16,725 research outputs found
Interactions and magnetic moments near vacancies and resonant impurities in graphene
The effect of electronic interactions in graphene with vacancies or resonant
scatterers is investigated. We apply dynamical mean-field theory in combination
with quantum Monte Carlo simulations, which allow us to treat
non-perturbatively quantum fluctuations beyond Hartree-Fock approximations. The
interactions narrow the width of the resonance and induce a Curie magnetic
susceptibility, signaling the formation of local moments. The absence of
saturation of the susceptibility at low temperatures suggests that the coupling
between the local moment and the conduction electrons is ferromagnetic
The absoption refrigerator as a thermal transformer
The absorption refrigerator can be considered a thermal transformer, i.e. a
device that is analogous to the electric transformer. The analogy is based on a
correspondence between the extensive quantities entropy and electric charge and
that of the intensive variables temperature and electric potential
Scalar and vector decomposition of the nucleon self-energy in the relativistic Brueckner approach
We investigate the momentum dependence of the nucleon self-energy in nuclear
matter. We apply the relativistic Brueckner-Hartree-Fock approach and adopt the
Bonn A potential. A strong momentum dependence of the scalar and vector
self-energy components can be observed when a commonly used pseudo-vector
choice for the covariant representation of the T-matrix is applied. This
momentum dependence is dominated by the pion exchange. We discuss the problems
of this choice and its relations to on-shell ambiguities of the T-matrix
representation. Starting from a complete pseudo-vector representation of the
T-matrix, which reproduces correctly the pseudo-vector pion-exchange
contributions at the Hartree-Fock level, we observe a much weaker momentum
dependence of the self-energy. This fixes the range of the inherent uncertainty
in the determination of the scalar and vector self-energy components. Comparing
to other work, we find that extracting the self-energy components by a fit to
the single particle potential leads to even more ambiguous results.Comment: 35 pages RevTex, 7 PS figures, replaced by a revised and extended
versio
Spectral properties of the three-dimensional Hubbard model
We present momentum resolved single-particle spectra for the
three-dimensional Hubbard model for the paramagnetic and antiferromagnetically
ordered phase obtained within the dynamical cluster approximation. The
effective cluster problem is solved by continuous-time Quantum Monte Carlo
simulations. The absence of a time discretization error and the ability to
perform Monte Carlo measurements directly in Matsubara frequencies enable us to
analytically continue the self-energies by maximum entropy, which is essential
to obtain momentum resolved spectral functions for the N'eel state. We
investigate the dependence on temperature and interaction strength and the
effect of magnetic frustration introduced by a next-nearest neighbor hopping.
One particular question we address here is the influence of the frustrating
interaction on the metal insulator transition of the three-dimensional Hubbard
model.Comment: 16 pages, 14 figure
Complex joint probabilities as expressions of determinism in quantum mechanics
The density operator of a quantum state can be represented as a complex joint
probability of any two observables whose eigenstates have non-zero mutual
overlap. Transformations to a new basis set are then expressed in terms of
complex conditional probabilities that describe the fundamental relation
between precise statements about the three different observables. Since such
transformations merely change the representation of the quantum state, these
conditional probabilities provide a state-independent definition of the
deterministic relation between the outcomes of different quantum measurements.
In this paper, it is shown how classical reality emerges as an approximation to
the fundamental laws of quantum determinism expressed by complex conditional
probabilities. The quantum mechanical origin of phase spaces and trajectories
is identified and implications for the interpretation of quantum measurements
are considered. It is argued that the transformation laws of quantum
determinism provide a fundamental description of the measurement dependence of
empirical reality.Comment: 12 pages, including 1 figure, updated introduction includes
references to the historical background of complex joint probabilities and to
related work by Lars M. Johanse
Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors
We report resistivity and upper critical field B_c2(T) data for disordered
(As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field
range up to 60 T. These samples exhibit a slightly enhanced superconducting
transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4
T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K
above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0)
approximately 63 T. We compare our results with B_c2(T)-data from the
literature for clean and disordered samples. Whereas clean samples show almost
no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced
flattening of B_c2(T) for applied fields H II ab observed for several
disordered closely related systems is interpreted also as a manifestation of
PLB. Consequences are discussed in terms of disorder effects within the frames
of (un)conventional superconductivity, respectively.Comment: 2 pages, 3 figures, submitted to M2S Tokyo 0
Collective modes of doped graphene and a standard 2DEG in a strong magnetic field: linear magneto-plasmons versus magneto-excitons
A doped graphene layer in the integer quantum Hall regime reveals a highly
unusual particle-hole excitation spectrum, which is calculated from the
dynamical polarizability in the random phase approximation. We find that the
elementary neutral excitations in graphene in a magnetic field are unlike those
of a standard two-dimensional electron gas (2DEG): in addition to the
upper-hybrid mode, the particle-hole spectrum is reorganized in linear
magneto-plasmons that disperse roughly parallel to , instead of
the usual horizontal (almost dispersionless) magneto-excitons. These modes
could be detected in an inelastic light scattering experiment.Comment: 8 pages, 3 figures. Version accepted for publication in Phys. Rev.
Von Bezold assimilation effect reverses in stereoscopic conditions
Lightness contrast and lightness assimilation are opposite phenomena: in contrast,
grey targets appear darker when bordering bright surfaces (inducers) rather than dark ones; in
assimilation, the opposite occurs. The question is: which visual process favours the occurrence
of one phenomenon over the other? Researchers provided three answers to this question. The
first asserts that both phenomena are caused by peripheral processes; the second attributes their
occurrence to central processes; and the third claims that contrast involves central processes,
whilst assimilation involves peripheral ones. To test these hypotheses, an experiment on an IT
system equipped with goggles for stereo vision was run. Observers were asked to evaluate the
lightness of a grey target, and two variables were systematically manipulated: (i) the apparent
distance of the inducers; and (ii) brightness of the inducers. The retinal stimulation was kept
constant throughout, so that the peripheral processes remained the same. The results show that
the lightness of the target depends on both variables. As the retinal stimulation was kept constant, we
conclude that central mechanisms are involved in both lightness contrast and lightness assimilation
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