1,264 research outputs found

### The Development of Staging Mechanisms for the Japanese Satellite Launcher Mu-3SII

The staging mechanisms of the Japanese satellite launch vehicle Mu-3SII involving a unique separation and jettison mechanism for the nose fairing are described. The design requirements, the design features and the development problems are presented together with their solutions

### Dynamics of localized spins coupled to the conduction electrons with charge/spin currents

The effects of the charge/spin currents of conduction electrons on the
dynamics of the localized spins are studied in terms of the perturbation in the
exchange coupling $J_{K}$ between them. The equations of motion for the
localized spins are derived exactly up to $O(J_{K}^2)$, and the equations for
the two-spin system is solved numerically. It is found that the dynamics
depends sensitively upon the relative magnitude of the charge and spin
currents, i.e., it shows steady state, periodic motion, and even chaotic
behavior. Extension to the multi-spin system and its implications including
possible ``spin current detector'' are also discussed.Comment: 5 pages, 4 figures, REVTe

### Quantum fluctuations in the effective pseudospin-1/2 model for magnetic pyrochlore oxides

The effective quantum pseudospin-1/2 model for interacting rare-earth
magnetic moments, which are locally described with atomic doublets, is studied
theoretically for magnetic pyrochlore oxides. It is derived microscopically for
localized Pr^{3+} 4f moments in Pr_2TM_2O_7 (TM = Zr, Sn, Hf, and Ir) by
starting from the atomic non-Kramers magnetic doublets and performing the
strong-coupling perturbation expansion of the virtual electron transfer between
the Pr 4f and O 2p electrons. The most generic form of the nearest-neighbor
anisotropic superexchange pseudospin-1/2 Hamiltonian is also constructed from
the symmetry properties, which is applicable to Kramers ions Nd^{3+}, Sm^{3+},
and Yb^{3+} potentially showing large quantum effects. The effective model is
then studied by means of a classical mean-field theory and the exact
diagonalization on a single tetrahedron and on a 16-site cluster. These
calculations reveal appreciable quantum fluctuations leading to quantum phase
transitions to a quadrupolar state as a melting of spin ice for the Pr^{3+}
case. The model also shows a formation of cooperative quadrupole moment and
pseudospin chirality on tetrahedrons. A sign of a singlet quantum spin ice is
also found in a finite region in the space of coupling constants. The relevance
to the experiments is discussed.Comment: 18 pages including 14 figures; Comparison with the magnetization
curve on Pr2Ir2O7 included; to appear in Phys. Rev.

### Renormalization of the electron-phonon coupling in the one-band Hubbard model

We investigate the effect of electronic correlations on the coupling of
electrons to Holstein phonons in the one-band Hubbard model. We calculate the
static electron-phonon vertex within linear response of Kotliar-Ruckenstein
slave-bosons in the paramagnetic saddle-point approximation. Within this
approach the on-site Coulomb interaction U strongly suppresses the coupling to
Holstein phonons at low temperatures. Moreover the vertex function does not
show particularly strong forward scattering. Going to larger temperatures
kT\sim t we find that after an initial decrease with U, the electron-phonon
coupling starts to increase with U, confirming a recent result of Cerruti,
Cappelluti, and Pietronero. We show that this behavior is related to an unusual
reentrant behavior from a phase separated to a paramagnetic state upon
decreasing the temperature.Comment: 4 pages, 6 figure

### Tuning phase transition between quantum spin Hall and ordinary insulating phases

An effective theory is constructed for analyzing a generic phase transition
between the quantum spin Hall and the insulator phases. Occurrence of
degeneracies due to closing of the gap at the transition are carefully
elucidated. For systems without inversion symmetry the gap-closing occurs at
\pm k_0(\neq G/2) while for systems with inversion symmetry, the gap can close
only at wave-numbers k=G/2, where G is a reciprocal lattice vector. In both
cases, following a unitary transformation which mixes spins, the system is
represented by two decoupled effective theories of massive two-component
fermions having masses of opposite signs. Existence of gapless helical modes at
a domain wall between the two phases directly follows from this formalism. This
theory provides an elementary and comprehensive phenomenology of the quantum
spin Hall system.Comment: 6 pages, 2 figures, to appear in Phys. Rev.

