529 research outputs found
High pressure effect on structure, electronic structure and thermoelectric properties of MoS
We systematically study the effect of high pressure on the structure,
electronic structure and transport properties of 2H-MoS, based on
first-principles density functional calculations and the Boltzmann transport
theory. Our calculation shows a vanishing anisotropy in the rate of structural
change at around 25 GPa, in agreement with the experimental data. A conversion
from van der Waals(vdW) to covalent-like bonding is seen. Concurrently, a
transition from semiconductor to metal occurs at 25 GPa from band structure
calculation. Our transport calculations also find pressure-enhanced electrical
conductivities and significant values of the thermoelectric figure of merit
over a wide temperature range. Our study supplies a new route to improve the
thermoelectric performance of MoS and of other transition metal
dichalcogenides by applying hydrostatic pressure.Comment: 6 pages, 6 figures; published in JOURNAL OF APPLIED PHYSICS 113, xxxx
(2013
Experimental study on diesel spray with single and multiple injection under room temperature and low temperature
Multiple-injection strategy can downsize the penetration and boost the atomization, causing higher IMEP and lower emissions. Multiple-injection however is complex, especially under cold condition.
The injection characteristics were first studied by applying long-tube measuring instrument. Single and split injection strategies were employed. The flow rate and fuel mass were quantitatively studied. Furthermore, the interaction among splits was linked to dwell interval and injection duration of each split. The influences of temperature were also studied.
A microscope and a CCD camera were then employed to study the primary breakup. The regime for the mushroom spray head was analyzed and the dispersion quality was quantified. The breakup-regime study was carried out. Flow regimes in nozzle dominate the breakup. Dwell interval, injection pressure, the number of injections and fuel temperature determined the interaction.
The following is the investigation of macroscopic characteristics with high speed imaging technique. The effects of back pressure and the interaction between injections were investigated. The impact of cold fuel temperature was investigated and various correlations were employed to probe the influence of fuel temperature.
The velocities and sizes of droplets were finally studied with Phase Doppler Particle Analyzer
Ballistic magnon transport and phonon scattering in the antiferromagnet NdCuO
The thermal conductivity of the antiferromagnet NdCuO was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as , from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as ,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency.Comment: 4 pages, 3 figures, one figure modifie
Elastic collapse in disordered isostatic networks
Isostatic networks are minimally rigid and therefore have, generically,
nonzero elastic moduli. Regular isostatic networks have finite moduli in the
limit of large sizes. However, numerical simulations show that all elastic
moduli of geometrically disordered isostatic networks go to zero with system
size. This holds true for positional as well as for topological disorder. In
most cases, elastic moduli decrease as inverse power-laws of system size. On
directed isostatic networks, however, of which the square and cubic lattices
are particular cases, the decrease of the moduli is exponential with size. For
these, the observed elastic weakening can be quantitatively described in terms
of the multiplicative growth of stresses with system size, giving rise to bulk
and shear moduli of order exp{-bL}. The case of sphere packings, which only
accept compressive contact forces, is considered separately. It is argued that
these have a finite bulk modulus because of specific correlations in contact
disorder, introduced by the constraint of compressivity. We discuss why their
shear modulus, nevertheless, is again zero for large sizes. A quantitative
model is proposed that describes the numerically measured shear modulus, both
as a function of the loading angle and system size. In all cases, if a density
p>0 of overconstraints is present, as when a packing is deformed by
compression, or when a glass is outside its isostatic composition window, all
asymptotic moduli become finite. For square networks with periodic boundary
conditions, these are of order sqrt{p}. For directed networks, elastic moduli
are of order exp{-c/p}, indicating the existence of an "isostatic length scale"
of order 1/p.Comment: 6 pages, 6 figues, to appear in Europhysics Letter
Low-temperature phonon thermal conductivity of cuprate single crystals
The effect of sample size and surface roughness on the phonon thermal
conductivity of NdCuO single crystals was studied down to 50
mK. At 0.5 K, is proportional to , where is the
cross-sectional area of the sample. This demonstrates that is
dominated by boundary scattering below 0.5 K or so. However, the expected
dependence of is not observed down to 50 mK. Upon roughing the
surfaces, the dependence is restored, showing that departures from
are due to specular reflection of phonons off the mirror-like sample surfaces.
