1,659 research outputs found
Inter-site Coulomb interaction and Heisenberg exchange
Based on exact diagonalization results for small clusters we discuss the
effect of inter-site Coulomb repulsion in Mott-Hubbard or charge transfer
insulators. Whereas the exchange constant J for direct exchange is
substantially enhanced by inter-site Coulomb interaction, that for
superexchange is suppressed. The enhancement of J in the single-band models
holds up to the critical value for the charge density wave (CDW) instability,
thus opening the way for large values of J. Single-band Hubbard models with
sufficiently strong inter-site repulsion to be near a CDW instability thus may
provide `physical' realizations of t-J like models with the `unphysical'
parameter ratio J/t=1.Comment: Revtex file, 4 PRB pages, with 5 embedded ps-files. To appear in PRB,
rapid communications. Hardcopies of figures or the entire manuscript may also
be obtained by e-mail request to: [email protected]
Charged excitons in doped extended Hubbard model systems
We show that the charge transfer excitons in a Hubbard model system including
nearest neighbor Coulomb interactions effectively attain some charge in doped
systems and become visible in photoelectron and inverse photoelectron
spectroscopies. This shows that the description of a doped system by an
extended Hubbard model differs substantially from that of a simple Hubbard
model. Longer range Coulomb interactions cause satellites in the one electron
removal and addition spectra and the appearance of spectral weight if the gap
of doped systems at energies corresponding to the excitons of the undoped
systems. The spectral weight of the satellites is proportional to the doping
times the coordination number and therefore is strongly dependent on the
dimension.Comment: 10 pages revtex, 5 figures ps figures adde
Hamiltonian and Linear-Space Structure for Damped Oscillators: I. General Theory
The phase space of damped linear oscillators is endowed with a bilinear
map under which the evolution operator is symmetric. This analog of
self-adjointness allows properties familiar from conservative systems to be
recovered, e.g., eigenvectors are "orthogonal" under the bilinear map and obey
sum rules, initial-value problems are readily solved and perturbation theory
applies to the_complex_ eigenvalues. These concepts are conveniently
represented in a biorthogonal basis.Comment: REVTeX4, 10pp., 1 PS figure. N.B.: `Alec' is my first name, `Maassen
van den Brink' my family name. v2: extensive streamlinin
Quantum limited sensitivity of SET-based displacement detectors
We consider a model of a quantum-mechanical resonator capacitively coupled to
a single electron transistor (SET). The tunnel current in the SET is modulated
by the vibrations of the resonator, and thus the system operates as a
displacement detector. We analyze the effect of the back-action noise of charge
fluctuations in the SET onto the dynamics of the resonator and evaluate the
displacement sensitivity of the system. The relation between the "classical"
and "quantum" parts of the SET charge noise and their effect on the measured
system are also discussed.Comment: 4 pages, 2 eps fig
Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made
remarkable progress as a spectroscopic technique. This is a direct result of
the availability of high-brilliance synchrotron X-ray radiation sources and of
advanced photon detection instrumentation. The technique's unique capability to
probe elementary excitations in complex materials by measuring their energy-,
momentum-, and polarization-dependence has brought RIXS to the forefront of
experimental photon science. We review both the experimental and theoretical
RIXS investigations of the past decade, focusing on those determining the
low-energy charge, spin, orbital and lattice excitations of solids. We present
the fundamentals of RIXS as an experimental method and then review the
theoretical state of affairs, its recent developments and discuss the different
(approximate) methods to compute the dynamical RIXS response. The last decade's
body of experimental RIXS data and its interpretation is surveyed, with an
emphasis on RIXS studies of correlated electron systems, especially transition
metal compounds. Finally, we discuss the promise that RIXS holds for the near
future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure
Saturation field of frustrated chain cuprates: broad regions of predominant interchain coupling
An efficient and precise thermodynamic method to extract the interchain
coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their
empirical saturation field H_s (T=0) is proposed. Using density-matrix
renormalization group, hard-core boson, and spin-wave theory we study how H_s
is affected by an antiferromagnetic (AFM) IC between frustrated chains
described in the J_1-J_2-spin model with ferromagnetic 1st and AFM 2nd neighbor
in-chain exchange. A complex 3D-phase diagram has been found. For Li2CuO2 and
Y2Ca2Cu5O10, we show that H_s is solely determined by the IC and predict H_s
approx 61 T for the latter.Using H_s approx 55 T from our high-field pulsed
measurements one reads out a weak IC for Li2CuO2 close to that from neutron
scattering.Comment: 4 pages, 6 figures, slightly revised version including a slightly
changed title and abstract, one new figure and an EPAPS-supplementatary part
have been adde
From Hierarchies to Levels: New Solutions for Games with Hierarchical Structure
Recently, applications of cooperative game theory to economic allocation problems have gained popularity. In many of these problems, players are organized according to either a hierarchical structure or a levels structure that restrict players ’ possibilities to cooperate. In this paper, we propose three new solutions for games with hierarchical structure and characterize them by properties that relate a player’s payoff to the payoffs of other players located in specific positions in the structure relative to that player. To define each of these solutions, we consider a certain mapping that transforms any hierarchical structure into a levels structure, and then we apply the standard generalization of the Shapley Value to the class of games with levels structure. The transformations that map the set of hierarchical structures to the set of levels structures are also studied from an axiomatic viewpoint by means of properties that relate a player’s position in both types of structure
Photoemission spectra of LaMnO3 controlled by orbital excitations
We investigate the spectral function of a hole moving in the orbital-ordered
ferromagnetic planes of LaMnO, and show that it depends critically on the
type of orbital ordering. While the hole does not couple to the spin
excitations, it interacts strongly with the excitations of orbitals
(orbitons), leading to new type of quasiparticles with a dispersion on the
orbiton energy scale and with strongly enhanced mass and reduced weight.
Therefore we predict a large redistribution of spectral weight with respect to
the bands found in local density approximation (LDA) or in LDA+U.Comment: 4 pages, 4 figures, 3 figures embedded, figure 3 correcte
Order Monotonic Solutions for Generalized Characteristic Functions
Generalized characteristic functions extend characteristic functions of 'classical' TU-games by assigning a real number to every ordered coalition being a permutation of any subset of the player set. Such generalized characteristic functions can be applied when the earnings or costs of cooperation among a set of players depends on the order in which the players enter a coalition. In the literature, the two main solutions for generalized characteristic functions are the one of Nowak and Radzik (1994), shortly called NR-value, and the one introduced by Sanchez and Bergantinos (1997), shortly called SB-value. In this paper, we introduce the axiom of order monotonicity with respect to the order of the players in a unanimity coalition, requiring that players who enter earlier should get not more in the corresponding (ordered) unanimity game than players who enter later. We propose several classes of order monotonic solutions for generalized characteristic functions that contain the NR-value and SB-value as special (extreme) cases. We also provide axiomatizations of these classes
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