24,446 research outputs found
Polarization entanglement visibility of photon pairs emitted by a quantum dot embedded in a microcavity
We study the photon emission from a quantum dot embedded in a microcavity.
Incoherent pumping of its excitons and biexciton provokes the emission of leaky
and cavity modes. By solving a master equation we obtain the correlation
functions required to compute the spectrum and the relative efficiency among
the emission of pairs and single photons. A quantum regime appears for low
pumping and large rate of emission. By means of a post-selection process, a two
beams experiment with different linear polarizations could be performed
producing a large polarization entanglement visibility precisely in the quantum
regime.Comment: 13 pages and 6 figure
Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains
We numerically investigate elementary excitations of the Heisenberg
alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically
coupled to each other. Employing a recently developed efficient Monte Carlo
technique as well as an exact diagonalization method, we verify the spin-wave
argument that the model exhibits two distinct excitations from the ground state
which are gapless and gapped. The gapless branch shows a quadratic dispersion
in the small-momentum region, which is of ferromagnetic type. With the
intention of elucidating the physical mechanism of both excitations, we make a
perturbation approach from the decoupled-dimer limit. The gapless branch is
directly related to spin 1's, while the gapped branch originates from
cooperation of the two kinds of spins.Comment: 7 pages, 7 Postscript figures, RevTe
Competing Ground States of the New Class of Halogen-Bridged Metal Complexes
Based on a symmetry argument, we study the ground-state properties of
halogen-bridged binuclear metal chain complexes. We systematically derive
commensurate density-wave solutions from a relevant two-band Peierls-Hubbard
model and numerically draw the the ground-state phase diagram as a function of
electron-electron correlations, electron-phonon interactions, and doping
concentration within the Hartree-Fock approximation. The competition between
two types of charge-density-wave states, which has recently been reported
experimentally, is indeed demonstrated.Comment: 4 pages, 5 figures embedded, to appear in J. Phys. Soc. Jp
Universal relationship between crystallinity and irreversibility field of MgB2
The relationship between irreversibility field, Hirr, and crystallinity of
MgB2 bulks including carbon substituted samples was studied. The Hirr was found
to increase with an increase of FWHM of MgB2 (110) peak, which corresponds to
distortion of honeycomb boron sheet, and their universal correlation was
discovered even including carbon substituted samples. Excellent Jc
characteristics under high magnetic fields were observed in samples with large
FWHM of (110) due to the enhanced intraband scattering and strengthened grain
boundary flux pinning. The relationship between crystallinity and Hirr can
explain the large variation of Hirr for MgB2 bulks, tapes, single crystals and
thin films.Comment: 3 pages, 4 figures, to be published in Appl. Phys. Lett. (in press
Dynamics of the excitations of a quantum dot in a microcavity
We study the dynamics of a quantum dot embedded in a three-dimensional
microcavity in the strong coupling regime in which the quantum dot exciton has
an energy close to the frequency of a confined cavity mode. Under the
continuous pumping of the system, confined electron and hole can recombine
either by spontaneous emission through a leaky mode or by stimulated emission
of a cavity mode that can escape from the cavity. The numerical integration of
a master equation including all these effects gives the dynamics of the density
matrix. By using the quantum regression theorem, we compute the first and
second order coherence functions required to calculate the photon statistics
and the spectrum of the emitted light. Our main result is the determination of
a range of parameters in which a state of cavity modes with poissonian or
sub-poissonian (non-classical) statistics can be built up within the
microcavity. Depending on the relative values of pumping and rate of stimulated
emission, either one or two peaks close to the excitation energy of the dot
and/or to the natural frequency of the cavity are observed in the emission
spectrum. The physics behind these results is discussed
Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound
Ferromagnetism and accompanying large negative magnetoresistance in
Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail.
Recent structural analysis of (Bi,Pb)SrCoO, which has
been believed to be a Co analogue of
BiSrCaCuO, revealed that it has a more complex
structure including a CoO hexagonal layer [T. Yamamoto {\it et al.}, Jpn.
