RF impedance of intrinsic Josephson junction in flux-flow state with a periodic pinning potential and its optimum condition for RF radiation

We reported dynamics of Josephson vortices interacting with electromagnetic waves in strongly coupled long Josephson junctions stack, such as an intrinsic Josephson junction (IJJ), by numerical simulations based on coupled sine-Gordon equations considering a periodic pinning potential of sinusoidal form. The numerical simulation results for the influence of the electromagnetic waves on flux-flow properties show that the periodic pinning potential induces an in-phase motion of Josephson vortices over the junction stacks, which achieve high performances of IJJ flux-flow oscillator. In order to prove it from another viewpoint, we calculate RF impedance of long Josephson junction stacks in flux-flow state. A remarkable negative real part region of RF impedance appears at 1st harmonic step, it means that the long Josephson junction stacks in flux-flow state acts as an oscillator at the negative real part region. In this study, we evaluate the optimum condition for RF radiation with the periodic pinning potential.     

高温超导约瑟夫森结阵列的相位锁定

根据交流约瑟夫森效应,研究了嵌入Fabry-Perot谐振腔里的约瑟夫森结阵列在微波辐照下的相位锁定问题。结阵列是由生长在双晶钇稳定氧化锆(YSZ)基片上的YB2Cu3O7(YBCO)超导薄膜光刻成微桥得到的。通过优化设计,在温度为79.2K辐照频率为77.465GHz的条件下,包含620个串联约瑟夫森双晶结的结阵列在外加微波辐照下实现了相位锁定,得到了陡峭的夏皮罗台阶。其第一级夏皮罗台阶的电压约为0.1V,台阶高度约为0.17mA。试验结果表明,这种结阵列结构在电压基准的应用上有极大的优势,而且这种准光学耦合方法在太赫兹信号发生和检测方面有很好的应用前景。     

Josephson effect on YBCO break junctions

We have investigated dc and ac Josephson effects on YBa₂Cu₃O_7–x ceramic samples using the break junction technique. The critical current was measured as a function of external magnetic field and temperature. The field dependence shows superposition effects of junction and SQUID behavior. The critical current depends linearly on temperature like for SNS junctions. We observed Shapiro steps with microwave radiation with a frequency of 35 GHz at 4.2 K and 77 K, respectively. Deviations of the step height from Bessel behavior at low microwave power are explained within the Stewart-McCumber model using small capacitances of the junction in simulation calculations.     

微波场中Josephson结的微波发射与吸收研究

采用数值仿真方法,研究微波场中约瑟夫森结微波感应台阶处的功率特性。发现微波场中约瑟夫森结不止吸收辐射,也发射辐射的情况。得到微波辐照和结电阻对功率吸收和发射的影响规律。结果表明,处于发射功率的状态对应的偏置电流范围占整个台阶总电流范围的比例可以趋于50%。这一发现对约瑟夫森结理论和应用研究具有重要的意义。     

Millimeter wave response of Nb-Pb-Nb planar Josephson junction

The response characteristics of Nb-Pb-Nb planar Josephson junction under millimeter wave radiation is discussed. We could observe the third Shapiro step.     

Specular Andreev reflection at the edge of an InAs/GaSb double quantum well with band inversion

We experimentally investigate transport through the side junction between a niobium superconductor and the mesa edge of a two-dimensional system, realized in an InAs/GaSb double quantum well with band inversion. We demonstrate, that different transport regimes can be achieved by variation of the mesa step. We observe anomalous behavior of Andreev reflection within a finite low-bias interval, which is invariant for both transport regimes. We connect this behavior with the transition from retro-(at low biases) to specular (at high ones) Andreev reflection channels in an InAs/GaSb double quantum well with band inversion.     

Enhancement of Josephson phase diffusion by microwaves

We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position V(top) of the peak increases with the power P of microwave radiation as V(top) proportional to sqrt[P], while its current amplitude weakly decreases with P. As the microwave frequency increases, the peak feature evolves into Shapiro steps with a finite slope. Our theoretical analysis, taking into account the enhancement of incoherent superconducting current by multiphoton absorption, is in good agreement with experimental data.     

