840 research outputs found
Perturbation theory for optical excitations in the one-dimensional extended Peierls--Hubbard model
For the one-dimensional, extended Peierls--Hubbard model we calculate
analytically the ground-state energy and the single-particle gap to second
order in the Coulomb interaction for a given lattice dimerization. The
comparison with numerically exact data from the Density-Matrix Renormalization
Group shows that the ground-state energy is quantitatively reliable for Coulomb
parameters as large as the band width. The single-particle gap can almost
triple from its bare Peierls value before substantial deviations appear. For
the calculation of the dominant optical excitations, we follow two approaches.
In Wannier theory, we perturb the Wannier exciton states to second order. In
two-step perturbation theory, similar in spirit to the GW-BSE approach, we form
excitons from dressed electron-hole excitations. We find the Wannier approach
to be superior to the two-step perturbation theory. For singlet excitons,
Wannier theory is applicable up to Coulomb parameters as large as half band
width. For triplet excitons, second-order perturbation theory quickly fails
completely.Comment: 32 pages, 12 figures, submtted to JSTA
Multi-band Gutzwiller wave functions for general on-site interactions
We introduce Gutzwiller wave functions for multi-band models with general
on-site Coulomb interactions. As these wave functions employ correlators for
the exact atomic eigenstates they are exact both in the non-interacting and in
the atomic limit. We evaluate them in infinite lattice dimensions for all
interaction strengths without any restrictions on the structure of the
Hamiltonian or the symmetry of the ground state. The results for the
ground-state energy allow us to derive an effective one-electron Hamiltonian
for Landau quasi-particles, applicable for finite temperatures and frequencies
within the Fermi-liquid regime. As applications for a two-band model we study
the Brinkman-Rice metal-to-insulator transition at half band-filling, and the
transition to itinerant ferromagnetism for two specific fillings, at and close
to a peak in the density of states of the non-interacting system. Our new
results significantly differ from those for earlier Gutzwiller wave functions
where only density-type interactions were included. When the correct spin
symmetries for the two-electron states are taken into account, the importance
of the Hund's-rule exchange interaction is even more pronounced and leads to
paramagnetic metallic ground states with large local magnetic moments.
Ferromagnetism requires fairly large interaction strengths, and the resulting
ferromagnetic state is a strongly correlated metal.Comment: 37 pages, 10 figures; accepted for publication in Phys. Rev. B 57
(March 15, 1998
Optical conductivity of the half-filled Hubbard chain
We combine well-controlled analytical and numerical methods to determine the
optical conductivity of the one-dimensional Mott-Hubbard insulator at zero
temperature. A dynamical density-matrix renormalization group method provides
the entire absorption spectrum for all but very small coupling strengths. In
this limit we calculate the conductivity analytically using exact
field-theoretical methods. Above the Lieb-Wu gap the conductivity exhibits a
characteristic square-root increase. For small to moderate interactions, a
sharp maximum occurs just above the gap. For larger interactions, another weak
feature becomes visible around the middle of the absorption band.Comment: 4 pages with 3 eps figures, published version (changes in text and
references
Effects of the Nearest-Neighbour Coulomb Interactions on the Ground State of the Periodic Anderson Model
The magnetic and non-magnetic ground states of the periodic Anderson model
with Coulomb interaction between -electrons on the nearest-neighbour(NN)
sites are investigated using a variational method, which gives exact
calculation of the expectation values in the limit of infinite dimensions. It
is shown that for a critical value of NN Coulomb interactions the magnetic
ground state of the periodic Anderson model in the Kondo regime is unstable.
Factors in terms of the physical processes responsible for instability of the
magnetic ground state are also discussed. Our study indicates the importance of
the NN Coulomb interactions for correlated two band models.Comment: RevTeX, 6 pages, 5 figures, to appear in Phys. Rev.
