Symmetry interaction and many-body correlations

Many-body correlations generated by the symmetry potential naturally arise in the molecular-dynamics CoMD-II model. The effect of these correlations on the collision dynamics at Fermi energies is discussed. In particular we show that two-body correlations generated by the symmetry potential are able to produce large effects in simple observables such as the charge distributions. A comparison with the predictions, based on EOS static calculations, is also discussed.     

The barrier height inhomogeneity in Al/p-Si Schottky barrier diodes with native insulator layer

The current-voltage (I-V) characteristics of Al/p-Si Schottky barrier diodes (SBDs) with native insulator layer were measured in the temperature range of 150-375 K. The estimated zero-bias barrier height Φ_B0 and the ideality factor n assuming thermionic emission (TE) theory show strong temperature dependence. Evaluation of the forward I-V data reveals an increase of zero-bias barrier height Φ_B0 but decrease of ideality factor n with increase in temperature. The conventional Richardson plot exhibits non-linearity below 250 K with the linear portion corresponding to activation energy of 0.41 eV and Richardson constant (A^*) value of 1.3 × 10⁻⁴ A cm⁻² K⁻² is determined from intercept at the ordinate of this experimental plot, which is much lower than the known value of 32 A cm² K² for holes in p-type Si. Such behavior is attributed to Schottky barrier inhomogene ties by assuming a Gaussian distribution of barrier heights (BHs) due to barrier height inhomogeneities that prevail at interface. Also, Φ_B0 versus q/2kT plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of Φ_B0 = 1.055 eV and σ₀ = 0.13 V for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Thus, the modified versus q/kT plot gives Φ_B0 and A^* as 1.050 eV and 40.08 A cm⁻² K⁻², respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 40.03 A cm⁻² K⁻² is very close to the theoretical value of 32 A K⁻² cm⁻² for p-type Si. Hence, it has been concluded that the temperature dependence of the forward I-V characteristics of the Al/p-Si Schottky barrier diodes with native insulator layer can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights.     

Two-temperature relaxation and melting after absorption of femtosecond laser pulse

The theory and experiments concerned with the electron-ion thermal relaxation and melting of overheated crystal lattice constitute the subject of this paper. The physical model includes two-temperature (2T) equation of state, many-body interatomic potential, the electron-ion energy exchange, electron thermal conductivity, and optical properties of solid, liquid, and two phase solid-liquid mixture. Two-temperature hydrodynamics and molecular dynamics codes are used. An experimental setup with pump-probe technique is used to follow evolution of an irradiated target with a short time step 100 fs between the probe femtosecond laser pulses. Accuracy of measurements of reflection coefficient and phase of reflected probe light are 1% and ∼1 nm, respectively. It is found that, firstly, the electron-electron collisions make a minor contribution to a light absorption in solid Al at moderate intensities; secondly, the phase shift of a reflected probe results from heating of ion subsystem and kinetics of melting of Al crystal during  ps, where t is time delay between the pump and probe pulses measured from the maximum of the pump; thirdly, the optical response of Au to a pump shows a marked contrast to that of Al on account of excitation of d-electrons.     

Numerical irreversibility in self-gravitating small N-body systems, II: Influence of instability affected by softening parameters

We investigate the fundamental characteristics of numerical irreversibility appearing in self-gravitating small N-body systems by means of a molecular dynamics method from the viewpoint of time-reversible dynamics. We reconsider a closed spherical system consisting of 250 point-particles interacting through the Plummer softened potential. To investigate the characteristics of numerical irreversibility, we examine the influence of the instability affected by the softening parameter for the softened potential (the instability considered here is the instability of a dynamical system in chaos theory, e.g., a separation rate of the distance between two nearby trajectories in phase space or speed space). To this end, under the restriction of constant initial energy, the softening parameter for the Plummer softened potential is varied from 0.005R to 0.050R, where R is the radius of the spherical container. We first confirm that the size of the softening parameter, i.e., the deviation of the potential from a pure gravitational potential, influences the virial ratio, the density, the pressure on the spherical container, etc., during an early stage of the relaxation process. Through a time-reversible simulation based on a velocity inversion technique, we demonstrate that numerical irreversibility due to round-off errors appears more rapidly with decreasing softening parameter. This means that the higher the instability of the system or the higher the separation rate of the distance between two nearby trajectories, the earlier the memory of the initial conditions is lost. We show that the memory loss time , when the simulated trajectory completely forgets its initial conditions, increases approximately linearly with the timescale of the chaotic system, i.e., the Lyapunov time t_λ. In a small self-gravitating system, propagation of numerical irreversibility or loss of reversibility depends on both the energy state of the system and the instability affected by the softening parameter.     

Molecular dynamics simulation of AFM studies of a single polymer chain

Single polymer chain force-extension behavior measured by Atomic Force Microscopy (AFM) was interpreted by molecular dynamics (MD) simulation performed by applying a bead-spring (coarse-graining) model in which the bond potential function between adjacent beads is described by a worm-like chain (WLC) model. Simulation results indicate that caution should be applied when interpreting experimental AFM data, because the data vary depending on the point of AFM tip-polymer chain attachment. This approach offers an effective way for eventual analysis of the mechanical behavior of complex polymer networks.     

Mechanism of current hysteresis in reduced rutile TiO2 crystals for resistive memory

Memory devices based on the reversible resistance switching of various materials are attractive for today’s semiconductor technology. The reproducible current hysteresis (resistance switching) characteristics of reduced TiO₂ single crystal are demonstrated. Basic models concerning the filamentary and Schottky barrier models are discussed. Good retention characteristics are exhibited by the accurate controlling of the annealing parameters.     

Molecular dynamics simulation of ultrafast laser ablation of fused silica film

Ultrafast laser ablation of fused silica is studied using molecular dynamics simulations. Ionization and generation of free electrons, absorption of the laser energy by free electrons and energy coupling between free electrons and ions are considered. The BKS potential is applied and modified to describe molecular interactions and the effect of free electrons. Smooth particle mesh of the Ewald method (SPME) is adopted to calculate the Coulomb force. It is found that the electrostatic Coulomb force, which is caused by the ionization, plays an important role in the laser ablation process.     

Determination of the laterally homogeneous barrier height of thermally annealed and unannealed Au/p-InP/Zn-Au Schottky barrier diodes

We have identically prepared Au/p-InP Schottky barrier diodes (SBDs). The diodes were annealed up to 400 °C thermally. The barrier height (BH) for the as-deposited Au/p-InP/Zn-Au SBDs from the current-voltage characteristics have varied from 0.58 to 0.72 eV, and ideality factor n from 1.14 to 1.47. The BH for the annealed SBDs from the current-voltage characteristics have varied from 0.76 to 0.82 eV, and ideality factor n from 1.17 to 1.39. As a result of the thermal annealing, it has been seen that the BH values of the annealed SBDs are larger than those of the as-deposited SBDs. We have determined a lateral homogeneous BH value of 0.72 eV for the as-deposited Au/p-InP SBD from the experimental linear relationship between barrier heights and ideality factors, and a value of 0.85 eV for the annealed Au/p-InP SBD. The increase of 0.13 eV in the BH value by means of 400 °C annealing has been ascribed to the formation of the excess charges that electrically actives on the semiconductor surface.