Narrow-band Phenomenon Research Articles

Aggregation‐induced narrowband isomeric fluorophores for ultraviolet nondoped OLEDs by engineering multiple nonbonding interactions

AbstractTraditional donor‐acceptor type organic luminescent materials usually suffer from unfavorable spectral broadening and fluorescence quenching problems arising from strong inter/intra‐chromophore interactions in aggregation state. Here, two ultraviolet carbazole‐pyrimidine isomers (named o‐DCz‐Pm and m‐DCz‐Pm) with novel aggregation‐induced narrowband phenomenon are constructed and systematic investigated by experiments and theoretical simulations. Benefitting from strengthened steric hindrance and multiple noncovalent interactions, the nonradiative decay, vibrational motion, and structural relaxation of singlet state can be effectively suppressed in aggregation state. Consequently, the electroluminescence peak of 397 nm, full width at half maximum of 21 nm and external quantum efficiency of 3.4% are achieved simultaneously in nondoped o‐DCz‐Pm‐based device. This work paves an avenue toward the development of high‐performance narrowband nondoped ultraviolet materials and organic light‐emitting diodes.

PSR B0943+10: low-frequency study of subpulse periodicity in the Bright mode with LOFAR

We use broadband sensitive LOFAR observations in the 25–80 MHz frequency range to study the single-pulse emission properties of the mode-switching pulsar B0943+10. We review the derivation of magnetospheric geometry, originally based on low-frequency radio data, and show that the geometry is less strongly constrained than previously thought. This may be used to help explain the large fractional amplitudes of the observed thermal X-ray pulsations from the polar cap, which contradicted the almost aligned rotator model of PSR B0943+10. We analyse the properties of drifting subpulses in the Bright mode and report on the short-scale (minutes) variations of the drift period. We searched for the periodic amplitude modulation of drifting subpulses, which is a vital argument for constraining several important system parameters: the degree of aliasing, the orientation of the line-of-sight vector with respect to magnetic and spin axes, the angular velocity of the carousel, and thus, the gradient of the accelerating potential in the polar gap. The periodic amplitude modulation was not detected, indicating that it may be a rare or narrow-band phenomenon. Based on our non-detection and review of the available literature, we chose to leave the aliasing order unconstrained and derived the number of sparks under different assumptions about the aliasing order and geometry angles. Contrary to the previous findings, we did not find a large (of the order of 10%) gradual variation of the separation between subpulses throughout Bright mode. We speculate that this large variation of subpulse separation may be due to the incorrect accounting for the curvature of the line of sight within the on-pulse window. Finally, we report on the frequency-dependent drift phase delay, which is similar to the delay reported previously for PSR B0809+74. We provide a quantitative explanation of the observed frequency-dependent drift phase delay within the carousel model.

Characterizing Wing Rock with Variations in Size and Configuration of Vertical Tail

Flight at high angle-of-attack conditions, which range beyond wing stall, has numerous advantages for mission performance; however, that flight regime is often affected by adverse behaviors. A particularly notable behavior is associated with uncommanded oscillations primarily about the roll axis and is known as wing rock. This paper investigates wing rock for a small, propeller-powered aerobatic unmanned aerial vehicle with a high degree of vertical symmetry flown at high angle-of-attack conditions. Most importantly, the investigation characterizes the nature of wing rock as a function of size and configuration of the vertical tail. The flight data indicate that wing rock is dependent on the configuration but not on the size of the vertical tail: wing rock oscillations are present for upright but not inverted regardless of vertical tail size. Furthermore, a time-frequency analysis indicates that the wing rock is actually a narrowband phenomenon for which the central frequency varies with time.

Theory of Dicke narrowing in coherent population trapping

The Doppler effect is one of the dominant broadening mechanisms in thermal vapor spectroscopy. For two-photon transitions one would naively expect the Doppler effect to cause a residual broadening, proportional to the wave-vector difference. In coherent population trapping (CPT), which is a two-photon narrow-band phenomenon, such broadening was not observed experimentally. This has been commonly attributed to frequent velocity-changing collisions, known to narrow Doppler-broadened one-photon absorption lines (Dicke narrowing). Here we show theoretically that such a narrowing mechanism indeed exists for CPT resonances. The narrowing factor is the ratio between the atom's mean free path and the wavelength associated with the wave-vector difference of the two radiation fields. A possible experiment to verify the theory is suggested.

Underwater audiogram of a Pacific walrus (Odobenus rosmarus divergens) measured with narrow-band frequency-modulated signals

The underwater hearing sensitivity of an 18-year-old male Pacific walrus was measured in a pool by using a go/no-go response paradigm and the up-down staircase method. Auditory sensitivity was measured using narrow-band, frequency-modulated signals (1500 ms duration) with center frequencies ranging from 0.125 to 15 kHz. The resulting underwater audiogram (50% detection thresholds) for this individual walrus shows the typical mammalian U-shape. Maximum sensitivity (67 dB re 1 microPa) occurred at 12 kHz. The range of best hearing (10 dB from the maximum sensitivity) was from 1 to 12 kHz. Sensitivity fell gradually below 1 kHz and dropped off sharply above 12 kHz. The animal showed a peculiar insensitivity for 2 kHz signals. His much higher sensitivity for 1.5- and 3-kHz signals indicated that this is a narrow-band phenomenon. Walrus hearing is relatively sensitive to low frequency sound, thus the species is likely to be susceptible to anthropogenic noise. The thresholds found during a small test with four frequencies with signal durations of 300 ms did not differ significantly from those obtained with signal durations of 1500 ms.

Semiparametric fractional cointegration analysis

Fractional cointegration is viewed from a semiparametric viewpoint as a narrow-band phenomenon at frequency zero. We study a narrow-band frequency domain least squares estimate of the cointegrating vector, and related semiparametric methods of inference for testing the memory of observables and the presence of fractional cointegration. These procedures are employed in analysing empirical macroeconomic series; their usefulness and feasibility in finite samples is supported by results of a Monte Carlo experiment.