Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.     

Superconducting quantum interference device without Josephson junctions

A new type of a superconducting quantum interference device (SQUID) based on a single superconducting loop without Josephson junctions and with asymmetric contacts has been proposed. This SQUID offers advantages in simplicity of fabrication and a steeper dependence of measured quantities on the magnetic flux. To confirm the possibility of making this type of SQUID, the magnetic field dependence of the critical current in an aluminum ring with asymmetric contacts has been experimentally investigated.     

Confinement into a state with persistent current by thermal quenching of loop of Josephson junctions

We study a loop of Josephson junctions that is quenched through its critical temperature. For three or more junctions, symmetry breaking states can be achieved without thermal activation, in spite of the fact that the relaxation time is practically constant when the critical temperature is approached from above. The probability for these states decreases with quenching time, but, for a large number of junctions, cooling does not have to be fast. For this case, we evaluate the standard deviation of the induced flux. Our results are consistent with the available experimental data.     

Observation of spontaneous flux generation in a multi-josephson-junction loop

We describe observations of spontaneous flux generation inside a YBa(2)Cu(3)O(7-delta) loop made of 214 Josephson junctions in series. The flux is generated spontaneously during cooldown into the superconducting state. The experiment is motivated by the Kibble-Zurek scenario of formation of topological defects in condensed matter systems. The transition from decoupled superconducting segments into a coherent loop is determined by the strength of thermal fluctuations in the junctions. Values of the flux measured at the end of each cooldown follow a normal distribution, and are consistent with the instantaneous phase differences across the junctions adding up as the loop becomes coherent.     

Superconducting Josephson structures with high linearity of transformation of magnetic signal into voltage

Possible approaches to the creation of Josephson superconducting circuits with a high linearity of transformation of a magnetic signal into voltage and a wide dynamic range have been described. Realization of such circuit by the creation of a special spatial inhomogeneity in chains of Josephson junctions and connection of these chains into a differential circuit has been considered. A new method for linearizing the transfer function of a single two-junction superconducting quantum interference device, which is an elementary cell of the chain, by connecting to it an additional loop playing the role of a nonlinear transformer of the magnetic flux has been described. For both approaches, the basic analytical expressions describing the synthesized circuits are presented. The data on numerical calculation and experimental measurements of samples produced on the basis of niobium technology with the critical current density of Josephson junctions of 4.5 kA/cm² are reported. The approaches developed in the work can serve as a basis for the creation of highly efficient gigahertz superconducting amplifiers for cellular and satellite communications and can be applied for improving the characteristics of SQUID-based devices.     

General Relativistic Thermoelectric Effects in Superconductors

We discuss the general-relativisticcontributions which occur in the electromagneticproperties of a superconductor with a heat flow. Theappearance of a general-relativistic contribution to themagnetic flux through a superconducting thermoelectricbimetallic circuit is shown. The response of theJosephson junctions to a heat flow is investigated inthe general-relativistic framework. Somegravitothermoelectric effects which are observable in thesuperconducting state in the Earth's gravitational fieldare considered.     

Attojoule calorimetry of mesoscopic superconducting loops

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system--a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of approximately 20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with a period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants kB.     

Josephson effect without superconductivity: realization in quantum Hall bilayers

We show that a quantum Hall bilayer with the total filling nu = 1 should exhibit a dynamical regime similar to the flux flow in large Josephson junctions. This analogy may explain a conspicuous peak in the interlayer tunneling conductance [Phys. Rev. Lett. 84, 5808 (2000)]. The flux flow is likely to be spatiotemporally chaotic at low-bias voltage, which will manifest itself through broadband noise. The peak position can be controlled by an in-plane magnetic field.     

Experimental investigation of magnetohydrodynamic action on a heat flux toward the surface of a model

Experimental data for magnetohydrodynamic (MHD) action on a supersonic nitrogen flow about an axisymmetric model are presented. The experiments were carried out in the Big Shock Tube (Ioffe Physical-Technical Institute), at the end of which a test section equipped with a supersonic nozzle was mounted. A test conic model coupled with a cylinder is attached to the output cross section of the nozzle. A magnetic field is produced by a solenoid placed on the cylindrical part of the model through which a pulsed current due to an external voltage source discharging passes. Electrodes on the conic part of the model initiate a gas discharge, which rotates about the axis of the model in the solenoidal magnetic field. The influence of the magnetic field on the gasdynamic pattern of the flow near the model and on the heat flux toward its surface is investigated. Schlieren patterns of the flow about the model, photographic scans of the discharge glow, and heat flux measurements are taken. It is found that the magnetic field has an effect on the gasdynamic pattern of the flow near the model and on the heat flux toward its surface. The dependence of MHD effects on the external voltage polarity is also revealed.     

Superradiant dissipative tunneling in a double p–i–n semiconductor heterostructure with thermal injection of electrons

We propose a semiconductor device with two p–i–n junctions maintained at two different temperatures. When the current injected in the device due to this temperature difference exceeds a threshold value, a superradiant field is created in the first gate that induces an additional current in the second gate. The injection current is amplified by this reaction loop. In this way, the heat flow between the two junctions is partially transformed in superradiant power.     

