We construct nonlinear integrable couplings of discrete soliton hierarchy, then the infinite conservation laws for the nonlinear integrable couplings of the lattice hierarchy are established. For explicit application of the method proposed, the infinite conservation laws of nonlinear integrable couplings of the Toda lattice hierarchy are presented. The obtained integrable couplings of the Toda lattice equations and conservation laws can be used to describe the possible formation mechanisms for hydrodynamics, solid state physics and plasma physics, respectively.

Employing the general ordering theorem (GOT), operational methods and incomplete 2-D Hermite polynomials, we derive the t-ordered expansion of Fock space projectors. Using the result, the general ordered form of the coherent state projectors is obtained. This indeed gives a new integration formula regarding incomplete 2-D Hermite polynomials. In addition, the orthogonality relation of the incomplete 2-D Hermite polynomials is derived to resolve Dattoli's failure.

The energy spectra and the wave function depending on the c-factor are investigated for a more general Woods–Saxon potential (MGWSP) with an arbitrary l-state. The wave functions are expressed in terms of the Jacobi polynomials. Two potentials are obtained from this MGWSP as the special cases. These special potentials are Hulthen and the standard Woods–Saxon potentials. We also discuss the energy spectrum and wave function for the special cases.

By two direct assumption methods and symbolic computation, we present two families of one-soliton solutions and a family of two-soliton solutions with some arbitrary functions for the three-dimensional Gross–Pitaevskii equation with time-space modulation. Then we investigate the dynamics of these matter-wave solitons in three-dimensional Bose–Einstein condensates. We can see that the intensities of both one-solitons and two-solitons first increase rapidly to the condensation peak value, then decay very slowly to the background value. Thus these matter-wave solitons in three-dimensional Bose–Einstein condensates can remain for a sufficiently long time to be fully observed and modulated for real applications in today's experiments.

We propose a more efficient and reliable scheme for transferring quantum states between distant atoms than previous schemes based on resonant atom-cavity interaction. In addition, our system can also be used to implement a controlled phase gate with high fidelity.

An adaptive learning rule of synapses is proposed for a general asymmetric non-identical neural network. Its feasibility is proved by the Lasalle principle. Numerical simulation results show that synaptic connection weight can converge to an appropriate strength and the identical network comes to synchronization. Furthermore, by this approach of learning, a non-identical neural population can still reach synchronization. This means that the learning rule has robustness on mismatch parameters. The firing rhythm of the neural population is totally dependent on topological properties, which promotes our understanding of neuron population activities.

After being pumped from the 5s_{1/2} ground state to the 6p_{1/2} state, the population inversion between 6s_{1/2} and 5p_{1/2,3/2} can be established for a rubidium four-level active optical clock. We calculate the ac Stark shift due to lattice trapping laser which dominates the frequency shift of clock transition in a lattice trapped rubidium four-level active optical clock. Several magic wavelengths are found, which can form desired optical lattice trapping potential. By choosing a proper intensity and linewidth of the trapping laser, the fractional frequency uncertainty of clock transition due to the ac Stark shift of the trapping laser, is estimated to be below 10^{−18}.

Applying the effective Lagrangian method, we analyze the radiative contributions of the Kaluza–Klein (KK) modes to the muon magnetic dipole moments in the Appelquist–Cheng–Dobrescu model. Summing over the infinite series composed by the KK towers, we verify the final results satisfying the decoupling theorem in the limit R^{?1}→∞. For the compactification scale R^{?1}=300 GeV, we obtain the electroweak radiative corrections from the KK modes to the muon MDM amount to 6.72×10^{?12} at one loop level.

Using an isobaric method, the symmetry-energy coefficient (a_{sym}) of a neutron-rich nucleus is obtained from experimental binding energies. The shell effects are shown in a_{sym}^{*}/A≡4a_{sym}/A of nuclei. A (sub)magic neutron magic number N=40 is suggested in a very neutron-rich nucleus, and a_{sym}^{*}/A of a nucleus is found to decrease when its mass increases. The a_{sym}^{*}/A of a very neutron-rich nucleus with large mass saturates. The volume-symmetry coefficients (b_{v}) and surface-symmetry coefficients (b_{s}) of a neutron-rich nucleus are extracted from a_{ sym}^{*}/A by a correlation a_{sym}^{*}/A=b_{v}/A?b_{ s}/A^{4/3}. It is found that b_{v} and b_{s} decrease when the nucleus becomes more neutron-rich, and tend to saturate in the very neutron-rich nucleus. A linear correlation between b_{ v} and b_{s} is obtained in nuclei with different neutron-excess I, and b_{v} of I>7 nuclei is found to coincide with the results of infinite nuclear matter a_{ sym}=32 ±4 MeV, and b_{s}/b_{v} of the nucleus is found to coincide with the results of the finite-range liquid-drop model results.

