283 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
Thermal expansion and effect of pressure on superconductivity in CuxTiSe2
We report measurements of thermal expansion on a number of polycrystalline
CuxTiSe2 samples corresponding to the parts of x - T phase diagram with
different ground states, as well as the pressure dependence of the
superconducting transition temperature for samples with three different values
of Cu-doping. Thermal expansion data suggest that the x - T phase diagram may
be more complex than initially reported. T_c data at elevated pressure can be
scaled to the ambient pressure CuxTiSe2 phase diagram, however, significantly
different scaling factors are needed to accommodate the literature data on the
charge density wave transition suppression under pressure
Magnetic field-induced quantum critical point in YbPtIn and YbPtIn single crystals
Detailed anisotropic (Hab and Hc) resistivity and
specific heat measurements were performed on online-grown YbPtIn and
solution-grown YbPtIn single crystals for temperatures down to 0.4 K,
and fields up to 140 kG; Hab Hall resistivity was also measured on
the YbPtIn system for the same temperature and field ranges. All these
measurements indicate that the small change in stoichiometry between the two
compounds drastically affects their ordering temperatures (T
K in YbPtIn, and K in YbPtIn). Furthermore, a field-induced
quantum critical point is apparent in each of these heavy fermion systems, with
the corresponding critical field values of YbPtIn (H around
35-45 kG and H kG) also reduced compared to the analogous
values for YbPtIn (H kG and H kG
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
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
Conditional Hardness of Earth Mover Distance
The Earth Mover Distance (EMD) between two sets of points A, B subseteq R^d with |A| = |B| is the minimum total Euclidean distance of any perfect matching between A and B. One of its generalizations is asymmetric EMD, which is the minimum total Euclidean distance of any matching of size |A| between sets of points A,B subseteq R^d with |A| <= |B|. The problems of computing EMD and asymmetric EMD are well-studied and have many applications in computer science, some of which also ask for the EMD-optimal matching itself. Unfortunately, all known algorithms require at least quadratic time to compute EMD exactly. Approximation algorithms with nearly linear time complexity in n are known (even for finding approximately optimal matchings), but suffer from exponential dependence on the dimension.
In this paper we show that significant improvements in exact and approximate algorithms for EMD would contradict conjectures in fine-grained complexity. In particular, we prove the following results:
- Under the Orthogonal Vectors Conjecture, there is some c>0 such that EMD in Omega(c^{log^* n}) dimensions cannot be computed in truly subquadratic time.
- Under the Hitting Set Conjecture, for every delta>0, no truly subquadratic time algorithm can find a (1 + 1/n^delta)-approximate EMD matching in omega(log n) dimensions.
- Under the Hitting Set Conjecture, for every eta = 1/omega(log n), no truly subquadratic time algorithm can find a (1 + eta)-approximate asymmetric EMD matching in omega(log n) dimensions
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