We propose an empirical power law fit, to (where
) in cuprate single crystals. Using this method, we show that
recent thermal conductivity studies of Zn doping in YBaCuO
re-affirm the universal heat conductivity of d-wave quasiparticles at .Comment: 4 pages, 4 figure
Low temperature resistivity in a nearly half-metallic ferromagnet
We consider electron transport in a nearly half-metallic ferromagnet, in
which the minority spin electrons close to the band edge at the Fermi energy
are Anderson-localized due to disorder. For the case of spin-flip scattering of
the conduction electrons due to the absorption and emission of magnons, the
Boltzmann equation is exactly soluble to the linear order. From this solution
we calculate the temperature dependence of the resistivity due to single magnon
processes at sufficiently low temperature, namely , where is
the Anderson localization length and is the magnon stiffness. And depending
on the details of the minority spin density of states at the Fermi level, we
find a or scaling behavior for resistivity. Relevance to the
doped perovskite manganite systems is discussed
Anisotropic magnetotransport of superconducting and normal state in an electron-doped Nd_{1.85}Ce_{0.15}CuO_{4-\delta} single crystal
The anisotropic properties of an optimally doped
Nd_{1.85}Ce_{0.15}CuO_{4-\delta} single crystal have been studied both below
and above the critical temperature Tc via the resistivity measurement in
magnetic field H up to 12 T. By scaling the conductivity fluctuation around the
superconducting transition, the upper critical field H_{c2}(T) has been
determined for field parallel to the c-axis or to the basal ab-plane. The
anisotropy factor \gamma={H_c2||ab}/{H_c2||c} is estimated to be about 8. In
the normal state (50=<T=<180 K), the magnetoresistance (MR) basically follows
an H^2 dependence and for H||c it is almost 10 times larger than that for
H||ab. Comparing with hole-doped cuprates it suggests that the optimally doped
Nd_{1.85}Ce_{0.15}CuO_{4-\delta} cuprate superconductor has a moderate
anisotropy.Comment: 5 pages, 4 figure
Superconductivity induced by oxygen deficiency in Sr-doped LaOFeAs
We synthesized Sr-doped sample with single phase,
and systematically studied the effect of oxygen deficiency in the Sr-doped
LaOFeAs system. It is found that substitution of Sr for La indeed induces the
hole carrier evidenced by positive thermoelectric power (TEP), but no bulk
superconductivity is observed. The superconductivity can be realized by
annealing the as-grown sample in vacuum to produce the oxygen deficiency. With
increasing the oxygen deficiency, the superconducting transition temperature
() increases and maximum reaches about 26 K the same as that in
La(O,F)FeAs. TEP dramatically changes from positive to negative in the
nonsuperconducting as-grown sample to the superconducting samples with oxygen
deficiency. While is always negative for all samples (even for Sr-doped
as grown sample). It suggests that the is
still electron-type superconductor.Comment: 4 pages, 4 figure
Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells
We have studied a series of InAs/GaSb coupled quantum wells using
magneto-infrared spectroscopy for high magnetic fields up to 33T within
temperatures ranging from 4K to 45K in both Faraday and tilted field
geometries. This type of coupled quantum wells consists of an electron layer in
the InAs quantum well and a hole layer in the GaSb quantum well, forming the
so-called two dimensional electron-hole bilayer system. Unlike the samples
studied in the past, the hybridization of the electron and hole subbands in our
samples is largely reduced by having narrower wells and an AlSb barrier layer
interposed between the InAs and the GaSb quantum wells, rendering them weakly
hybridized. Previous studies have revealed multiple absorption modes near the
electron cyclotron resonance of the InAs layer in moderately and strongly
hybridized samples, while only a single absorption mode was observed in the
weakly hybridized samples. We have observed a pair of absorption modes
occurring only at magnetic fields higher than 14T, which exhibited several
interesting phenomena. Among which we found two unique types of behavior that
distinguishes this work from the ones reported in the literature. This pair of
modes is very robust against rising thermal excitations and increasing magnetic
fields alligned parallel to the heterostructures. While the previous results
were aptly explained by the antilevel crossing gap due to the hybridization of
the electron and hole wavefunctions, i.e. conduction-valence Landau level
mixing, the unique features reported in this paper cannot be explained within
the same concept. The unusual properties found in this study and their
connection to the known models for InAs/GaSb heterostructures will be
disccused; in addition, several alternative ideas will be proposed in this
paper and it appears that a spontaneous phase separation can account for most
of the observed features
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