Qingbin Tang

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.

Transport properties and anisotropy of superconducting (Li1−x Fe x )OHFeSe single crystals

Large size single crystals of (Li1−x Fe x )OHFeSe have been synthesized via a hydrothermal ion-exchange technique using K0.8Fe2Se2 single crystals as the main raw material. The onset superconducting transition temperature is up to 40.3 K. The critical current density is as large as 1.9 × 105 A cm−2 at 5 K and self field. The upper critical fields have been determined by analyzing the relationship between resistivity and temperature under different applied fields along c-axis and ab-plane, respectively. An anisotropic factor about 11 is obtained, which is further confirmed by fitting the data obtained from the angle-dependent resistivity according to the anisotropic Ginzburg–Landau theory. The flux pinning potential (U 0/k B) is as large as 6429 and 3115 K at 0.1 T in H//ab and H//c orientation, respectively. However, the flux pinning potentials are very sensitive to the applied magnetic field and will fast decrease with increase of the magnetic field in both directions, which are obviously different from the Ba1−x K x Fe2As2 superconductor indicating a different flux pinning mechanism in the two kinds of superconductors.

Vortex phase transition and anisotropy behavior of optimized (Li1−x Fe x OH)FeSe single crystals

(Li1−x Fe x OH)FeSe single crystals have been successfully synthesized by a hydrothermal ion-exchange technique. We have carried out systematic research on the influence of the concentration of selenourea (c m) on the superconducting properties of (Li1−x Fe x OH)FeSe single crystals. The optimized specimens possess an onset superconducting transition temperature T c of up to 42 K obtained at c m = 0.8 mol l−1, and the corresponding residual resistivity ratio (RRR = ρ (300 K)/ρ (T c)) is estimated to be 11. The vortex liquid-to-glass phase transition has been discussed based on the collective pinning model. The large distance between the H g and H c2 lines, as well as the much narrower region of the pinned vortex-liquid phase, indicate a weak vortex-pinning ability in (Li1−x Fe x OH)FeSe single crystals. The big out-plane anisotropy behavior versus temperature has been explored according to the anisotropic Ginzburg–Landau theory, and the anisotropy factor Γ displays linear behavior towards temperature similar to that of SmFeAsO1−x F x superconductors.

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.

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).