174 research outputs found
Conditions for electron-cyclotron maser emission in the solar corona
Context. The Sun is an active source of radio emission ranging from long
duration radio bursts associated with solar flares and coronal mass ejections
to more complex, short duration radio bursts such as solar S bursts, radio
spikes and fibre bursts. While plasma emission is thought to be the dominant
emission mechanism for most radio bursts, the electron-cyclotron maser (ECM)
mechanism may be responsible for more complex, short-duration bursts as well as
fine structures associated with long-duration bursts. Aims. We investigate the
conditions for ECM in the solar corona by considering the ratio of the electron
plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The
ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods.
Two-dimensional electron density, magnetic field, plasma frequency, and
electron cyclotron frequency maps of the off- limb corona were created using
observations from SDO/AIA and SOHO/LASCO, together with potential field
extrapolations of the magnetic field. These maps were then used to calculate
{\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results.
We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is
possible at heights < 1.07 R_sun in an active region near the limb; that is,
where magnetic field strengths are > 40 G and electron densities are greater
than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities
(> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region.
Conclusions. This demonstrates that the condition for ECM emission is satisfied
within areas of the corona containing large magnetic fields, such as the core
of a large active region. Therefore, ECM could be a possible emission mechanism
for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure
Field-Dependent Hall Effect in Single Crystal Heavy Fermion YbAgGe below 1K
We report the results of a low temperature (T >= 50 mK) and high field (H <=
180 kOe) study of the Hall resistivity in single crystals of YbAgGe, a heavy
fermion compound that demonstrates field-induced non-Fermi-liquid behavior near
its field-induced quantum critical point. Distinct features in the anisotropic,
field-dependent Hall resistivity sharpen on cooling down and at the base
temperature are close to the respective critical fields for the field-induced
quantum critical point. The field range of the non-Fermi-liquid region
decreases on cooling but remains finite at the base temperature with no
indication of its conversion to a point for T -> 0. At the base temperature,
the functional form of the field-dependent Hall coefficient is field direction
dependent and complex beyond existing simple models thus reflecting the
multi-component Fermi surface of the material and its non-trivial modification
at the quantum critical point
Very large magnetoresistance in FeTaS single crystals
Magnetic moments intercalated into layered transition metal dichalcogenides
are an excellent system for investigating the rich physics associated with
magnetic ordering in a strongly anisotropic, strong spin-orbit coupling
environment. We examine electronic transport and magnetization in
FeTaS, a highly anisotropic ferromagnet with a Curie temperature
K. We find anomalous Hall data confirming a
dominance of spin-orbit coupling in the magnetotransport properties of this
material, and a remarkably large field-perpendicular-to-plane MR exceeding 60%
at 2 K, much larger than the typical MR for bulk metals, and comparable to
state-of-the-art GMR in thin film heterostructures, and smaller only than CMR
in Mn perovskites or high mobility semiconductors. Even within the
FeTaS series, for the current = 0.28 single crystals the MR is
nearly higher than that found previously in the commensurate
compound FeTaS. After considering alternatives, we argue that
the large MR arises from spin disorder scattering in the strong spin-orbit
coupling environment, and suggest that this can be a design principle for
materials with large MR.Comment: 8 pages, 8 figures, accepted in PR
Angular dependent planar metamagnetism in the hexagonal compounds TbPtIn and TmAgGe
Detailed magnetization measurements, M(T,H,theta), were performed on single
crystals of TbPtIn and TmAgGe (both members of the hexagonal Fe_2P/ZrNiAl
structure type), for the magnetic field H applied perpendicular to the
crystallographic c axis. These data allowed us to identify, for each compound,
the easy-axes for the magnetization, which coincided with high symmetry
directions ([120] for TbPtIn and [110] for TmAgGe). For fixed orientations of
the field along each of the two six-fold symmetry axes, a number of
magnetically ordered phases is being revealed by M(H,T) measurements below T_N.