ZHANG Qiao-Li, YUAN Da-Qing, ZHANG Huan-Qiao, FAN Ping, ZUO Yi, ZHENG Yong-Nan, K. Masuta, M. Fukuda, M. Mihara, T. Minamisono, A. Kitagawa, ZHU Sheng-Yun

Chin. Phys. Lett. 2012, 29 (9):
092102
.
DOI: 10.1088/0256-307X/29/9/092102

An ab initio calculation of the electric-field gradient (EFG) at the site of a phosphorous impurity substituting an Al atom in α-Al_{2}O_{3} is carried out using the WIEN2k code with the full-potential linearized augmented plane wave plus local orbital method (LAPW+lo) in the frame of density functional theory. The atomic lattice relaxations caused by the implanted impurities were calculated for two different charged states to well describe the electronic structure of the doped system. The EFG at the site of the phosphorous impurity in the charged supercell calculated with the exchange-correlation potential of the Wu-Cohen generalized gradient approximation (WC-GGA) is 0.573×10^{21} V/m^{2}. Then, the nuclear quadrupole moment of the I=3 state in ^{28}P is deduced to be 137 mb from the quadrupole interaction frequency of 190 kHz measured recently by the β-NQR method.

Excited states of the odd-odd nucleus ^{122}I are investigated via the fusion-evaporation reaction ^{116}Cd(^{11}B,5n) at a beam energy of 68 MeV. The two most strongly populated bands in ^{122}I are observed up to excitation energies around 10.5 MeV. Their possible configurations at lower spins are assigned to πh_{11/2}?νh_{11/2} and πh_{11/2}?νd_{5/2} based on the observed population, B(M1)/B(E2) values, alignments and signature splitting. E1 transitions are observed connecting the πh_{11/2}?νd_{5/2} band to the πh_{11/2}?νh_{11/2} band, which may be evidence of octupole collectivity. Band terminations caused by the full alignment of the valence nucleons outside the ^{114}Sn core are observed in both bands.

The influences of Δ quartet, especially Δ^{?} and Δ^{0} particles, on the neutrino scatting and cooling properties of neutron stars are discussed. The results show that Δ quartet can change the density range of direct Urca process of nucleons for muons, as well as the neutrino emissivity and luminosity. The role of Δ on the cooling rates is complicated. It is found that Δ quartet slows the neutron star cooling obviously in hyperonic neutron star matter. However in nuclear matter, Δ can not influence the cooling too much, unless the neutron star is quite massive. The Δ-meson coupling constants can also impact on the results, and the relatively smaller value, the stronger effects of Δ quartet.

The electron detachment of negative hydrogen ions exposed to a few-cycle linearly polarized laser pulse is investigated in the context of the strong field approximation evaluated by the saddle-point method. The results show that the angular distributions of the laser-induced photoelectron obtained by the saddle-point method are in good agreement with both the experimental results and the numerical-integration results. More importantly, we show that the unusual maximum in the angular distributions of photoelectrons for negative hydrogen ions can be explained by the quantum interference effect based on the saddle-point method.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

We study the application of the exp-function method to solve a relativistic Toda lattice system. To reduce the original equation into a convenient form, a suitable transformation is used so that the so-called exp-function method can be applied. By using the homogeneous balance principle and some further analysis, we succeed in calculating some exact analytic solutions.

The peaks of exciton and biexciton from the single quantum dot (QD) micro-photoluminescence spectra are identified by observing the intensity of those peaks in relation to increasing excitation power. In order to further verify the properties of the exciton and biexciton, we perform auto- and cross correlation measurements. Using the former, we confirm the antibunching property of the signal light emitted from the single QD. Using cross correlation measurement, we verify that the exciton and biexciton emissions originate from the same QD and they are strongly correlated with each other. Lastly, we analyze the behavior of the cross correlation function in both the cases of saturated and unsaturated excitation of the QD.

We report the high power acousto-optically Q-switched operation of a dual-end-pumped Ho:YAG laser at room temperature. For the Q-swithched mode, a maximum pulse energy of 2.4 mJ and a minimum pulse width of 23 ns at the repetition rate of 10 kHz are achieved, resulting in a peak power of 104.3 kW. The beam quality factor of M ^{2}～1.5, which is demonstrated by a knife-edge method. In addition, the Ho:YAG laser is employed as a pumping source of ZGP optical parametric oscillator, and its total average output power is 13.2 W at 3.9 μm and 4.4 μm with a slope efficiency of 68.4%.

A refractive index (RI) sensor based on a cladding-etched thin-core single-mode fiber (TCSMF) sandwiched between two single-mode fibers is demonstrated. The experimental results show that the sensitivity, within the RI range of 1.333–1.340, is enhanced at least 6 times by etching. It increases with the surrounding RI and reaches 857.5 nm/RIU at RI of 1.3684, and it can be expected to be higher with the decrease of the cladding diameter of the TCSMF.

