Entanglement entropies in the ground states of helium-like atoms

We examine the entanglement in the ground states of helium and helium-like ions using an original Hylleraas expansion. The von Neumann and linear entropies of the reduced density matrix are accurately computed by performing the Schmidt decomposition of the S singlet spatial wavefunctions. The results presented are more accurate than currently available in published literature.Comment: 6 pages, 1 figur

A Variational Expansion for the Free Energy of a Bosonic System

In this paper, a variational perturbation scheme for nonrelativistic many-Fermion systems is generalized to a Bosonic system. By calculating the free energy of an anharmonic oscillator model, we investigated this variational expansion scheme for its efficiency. Using the modified Feynman rules for the diagrams, we obtained the analytical expression of the free energy up to the fourth order. Our numerical results at various orders are compared with the exact and other relevant results.Comment: 9 pages, 3 EPS figures. With a few typo errors corrected. to appear in J. Phys.

Scaling property of variational perturbation expansion for general anharmonic oscillator

We prove a powerful scaling property for the extremality condition in the recently developed variational perturbation theory which converts divergent perturbation expansions into exponentially fast convergent ones. The proof is given for the energy eigenvalues of an anharmonic oscillator with an arbitrary $x^p$-potential. The scaling property greatly increases the accuracy of the results

Gaussian Effective Potential and the Coleman's normal-ordering Prescription : the Functional Integral Formalism

For a class of system, the potential of whose Bosonic Hamiltonian has a Fourier representation in the sense of tempered distributions, we calculate the Gaussian effective potential within the framework of functional integral formalism. We show that the Coleman's normal-ordering prescription can be formally generalized to the functional integral formalism.Comment: 6 pages, revtex; With derivation details and an example added. To appear in J. Phys.

Mass generation without phase coherence in the Chiral Gross-Neveu Model at finite temperature and small N in 2+1 dimensions

The chiral Gross-Neveu model is one of the most popular toy models for QCD. In the past, it has been studied in detail in the large-N limit. In this paper we study its small-N behavior at finite temperature in 2+1 dimensions. We show that at small N the phase diagram of this model is {\it principally} different from its behavior at $N\to \infty$. We show that for a small number $N$ of fermions the model possesses two characteristic temperatures $T_{KT}$ and $T^*$. That is, at small N, along with a quasiordered phase $0<T<T_{KT}$ the system possesses a very large region of precursor fluctuations $T_{KT}<T<T^*$ which disappear only at a temperature $T^*$, substantially higher than the temperature $T_{KT}$ of Kosterlitz-Thouless transition.Comment: a factor 2 corrected. An extended discussion of similarities and differences of low-N behavior of the chiral GN model and various models of superconductivity is currently in preparation and will be presented in additional articl

Variational Interpolation Algorithm between Weak- and Strong-Coupling Expansions

For many physical quantities, theory supplies weak- and strong-coupling expansions of the types $\sum a_n \alpha ^n$ and \alpha ^p\sum b_n (\alpha^{-2/q) ^n, respectively. Either or both of these may have a zero radius of convergence. We present a simple interpolation algorithm which rapidly converges for an increasing number of known expansion coefficients. The accuracy is illustrated by calculating the ground state energies of the anharmonic oscillator using only the leading large-order coefficient $b_0$ (apart from the trivial expansion coefficent $a_0=1/2$). The errors are less than 0.5 for all g. The algorithm is applied to find energy and mass of the Fr\"ohlich-Feynman polaron. Our mass is quite different from Feynman's variational approach.Comment: PostScript, http://www.physik.fu-berlin.de/kleinert.htm

Variational perturbation approach to the Coulomb electron gas

The efficiency of the variational perturbation theory [Phys. Rev. C {\bf 62}, 045503 (2000)] formulated recently for many-particle systems is examined by calculating the ground state correlation energy of the 3D electron gas with the Coulomb interaction. The perturbation beyond a variational result can be carried out systematically by the modified Wick's theorem which defines a contraction rule about the renormalized perturbation. Utilizing the theorem, variational ring diagrams of the electron gas are summed up. As a result, the correlation energy is found to be much closer to the result of the Green's function Monte Carlo calculation than that of the conventional ring approximation is.Comment: 4 pages, 3 figure

Sine-Gordon Expectation Values of Exponential Fields With Variational Perturbation Theory

In this paper, expectation values of exponential fields in the 2-dimensional Euclidean sine-Gordon field theory are calculated with variational perturbation approach up to the second order. Our numerical analysis indicates that for not large values of the exponential-field parameter $a$, our results agree very well with the exact formula conjectured by Lukyanov and Zamolodchikov in Nucl. Phys. B 493, 571 (1997).Comment: only abbreviated the original, 12 pages, 2 EPS figures, to be published in Phys. Lett.

QCD Running Coupling Constant in the Timelike Region

By using a non-perturbative expansion and the dispersion relation for the Adler $D$--function we propose a new method for constructing the QCD effective coupling constant in the timelike region.Comment: LaTeX, 11 pages, 4 figure