Moreover, T ~ 2 K, M(H)|_theta measurements for both compounds (with H applied
parallel to the basal plane), as well as T = 20 K data for TbPtIn, reveal five
metamagnetic transitions with simple angular dependencies: H_{ci,j} ~
1/cos(theta +/- phi), where phi = 0^0 or 60^0. The high field magnetization
state varies with theta like 2/3*mu_{sat}(R^{3+})*cos(theta), and corresponds
to a crystal field limited saturated paramagnetic, CL-SPM, state. Analysis of
these data allowed us to model the angular dependence of the locally saturated
magnetizations M_{sat} and critical fields H_c with a three coplanar Ising-like
model, in which the magnetic moments are assumed to be parallel to three
adjacent easy axes. Furthermore, net distributions of moments were inferred
based on the measured data and the proposed model
Semimetal to semimetal charge density wave transition in 1T-TiSe
We report an infrared study on 1-TiSe, the parent compound of the
newly discovered superconductor CuTiSe. Previous studies of this
compound have not conclusively resolved whether it is a semimetal or a
semiconductor: information that is important in determining the origin of its
unconventional CDW transition. Here we present optical spectroscopy results
that clearly reveal that the compound is metallic in both the high-temperature
normal phase and the low-temperature CDW phase. The carrier scattering rate is
dramatically different in the normal and CDW phases and the carrier density is
found to change with temperature. We conclude that the observed properties can
be explained within the scenario of an Overhauser-type CDW mechanism.Comment: 4 pages, 4 page
Superconductivity in Mg10Ir19B16
Mg10Ir19B16, a previously unreported compound in the Mg-Ir-B chemical system,
is found to be superconducting at temperatures near 5 K. The fact that the
compound exhibits a range of superconducting temperatures between 4 and 5 K
suggests that a range of stoichiometries is allowed, though no structural
evidence for this is observed. The compound has a large, noncentrosymmetric,
body centered cubic unit cell with a = 10.568 Angstrom, displaying a structure
type for which no previous superconductors have been reported.Comment: submitted to PR
Emergence of Fermi pockets in an excitonic CDW melted novel superconductor
A superconducting (SC) state (Tc ~ 4.2K) has very recently been observed upon
successful doping of the CDW ordered triangular lattice TiSe, with copper.
Using high resolution photoemission spectroscopy we identify, for the first
time, the momentum space locations of the doped electrons that form the Fermi
sea of the parent superconductor. With doping, we find that the kinematic
nesting volume increases whereas the coherence of the CDW order sharply drops.
In the superconducting doping, we observe the emergence of a large density of
states in the form of a narrow electron pocket near the \textit{L}-point of the
Brillouin Zone with \textit{d}-like character. The \textit{k}-space electron
distributions highlight the unconventional interplay of CDW to SC cross-over
achieved through non-magnetic copper doping.Comment: 4+ pages, 5 figures; Accepted for publication in Phys. Rev. Lett.
(2007
Extended radio emission associated with a breakout eruption from the back side of the Sun
Context. Coronal mass ejections (CMEs) on the Sun are the largest explosions in the Solar System that can drive powerful plasma shocks. The eruptions, shocks, and other processes associated to CMEs are efficient particle accelerators and the accelerated electrons in particular can produce radio bursts through the plasma emission mechanism. Aims. Coronal mass ejections and associated radio bursts have been well studied in cases where the CME originates close to the solar limb or within the frontside disc. Here, we study the radio emission associated with a CME eruption on the back side of the Sun on 22 July 2012. Methods. Using radio imaging from the Nancay Radioheliograph, spectroscopic data from the Nancay Decametric Array, and extreme-ultraviolet observations from the Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft, we determine the nature of the observed radio emission as well as the location and propagation of the CME. Results. We show that the observed low-intensity radio emission corresponds to a type II radio burst or a short-duration type IV radio burst associated with a CME eruption due to breakout reconnection on the back side of the Sun, as suggested by the pre-eruptive magnetic field configuration. The radio emission consists of a large, extended structure, initially located ahead of the CME, that corresponds to various electron acceleration locations. Conclusions. The observations presented here are consistent with the breakout model of CME eruptions. The extended radio emission coincides with the location of the current sheet and quasi-separatrix boundary of the CME flux and the overlying helmet streamer and also with that of a large shock expected to form ahead of the CME in this configuration.Peer reviewe
Anomalous metallic state of CuTiSe: an optical spectroscopy study
We report an optical spectroscopy study on the newly discovered
superconductor CuTiSe. Consistent with the development from a
semimetal or semiconductor with a very small indirect energy gap upon doping
TiSe, it is found that the compound has a low carrier density. Most
remarkably, the study reveals a substantial shift of the "screened" plasma edge
in reflectance towards high energy with decreasing temperature. This
phenomenon, rarely seen in metals, indicates either a sizeable increase of the
conducting carrier concentration or/and a decrease of the effective mass of
carriers with reducing temperature. We attribute the shift primarily to the
later effect.Comment: 4 figures, 4+ page
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