### Singlet levels of the NV$^{-}$ centre in diamond

The characteristic transition of the NV- centre at 637 nm is between
${}^3\mathrm{A}_2$ and ${}^3\mathrm{E}$ triplet states. There are also
intermediate ${}^1\mathrm{A}_1$ and ${}^1\mathrm{E}$ singlet states, and the
infrared transition at 1042 nm between these singlets is studied here using
uniaxial stress. The stress shift and splitting parameters are determined, and
the physical interaction giving rise to the parameters is considered within the
accepted electronic model of the centre. It is established that this
interaction for the infrared transition is due to a modification of
electron-electron Coulomb repulsion interaction. This is in contrast to the
visible 637 nm transition where shifts and splittings arise from modification
to the one-electron Coulomb interaction. It is also established that a dynamic
Jahn-Teller interaction is associated with the singlet ${}^1\mathrm{E}$ state,
which gives rise to a vibronic level 115 $\mathrm{cm}^{-1}$ above the
${}^1\mathrm{E}$ electronic state. Arguments associated with this level are
used to provide experimental confirmation that the ${}^1\mathrm{A}_1$ is the
upper singlet level and ${}^1\mathrm{E}$ is the lower singlet level.Comment: 19 pages, 6 figure

### Effective mass staircase and the Fermi liquid parameters for the fractional quantum Hall composite fermions

Effective mass of the composite fermion in the fractional quantum Hall
system, which is of purely interaction originated, is shown, from a numerical
study, to exhibit a curious nonmonotonic behavior with a staircase correlated
with the number (=2,4,...) of attached flux quanta. This is surprising since
the usual composite-fermion picture predicts a smooth behavior. On top of that,
significant interactions are shown to exist between composite fermions, where
the excitation spectrum is accurately reproduced in terms of Landau's Fermi
liquid picture with negative (i.e., Hund's type) orbital and spin exchange
interactions.Comment: 4 pages, 3 figures, REVTe

### Spin Hall effect of conserved current: Conditions for a nonzero spin Hall current

We study the spin Hall effect taking into account the impurity scattering
effect as general as possible with the focus on the definition of the spin
current. The conserved bulk spin current (Shi et al. [Phys. Rev. Lett. 96,
076604 (2006)]) satisfying the continuity equation of spin is considered in
addition to the conventional one defined by the symmetric product of the spin
and velocity operators. Conditions for non-zero spin Hall current are
clarified. In particular, it is found that (i) the spin Hall current is
non-zero in the Rashba model with a finite-range impurity potential, and (ii)
the spin Hall current vanishes in the cubic Rashba model with a
$\delta$-function impurity potential.Comment: 5 pages, minor change from the previous versio

### Temperature dependent magnetotransport around $\nu$= 1/2 in ZnO heterostructures

The sequence of prominent fractional quantum Hall states up to $\nu$=5/11
around $\nu$=1/2 in a high mobility two-dimensional electron system confined at
oxide heterointerface (ZnO) is analyzed in terms of the composite fermion
model. The temperature dependence of \Rxx oscillations around $\nu$=1/2
yields an estimation of the composite fermion effective mass, which increases
linearly with the magnetic field. This mass is of similar value to an enhanced
electron effective mass, which in itself arises from strong electron
interaction. The energy gaps of fractional states and the temperature
dependence of \Rxx at $\nu$=1/2 point to large residual interactions between
composite fermions.Comment: 5 pages, 4 Figure

### Spin Chirality Fluctuation and Anomalous Hall Effect in Itinerant Ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is
studied theoretically in a Kondo lattice model without the relativistic
spin-orbit interaction. Even without the correlations of the localized spins,
$\sigma_{xy}$ can emerge depending on the lattice structure and the spin
anisotropy. We reveal the condition for this chirality-fluctuation driven
mechanism for $\sigma_{xy}$. Our semiquantitative estimates for a pyrochlore
oxide Nd$_2$Mo$_2$O$_7$ give a finite \sigma_{xy} \sim 10 \Ohm^{-1} \cm^{-1}
together with a high resistivity \rho_{xx} \sim 10^{-4}-10^{-3} \Ohm \cm, in
agreement with experiments.Comment: 5 pages, including 4 figure

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