Magnetic Reconnection Triggered by the Parker Instability in the Galaxy: Two-Dimensional Numerical Magnetohydrodynamic Simulations and Application to the Origin of X-Ray Gas in the Galactic Halo

We propose the Galactic flare model for the origin of the X-ray gas in the Galactic halo. For this purpose, we examine the magnetic reconnection triggered by Parker instability (magnetic buoyancy instability), by performing the two-dimensional resistive numerical magnetohydrodynamic simulations. As a result of numerical simulations, the system evolves as following phases: Parker instability occurs in the Galactic disk. In the nonlinear phase of Parker instability, the magnetic loop inflates from the Galactic disk into the Galactic halo, and collides with the anti-parallel magnetic field, so that the current sheets are created in the Galactic halo. The tearing instability occurs, and creates the plasmoids (magnetic islands). Just after the plasmoid ejection, further current-sheet thinning occurs in the sheet, and the anomalous resistivity sets in. Petschek reconnection starts, and heats the gas quickly in the Galactic halo. It also creates the slow and fast shock regions in the Galactic halo. The magnetic field ($B\sim 3 \mu$G), for example, can heat the gas ($n\sim 10^{-3}$ cm$^{-3}$) to temperature of $\sim 10^6$ K via the reconnection in the Galactic halo. The gas is accelerated to Alfv\'en velocity ($\sim 300$ km s$^{-1}$). Such high velocity jets are the evidence of the Galactic flare model we present in this paper, if the Doppler shift of the bipolar jet is detected in the Galactic halo. Full size figures are available at http://www.kwasan.kyoto-u.ac.jp/~tanuma/study/ApJ2002/ApJ2002.htmlComment: 13 pages, 12 figures, uses emulateapj.sty, accepted by Ap

Odd-frequency pairing in normal metal/superconductor junctions

We study the induced odd-frequency pairing states in ballistic normal metal/superconductor (N/S) junctions where a superconductor has even-frequency symmetry in the bulk and a normal metal layer has an arbitrary length. Using the quasiclassical Green's function formalism, we demonstrate that, quite generally, the pair amplitude in the junction has an admixture of an odd-frequency component due to the breakdown of translational invariance near the N/S interface where the pair potential acquires spatial dependence. If a superconductor has even-parity pair potential (spin-singlet s-wave state), the odd-frequency pairing component with odd-parity is induced near the N/S interface, while in the case of odd-parity pair potential (spin-triplet $p_{x}$-wave or spin-singlet $d_{xy}$-wave) the odd-frequency component with even-parity is generated. We show that in conventional s-wave junctions, the amplitude of the odd-frequency pairing state is enhanced at energies corresponding to the peaks in the local density of states (LDOS). In $p_x$- and $d_{xy}$-wave junctions, the amplitude of the odd-frequency component on the S side of the N/S interface is enhanced at zero energy where the midgap Andreev resonant state (MARS) appears due to the sign change of the pair potential. The odd-frequency component extends into the N region and exceeds the even-frequency component at energies corresponding to the LDOS peak positions, including the MARS.Comment: 27 pages, 12 figure

Fractional ac Josephson effect in unconventional superconductors

For certain orientations of Josephson junctions between two p_x-wave or two d-wave superconductors, the subgap Andreev bound states produce a 4pi-periodic relation between the Josephson current I and the phase difference phi: I ~ sin(phi/2). Consequently, the ac Josephson current has the fractional frequency eV/h, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current between unconventional superconductors is carried by single electrons, rather than by Cooper pairs. The fractional ac Josephson effect can be observed experimentally by measuring frequency spectrum of microwave radiation from the junction.Comment: 8 pages, 3 figures, RevTEX 4; v2. - minor typos corrected in proof

Temperature-dependence of spin-polarized transport in ferromagnet / unconventional superconductor junctions

Tunneling conductance in ferromagnet / unconventional superconductor junctions is studied theoretically as a function of temperatures and spin-polarization in feromagnets. In d-wave superconductor junctions, the existence of a zero-energy Andreev bound state drastically affects the temperature-dependence of the zero-bias conductance (ZBC). In p-wave triplet superconductor junctions, numerical results show a wide variety in temperature-dependence of the ZBC depending on the direction of the magnetic moment in ferromagnets and the pairing symmetry in superconductors such as $p_{x}$, $p_{y}$ and $p_{x}+ip_{y}$-wave pair potential. The last one is a promising symmetry of Sr$_2$RuO$_4$. From these characteristic features in the conductance, we may obtain the information about the degree of spin-polarization in ferromagnets and the direction of the $d$-vector in triplet superconductors

