539 research outputs found
Competition and symmetry in an artificial word learning task
Natural language involves competition. The sentences we choose to utter activate alternative sentences (those we chose not to utter), which hearers typically infer to be false. Hence, as a first approximation, the more alternatives a sentence activates, the more inferences it will trigger. But a closer look at the theory of competition shows that this is not quite true and that under specific circumstances, so-called symmetric alternatives cancel each other out. We present an artificial word learning experiment in which participants learn words that may enter into competition with one another. The results show that a mechanism of competition takes place, and that the subtle prediction that alternatives trigger inferences, and may stop triggering them after a point due to symmetry, is borne out. This study provides a minimal testing paradigm to reveal competition and some of its subtle characteristics in human languages and beyond
Observation of inter-Landau-level quantum coherence in semiconductor quantum wells
Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a
non-radiative coherence between the discrete Landau levels of an undoped
quantum well and study its dynamics. We observe quantum beats that reflect the
time evolution of the coherence between the two lowest Landau level
magnetoexcitons. We interpret our observations using a many-body theory and
find that the inter Landau level coherence decays with a new time constant,
substantially longer than the corresponding interband magnetoexciton dephasing
times. Our results indicate a new intraband excitation dynamics that cannot be
described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication
On the role of alternatives in the acquisition of simple and complex disjunctions in French and Japanese
International audienc
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
Constructing Modular and Universal Single Molecule Tension Sensor Using Protein G to Study Mechano-sensitive Receptors
Recently a variety of molecular force sensors have been developed to study cellular forces acting through single mechano-sensitive receptors. A common strategy adopted is to attach ligand molecules on a surface through engineered molecular tethers which report cell-exerted tension on receptor-ligand bonds. This approach generally requires chemical conjugation of the ligand to the force reporting tether which can be time-consuming and labor-intensive. Moreover, ligand-tether conjugation can severely reduce the activity of protein ligands. To address this problem, we developed a Protein G (ProG)-based force sensor in which force-reporting tethers are conjugated to ProG instead of ligands. A recombinant ligand fused with IgG-Fc is conveniently assembled with the force sensor through ProG:Fc binding, therefore avoiding ligand conjugation and purification processes. Using this approach, we determined that molecular tension on E-cadherin is lower than dsDNA unzipping force (nominal value: 12 pN) during initial cadherin-mediated cell adhesion, followed by an escalation to forces higher than 43 pN (nominal value). This approach is highly modular and potentially universal as we demonstrate using two additional receptor-ligand interactions, P-selectin & PSGL-1 and Notch & DLL1
Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap
The superfluid phase transition of bosons in a two-dimensional (2D) system
with disorder and an external parabolic potential is studied. The theory is
applied to experiments on indirect excitons in coupled quantum wells. The
random field is allowed to be large compared to the dipole-dipole repulsion
between excitons. The slope of the external parabolic trap is assumed to change
slowly enough to apply the local density approximation (LDA) for the superfluid
density, which allows us to calculate the Kosterlitz-Thouless temperature
at each local point of the trap. The superfluid phase occurs
around the center of the trap () with the normal phase outside
this area. As temperature increases, the superfluid area shrinks and disappears
at temperature . Disorder acts to deplete the condensate; the
minimal total number of excitons for which superfluidity exists increases with
disorder at fixed temperature. If the disorder is large enough, it can destroy
the superfluid entirely. The effect of magnetic field is also calculated for
the case of indirect excitons. In a strong magnetic field , the superfluid
component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure
- …