1,418 research outputs found
Band-monitoring Payload for a CubeSat Satellite
During changing sun activity, the ionosphere is responding accordingly and therefore it is interesting to observe the propagation behavior of shortwave bands. For the above mentioned purpose we have designed a band-monitoring payload for an experimental CubeSat satellite. The payload consists of a receiver, which is able to receive SSB modulated narrowband signals in 28 MHz uplink band, and a transmitter with FM modulation in UHF downlink band. The receiver frequency is selected to be at the center of radio amateur activity with low data rate digital modulations
Fast Incomplete Decoherence of Nuclear Spins in Quantum Hall Ferromagnet
A scenario of quantum computing process based on the manipulation of a large
number of nuclear spins in Quantum Hall (QH) ferromagnet is presented. It is
found that vacuum quantum fluctuations in the QH ferromagnetic ground state at
filling factor , associated with the virtual excitations of spin waves,
lead to fast incomplete decoherence of the nuclear spins. A fundamental upper
bound on the length of the computer memory is set by this fluctuation effect
Hyperfine interaction induced critical exponents in the quantum Hall effect
We study localization-delocalization transition in quantum Hall systems with
a random field of nuclear spins acting on two-dimensional (2d) electron spins
via hyperfine contact (Fermi) interaction. We use Chalker-Coddington network
model, which corresponds to the projection onto the lowest Landau level. The
inhomogeneous nuclear polarization acts on the electrons as an additional
confining potential, and, therefore, introduces additional parameter (the
probability to find a polarized nucleus in the vicinity of a saddle point of
random potential) responsible for the change from quantum to classical
behavior. In this manner we obtain two critical exponents corresponding to
quantum and classical percolation. We also study how the 2d extended state
develops into the one-dimensional (1d) critical state.Comment: 9 pages, 3 figure
Electrical read-out of the local nuclear polarization in the quantum Hall effect
It is demonstrated that the now well-established `flip-flop' mechanism of
spin exchange between electrons and nuclei in the quantum Hall effect can be
reversed. We use a sample geometry which utilizes separately contacted edge
states to establish a local nuclear spin polarization --close to the maximum
value achievable-- by driving a current between electron states of different
spin orientation. When the externally applied current is switched off, the
sample exhibits an output voltage of up to a few tenths of a meV, which decays
with a time constant typical for the nuclear spin relaxation. The surprizing
fact that a sample with a local nuclear spin polarization can act as a source
of energy and that this energy is well above the nuclear Zeeman splitting is
explained by a simple model which takes into account the effect of a local
Overhauser shift on the edge state reconstruction.Comment: Submitted to Phys. Rev. Let
Localized states in 2D semiconductors doped with magnetic impurities in quantizing magnetic field
A theory of magnetic impurities in a 2D electron gas quantized by a strong
magnetic field is formulated in terms of Friedel-Anderson theory of resonance
impurity scattering. It is shown that this scattering results in an appearance
of bound Landau states with zero angular moment between the Landau subbands.
The resonance scattering is spin selective, and it results in a strong spin
polarization of Landau states, as well as in a noticeable magnetic field
dependence of the factor and the crystal field splitting of the impurity
levels.Comment: 12 pages, 4 figures Submitted to Physical Review B This version is
edited and updated in accordance with recent experimental dat
Massive Spin Collective Mode in Quantum Hall Ferromagnet
It is shown that the collective spin rotation of a single Skyrmion in quantum
Hall ferromagnet can be regarded as precession of the entire spin texture in
the external magnetic field, with an effective moment of inertia which becomes
infinite in the zero g-factor limit. This low-lying spin excitation may
dramatically enhance the nuclear spin relaxation rate via the hyperfine
interaction in the quantum well slightly away from filling factor equal one.Comment: 4 page
Efforts for aligning the Brazilian Height System to the International Height Reference System
The International Height Reference System (IHRS) consolidated the idea of a world height system (WHS) according to the IAG Resolution 1/2015. Since 2000, the majority of activities related to the modernization of the Brazilian Height System (BHS) has referred to a physical meaning for the heights and have linked two existing vertical datums in the country to a same equipotential surface. Today, these activities relate to the BHS and its realization by the Brazilian Vertical Reference Network (BVRN) agrees with the IHRS precepts. In this context, the main developed activities related to BHS/BVRN and achieved goals are described in this work
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