We present transport characterizations of a fully tunable double quantum dot over the few electron number down to zero, which is defined by means of surface gates on top of a GaAs/AlGaAs heterostructure. We also perform numerical simulations to map out the charge stability diagram, which is consistent with the measurements. The results demonstrate that the electric control in the double dot can be significantly enhanced by improving the design of the device structure, which provides potential advantages for quantum information processing.

In a normal form, prisoners' dilemma (PD) is represented by a payoff matrix showing players' strategies and payoffs. To obtain the distinguishing trait and strategic form of PD, certain constraints are imposed on the elements of its payoff matrix. We quantize PD by a generalized quantization scheme to analyze its strategic behavior in the quantum domain. The game starts with a general entangled state of the form |ψ>=cos(ξ/2)|00>+sin(ξ/2)|11> and the measurement for payoffs is performed in entangled and product bases. We show that for both measurements, there exist respective cutoff values of entanglement of the initial quantum state up to which the strategic form of the game remains intact. Beyond these cutoffs the quantized PD behaves like the chicken game (CG) up to another cutoff value. For the measurement in the entangled basis the dilemma is resolved for sinξ >1/7 with Q⊗Q as a Nash Equilibrium (NE). However, the quantized game behaves like PD when sinξ >1/3; whereas in the range 1/7<sinξ <1/3 it behaves like CG with Q⊗Q as an NE. For the measurement in the product basis the quantized PD behaves like classical PD for sin^{2}(ξ/2) <1/3 with D⊗D as an NE. In region 1/3<sin^{2}(ξ/2)<3/7, the quantized PD behaves like classical CG with C⊗D and D⊗C as NEs.

We investigate the dynamics of quantum discord for a two-qubit system coupled to a spin chain with three-site interaction in the weak-coupling region. For the case of the initial two-qubit pure state, the decay of quantum discord can be delayed in a special interval of the three-site coupling strength. In the absence of the transverse field, with the increase of three-site interaction, quantum discord tends to be stabler. For the case of the initial mixed state, the increase of quantum discord is delayed in the special interval of three-site interaction. In addition, a sudden transition between classical and quantum decoherence is also observed, with the transition time determined by the three-site interaction and degree of anisotropy.

We propose a novel scheme for generating N-qubit GHZ entangled state with a hybrid quantum system, which consists of N nitrogen-vacancy centers, N transmission line resonators, a current-biased Josephson junction superconducting qubit, and three kinds of interaction Hamiltonians. The proposal requires no adjustment of the qubit level spacings during the entire operation. Moreover, it is shown that the operation time is independent of the number of qubits. The present proposal is quite useful, and is a promising step to realize the large-sized quantum networks for quantum information processing and quantum computation.

Understanding the dynamics of gas-liquid two-phase flows is a challenge in the fields of nonlinear dynamics. We first construct and analyze a recurrence network from Chen's chaotic system and find that the network local statistic is feasible to characterize chaotic dynamics associated with unstable periodic orbits. Then we construct recurrence networks from gas-liquid two-phase flow experimental signals and associate the network topological statistic with the flow pattern dynamics. The results indicate that the recurrence network could be a powerful tool for the dynamic characterization of experimental gas-liquid two-phase flows.

We investigate frequency shift of a quartz crystal microbalance (QCM) sensor introduced by mass effect, and properties of material of its coated viscoelastic film are discussed. The validity of the Sauerbrey relation cannot be held if the viscoelastic properties of the contacting medium are considered. When the QCM sensor with a viscoelastic film works in the gas phase, the viscoelastic properties will introduce an extra mass effect. While in the liquid phase, the missing mass effect can be observed. The experimental results demonstrate that the QCM sensor is sensitive to the viscoelastic properties of the coating film. Properties of the viscoelastic contacting medium should be considered.

We calculate the quark number density and quark number susceptibility (QNS) of QCD at finite chemical potential μ and finite temperature T in the framework of a new nonperturbative QCD model. Analysis and discussions of the calculated results of the QNS are given. It is found that the quark number density has a singularity when μ comes close to a critical value μ_{0}, and the QNS χ(μ,T) becomes discontinuous at some values of T. At high temperature the QNS approaches the free quark gas result, while at very low temperature the QNS equals zero. Importantly, the QNS shows a sudden increase near some temperature (T～120 MeV), which may be regarded as the signal of a crossover.

