1,643 research outputs found
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High efficiency single Ag nanowire/p-GaN substrate Schottky junction-based ultraviolet light emitting diodes
We report a high efficiency single Ag nanowire (NW)/p-GaN substrate Schottky junction-based ultraviolet light emitting diode (UV-LED). The device demonstrates deep UV free exciton electroluminescence at 362.5 nm. The dominant emission, detectable at ultralow (<1 μA) forward current, does not exhibit any shifts when the forward current is increased. External quantum efficiency (EQE) as high as 0.9% is achieved at 25 μA current at room temperature. Experiments and simulation analysis show that devices fabricated with thinner Ag NWs have higher EQE. However, for very thin Ag NWs (diameter < 250 nm), this trend breaks down due to heat accumulation in the NWs. Our simple device architecture offers a potentially cost-effective scheme to fabricate high efficiency Schottky junction-based UV-LEDs.This work was supported by National Key Basic Research Program of China (No. 2013CB328703), National Natural Science Foundation of China (Nos. 51372220 and 61177062), and the Fundamental Research Funds for the Central Universities. T.H. acknowledges funding from the Royal Academy of Engineering (Graphlex).This is the accepted manuscript of a paper published in Applied Physics Letters (Wu Y, Hasan T, Li X, Xu P, Wang Y, Shen X, Liu X, Yang Q, Applied Physics Letters, 2015, 106, 051108, doi:10.1063/1.4907568). The final version is available at http://dx.doi.org/10.1063/1.490756
Broadly defining lasing wavelengths in single bandgap-graded semiconductor nanowires.
Designing lasing wavelengths and modes is essential to the practical applications of nanowire (NW) lasers. Here, according to the localized photoluminescence spectra, we first demonstrate the ability to define lasing wavelengths over a wide range (up to 119 nm) based on an individual bandgap-graded CdSSe NW by forward cutting the NW from CdSe to CdS end. Furthermore, free spectral range (FSR) and modes of the obtained lasers could be controlled by backward cutting the NW from CdS to CdSe end step-by-step. Interestingly, single-mode NW laser with predefined lasing wavelength is realized in short NWs because of the strong mode competition and increase in FSR. Finally, the gain properties of the bandgap-graded NWs are investigated. The combination of wavelength and mode selectivity in NW lasers may provide a new platform for the next generation of integrated optoelectronic devices.This work is supported by National Key Basic Research Program of China (No.
2013CB328703), National Natural Science Foundation of China (No. 51372220,
61177062, 61125402 and 51172004), the Fundamental Research Funds for the
Central Universities, the Program for Zhejiang Leading Team of S&T Innovation and
the Fundamental Research Funds for the Central Universities.This is the author accepted manuscript. The final version can be found on the publisher's website at: http://pubs.acs.org/doi/abs/10.1021/nl500432m Copyright © 2014 American Chemical Societ
Image analysis of non-aqueous phase liquid migration in aggregated kaolin
Double-porosity is an important feature in soil due to its influence on the migration of fluids within the soil. Conventional ways of measuring fluid saturation involves intrusive use of equipment that may disturb the original setting of the sample being measured. The use of image analysis has overcome this problem but has rarely been applied in research concerning double-porosity soil media. The study presented in this article applies image analysis to study the migration of non-aqueous phase liquid (NAPL) in soil with double-porosoity features. In this study, the laboratory experiments were conducted in a three-dimensional rectangular acrylic model and images were acquired using the photographic technique. Immiscible NAPL was chosen as the fluid applied as it is relatively less studied in double-porosity media compared to miscible contaminants. Aggregated kaolin was used as the double-porosity soil samples. Image analysis was utilized to observe the migration of the NAPL based on migration area coverage, the optical saturation of the NAPL as well as the intensity of the NAPL during migration. The experiments were performed over a range of different moisture content contained in the aggregated soil samples and the effect of different soil moisture content on the migration of NAPL in double-porosity soil was analyzed. The experimental results showed that the rate of NAPL migration will increase as the moisture content increases. In summary, image analysis was found to be a viable method in observing and visualizing the migration of NAPL based on optical saturation, intensity, and area invaded by NAPL in double-porosity soil
Topological phase diagram and saddle point singularity in a tunable topological crystalline insulator
We report the evolution of the surface electronic structure and surface
material properties of a topological crystalline insulator (TCI) Pb1-xSnxSe as
a function of various material parameters including composition x, temperature
T and crystal structure. Our spectroscopic data demonstrate the electronic
groundstate condition for the saddle point singularity, the tunability of
surface chemical potential, and the surface states' response to circularly
polarized light. Our results show that each material parameter can tune the
system between trivial and topological phase in a distinct way unlike as seen
in Bi2Se3 and related compounds, leading to a rich and unique topological phase
diagram. Our systematic studies of the TCI Pb1-xSnxSe are valuable materials
guide to realize new topological phenomena.Comment: 10 pages, 7 figures. Expanded version of arXiv:1403.156
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102 fs pulse generation from a long-term stable, inkjet-printed black phosphorus-mode-locked fiber laser.
