2,502 research outputs found
Evolution of structural and magnetic properties in Ta/Ni_81Fe_(19) multilayer thin films
The interdiffusion kinetics in short period (12.8 nm) Ta/Ni81Fe19 polycrystalline multilayer films has been investigated and related to the evolution of soft magnetic properties upon thermal annealing in the temperature range 300-600-degrees-C. Small angle x-ray diffraction and transmission electron microscopy were used to estimate the multilayer period. Interdiffusion in the multilayers was directly computed from the decay of the satellites near (000) in a small angle x-ray diffraction spectrum. A kinetic analysis of interdiffusion suggests that grain growth is concurrent with grain boundary diffusion of Ta in Ni81Fe19. The evolution of soft magnetic properties of Ni81Fe19, i.e., lowering of 4piM(s) and increase in coercivity H(c), also lend support to the above analysis
Probing the inter-layer exciton physics in a MoS/MoSe/MoS van der Waals heterostructure
Stacking atomic monolayers of semiconducting transition metal dichalcogenides
(TMDs) has emerged as an effective way to engineer their properties. In
principle, the staggered band alignment of TMD heterostructures should result
in the formation of inter-layer excitons with long lifetimes and robust valley
polarization. However, these features have been observed simultaneously only in
MoSe/WSe heterostructures. Here we report on the observation of long
lived inter-layer exciton emission in a MoS/MoSe/MoS trilayer van
der Waals heterostructure. The inter-layer nature of the observed transition is
confirmed by photoluminescence spectroscopy, as well as by analyzing the
temporal, excitation power and temperature dependence of the inter-layer
emission peak. The observed complex photoluminescence dynamics suggests the
presence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. We
show that circularly polarized optical pumping results in long lived valley
polarization of inter-layer exciton. Intriguingly, the inter-layer exciton
photoluminescence has helicity opposite to the excitation. Our results show
that through a careful choice of the TMDs forming the van der Waals
heterostructure it is possible to control the circular polarization of the
inter-layer exciton emission.Comment: 19 pages, 3 figures. Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03184
Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3
We studied the defects of Bi2Se3 generated from Bridgman growth of
stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size,
and transport properties are strongly affected by the types of defect
generated. Major defect types of Bi_Se antisite and partial Bi_2-layer
intercalation are identified through combined studies of direct atomic-scale
imaging with scanning transmission electron microscopy (STEM) in conjunction
with energy-dispersive X-ray spectroscopy (STEM-EDX), X-ray diffraction, and
Hall effect measurements. We propose a consistent explanation to the origin of
defect type, growth morphology, and transport property.Comment: 5 pages, 5 figure
Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor
Systematic ac susceptibility measurements have been performed on a MgB
bulk sample. We demonstrate that the flux creep activation energy is a
nonlinear function of the current density , indicating a
nonlogarithmic relaxation of the current density in this material. The
dependence of the activation energy on the magnetic field is determined to be a
power law , showing a steep decline in the activation
energy with the magnetic field, which accounts for the steep drop in the
critical current density with magnetic field that is observed in MgB. The
irreversibility field is also found to be rather low, therefore, the pinning
properties of this new material will need to be enhanced for practical
applications.Comment: 11 pages, 6 figures, Revtex forma
Luminosity Functions of Lyman-Break Galaxies at z~4 and 5 in the Subaru Deep Field
We investigate the luminosity functions of Lyman-break galaxies (LBG) at z~4
and 5 based on the optical imaging data obtained in the Subaru Deep Field
Project. Three samples of LBGs in a contiguous 875 arcmin^2 area are
constructed. One consists of 3,808 LBGs at z~4 down to i'=26.85 selected with
the B-R vs R-i' diagram. The other two consist of 539 and 240 LBGs at z~5 down
to z'=26.05 selected with two kinds of two-color diagrams: V-i' vs i'-z' and
R-i' vs i'-z'. The adopted selection criteria are proved to be fairly reliable
by spectroscopic observation. We derive the luminosity functions of the LBGs at
rest-frame ultraviolet wavelengths down to M_{UV}=-19.2 at z~4 and M_{UV}=-20.3
at z~5. We find clear evolution of the luminosity function over the redshift
range of 0<z<6, which is accounted for by a sole change in the characteristic
magnitude, M^*. The cosmic star formation rate (SFR) density at z~4 and z~5 is
measured from the luminosity functions. We examine the evolution of the cosmic
SFR density and its luminosity dependence over 0<z<6. The SFR density
contributed from brighter galaxies is found to change more drastically with
cosmic time. The contribution from brighter galaxies has a sharp peak around
z=3-4, while that from fainter galaxies evolves relatively mildly with a broad
peak at earlier epoch. Combining the observed SFR density with the standard
Cold Dark Matter model, we compute the cosmic SFR per unit baryon mass in dark
haloes, i.e., the specific SFR. The specific SFR is found to scale with
redshift as (1+z)^3 up to z~4, implying that the efficiency of star formation
is on average higher at higher redshift in proportion to the cooling rate
within dark haloes, while this is not simply the case at z>4.Comment: 28 pages, 25 figures, accepted for publication in ApJ, a high
resolution version of Figs.7,8,9 is available at
http://hikari.astron.s.u-tokyo.ac.jp/~yoshida/sdflbglf
Electrical characteristics of amorphous molybdenum-nickel contacts to silicon
The electrical characteristics of sputtered, amorphous Mo-Ni contacts have been measured on both p- and n-type Si, as functions of composition (30, 54, and 58 at. % Mo). The contact resistivity on both p+ and n+ Si is in the 10^−6 Ω cm^2 range. The barrier height for as-deposited samples varies between φbp=0.47–0.42 V on p-type Si and between φbn=0.63–0.68 V on n-type Si, as the composition of the amorphous layer goes from Ni-rich to Mo-rich. The sum φbp+φbn always equals 1.12 V, within experimental error. After thermal treatment at 500 °C for 1/2 h, the contact resistivity changes by a factor of two or less, while the barrier height changes by at most ~0.05 V. In light of these results, the amorphous Mo-Ni film makes good ohmic contacts to silicon
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