612 research outputs found
Rational expectations and the paradox of policy-relevant natural experiments
Policy experiments using large microeconomic datasets have recently gained ground in macro- economics. Imposing rational expectations, we examine robustness of evidence derived from ideal natural experiments applied to atomistic agents in dynamic settings. Paradoxically, once experi-mental evidence is viewed as su¢ ciently clean to use, it then becomes contaminated byex post endo- geneity: Measured responses depend upon priors and the objective function into which evidence is fed. Moreover, agentsípolicy beliefs become endogenously correlated with their causal parameters, severely clouding inference, e.g. sign reversals and non-invertibility may obtain. Treatment-control di§erences are contaminated for non-quadratic adjustment costs. Constructively, we illustrate how inference can be corrected accounting for feedback and highlight factors mitigating contamination
Charge transport and phase transition in exciton rings
The macroscopic exciton rings observed in the photoluminescence (PL) patterns
of excitons in coupled quantum wells (CQWs) are explained by a series of
experiments and a theory based on the idea of carrier imbalance, transport and
recombination. The rings are found to be a source of cold excitons with
temperature close to that of the lattice. We explored states of excitons in the
ring over a range of temperatures down to 380 mK. These studies reveal a sharp,
albeit continuous, second order phase transition to a low-temperature ordered
exciton state, characterized by ring fragmentation into a periodic array of
aggregates. An instability at the onset of degeneracy in the cold exciton
system, due to stimulated exciton formation, is proposed as the transition
mechanism.Comment: 8 pages including 4 figure
Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR
Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear
optical response of excitons to sequences of ultrafast pulses, has the
potential to provide some unique insights into carrier dynamics in
semiconductors. The most prominent feature of 2DCS, cross peaks, can best be
understood using a sum-over-states picture involving the many-body eigenstates.
However, the optical response of semiconductors is usually calculated by
solving truncated equations of motion for dynamical variables, which result in
a quasiparticle picture. In this work we derive Green's function expressions
for the four wave mixing signals generated in various phase-matching directions
and use them to establish the connection between the two pictures. The formal
connection with Frenkel excitons (hard-core bosons) and vibrational excitons
(soft-core bosons) is pointed out.Comment: Accepted to Phys. Rev.
Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy
We present the first study of nonlinear optical third harmonic generation in
the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental
excitation in the near-infrared, the THG spectrum reveals a strongly resonant
response for photon energies near 0.7 eV. Polarization analysis reveals this
novel resonance to be only partially accounted for by three-photon excitation
to the optical charge-transfer exciton, and indicates that an even-parity
excitation at 2 eV, with a_1g symmetry, participates in the third harmonic
susceptibility.Comment: Requires RevTeX v4.0beta
Third-person knowledge ascriptions: a crucial experiment for contextualism
In the past few years there has been a turn towards evaluating the empirical foundation of epistemic contextualism using formal (rather than armchair) experimental methods. By-and-large, the results of these experiments have not supported the original motivation for epistemic contextualism. That is partly because experiments have only uncovered effects of changing context on knowledge ascriptions in limited experimental circumstances (when contrast is present, for example), and partly because existing experiments have not been designed to distinguish between contextualism and one of its main competing theories, subject-sensitive invariantism. In this paper, we discuss how a particular, “third-person”, experimental design is needed to provide evidence that would support contextualism over subject-sensitive invariantism. In spite of the theoretical significance of third-person knowledge ascriptions for debates surrounding contextualism, no formal experiments evaluating such ascriptions that assess contextualist claims have previously been conducted. In this paper, we conduct an experiment specifically designed to examine that central gap in contextualism’s empirical foundation. The results of our experiment provide crucial support for epistemic contextualism over subject-sensitive invariantism
Radiative corrections to the excitonic molecule state in GaAs microcavities
The optical properties of excitonic molecules (XXs) in GaAs-based quantum
well microcavities (MCs) are studied, both theoretically and experimentally. We
show that the radiative corrections to the XX state, the Lamb shift
and radiative width , are
large, about of the molecule binding energy , and
definitely cannot be neglected. The optics of excitonic molecules is dominated
by the in-plane resonant dissociation of the molecules into outgoing
1-mode and 0-mode cavity polaritons. The later decay channel,
``excitonic molecule 0-mode polariton + 0-mode
polariton'', deals with the short-wavelength MC polaritons invisible in
standard optical experiments, i.e., refers to ``hidden'' optics of
microcavities. By using transient four-wave mixing and pump-probe
spectroscopies, we infer that the radiative width, associated with excitonic
molecules of the binding energy meV, is
meV in the microcavities and
meV in a reference GaAs single quantum
well (QW). We show that for our high-quality quasi-two-dimensional
nanostructures the limit, relevant to the XX states, holds at
temperatures below 10 K, and that the bipolariton model of excitonic molecules
explains quantitatively and self-consistently the measured XX radiative widths.
We also find and characterize two critical points in the dependence of the
radiative corrections against the microcavity detuning, and propose to use the
critical points for high-precision measurements of the molecule bindingenergy
and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons
We study theoretically the ultrafast nonlinear optical response of quantum
well excitons in a perpendicular magnetic field. We show that for
magnetoexcitons confined to the lowest Landau levels, the third-order
four-wave-mixing (FWM) polarization is dominated by the exciton-exciton
interaction effects. For repulsive interactions, we identify two regimes in the
time-evolution of the optical polarization characterized by exponential and
{\em power law} decay of the FWM signal. We describe these regimes by deriving
an analytical solution for the memory kernel of the two-exciton wave-function
in strong magnetic field. For strong exciton-exciton interactions, the decay of
the FWM signal is governed by an antibound resonance with an
interaction-dependent decay rate. For weak interactions, the continuum of
exciton-exciton scattering states leads to a long tail of the time-integrated
FWM signal for negative time delays, which is described by the product of a
power law and a logarithmic factor. By combining this analytic solution with
numerical calculations, we study the crossover between the exponential and
non-exponential regimes as a function of magnetic field. For attractive
exciton-exciton interaction, we show that the time-evolution of the FWM signal
is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig
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