Josephson结跳变电流在不同温度下的统计分布

本工作提出了一种通过对时间的测量来获得单个Josephson结的跳变电流统计分布P(I)的电路,该电路具有很好的精度和速度.利用该电路我们首次从4.2K到30mK的不同温度下,对NbN/AlN/AbN结和Nb/AlOx/Nb结的跳变电流统计分布进行了测量,得到了一系列P(I).在一定的温度范围内,P(I)和理论值符合的很好.该电路结合其他的实验手段,将在今后研究Josephson结其他物理性质(例如Rabi振荡)中发挥很好的作用.     

微波感应直流电压现象的实验研究

本义报道了微波感应直流电压现象(或称逆交流约瑟夫逊效应)的某些实验研究。实验中采用的样品为具有良好迦滞的Nb-NbO_x-Pb谐振型隧道结;结与微波的耦合采用磁耦合方式,微波工作频率为x频带.在零电流偏置时,结的直流电压可锁定在一系列量子化电压值中的某一个。实验观测到2K时单结最大电压为0.6mV,电流台阶高度为200μA。实验结果与逆交流约瑟夫逊效应理论相符,并进一步支持了磁耦合的观点。     

超导约瑟夫森结物理参数的实验推算

超导约瑟夫森结是实现超导量子计算和微波单光子探测的核心器件,其物理参数很难直接测定.与之前常用的测量结微波激励效应估计方法不同,本文通过实验测量低频电流驱动下的约瑟夫森结Ⅰ-Ⅴ曲线及其跳变电流统计分布,并与基于标准电阻电容分路结模型数值模拟进行比对,推算出了约瑟夫森结的临界电流I_c、电容C、电阻R及阻尼参数β_c等物理参数.结果表明,所推算的参数值与基于微观理论推导所得到的Ambgaokar-Baratoff公式基本符合,可供约瑟夫森结的器件参数按需设计和制备工艺的参数设置等参考.     

金刚石膜α粒子探测器的电学性能研究

从外加偏压、预辐照处理等方面对三明治结构金刚石膜探测器在α粒子辐照下的电学性能进行了研究.电流-电压特性和脉冲高度分布测试和分析表明，金刚石膜探测器在能量为5.5MeV的²⁴¹Am α粒子辐照一定时间后，其暗电流有所增加.探测器顶电极施加负偏压时，在α粒子辐照下得到的净电流和信噪比均较大.Raman光谱测试表明，造成上述现象的原因很可能是金刚石膜厚度方向的不均匀性分布.负偏压下探测器对α粒子的能量分辨率为25.0%，优于正偏压下的能量分辨率(38.4%).随着α粒子辐照时间的延长，探测器的净电流和电荷收集效率均有明显增加. 关键词： 金刚石薄膜 辐射探测器 电学性能 脉冲高度分布     

硅烯/dx2-y2+idxy混合波超导隧道结隧道输运性质研究

运用Blonder-Tinkham-Klapwijk(BTK)理论研究了硅烯/dx2-y2 +idxy 混合波超导隧道结的隧穿性质.研究发现:垂直施加的电场、超导配对势的方向角和两种混合波配对能隙的比值Δ1/Δ0 强烈地影响正常反射、Andreev反射和隧穿电导的值;当两种混合波的序参量比值较大时,隧道谱线在外加偏压E =Δ1 处出现谐振峰;系统的隧穿电导、正常反射幅和Andreev反射幅随超导方向角成周期性变化,变化周期为π/2;由于dxy-波的存在,通过改变外加电场可以对隧穿电流加以调控。     

谐振腔中的超导弱连接在微波辐照下Josephson电流的行为

本文研究了微波辐照下置于谐振腔中的超导弱连接的Joscphson电流特性,并就微波频率与腔的本征频率公度与非公度两种情况进行了详细的理论计算。结果表明,外加的微波辐照总是抑制腔本征振荡的反馈作用。在非公度时,形式上类似于两微波的混频结果,但存在电流随外磁场在一个磁通量子φ_0内的多次阶跃;在公度时,只要微波幅度足够强,这种电流的小周期振荡就会消失。     

一种通过约瑟夫森结非线性频率响应确定微波耗散的方法

通过对电流偏置超导约瑟夫森结的微波驱动行为的研究,提出了一个确定约瑟夫森结微波耗散的方法.结的微波耗散由它的品质因子描述.微波耗散严重影响约瑟夫森器件如参量放大器、超导量子比特等的性能.对电流偏置的约瑟夫森结势阱采用四阶近似后,可以得到在较强微波驱动下约瑟夫森结非线性微波响应方程.该方程定量描述了非线性共振频率随外加微波功率变化关系:非线性共振频率与结等离子频率的差别依赖于约瑟夫森结的微波品质因子.对电流偏置的约瑟夫森结的微波运动行为进行了数值模拟.模拟结果确证了微波品质因子与非线性共振频率-等离子频率差别的定量关系可以应用于约瑟夫森结中.用这种非线性频率响应方法来确定约瑟夫森结的微波耗散没有严格的温度要求,可在单个电流偏置的结中完成,实验上具有简单可靠性.     

硅烯/d_(x~2-y~2)+id_(xy)混合波超导隧道结隧道输运性质研究

运用Blonder-Tinkham-Klapwijk (BTK)理论研究了硅烯/d_(x~2-y~2)+id_(xy)混合波超导隧道结的隧穿性质.研究发现:垂直施加的电场、超导配对势的方向角和两种混合波配对能隙的比值Δ_1/Δ_0强烈地影响正常反射、Andreev反射和隧穿电导的值;当两种混合波的序参量比值较大时,隧道谱线在外加偏压E=Δ_1处出现谐振峰;系统的隧穿电导、正常反射幅和Andreev反射幅随超导方向角成周期性变化,变化周期为π/2;由于d_(xy)-波的存在,通过改变外加电场可以对隧穿电流加以调控.     

