5,861 research outputs found
Simultaneous VHF and UHF radar observation of the mesosphere at Arecibo during a solar flare: A check on the gradient-mixing hypothesis
The results of a two wavelength (VHF and UHF) mesosphere experiment performed at the Arecibo Observatory on January 5, 1981 are discussed. The 46.8-MHz VHF radar (3.21 m Bragg scale) was operated to provide spectral measurements of signals scattered from refractivity fluctuations due to turbulence. Other physical parameters such as radial velocities, scattered signal power, and Doppler spread due to turbulence can be derived from signal spectra. The 430-MHz UHF radar (0.36 m Bragg scale) was used for D-region electron-density measurements using the incoherent scatter technique with a comparable height resolution. The radars were pointed symmetrically about the vertical with a beam spacing of 5.5 degree in the meridional plane. Occurrence of a type 4 solar flare during the experiment produced enhanced D-region electron-density gradients. This was a unique circumstance that provided the possibility of testing the basic premises of the turbulent gradient-mixing hypothesis
Usefulness of multifrequency MST radar measurements, part 2.6B
Scattering of radio waves from atmospheric refractive-index irregularities induced by turbulence was invoked almost four decades ago to explain the characteristics of signals received on VHF/UHF ionospheric and tropospheric forward-scatter links. Due to the bistatic geometry of these links a slender, horizontally extended, common volume or cell is formed in space. The principal contribution to scattering arises from refractive-index fluctuations in this volume at the Bragg wave number K approx. sub B = K approx. sub i -k approx. sub s vectors. It has been surmised that the use of more than one frequency in probing the middle-atmosphere regions should help resolve several issues pertaining to the scattering mechanism. These issues are briefly re-examined in this note. The implications of the radar equation are discussed. The problems arising due to layered structure of turbulence and the choice of frequencies most suitable for multifrequency measurements are considered
Interference detection and correction applied to incoherent-scatter radar power spectrum measurement
A median filter based interference detection and correction technique is evaluated and the method applied to the Arecibo incoherent scatter radar D-region ionospheric power spectrum is discussed. The method can be extended to other kinds of data when the statistics involved in the process are still valid
Coherent storage and manipulation of broadband photons via dynamically controlled Autler-Townes splitting
The coherent control of light with matter, enabling storage and manipulation
of optical signals, was revolutionized by electromagnetically induced
transparency (EIT), which is a quantum interference effect. For strong
electromagnetic fields that induce a wide transparency band, this quantum
interference vanishes, giving rise to the well-known phenomenon of
Autler-Townes splitting (ATS). To date, it is an open question whether ATS can
be directly leveraged for coherent control as more than just a case of "bad"
EIT. Here, we establish a protocol showing that dynamically controlled
absorption of light in the ATS regime mediates coherent storage and
manipulation that is inherently suitable for efficient broadband quantum memory
and processing devices. We experimentally demonstrate this protocol by storing
and manipulating nanoseconds-long optical pulses through a collective spin
state of laser-cooled Rb atoms for up to a microsecond. Furthermore, we show
that our approach substantially relaxes the technical requirements intrinsic to
established memory schemes, rendering it suitable for broad range of platforms
with applications to quantum information processing, high-precision
spectroscopy, and metrology.Comment: 14 pages with 6 figures; 3 pages supplementary info with 2
supplementary figure
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
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