Utilizing PYTHIA we study non-pertubative QCD effects to jet transverse momentum shift in hadronic collisions at the RHIC and the LHC. The dependences of non-perturbative effects such as hadronization corrections and underlying event effect on jet radius R, colliding energy, color factor, transverse momentum are investigated by numerical simulations. Hadronization corrections give a negative contribution to the jet energy shift and its magnitude decreases with jet E_{T} and colliding energy √s as well as jet radius R. However, the underlying event effect gives a positive contribution to the jet energy shift and its contribution increases with jet radius and √s. Hadronization and underlying event effect offset each other and could be canceled completely at a specific jet radius dubbed R_{NP}_{=0}. It is observed that R_{NP}_{=0} decreases with colliding energy √s and is larger for a gluon jet than a quark jet at any fixed E_{T}.

The spin-flip response of finite nuclei is studied in a fully self-consistent Hartree–Fock plus random phase approximation (RPA) within the SLy5 Skyrme parameter set. The effect of J^{2} terms and spin-orbit interaction on the response function are discussed. The J^{2} terms are included/excluded at both the mean field and the excited state for self-consistency. The numerical results show that the J^{2} terms give a strong effect on the properties of the magnetic dipole response and the Gamow–Teller response. The spin-orbit residual interaction has almost no contribution to the spin-flip response according to our present study.

The Faddeev equations are solved to investigate the bound states of three-body systems with short-range two-body interactions. Different bound 0^{+} excited states are obtained depending on the mass ratios of particles, when there is no bound state for two-body sub-systems. On the one hand, a number of bound 0^{+} excited states appear in the system with two heavy particles and a light particle. On the other hand, a weakly bound 0^{+} excited state is obtained in the system with two light particles and a heavy particle, which has not been reported in previous works as far as we know.

The isoscalar giant dipole resonance (ISGDR) in nuclei is studied in the framework of a fully self-consistent relativistic continuum random phase approximation (RCRPA). In this method the contribution of the continuum spectrum to nuclear excitations is treated exactly by the single particle Green's function technique. We employ different type interactions (NL1, NL3, NL3^{*}, NL4, TM1, NLSH and PK1) corresponding to incompressibilities in the range 200–360 MeV. The results are discussed in comparison with the existing experimental data. It is found that the term η= 5/3<r^{2}> can remove spurious components from the admixture of the center of mass state perfectly. The ISGDR distribution has two components, the lower-energy component and the higher-energy component. There is a large amount of very sharp peaks in the lower-energy region in RCRPA calculations. Only the higher-energy component is sensitive to the value of nuclear incompressibility employed in the calculations; the position of the lower-energy component is completely independent of the nuclear incompressibility K_{nm}.

The "finite-size" effects in the isobaric yield ratio (IYR), which are shown in the standard grand-canonical and canonical statistical ensembles (SGC/CSE) method, are claimed to prevent the actual values of physical parameters from being obtained. The conclusion of SGC/CSE may be questionable for neutron-rich nucleus-induced reactions. To investigate whether the IYR has finite-size effects, we re-examine the IYR for the mirror nuclei [IYR(m)] using a modified statistical abrasion-ablation (SAA) model. It is found that when the projectile is not so neutron-rich, the IYR(m) depends on the isospin of the projectile, but size dependence can not be excluded. In reactions induced by the very neutron-rich projectiles, contrary results to those of the SGC/CSE models are obtained, i.e., the dependence of the IYR(m) on the size and the isospin of the projectile is weakened and disappears in both the SAA and experimental results.

