1,996 research outputs found
Hall effect of quasi-hole gas in organic single-crystal transistors
Hall effect is detected in organic field-effect transistors, using
appropriately shaped rubrene (C42H28) single crystals. It turned out that
inverse Hall coefficient, having a positive sign, is close to the amount of
electric-field induced charge upon the hole accumulation. The presence of the
normal Hall effect means that the electromagnetic character of the surface
charge is not of hopping carriers but resembles that of a two-dimensional
hole-gas system
Ferroelectric polarization flop in a frustrated magnet MnWO induced by magnetic fields
The relationship between magnetic order and ferroelectric properties has been
investigated for MnWO with long-wavelength magnetic structure. Spontaneous
electric polarization is observed in an elliptical spiral spin phase. The
magnetic-field dependence of electric polarization indicates that the
noncollinear spin configuration plays a key role for the appearance of
ferroelectric phase. An electric polarization flop from the b direction to the
a direction has been observed when a magnetic field above 10T is applied along
the b axis. This result demonstrates that an electric polarization flop can be
induced by a magnetic field in a simple system without rare-earth f-moments.Comment: 9 pages, 4 figure
Diffraction from Ordered States of Higher Multipoles
Possible ways of identification are discussed of an electronic order of
higher multipoles such as octupoles and hexadecapoles. A particularly powerful
method is resonant X-ray scattering (RXS) using quadrupolar resonance processes
called E2.The characteristic azimuthal angle dependence of
CeLaB is interpreted as evidence of antiferro-octupole
order. For PrRuP, eightfold pattern against azimuthal angle is
predicted if its metal-insulator transition is a consequence of a hexadecapole
order. In non-resonant superlattice Bragg scattering, hexadecapole contribution
may also be identified because of absence of quadrupole component.Comment: Invited paper to be published in Proc. Hiroshima Workshop on Novel
Functional Materials with Multinary Freedoms (Physica B, 2006
Low-energy excitations in electron-doped metal phthalocyanine from NMR in LiMnPc
Li and H NMR and magnetization measurements in \lpc
(PcCHN), recently proposed as a strongly correlated
metal, are presented. Two different low-frequency dynamics are evidenced. The
first one, probed by H nuclei gives rise to a slowly relaxing magnetization
at low temperature and is associated with the freezing of MnPc spins.
This dynamic is similar to the one observed in pristine -MnPc and
originates from Li depleted chain segments. The second one, evidenced by Li
spin-lattice relaxation rate, is associated with the hopping of the electrons
along Li-rich chains. The characteristic correlation times for the two dynamics
are derived and the role of disorder is briefly discussed.Comment: 7 two-columns pages, 11 figure
Spin frustration and magnetic ordering in theS=12molecular antiferromagnetfcc−Cs3C60
We have investigated the low-temperature magnetic state of face-centered-cubic (fcc) Cs3C60, a Mott insulator and the first molecular analog of a geometrically frustrated Heisenberg fcc antiferromagnet with S=1/2 spins. Specific heat studies reveal the presence of both long-range antiferromagnetic ordering and a magnetically disordered state below TN=2.2 K, which is in agreement with local probe experiments. These results together with the strongly suppressed TN are unexpected for conventional atom-based fcc antiferromagnets, implying that the fulleride molecular degrees of freedom give rise to the unique magnetic ground state
Cell-cell signalling in sexual chemotaxis: a basis for gametic differentiation, mating types and sexes
While sex requires two parents, there is no obvious need for them to be differentiated into distinct mating types or sexes. Yet this is the predominate state of nature. Here, we argue that mating types could play a decisive role because they prevent the apparent inevitability of self-stimulation during sexual signalling. We rigorously assess this hypothesis by developing a model for signaller-detector dynamics based on chemical diffusion, chemotaxis and cell movement. Our model examines the conditions under which chemotaxis improves partner finding. Varying parameter values within ranges typical of protists and their environments, we show that simultaneous secretion and detection of a single chemoattractant can cause a multifold movement impediment and severely hinder mate finding. Mutually exclusive roles result in faster pair formation, even when cells conferring the same roles cannot pair up. This arrangement also allows the separate mating types to optimize their signalling or detecting roles, which is effectively impossible for cells that are both secretors and detectors. Our findings suggest that asymmetric roles in sexual chemotaxis (and possibly other forms of sexual signalling) are crucial, even without morphological differences, and may underlie the evolution of gametic differentiation among both mating types and sexes
SIGNALING EFFICACY DRIVES THE EVOLUTION OF LARGER SEXUAL ORNAMENTS BY SEXUAL SELECTION.
Why are there so few small secondary sexual characters? Theoretical models predict that sexual selection should lead to reduction as often as exaggeration, and yet we mainly associate secondary sexual ornaments with exaggerated features such as the peacock's tail. We review the literature on mate choice experiments for evidence of reduced sexual traits. This shows that reduced ornamentation is effectively impossible in certain types of ornamental traits (behavioral, pheromonal, or color-based traits, and morphological ornaments for which the natural selection optimum is no trait), but that there are many examples of morphological traits that would permit reduction. Yet small sexual traits are very rarely seen. We analyze a simple mathematical model of Fisher's runaway process (the null model for sexual selection). Our analysis shows that the imbalance cannot be wholly explained by larger ornaments being less costly than smaller ornaments, nor by preferences for larger ornaments being less costly than preferences for smaller ornaments. Instead, we suggest that asymmetry in signaling efficacy limits runaway to trait exaggeration
- …