A channel-selectable optical link based on a silicon microring resonator is proposed and demonstrated. This optical link consists of the wavelength-tunable microring modulators and the filters, defined on a silicon-on-insulator (SOI) platform. With a p–i–n junction embedded in the microring modulator, light at the resonant wavelength of the ring resonator is modulated. The 2^{nd}-order microring add-drop filter routes the modulated light. The channel selectivity is demonstrated by heating the microrings. With a thermal tuning efficiency of 5.9 mW/nm, the filter drop port response was successfully tuned with 0.8 nm channel spacing. We also show that modulation can be achieved in these channels. This device aims to offer flexibility and increase the bandwidth usage efficiency in optical interconnection.

We present a surface metal grating distributed feedback quantum cascade laser emitting at λ～8.3 μm and operating above 400 K in pulsed mode. A very high peak power of 463 mW at 290 K and still more than 18 mW at 400 K is achieved for a high-reflectivity-coated 13-μm -wide and 2.5-mm-long laser. Single-mode emission with a high side mode suppression ratio and wide temperature tuning range is observed. Such a high-power single-mode laser with a simple device process is important for practical applications.

A fiber-optic solution concentration sensor based on a pressure-induced long-period grating (LPG) in a composite optical waveguide is proposed. The composite waveguide consists of a standard single-mode fiber with its coating stripped away, a teflon-cannula and the medium to be measured. An experiment has been carried out to measure the concentration of the sodium chloride (NaCl) solution. The results show that the central resonant wavelengths of the LPG shift towards shorter wavelengths when the concentration of the NaCl solution increases. The central resonant wavelength of the LP_{14} cladding mode exhibits a total blue shift of 4.13 nm in the NaCl solution concentration range of 0–25%, which corresponds to a sensing sensitivity of 0.17 nm/%.

High-power cw green laser radiation is generated by intra-cavity frequency doubling of a diode-pumped Nd:GdVO_{4} laser with a MgO-doped periodically-poled LiNbO_{3} (MgO:PPLN) crystal at room temperature. An average power of 2.4 W at 0.53 μm is obtained under the pump 15 W at 808 nm, corresponding to an overall optical-to-optical conversion efficiency of 16%. The M^{2} factor of the green beam is 3.90 and 1.34 for the horizontal and vertical direction, respectively. In addition, the power fluctuation is measured to be about ±5%.

The phase tuning characteristics of a double-longitudinal-mode He-Ne laser with optical feedback are demonstrated. The phase behaviors of the two adjacent longitudinal modes are studied with different laser cavity lengths at the weak feedback level, and it is found that the phase difference of the two modes can be 0 to 180° along with the changing of the length of external cavity. The high-density intensity fringes are obtained with asymmetric feedback cavity at a strong feedback level. In particular, the phase difference is also observed between the two high-density fringes of adjacent modes, which have the potential to develop a direction sensitive laser feedback interferometer with high resolution. A simple theoretical analysis is presented, which fits well with the experimental results.

An ultrabroad and sharp transition bandpass flexible terahertz (THz) filter is designed using a multiple-layered metamaterial. This bandpass filter has excellent filtering capability, with a 3 dB bandwidth of about 0.47 THz and sharp band-edge transitions of 80 dB/THz and 96 dB/THz, respectively, and it can be realized by a coupling individual resonance mode. We find that the geometry parameters have an influence on the transmission profile, which are capable of giving us meaningful guidance in design of high profile bandpass THz filters. The numerical results show that the multiple-layered flexible metamaterial provides an effective way to achieve ultrabroad THz devices.

We present a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation (SHG) with a periodically poled KTP crystal (PPKTP). The cavity is a bow-tie ring configuration with a unitized structure. When the impedance matching is optimized, the coupling efficiency of the fundamental is as high as 95%. Taking into account both the high power output of the second harmonic and the stability of the system, we obtain over 500 mW green passing through the output cavity mirror, corresponding to a net conversion efficiency higher than 75.2%. Under these operating conditions, the power stability is better than ±0.25% during 5 h. It is the highest conversion efficiency and power stability ever produced in the bow-tie ring cavity with PPKTP for 532 nm generation.

The formula construction of the unified model with the squirt and the Biot mechanism, i.e. the BISQ model [Geophysics 58 (1993) 524], is discussed. The inconsistent formula construction caused by the self-contradictions of key assumptions is pointed out and demonstrated. Once the deformation of the solid skeleton is assumed to be in the uniaxial mode, the deformation of pore fluid is not able to be non-uniaxial at all, but must be uniaxial. Therefore, the expected results in the BISQ model's paper are doubtful and this model needs to be improved. A combination of the two mechanisms simultaneously in theory is significant and still needs more research.

