Benhai Yu

ELASTIC AND THERMODYNAMIC PROPERTIES OF NiAl AND Ni3Al FROM FIRST-PRINCIPLES CALCULATIONS

The elastic and thermodynamic characteristics of NiAl and Ni3Al crystals have been investigated by using a method of density functional theory within the generalized gradient approximation. The three independent elastic constants are C11 = 229.8 GPa, C12 = 124.7 GPa, C44 = 115.7 GPa for NiAl and C11 = 239.6 GPa, C12 = 151.7 GPa, C44 = 123.4 GPa for Ni3Al. The bulk moduli, shear moduli, Youngs moduli, Poissons ratios and ratios of B/G of NiAl and Ni3Al are also calculated. In addition, the dependences of the bulk moduli on temperatures and pressures as well as the linear thermal expansion coefficients (αL) versus temperatures are evaluated and discussed. Both NiAl and Ni3Al crystals are elastically anisotropic. Especially for NiAl crystal, which should be much brittler than Ni3Al, lies at the critical point of brittle behavior. In addition, the present results are well in line with experimental and other theoretical results.

Transition of recollision trajectories from linear to elliptical polarization.

Using a classical ensemble method, we revisit the topic of recollision and nonsequential double ionization with elliptically polarized laser fields. We focus on how the recollision mechanism transitions from short trajectories with linear polarization to long trajectories with elliptical polarization. We propose how this transition can be observed by meansuring the carrier-envelop-phase dependence of the correlated electron momentum spectra using currently available few-cycle laser pulses.

Role of e–e repulsion interaction in nonsequential double ionization of Xe by linearly and elliptically polarized laser pulses

We investigated the nonsequential double ionization (NSDI) of Xe by linearly polarized (LP) and elliptically polarized (EP) laser pulses using the classical ensemble model. For the two cases, the ensemble from NSDI was separated into two kinds of subensemble according to the relative momentum along the y-axis , respectively. The results show only for the case of relatively small , the momentum distribution along the x-axis can show a V-like shape obviously, which is according with the experiment [Phys. Rev. Lett.99, 263003 (2007)]. What is important is that we firstly examine the role of the repulsion interaction between two electrons for the two kinds of subensemble, and find that the process of NSDI for the case of relatively small exits a strong repulsion interaction which contributes significantly to the V-like shape. However, for the case of relatively big , the repulsion interaction between two electrons is relatively weak, and both the nuclear Coulomb attraction and electric field force dominate the process of NSDI.

Electron correlations in nonsequential double ionization of helium at intensity below the recollision threshold

With a classical three-dimensional ensemble model, we investigate the correlation behavior of the electrons from nonsequential double ionization of helium at intensities below the recollision threshold. At these laser intensities well below the recollision threshold, the two-electron momentum spectra pairs along the laser polarization show anticorrelated behaviors. This anticorrelated behavior becomes stronger when the laser intensity decreases, and then becomes weaker as the laser intensity further decreases. The responsible microscopic dynamics for the anticorrelated behavior and its intensity dependence is explored by back analyzing the double ionization trajectories.

Non-sequential double ionization of argon by elliptically polarized laser pulses

With a classical ensemble model, we investigated non-sequential double ionization (NSDI) of argon by elliptically polarized laser pulses. The results show that the correlation behaviors of two electrons depend strongly on the laser intensity. At relatively high laser intensity, the momentum spectra of two electrons along the long axis of the laser polarization plane are mainly distributed in the first and third quadrants and display V-like structures. However, at relatively low laser intensity, the momentum spectra of two electrons are mainly distributed in the second and fourth quadrants. By back analyzing the classical trajectories of NSDI, we find that all of the successful NSDI events still come from recollision in the cases of elliptically polarized laser pulses, and the final-state electron repulsion plays a decisive role for the V-like structure along the long axis of the laser polarization plane. In addition, we find that the initial velocity of the first electron at ionization along the short axis of the laser polarization plane are essential for the recollision, and the time delay between the first ionization and recollision depends on the ellipticity strongly.

Efficient generation of ultra broadband supercontinuum in a mid-infrared field

We theoretically investigate the generation of ultra broadband supercontinuum from helium atoms exposed to a linearly polarized mid-infrared field. By adopting a UV trigger pulse to the mid-infrared field, the continuous harmonic yields are significantly enhanced by 3.5 orders, and a supercontinuum with the width of 230 eV is observed. The spectrum can support a sub-20 as pulse, which is below one atomic unit of time (24 as). The short quantum path is selected by adjusting the time delay between the UV pulse and the mid-infrared pulse, then broadband single 70 as pulses with tunable central wavelengths are obtained, which can be extended to the ‘water window’ region (284–543 eV).