Mutual inductance effects in rf driven planar Josephson junctions arrays

In this paper we investigate the behavior of moderate size two-dimensional classical arrays of Josephson junctions in presence of an external oscillating field. We have included in the model the effects due to mutual inductance terms, and we have employed an explicit set of differential equations. We have found that the discretization parameter - i.e. the coupling term due to the inductance of the loops - is the most important parameter to determine the height of the Shapiro steps for a given amplitude and frequency of the rf-bias. The amplitude of the Shapiro steps in the case of zero frustration as a function of the coupling term shows a remarkable minimum for intermediate values when we retain all terms of the full model with mutual inductances, while the limits for very large and very small values of they are the same of the single Josephson junction. For the case of frustration 1/2 the Shapiro step becomes smaller in the rigid limit (i.e., small ) as expected for the XY model, and tends to the limit value of the single junctions for the decoupled case (i.e., large ). Received 9 November 1998 and Received in final form 6 April 1999     

GaP台面腐蚀研究

高效GaP绿色发光二极管制造中,器件的隔离和台面制作具有十分重要意义。本文介绍了用于GaP发光器件的隔离和台面制作的碱性铁氰化钾化学腐蚀技术。观察了腐蚀温度,腐蚀时间对腐蚀深度和表面形貌间关系。结果表明:碱性铁氰化钾对GaP台面腐蚀是一种优良的腐蚀剂,具有较高的腐蚀速率(2微米/分),并可获得光滑无凹坑或少凹坑的腐蚀面。对出现的实验现象从机理上作了解释。     

PIN限幅器的高功率微波单脉冲效应研究

基于PIN限幅器的等效电路模型,构建了PIN限幅器HPM效应ADS等效电路仿真模型,利用HPM注入实验和等效电路仿真相结合的方法,研究了单个微波脉冲作用下PIN限幅器的响应规律,获取了HPM作用结束后限幅器限幅持续时间与注入脉冲功率、脉宽的对应关系,并对限幅器的限幅持续过程进行了分析。仿真与实验结果表明：PIN限幅器限幅持续时间随着微波脉冲功率和脉宽的增大而变大,实验和仿真结果趋势一致,该研究使用的ADS等效电路模型可以应用于PIN限幅器的高功率微波瞬态响应特性分析研究。     

Josephson effect in MgB(2) break junctions

We present the first observation of the dc and ac Josephson effect in MgB(2) break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB(2) samples, show a nonhysteretic dc Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal Delta V value. The temperature dependence of the dc Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results directly prove the existence of pairs with charge 2e in MgB(2) and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.     

Butt-joint regrowth method by MOCVD for integration of evanescent wave coupled photodetector and multi-quantum well semiconductor optical amplifier

We have realized integration of evanescent wave coupled photodetector (ECPD) and multi-quantum well (MQW) semiconductor optical amplifier (SOA) on MOCVD platform by investigating butt-joint regrowth method of thick InP/InGaAsP waveguides to deep etched SOA mesas. The combination of inductively coupled plasma etching and wet chemical etching technique has been studied to define the final mesa shape before regrowth. By comparing the etching profiles of different non-selective etchants, we have obtained a controllable non-reentrant mesa shape with smooth sidewall by applying one step 2HBr:2H₃PO₄:K₂Cr₂O₇ wet etching. A high growth temperature of 680 ℃ is found helpful to enhance planar regrowth. By comparing the growth morphologies and simulating optical transmission along different directions, we determined that waveguides should travel across the regrowth interface along the [110] direction. The relation between growth rate and mask design has been extensively studied and the result can provide an important guidance for future mask design and vertical alignment between the active and passive cores. ECPD-SOA integrated device has been successfully achieved by this method without further regrowth steps and provided a responsivity of 7.8 A/W. The butt-joint interface insertion loss is estimated to be 1.05 dB/interface.     

Observation of macroscopic quantum tunneling in a single Bi2Sr2CaCu2O8+delta surface intrinsic Josephson junction

We report on the first unambiguous observation of macroscopic quantum tunneling (MQT) in a single submicron Bi(2)Sr(2)CaCu(2)O(8+delta) surface intrinsic Josephson junction (IJJ) by measuring its temperature-dependent switching current distribution. All relevant junction parameters were determined in situ in the classical regime and were used to predict the behavior of the IJJ in the quantum regime via MQT theory. Experimental results agree quantitatively with the theoretical predictions, thus confirming the MQT picture. Furthermore, the data also indicate that the surface IJJ, where the current flows along the c axis of the crystal, has the conventional sinphi current-phase relationship.     