Fluctuations of sub-grid scale (SGS) velocity and its influence on the motion of particles are important issues for large eddy simulation (LES) of gas-particle turbulence. We obtain the SGS statistics from the direct numerical simulation (DNS) data of homogeneous isotropic turbulent flow, which is filtered by the top filter in physical space. The result shows that the SGS kinetic energy seen by particles reach a minimum value when the Stokes number St closes to 1. Moreover, the turning point shifts towards a larger St as the filter width increases. Different from that observed in DNS, the Lagrangian integral time scale is larger than the Eulerian integral time scale in SGS. For small particles, the Lagrangian timescale of SGS fluid seen by particle is very close to that of the fluid itself. For particles which are large enough, it approaches the Eulerian timescale of SGS fluid. For the intermediate particles, the predicted curve of SGS fluid timescale seen by particle varies with St and its variation is non-monotonic.

The instability of a flexible filament immersed in uniform flow is studied. A numerical simulation based on the immersed boundary method is conducted on a two-dimensional uniform flow past a flapping filament. Different from the conventional bistability behavior, more regions of initial states of filament corresponding to different modes of motion are partitioned at each freestream velocity, and a new stable mode of motion with smaller flapping amplitude is observed. Mode selection highly depends on these initial states.

We present a new hybrid numerical scheme for two-dimensional (2D) ideal magnetohydrodynamic (MHD) equations. A simple conservation element and solution element (CESE) method is used to calculate the flow variables, and the unknown first-order spatial derivatives involved in the CESE method are computed with a finite volume scheme that uses the solution of the derivative Riemann problem with limited reconstruction to evaluate the numerical flux at cell interface position. To show the validation and capacity of its application to 2D MHD problems, we study several benchmark problems. Numerical results verify that the hybrid scheme not only performs well, but also can retain the solution quality even if the Courant number ranges from close to 1 to less than 0.01.

We report the detailed information of a purely elastic instability and its mixing enhancement effect in a viscoelastic fluid flow driven by a constant pressure gradient along a curvilinear channel obtained for the first time from the three-dimensional direct numerical simulations (DNSs) of this kind of flow geometry. Three-dimensional unstable flow structures similar to those visualized in experiments are realized through the DNSs. The inception of elastic instability, conformation of microstructures, statistical and dynamic information of velocity field and the mixing enhancement effect caused by elastic instability are then discussed.

In comparison with the phenomenon of negative index refraction observed in artificial meta-materials, it is interesting to ask if this type of behavior also exists or not in reaction-diffusion systems that support nonlinear chemical waves. Previous studies indicate that the negative index refraction could occur on a interface between a medium of a normal wave and a medium that supports anti-waves. Here we investigate the phenomenon in the complex Ginzburg–Landau equation (CGLE) in a close relationship with the quantitative model for the chlorite-iodide-malonic acid (CIMA) reaction. The amplitude equation CGLE is deduced from the CIMA reaction, and simulations with mapped parameters from the reaction-diffusion equation reveal that the competition between normal waves and anti-waves on the interface determines whether the negative index refraction occurs or not.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Radiation flow through gold-doped hydrocarbon foam is investigated and a model is presented to calculate effective opacity for an inhomogeneous, pressure-equilibrated gold/foam mixture based on the Levermore–Pomraning method for binary stochastic media. The effective opacity dependance on the size of the gold particles and the foam temperature are studied. The results suggest that when the mixture temperature is lower than 250 eV, the opacity difference between the 5 μm particle mix case and the atomic mix case is large enough to induce a significant discrepancy in radiation transport, which is confirmed by the hydrodynamic simulation.

The effect of viscosity on the coupling coefficient and specific impulse is investigated with water and glycerol as propellants in laser plasma propulsion. It is found that the propulsion is much more correlated with the liquid viscosity. For water in particular, the coupling coefficient and specific impulse presents nearly a linear relationship with the viscosity. The weak dependence of the coupling coefficient on the laser energy at a high viscosity is observed. These results indicate that a liquid propellant with suitable viscosity can be used in laser plasma thruster.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

Dislocation behaviors are analyzed in AlGaN/GaN multiple-quantum-well films grown with different strain-modified interlayers. In the case of multiple-quantum-well layers grown on a GaN buffer layer without the interlayer, many threading dislocations interact and annihilate within about 100 nm below the multiple quantum well layer. For multiple-quantum-well layers grown with the AlGaN interlayer, misfit dislocations between the GaN buffer layer and the AlGaN interlayer enter multiple-quantum-well layers and result in an increase of threading dislocation density. Besides misfit dislocations, the edge-type dislocation is another dislocation origin attributed to the dissociation of Shockley partials bounding the stacking fault in AlN/GaN superlattices below the interlayer interface.

The total energy of bcc Fe containing Cu clusters with different sizes and number densities are calculated using the molecular dynamics (MD) method. The results indicate that the Cu atoms prefer to form Cu clusters instead of being uniformly distributed in the bcc Fe matrix. The binding energy of substitutional Cu to Cu clusters is also found to increase with the number of Cu atoms. For a large-sized Cu precipitate, the change of the local stress state is found to relate to the phase transition from bcc to fcc Cu based on MD and common neighbor analysis.

