1,264 research outputs found

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

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    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

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    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 JKJ_{K} between them. The equations of motion for the localized spins are derived exactly up to O(JK2)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

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    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

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    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

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    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

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    The characteristic transition of the NV- centre at 637 nm is between 3A2{}^3\mathrm{A}_2 and 3E{}^3\mathrm{E} triplet states. There are also intermediate 1A1{}^1\mathrm{A}_1 and 1E{}^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 1E{}^1\mathrm{E} state, which gives rise to a vibronic level 115 cm1\mathrm{cm}^{-1} above the 1E{}^1\mathrm{E} electronic state. Arguments associated with this level are used to provide experimental confirmation that the 1A1{}^1\mathrm{A}_1 is the upper singlet level and 1E{}^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

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    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

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    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 δ\delta-function impurity potential.Comment: 5 pages, minor change from the previous versio

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

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    The sequence of prominent fractional quantum Hall states up to ν\nu=5/11 around ν\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 ν\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 ν\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

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    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, σxy\sigma_{xy} can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for σxy\sigma_{xy}. Our semiquantitative estimates for a pyrochlore oxide Nd2_2Mo2_2O7_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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