XING Qing-Zi, DU Lei, ZHENG Shu-Xin, GUAN Xia-Ling, LI Jian, CAI Jin-Chi, GONG Cun-Kui, WANG Xue-Wu, TANG Chuan-Xiang, James Billen, James Stovall, Lloyd Young

Chin. Phys. Lett. 2013, 30 (5):
052901
.
DOI: 10.1088/0256-307X/30/5/052901

A four-vane radio-frequency quadrupole (RFQ) accelerator is under construction for the Compact Pulsed Hadron Source (CPHS) project at Tsinghua University. The 3 m-long RFQ will accelerate a 50 keV proton beam from the ECR source to 3 MeV, and deliver it to the downstream drift tube linac (DTL) with a peak current of 50 mA, pulse length of 0.5 ms and beam duty factor of 2.5%. The inter-vane voltage is designed to increase with the longitudinal position to produce a short RFQ. Coupling plates are therefore not necessary. The cavity cross section and vane-tip geometry are tailored as a function of the longitudinal position, while limiting the peak surface electric field to 1.8 Kilpatrick. The RFQ is designed, manufactured, and installed at Tsinghua University. We also present the tuning and cold test results of the RFQ accelerator. After final tuning, the relative error of the quadrupole field is within 2%, and the admixture of the two dipole modes are less than 2% of the quadrupole mode.

The electronic structures and optical properties of Tb_{1?x}Yb_{x}MnO_{3} as a function of Yb element content x are investigated by employing the first-principles method. The calculation results indicate that the electronic states associated with the Yb element are dominated gradually and the inner electronic states move to the lower energy level with the increasing x. The dielectric function and other optical properties of Tb_{1?x}Yb_{x}MnO_{3} such as the reflectance spectra are obtained and the results indicate that the static dielectric function reaches a maximum at about x=0.75, while the fitting expression predicts a maximum when x=0.69. In addition, it is found that one peak appearing at about 3.60 eV in the reflectance spectra of TbMnO_{3} is driven to shift linearly towards higher energy level with the increasing x. However, another peak at about 29.63 eV moves nonlinearly in the same situation.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

Single-photon scattering in a pair of coupled-resonator arrays of waveguides linked by a nanocavity embedded with a two-level system is investigated theoretically. By using the discrete coordinates approach, we deduce the analytical expressions for the transmission and reflection amplitudes. The calculations reveal that the transport properties of the single photon can be controlled by adjusting the coupled strength between the nanocavity and the coupled-resonator array. Quantum switching and a 1/4 beam splitter for the band-edge photon based on this structure are also discussed.

Generalized ptychography is established with diverse probes, which not only extend the core spirit of ptychography that is the overlapping of neighboring probes, but also enhance the imaging quality. We find that in one of suggested configurations with visible light, the performances of two types of diversified probes are improved in comparison with the traditional circular probes. This may open up some new possibilities of ptychographic imaging.

We propose and numerically simulate a time-spectrum coding method to measure the fine structure of long pulses. By a difference-frequency (DF) interaction between the pending-measured long pulse and a linearly chirped pulse, the spectrum of the generated DF pulse contains the intensity information of the pending-measured pulse and the frequency information of the linearly chirped pulse. The numerical simulation results show that the fine structure of the pending-measured pulse can be derived from the spectra of the DF pulse and the linearly chirped pulse.

Multiple phase-shifted (MPS) diffraction grating is an effective way proposed to overcome the spatial hole burning (SHB) effect in a distributed feedback (DFB) laser. We present two symmetric λ/8 phase-shifted DFB lasers by using nanoimprint lithography (NIL). The threshold current of a typical laser is less than 15 mA. The side mode suppression ratio (SMSR) is still above 42 dB even at 100 mA current injection. To show the versatility of NIL, eight different wavelength MPS-DFB lasers on this single chip are also demonstrated. Our results prove that NIL is a promising tool for fabricating high performance complex grating DFB lasers.

The negative refraction and imaging effects in photonic crystals can be used to solve diffraction limit problem in near-field optics. Improving transmission efficiency and image resolution is a critical work for negative refraction imaging. We theoretically investigate the band structures, equi-frequency surfaces, electromagnetic wave propagation, and the image intensity distributions in a two-dimensional hexagonal photonic crystal consisting of hollow components. It is found that, in contrast to a hexagonal photonic crystal consisting of solid dielectric cylinders of the same radius, photonic crystals with hollow components can be used to optimize the all-angle negative refraction. Numerical simulations show that the transmission efficiency and resolution of image can be enhanced by changing the radii of the hollow air rods.

