4,859 research outputs found
Electricâfield dependence of interband transitions in In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)As single quantum wells by roomâtemperature electrotransmittance
Roomâtemperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250âĂ
âthick In_(0.53)Ga_(0.47)As/In_(0.52)Al_(0.48)As singleâquantumâwell system as a function of an externally applied electric field. Parity forbidden transitions, involving conductionâband states with quantum numbers up to n=5, which become more pronounced at high electric fields were observed. The groundâstate and the forbidden transitions showed a significant red shift due to the quantum confined Stark effect. A comparison with previously reported results on thinner InGaAs/InAlAs quantum wells indicated that the wideâwell sample exhibits the largest shift, as expected from theory. Despite the appreciable Stark shift, the rather large, fieldâinduced linewidth broadening and the relatively low electric field at which the groundâstate exciton is ionized poses limitations on using this wideâquantumâwell system for electroâoptic applications
Type I superconductivity in the Dirac semimetal PdTe2
The superconductor PdTe was recently classified as a Type II Dirac
semimetal, and advocated to be an improved platform for topological
superconductivity. Here we report magnetic and transport measurements conducted
to determine the nature of the superconducting phase. Surprisingly, we find
that PdTe is a Type I superconductor with K and a critical
field mT. Our crystals also exhibit the intermediate
state as demonstrated by the differential paramagnetic effect. For we
observe superconductivity of the surface sheath. This calls for a close
examination of superconductivity in PdTe in view of the presence of
topological surface states.Comment: 5 page
Muon spin rotation study of the topological superconductor SrxBi2Se3
We report transverse-field (TF) muon spin rotation experiments on single
crystals of the topological superconductor SrBiSe with nominal
concentrations and ( K). The TF spectra (
mT), measured after cooling to below in field, did not show any
additional damping of the muon precession signal due to the flux line lattice
within the experimental uncertainty. This puts a lower bound on the magnetic
penetration depth m. However, when we induce disorder in
the vortex lattice by changing the magnetic field below a sizeable
damping rate is obtained for . The data provide microscopic
evidence for a superconducting volume fraction of in the
crystal and thus bulk superconductivity.Comment: 6 pages, includes 4 figure
Recovering the unsigned photospheric magnetic field from Ca II K observations
We reassess the relationship between the photospheric magnetic field strength
and the Ca II K intensity for a variety of surface features as a function of
the position on the disc and the solar activity level. This relationship can be
used to recover the unsigned photospheric magnetic field from images recorded
in the core of Ca II K line. We have analysed 131 pairs of high-quality,
full-disc, near-co-temporal observations from SDO/HMI and Rome/PSPT spanning
half a solar cycle. To analytically describe the observationally-determined
relation, we considered three different functions: a power law with an offset,
a logarithmic function, and a power law function of the logarithm of the
magnetic flux density. We used the obtained relations to reconstruct maps of
the line-of-sight component of the unsigned magnetic field (unsigned
magnetograms) from Ca II K observations, which were then compared to the
original magnetograms. We find that both power-law functions represent the data
well, while the logarithmic function is good only for quiet periods. We see no
significant variation over the solar cycle or over the disc in the derived fit
parameters, independently of the function used. We find that errors in the
independent variable, usually not accounted for, introduce attenuation bias. To
address this, we binned the data with respect to the magnetic field strength
and Ca II K contrast separately and derived the relation for the bisector of
the two binned curves. The reconstructed unsigned magnetograms show good
agreement with the original ones. RMS differences are less than 90 G. The
results were unaffected by the stray-light correction of the SDO/HMI and
Rome/PSPT data. Our results imply that Ca~II~K observations, accurately
processed and calibrated, can be used to reconstruct unsigned magnetograms by
using the relations derived in our study.Comment: 18 pages, 22 figures, accepted in A&
Social-Aware Edge Caching in Fog Radio Access Networks