环形管环状流临界热负荷解析模型

根据环形管内流动与传热的特点,重新定义了环形管的水力直径和热周直径,基于圆管环状流CHF解析模型,得到了环形管内环状流临界热负荷解析模型。该模型可以预测环形管内沸腾两相流环状流时在不同加热方式下发生的临界热负荷,加热方式包括有内侧加热、外侧加热、两侧同时加热等。数值预测结果与文献数据比较表明本文的解析模型在P=0.57-15.01.MPa,G=198-3789.5 kgm-2s-1,xc=0.068-0.855,L/Dhe=19.3-539.5范围内预测良好。     

环路热虹吸管间歇沸腾可视化实验研究

针对两相环路热虹吸管中出现的间歇沸腾不稳定现象,分别以R134a、水和无水乙醇作为工质,通过流场可视化实验观测,探究了间歇沸腾出现的条件及其对环路传热特性的影响.实验结果表明,在中等充液率和中等加热热流密度条件下更容易发生间歇沸腾现象;流型的周期性变化引起环路内部压力和温度波动,同时会增加环路的均温性;流型变化和波动特...     

Observation of flux qubit states with the help of a superconducting differential double contour interferometer

The critical current of a new structure, the superconducting differential double contour interferometer (DDCI), investigated recently, depends on the parity of the sum of quantum numbers of the two superconducting loops connected in two points by two Josephson junctions. The theory confirms that the DDCI structure can be used for the ideal readout of quantum states of the flux qubit. Large jumps in the critical current and voltage enables to observe continuously the change in time the state of the flux qubit. Such observations can have fundamental importance for the investigation of macroscopic quantum systems with strongly discrete spectrum such as the flux qubit. The DDCI structure can also be used for precise measurement of a very weak magnetic field.     

Noise properties of SQUIDs with Superconductor-Semiconductor-Superconductor proximity effect Josephson junctions

SQUIDs using Superconductor-Semiconductor-Superconductor (SSmS) proximity effect Josephson junctions were prepared and noise measurements were carried out. Since SSmS junctions are basic elements of Josephson field effect transistors (JoFETs), information about dynamic properties of JoFETs can be gained in this way. A planar geometry was used for the SSmS junctions, with a single crystalline silicon wafer acting as both, substrate and proximity layer. Rf- and dc-SQUIDs could be realized. When the SQUIDs were operated in a flux locked loop, flux noise values comparable to conventional tunnel junction SQUIDs were measured.     

Numerical investigation of subcooled boiling characteristics of magnetic nanofluid under the effect of quadrupole magnetic field

This paper investigates numerically the characteristics of subcooled flow boiling of a magnetic nanofluid (refrigerant-113 and 4 vol% Fe₃O₄) in a vertical annulus, which is exposed to a nonuniform transverse magnetic field generated by the quadrupole magnet. A control volume technique and SIMPLEC algorithm have been used for discretizing the governing equations and pressure-velocity coupling, respectively. The two-fluid model has been used to simulate subcooled flow boiling of the refrigerant-113. The results indicate that subcooled flow boiling characteristics change not only by using nanofluid as the working fluid, but also by applying the nonuniform transverse magnetic field. In the presence of the aforementioned magnetic field due to the Kelvin force, the fluid attracted to the outer wall. This leads to higher bubble detachment frequency so that the heat pumping is increased and the void fraction on the heated wall is decreased. Thus, the critical heat flux as one of the most important parameters in boiling processes will be increased.     

Superconducting quantum interference devices made of Sb-doped Bi2Se3 topological insulator nanoribbons

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) made of Sb-doped Bi₂Se₃ topological insulator (TI) nanoribbon (NR) contacted with PbIn superconducting electrodes. When an external magnetic field was applied along the NR axis, the TI NR exhibited periodic magneto-conductance oscillations, the so-called Aharonov-Bohm oscillations, owing to one-dimensional subbands. Below the superconducting transition temperature of PbIn electrodes, we observed supercurrent flow through TI NR-based SQUID. The critical current periodically modulates with a magnetic field perpendicular to the SQUID loop, revealing that the periodicity corresponds to the superconducting flux quantum. Our experimental observations can be useful to explore Majorana bound states (MBS) in TI NR, promising for developing topological quantum information devices.     

并联管两相流不稳定性的研究

在高压汽水回路上对垂直并联管中汽液两相流不稳定性进行了试验研究．确定了压力、质量流速、入口过冷度、热负荷及热负荷不对称分布、进口及出口节流、可压缩容积等对不稳定性的影响．得出了压力降型和密度波型不稳定性的界限，给出了并联管中计算不稳定性起始条件的无因次方程，为大型直流锅炉和蒸汽发生器的设计提供依据．     

超临界二氧化碳在突扩管中对流换热的流动特性研究

对流动分岔后的超临界二氧化碳在平面对称突扩管中进行强迫对流换热进行了数值模拟,研究了热流密度在流体发生流动分岔现象后流动特性的影响。计算结果表明:随着热流密度的增加,临界雷诺数和转换雷诺数减小,流动稳定性遭到削弱;对应于相同的雷诺数,由于流动分岔引起的不对称压力分布随着热流密度的增加对应于突扩管上、下半部有不同变化规律,这使得对应回流区的大小分别减小和增大。     

Ferromagnetic 0–<Emphasis Type="BoldItalic">π</Emphasis> Josephson junctions

We present a study on low-T_c superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop of either 0 or π. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0–π Josephson junction, in which 0 and π ground states compete with each other. At a certain temperature the 0 and π parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum Φ₀. PACS  74.50.+r; 74.78.Fk; 74.81.-g; 85.25.Cp