J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only
introduces holes into the conducting CoO layers but also creates a
certain amount of localized spins. Ferromagnetic transition appears at =
3.2 K with small spontaneous magnetization along the axis, and around the
transition temperature large and anisotropic negative magnetoresistance was
observed. This compound is the first example which shows ferromagnetic
long-range order in a two-dimensional metallic hexagnonal CoO layer.Comment: 8 pages including eps figures. To be published in J. Phys. Soc. Jp
Dynamics of Quasi-ordered Structure in a Regio-regulated pi-Conjugated Polymer:Poly(4-methylthiazole-2,5-diyl)
Dynamics of regio-regulated Poly(4-methylthiazole-2,5-diyl) [HH-P4MeTz] was
inves tigated by solid-state 1H, 2D, 13C NMR spectroscopies, and differential
scanning calorimetry(DSC) measurements. DSC, 2D quadrupolar echo NMR, 13C
cross-polarization and magic-angle spinning(CPMAS) NMR, and 2D spin-echo(2DSE)
CPMAS NMR spectroscopy suggest existence of a quasi-ordered phase in which
backbone twists take place with weakened pi-stackings. Two-dimensional exchange
2D NMR(2DEX) detected slow dynamics with a rate of an order of 10^2Hz for the
CD_3 group in d_3-HH-P4MeTz at 288K. The frequency dependence of proton
longitudinal relaxation rate at 288K shows a omega^-1/2 dependence, which is
due to the one-dimensional diffusion-like motion of backbone conformational
modulation waves. The diffusion rate was estimated as 3+/-2 GHz, which was
approximately 10^7 times larger than that estimated by 2DEX NMR measurements.
These results suggest that there exists anomalous dispersion of modulation
waves in HH-P4MeTz. The one-dimensional group velocity of the wave packet is
responsible for the behavior of proton longitudinal relaxation time. On the
other hand, the 2DEX NMR is sensitive to phase velocity of the nutation of
methyl groups that is associated with backbone twists. From proton T_1 and T_2
measurements, the activation energy was estimated as 2.9 and 3.4 kcal/mol,
respectively. These were in agreement with 3.0 kcal/mol determined by
Moller-Plesset(MP2) molecular orbital(MO) calculation. We also performed
chemical shielding calculation of the methyl-carbon in order to understand
chemical shift tensor behavior, leading to the fact that a quasi-ordered phase
coexist with the crystalline phase.Comment: 14 pages, 11 figures, to appear in Phys.Rev.
Nuclear Magnetic Relaxation in the Haldane-Gap Antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_)
A new theory is proposed to interpret nuclear spin-lattice relaxation-time
(T_1_) measurements on the spin-1 quasi-one-dimensional Heisenberg
antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_) (NENP). While Sagi and Affleck
pioneeringly discussed this subject in terms of field-theoretical languages,
there is no theoretical attempt yet to explicitly simulate the novel
observations of 1/T_1_ reported by Fujiwara et al.. By means of modified spin
waves, we solve the minimum of 1/T_1_ as a function of an applied field,
pending for the past decade.Comment: to be published in J. Phys. Soc. Jpn. 73, No. 4 (2004
Cancellation of UV Divergences in the N=4 SUSY Nonlinear Sigma Model in Three Dimensions
We study the UV properties of the three-dimensional SUSY
nonlinear sigma model whose target space is (the cotangent
bundle of ) to higher orders in the 1/N expansion. We calculate the
-function to next-to-leading order and verify that it has no quantum
corrections at leading and next-to-leading orders.Comment: 10 pages, 2 figures. references adde
Effect of interfacial strain on spin injection and spin polarization of Co2CrAl/NaNbO3/Co2CrAl magnetic tunneling junction
First-principles calculations were carried out to investigate interfacial
strain effects on spin injection and spin polarization of a magnetic tunnel
junction consisting of half-metallic full-Heusler alloy Co2CrAl and
ferroelectric perovskite NaNbO3. Spin-dependent coherent tunneling was
calculated within the framework of non-equilibrium Green's function technique.
Both spin polarization and tunnel magnetoresistance (TMR) are affected by the
interfacial strain but their responses to compressive and tensile strains are
different. Spin polarization across the interface is fully preserved under a
compressive strain due to stronger coupling between interfacial atoms, whereas
a tensile strain significantly enhances interface states and lead to
substantial drops in spin polarization and TMR
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