Random phase approximation for multi-band Hubbard models
We derive the random-phase approximation for spin excitations in general
multi-band Hubbard models, starting from a collinear ferromagnetic Hartree-Fock
ground state. The results are compared with those of a recently introduced
variational many-body approach to spin-waves in itinerant ferromagnets. As we
exemplify for Hubbard models with one and two bands, the two approaches lead to
qualitatively different results. The discrepancies can be traced back to the
fact that the Hartree-Fock theory fails to describe properly the local moments
which naturally arise in a correlated-electron theory.Comment: 25 pages, 2 figure
Humanitarian aid as an integral part of the European Union's external action: the challenge of reconciling coherence and independence
The article focuses on the European Union's (EU) humanitarian aid policy. It addresses the challenge for the EU to deliver independent humanitarian aid while simultaneously seeking to establish more coherence between its external policies. The article examines how the EU tries to reconcile these potentially conflicting policy goals, both de jure and in practice. Empirically, it explores the interaction between EU humanitarian aid and development cooperation, the Common Foreign and Security Policy, and trade policy. While the independence of the humanitarian aid delivery is, for the most part, not being undermined, it remains difficult to establish positive synergies with other external policies because of institutional hurdles and legal constraints, as well as political obstacles and operational incompatibilities
Modulation spectroscopy with ultracold fermions in an optical lattice
We propose an experimental setup of ultracold fermions in an optical lattice
to determine the pairing gap in a superfluid state and the spin ordering in a
Mott-insulating state. The idea is to apply a periodic modulation of the
lattice potential and to use the thereby induced double occupancy to probe the
system.
We show by full time-dependent calculation using the adaptive time dependent
density-matrix renormalization group method that the position of the peak in
the spectrum of the induced double occupancy gives the pairing energy in a
superfluid and the interaction energy in a Mott-insulator, respectively. In the
Mott-insulator we relate the spectral weight of the peak to the spin ordering
at finite temperature using perturbative calculations
Photoemission study of (VM)O (M=Cr, Ti)
We present high-resolution bulk-sensitive photoemission spectra of
(VM)O (M=Cr, Ti). The measurements were made for the
paramagnetic metal (PM), paramagnetic insulator (PI), and antiferromagnetic
insulator (AFI) phases of (VM)O with the samples of =
0, 0.012, and 0.028 for Cr-doping and = 0.01 for Ti-doping. In the PM
phase, we observe a prominent quasiparticle peak in general agreement with
theory, which combines dynamical mean-field theory with the local density
approximation (LDA+DMFT). The quasiparticle peak shows a significantly larger
peak width and weight than in the theory. For both the PI and AFI phases, the
vanadium 3d parts of the valence spectra are not simple one peak structures.
For the PI phase, there is not yet a good theoretical understanding of these
structures. The size of the electron removal gap increases, and spectral weight
accumulates in the energy range closer to the chemical potential, when the PI
to AFI transition occurs. Spectra taken in the same phases with different
compositions show interesting monotonic changes as the dopant concentration
increases, regardless of the dopant species. With increased Cr-doping, the AFI
phase gap decreases and the PI phase gap increases.Comment: 13 pages, 16 figures. accepted for publication in Physical Review
Exact Solution of the strong coupling t-V model with twisted boundary conditions
We present the solution of the one-dimensional t-V model with twisted
boundary conditions in the strong coupling limit, t<<V and show that this model
can be mapped onto the strong coupling Hubbard chain threaded by a fictitious
flux proportional to the total momentum of the charge carriers. The high energy
eigenstates are characterized by a factorization of degrees of freedom
associated with configurations of soliton and antisoliton domains and degrees
of freedom associated with the movement of ``holes'' through these domains. The
coexistence of solitons and antisolitons leads to a strange flux dependence of
the eigenvalues. We illustrate the use of this solution, deriving the full
frequency dependence of the optical conductivity at half-filling and zero
temperature.Comment: 11 pages, 1 figure; to be published in Physical Review
Symmetry-projected variational approach for ground and excited states of the two-dimensional Hubbard model
We present a symmetry-projected configuration mixing scheme to describe
ground and excited states, with well defined quantum numbers, of the
two-dimensional Hubbard model with nearestneighbor hopping and periodic
boundary conditions. Results for the half-filled 2{\times}4, 4{\times}4, and
6{\times}6 lattices, as well as doped 4 {\times} 4 systems, compare well with
available results, both exact and from other state-of-the-art approximations.
We report spectral functions and density of states obtained from a
well-controlled ansatz for the (Ne {\pm} 1)-electron system. Symmetry projected
methods have been widely used for the many-body nuclear physics problem but
have received little attention in the solid state community. Given their
relatively low (mean-field) computational cost and the high quality of results
here reported, we believe that they deserve further scrutiny
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