Josephson结开关电流分布的测量方案探讨

Josephson结的开关电流存在着一定的分布.利用开关电流的分布,我们可以推算出Josephson结的逸出率.进一步结合合适的微波辐照,还可以获得结的诸如能级、拉比振荡等许多相关的量子特性.Josephson结的开关电流分布的获得,对于研究超导量子比特,包括相位量子比特、电荷量子比特、磁通量子比特和涡流量子比特以及他们的组合量子比特都有着重要意义.我们提出了三种测量方案,对这三种方案进行了比较,并初步的对自制的NbN/AlNx/NbN Josephson结的开关电流进行了多次(104次)测量,得到一定温度下的开关电流分布的直方图.针对三种方案各自的优缺点及已有的结果,我们提出了需要进一步改进的措施,对于下一步开展在极低温下(mK)Josephson结的开关电流分布的测量有着重要的意义.     

用于电压基准的Nb/NbxSi1-x/Nb约瑟夫森单结的研制

现代可编程约瑟夫森电压基准的核心器件是约瑟夫森结阵.目前最具有优势的约瑟夫森结阵是 Nb/Nb_xSi_1-x/Nb材料的结阵. Nb/Nb_xSi_1-x/Nb材料的约瑟夫森结 具有三层薄膜的制作过程简便, Nb和Nb_xSi_1-x刻蚀工艺相同以及Nb_xSi_1-x 势垒层成分可调等优点.中国计量科学研究院设计制作了Nb/Nb_xSi_1-x/Nb约瑟夫森单结. 通过在4.2 K低温下对所做单结进行直流电流-电压特性测量,观测到了清晰的超导隧穿电流和 从零电压态向电压态的跳变,最后就测量结果进行了分析讨论.此项工作属于国内首个开展 Nb/Nb_xSi_1-x/Nb材料约瑟夫森单结研究的工作.     

Multiphoton transitions between energy levels in a current-biased Josephson tunnel junction

The escape of a current-biased Josephson tunnel junction from the zero-voltage state in the presence of weak microwave radiation is investigated experimentally at low temperatures. The measurements of the junction switching current distribution indicate the macroscopic quantum tunneling of the phase below a crossover temperature of T small star, filled approximately 280 mK. At temperatures below T small star, filled we observe both single-photon and multiphoton transitions between the junction energy levels by applying microwave radiation in the frequency range between 10 and 38 GHz to the junction. These observations reflect the anharmonicity of the junction potential containing only a small number of levels.     

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。     

Quantum transitions of a small Josephson junction array

We have experimentally studied a small Josephson junction array in the presence of microwave irradiation. The array has comparable energy scales for single-charge effects and the Josephson effect, resulting in a discrete set of macroscopic eigenenergy levels. Excitation of the array by low-power microwaves is possible at frequencies where the photon energy matches the level spacing. The microwave frequency and amplitude dependence show that the excitation mechanism involves resonant quantum coherent dynamics of the array.     

THE SUPERCONDUCTING TUNNELLING CURRENT UNDER MICROWAVE IRRADIATION

In this article, the non-eguilibrium Green's function method developed by Keldysh^[1]and Larkin-Ovchinnikov^[2]is used to derive the expressions for quasi-particle and Josephson current of a superconducting junction under microwave irradiation. As an example, the results up to second order of irradiation field have been calculated. The influences of the microwave have been discussed briefly.     

Very high frequency spectroscopy and tuning of a single-cooper-pair transistor with an on-chip generator

A single-Cooper-pair transistor (SCPT) is coupled capacitively to a voltage biased Josephson junction, used as a high-frequency generator. Thanks to the high energy of photons generated by the Josephson junction, transitions between energy levels, not limited to the first two levels, were induced and the effect of this irradiation on the dc Josephson current of the SCPT was measured. The phase and gate bias dependence of energy levels of the SCPT at high energy is probed. Because the energies of photons can be higher than the superconducting gap we can induce not only transfer of Cooper pairs but also transfer of quasiparticles through the island of the SCPT, thus controlling the poisoning of the SCPT. This can both decrease and increase the average Josephson energy of the SCPT: its supercurrent is then controlled by high-frequency irradiation.     

Microwave-induced thermal escape in Josephson junctions

We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multivalued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.     

Phase escape of current-biased Josephson junctions

Switching current distributions of an Nb/Al--AlO_x/Nb Josephson junction are measured in a temperature range from 25~mK to 800~mK. We analyse the phase escape properties by using the theory of Larkin and Ovchinnikov (LO) which takes discrete energy levels into account. Our results show that the phase escape can be well described by the LO approach for temperatures near and below the crossover from thermal activation to macroscopic quantum tunneling. These results are helpful for further study of macroscopic quantum phenomena in Josephson junctions where discrete energy levels need to be considered.     

Quantum dissociation of a vortex-antivortex pair in a long josephson junction

The thermal and the quantum dissociation of a single vortex-antivortex (VAV) pair in an annular Josephson junction is experimentally observed and theoretically analyzed. In our experiments, the VAV pair is confined in a pinning potential controlled by external magnetic field and bias current. The dissociation of the pinned VAV pair manifests itself in a switching of the Josephson junction from the superconducting to the resistive state. The observed temperature and field dependence of the switching current distribution is in agreement with the analysis. The crossover from the thermal to the macroscopic quantum tunneling mechanism of dissociation occurs at a temperature of about 100 mK. We also predict the specific magnetic field dependence of the oscillatory energy levels of the pinned VAV state.