Direct Hopping Research Articles

Hole and electron dynamics in a triangular-lattice antiferromagnet: Interplay of frustration and spin fluctuations

Single-particle dynamics in the 120$^{\circ}$ ordered antiferromagnetic state of the triangular-lattice Hubbard model is studied using a physically transparent fluctuation approach in terms of multiple magnon emission and absorption processes within the noncrossing approximation. Hole and electron spectral features are evaluated at intermediate $U$, and analyzed in terms of a competition between the frustration-induced direct hopping and the virtual hopping terms. Finite $U$-induced competing interactions and frustration effects contributing through the magnon dispersion are also discussed. Finite contribution to self-energy correction from long-wavelength (Goldstone) modes, together with the high density of electron scattering states in the narrow, sharp peak in the upper band, result in strong fermion-magnon scattering leading to pronounced incoherent behaviour in the electron dynamics. The fluctuation-induced first-order metal-insulator transition due to vanishing band gap is also discussed.

Resonant mechanisms for negative ionization of secondary emitted atoms from sputtered metals

A model is presented to describe negative ionization of low energy, secondary atomic particles ejected from sputtered metal surfaces. Focus is made on the diatomic systems formed, in the collision cascade generated by the primary ion beam, between secondary emitted atoms and their nearest-neighbor substrate atoms that provide the initial impulse for ejection. Two different resonant ionization mechanisms are investigated such that a conduction electron may tunnel into the affinity orbital of the ejected atom either by direct hopping or after an intermediate transition via the affinity orbital of the substrate atom. A numerical method is outlined to calculate the negative ionization probability of secondary emitted atoms. A good agreement is found with van Der Heide’s measurements of the Cu− population sputtered from a clean Cu-surface, at emission energies below 100eV.

Calculation of exchange constants in manganese ferrite (MnFe 2O 4)

The exchange constants and electronic structure of manganese ferrite (MnFe 2O 4) were calculated using Becke's density functional. The total exchange energy consists of Hartree–Fock (HF) and Becke's density functional terms. We introduced one parameter w as the weight of HF's contribution. We also introduced a parameter α to scale the radial part of the 3d wave functions of Fe 3+ ions. By varying w and α the calculated exchange constants were quantitatively fitted to the experimental values of a spinel ferrite for the first time. Direct (d–d) and indirect (d–p–d) hopping are controlled by the parameters w and α.

Frequency adaptation efficiency of spread spectrum communications under narrowband interference

We investigate a new possibility of frequency adaptation for spread spectrum communication systems. A special index is introduced to characterize the adaptation gain referring to the nonadaptive case. An existence of frequency band limit is developed for the allocated frequency band for possible frequency maneuver. The investigation method can be adapted both to direct sequence and frequency hopping spread spectrum techniques. In the paper only the narrowband interference case is treated

Direct Current Hopping Conductivity in One-DimensionalNanometre Systems

A one-dimensional random nanocrystalline chain modelis established. A dc electron-phonon-field conductance model of electrontunnelling transfer is set up, and a new dc conductance formula inone-dimensional nanometre systems is derived. By calculating the dcconductivity, the relationship among the electric field, temperature andconductivity is analysed, and the effect of the crystalline grain sizeand the distortion of interfacial atoms on the dc conductance isdiscussed. The result shows that the nanometre system appears thecharacteristic of negative differential dependence of resistance andtemperature at low temperature. The dc conductivity of nanometre systemsvaries with the change of electric field and trends to rise as thecrystalline grain size increases and to decrease as the distorted degreeof interfacial atoms increases.

Exact ground-state for the periodic Anderson model in $ \mathsf {D=2}$ dimensions at finite value of the interaction and absence of the direct hopping in the correlated f-band

We report for the first time exact ground-states deduced for the D=2 dimensional generic periodic Anderson model at finite $U$, the Hamiltonian of the model not containing direct hopping terms for $f$-electrons $(t^f = 0)$. The deduced itinerant phase presents non-Fermi liquid properties in the normal phase, emerges for real hybridization matrix elements, and not requires anisotropic unit cell. In order to deduce these results, the plaquette operator procedure has been generalised to a block operator technique which uses blocks higher than an unit cell and contains $f$-operator contributions acting only on a single central site of the block.