Element substitution is usually used to improve the glass forming ability (GFA) and magnetic properties of alloys. We obtain Gd_{55}Al_{20}Ni_{20}Co_{5} bulk metallic glass (BMG) by minor Co substitution for Ni in the Gd_{55}Al_{20}Ni_{25} glass-forming alloy. The Gd_{55}Al_{20}Ni_{20}Co_{5} BMG exhibits a better GFA and magnetocaloric effect (MCE) than the Gd_{55}Al_{20}Ni_{25} BMG. The mechanism for the enhanced magnetic entropy change and refrigeration capacity of the Gd_{55}Al_{20}Ni_{20}Co_{5} BMG is investigated.

We study the materials that are composed of metals and insulators. These plasmonic and non-plasmonic materials are ordered in geometric arrangements with dimensions that are fractions of the wavelength of light. A theoretical model is developed to simulate the refractive index of these nano-structured electromagnetic materials which support surface plasmon resonances. Factors contributing to the refractive index sensitivity are explored phenomenologically. Particles with size parameters smaller by much less than 1 have optical properties accurately predicted by the quasi-electrostatic theory while particles with larger size parameters necessitate electrodynamics. We simulate a type of refractive index material composed of metal and insulating dielectric nano-spheres, which are able to sustain the propagation of infrared or visible frequency electromagnetic waves known as surface plasmon polaritons.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Optical and electrical properties of single-crystal Si supersaturated with Se by ion implantation and followed by different thermal annealing conditions are reported. Si is implanted with 1×10^{16} cm^{?2} Se ions at 100 keV. The total substitutional fraction of Se atoms in Si is 45% under the annealing at 800°C for 30 min and the peak concentration of substitutional Se atoms is exceeded 1×10^{20} cm^{?3}. A temperature-independent carrier concentration of 3×10^{19} cm^{?3} is measured and the near-infrared absorption is closed to 30%. These results indicate the insulator-to-metal transition of the doped layer and the formation of impurity bands in the Si band gap.

A first-principles technique capable of describing the nearly excited states of semiconductors and insulators, namely the modified Becke–Johnson (mBJ) potential approximation, is used to investigate the electronic band structure and optical properties of spinel oxides: GeZn_{2}O_{4}. The predicted band gaps using the mBJ approximation are significantly more accurate than the proposed previous theoretical work using the common LDA and GGA. Band gap dependent optical parameters, like the dielectric constant, index of refraction, reflectivity and optical conductivity are calculated and analyzed. The results from the dielectric constant shows that the numerical value of the static dielectric, after dropping constantly, becomes less than zero and the material exhibits metallic behavior. The refractive index also drops below unity for photons higher than 18 eV, which indicates that the velocities of incident photons are greater than the velocity of light. However, these phenomena can be explained by the fact that a signal must be transmitted as a wave packet rather than a monochromatic wave. This comprehensive theoretical study of the optoelectronic properties predicts that these materials can effectively be used in optical devices.

In order to investigate the carrier redistribution mechanisms in InAs/GaAs self-assembled quantum dots, the photoluminescent energy peak shift is studied with increasing excitation power. Unusually for samples of relatively low density, it is shown that the energy peak position could recover slowly after a fast redshift, associated with the increasing excitation power. A theoretical model is presented, which involves the Auger effect assisting carrier recapture as important mechanisms during the relaxation process.

An improved analytical model of the drift field suppressed by the Dember field due to ambipolar diffusion in the p-type quasineutral region (p-QNR) of a forward silicon p–n junction at low injection levels is presented with a good fit to the numerical simulation results. Considering ambipolar transport of both carriers, the diode current in the p-QNR is found to consist of a minority-electron diffusion-current component and majority-hole drift- and diffusion-current components, and the Dember field plays a dominant role in balancing all the components mentioned above to keep the current constant. The analytical model is beneficial to completely understand ambipolar current transport mechanisms in semiconductor p–n junction devices.

Based on the principles of the Hall effect for ultrasound excitation and wave propagation in a static magnetic field, the theory of ultrasound induced Hall voltage generation is derived in explicit formulae with consideration of the acoustic radiation for a planar transducer. It is proved by numerical simulations that the induced Hall voltage is mainly generated at the conductivity boundary and can be used to map the spatial variation of the conductivity value along the acoustic transmission path. Both the simulated Hall voltage and the reconstructed image show good agreement with the experimental results of Wen [Ultrasonic Imaging 20 (1998) 206, 21 (1999) 186]. The promising simulation results suggest the potential of implementing medical electrical impedance imaging by means of ultrasound induced Hall effect imaging.

The electron spin dynamics is investigated by the time-resolved Kerr rotation technique in a pair of special GaAs/AlGaAs asymmetric quantum well samples grown on (111)-oriented substrates, whose structures are the same except for their opposite directions of potential asymmetry. A large difference of spin lifetimes between the two samples is observed at low temperature. This difference is interpreted in terms of a cancellation effect between the Dresselhaus spin-splitting term in the conduction band and another term induced by interface inversion asymmetry. The deviation decreases with the increasing temperature, and almost disappears when T>100 K because the cubic Dresselhaus term becomes more important.

