Compact solid-state lamps based on light-emitting diodes (LEDs)
are of current technological interest as an alternative to conventional light
bulbs. The brightest LEDs available so far emit red light ...and exhibit higher
luminous efficiency than fluorescent lamps. If this luminous efficiency could
be transferred to white LEDs, power consumption would be dramatically reduced,
with great economic and ecological consequences. But the luminous efficiency
of existing white LEDs is still very low, owing to the presence of electrostatic
fields within the active layers. These fields are generated
by the spontaneous and piezoelectric polarization along the 0001 axis of
hexagonal group-III nitrides-the commonly used materials for light generation. Unfortunately, as this crystallographic orientation corresponds
to the natural growth direction of these materials deposited on currently
available substrates. Here we demonstrate that the epitaxial
growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction
allows the fabrication of structures free of electrostatic fields, resulting
in an improved quantum efficiency. We expect that this approach will pave
the way towards highly efficient white LEDs.
Vertical GaN nanowires are grown in a self-induced way on a sputtered Ti film by plasma-assisted molecular beam epitaxy. Both in situ electron diffraction and ex situ ellipsometry show that Ti is ...converted to TiN upon exposure of the surface to the N plasma. In addition, the ellipsometric data demonstrate this TiN film to be metallic. The diffraction data evidence that the GaN nanowires have a strict epitaxial relationship to this film. Photoluminescence spectroscopy of the GaN nanowires shows excitonic transitions virtually identical in spectral position, line width, and decay time to those of state-of-the-art GaN nanowires grown on Si. Therefore, the crystalline quality of the GaN nanowires grown on metallic TiN and on Si is equivalent. The freedom to employ metallic substrates for the epitaxial growth of semiconductor nanowires in high structural quality may enable novel applications that benefit from the associated high thermal and electrical conductivity as well as optical reflectivity.
We demonstrate the frequency stabilization of a terahertz quantum-cascade laser (QCL) to the Lamb dip of the absorption line of a D
O rotational transition at 3.3809309 THz. To assess the quality of ...the frequency stabilization, a Schottky diode harmonic mixer is used to generate a downconverted QCL signal by mixing the laser emission with a multiplied microwave reference signal. This downconverted signal is directly measured by a spectrum analyzer showing a full width at half maximum of 350 kHz, which is eventually limited by high-frequency noise beyond the bandwidth of the stabilization loop.
There is strong circumstantial evidence that certain heavy, unstable atomic nuclei are 'octupole deformed', that is, distorted into a pear shape. This contrasts with the more prevalent rugby-ball ...shape of nuclei with reflection-symmetric, quadrupole deformations. The elusive octupole deformed nuclei are of importance for nuclear structure theory, and also in searches for physics beyond the standard model; any measurable electric-dipole moment (a signature of the latter) is expected to be amplified in such nuclei. Here we determine electric octupole transition strengths (a direct measure of octupole correlations) for short-lived isotopes of radon and radium. Coulomb excitation experiments were performed using accelerated beams of heavy, radioactive ions. Our data on (220)Rn and (224)Ra show clear evidence for stronger octupole deformation in the latter. The results enable discrimination between differing theoretical approaches to octupole correlations, and help to constrain suitable candidates for experimental studies of atomic electric-dipole moments that might reveal extensions to the standard model.
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