1,935 research outputs found
Magnetic translation groups in an n-dimensional torus
A charged particle in a uniform magnetic field in a two-dimensional torus has
a discrete noncommutative translation symmetry instead of a continuous
commutative translation symmetry. We study topology and symmetry of a particle
in a magnetic field in a torus of arbitrary dimensions. The magnetic
translation group (MTG) is defined as a group of translations that leave the
gauge field invariant. We show that the MTG on an n-dimensional torus is
isomorphic to a central extension of a cyclic group Z_{nu_1} x ... x
Z_{nu_{2l}} x T^m by U(1) with 2l+m=n. We construct and classify irreducible
unitary representations of the MTG on a three-torus and apply the
representation theory to three examples. We shortly describe a representation
theory for a general n-torus. The MTG on an n-torus can be regarded as a
generalization of the so-called noncommutative torus.Comment: 29 pages, LaTeX2e, title changed, re-organized, to be published in
Journal of Mathematical Physic
Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks
We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.Comment: ApJ (in press), 32 pages, Higher quality version available at
http://www-laog.obs.ujf-grenoble.fr/~fcass
Effect of Band Structure on the Symmetry of Superconducting States
Effects of the band structure on the symmetry of superconducting (SC) states
are studied. For a square lattice system with a nearest-neighbor attractive
interaction, SC states with various symmetries are found by changing the band
structure, or, the shape of the Fermi surface. The spin-triplet (-wave) and spin-singlet (- or s-wave) SC states, and states with their
coexistence (, ) can be stabilized within the same type of
interaction. The stability of interlayer-pairing states with line nodes is also
examined, and its relation to the SC state of SrRuO is discussed.Comment: 4 pages, 4 figure
Spin-triplet superconductivity due to antiferromagnetic spin-fluctuation in Sr_2RuO_4
A mechanism leading to the spin-triplet superconductivity is proposed based
on the antiferromagnetic spin fluctuation. The effects of anisotropy in spin
fluctuation on the Cooper pairing and on the direction of d vector are examined
in the one-band Hubbard model with RPA approximation. The gap equations for the
anisotropic case are derived and applied to Sr_2RuO_4. It is found that a
nesting property of the Fermi surface together with the anisotropy leads to the
triplet superconductivity with the d=z(sin{k_x}\pm isin{k_y}), which is
consistent with experiments.Comment: 4 pages, 3 eps figures, revte
Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015
We analyze the short cosmic ray intensity increase ("cosmic ray burst": CRB)
on June 22, 2015 utilizing a global network of muon detectors and derive the
global anisotropy of cosmic ray intensity and the density (i.e. the
omnidirectional intensity) with 10-minute time resolution. We find that the CRB
was caused by a local density maximum and an enhanced anisotropy of cosmic rays
both of which appeared in association with Earth's crossing of the heliospheric
current sheet (HCS). This enhanced anisotropy was normal to the HCS and
consistent with a diamagnetic drift arising from the spatial gradient of cosmic
ray density, which indicates that cosmic rays were drifting along the HCS from
the north of Earth. We also find a significant anisotropy along the HCS,
lasting a few hours after the HCS crossing, indicating that cosmic rays
penetrated into the inner heliosphere along the HCS. Based on the latest
geomagnetic field model, we quantitatively evaluate the reduction of the
geomagnetic cut-off rigidity and the variation of the asymptotic viewing
direction of cosmic rays due to a major geomagnetic storm which occurred during
the CRB and conclude that the CRB is not caused by the geomagnetic storm, but
by a rapid change in the cosmic ray anisotropy and density outside the
magnetosphere.Comment: accepted for the publication in the Astrophysical Journa
Teaching controversial issues in Japan: An exploration of contextual gatekeeping
This qualitative case study sought to understand the extent to which Japanese high school Social Studies teachers grapple with controversial issues in their classrooms. Situated within a curricular-instructional gatekeeping framework, we conducted semi-structured interviews with eight respondents of varying backgrounds and schools in Okayama, Japan. The findings indicated that although the respondents believe they can teach controversial issues in their classrooms they generally do not because of specific contextual forces. Although we found controversial issue instruction to be generally moribund one respondent provided a rationale that reconceptualized exam preparation curriculum and instructio
Electronic theory for the normal state spin dynamics in SrRuO: anisotropy due to spin-orbit coupling
Using a three-band Hubbard Hamiltonian we calculate within the
random-phase-approximation the spin susceptibility, , and
NMR spin-lattice relaxation rate, 1/T, in the normal state of the triplet
superconductor SrRuO and obtain quantitative agreement with
experimental data. Most importantly, we find that due to spin-orbit coupling
the out-of-plane component of the spin susceptibility becomes at
low temperatures two times larger than the in-plane one. As a consequence
strong incommensurate antiferromagnetic fluctuations of the
quasi-one-dimensional - and -bands point into the -direction. Our
results provide further evidence for the importance of spin fluctuations for
triplet superconductivity in SrRuO.Comment: revised versio
Electron-Phonon Interaction and Ultrasonic Attenuation in the Ruthenate and Cuprate superconductors
This article derives an electron-phonon interaction suitable for interpreting
ultrasonic attenuation measurements in the ruthenate and cuprate
superconductors. The huge anisotropy found experimentally (Lupien et al., 2001)
in Sr2RuO4 in the normal state is accounted for in terms of the layered
square-lattice structure of Sr2RuO4, and the dominant contribution to the
attenuation in Sr2RuO4 is found to be due to electrons in the gamma band. The
experimental data in the superconducting state is found to be inconsistent with
vertical lines nodes in the gap in either (100) or (110) planes. Also, a
general method, based on the use of symmetry, is developed to allow for the
analysis of ultrasonic attenuation experiments in superconductors in which the
electronic band structure is complicated or not known. Our results, both for
the normal-state anisotropy, and relating to the positions of the gap nodes in
the superconducting state, are different from those obtained from analyses
using a more traditional model for the electron-phonon interaction in terms of
an isotropic electron stress tensor. Also, a brief discussion of the ultrasonic
attenuation in UPt3 is given.Comment: 12 pages. Comments have been added to the original version of this
article showing how, for the ultrasonic attenuation for a hexagonal crystal
(which must be isotropic with respect to rotations about the c axis) our
approach reproduces the results of the traditional isotropic electron stress
tensor mode
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