Macroscopic quantum tunneling in "small" Josephson junctions in a magnetic field

We study the phenomenon of macroscopic quantum tunneling (MQT) in small Josephson junctions (JJ) with an externally applied magnetic field. The latter results in the appearance of the Fraunhofer type modulation of the current density along the barrier. The problem of MQT for a pointlike JJ is reduced to the motion of the quantum particle in the washboard potential. In the case of a finite size JJ under consideration, this problem corresponds to a MQT in a potential which itself, besides the phase, depends on space variables. The general expression for the crossover temperature T0 between thermally activated and macroscopic quantum tunneling regimes and the escaping time tau(esc) have been calculated.     

Band-structure modulation in carbon nanotube T junctions

We show that the band structure of metallic carbon nanotubes can be dramatically altered by the local electrostatic field. This is realized by coupling chemically functionalized nanotubes to form T junctions. The bar of the T is the conducting channel and the leg of the T is used for local gating. Transport measurements reveal that an energy gap develops upon application of a local electric field in both devices with or without linker molecules at the junction. We propose that the mechanism of the band gap modulation in the T junctions without linker molecules is the field effect, with the linker molecules introducing additional electromechanical and chemical effects.     

Anomalous behavior of superconducting-normal mesoscopic structures near T(c)

We observe a maximum in the conductance of Al/n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature (T(c)). This is the first observation of a peak in the conductance near the superconducting transition in superconducting-normal (S/N) junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low-temperature peak at temperatures on the order of the Thouless energy, we find a maximum near T(c). This peak has the same origin as the subgap conductance observed in S/N junctions at low temperatures.     

Degenerate ground state in a mesoscopic YBa2Cu3O(7-x) grain boundary Josephson junction

We have measured the current-phase relationship I(varphi) of symmetric 45 degrees YBa2Cu3O7-x grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T, a local minimum at a temperature T*. (ii) At T approximately T*, the first harmonic of I(phi) changes sign. (iii) For T相似文献   

Coulomb interaction and first-order superconductor-insulator transition

The superconductor-insulator transition (SIT) in regular arrays of Josephson junctions is studied at low temperatures. We derived an imaginary time Ginzburg-Landau-type action properly describing the Coulomb interaction. The renormalization group analysis at zero temperature T=0 in the space dimensionality d=3 shows that the SIT is always of the first order. At finite T, a tricritical point separates the lines of the first- and second-order phase transitions. The same conclusion holds for d=2 if the mutual capacitance is larger than the distance between junctions.     

Collective responses of Bi-2212 stacked junction to 100 GHz microwave radiation under magnetic field oriented along the <Emphasis Type="Italic">c</Emphasis>-axis

We studied a response of Bi-2212 mesa type structures to 100 GHz microwave radiation. We found that applying magnetic field of about 0.1 T across the layers enables to observe collective Shapiro step response corresponding to a synchronization of all 50 intrinsic Josephson junctions (IJJ) of the mesa. At high microwave power we observed up to 10th harmonics of the fundamental Shapiro step. Besides, we found microwave induced flux-flow step position of which is proportional to the square root of microwave power and that can exceed at high enough powers 1 THz operating frequency of IJJ oscillations. The article is published in the original.     

Tunnel spectroscopy into the YBa2Cu4O8 compound

Il Nuovo Cimento D - We have realized planar tunnel junctions using both Y123 and Y124 single crystals as base electrodes and Pb as counterelectrode. The crystals, with T c=90 K (Y123) and T c=80 K...     

Rectification properties of carbon nanotube "Y-junctions"

Quantum conductivity of single-wall carbon nanotube Y-junctions is calculated. The current versus voltage characteristics of these junctions show asymmetry and rectification, in agreement with recent experimental results. Furthermore, rectification is found to be independent of the angle between the branches of these junctions, indicating this to be an intrinsic property of symmetric Y-junctions. The implications for the Y-junction to function as a nanoscale molecular electronic switch are investigated.     

Transmission resonance via quantum bound states in confined arrays of antidots

We have studied the transmission resonances for a confined array of antidots, using the lattice Green's function method. Two kinds of resonant peaks via quasibound states are found. One kind of resonant peak corresponds to the split quasibound states. The split states originate from the superposition of quasibound states respectively localized in different (T or crossed) junctions, while the number of quasibound states in each junction is associated with the arm-width of the junction. Electrons in these split states are mainly localized in the junctions. The other kind of resonant peaks correspond to the high quasibound states which exist in (transverse and longitude) multi-period confined arrays of antidots. It is interesting to note that electrons in some of the high quasibound states are mainly localized in the intersection of the junctions rather than in the junctions themselves.     

Geometrically mediated breakup of drops in microfluidic devices

Microfluidic technology offers capabilities for the precise handling of small fluid volumes dispersed as droplets. To fully exploit this potential requires simultaneous generation of multiple size droplets. We demonstrate two methods for passively breaking larger drops into precisely controlled daughter drops using pressure-driven flow in simple microfluidic configurations: (i) a T junction and (ii) flow past isolated obstacles. We quantify conditions for breakup at a T junction and illustrate sequential breakup at T junctions for making small drops at high dispersed phase volume fractions.     

Field induced resistivity in crystalline YBa2Cu3O7-δ: evidence for nano-scale junction structures

We have carried out a series of measurements of the resistivity of a high quality single crystal of Y-Ba-Cu-O, near the superconducting transition temperature, as a function of field varying to a maximum of 5 T applied in the a-b plane of the crystal. The results of the measurements are described accurately in terms of a model for phase-slip resistivity given by Tinkham which features phase-slip dissipation in small Josephson junctions. Since this is a crystal, the junctions are not present as a consequence of grain boundaries, but are believed to be a consequence of defects in oxygen (vacancy) ordering in the charge reservoir layers. As a result, the junctions are believed to be intrinsic to the material. The defects which give rise to the junctions are likely candidates for intrinsic pinning centers. This model is compared to a vortex-liquid state model discussed by Chien et al.     