We demonstrate a long-term stable, all-fiber, erbium-doped femtosecond laser mode-locked by a black phosphorus saturable absorber. The saturable absorber, fabricated by scalable and highly controllable inkjet printing technology, exhibits strong nonlinear optical response and is stable for long-term operation against intense irradiation, overcoming a key drawback of this material. The oscillator delivers self-starting, 102 fs stable pulses centered at 1555 nm with 40 nm spectral bandwidth. This represents the shortest pulse duration achieved from black phosphorus in a fiber laser to date. Our results demonstrate the great potential for black phosphorus as an excellent candidate for long-term stable ultrashort pulse generation
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
Three dimensional topological insulators embody a newly discovered state of
matter characterized by conducting spin-momentum locked surface states that
span the bulk band gap as demonstrated via spin-resolved ARPES measurements .
This highly unusual surface environment provides a rich ground for the
discovery of novel physical phenomena. Here we present the first controlled
study of the topological insulator surfaces under strong Coulomb, magnetic and
disorder perturbations. We have used interaction of iron, with a large Coulomb
state and significant magnetic moment as a probe to \textit{systematically test
the robustness} of the topological surface states of the model topological
insulator BiSe. We observe that strong perturbation leads to the
creation of odd multiples of Dirac fermions and that magnetic interactions
break time reversal symmetry in the presence of band hybridization. We also
present a theoretical model to account for the altered surface of BiSe.
Taken collectively, these results are a critical guide in manipulating
topological surfaces for probing fundamental physics or developing device
applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with
arXiv:1009.621
Spin-Rotation Symmetry Breaking in the Superconducting State of CuxBi2Se3
Spontaneous symmetry breaking is an important concept for understanding
physics ranging from the elementary particles to states of matter. For example,
the superconducting state breaks global gauge symmetry, and unconventional
superconductors can break additional symmetries. In particular, spin rotational
symmetry is expected to be broken in spin-triplet superconductors. However,
experimental evidence for such symmetry breaking has not been conclusively
obtained so far in any candidate compounds. Here, by 77Se nuclear magnetic
resonance measurements, we show that spin rotation symmetry is spontaneously
broken in the hexagonal plane of the electron-doped topological insulator
Cu0.3Bi2Se3 below the superconducting transition temperature Tc=3.4 K. Our
results not only establish spin-triplet superconductivity in this compound, but
may also serve to lay a foundation for the research of topological
superconductivity
Fractional quantum Hall effect in the absence of Landau levels
It has been well-known that topological phenomena with fractional
excitations, i.e., the fractional quantum Hall effect (FQHE) \cite{Tsui1982}
will emerge when electrons move in Landau levels. In this letter, we report the
discovery of the FQHE in the absence of Landau levels in an interacting fermion
model. The non-interacting part of our Hamiltonian is the recently proposed
topologically nontrivial flat band model on the checkerboard lattice
\cite{sun}. In the presence of nearest-neighboring repulsion (), we find
that at 1/3 filling, the Fermi-liquid state is unstable towards FQHE. At 1/5
filling, however, a next-nearest-neighboring repulsion is needed for the
occurrence of the 1/5 FQHE when is not too strong. We demonstrate the
characteristic features of these novel states and determine the phase diagram
correspondingly.Comment: 6 pages and 4 figure
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