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Volume 35 Issue 2
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Interaction between Breathers and Rogue Waves in a Nonlinear Optical Fiber
Xiang-Shu Liu, Li-Chen Zhao, Liang Duan, Peng Gao, Zhan-Ying Yang, Wen-Li Yang
Chin. Phys. Lett. 2018, 35 (2): 020501 .   DOI: 10.1088/0256-307X/35/2/020501
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We study the interaction between breather and $N$-order rogue waves in a nonlinear optical fiber. The impacts of the relative phase and the interaction distance between breathers and rogue waves are discussed in detail. Specifically, the breather can reduce the maximum hump value of high-order rogue waves greatly in the cases of nonzero relative phase or nonzero interaction distance. The characteristic of exclusion between breathers and rogue waves is described qualitatively in the situation of different interaction distances, which can be used to change the temporal-spatial distribution of rogue waves. Their interaction properties are characterized by the trajectory of localized waves' valleys and humps. It is shown that the interaction changes the dynamical evolution trajectory of rogue waves and breathers. These results provide some possible ways to control high-order rogue waves.
Two-Point Resistance on the Centered-Triangular Lattice
M. Q. Owaidat, A. A. Al-Badawi, J. H. Asad, Mohammed Al-Twiessi
Chin. Phys. Lett. 2018, 35 (2): 020502 .   DOI: 10.1088/0256-307X/35/2/020502
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The resistance between any two lattice points in an infinite, centered-triangular lattice of equal resistors is determined using the lattice Green function method. It is shown that the two-point resistance on the centered-triangular lattice is expressed in terms of the resistance of a triangular lattice. Some exact values for the resistance near the origin of the lattice are presented. For large separation between lattice points the asymptotic forms of the resistance are calculated.
Orbital Angular Momentum Generation Using Circular Ring Resonators in Radio Frequency
Fu-Chun Mao, Ming Huang, Cheng-Fu Yang, Ting-Hua Li, Jia-Lin Zhang, Si-Yu Chen
Chin. Phys. Lett. 2018, 35 (2): 020701 .   DOI: 10.1088/0256-307X/35/2/020701
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Electromagnetic field generators based on circular ring resonators, whose perimeters are integer times of equivalent wavelength, are well known to have attractive potential for producing radio vortexes carrying orbital angular momentum (OAM). We study the radiation characteristics of the generators based on radiation vector and antenna array theory. The behaviors of radiation patterns, field intensity and phase distribution are investigated in detail, and show classical features of OAM beams. The evolution of the generators performance versus the OAM state is also analyzed. The proposed generators can be realized by all kinds of microwave transmission lines, verified by two different prototypes. The discussions and conclusions drawn in this study are useful and meaningful for the radio OAM generator design.
Frequency Stabilization of Pulsed Injection-Seeded OPO Based on Optical Heterodyne Technique
Xiao Chen, Xiao-Lei Zhu, Shi-Guang Li, Xiu-Hua Ma, Wei Xie, Ji-Qiao Liu, Wei-Biao Chen, Ren Zhu
Chin. Phys. Lett. 2018, 35 (2): 024201 .   DOI: 10.1088/0256-307X/35/2/024201
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A frequency stabilizing system for a pulsed injection seeded 1550 nm optical parametric oscillator (OPO) at 20 Hz repetition rate is demonstrated. The optical heterodyne method is used to measure the frequency difference between the seed laser and the OPO output. Using the frequency difference as the error signal, a proportional-integral controller in combination with a scanner is applied to stably match the OPO cavity length to the seed laser frequency. The root-mean-square (rms) error of the frequency discrimination method is $ < $0.07 MHz according to a 'frequency shifting-chopping-beat' evaluation. The frequency fluctuation of the frequency-stabilized OPO is 0.29 MHz (rms), and the Allan deviation is less than 20 kHz for averaging time of more than 3 s.
Quantitative and Spatially Resolved Measurement of Atomic Potassium in Combustion Using Diode Laser
Qiang Gao, Wu-Bin Weng, Bo Li, Zhong-Shan Li
Chin. Phys. Lett. 2018, 35 (2): 024202 .   DOI: 10.1088/0256-307X/35/2/024202
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A compact optical setup for quantitative and spatially resolved measurement of atomic alkali concentration in combustion is demonstrated. Tunable diode laser absorption spectroscopy and laser-induced fluorescence are combined using a single continuous wave diode laser to measure the line-integration concentration and the relative distribution simultaneously, thereby obtaining the absolute concentration distribution along the laser beam. The results indicate the good performance of this method for one-dimensional quantitative measurement.