Based on the differential Ohm's law and Poisson's equation, an analytical model of the drain current for a-Si:H thin-film transistors is developed. This model is proposed to elaborate the temperature effect on the drain current, which indicates that the drain current is linear with temperature in the range of 290–360 K, and the results fit well with the experimental data.

AlGaN-based back-illuminated solar-blind ultraviolet (UV) p–i–n photodetectors (PDs) with high quantum efficiency are fabricated on sapphire substrates. To improve the overall performance of the PD, a series of structural design considerations and growth procedures are implemented in the epitaxy process. A distinct wavelength-selective photo-response peak of the PD is obtained in the solar-blind region. When operating in photovoltaic mode, the PD exhibits a solar-blind/UV rejection ratio of up to 4 orders of magnitude and a peak responsivity of ～113.5 mA/W at 270 nm, which corresponds to an external quantum efficiency of ～52%. Under a reverse bias of ?5 V, the PD shows a low dark current of ～1.8 pA and an enhanced peak quantum efficiency of ～64%. The thermal noise limited detectivity is estimated to be ～3.3×10^{13} cm?Hz^{1/2}W^{?1}.

Using a double templating method by electroless deposition within a templating organic porous mold, we fabricate a monolayer of hexagonal-close-packed metallic nanoshells, each with a small opening. Light transmission spectra of the metallic nanoshell arrays are measured, which show transmission resonances at specific wavelengths whose positions are observed to be independent of the incident angle as well as light polarizations. More interestingly, the resonance wavelengths of Mie plasmon modes are also independent of the surrounding medium. Further numerical simulations confirm these transmission resonances and reveal that they are attributed to the excitations of highly localized dipolar, quadrupolar and hexapolar Mie plasmon modes, which are highly confined within metallic nanoshells.

WEN Xiao-Xia, YANG Xiao-Dong, HE Miao, LI Yang, WANG Geng, LU Ping-Yuan, QIAN Wei-Ning, LI Yun, ZHANG Wei-Wei, WU Wen-Bo, CHEN Fang-Sheng, DING Li-Zhen

Chin. Phys. Lett. 2012, 29 (9):
097304
.
DOI: 10.1088/0256-307X/29/9/097304

GaN-based light-emitting devices (LEDs) with different electron blocking layers are theoretically studied and compared by using the advanced physical models of a semiconductor device simulation program. It is found that the structure with an AlInN electron blocking layer shows improved light output power, lower current leakage and efficiency droop. Based on numerical simulation and analysis, these improvements of the electrical and optical characteristics are mainly accounted for by efficient electron blocking. It can be concluded that Auger recombination is responsible for the dominant origin of the efficiency droop of a GaN-based LED as current increases.

Polycrystalline BiFe_{1-x}Mn_{x}O_{3} films with x up to 0.50 are prepared on LaNiO_{3} buffered surface oxidized Si substrates. The doped Mn is confirmed to be partially in a +4 valence state. A clear exchange bias effect is observed with a 3.6 nm Ni_{81}Fe_{19} layer deposited on the top BiFe_{1-x}Mn_{x}O_{3} layer, which decreases drastically with increasing Mn doping concentration and finally to zero when x is above 0.20. These results clearly demonstrate that the exchange bias field comes from the net spins due to the canted antiferromagnetic spin structure in polycrystalline BiFe_{1-x}Mn_{x}O_{3} films, which transforms to a collinear antiferromagnetic spin structure when the Mn doping concentration is larger than 0.20.

The effect of high energy electron irradiation on the electrical and optical properties of n-GaN is studied. Hall and photoluminescence measurements are carried out on the samples irradiated with different doses. The results obtained from Hall measurements show that electron concentration and mobility are proportional to electron irradiation doses. The PL results show that the near-band-edge intensity and yellow luminescence intensity decrease continually with the increasing electron irradiation. However, it is found that the ratio of the yellow luminescence intensity to the near-band-edge intensity increases with the increasing of electron irradiation dose. To interpret this phenomenon, we propose two theoretical models based on the charge transport mechanism and rate equations, respectively, and they are in good agreement with the experimental observations.

The role of F_{4}TCNQ as an exciton quenching material in thin organic light-emitting films is investigated by means of in situ photoluminescence measurements. C_{60} was used as another quenching material in the experiment for comparison, with Alq_{3} as a common organic light-emitting material. The effect of the growth sequence of the materials on quenching was also examined. It is found that the radius of F?rster energy transfer between F_{4}TCNQ and Alq_{3} is close to 0 nm and Dexter energy transfer dominates in the quenching process.

