1,472 research outputs found
Tunnelling Characteristics of Stone-Wales Defects in Monolayers of Sn and Group-V Elements
Topological defects in ultrathin layers are often formed during synthesis and
processing, thereby, strongly influencing their electronic properties . In this
paper, we investigate the role of Stone-Wales (SW) defects in modifying the
electronic properties of the monolayers of Sn and group-V elements. The
calculated results find the electronic properties of stanene (monolayer of Sn
atoms) to be strongly dependent on the concentration of SW-defects e.g.,
defective stanene has nearly zero band gap (~ 0.03 eV) for the defect
concentration of 2.2 x 10^13 cm^-2 which opens up to 0.2 eV for the defect
concentration of 3.7 x 10^13 cm^-2. In contrast, SW-defects appear to induce
conduction states in the semiconducting monolayers of group-V elements. These
conduction states act as channels for electron tunnelling, and the calculated
tunnelling characteristics show the highest differential conductance for the
negative bias with the asymmetric current-voltage characteristics. On the other
hand, the highest differential conductance was found for the positive bias in
stanene. Simulated STM topographical images of stanene and group-V monolayers
show distinctly different features in terms of their cross-sectional views and
distance-height profiles which can serve as fingerprints to identify the
topological defects in the monolayers of group-IV and group-V elements in
experiments.Comment: 18 pages, 5 figures, 1 tabl
Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2
In present study, we report an inter-comparison of various physical and
electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve
for MgB2 the same is -ve for AlB2. This is consistent our band structure plots.
We fitted the experimental specific heat of MgB2 to Debye Einstein model and
estimated the value of Debye temperature (theta) and Sommerfeld constant
(gamma) for electronic specific heat. Further, from gamma the electronic
density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of
experimental N (EF) and the one being calculated from DFT, we obtained value of
Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The
electronic specific heat of MgB2 is also fitted below Tc using pi-model and
found that it is a two gap superconductor. The calculated values of two gaps
are in good agreement with earlier reports. Our results clearly demonstrate
that the superconductivity of MgB2 is due to very large phonon contribution
from its stretched lattice. The same two effects are obviously missing in AlB2
and hence it is not superconducting. DFT calculations demonstrated that for
MgB2 the majority of states come from Sigma and Pi 2p states of boron on the
other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak
electron phonon coupling and also to hole deficiency as Pi bands are known to
be of electron type and hence obviously the AlB2 is not superconducting. The
DFT calculations are consistent with the measured physical properties of the
studied borides, i.e., MgB2 and AlB2Comment: 16 pages Text + Figs: comments/suggestions welcome
([email protected])/www.freewebs.com/vpsawana
Anomalous Thermoelectric power of over-doped Bi2Sr2CaCu2O8 superconductor
Temperature dependence of thermoelectric power S(T) of three differently
processed Bi2Sr2CaCu2O8 (Bi2212) samples, viz. as-processed melt quenched
(Bi2212-MQ), 6000C N2-annealed (Bi2212-N2) and 6000C O2-annealed (Bi2212-O2) is
reported here. All the samples possess single-phase character and their
superconducting transition temperatures (TcR=0) are 85 K, 90 K and 72 K
respectively for Bi2212-MQ, Bi2212-N2 and Bi2212-O2. While Bi2212-MQ and
Bi2212-N2 samples are in near optimum doping regime, Bi2212-O2 is an over-doped
sample. TcS=0 values obtained through S(T) data are also in line with those
deduced from the temperature dependence of resistance and DC magnetization.