Experimental Observation of Bright and Dark Solitons Mode-Locked with Zirconia-Based Erbium-Doped Fiber Laser
A. M. Markom, S. J. Tan, H. Haris, M. C. Paul, A. Dhar, S. Das, S. W. Harun
Chin. Phys. Lett. 2018, 35 (2): 024203 .   DOI: 10.1088/0256-307X/35/2/024203
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We demonstrate the generation of dark and bright solitons with our homemade zirconia-based erbium-doped fiber and graphene oxide (GO) saturable absorber in anomalous dispersion region. The GO is fabricated using an abridged Hummer's method, which is combined with polyethylene oxide to produce a composite film. The film is sandwiched between two optical ferrules and embedded in the laser cavity to enhance its birefringence and nonlinearity. The self-starting bright soliton is easily generated at pump power of 78 mW with the whole length cavity of 14.7 m. The laser produces the bright pulse train with repetition rate, pulse width, pulse energy and central wavelength being 13.9 MHz, 0.6 ps, 2.74 pJ and 1577.46 nm, respectively. Then, by adding the 10 m of single mode fiber into the laser cavity, dark soliton pulse is produced. For the formation of dark pulse train, the measured repetition rate, pulse width, pulse energy and central wavelength are 8.3 MHz, 20 ns and 4.98 pJ and 1596.82 nm, respectively. Both pulses operate in the anomalous region.
Phase Control of Transient Optical Properties of Double Coupled Quantum-Dot Nanostructure via Gaussian Laser Beams
J. Shiri, F. Shahi, M. R. Mehmannavaz, L. Shahrassai
Chin. Phys. Lett. 2018, 35 (2): 024204 .   DOI: 10.1088/0256-307X/35/2/024204
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We theoretically analyze the transient properties of a probe field absorption and dispersion in a coupled semiconductor double-quantum-dot nanostructure. We show that in the presence of the Gaussian laser beams, absorption and dispersion of the probe field can be dramatically influenced by the relative phase between applied fields and intensity of the Gaussian laser beams. Transient and steady-state behaviors of the probe field absorption and dispersion are discussed to estimate the required switching time. The estimated range is between 5–8 ps for subluminal to superluminal light propagation.
Transparent and Ultra-lightweight Design for Ultra-Broadband Asymmetric Transmission of Airborne Sound
Jie Hu, Bin Liang, Xiao-Jun Qiu
Chin. Phys. Lett. 2018, 35 (2): 024301 .   DOI: 10.1088/0256-307X/35/2/024301
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Acoustic one-way manipulations have recently attracted significant attention due to the deep implications in many diverse fields such as biomedical imaging and treatment. However, the previous mechanisms of asymmetric manipulation of airborne sound need to use elaborate heavyweight structures and only work in certain frequency ranges. We propose a mechanism for designing an ultra-lightweight and optically transparent structure with asymmetric transmission property for normally incident plane waves. Instead of fabricating solids into complicated artificial structures with limited bandwidth and heavy weight, we simply use xenon to fill a spatial region of asymmetric shape which allows the incident plane wave to pass along one direction while reflecting the reversed wave regardless of frequency. We demonstrate both analytically and numerically its effectiveness of producing highly-asymmetric transmission within an ultra-broad band. Our design offers new possibility for the design of one-way devices and may have far-reaching impact on various scenarios such as noise control.
Particle Trajectory Integrator in Guiding Center Phase Space
Jing-Bo Lin, Wen-Lu Zhang, Peng-Fei Liu, Chao Dong, Jin-Tao Cao, Ding Li
Chin. Phys. Lett. 2018, 35 (2): 025201 .   DOI: 10.1088/0256-307X/35/2/025201
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A trajectory integrator is developed based on a particle's guiding center Hamiltonian. It is verified by a series of benchmarks, which are in good accordance with theoretical prediction. This integrator can be used as the guiding center trajectory integrator of a particle-in-cell simulation platform, such as the newly developed VirtEx. It can also be used as a stand-alone tool to investigate particle dynamics in a given background field.