A new type of germanate-tellurite glass with doped Tm^{3+} ions is synthesized and its 1.8 μm emission properties have been studied for its application as a laser material. The Judd–Ofelt intensity parameters, emission cross section, absorption cross section, and gain coefficient of Tm^{3+} ions are calculated and analyzed. Germanate-tellurite glass with 1.0 mol% Tm_{2}O_{3} possesses the highest spontaneous transition probability (423.4 s^{?1}) and the largest calculated emission cross section (6.91×10^{?21} cm^{2}) at 1.8 μm corresponding to the ^{3}F_{4}→^{3}H_{6} transition. The good 1.8 μm emission performance suggests that this glass may become an attractive host for developing solid state lasers operating in the mid-infrared.

We present an all-fiber 1.94 μm nanosecond pulse laser in master oscillation-power amplifier configuration. A dual-FBGs gain-switched Tm^{3+}-doped fiber laser is built and is used as the seed laser. The output characteristics of the amplifier are studied at different pulse repetition rates, and the maximum output energy of 500 μJ is achieved under incident pump power of 39 W. The emitting spectra are checked at 5 and 10 kHz, and no obvious nonlinear effects are observed in the spectrum of the amplified laser. The beam quality factor is M^{2}=2.1±0.03 measured by the traveling knife-edge method, and the laser beam spot is also monitored by an infrared vidicon camera.

An algorithm is proposed to enhance the resolution of digital holography by retrieving the frequency components lost in common holograms. A pinhole is placed directly behind the specimen to record the hologram, and an iterative scheme commonly used in coherent diffraction imaging is adopted for the reconstruction. Since some of the frequency components lost in common digital holography can be properly retrieved, the resolution of the reconstructed image is remarkably improved. Theoretical analysis and computer simulations are presented to demonstrate the feasibility of this proposed technique.

We demonstrate a simple, compact and low-cost mode-locked erbium-doped fiber laser (EDFL) using a single-wall carbon nanotube (SWCNT) poly-ethylene oxide (PEO) composite as a passive saturable absorber (SA). The composite with an SWCNT concentration of 18wt% is prepared by mixing the SWCNT homogeneous solution with a diluted PEO polymer solution. A droplet of the polymer composite is applied on the fiber ferrule end, which is then mated to another clean ferrule connector to construct an SA. The SA is then integrated into the laser system to self-start stable mode locking at 1557 nm without employing a polarization controller. The EDFL generates a stable soliton pulse train with a duration of 0.81 ps, repetition rate of 44 MHz and average output power of 92.4 μW at a 980 nm pump power of 26.8 mW. The soliton laser starts to lase at a pump power threshold of 14.6 mW.

Based on vectorial Debye diffraction theory, the spatial correlation properties in the focal region of a J_{0} -correlated azimuthally polarized vortex beam through a high numerical aperture (NA) are analyzed. The expressions for a pair of points on the axis of symmetry and for a pair of points in the focal plane are derived. It is found that the longitudinal and transverse coherence lengths in the focal region change with the variation in the topological charge and coherence parameter of the vortex field, together with the NA values. In addition, the degree of coherence is shown to possess phase singularities.

A compact high-resolution structure for plasmonic wavelength demultiplexers with cascading square resonators is proposed and demonstrated numerically by using the two-dimensional finite element method. It is found that the full width at half maximum of the transmission spectrum can be narrower (～10 nm) than any results reported before. The simulation results can be explained by the temporal coupled-mode theory. This structure can be easily extended to 1×N channels, which has an important role in the wavelength division multiplexing system in nanoscale.