Interestingly, S(T) behaviour of the optimally-doped Bi2212-MQ and Bi2212-N2
samples is seen to be positive in whole temperature range, it is found negative
for the over-doped Bi2212-O2 sample above TcS=0. These results have been seen
in the light of the recent band structure calculations and the ensuing split
Fermi surface as determined by angle-resolved photoelectron spectroscopy
(ARPES).Comment: 11 Pages Text + Figs: comments welcome ([email protected]
Neutrino oscillations with disentanglement of a neutrino from its partners
We bring attention to the fact that in order to understand existing data on
neutrino oscillations, and to design future experiments, it is imperative to
appreciate the role of quantum entanglement. Once this is accounted for, the
resulting energy-momentum conserving phenomenology requires a single new
parameter related to disentanglement of a neutrino from its partners. This
parameter may not be CP symmetric. We illustrate the new ideas, with
potentially measurable effects, in the context of a novel experiment recently
proposed by Gavrin, Gorbachev, Veretenkin, and Cleveland. The strongest impact
of our ideas is on the resolution of various anomalies in neutrino oscillations
and on neutrino propagation in astrophysical environments.Comment: 6 page
Chalcogen Height Dependence of Magnetism and Fermiology in FeTe_xSe_{1-x}
FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using
density functional theory. Our results show that for FeSe, LDA seems better
approximation in terms of magnitude of magnetic energy whereas GGA
overestimates it largely. On the other hand for FeTe, GGA is better
approximation that gives experimentally observed magnetic state. It has been
shown that the height of chalcogen atoms above Fe layers has significant effect
on band structure, electronic density of states (DOS) at Fermi level N(EF) and
Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner
criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the
value of N(EF) is large, the same is close to be satisfied. Force minimization
done for FeTexSe1-x using supercell approach shows that in disordered system Se
and Te do not share same site and have two distinct z coordinates. This has
small effect on magnetic energy but no significant difference in band structure
and DOS near EF when calculated using either relaxed or average value of z for
chalcogen atoms. Thus substitution of Se at Te site decreases average value of
chalcogen height above Fe layers which in turn affect the magnetism and
Fermiology in the system. By using coherent-potential approximation for
disordered system we found that height of chalcogen atoms above Fe layer rather
than chalcogen species or disorder in the anion planes, affect magnetism and
shape of Fermi surfaces (FS), thus significantly altering nesting conditions,
which govern antiferromagnetic spin fluctuations in the system.Comment: 24 pages Text+Figs: comments/suggestions welcome
([email protected]
Two Dimensional Allotropes of Arsenene with Wide Range of High and Anisotropic Carrier Mobility
Considering the rapid development of experimental techniques for fabricating
2D materials in recent years, various monolayers are expected to be
experimentally realized in the near future. Motivated by the recent research
activities focused on the honeycomb arsenene monolayers, stability and carrier
mobility of non-honeycomb and porous allotropic arsenene are determined using
first principles calculations. In addition to five honeycomb structures of
arsenene, a total of eight other structures are considered in this study. An
extensive analysis comprising energetics, phonon spectra and mechanical
properties confirms that these structures are energetically and dynamically
stable. All these structures are semiconductors with a broad range of band gap
varying from ~1 eV to ~2.5 eV. Significantly, these monolayer allotropes
possess anisotropic carrier mobilities as high as several hundred
cm^{2}V^{-1}s^{-1} which is comparable with the well-known 2D materials such as
black phosphorene and monolayer MoS_{2}. Combining such broad band gaps and
superior carrier mobilities, these monolayer allotropes can be promising
candidates for the superior performance of the next generation nanoscale
devices. We further explore these monolayer allotropes for photocatalytic water
splitting and find that arsenene monolayers have potential for usage as visible
light driven photocatalytic water splitting.Comment: 31 pages, 8 figures, 3 table
Randomized Trial Comparing the Effectiveness of 2 Electric Breast Pumps in the NICU.
Background:Mothers with preterm infants may need to express milk for considerable periods. Research to improve breast pump design has focused on compression stimuli, frequencies, and vacuums.Objective:This study aimed to compare the effectiveness of 2 electric pumps: Medela Symphony (pump S) and a novel pump (Philips AVENT Twin electronic pump; pump A). Both offer flexibility of rate and suction; pump A also incorporates petal compression cushions. Primary outcomes were (1) milk weight expressed during 10-day study period and (2) weight of milk expressed in a 15-minute test.Methods:Seventy-one mothers with preterm infants < 34 weeks were randomized. Mothers completed 10-day diaries including weight of milk expressed. Milk weight expressed during a single 15-minute test period and data on pumping mode, skin-to-skin contact, breastfeeding at infant discharge, and mothers' opinions of the pump were recorded.Results:There was no significant difference in milk expressed during the first 10 days between groups. Pump S mothers expressed significantly more milk during a fixed 15-minute period. Mothers using pump A awarded higher scores for certain characteristics of the pump, notably location of control button and ease of use. Similar proportions of infants received breast milk at discharge, but pump A mothers were more likely to be directly breastfeeding (odds ratio, 4.27 [95% confidence interval, 1.29, 14.1]).Conclusion:The breast pumps showed similar effectiveness in terms of milk expression and maternal opinions. The finding that breast pump design may influence breastfeeding at infant discharge merits further investigation
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