Effects of Grain Boundary Characteristics on Its Capability to Trap Point Defects in Tungsten
Wen-Hao He, Xing Gao, Ning Gao, Ji Wang, Dong Wang, Ming-Huan Cui, Li-Long Pang, Zhi-Guang Wang
Chin. Phys. Lett. 2018, 35 (2): 026101 .   DOI: 10.1088/0256-307X/35/2/026101
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As recombination centers of vacancies (Vs) and self-interstitial atoms (SIAs), firstly grain boundaries (GBs) should have strong capability of trapping point defects. In this study, abilities to trap Vs and SIAs of eight symmetric tilt GBs in tungsten are investigated through first-principles calculations. On the one hand, vacancy formation energy $E_{\rm V}^{\rm f}$ rapidly increases then slowly decreases as the hard-sphere radius $r_0$ of the vacancy increases. The value of $E_{\rm V}^{\rm f}$ is the largest when $r_0$ is about 1.38 Å, which is half the distance between the nearest atoms in equilibrium single crystal tungsten. That is, any denser or looser atomic configuration around GBs than that in bulk is helpful to form a vacancy. On the other hand, SIA formation energy $E_{\rm SIA}^{\rm f}$ at GBs decreases monotonically with increasing the hard-sphere radius of the interstitial sites, which indicates that GBs with larger interstitial sites have stronger ability to trap SIAs. Based on the data obtained for GBs investigated in this study, it is found that the ability to trap Vs increases as the GB energy increases, and the capability of trapping SIAs linearly increases as the excess volume of GB increases. Due to its lowest GB energy and smallest excess volume among all GBs studied, twin GB $\sum$3(110)[111] has the weakest capability to trap both Vs and SIAs.
Structural Distortion and Defects in Ti$_{3}$AlC$_{2}$ irradiated by Fe and He Ions
Li-Long Pang, Bing-Sheng Li, Tie-Long Shen, Xing Gao, Xue-Song Fang, Ning Gao, Cun-Feng Yao, Kong-Fang Wei, Ming-Huan Cui, Jian-Rong Sun, Hai-Long Chang, Wen-Hao He, Qing Huang, Zhi-Guang Wang
Chin. Phys. Lett. 2018, 35 (2): 026102 .   DOI: 10.1088/0256-307X/35/2/026102
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Ti$_{3}$AlC$_{2}$ samples are irradiated in advance by 3.5 MeV Fe-ion to the fluence of 1.0$\times$10$^{16}$ ion/cm$^{2}$, and then are implanted by 500 keV He-ion with the fluence of 1.0$\times$10$^{17}$ ion/cm$^{2}$ at room temperature. The irradiated samples are investigated by grazing incidence x-ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD results show serious structural distortion, but without amorphization in the irradiated samples. Fe-ion irradiation and He-ion implantation create much more serious structural distortion than single Fe-ion irradiation. TEM results reveal that there are a large number of defect clusters in the damage region, and dense spherical He bubbles appear in the He depositional region. It seems that the pre-damage does not influence the growth of He bubbles, but He-ion implantation influences the pre-created defect configurations.
Influence of Flexoelectric Effect on Director Alignment of Nematic Liquid Crystals in Axial Arrangement Cylindrical Cells
Hong-Hong Liu, Yan-Jun Zhang, Huai-Rui Yue, Li-Zhi Zhu, Rui-Xia Yang
Chin. Phys. Lett. 2018, 35 (2): 026103 .   DOI: 10.1088/0256-307X/35/2/026103
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A positive nematic liquid crystal (5CB) sample is confined in cylindrical cells under strong or weak axial anchoring boundary conditions when a radial nonuniform low-frequency electric field is applied and the flexoelectric effect is taken into account. Based on the Frank elastic free energy, the surface energy of the Rapini–Papoular approximation, the polarization free energy and the flexoelectric free energy caused by electric field, we obtain the free energy density of the nematic and solve the corresponding Euler–Lagrange equation numerically. We investigate the director distribution, the critical voltage and the critical exponent of nematic liquid crystal in cylindrical cells. It follows that the critical exponent is the classical one. It is also shown that the critical voltage in the system is affected by the flexoelectric effect, the geometric effect and radial weak anchoring effect on the cylindrical surfaces. A new type of director transition caused by the flexoelectric effect, the dielectric coupling effect and the radial weak anchoring effect is found.
