2,711 research outputs found
Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films
Quantum-well (QW) states in {\it nonmagnetic} metal layers contained in
magnetic multilayers are known to be important in spin-dependent transport, but
the role of QW states in {\it magnetic} layers remains elusive. Here we
identify the conditions and mechanisms for resonant tunneling through QW states
in magnetic layers and determine candidate structures. We report
first-principles calculations of spin-dependent transport in epitaxial
Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr tunnel junctions. We demonstrate the
formation of sharp QW states in the Fe layer and show discrete conductance
jumps as the QW states enter the transport window with increasing bias. At
resonance, the current increases by one to two orders of magnitude. The
tunneling magnetoresistance ratio is several times larger than in simple spin
tunnel junctions and is positive (negative) for majority- (minority-) spin
resonances, with a large asymmetry between positive and negative biases. The
results can serve as the basis for novel spintronic devices.Comment: 4 figures in 5 eps file
Very Old Isolated Compact Objects as Dark Matter Probes
Very old isolated neutron stars and white dwarfs have been suggested to be
probes of dark matter. To play such a role, two requests should be fulfilled,
i.e., the annihilation luminosity of the captured dark matter particles is
above the thermal emission of the cooling compact objects (request-I) and also
dominate over the energy output due to the accretion of normal matter onto the
compact objects (request-II). Request-I calls for very dense dark matter medium
and the critical density sensitively depends on the residual surface
temperature of the very old compact objects. The accretion of
interstellar/intracluster medium onto the compact objects is governed by the
physical properties of the medium and by the magnetization and rotation of the
stars and may outshine the signal of dark matter annihilation. Only in a few
specific scenarios both requests are satisfied and the compact objects are dark
matter burners. The observational challenges are discussed and a possible way
to identify the dark matter burners is outlined.Comment: 9 pages including 1 Figure, to appear in Phys. Rev.
Inert Higgs Dark Matter for New CDF W-boson Mass and Detection Prospects
The -boson mass, which was recently measured at FermiLab, suggests the
presence of new multiplets beyond the Standard Model (SM). One of the minimal
extensions of the SM is to introduce an additional scalar doublet, in which the
non-SM scalars can enhance -boson mass via the loop corrections. On the
other hand, with a proper discrete symmetry, the lightest new scalar in the
doublet can be stable and play the role of dark matter particle. We show that
the inert two Higgs doublet model can naturally handle the new -boson mass
without violating other constraints, and the preferred dark matter mass is
between and GeV. We identify three feasible parameter regions for the
thermal relic density: the co-annihilation, the Higgs resonance, and the
annihilation. We find that the first region can be fully tested
by the HL-LHC, the second region will be tightly constrained by direct
detection experiments, and the third region could yield detectable GeV
gamma-ray and antiproton signals in the Galaxy that may have been observed by
Fermi-LAT and AMS-02.Comment: 8 pages, 5 figure
Neutron Scattering Measurements of Spatially Anisotropic Magnetic Exchange Interactions in Semiconducting K0.85Fe1.54Se2 (TN=280 K)
We use neutron scattering to study the spin excitations associated with the
stripe antiferromagnetic (AFM) order in semiconducting
KFeSe (= K). We show that the spin wave spectra
can be accurately described by an effective Heisenberg Hamiltonian with highly
anisotropic in-plane couplings at = K. At high temperature (=
K) above , short range magnetic correlation with anisotropic correlation
lengths are observed. Our results suggest that, despite the dramatic difference
in the Fermi surface topology, the in-plane anisotropic magnetic couplings are
a fundamental property of the iron based compounds; this implies that their
antiferromagnetism may originate from local strong correlation effects rather
than weak coupling Fermi surface nesting.Comment: 5 pages, 4 figure
Double In Situ Approach for the Preparation of Polymer Nanocomposite with Multi-functionality
A novel one-step synthetic route, the double in situ approach, is used to produce both TiO2nanoparticles and polymer (PET), and simultaneously forming a nanocomposite with multi-functionality. The method uses the release of water during esterification to hydrolyze titanium (IV) butoxide (Ti(OBu)4) forming nano-TiO2in the polymerization vessel. This new approach is of general significance in the preparation of polymer nanocomposites, and will lead to a new route in the synthesis of multi-functional polymer nanocomposites
Electronic structure of Fe1.04(Te0.66Se0.34)
We report the electronic structure of the iron-chalcogenide superconductor,
Fe1.04(Te0.66Se0.34), obtained with high resolution angle-resolved
photoemission spectroscopy and density functional calculations. In
photoemission measurements, various photon energies and polarizations are
exploited to study the Fermi surface topology and symmetry properties of the
bands. The measured band structure and their symmetry characters qualitatively
agree with our density function theory calculations of Fe(Te0.66Se0.34),
although the band structure is renormalized by about a factor of three. We find
that the electronic structures of this iron-chalcogenides and the
iron-pnictides have many aspects in common, however, significant differences
exist near the Gamma-point. For Fe1.04(Te0.66Se0.34), there are clearly
separated three bands with distinct even or odd symmetry that cross the Fermi
energy (EF) near the zone center, which contribute to three hole-like Fermi
surfaces. Especially, both experiments and calculations show a hole-like
elliptical Fermi surface at the zone center. Moreover, no sign of spin density
wave was observed in the electronic structure and susceptibility measurements
of this compound.Comment: 7 pages, 9 figures. submitted to PRB on November 15, 2009, and
accepted on January 6, 201
Charge qubit dynamics in a double quantum dot coupled to phonons
The dynamics of charge qubit in a double quantum dot coupled to phonons is
investigated theoretically in terms of a perturbation treatment based on a
unitary transformation. The dynamical tunneling current is obtained explicitly.
The result is compared with the standard perturbation theory at Born-Markov
approximation. The decoherence induced by acoustic phonons is analyzed at
length. It is shown that the contribution from deformation potential coupling
is comparable to that from piezoelectric coupling in small dot size and large
tunneling rate case. A possible decoupling mechanism is predicted.Comment: 8 pages, 6 figure
Unconventional superconductivity of NdFeAsO0.82F0.18 indicated by the low temperature dependence of the lower critical field Hc1
We measured the initial M-H curves for a sample of the newly discovered
superconductor NdFeAsO0.82Fe0.18, which had a critical temperature, Tc, of 51
K, and was fabricated at the high pressure of 6 GPa. The lower critical field,
Hc1, was extracted from the deviation point of the Meissner linearity in the
M-H curves, which show linear temperature dependence in the low temperature
region down to 5 K. The Hc1(T) indicates no s-wave superconductivity, but
rather an unconventional superconductivity with a nodal gap structure.
Furthermore, the linearity of Hc1 at low temperature does not hold at high
temperature, but shows other characteristics, indicating that this
superconductor might have multi-gap features. Based on the low temperature
nodal gap structure, we estimate that the maximum gap magnitude delta 0 =
(1.6+- 0.2) kBTc.Comment: 8 pages, 3 figure
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