Magnetic-fluorescent bifunctional Fe_{3}O_{4}/SiO_{2}-CdTeS nanocomposites are synthesized. Fe_{3}O_{4} superparamagnetic nanoparticles are firstly prepared through the thermal decomposition of Fe oleate precursors and coated with a mesoporous silica shell using the St?ber method, and the silica surface is then modified with positively charged amino groups by adding 3-aminopropyltrimethoxysilane. Finally, negatively charged CdTeS quantum dots are linked and assembled onto the positively charged surface of Fe_{3}O_{4}/SiO_{2} through electrostatic interactions. X-ray diffraction, transmission electron microscopy, photoluminescence spectroscopy, and magnetometry are applied to characterize the nanocomposites. The results show that the bifunctional nanocomposites combine the optical properties of near-infrared CdTeS quantum dots with the superparamagnetic properties of Fe_{3}O_{4} perfectly, expressing the potential application as a biocompatible magnetofuorescent nanoprobe for in vivo labelling.

A separated absorption and multiplication GaN p–i–p–i–n avalanche photo-diode (APD) with a 25 μm diameter mesa is proposed and demonstrated. Compared to the conventional p–i–n APD, the p–i–p–i–n structure reduces the probability of premature micro-plasma breakdown, raises the gain from 30 to 400 and reduces the work voltage from 93 to 48 V. The temperature test is set on p–i–p–i–n APDs, and the positive coefficient of 30 mV/K shows that avalanche breakdown happens in the devices. The peak responsivity of p–i–p–i–n APDs is 0.11 A/W under a wavelength of 358 nm.

We report on a white hybrid light-emitting device employing a mixture of ternary CdSe/ZnS quantum dots (QDs) as an emitting layer (EML) and a small molecular material tris(8-hydroxyquinoline) aluminum (Alq_{3}) as an electron-transporting layer. The film morphology of the spin-coated mixture of QDs is strongly improved via thermal annealing, and therefore a close-packed QD-EML is realized between organic charge-transporting layers. As a result, compared to the device with an unannealed QD-EML, the emission of Alq_{3} is deeply suppressed. In addition, a maximum luminance of more than 1000 cd/m^{2} and a maximum luminous efficiency of 2.2 cd/A are achieved.

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Although grating fabrication technologies on solid materials are well developed, grating fabrication on free standing films is rather more difficult. We propose a new film grating fabrication method based on UV-nanoimprint lithography. This method combines the grating fabrication technique using nanoimprint lithography with thin film preparation technology. It involves the fabrication of a PMMA grating by UV-nanoimprint lithography, followed by the preparation of a thin metal film on the PMMA grating and the patterning of the tensile film specimen through photolithography. After dissolving the PMMA layer, the tensile film specimen becomes a free standing structure. To identify the quality of the thin film specimen as well as the grating, the specimen is loaded with uniaxial tensile stress. The Moiré method is adopted to measure the full-field deformation and the mechanical parameters of the film specimen. The successful results verify the potential of this method in grating fabrication on other film-like specimens.

Using lateral phase change random access memory (PCRAM) for demonstration, we report a self-aligned process to fabricate a metal electrode-quantum dot(QD)/nanowire(NW)-metal electrode structure. Due to the good confinement and coupling between the Ge_{2}Sb_{2}Te_{5} (GST) QD and the tungsten electrodes, the device shows a threshold current and voltage as small as 2.50 μA and 1.08 V, respectively. Our process is highlighted with good controllability and repeatability with 100% yield, making it a promising fabrication process for nanoelectronics.

We propose a solvable model of topological characteristics for a microwave signal transmitting system, which is named as a repairable linear m-consecutive-k-out-of-n:F system with l repairers. It is assumed that both the working time and repair time of each component are exponentially distributed, and the repair is perfect. The transition rate matrix is determined by defining the generalized transition probability. Furthermore, some important reliability indices are evaluated. Finally, the proposed approach is illustrated by a numerical example. The results are helpful to guide the stability of network nodes and the reliability of microwave signal transmitting systems.

Al nanoparticles (NPs) are incorporated in the active layers to enhance the performance of organic solar cells (OSCs). The improved short circuit current J_{sc} and power conversion efficiency (PCE) for OSCs with Al NPs are observed. A final PCE of 3.66wt% is achieved, which is an improvement of more than 30wt% compared to a standard cell with a PCE of 2.84wt%. When the mass of Al NPs is 10wt% of OSCs, the device performance is best. The optical performance of OSCs reveals that the absorption from sunlight increases. The external quantum efficiency spectra suggests that the Al NPs in the active layers influence the efficiency of converting photons into electrons, which leads to the improvements in the photocurrent. The enhanced photovoltaic performance induced by incorporating Al NPs in the active layer is discussed in the terms of increasing charge separation at the donor-acceptor interface and the effectively decreasing transmission distance of charge in polymer.