The effects of gravity on the efficiency of thermoacoustic engines are investigated theoretically and experimentally, especially for thermoacoustic pulse tube refrigerators. The significant effects of gravity are found to be due to the presence of natural convection in the thermoacoustic pulse tube when the hot side of the tube is lower than the cold side. This kind of natural convection influences and reduces the efficiency of the thermoacoustic working system. Thus, how to suppress this natural convection becomes important for increasing the efficiency of thermoacoustic engines. Unlike the method of inserting a silk screen in a pulse tube, the present study uses a numerical simulation method to research the natural convection in pulse tubes, and we try to change the shape of the pulse tube to suppress this convection.

A new assembly for ultrasonic measurements of water and ice on multi-anvil apparatus has been designed, and the ultrasonic compressional wave velocities in water and ice up to 4.2 GPa and 500 K are achieved. The pressure of the sample is calibrated by the melting curve of ice VII and the transformation pressure of liquid to solid at ambient temperature. The continuous changing process of the sound velocity transforming from water into ice at high pressure is achieved, and the experimental results of sound velocities at high pressure at room temperature on the melting curve of water are consistent with the previous works by Brillouin scattering. It is believed that our new method of ultrasonic measurements of water is reliable, and worth being used for studying more liquids at high pressure.

Detonation is initiated through a hot jet in a supersonic premixed mixture of H_{2} and air, which is produced by using a air heater. The results show that initiation fails in the low-equivalence-ratio premixed gas. With the increase of equivalence ratio, the hot jet can induce deflagration to detonation transition (DDT) in the premixed mixture, which an indirect initiation of detonation. Further studies show that the DDT process is due to the combined effect of a local hemispherical explosion shock wave, the bow shock, and the flame produced by the hot jet.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

We study the physical process for the pick-up of minor ions by obliquely propagating low-frequency Alfvén waves. It is demonstrated that minor ions can be picked up by the intrinsic low-frequency Alfvén waves observed in the solar wind. When the wave amplitude exceeds the threshold condition for stochasticity, a minor ion can gain a high magnetic moment through the stochastic heating process. Then, the ion with a large magnetic moment can be trapped in the magnetic mirror-like field structures formed by the large-amplitude low-frequency Alfvén waves in the wave frame. As a result, the ion is picked up by the Alfvén waves.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

We present embedded atom method-based geometry optimization calculations for Fe, Cr, Mo, Nb, Ta, V and W body-centered cubic metals with Finnis–Sinclair potentials. After the optimization, we determine their typical elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratios, elastic wave velocities and cohesive energies. Additionally, we perform a benchmark between the experiments and the available density functional theory results. In general, our results show a good consistency with previous findings on the elastic and cohesive energy properties of the considered metals.

GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K_{2}S_{2}O_{8} under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {1011} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

Interaction with the substrate plays an essential role in determining the structure and electronic property of graphene supported by a surface. We observe a maze-like reconstruction pattern in graphene on flat copper foil. With functionalized scanning tunneling microscope tips, a triangular three-for-six structure of graphene and a mixed (2√2 ×√2 )R45° reconstruction of a Cu(100) surface are separately visualized at the atomic scale. Substrate-induced changes in the structure and electronic property are further illustrated by micro-Raman spectroscopy and scanning tunneling spectroscopy. This finding suggests a new method to effectively induce partial sp^{3} hybridization in a single-layer graphene and therefore to tune its electronic property through interaction with the substrate.

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

The valence band offset (VBO) of an Al_{0.17}Ga_{0.83}N/GaN heterojunction is determined to be 0.13±0.07 eV by x-ray photoelectron spectroscopy. From the obtained VBO value, the conduction band offset (CBO) of ～0.22 eV is obtained. The results indicate that the Al_{0.17}Ga_{0.83}N/GaN heterojunction exhibits a type-I band alignment.

A spin-polarized current direction controller scheme is proposed based on a nonuniform Rashba quantum wire. It is shown that |P_{z}^{LR}|=|P_{z}^{RL}|, whereas their signs are opposite, and the effect of this phenomenon is due to the two broken symmetries and the unbroken C_{2}-rotation symmetry of the investigated system. In addition, the spin-polarized current is nonzero even with a strong disorder strength, which demonstrates that this structure may be utilized for potential applications.

