2,328 research outputs found
Doping dependence of magnetic excitations of 1D cuprates as probed by Resonant Inelastic x-ray Scattering
We study the dynamical, momentum dependent two- and four-spin response
functions in doped and undoped 1D cuprates, as probed by resonant inelastic
x-ray scattering, using an exact numerical diagonalization procedure. In the
undoped system the four-spin response vanishes at , whereas the
two-spin correlator is peaked around , with generally larger spectral
weight. Upon doping spectra tend to soften and broaden, with a transfer of
spectral weight towards higher energy. However, the total spectral weight and
average peak position of either response are only weakly affected by doping up
to a concentration of 1/8. Only the two-spin response at changes
strongly, with a large reduction of spectral weight and enhancement of
excitation energy. At other momenta the higher-energy, generic features of the
magnetic response are robust against doping. It signals the presence of strong
short-range antiferromagnetic correlations, even after doping mobile holes into
the system. We expect this to hold also in higher dimensions.Comment: 7 pages, 5 figure
Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals
From first principles calculations we determine the Coulomb interaction
between two holes on oligo-acene and -thiophene molecules in a crystal, as a
function of the oligomer length. The relaxation of the molecular geometry in
the presence of holes is found to be small. In contrast, the electronic
polarization of the molecules that surround the charged oligomer, reduces the
bare Coulomb repulsion between the holes by approximately a factor of two. In
all cases the effective hole-hole repulsion is much larger than the calculated
valence bandwidth, which implies that at high doping levels the properties of
these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure
Absence of helical surface states in bulk semimetals with broken inversion symmetry
Whereas the concept of topological band-structures was developed originally
for insulators with a bulk bandgap, it has become increasingly clear that the
prime consequences of a non-trivial topology -- spin-momentum locking of
surface states -- can also be encountered in gapless systems. Concentrating on
the paradigmatic example of mercury chalcogenides HgX (X = Te, Se, S), we show
that the existence of helical semimetals, i.e. semimetals with topological
surface states, critically depends on the presence of crystal inversion
symmetry. An infinitesimally small broken inversion symmetry (BIS) renders the
helical semimetallic state unstable. The BIS is also very important in the
fully gapped regime, renormalizing the surface Dirac cones in an anisotropic
manner. As a consequence the handedness of the Dirac cones can be flipped by a
biaxial stress field.Comment: 7 pages, 4 figure
Switchable Quantum Anomalous Hall state in a strongly frustrated lattice magnet
We establish that the interplay of itinerant fermions with localized magnetic
moments on a checkerboard lattice leads to magnetic flux-phases. For weak
itineracy the flux-phase is coplanar and the electronic dispersion takes the
shape of graphene-like Dirac fermions. Stronger itineracy drives the formation
of a non-coplanar, chiral flux-phase, in which the Dirac fermions acquire a
topological mass that is proportional to a ferromagnetic spin polarization.
Consequently the system self-organizes into a ferromagnetic Quantum Anomalous
Hall state in which the direction of its dissipationless edge-currents can be
switched by an applied magnetic field.Comment: 4.5 pages, 3 figure
Measuring power and satisfaction in societies with opinion leaders: An axiomatization
URL des Documents de travail : http://centredeconomiesorbonne.univ-paris1.fr/documents-de-travail/Documents de travail du Centre d'Economie de la Sorbonne 2011.18 - ISSN : 1955-611XA well-known model in sociology and marketing is that of opinion leadership. Opinion leaders are actors who are able to affect the behavior of their followers. Hence, opinion leaders have some power over their followers, and they can exercise this power by influencing their followers choice of action. We study a two-action model for a society with opinion leaders. We assume that each member of the society has an inclination to choose one of these actions and that the collective choice is made by simple majority of the actions chosen by each member. For this model, we axiomatize satisfaction and power scores, which allow us to investigate the effects of different opinion leader-follower structures.Un modèle bien connu en sociologie et gestion est celui du leadership d'opinion. Les leaders d'opinion sont des acteurs qui peuvent influer sur les comportements de leurs disciples. En conséquence, les leaders d'opinion ont un certain pouvoir sur leurs disciples et ils peuvent exercer ce pouvoir en influençant le choix d'action de leurs disciples. Nous étudions un modèle de deux actions pour une société avec des leaders d'opinion. Nous supposons que chaque membre de la société a une inclination de choisir une des actions et que le choix collectif est fait par la majorité simple des actions choisies par chaque membre. Pour ce modèle, nous axiomatisons les scores de satisfaction et de pouvoir, ce qui nous permet d'examiner les effets des différentes structures de leader d'opinion - disciples
Measuring power and satisfaction in societies with opinion leaders: Dictator and opinion leader properties
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