Superexchange Mediated Charge Hopping in DNA

We explore the relationship between the electronic-nuclear level structure, the electronic couplings, and the dynamics of hole hopping transport in DNA. We utilized the electronic coupling matrix elements for hole transfer between nearest-neighbor nucleobases in DNA [Voityuk, A. A.; Jortner, J.; Bixon, M.; Rösch, N. J. Chem. Phys. 2001, 114, 5614] to evaluate intrastrand and interstrand superexchange electronic couplings, which determine hole hopping rates within the framework of a semiempirical quantum mechanical-kinetic model. Calculations of the exponential distance (R) dependence of the superexchange mediated intrastrand electronic couplings |Vsuper|2 ∝ exp(−βR) between guanines (G) in “short” G+(T−A)nG (n ≲ 3) duplexes result in β = 0.8−0.9 Å-1. We interpret the experimental data on time-resolved hole transport in the presence of a site-specifically bound methyl transferase mutant in DNA [Wagenknecht, H.-A.; Rajski, S. R.; Pascally, M.; Stemp, E. D. A.; Barton, J. K. J. Am. Chem. Soc. 2001, 123, 4400] in terms of composite sequential, interstrand and intrastrand superexchange mediated, and direct interstrand hole hopping. This mechanism accounts for the rate determining step, for the weak duplex size dependence of the rate, and for the long-range charge transport induced by interstrand superexchange via short (T−A) bridges, containing a single mediating nucleobase. For hole transfer via longer (T−A)n (n ≳ 3) bridges, the superexchange mechanism is replaced by the parallel mechanism of thermally induced hole hopping (TIH) via long (A)n chains. A kinetic analysis of the experimental data for hole transport through seven GG pairs separated by (T−A)n (n = 2−5) bridges across the 3‘−5‘ strand of the DNA duplex [Sartor, V.; Boone, E.; Schuster, G. B. J. Phys. Chem. B, 2001, 105, 11057] reveals that the superexchange−TIH crossover occurs at n = nx = 3. The explorations of the range of applicability and the breakdown of the superexchange mechanism in DNA lay the foundations for the scrutiny of the universality and system specificity of this mechanism in large-scale chemical and biophysical systems.

The c-Axis Infrared Conductivity of a dx2-y2-Wave Superconductor with the Hopping Assisted by the Spin Fluctuations**The project supported by National Natural Science Foundation of China under Grant No. 19974001 and Ministry of Science andTechnology of China (NKBRSF-G19990646)

A theory of the c-axis infrared conductivity of a dx2-y2-wave superconductor due to the competition between the interlayer direct hopping and the hopping assisted by the spin fluctuations has been developed. The prediction of our theory captures the main feature of the experiment. Thus we argue that the anomalous behavior of the c-axis infrared conductivity of the underdoped cuprates in superconducting state may be properly understood within the theory.

Direct comparisons of rates for low temperature diffusion of hydrogen and deuterium on Cu(001) from quantum mechanical calculations and scanning tunneling microscopy experiments

Recent experiments by Lauhon and Ho using scanning tunneling microscopy (STM) observed the direct hopping of H and D on Cu(001) as a function of temperature. They found nearly temperature independent tunneling for H below 60 K, but could not detect the tunneling threshold for D (it is at least 1000 times lower than for H). The availability of such direct and accurate measurements provides the opportunity for validating the level of theory required to predict the diffusion of adsorbates on surfaces. Thus, we carried out density functional theory (DFT) using the generalized gradient approximation (GGA-II) on periodic slabs. The calculated tunneling rate of 4.74×10−4 s−1 for H is in close agreement with the experimental value of 4.4×10−4 s−1. We predict 4.66×10−9 s−1 for the tunneling rate of D (one hop every 83 months!). Between 60 and 80 K, the calculated thermally activated diffusion rate of H is 1012.88 exp(−0.181 eV/kT) s−1 in close agreement with the STM value: 1012.9±0.3 exp(−0.197 eV/kT). For deuterium, between 50 and 80 K, the calculated rate is 1012.70 exp(−0.175 eV/kT) s−1 in close agreement with the STM value: 1012.7±0.2 exp(−0.194 eV/kT) s−1. These results validate that such first principles theory can be used to predict the diffusion (including tunneling) for adsorbates on surfaces, providing important data needed to unravel surface processes in catalysis and crystal growth.