A novel symmetric plasmonic structure consisting of a metal-insulator-metal waveguide and a rectangular cavity is proposed to investigate Fano resonance performance by adjusting the size of the structure. The Fano resonance originates from the interference between a local quadrupolar and a broad spectral line in the rectangular cavity. The tuning of the Fano profile is realized by changing the size of the rectangular cavity. The nanostructure is expected to work as an excellent plasmonic sensor with a high sensitivity of about 530 nm/RIU and a figure of merit of about 650.

We describe a single level quantum dot driven by an external stochastic force which works as a nano-thermoelectric refrigerator. Based on the model, expressions for the cooling rate, power input, and coefficient of performance (COP) are derived. The effects of the energy level and energy space on the refrigerator are revealed. The optimal performance characteristics are analyzed by numerical calculation. Furthermore, the practical operating regions of the refrigerator are determined.

Edge termination is one of the key technologies for fabricating high voltage Schottky barrier diodes (SBDs), which could effectively reduce the peak electric field along the Schottky contact edge and enhance the breakdown voltage. We adopt a high-resistivity ring structure as the edge termination for planar GaN SBDs. The edge termination is formed by self-aligned boron implantation on the edge of devices to form a highly damaged layer. In the implant dose and energy ranges studied experimentally, the GaN SBDs show improved blocking characteristics in terms of reverse leakage current and breakdown voltage at higher implant dose or implant energy. Meanwhile, the forward turn-on characteristics of the GaN SBDs exhibit no apparent change.

We introduce a generalized joint density of states (GJDOS), which incorporates the coherent factors into the JDOS, to study quasiparticle interference (QPI) in superconductors. The intimate relation between the Fourier-transformed local density of states and GJDOS is revealed: they correspond respectively to the real and imaginary parts of a generalized impurity-response function, and particularly share the same angular factors and singular boundaries, as seen from our approximate analytic results for d-wave superconductors. Remarkably, our numerical GJDOS analysis agrees well with the QPI patten of d-wave cuprates and s_{±}-wave iron-based superconductors. Moreover, we illustrate that the present GJDOS scenario can uncover the sign features of the superconducting gap and thus can be used to explore the pairing symmetry of the A_{1?x}Fe_{2?y}Se_{2} (A=K,Cs, etc) superconductors.

Complex perovskite Ba(Fe_{1/2}Nb_{1/2})O_{3} thin films are grown on Pt/TiO_{2}/SiO_{2}/Si substrates by pulsed laser deposition. Non-linear polarization–electric-field (P–E) loops are observed at low temperatures under different electric fields and frequencies. The broad peak in the corresponding current density–electric field (J–E) loop confirms the existence of domain switching. The remnant polarization decreases slightly when the temperature decreases from 153 to 123 K, which differentiates Ba(Fe_{1/2}Nb_{1/2})O_{3} from normal ferroelectrics. The Raman spectra of Ba(Fe_{1/2}Nb_{1/2})O_{3} thin films show two-mode-like behavior in the high-wavenumber region, which can be caused by the local chemically heterogeneous zones of NbO_{6} and FeO_{6}. The weak ferroelectricity observed in the Ba(Fe_{1/2}Nb_{1/2})O_{3} thin films might originate from the composition fluctuation and subsequent symmetry breaking in the material. The present results might provide a new aspect to the understanding of the intrinsic dielectric nature in Ba(Fe_{1/2}Nb_{1/2})O_{3}.

We prepare a tandem (OLED) with two independent units and three electrodes. One of the units consists of ITO/TPD/Alq_{3}:DCJTB (5%)/LiF/Al/Au, and the other is Al/Au/TPD/Alq_{3}/LiF/Al. Two units sharing one transparent intermediate electrode Al/Au, together constitute the entire device. With the same current density, the voltage of the entire device is approximately equal to the sum of two units. The current efficiency of the entire device is several times higher than the sum up of two components. Without directly contacting an external power supply, the bi-metal intermediate electrode has the ability to produce electrons and holes for two adjacent emitting units, respectively. This kind of tandem device is characterized by controllable emission color and higher current efficiency than its two components added up.