Polar Dependence of Threading Dislocation Density in GaN Films Grown by Metal-Organic Chemical Vapor Deposition
Zhi-Yu Lin, Zhi-Bin Chen, Jin-Cheng Zhang, Sheng-Rui Xu, Teng Jiang, Jun Luo, Li-Xin Guo, Yue Hao
Chin. Phys. Lett. 2018, 35 (2): 026104 .   DOI: 10.1088/0256-307X/35/2/026104
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We investigate the threading dislocation (TD) density in N-polar and Ga-polar GaN films grown on sapphire substrates by metal-organic chemical vapor deposition. X-ray diffraction results reveal that the proportion of screw type TDs in N-polar GaN is much larger and the proportion of edge type TDs is much smaller than that in Ga-polar. Transmission electron microscope results show that the interface between the AlN nucleation layer and the GaN layer in N-polar films is smoother than that in Ga-polar films, which suggests different growth modes of GaN. This observation explains the encountered difference in screw and edge TD density. A model is proposed to explain this phenomenon.
Effect of Nickel Distributions Embedded in Amorphous Carbon Films on Transport Properties
Vali Dalouji, Dariush Mehrparvar, Shahram Solaymani, Sahar Rezaee
Chin. Phys. Lett. 2018, 35 (2): 026501 .   DOI: 10.1088/0256-307X/35/2/026501
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Electrical properties of C/Ni films are studied using four mosaic targets made of pure graphite and stripes of nickel with the surface areas of 1.78, 3.21, 3.92 and 4.64%. The conductivity data in the temperature range of 400–500 K shows the extended state conduction. The conductivity data in the temperature range of 150–300 K shows the multi-phonon hopping conduction. The Berthelot-type conduction dominates in the temperature range of 50–150 K. The conductivity of the films in the temperature range about $T < 50$ K is described in terms of variable-range hopping conduction. In low temperatures, the localized density of state around Fermi level $N(E_{\rm F})$ for the film deposition with 3.92% nickel has a maximum value of about $56.2\times10^{17}$ cm$^{-3}$eV$^{-1}$ with the minimum average hopping distance of about $3.43\times10^{-6}$ cm.
Thickness Effect on (La$_{0.26}$Bi$_{0.74}$)$_{2}$Ti$_{4}$O$_{11}$ Thin-Film Composition and Electrical Properties
Hui-Zhen Guo, An-Quan Jiang
Chin. Phys. Lett. 2018, 35 (2): 026801 .   DOI: 10.1088/0256-307X/35/2/026801
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Highly oriented (00l) (La$_{0.26}$Bi$_{0.74}$)$_{2}$Ti$_{4}$O$_{11}$ thin films are deposited on (100) SrTiO$_{3}$ substrates using the pulsed laser deposition technique. The grains form a texture of bar-like arrays along SrTiO$_{3}$ $\langle 110\rangle$ directions for the film thickness above 350 nm, in contrast to spherical grains for the reduced film thickness below 220 nm. X-ray diffraction patterns show that the highly ordered bar-like grains are the ensemble of two lattice-matched monoclinic (La,Bi)$_{4}$Ti$_{3}$O$_{12}$ and TiO$_{2}$ components above a critical film thickness. Otherwise, the phase decomposes into the random mixture of Bi$_{2}$Ti$_{2}$O$_{7}$ and Bi$_{4}$Ti$_{3}$O$_{4}$ spherical grains in thinner films. The critical thickness can increase up to 440 nm as the films are deposited on LaNiO$_{3}$-buffered SrTiO$_{3}$ substrates. The electrical measurements show the dielectric enhancement of the multi-components, and comprehensive charge injection into interfacial traps between (La,Bi)$_{4}$Ti$_{3}$O$_{12}$ and TiO$_{2}$ components occurs under the application of a threshold voltage for the realization of high-charge storage.