Effect of Spin Fluctuations on the c‐Axis Optical Conductivity in Underdoped Cuprate

A theory of optical conductivity due to the competition between interlayer direct hopping and hopping assisted by spin fluctuations has been developed. The prediction of our theory captures the main feature of the experiment. Thus, we argue that the anomalous behavior of the c-axis optical conductivity in the underdoped cuprates may be properly understood within the theory.

Effect of spin fluctuations on thec-axis thermoelectric power in underdopedLa2−xSrxCuO4+δ

A theory of the thermoelectric power due to the competition between interlayer direct hopping and the hopping assisted by the spin fluctuations has been developed. The prediction of the theory captures the main feature of experiment. Thus we argue that the c-axis thermoelectric power exhibits metallic behavior while the c-axis electronic conductivity appears to be nonmetallic in the underdoped LaSrCuO and may be properly understood within the theory.

Electronic Coupling for Charge Transfer and Transport in DNA

We calculated electronic matrix elements for hole transfer between adjacent nucleobases in DNA. Calculations of the matrix elements for intrastrand and interstrand transfer were performed at the Hartree−Fock level employing the 6-31G* and 6-311G** basis sets. The matrix elements for intrastrand hole transfer, for which a wealth of experimental solution data is available, are almost independent of the basis set and exhibit an exponential interbase distance dependence, sensitivity to the donor−acceptor geometry, and dependence on 5‘ → 3‘ direction base sequence. The calculated intrastrand hole transfer matrix elements between adjacent thymines, v+(T,T) = 0.16 eV, is in good agreement with the experimental estimate, v+(T,T) = 0.18 eV, inferred from hole hopping in G+(T)mGGG (m = 1−3). The features of the nucleobase bridge specificity for superexchange-induced hole hopping between guanines in G+XY...G (X,Y = T or A) were elucidated, with the prediction of enhanced efficiency of thymine relative to adenine as mediator. Information on superexchange-mediated intrastrand and direct interstrand hole hopping between guanine bases was also inferred. Our results for interstrand, adjacent G+G coupling predict the existence of zigzagging pathways for hole hopping, in line with experiment.

Effect of spin fluctuations on the c-axis electronic conduction in YBaCU2O7-delta

A theory of the c-axis electronic conduction due to the competition between interlayer direct hopping and the hopping assisted by the spin fluctuations has been developed and on this basis the experimental results of YBa2Cu3O7-delta have been interpreted. Our theoretical analysis fits the experimental results excellently and provides good evidence that the spin fluctuations affect the c-axis electronic conduction and lead to the anomalous behaviour.

Profile of a decaying crystalline cone

The decay of a crystalline cone below the roughening transition is studied. We consider local mass transport through surface diffusion, focusing on the two cases of diffusion limited and attachment-detachment limited step kinetics. In both cases, we describe the decay kinetics in terms of step flow models. Numerical simulations of the models indicate that in the attachment-detachment limited case the system undergoes a step bunching instability if the repulsive interactions between steps are weak. Such an instability does not occur in the diffusion limited case. In stable cases the height profile, h(r,t), is flat at radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz for the time-dependent profile of the cone yields analytical values for the scaling exponents and a differential equation for the scaling function. In the long time limit this equation provides an exact description of the discrete step dynamics. It admits a family of solutions and the mechanism responsible for the selection of a unique scaling function is discussed in detail. Finally we generalize the model and consider permeable steps by allowing direct adatom hops between neighboring terraces. We argue that step permeability does not change the scaling behavior of the system, and its only effect is a renormalization of some of the parameters.

Ab initioand semiempirical calculations ofH−centers in MgO crystals

The atomic and electronic structure of ${\mathrm{H}}^{\ensuremath{-}}$ ions substituting for ${\mathrm{O}}^{2\ensuremath{-}}$ ions in regular sites in MgO crystals are calculated using an ab initio Hartree-Fock (HF) cluster approach and its semiempirical version, intermediate neglect of the differential overlap. The theoretical optical absorption energy is predicted to be 10 eV, which is supported by analysis of experimental data for the ${\mathrm{H}}^{\ensuremath{-}}$ centers in a series of ionic crystals. The HF simulations of ${\mathrm{H}}^{\ensuremath{-}}$ ion diffusion via direct interstitial hops along the [100] axis predict an activation energy of about 3 eV.