Subharmonic shapiro steps and assisted tunneling in superconducting point contacts

We analyze the current in a superconducting point contact of arbitrary transmission in the presence of a microwave radiation. The interplay between the ac Josephson current and the microwave signal gives rise to Shapiro steps at voltages V = (m/n)h(omega)(r)/2e, where n,m are integer numbers and omega(r) is the radiation frequency. The subharmonic steps (n not equal 1) are a consequence of multiple Andreev reflections (MAR) and provide a signature of the peculiar ac Josephson effect at high transmission. Moreover, the dc current exhibits a rich subgap structure due to photon-assisted MARs.     

Generation of a uniform high-density microwave plasma for CO2 lasers using orthogonal electric fields

To realize a CO₂ laser using a fast-axial-flow high-output-power microwave discharge excitation, we devised a technology for making the microwave discharge uniform by varying the oscillation direction of an electric field with time. We also verified the effectiveness of this technology. As a result, we succeeded in increasing the discharge uniformity to 70% of the laser-tube cross-sectional area and realized a high laser output power and a high laser efficiency. In the case of a microwave input power of 1450 W, a maximum laser output power of 273 W and a laser efficiency of 18.8% were achieved; in the case of a microwave input power of 1070 W, a laser output power of 214 W and a laser efficiency of 20.0% were achieved. At the time of maximum output power, a high input power density of 280 W/cm³, which is approximately 20 times that in a dc discharge method, was achieved. Thus, a high-output-power microwave-discharge-excited CO₂ laser has become feasible. PACS 42.60.By; 52.80.Pi     

热噪声对约瑟夫森结I-V特性及微波感应台阶的影响研究

采用数值仿真的方法研究了热噪声对约瑟夫森结I-V特性及微波感应台阶的影响。研究表明,热噪声导致约瑟夫森结的I-V特性曲线呈现"圆拱化",也使得微波感应台阶高度减小。得出了取不同约瑟夫森结临界电流和结电阻时,热噪声对台阶的影响规律     

Ac Josephson effect in Corbino-geometry Josephson junctions constructed on Bi2Te3 surface

Recently, a Corbino-geometry type of Josephson junction constructed on the surface of topological insulators has been proposed for hosting and braiding Majorana zero modes. As a first step to test this proposal, we successfully fabricated Corbino-geometry Josephson junctions (JJs) on the surface of Bi₂Te₃ flakes. Ac Josephson effect with fractional Shapiro steps was observed in the Corbino-geometry JJs while the flux in the junction area was quantized. By analyzing the experimental data using the resistively shunted Josephson junction model, we found that the Corbino-geometry JJs exhibit a skewed current—phase relation due to its high transparency. The results suggest that Corbino-geometry JJs constructed on the surface of topological insulators may provide a promising platform for studying Majorana-related physics.     

Role of MW-ECR hydrogen plasma on dopant deactivation and open-circuit voltage in crystalline silicon solar cells

Plasma hydrogenation is an efficient method to passivate intergrain and intragrain defects of polycrystalline silicon (pc-Si) solar cells. The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system controlling different operating parameters such as microwave power (P _MW), process time (t _H) and hydrogenation temperature (T _H) for a fixed hydrogen flux of 30 sccm. The hydrogenation of n⁺pp⁺ pc-Si solar cells resulted in an improvement in the open-circuit voltage. The improvement was correlated with the dopant deactivation due to the formation of boron–hydrogen bonding. This was demonstrated from the changes in the doping level after hydrogenation of n⁺p diode structures made using single crystalline silicon as a reference material. It was found that deactivation of boron was more pronounced at high microwave plasma power, in good agreement with the high open-circuit voltage values obtained on pc-Si mesa cells. On the other hand, the effect of longer hydrogenation time and higher temperature resulted in a decrease of boron deactivation, while an increase in V _oc with a tendency of saturation at high T _H was observed. Reasons for such behavior were thoroughly explained.     

Effects of capacitive coupling on the escape rate in intrinsic Josephson junction stacks

We formulate a macroscopic quantum theory that can describe the macroscopic quantum tunneling (MQT) in intrinsic Josephson junctions (IJJs). The capacitively coupled IJJ model is quantized in terms of the canonical quantization method. The multi-junction effect for the MQT to the first resistive branch is clarified. It is shown that the escape rate is greatly enhanced by the capacitive coupling between junctions.