Surface Structure and Reconstructions of HgTe (111) Surfaces
Xin-Yi Yang, Guan-Yong Wang, Chen-Xiao Zhao, Zhen Zhu, Lu Dong, Ai-Min Li, Yang-Yang Lv, Shu-Hua Yao, Yan-Bin Chen, Dan-Dan Guan, Yao-Yi Li, Hao Zheng, Dong Qian, Canhua Liu, Yu-Lin Chen, Jin-Feng Jia
Chin. Phys. Lett. 2018, 35 (2): 026802 .   DOI: 10.1088/0256-307X/35/2/026802
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HgTe (111) surface is comprehensively studied by scanning tunneling microscopy/spectroscopy (STS). In addition to the primitive $(1\times 1)$ hexagonal lattice, six reconstructed surface structures are observed: $(2\times 2)$, $2\times 1$, $4\times 1$, $3\times \sqrt{3}$, $2\sqrt{2}\times 2$ and $\sqrt{11}\times 2$. The $(2\times 2)$ reconstructed lattice maintains the primitive hexagonal symmetry, while the lattices of the other five reconstructions are rectangular. Moreover, the topographic features of the $3\times \sqrt{3}$ reconstruction are bias dependent, indicating that they have both topographic and electronic origins. The STSs obtained at different reconstructed surfaces show a universal dip feature with size $\sim $100 mV, which may be attributed to the surface distortion. Our results reveal the atomic structure and complex reconstructions of the cleaved HgTe (111) surfaces, which paves the way to understand the rich properties of HgTe crystal.
Interaction-Induced Characteristic Length in Strongly Many-Body Localized Systems
Rong-Qiang He, Zhong-Yi Lu
Chin. Phys. Lett. 2018, 35 (2): 027101 .   DOI: 10.1088/0256-307X/35/2/027101
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A complete set of local integrals of motion (LIOM) is a key concept for describing many-body localization (MBL), which explains a variety of intriguing phenomena in MBL systems. For example, LIOM constrain the dynamics and result in ergodicity violation and breakdown of the eigenstate thermalization hypothesis. However, it is difficult to find a complete set of LIOM explicitly and accurately in practice, which impedes some quantitative structural characterizations of MBL systems. Here we propose an accurate numerical method for constructing LIOM, discover through the LIOM an interaction-induced characteristic length $\xi_+$, and prove a 'quasi-product-state' structure of the eigenstates with that characteristic length $\xi_+$ for MBL systems. More specifically, we find that there are two characteristic lengths in the LIOM. The first one is governed by disorder and is of Anderson-localization nature. The second one is induced by interaction but shows a discontinuity at zero interaction, showing a nonperturbative nature. We prove that the entanglement and correlation in any eigenstate extend not longer than twice the second length and thus the eigenstates of the system are the quasi-product states with such a localization length.
Clustering and Dissolution of Small Helium Clusters in Bulk Tungsten
Chu-Bin Wan, Su-Ye Yu, Xin Ju
Chin. Phys. Lett. 2018, 35 (2): 027102 .   DOI: 10.1088/0256-307X/35/2/027102
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Density functional theory calculations are conducted to investigate the stability and interactions among small helium (He) clusters in bulk tungsten (W). The lowest energy structure of each cluster for sizes $n=1$ to 6 is determined. With the formation of He clusters, He defects form in bulk W. The thermodynamics of the clusters are investigated in the temperature range of 1000–2300 K using molecular dynamics. This study provides the information essential to understand small He cluster behavior in bulk W.
Electron Transport Behavior of Multiferroic Perovskite BiMnO$_{3}$ Prepared by Co-Precipitation Method
Hong-Jun Wang, Yuan-Yuan Zhu, Jing Zhou
Chin. Phys. Lett. 2018, 35 (2): 027201 .   DOI: 10.1088/0256-307X/35/2/027201
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Perovskite BiMnO$_{3}$ samples are successfully synthesized by the co-precipitation method at relatively low pressure and moderate temperature. The temperature dependences of resistivity are measured and systematically investigated. It is shown that the electrical resistivity increases sharply with the decrease of temperature above 210 K and the fitted results demonstrate that the thermally activated conduction model is the dominant conduction mechanism for the electron transport behaviors in this temperature region. A dual conducting mechanism, i.e., the variable range hopping and thermal activated conduction, is suggested to be responsible for the transport behaviors of BiMnO$_{3}$ in the region of 180–200 K. Moreover, the resistivity increases slightly with the decrease of temperature below 180 K and the transport is governed by the variable range hopping mechanism.
