661 research outputs found
Persistence of planetary wave type oscillations in the mid-latitude ionosphere
Planetary wave type oscillations have been observed in the lower and middle atmosphere but also in the ionosphere,
including the ionospheric F2 layer. Here we deal with the oscillations in foF2 analysed for two Japanese
and two US stations over a solar cycle (1979-1989) with the use of the Morlet and Paul wavelet transforms.
Waves with periods near 5, 10 and 16 days are studied. Only events of duration of three wave-cycles and more
are considered. The results are compared with the results of a similar analysis made for foF2 and the lower ionosphere
over Europe (Lasˇtovicˇka et al., 2003a,b). The 5-day period wave events display a typical duration of 4
cycles, while the 10- and 16-day wave events are less persistent with typical duration of about 3.5 cycles and
rather 3 cycles, respectively, in all three geographic regions. The persistence pattern in terms of number of cycles
and in terms of number of days is different. In terms of number of cycles, the typical persistence of oscillations
decreases with increasing period. On the other hand, in terms of number of days the typical persistence evidently
increases with increasing period. The spectral distribution of event duration is too broad to allow for a
reasonable prediction of event duration. Thus the predictability of the planetary wave type oscillations in foF2
seems to be very questionable. The longitudinal size of the planetary wave type events increases with increasing
wave period. The persistence of the planetary wave type events in foF2 and the lower ionosphere is similar in
Europe, but the similarity in occurrence of individual events in foF2 and the lower ionosphere is rather poor
Anomalous conductivity tensor in the Dirac semimetal Na_3Bi
Na3Bi is a Dirac semimetal with protected nodes that may be sensitive to the
breaking of time-reversal invariance in a magnetic field B. We report
experiments which reveal that both the conductivity and resistivity tensors
exhibit robust anomalies in B. The resistivity is B-linear up to 35
T, while the Hall angle exhibits an unusual profile approaching a
step-function. The conductivities and share
identical power-law dependences at large B. We propose that these significant
deviations from conventional transport result from an unusual sensitivity of
the transport lifetime to B. Comparison with Cd3As2 is made.Comment: 8 pages, 5 figure
Heat capacity anomaly at the quantum critical point of the Transverse Ising Magnet CoNb_2O_6
The transverse Ising magnet Hamiltonian describing the Ising chain in a
transverse magnetic field is the archetypal example of a system that undergoes
a transition at a quantum critical point (QCP). The columbite CoNbO is
the closest realization of the transverse Ising magnet found to date. At low
temperatures, neutron diffraction has observed a set of discrete collective
spin modes near the QCP. We ask if there are low-lying spin excitations
distinct from these relatively high energy modes. Using the heat capacity, we
show that a significant band of gapless spin excitations exists. At the QCP,
their spin entropy rises to a prominent peak that accounts for 30 of the
total spin degrees of freedom. In a narrow field interval below the QCP, the
gapless excitations display a fermion-like, temperature-linear heat capacity
below 1 K. These novel gapless modes are the main spin excitations
participating in, and affected, by the quantum transition.Comment: 14 pages total, 8 figure
A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNbO investigated by high resolution time-domain terahertz spectroscopy
Kink bound states in the one dimensional ferromagnetic Ising chain compound
CoNbO have been studied using high resolution time-domain terahertz
spectroscopy in zero applied magnetic field. When magnetic order develops at
low temperature, nine bound states of kinks become visible. Their energies can
be modeled exceedingly well by the Airy function solutions to a 1D
Schr\"odinger equation with a linear confining potential. This sequence of
bound states terminates at a threshold energy near two times the energy of the
lowest bound state. Above this energy scale we observe a broad feature
consistent with the onset of the two particle continuum. At energies just below
this threshold we observe a prominent excitation that we interpret as a novel
bound state of bound states -- two pairs of kinks on neighboring chains
Tests of a proximity focusing RICH with aerogel as radiator
Using aerogel as radiator and multianode PMTs for photon detection, a
proximity focusing Cherenkov ring imaging detector has been constructed and
tested in the KEK 2 beam. The aim is to experimentally study the basic
parameters such as resolution of the single photon Cherenkov angle and number
of detected photons per ring. The resolution obtained is well approximated by
estimates of contributions from pixel size and emission point uncertainty. The
number of detected photons per Cherenkov ring is in good agreement with
estimates based on aerogel and detector characteristics. The values obtained
turn out to be rather low, mainly due to Rayleigh scattering and to the
relatively large dead space between the photocathodes. A light collection
system or a higher fraction of the photomultiplier active area, together with
better quality aerogels are expected to improve the situation. The reduction of
Cherenkov yield, for charged particle impact in the vicinity of the aerogel
tile side wall, has also been measured.Comment: 4 pages, 8 figure
Bulk crystal growth and electronic characterization of the 3D Dirac Semimetal Na3Bi
High quality hexagon plate-like Na3Bi crystals with large (001) plane
surfaces were grown from a molten Na flux. The freshly cleaved crystals were
analyzed by low temperature scanning tunneling microscopy (STM) and
angle-resolved photoemission spectroscopy (ARPES), allowing for the
characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior
and the observation of the topological surface states. Landau levels (LL) were
observed, and the energy-momentum relations exhibited a linear dispersion
relationship, characteristic of the 3D TDS nature of Na3Bi. In transport
measurements on Na3Bi crystals the linear magnetoresistance and Shubnikov-de
Haas (SdH) quantum oscillations are observed for the first time.Comment: To be published in a special issue of APL Material
Longitudinal polarization asymmetry of leptons in the pure leptonic B decays
Longitudinal lepton polarization asymmetry in B_q -> l l (q = d, s$ and l =
e, mu, tau) decays is investigated. The analysis is done in a general manner by
using the effective operators approach. It is shown that the longitudinal
lepton polarization asymmetry would provide a direct search for the scalar and
pseudoscalar type interactions, which are induced in all variants of
Higgs-doublet models.Comment: 7 page
- …