an equivalent initial value problem.

We report a measurement of the branching fraction of the decay B⁰→D^*+D^*- and of the CP-odd component of its final state using the BABAR detector. With data corresponding to an integrated luminosity of 20.4  fb^-1 collected at the Υ(4S) resonance during 1999–2000, we have reconstructed 38 candidate signal events in the mode B⁰→D^*+D^*- with an estimated background of 6.2±0.5 events. From these events, we determine the branching fraction to be B(B⁰→D^*+D^*-)=[8.3±1.6(stat)±1.2(syst)]×10^-4. The measured CP-odd fraction of the final state is 0.22±0.18(stat)±0.03(syst).

Insulator-quantum Hall conductor transitions at low magnetic field B were studied with a gated GaAs-AlGaAs heterostructure. A low field disorder-magnetic field phase diagram was constructed based on the experimental results. This phase diagram shows no floating up of the extended state and allows transitions from the insulating state directly to any Landau level states. The critical filling factor can change from 16 to 6 as the disorder in the sample increases. By inspecting the raw data from this and the other samples and analyzing the scaling behaviors near the transition points, we found that the observed transition has the properties of a genuine phase transition.

Polarized Raman spectra were obtained from a rope of aligned semiconducting single-wall nanotubes (SWNTs) in the vicinity of the D band and the G band. Based on group theory analysis and related theoretical predictions, the G-band profile was deconvolved into four intrinsic SWNT components with the following symmetry assignments: 1549 cm^-1 [E₂(E_2g)], 1567 cm^-1 [A(A_1g)+E₁(E_1g)], 1590 cm^-1 [A(A_1g)+E₁(E_1g)] and 1607 cm^-1 [E₂(E_2g)]. The frequency shifts of the tangential G modes from the 2D graphitelike E_2g2 frequency are discussed in terms of the nanotube geometry.

Raman spectra of radial breathing modes (RBM’s) of single-walled carbon nanotubes (SWNT’s) are reported to exhibit a different resonantly enhanced behavior between the Stokes and anti-Stokes Raman-scattering components, from which we determine the electronic transition energy of individual SWNT’s that is involved in the resonant process. By comparing the measured electronic transition energy with the theoretical energy separations between singularities in the one-dimensional density of electronic states for metallic or semiconducting SWNT’s, the conducting category of observed SWNT’s is identified. Moreover, we find that the relative intensity of each RBM does not reflect the proportion of a particular SWNT due to the coexistence of resonant and nonresonant Raman-scattering processes for different diametric SWNT’s.

We report both experimental and theoretical studies on Si_1-xGe_x/Si multiple quantum wells. A self-consistent calculation is employed to model the excitonic transition. It shows that, in the large conduction-band-offset region the Δ₂–heavy-hole (hh) exciton is the lowest transition, while in the small-offset region the Δ₄-hh exciton is the lower. From an analysis of the data, a type-II conduction-band-offset ratio of 30±3% is concluded.

By solving numerically the full set of hydrodynamic equations governing the pulsation of a bubble, we show that shock waves are often absent in a stable sonoluminescing bubble. Nevertheless, for a wide range of physical parameters, a continuous compressional wave emerges and heats up the bubble, and the resulting black-body radiations have pulse heights and widths that agree with experimental data. Shock waves, being much less robust, are not essential for stable single-bubble sonoluminescence.

We present results of experimental studies of the nonlinear dynamics of synchrotron oscillations in the presence of phase modulation in an electron storage ring. A streak camera is used to observe the longitudinal distribution of an electron bunch directly as it forms two stable resonant islands. The positions of the fixed points as a function of modulation frequency agree well with theory. We also present measurements of the diffusion rate from one stable island to the other for a fixed modulation frequency, which show agreement with the diffusion rates expected from large-angle intrabeam (Touschek) scattering. These results also explain anomalous results of beam transfer function diagnostic measurements obtained at other electron storage rings.

We studied the dependence of thermodynamic variables in a sonoluminescing (SL) bubble on various physical factors, which include viscosity, thermal conductivity, surface tension, the equation of state of the gas inside the bubble, as well as the compressibility of the surrounding liquid. The numerical solutions show that the existence of shock waves in the SL parameter regime is very sensitive to these factors. Furthermore, we show that even without shock waves, the reflection of continuous compressional waves at the bubble center can produce the high temperature and picosecond time scale light pulse of the SL bubble, which implies that SL may not necessarily be due to shock waves.

We report measurements of magnetoreflectivity in CdTe/Cd_1-xMn_xTe superlattices with varying manganese concentration in magnetic fields up to 45 T. From an analysis of the 1s heavy-hole σ₊ transition, we observe that the energy shift is directly equal to the shift in the valence-band edge of the barrier layer. This provides a clear proof of type-I–type-II crossover in this system. The critical magnetic field that leads to the formation of a type-II exciton is also estimated. We point out that the criterion for the observation of a type-II exciton is that the type-II valence-band offset has to be larger than the binding energy of an exciton.

When a composite is subjected to a constant applied electric, thermal, or stress field, the associated local fields exhibit strong spatial fluctuations. In this paper, we evaluate the distribution of the local electric field (i.e., all moments of the field) for continuum (off-lattice) models of random dielectric composites. The local electric field in the composite is calculated by solving the governing partial differential equations using efficient and accurate integral equation techniques. We consider three different two-dimensional dispersions in which the inclusions are either (i) circular disks, (ii) squares, or (iii) needles. Our results show that in general the probability density function associated with the electric field for disks and squares exhibits a double-peak character. Therefore, the variance or second moment of the field is inadequate in characterizing the field fluctuations in the composite. Moreover, our results suggest that the variances for each phase are generally not equal to each other. In the case of a dilute concentration of needles, the probability density function is a singly peaked one, but the higher-order moments are appreciably larger for needles than for either disks or squares.