Effect of Mg-Preflow for p-AlGaN Electron Blocking Layer on the Electroluminescence of Green LEDs with V-Shaped Pits
Ai-Xing Li, Chun-Lan Mo, Jian-Li Zhang, Xiao-Lan Wang, Xiao-Ming Wu, Guang-Xu Wang, Jun-Lin Liu, Feng-Yi Jiang
Chin. Phys. Lett. 2018, 35 (2): 027301 .   DOI: 10.1088/0256-307X/35/2/027301
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InGaN-based green light-emitting diodes (LEDs) with and without Mg-preflow before the growth of p-AlGaN electron blocking layer (EBL) are investigated experimentally. A higher Mg doping concentration is achieved in the EBL after Mg-preflow treatment, effectively alleviating the commonly observed efficiency collapse and electrons overflowing at cryogenic temperatures. However, unexpected decline in quantum efficiency is observed after Mg-preflow treatment at room temperature. Our conclusions are drawn such that the efficiency decline is probably the result of different emission positions. Higher Mg doping concentration in the EBL after Mg-preflow treatment will make it easier for a hole to be injected into multiple quantum wells with emission closer to p-GaN side through the $c$-plane rather than the V-shape pits, which is not favorable to luminous efficiency due to the preferred occurrence of accumulated strain relaxation and structural defects in upper QWs closer to p-GaN. Within this framework, apparently disparate experimental observations regarding electroluminescence properties, in this work, are well reconciled.
Concise Modeling of Amorphous Dual-Gate In-Ga-Zn-O Thin-Film Transistors for Integrated Circuit Designs
Can Li, Cong-Wei Liao, Tian-Bao Yu, Jian-Yuan Ke, Sheng-Xiang Huang, Lian-Wen Deng
Chin. Phys. Lett. 2018, 35 (2): 027302 .   DOI: 10.1088/0256-307X/35/2/027302
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An analytical model for current–voltage behavior of amorphous In-Ga-Zn-O thin-film transistors (a-IGZO TFTs) with dual-gate structures is developed. The unified expressions for synchronous and asynchronous operating modes are derived on the basis of channel charges, which are controlled by gate voltage. It is proven that the threshold voltage of asynchronous dual-gate IGZO TFTs is adjusted in proportion to the ratio of top insulating capacitance to the bottom insulating capacitance $(C_{\rm TI}/C_{\rm BI})$. Incorporating the proposed model with Verilog-A, a touch-sensing circuit using dual-gate structure is investigated by SPICE simulations. Comparison shows that the touch sensitivity is increased by the dual-gate IGZO TFT structure.
Observation of Tunneling Gap in Epitaxial Ultrathin Films of Pyrite-Type Copper Disulfide
Chong Liu, Haohao Yang, Can-Li Song, Wei Li, Ke He, Xu-Cun Ma, Lili Wang, Qi-Kun Xue
Chin. Phys. Lett. 2018, 35 (2): 027303 .   DOI: 10.1088/0256-307X/35/2/027303
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We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide. Layer-by-layer growth of CuS$_{2}$ films with a preferential orientation of (111) on SrTiO$_{3}$(001) and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ substrates is achieved by molecular beam epitaxy growth. For ultrathin films on both kinds of substrates, we observe symmetric tunneling gap around the Fermi level that persists up to $\sim$15 K. The tunneling gap degrades with either increasing temperature or increasing thickness, suggesting new matter states at the extreme two-dimensional limit.
Fe$^{3+}$-Doped Anatase TiO$_{2}$ Study Prepared by New Sol-Gel Precursors
Bahram Khoshnevisan, Mohammad Bagher Marami, Majid Farahmandjou
Chin. Phys. Lett. 2018, 35 (2): 027501 .   DOI: 10.1088/0256-307X/35/2/027501
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Fe$_{x}$Ti$_{1-x}$O$_{2}$ ($x=0.00$, 0.05, 0.10) nanocomposites are synthesized using a sol-gel method involving an ethanol solvent in the presence of ethylene glycol as the stabilizer, and acetic acid as the chemical reagent. Their structural and optical analyses are studied to reveal their physicochemical properties. Using the x-ray diffractometer (XRD) analysis, the size of the nanoparticles (NPs) is found to be 18–32 nm, where the size of the NPs decreases down to 18 nm when Fe impurity of up to 10% is added, whereas their structure remains unchanged. The results also indicate that the structure of the NPs is tetragonal in the anatase phase. The Fourier transform infrared spectroscopy analysis suggests the presence of a vibration bond (Ti–O) in the sample. The photoluminescence analysis indicates that the diffusion of Fe$^{3+}$ ions into the TiO$_{2}$ matrix results in a decreasing electron–hole recombination, and increases the photocatalytic properties, where the best efficiency appears at an impurity of 10%. The UV-diffuse reflection spectroscopy analysis indicates that with the elevation of iron impurity, the band gap value decreases from 3.47 eV for the pure sample to 2.95 eV for the 10 mol% Fe-doped TiO$_{2}$ NPs.
