Plasmon signatures in high harmonic generation

High harmonic generation in polarizable multi-electron systems is investigated in the framework of multi-configuration time-dependent Hartree-Fock. The harmonic spectra exhibit two cut offs. The first cut off is in agreement with the well established, single active electron cut off law. The second cut off presents a signature of multi-electron dynamics. The strong laser field excites non-linear plasmon oscillations. Electrons that are ionized from one of the multi-plasmon states and recombine to the ground state gain additional energy, thereby creating the second plateau.Comment: Major revision, 12 pages, 5 figures, submitted to J. Phys. B (2005), accepte

Phase Space Approach to Laser-driven Electronic Wavepacket Propagation

We propose a phase space method to propagate a quantum wavepacket driven by a strong external field. The method employs the so-called biorthogonal von Neumann basis recently introduced for the calculation of the energy eigenstates of time-independent quantum systems [A. Shimshovitz and D.J. Tannor, arXiv:1201.2299v1]. While the individual elements in this basis set are time-independent, a small subset is chosen in a time-dependent manner to adapt to the evolution of the wavepacket in phase space. We demonstrate the accuracy and efficiency of the present propagation method by calculating the electronic wavepacket in a one-dimensional soft-core atom interacting with a superposition of an intense, few-cycle, near-infrared laser pulse and an attosecond extreme-ultraviolet laser pulse.Comment: 4 pages, 4 figures. The following article has been submitted to the Journal of Chemical Physics. After it is published, it will be found at http://jcp.aip.org

Two Component Heat Diffusion Observed in CMR Manganites

We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434 v2: Abstract correcte

Frank's constant in the hexatic phase

Using video-microscopy data of a two-dimensional colloidal system the bond-order correlation function G6 is calculated and used to determine the temperature-dependence of both the orientational correlation length xi6 in the isotropic liquid phase and the Frank constant F_A in the hexatic phase. F_A takes the value 72/pi at the hexatic to isotropic liquid phase transition and diverges at the hexatic to crystal transition as predicted by the KTHNY-theory. This is a quantitative test of the mechanism of breaking the orientational symmetry by disclination unbinding

Two-photon ionization of Helium studied with the multiconfigurational time-dependent Hartree-Fock method

The multiconfigurational time-dependent Hartree-Fock method (MCTDHF) is applied for simulations of the two-photon ionization of Helium. We present results for the single- and double ionization from the groundstate for photon energies in the non-sequential regime, and compare them to direct solutions of the Schr\"odinger equation using the time-dependent (full) Configuration Interaction method (TDCI). We find that the single-ionization is accurately reproduced by MCTDHF, whereas the double ionization results correctly capture the main trends of TDCI

Many-body theory for systems with particle conversion: Extending the multiconfigurational time-dependent Hartree method

We derive a multiconfigurational time-dependent Hartree theory for systems with particle conversion. In such systems particles of one kind can convert to another kind and the total number of particles varies in time. The theory thus extends the scope of the available and successful multiconfigurational time-dependent Hartree methods -- which were solely formulated for and applied to systems with a fixed number of particles -- to new physical systems and problems. As a guiding example we treat explicitly a system where bosonic atoms can combine to form bosonic molecules and vise versa. In the theory for particle conversion, the time-dependent many-particle wavefunction is written as a sum of configurations made of a different number of particles, and assembled from sets of atomic and molecular orbitals. Both the expansion coefficients and the orbitals forming the configurations are time-dependent quantities that are fully determined according to the Dirac-Frenkel time-dependent variational principle. Particular attention is paid to the reduced density matrices of the many-particle wavefunction that appear in the theory and enter the equations of motion. There are two kinds of reduced density matrices: particle-conserving reduced density matrices which directly only couple configurations with the same number of atoms and molecules, and particle non-conserving reduced density matrices which couple configurations with a different number of atoms and molecules. Closed-form and compact equations of motion are derived for contact as well as general two-body interactions, and their properties are analyzed and discussed.Comment: 46 page

Time-dependent quantum Monte Carlo: preparation of the ground state

We study one-dimensional (1D) and two-dimensional (2D) Helium atoms using a new time-dependent quantum Monte Carlo (TDQMC) method. The TDQMC method employs random walkers, with a separate guiding wave attached to each walker. The ground state is calculated by a self-consistent solution of complex-time Schroedinger equations for the guiding waves and of equations for the velocity fields of the walkers. Our results show that the many-body wavefunction and the ground state energy of the model atoms are very close to those predicted by the standard diffusion quantum Monte Carlo method. The obtained ground state can further be used to examine correlated time-dependent processes which include, for example, interaction of atoms and molecules with external electromagnetic fields.Comment: 9 pages, 5 figure

The multi-configurational time-dependent Hartree method for bosons: Many-body dynamics of bosonic systems

The evolution of Bose-Einstein condensates is amply described by the time-dependent Gross-Pitaevskii mean-field theory which assumes all bosons to reside in a single time-dependent one-particle state throughout the propagation process. In this work, we go beyond mean-field and develop an essentially-exact many-body theory for the propagation of the time-dependent Schr\"odinger equation of $N$ interacting identical bosons. In our theory, the time-dependent many-boson wavefunction is written as a sum of permanents assembled from orthogonal one-particle functions, or orbitals, where {\it both} the expansion coefficients {\it and} the permanents (orbitals) themselves are {\it time-dependent} and fully determined according to a standard time-dependent variational principle. By employing either the usual Lagrangian formulation or the Dirac-Frenkel variational principle we arrive at two sets of coupled equations-of-motion, one for the orbitals and one for the expansion coefficients. The first set comprises of first-order differential equations in time and non-linear integro-differential equations in position space, whereas the second set consists of first-order differential equations with time-dependent coefficients. We call our theory multi-configurational time-dependent Hartree for bosons, or MCTDHB($M$), where $M$ specifies the number of time-dependent orbitals used to construct the permanents. Numerical implementation of the theory is reported and illustrative numerical examples of many-body dynamics of trapped Bose-Einstein condensates are provided and discussed.Comment: 30 pages, 2 figure