Polaron and ion diffusion in a poly(3-hexylthiophene) thin-film transistor gated with polymer electrolyte dielectric

Electrolytes are finding applications as dielectric materials in low-voltage organic thin-film transistors (OTFT). The presence of mobile ions in these materials (polymer electrolytes or ion gels) gives rise to very high capacitance (>10 μF/cm²) and thus low transistor turn-on voltage. In order to establish fundamental limits in switching speeds of electrolyte gated OFETs, we carry out in situ optical spectroscopy measurement of a poly(3-hexylthiophene) (P3HT) OTFT gated with a LiClO₄:poly(ethyleneoxide) (PEO) dielectric. Based on spectroscopic signatures of molecular vibrations and polaron transitions, we quantitatively determine charge carrier concentration and diffusion constants. We find two distinctively different regions: at V _G≥−1.5 V, drift-diffusion (parallel to the semiconductor/dielectric interface) of hole-polarons in P3HT controls charging of the device; at V _G<−1.5 V, electrochemical doping of the entire P3HT film occurs and charging is controlled by drift/diffusion (perpendicular to the interface) of ClO₄ ⁻ counter ions into the polymer semiconductor.     

Search for pi(0)-->nu(mu)nu(mu) decays in the LSND detector

We observe a net beam excess of 8.7+/-6.3(stat)+/-2.4(syst) events, above 160 MeV, resulting from the charged-current reaction of nu(micro) and/or nu;(mu) on C and H in the LSND detector. No beam-related muon background is expected in this energy regime. Within an analysis framework of pi(0)-->nu(mu)nu;(mu), we set a direct upper limit for this branching ratio of Gamma(pi(0)-->nu(mu)nu;(mu))/Gamma(pi(0)-->all)<1.6 x 10(-6) at 90% confidence level.     

The ν2, 2ν2, 3ν2, ν4, and ν2 + ν4 bands of 15NH3

The ir absorption of gaseous ¹⁵NH₃ between 510 and 3040 cm^?1 was recorded with a resolution of 0.06 cm^?1. The ν₂, 2ν₂, 3ν₂, ν₄, and ν₂ + ν₄ bands were measured and analyzed on the basis of the vibration-rotation Hamiltonian developed by V. ?pirko, J. M. R. Stone, and D. Papou?ek (J. Mol. Spectrosc.60, 159–178 (1976)). A set of effective molecular parameters for the ν₂ = 1, 2, 3 states was derived, which reproduced the transition frequencies within the accuracy of the experimental measurements. For ν₄ and ν₂ + ν₄ bands the standard deviation of the calculated spectrum is about four times larger than the measurements accuracy: a similar result was found for ν₄ in ¹⁴NH₃ by ?. Urban et al. (J. Mol. Spectrosc.79, 455–495 (1980)). This result suggests that the present treatment takes into account only the most significant part of the rovibration interaction in the doubly degenerate vibrational states of ammonia.     

Alternating sign matrices and descending plane partitions

An alternating sign matrix is a square matrix such that (i) all entries are 1, ?1, or 0, (ii) every row and column has sum 1, and (iii) in every row and column the nonzero entries alternate in sign. Striking numerical evidence of a connection between these matrices and the descending plane partitions introduced by Andrews (Invent. Math.53 (1979), 193–225) have been discovered, but attempts to prove the existence of such a connection have been unsuccessful. This evidence, however, did suggest a method of proving the Andrews conjecture on descending plane partitions, which in turn suggested a method of proving the Macdonald conjecture on cyclically symmetric plane partitions (Invent. Math.66 (1982), 73–87). In this paper is a discussion of alternating sign matrices and descending plane partitions, and several conjectures and theorems about them are presented.     

Laser excitation of positronium in the Paschen-Back regime

Zeeman mixing of singlet and triplet 2P states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the n=2 states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field >0.01 T and is essentially eliminated above ～2 T. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate.     

Sensitive and automated detection of iron-oxide-labeled cells using phase image cross-correlation analysis

Superparamagnetic iron oxide (SPIO) nanoparticles are increasingly being used to noninvasively track cells, target specific molecules and monitor gene expression in vivo. Contrast changes that are subtle relative to intrinsic sources of contrast present a significant detection challenge. Here, we describe a postprocessing algorithm, called Phase map cross-correlation Detection and Quantification (PDQ), with the purpose of automating identification and quantification of localized accumulations of SPIO agents. The method is designed to sacrifice little flexibility - it works on previously acquired data and allows the use of conventional high-SNR pulse sequences with no extra scan time. We first investigated the theoretical detection limits of PDQ using a simulated dipole field. This method was then applied to three-dimensional (3D) MRI data sets of agarose gel containing isolated dipoles and ex vivo transplanted allogenic rat hearts infiltrated by numerous iron-oxide-labeled macrophages as a result of organ rejection. A simulated dipole field showed this method to be robust in very low signal-to-noise ratio images. Analysis of agarose gel and allogenic rat heart shows that this method can automatically identify and count dipoles while visualizing their biodistribution in 3D renderings. In the heart, this information was used to calculate a quantitative index that may indicate its degree of cellular infiltration.     

Potential energy surface intersections for triatomic molecules

The different types of surface intersection which may occur in linear configurations of triatomic molecules are reviewed, particularly with regard to the way in which the degeneracy is split as the molecule bends. The Renner-Teller effect in states of symmetry Π, Δ, Φ, etc., and intersections between Σ and Π, Σ and Δ, and Π and Δ states are discussed. A general method of modelling such intersecting potential surfaces is proposed, as a development of the model previously used by Murrell and Carter and co-workers for single-valued surfaces. Some of the lower energy surfaces of H₂O, NH₂, O₃, C₃, and HNO are discussed as examples.     

Gibbs ensemble simulation of nematic-isotropic coexistence in a liquid crystal mixture

The results are presented of Gibbs ensemble Monte Carlo (GEMC) simulations examining the nematic–isotropic (N–I) coexistence envelope of a liquid crystal mixture. The system studied is a 50:50 mixture of 1000 generalized Gay–Berne particles with axial ratios 2.0:1 and 2.5:1. For this system, stable N–I coexistence is observed over run lengths in excess of 5 x 10⁵ GEMC sweeps. There is unambiguous evidence of fractionation, the mole fractions of long particles in the coexisting phases typically differing by 11%. The measured coexistence envelope does not conform to the lens shape predicted by Onsager theory; qualitative finite size arguments are given in explanation of this distortion.