Fog radio access networks (F-RANs) are becoming an emerging and promising paradigm for fifth generation cellular communication systems. In F-RANs, distributed edge caching techniques among remote radio heads (RRHs) and user equipment (UE) can effectively alleviate the burdens on the fronthaul toward the base band unit pool and the bandwidth of the RANs. However, it is still not clear as to how social relationships affect the performance of edge caching schemes. This paper attempts to analyze the impact of mobile social networks (MSNs) on the performance of edge caching in F-RANs. We propose a Markov-chain-based model to analyze edge caching among edge nodes (i.e., RRHs and MSNs), as well as data sharing among the potential MSNs from the viewpoint of content diffusion in the F-RANs. Moreover, we analyze the edge caching schemes among UE to minimize the bandwidth consumption in the RANs. Finally, the optimal edge caching strategies among RRHs in terms of caching locations and time are introduced to minimize the bandwidth consumption of fronthaul and storage costs in the F-RANs. Simulation results show that the proposed edge caching schemes among UE and RRHs are able to reduce the bandwidth consumption of RANs and fronthaul effectively
Joint Deployment and Mobility Management of Energy Harvesting Small Cells in Heterogeneous Networks
Small heterogeneous cells have been introduced to improve the system capacity and provide the ubiquitous service requirements. In order to make flexible deployment and management of massive small cells, the utilization of self-powered small cell base stations with energy harvesting (EH-SCBSs) is becoming a promising solution due to low-cost expenditure. However, the deployment of static EH-SCBSs entails several intractable challenges in terms of the randomness of renewable energy arrival and dynamics of traffic load with spatio-temporal fluctuation. To tackle these challenges, we develop a tractable framework of the location deployment and mobility management of EH-SCBSs with various traffic load distributions an environmental energy models. In this paper, the joint optimization problem for location deployment and mobile management is investigated for maximizing the total system utility of both users and network operators. Since the formulated problem is a NP-hard problem, we propose a low-complex algorithm that decouples the joint optimization into the location updating approach and the association matching approach. A suboptimal solution for the optimization problem can be guaranteed using the iteration of two stage approaches. Performance evaluation shows that the proposed schemes can efficiently solve the target problems while striking a better overall system utility, compared with other traditional deployment and management strategies
Superconductivity under pressure in the Dirac semimetal PdTe2
The Dirac semimetal PdTe was recently reported to be a type-I
superconductor (1.64 K, mT) with unusual
superconductivity of the surface sheath. We here report a high-pressure study,
GPa, of the superconducting phase diagram extracted from
ac-susceptibility and transport measurements on single crystalline samples.
shows a pronounced non-monotonous variation with a maximum 1.91 K around 0.91 GPa, followed by a gradual decrease to 1.27 K at 2.5 GPa.
The critical field of bulk superconductivity in the limit ,
, follows a similar trend and consequently the -curves
under pressure collapse on a single curve: .
Surface superconductivity is robust under pressure as demonstrated by the large
superconducting screening signal that persists for applied dc-fields . Surprisingly, for GPa the superconducting transition
temperature at the surface is larger than of the bulk. Therefore
surface superconductivity may possibly have a non-trivial nature and is
connected to the topological surface states detected by ARPES. We compare the
measured pressure variation of with recent results from band structure
calculations and discuss the importance of a Van Hove singularity.Comment: manuscript 9 pages with 8 figures + supplemental material 3 pages
with 6 figure
Constrained Shape Optimization of Cold-formed Steel Columns
The objective of this paper is to introduce appropriate constraints in the shape optimization of a cold- formed steel column such that the resulting optimized shapes retain the strength benefits of unconstrained optimal solutions combined with practical manufacturing and constructional needs. Unconstrained shape optimization of cold-formed steel columns, where the cross-section that maximizes axial capacity is found, has previously been performed. Here, practical manufacturing and construction constraints are introduced into the optimization algorithm. Members with three lengths: 2 ft, 4 ft, and 16 ft, are considered. Optimized sections from multiple runs show uniformity and bear a close resemblance to unconstrained results. A point-symmetric âSâ-shaped section has maximum capacity for long columns and a singly-symmetric âââ-shaped section with complex lips performs best for shorter columns. The observed strength loss from the unconstrained optimal design, to the constrained optimal design, is within ten percent. A simultaneous perturbation stochastic approximation algorithm, with the idea of injecting randomness in the gradient approximation to save computational cost, is adopted as the local optimizer. A systematic survey on a family of lipped channel cross-sections using the same amount of material was carried out. Comparison reveals that the optimized shapes have much larger capacities and exhibit the potential to seed a new generation of commercial products
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