Thin Ru_{21}Zr_{64}Si_{15} films deposited on Si substrates by radio frequency reactive magnetron sputtering are studied and evaluated as diffusion barriers for Cu metallization. Cu/Ru_{21}Zr_{64}Si_{15}/Si and Cu/Zr_{67}Si_{33}/Si structure samples are prepared under the same procedures for comparison. The thermal stability, phase formation, surface morphology and atomic depth profile of the Cu/Ru_{21}Zr_{64}Si_{15}/Si and Cu/Zr_{67}Si_{33}/Si structures before and after annealing at different temperatures are investigated. In conjunction with these analyses, the Cu/Ru_{21}Zr_{64}Si_{15}/Si contact system shows high thermal stability at least up to 650°C. The results obtained reveal that the incorporation of Ru atoms into the Zr_{67}Si_{33} barrier layer is shown to be beneficial for improving the thermal stability of the Cu/barrier/Si contact system.

Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By preparing the self-assemble nanoporous Ni template through rapid annealing of a thin Ni film, followed by a low damage dry etching process, a p-side-up LED with a roughened surface has been fabricated. Compared to a conventional LED with plane surface, the light output of LEDs with nanoporous p-GaN surface increases up to 71% and 36% at applied currents of 1 mA and 20 mA, respectively. Meanwhile, the electrical characteristics are not degraded obviously after surface roughening.

We fabricate N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide and pentacene heterostructure organic field effect transistors with a MoO_{3} ultrathin layer between Al source-drain electrode and active layer. By inserting the MoO_{3} layer, the injection barrier of hole carriers is lowered and the contact resistance is reduced. Thus, the performance of the device is improved. The device shows typical ambipolar transport characteristics with effective hole mobility of 4.838×10^{?3} cm^{2}/V?s and effective electron mobility of 1.909×10^{?3} cm^{2}/V?s, respectively. This result indicates that using a MoO_{3} ultrathin 1ayer is an effective way to improve the performance of ambipolar organic field effect transistors.

Inverted polymer solar cells with molybdenum oxide (MoO_{3}) as an anode buffer layer and different metals (Al or Ag) as anodes are studied. It is found that the inverted cell with a top Ag anode demonstrates enhanced charge collection and higher power conversion efficiency (PCE) compared to the cell with a top Al anode. An 18% increment of PCE is obtained by replacing Al with Ag as the top anode. Further studies show that an interfacial dipole pointing from MoO_{3} to Al is formed at MoO_{3}/Al interfaces due to electron transfer from Al to MoO_{3} while this phenomenon cannot be observed at MoO_{3}/Ag interfaces. It is speculated that the electric field at the MoO_{3}/Al interface would hinder hole extraction, and hence reduce the short-circuit current.

We propose a new definition of modularity, i.e. the Q^{d} function, for network analysis, which takes the edge density and topological structure of modules into account and is different from the original strategy of simply calculating the number of edges (the definition of modularity Q introduced by Newman and Girvan). Armed with this novel quality function Q^{d}, we implement an adaptive clustering algorithm for process optimization, and apply our strategy to several synthetic and real-world networks. The results of our exercises demonstrate a better performance in extracting accurate community ingredients from complex networks.

A cellular automaton model for pedestrian flow based on floor field is extended to take the effects of those walkers in the same and opposite directions into account simultaneously when they are in the view field of a walker, i.e., walkers tend to follow the leaders in the same direction and avoid conflicts with those opposite. Pedestrian counter flows on a crosswalk are investigated by the improved model under open boundary conditions. Numerical simulations show that lane formation is well reproduced and this process is pretty quick, which coincides with real pedestrian traffic. It is found that the effect of walkers in the same and opposite directions in the view field enhance walkers' intention to move to their preferential direction and separate pedestrian counter flow into lanes of uniform walking direction, especially when the number of walkers is large enough.

Based on empirical data, we develop a macro model with lane width and the number of lanes. For uniform flow, the model is formulated as a couple of algebra equations and solved by analytical deduction. The results show that the proposed model can capture the impacts of lane width, the number of lanes and the variation of the number of lanes on uniform flow.

The physical process in the Earth's polar region is very complex and still needs to be further studied. Using the data from Cluster satellite measurement, an analysis on field-aligned electrons in the mid-latitude cusp on 30 September 2001 has been performed. The satellite observed a down-flowing electron event in the low-latitude boundary and a sequential up-flowing electron event in the high-latitude boundary of the cusp region. The down-flowing electron had a velocity as high as 500 km/s and a flux of 2.0×10^{9} cm^{?2}?s^{?1}. The up-flowing electron had a velocity up to 1200 km/s and a flux of 4.9×10^{9} cm^{?2}?s^{?1}. Both the velocity and the flux observed in this event are the maximum values of the up-flowing electrons observed by all satellites to date. The electron is the main contributor for the field-aligned current in this event. The physical mechanism is also discussed. The down-flowing electron in the low-latitude boundary of the cusp region may result from solar wind injecting during the southward IMF, and the up-flowing electrons in the high-latitude boundary of cusp may result from mirroring of the solar wind, or from the ionospheric up-flowing electrons which have been accelerated.