Gate Tunable Supercurrent in Josephson Junctions Based on Bi_2Te_3 Topological Insulator Thin Films

We report transport measurements on Josephson junctions consisting of Bi_2Te_3 topological insulator(TI) thin films contacted by superconducting Nb electrodes.For a device with junction length L=134 nm,the critical supercurrent I_c can be modulated by an electrical gate which tunes the carrier type and density of the TI film.I_c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state.In the p-type regime the Josephson current can be well described by a short ballistic junction model.In the n-type regime the junction is ballistic at 0.7 K T 3.8 K while for T 0.7 K the diffusive bulk modes emerge and contribute a larger I_c than the ballistic model.We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p-n junctions.Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.     

IV-Curves of tunnel junction arrays at lowered temperature by numerical simulation

We have numerically investigated the effect of lowered T on the performance of the Coulomb blockade thermometer, which has been demostrated to yield primary thermometry in the high T limit where thermal excitations compete with the Coulomb blockade. The thermometer is formed by an array of tunnel junctions whose differentiated IV—curve is used to determine the temperature. The IV—curves were calculated by computer simulations and we have obtained corrections to the analytic high T behaviour due to the lowered temperature.     

Tunneling into the normal state of Pr2-xCexCuO4

The temperature dependence of the tunneling conductance was measured for various doping levels of Pr(2-x)CexCuO4 using planar junctions. A normal state gap is seen at all doping levels studied, x=0.11 to x=0.19. We find it to vanish above a certain temperature T*. T* is greater than T(c) for the underdoped region and it follows T(c) on the overdoped side. This behavior suggests finite pairing amplitude above T(c) on the underdoped side.     

Observation of high coherence in Josephson junction qubits measured in a three-dimensional circuit QED architecture

Superconducting quantum circuits based on Josephson junctions have made rapid progress in demonstrating quantum behavior and scalability. However, the future prospects ultimately depend upon the intrinsic coherence of Josephson junctions, and whether superconducting qubits can be adequately isolated from their environment. We introduce a new architecture for superconducting quantum circuits employing a three-dimensional resonator that suppresses qubit decoherence while maintaining sufficient coupling to the control signal. With the new architecture, we demonstrate that Josephson junction qubits are highly coherent, with T2 ～ 10 to 20 μs without the use of spin echo, and highly stable, showing no evidence for 1/f critical current noise. These results suggest that the overall quality of Josephson junctions in these qubits will allow error rates of a few 10(-4), approaching the error correction threshold.     

Direct measurement of the Josephson supercurrent in an ultrasmall Josephson junction

We have measured the supercurrent flowing through a nonhysteretic, ultrasmall, voltage-biased Josephson junction. In contrast with experiments performed so far on hysteretic Josephson junctions, we find a supercurrent peak whose maximum I(s max) increases as the temperature T decreases. The asymptotic T = 0 value of I(s max) agrees with the junction Ambegaokar-Baratoff critical current, as predicted by theory.     

Terahertz responses of intrinsic Josephson junctions in high T(c) superconductors

High frequency responses of intrinsic Josephson junctions up to 2.5 THz, including the observation of Shapiro steps under various conditions, are reported and discussed in this Letter. The sample was an array of intrinsic Josephson junctions singled out from inside a high T(C) superconducting Bi2Sr2CaCu2O(8+x) single crystal, with a bow-tie antenna integrated to it. The number of junctions in the array was controllable, the junctions were homogeneous, the distribution of applied irradiation among the junctions was even, and the junctions could synchronously respond to high frequency irradiation.     

Distribution of supercurrent switching in graphene under the proximity effect

We study the stochastic nature of switching current in hysteretic current-voltage characteristics of superconductor-graphene-superconductor junctions. We find that the dispersion of the switching current distribution scales with temperature as σ(I) proportional to T(α(G)) with α(G) as low as 1/3. This observation is in sharp contrast to the known Josephson junction behavior where σ(I) proportional to T(α(J)) with α(J)=2/3. We propose an explanation using a generalized version of Kurkij?rvi's theory for the flux stability in rf-SQUID and attribute this anomalous effect to the temperature dependence of the critical current which persists down to low temperatures.     

双势垒磁性隧道结中量子阱共振隧穿效应的第一性原理理论

磁性隧道结材料中自旋相关的量子阱态所导致的共振隧穿现象具有很重要的研究和应用价值.文章介绍了最近在Fe(001)/MgO/Fe/MgO/Fe双势垒磁性隧道结中存在的量子阱共振隧穿效应的理论研究工作.通过量子阱态的第一性原理的计算以及结合对中间Fe薄膜孤岛结构所导致Coulomb阻塞效应的分析,证实了最近Nozaki等人(Nozaki T et al. Phys. Rev. Lett., 2006,96:027208 )实验中得到的振荡效应确实来源于中间Fe层多数自旋电子在Γ点处形成的Δ1对称性的量子阱态.     

The <Emphasis Type="Italic">c</Emphasis>-axis charge traveling wave in a coupled system of Josephson junctions

We demonstrate a manifestation of the charge traveling wave along the c axis (TW) in current voltage characteristics of coupled Josephson junctions in high-T _c superconductors. The branches related to the TW with different wavelengths are found for the stacks with different number of Josephson junctions at different values of system’s parameters. Transitions between the TW branches and the outermost branch are observed. The electric charge in the superconducting layers and charge-charge correlation functions for TW and outermost branches show different behavior with bias current. We propose an experimental testing of the TW branching by microwave irradiation.