Gap States of ZnO Thin Films by New Methods: Optical Spectroscopy, Optical Conductivity and Optical Dispersion Energy
Vali Dalouji, Shahram Solaymani, Laya Dejam, Seyed Mohammad Elahi, Sahar Rezaee, Dariush Mehrparvar
Chin. Phys. Lett. 2018, 35 (2): 027701 .   DOI: 10.1088/0256-307X/35/2/027701
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The optical reflectance and transmittance spectra in the wavelength range of 300–2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600$^\circ\!$C. The values of the cross point between the curves of the real and imaginary parts of the optical conductivity $\sigma_{1}$ and $\sigma_{1}$ with energy axis of films exhibit values that correspond to optical gaps and are about 3.25–3.3 eV. The maxima of peaks in plots $dR/d\lambda$ and $dT/d\lambda$ versus wavelength of films exhibit optical gaps at about 3.12–3.25 eV. The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14–3.2 eV. It can be seen that films annealed at 600$^{\circ}\!$C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600$^{\circ}\!$C have Urbach's energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy $E_{\rm d}$ of films annealed at 500$^{\circ}\!$C has the minimum value of 43 eV.
Ultrafast Terahertz Probes of Charge Transfer and Recombination Pathway of CH$_{3}$NH$_{3}$PbI$_{3}$ Perovskites
Hui-Jie Yan, Zhi-Liang Ku, Xue-Feng Hu, Wan-Ying Zhao, Min-Jian Zhong, Qi-Biao Zhu, Xian Lin, Zuan-Ming Jin, Guo-Hong Ma
Chin. Phys. Lett. 2018, 35 (2): 028401 .   DOI: 10.1088/0256-307X/35/2/028401
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We use transient terahertz photoconductivity measurements to demonstrate that upon optical excitation of CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite, the hole transfer from CH$_{3}$NH$_{3}$PbI$_{3}$ into the organic hole-transporting material (HTM) Spiro-OMeTAD occurs on a sub-picosecond timescale. Second-order recombination is the dominant decay pathway at higher photo-excitation fluences as observed in neat CH$_{3}$NH$_{3}$PbI$_{3}$ films. In contrast, under similar experimental conditions, second-order recombination weakly contributes the relatively slow recombination between the electrons in the perovskite and the injected holes in HTM, as a loss mechanism at the CH$_{3}$NH$_{3}$PbI$_{3}$/Spiro-OMeTAD interface. Our results offer insights into the intrinsic photophysics of CH$_{3}$NH$_{3}$PbI$_{3}$-based perovskites with direct implications for photovoltaic devices and optoelectronic applications.
Ohmic Contact at Al/TiO$_{2}$/n-Ge Interface with TiO$_{2}$ Deposited at Extremely Low Temperature
Yi Zhang, Gen-Quan Han, Yan Liu, Huan Liu, Jin-Cheng Zhang, Yue Hao
Chin. Phys. Lett. 2018, 35 (2): 028501 .   DOI: 10.1088/0256-307X/35/2/028501
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TiO$_{2}$ deposited at extremely low temperature of 120$^\circ\!$C by atomic layer deposition is inserted between metal and n-Ge to relieve the Fermi level pinning. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy indicate that the lower deposition temperature tends to effectively eliminate the formation of GeO$_{x}$ to reduce the tunneling resistance. Compared with TiO$_{2}$ deposited at higher temperature of 250$^\circ\!$C, there are more oxygen vacancies in lower-temperature-deposited TiO$_{2}$, which will dope TiO$_{2}$ contributing to the lower tunneling resistance. Al/TiO$_{2}$/n-Ge metal-insulator-semiconductor diodes with 2 nm 120$^\circ\!$C deposited TiO$_{2}$ achieves 2496 times of current density at $-$0.1 V compared with the device without the TiO$_{2}$ interface layer case, and is 8.85 times larger than that with 250$^\circ\!$C deposited TiO$_{2}$. Thus inserting extremely low temperature deposited TiO$_{2}$ to depin the Fermi level for n-Ge may be a better choice.
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