27 research outputs found
Anisotropy and Magnetic Field Effects on the Entanglement of a Two Qubit Heisenberg \u3ci\u3eXY\u3c/i\u3e Chain
We investigate the entanglement of a two-qubit anisotropic Heisenberg XY chain in thermal equilibrium at temperature T in the presence of an external magnetic field B along the z axis. By means of the combined influences of anisotropic interactions and a magnetic field B, one is able to produce entanglement for any finite T, by adjusting the magnetic field strength. This contrasts with the isotropic interaction or the B = 0 cases, for which there is no entanglement above a critical temperature Tc that is independent of the external B field
Fermionic concurrence in the extended Hubbard dimer
In this paper, we introduce and study the fermionic concurrence in a two-site
extended Hubbard model. Its behaviors both at the ground state and finite
temperatures as function of Coulomb interaction (on-site) and
(nearest-neighbor) are obtained analytically and numerically. We also
investigate the change of the concurrence under a nonuniform field, including
local potential and magnetic field, and find that the concurrence can be
modulated by these fields.Comment: 5 pages, 7 figure
Entanglement, quantum phase transition and scaling in XXZ chain
Motivated by recent development in quantum entanglement, we study relations
among concurrence , SU(2) algebra, quantum phase transition and
correlation length at the zero temperature for the XXZ chain. We find that at
the SU(2) point, the ground state possess the maximum concurrence. When the
anisotropic parameter is deformed, however, its value decreases. Its
dependence on scales as in the XY metallic
phase and near the critical point (i.e. ) of the Ising-like
insulating phase. We also study the dependence of on the correlation length
, and show that it satisfies near the critical point. For
different size of the system, we show that there exists a universal scaling
function of with respect to the correlation length .Comment: 4 pages, 3 figures. to appear in Phys. Rev.
Sturmian bases for two-electron systems in hyperspherical coordinates
We give a detailed account of an spectral approach
for the calculation of energy spectra of two active electron atoms in a system
of hyperspherical coordinates. In this system of coordinates, the Hamiltonian
has the same structure as the one of atomic hydrogen with the Coulomb potential
expressed in terms of a hyperradius and the nuclear charge replaced by an angle
dependent effective charge. The simplest spectral approach consists in
expanding the hyperangular wave function in a basis of hyperspherical
harmonics. This expansion however, is known to be very slowly converging.
Instead, we introduce new hyperangular sturmian functions. These functions do
not have an analytical expression but they treat the first term of the
multipole expansion of the electron-electron interaction potential, namely the
radial electron correlation, exactly. The properties of these new functions are
discussed in detail. For the basis functions of the hyperradius, several
choices are possible. In the present case, we use Coulomb sturmian functions of
half integer angular momentum. We show that, in the case of H, the accuracy
of the energy and the width of the resonance states obtained through a single
diagonalization of the Hamiltonian, is comparable to the values given by
state-of-the-art methods while using a much smaller basis set. In addition, we
show that precise values of the electric-dipole oscillator strengths for
transitions in helium are obtained thereby confirming the
accuracy of the bound state wave functions generated with the present method.Comment: 28 pages, 4 figure
Angular Distributions for Double Ionization of \u3ci\u3eLi\u3c/i\u3e\u3csup\u3e-\u3c/sup\u3e by an Ultrashort, Intense Laser Pulse
We predict photoelectron angular distributions for double ionization of Li- by both weak and intense ultrashort, linearly polarized laser pulses by direct numerical integration of the three-dimensional, time-dependent Schrödinger equation. Li- is treated as a two-active electron system. Near threshold, for low intensity we recover general features of angular distributions for one-photon double ionization. For the intense field (multiphoton) case, the photoelectron angular distribution changes significantly, particularly in directions parallel and perpendicular to the laser polarization axis
Two-active electron approach to multi-electron systems in intense ultrashort laser pulses: Application to Li\u3csup\u3eâ\u3c/sup\u3e
The interaction of Liâ with an ultrashort intense laser pulse is investigated by solving the time-dependent Schrödinger equation (TDSE) in the dipole approximation using a two-active electron approach. We describe the numerical solution of the TDSE and give a more detailed presentation of a technique for obtaining angular distributions for double ionization by intense ultrashort laser pulses. We show how selection rules observed in the angular distributions for double ionization by an intense laser field can be derived from the symmetry properties of the wave function. In an attempt to elucidate the mechanisms for double ionization in the multiphoton regime, we have performed numerical experiments for ionization by single cycle and double half-cycle pulses. Our preliminary results unveil the important contribution of the shake-off mechanism, in addition to the rescattering and sequential ionization mechanisms
Multielectron system in an ultrashort, intense laser field: A nonperturbative, time-dependent two-active-electron approach
We present a two-active-electron (TAE) approach for solving the time-dependent Schrödinger equation (TDSE) for the interaction of a multi-electron system with an ultrashort, intense, and linearly polarized laser pulse [Lagmago Kamta and Starace, Phys. Rev. Lett. 86, 5687 (2001)]. A technique for obtaining angular distributions for double ionization by such pulses is also described. The approach for solving the TDSE in the TAE approximation is full dimensional and accounts for correlations between the two electrons, as well as the polarization of the core. It is based on a configuration-interaction expansion of the time-dependent wave function in terms of one-electron atomic orbitals. Applying the method to the lithium negative ion (Li-), we display the time-dependent dynamics of the photodetachment process. For low intensities, our results for the detachment yield follow expectations from lowest-order perturbation theory and agree satisfactorily with R-matrix calculations. Our results for angular distributions indicate that following multiphoton double ionization by an intense laser field, electrons are predominantly ejected along the laser polarization axis; however, a significant number are ejected perpendicularly to this axis. An angular momentum-based analysis of these angular distributions indicates that, in the dipole approximation and for an initial 1Se state interacting with a linearly polarized laser field, double ejection of both electrons along the direction perpendicular to the laser polarization axis can only occur following absorption of an even number of photons, whereas multiphoton absorption of an odd number of photons does not lead to double ejection at these angles