Zero-bias conductance peak splitting due to multiband effect in tunneling spectroscopy

We study how the multiplicity of the Fermi surface affects the zero-bias peak in conductance spectra of tunneling spectroscopy. As case studies, we consider models for organic superconductors $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ and (TMTSF)$_2$ClO$_4$. We find that multiplicity of the Fermi surfaces can lead to a splitting of the zero-bias conductance peak (ZBCP). We propose that the presence/absence of the ZBCP splitting is used as a probe to distinguish the pairing symmetry in $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$.Comment: 7 pages, 7 figure

The effect of interchain interaction on the pairing symmetry competition in organic superconductors (TMTSF)2_2X

We investigate the effect of interchain repulsive interaction on the pairing symmetry competition in quasi-one-dimensional organic superconductors (TMTSF)$_2$X by applying random phase approximation and quantum Monte Carlo calculation to an extended Hubbard model. We find that interchain repulsive interaction enhances the $2k_F$ charge fluctuations, thereby making the possibility of spin-triplet $f$-wave pairing dominating over spin-singlet d-wave pairing realistic.Comment: 4 page

Fast magnetic reconnection in free space: self-similar evolution process

We present a new model for time evolution of fast magnetic reconnection in free space, which is characterized by self-similarity. Reconnection triggered by locally enhanced resistivity assumed at the center of the current sheet can self-similarly and unlimitedly evolve until external factors affect the evolution. The possibility and stability of this type of evolution are verified by numerical simulations in a very wide spatial dynamic range. Actual astrophysical reconnection in solar flares and geomagnetospheric substorms can be treated as an evolutionary process in free space, because the resultant scale is much larger than the initial scale. In spite of this fact, most of the previous numerical works focused on the evolutionary characters strongly affected by artificial boundary conditions on the simulation boundary. Our new model clarifies a realistic evolution for such cases. The characteristic structure around the diffusion region is quite similar to the Petschek model which is characterized by a pair of slow-mode shocks and the fast-mode rarefaction-dominated inflow. However, in the outer region, a vortex-like return flow driven by the fast-mode compression caused by the piston effect of the plasmoid takes place. The entire reconnection system expands self-similarly.Comment: 17 Pages, 17 Figure

OVI, NV, and CIV in the Galactic Halo: I. Velocity-Dependent Ionization Models

We explore theoretical models of the ionization ratios of the Li-like absorbers NV, OVI, and CIV, in the Galactic halo. These ions are believed to form in nonequilibrium processes such as shocks, evaporative interfaces, or rapidly cooling gas, all of which trace the dynamics of the interstellar medium. As a useful new diagnostic, we focus on velocity-resolved signatures of several common physical structures: (1) a cooling Galactic fountain flow that rises, cools, and recombines as it returns to the disk; (2) shocks moving toward the observer; (3) a conductive interface with the observer located in the hotter gas. This last geometry occurs with the solar system inside a hot bubble, or when one looks out through the fragmenting top shell of our local bubble blown into the halo as part of the Galactic fountain. In Paper II, these models are compared to ionization-ratio data from FUSE and Hubble Space Telescope.Comment: ApJ in press 22 pages, 9 fi

Influence of impurity-scattering on tunneling conductance in d-wave superconductors with broken time reversal symmetry

Effects of impurity scattering on tunneling conductance in dirty normal-metal/insulator/superconductor junctions are studied based on the Kubo formula and the recursive Green function method. The zero-bias conductance peak (ZBCP) is a consequence of the unconventional pairing symmetry in superconductors. The impurity scattering in normal metals suppresses the amplitude of the ZBCP. The degree of the suppression agrees well with results of the quasiclassical Green function theory. When superconductors have $d$+is-wave pairing symmetry, the time-reversal symmetry is broken in superconductors and the ZBCP splits into two peaks. The random impurity scattering reduces the height of the two splitting peaks. The position of the splitting peaks, however, almost remains unchanged even in the presence of the strong impurity scattering. Thus the two splitting peaks never merge into a single ZBCP.Comment: 12 pages, 5 figures, using jpsj2.cls and overcite.st