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 Fe0.28_{0.28}TaS2_{2} 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 Fe$_{0.28}$TaS$_{2}$, a highly anisotropic ferromagnet with a Curie temperature $T_{\mathrm{C}} \sim 68.8~$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 Fe$_x$TaS$_2$ series, for the current $x$ = 0.28 single crystals the MR is nearly $100\times$ higher than that found previously in the commensurate compound Fe$_{0.25}$TaS$_{2}$. 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-TiSe2_2

We report an infrared study on 1$T$-TiSe$_2$, the parent compound of the newly discovered superconductor Cu$_x$TiSe$_2$. 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$_2$, 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 Cu0.07_{0.07}TiSe2_2: an optical spectroscopy study

We report an optical spectroscopy study on the newly discovered superconductor Cu$_{0.07}$TiSe$_2$. Consistent with the development from a semimetal or semiconductor with a very small indirect energy gap upon doping TiSe$_2$, 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