The thermoluminescence properties of CaF_{2}:Tm (TLD-300) are examined in detail after β-irradiation at room temperature. The glow curve of the sample shows two main dosimetric glow peaks: P3 (at ～150°C) and P5 (at ～250°C). The additive dose, variable heating rate, computer glow curve deconvolution, peak shape and three points methods are used to evaluate the trapping parameters, namely the order of kinetics (b), activation energy (E) and frequency factor (s) associated with the dosimetric thermoluminescent glow peaks (P3 and P5) of CaF_{2}:Tm (TLD-300) after different dose levels with β-irradiation.

Monodispersed spherical CdS nanoparticles embedded into polyvinyl alcohol (PVA) films are synthesized by using an in-situ gamma-irradiation-induced method. The formation mechanism of CdS nanoparticles capped by two united cells of PVA is purposed by means of surrounding the CdS nanoparticles with OH bonds of the PVA chain. CdS nanoparticles are found to possess an unusual orthorhombic structure in monoclinic crystalline PVA. The polymer matrix affords protection from agglomeration and controls the particle size. It is found that the distribution of the prepared nanoparticles increases and a narrower size distribution is observed when the gamma radiation is varied from 10 to 50 kGy. While the average size of the nanoparticles is found to be less affected by the variation of the gamma radiation doses. The size range of the synthesized nanoparticles is 14±1 nm. The optical absorption spectra of synthesized CdS nanoparticles in a polymer matrix reveal the blue shift in the band gap energy with respect to CdS bulk materials owing to quantum confinement effect. The photoluminescence study of nanocomposite films shows the green emission arising from the crystalline defects.

The plastic deformation of nanocrystalline Zn is investigated by positron annihilation lifetime spectroscopy. The deformation is carried out by exerting a pressure on the surface of the samples, which causes a thickness reduction of the samples. The different behavior of the relative thickness reduction with the increasing pressure between the nanocrystalline sample and the conventional bulk sample indicates that a distinct plastic deformation mechanism is operating in the nanocrystalline sample. The positron lifetime results confirm indirectly that the mechanism is grain boundary based, such as grain boundary sliding. Once the nanocrystalline sample is annealed at high temperature, the changes in the thickness reduction and the positron lifetime results become similar to those of the conventional bulk sample. Furthermore, vacancy clusters inside the nanocrystalline sample are found to be an impediment to the plastic deformation process.

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

GeTe_{4} films are deposited by using a dc magnetron sputtering technique, and its structural, thermal and electrical properties are investigated systematically. The prototypical phase-change memory cells are fabricated by using a focused ion beam and magnetron sputtering techniques. Compared with Ge_{2}Sb_{2}Te_{5}, the GeTe_{4} film exhibits a higher crystallization temperature (235°C), better data retention of ten years at 129°C, and larger activation energy (2.94 eV). GeTe_{4} phase change memory cells with an effective diameter of 1 μm show proper switching speed, low power consumption, and good resistance contrast. The Set and Reset operations are achieved by using a 200-ns 2.0-V pulse and a 30-ns 3.0-V pulse, respectively. The dynamic switching ratio between the OFF and ON states is larger than 1×10^{4}.

The internal optical loss in semiconductor optical amplifiers (SOAs) is investigated experimentally and theoretically over a wide injection current range. A new measurement method is proposed without noise figure spectra. Theoretically, considering the internal loss in an SOA, a modified formula of the input-output characteristics is deduced. Based on the practically measured input-output characteristic curves, the internal loss coefficient of an SOA is measured at different injection currents. The results demonstrate that the internal loss coefficient of SOAs decreases with increasing injection current.

