492 research outputs found
Stacking Faults, Bound States, and Quantum Hall Plateaus in Crystalline Graphite
We analyze the electronic properties of a simple stacking defect in Bernal
graphite. We show that a bound state forms, which disperses as |\bfk-\bfK|^3
in the vicinity of either of the two inequivalent zone corners \bfK. In the
presence of a strong c-axis magnetic field, this bound state develops a Landau
level structure which for low energies behaves as E\nd_n\propto |n B|^{3/2}.
We show that buried stacking faults have observable consequences for surface
spectroscopy, and we discuss the implications for the three-dimensional quantum
Hall effect (3DQHE). We also analyze the Landau level structure and chiral
surface states of rhombohedral graphite, and show that, when doped, it should
exhibit multiple 3DQHE plateaus at modest fields.Comment: 19 page
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 (G), for example, can heat the
gas ( cm) to temperature of K via the
reconnection in the Galactic halo. The gas is accelerated to Alfv\'en velocity
( km s). 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
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
, and -wave pair potential. The last one is a
promising symmetry of SrRuO. 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 -vector in
triplet superconductors
Layer dependent band dispersion and correlations using tunable Soft X-ray ARPES
Soft X-ray Angle-Resolved Photoemission Spectroscopy is applied to study
in-plane band dispersions of Nickel as a function of probing depth. Photon
energies between 190 and 780 eV were used to effectively probe up to 3-7
layers. The results show layer dependent band dispersion of the Delta_2
minority-spin band which crosses the Fermi level in 3 or more layers, in
contrast to known top 1-2 layers dispersion obtained using ultra-violet rays.
The layer dependence corresponds to an increased value of exchange splitting
and suggests reduced correlation effects in the bulk compared to the surface.Comment: 7 pages, 3 figures Revised text and figur
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 -(BEDT-TTF)Cu(NCS) and
(TMTSF)ClO. 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 -(BEDT-TTF)Cu(NCS).Comment: 7 pages, 7 figure
Local density of states for the corner geometry interface of d-wave superconductors, within the extended Hubbard model
The spatial variations of the order parameter, and the local density of
states (LDOS) on the corner of s-wave or -wave superconductors, as
well as in superconductor-insulator-normal metal interfaces, are calculated
self consistently using the Bogoliubov-deGennes formalism within the two
dimensional extended Hubbard model. The exact diagonalization method is used.
Due to the suppression of the dominant d-wave order parameter, the extended
s-wave order parameter is induced near the surface, that alternates its sign
for the topmost sites at adjacent edges of the lattice and decays to zero in
the bulk. The presence of surface roughness results into the appearance of the
zero band conduction peak (ZBCP) near the corner surface which lacks from the
predictions of the quasiclassical theory.Comment: 13 pages with 17 figure
A Hot Helium Plasma in the Galactic Center Region
Recent X-ray observations by the space mission Chandra confirmed the
astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9.
K (8 keV) found by previous surveys to extend over a few hundred parsecs
in the Galactic Centre region. This plasma coexists with the usual components
of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV)
component produced by supernova remnants, and its origin remains uncertain.
First, simple calculations using a mean sound speed for a hydrogen-dominated
plasma have suggested that it should not be gravitationally bound, and thus
requires a huge energy source to heat it in less than the escape time. Second,
an astrophysical mechanism must be found to generate such a high temperature.
No known source has been identified to fulfill both requirements. Here we
address the energetics problem and show that the hot component could actually
be a gravitationally confined helium plasma. We illustrate the new prospects
this opens by discussing the origin of this gas, and by suggesting possible
heating mechanisms.Comment: 9 pages, accepted for publication in APJ
A phenomenological theory of zero-energy Andreev resonant states
A conceptual consideration is given to a zero-energy state (ZES) at the
surface of unconventional superconductors. The reflection coefficients in
normal-metal / superconductor (NS) junctions are calculated based on a
phenomenological description of the reflection processes of a quasiparticle.
The phenomenological theory reveals the importance of the sign change in the
pair potential for the formation of the ZES. The ZES is observed as the
zero-bias conductance peak (ZBCP) in the differential conductance of NS
junctions. The split of the ZBCP due to broken time-reversal symmetry states is
naturally understood in the present theory. We also discuss effects of external
magnetic fields on the ZBCP.Comment: 12 page
Electronic structures of CeRu ( = Si, Ge) in the paramagnetic phase studied by soft X-ray ARPES and hard X-ray photoelectron spectroscopy
Soft and hard X-ray photoelectron spectroscopy (PES) has been performed for
one of the heavy fermion system CeRuSi and a -localized ferromagnet
CeRuGe in the paramagnetic phase. The three-dimensional band structures
and Fermi surface (FS) shapes of CeRuSi have been determined by soft
X-ray -dependent angle resolved photoelectron spectroscopy (ARPES). The
differences in the Fermi surface topology and the non- electronic
structures between CeRuSi and CeRuGe are qualitatively
explained by the band-structure calculation for both itinerant and
localized models, respectively. The Ce valences in CeRu ( = Si, Ge)
at 20 K are quantitatively estimated by the single impurity Anderson model
calculation, where the Ce 3d hard X-ray core-level PES and Ce 3d X-ray
absorption spectra have shown stronger hybridization and signature for the
partial contribution to the conduction electrons in CeRuSi.Comment: 8figure
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
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