Spin-polaron damping in the spin-fermion model for cuprate superconductors

A self-consistent, spin rotational invariant Green's function procedure has been developed to calculate the spectral function of carrier excitations in the spin-fermion model for the CuO2 plane. We start from the mean field description of a spin polaron in the Mori-Zwanzig projection method. In order to determine the spin polaron lifetime in the self-consistent Born approximation, the self-energy is expressed by an irreducible Green's function. Both, spin polaron and bare hole spectral functions are calculated. The numerical results show a well pronounced quasiparticle peak near the bottom of the dispersion at (pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather large damping away from the minimum and an asymmetry of the spectral function with respect to the antiferromagnetic Brillouin zone. These findings are in qualitative agreement with photoemission data for undoped cuprates. The direct oxygen-oxygen hopping is responsible for a more isotropic minimum at (pi/2,pi/2).

P43 Carcinome embryonnaire pur de l'ovaire. À propos de trois cas

The in situ conductivity of some poly(cyclopentadithiophenes) modified at the 4 position with reducible (p-nitrophenyl and p-N-methylpyridyl) or oxidizable (ferrocene) redox groups was investigated in acetonitrile. In the nitrophenyl- and pyridyl-functionalized polymers the reduction, which is confined to the pendant redox group, makes the polymer mixed-valence conductive. The conductivity ((3–6) × 10−3 S cm−1), which is the highest reported for a redox polymer, remains within the limits expected for a direct intersite electron hopping. In the ferrocene-functionalized polymers the oxidation involves both the redox group and the backbone. Redox conduction at the ferrocene group is enhanced by hopping through the conductive polymer backbone. The hopping rate is increased by a decrease of the ferrocene-backbone distance and by conjugation of ferrocene with the backbone itself making conductivity increase up to 1 S cm−1.

Quantitative theory of current-induced step bunching on Si(111)

We use a one-dimensional step model to study quantitatively the growth of step bunches on Si(111) surfaces induced by a direct heating current. Parameters in the model are fixed from experimental measurements near 900 $\ifmmode^\circ\else\textdegree\fi{}$C under the assumption that there is local mass transport through surface diffusion and that step motion is limited by the attachment rate of adatoms to step edges. The direct heating current is treated as an external driving force acting on each adatom. Numerical calculations show both qualitative and quantitative agreement with experiment. A force in the step down direction will destabilize the uniform step train towards step bunching. The average size of the step bunches grows with electromigration time $t$ as ${t}^{\ensuremath{\beta}}$, with $\ensuremath{\beta}\ensuremath{\approx}0.5$, in agreement with experiment and with an analytical treatment of the steady states. The model is extended to include the effect of direct hopping of adatoms between different terraces. Monte Carlo simulations of a solid-on-solid model, using physically motivated assumptions about the dynamics of surface diffusion and attachment at step edges, are carried out to study two-dimensional features that are left out of the present step model and to test its validity. These simulations give much better agreement with experiment than previous work. We find a step bending instability when the driving force is along the step edge direction. This instability causes the formation of step bunches and antisteps that is similar to that observed in experiment.

Study of Some Factors Affecting Thermodynamic Properties of the Insulating Phase of the Periodic Anderson Lattice Model

Within the framework of slave-boson mean-field theory, we study the thermodynamic properties of the periodic Anderson lattice model with half-filled conduction band and one 4f electron at each primitive cell and the degeneracy . It is found that after taking into account the direct nearest-neighbor hopping, such a periodic Anderson lattice model can exhibit both an insulating ground state and a heavy-fermion metal ground state depending on the value of the bare energy level , the hybridization matrix element , and the direct hopping strength . This is unlike the case neglecting the direct hopping, in which such a periodic Anderson lattice model will predict an insulating ground state only.

Transmission techniques for radio LAN's-a comparative performance evaluation using ray tracing

This paper uses the results of ray tracing in a typical indoor test area to compare the performance of major radio transmission techniques that are used as the air interface in evolving standards and major wireless local area network (WLAN) products. The performance of direct sequence (DS) and frequency hopping spread spectrum (FHSS) in the test area are compared with the performance of multicarrier modems, as well as modems using decision feedback equalization (DFE) and sectored antenna systems (SAS). The validity of using ray tracing for performance evaluation is examined by comparing the results with the results of performance evaluation obtained from the empirical measurement of the channel characteristics. Based on the maximum achievable data rate and minimum power requirement determined in the test area, operation of all modems in bandlimited and power limited applications are discussed.