MAO Wei, HAO Yue, YANG Cui, ZHANG Jin-Cheng, MA Xiao-Hua, WANG Chong, LIU Hong-Xia, YANG Lin-An, ZHANG Jin-Feng, ZHENG Xue-Feng, ZHANG Kai, CHEN Yong-He, YANG Li-Yuan

Chin. Phys. Lett. 2013, 30 (5):
058502
.
DOI: 10.1088/0256-307X/30/5/058502

We report the studies of In_{0.15}Al_{0.85}N/AlN/GaN metal-insulator-semiconductor (MIS) high electron mobility transistors with a field plate (FP) and a plasma-enhanced chemical vapor deposition (PECVD) SiN layer as the gate dielectric as well as the surface passivation layer (FP-MIS HEMTs). Compared with conventional In_{0.15}Al_{0.85}N/AlN/GaN high electron mobility transistors (HEMTs) of the same dimensions, the FP-MIS HEMTs exhibit a maximum drain current of 1211 mA/mm, a breakdown voltage of 120 V, an effective suppression of current collapse, about one order of magnitude reduction in reverse gate leakage, as well as more than five orders of magnitude reduction in forward gate leakage. These results confirm the potential of PECVD SiN in the application of the InAlN/AlN/GaN FP-MIS HEMTs.

A three-layer p-type Al_{0.82}In_{0.18}N–GaN–Al_{0.82}In_{0.18}N electron blocking layer (EBL) is designed to replace the original p-type AlGaN EBL in blue light emitting diodes (LEDs). The fabricated LEDs with Al_{0.82}In_{0.18}N–GaN–Al_{0.82}In_{0.18}N EBLs exhibit enhanced light output power and an alleviated efficiency drop compared to the original EBL. The improved performance is attributed to more effective electron confinement by this specially designed EBL and improved crystalline quality in the InGaN/GaN active region.

We study the statistical properties of leaders in growing networks with age. A leader of a network is defined as the node with the largest degree and the age of the node is trivially labeled by its index, i.e., the time it joins the network. As networks evolve with the addition of new nodes connecting to old ones with the possibility that is proportional to the index of the target, we investigate both the average number and index of leaders as well as the degree distribution of nodes. The average number of leaders first increases quickly with time and then saturates to a finite value and the average index of leaders increases algebraically with time. Both features result from the degree distribution with an exponential tail. Analytical calculations based on the rate equation are verified by numerical simulations.

Fast magnetosonic (MS) waves have been suggested to be able to effectively accelerate radiation belt electrons. We present gyro-averaged test-particle simulations to investigate counter-streaming interaction between MS wave and radiation belt electrons. It is found that an MS wave can significantly scatter counter-streaming electrons through a non-resonant process. The corresponding energy diffusion coefficients for counter-streaming interaction are always comparable to those for co-streaming interaction, independent of wave normal angle. The pitch-angle and cross diffusion coefficients of counter-streaming interaction are much smaller than those of co-streaming interaction for small normal angles, while they become comparable for large normal angles. Moreover, the bounce-averaged diffusion coefficients exhibit quite different distribution from those for co-streaming or counter-streaming alone in the pitch-angle-energy space. These results suggest that the non-resonant effect associated with counter-streaming interaction is indispensable for the acceleration processes driven by an MS wave.

The nature of pulsars is still unknown because of the non-perturbative effects of the fundamental strong interaction, so various models of pulsar inner structures are suggested, either for conventional neutron stars or quark stars. Additionally, a quark-cluster matter state is conjectured for cold matter at supranuclear density, and as a result pulsars can be quark-cluster stars. Besides understanding the different manifestations, the most important issue is to find an effective way to observationally differentiate these models. X-ray polarimetry plays an important role here. The thermal x-ray polarization of quark/quark-cluster stars is focused on, and while the thermal x-ray linear polarization percentage is typically higher than ～10% in normal neutron star models, the percentage of quark/quark-cluster stars is almost zero. This could then be an effective method to identify quark/quark-cluster stars by soft x-ray polarimetry. We are therefore expecting to detect thermal x-ray polarization in the coming decades.

The dynamics of slim disk under the influence of a large-scale ordered magnetic field is investigated. The global solutions show that the radial velocity increases and the disk temperature decreases with enhancing magnetic field. The fraction of mass loss becomes smaller when the accretion rate is higher. The ratio of the jet kinetic power to disk luminosity is less than 0.1, which indirectly supports the argument that radio-loud narrow-line Seyfert 1 galaxies share similarities with blazars.