Small Junctions Research Articles

Nonclassical Photon Pair Production in a Voltage-Biased Josephson Junction

We investigate electromagnetic radiation emitted by a small voltage-biased Josephson junction connected to a superconducting transmission line. At frequencies below the well-known emission peak at the Josephson frequency (2eV/h), extra radiation is triggered by quantum fluctuations in the transmission line. For weak tunneling couplings and typical Ohmic transmission lines, the corresponding photon-flux spectrum is symmetric around half the Josephson frequency, indicating that the photons are predominately created in pairs. By establishing an input-output formalism for the microwave field in the transmission line, we give further evidence for this nonclassical photon pair production, demonstrating that it violates the classical Cauchy-Schwarz inequality for two-mode flux cross correlations. In connection to recent experiments, we also consider a stepped transmission line, where resonances increase the signal-to-noise ratio.

NbSi Barrier Junctions Tuned for Metrological Applications up to 70 GHz: 20 V Arrays for Programmable Josephson Voltage Standards

PTB started using the robust NbxSi1-x barrier junction technology for the fabrication of large arrays for the Programmable Josephson Voltage Standard (PJVS) and for the Josephson Arbitrary Waveform Synthesizer 3 years ago. We demonstrate how Nb-doping of the amorphous Si barrier causes the transition from an underdamped to a desired overdamped junction behavior. Special dc SQUIDs have been used to evaluate junction capacitance and noise properties. The critical current of small junctions as a function of applied magnetic field has been investigated. On the basis of an existing 70 GHz design previously used for 10 V PJVS chips, we fabricated for the first time 20 V circuits with nearly 140 000 double-stacked Josephson junctions. A direct on-chip comparison between the two 10 V halves of the binary-divided array confirmed the metrological suitability of the 20 V circuits for dc and ac applications.

Nonequilibrium Green’s function method for quantum thermal transport

This review deals with the nonequilibrium Green’s function (NEGF) method applied to the problems of energy transport due to atomic vibrations (phonons), primarily for small junction systems. We present a pedagogical introduction to the subject, deriving some of the well-known results such as the Laudauer-like formula for heat current in ballistic systems. The main aim of the review is to build the machinery of the method so that it can be applied to other situations, which are not directly treated here. In addition to the above, we consider a number of applications of NEGF, not in routine model system calculations, but in a few new aspects showing the power and usefulness of the formalism. In particular, we discuss the problems of multiple leads, coupled left-right-lead system, and system without a center. We also apply the method to the problem of full counting statistics. In the case of nonlinear systems, we make general comments on the thermal expansion effect, phonon relaxation time, and a certain class of mean-field approximations. Lastly, we examine the relationship between NEGF, reduced density matrix, and master equation approaches to thermal transport.

A transparent electrode based on a metal nanotrough network

Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low infrared transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low conductivity. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω □(-1) at 90% transmission because of the high conductivity of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the number of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ~2Ω □(-1) at 90% transmission) and remarkable mechanical flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.

Asymmetric superconducting quantum interference devices for suppression of phase diffusion in small Josephson junctions

We have measured the switching current versus applied magnetic flux of a highly asymmetric, hysteretic dc superconducting quantum interference device (SQUID) from 30 mK to 400 mK. The SQUID loop had an inductance of 1 nH, and the capacitances of the two Al-Al2O3-Al Josephson junctions comprising the SQUID were 100 fF and 5 fF. As expected, the switching current of the SQUID displayed a small, periodic modulation with applied flux, from which the switching currents of the large and small junctions at 30 mK were determined to be 846.3 ± 0.1 nA and 0.7 ± 0.08 nA, respectively. Measurements of an identically prepared small junction exhibited a highly suppressed switching current (IS < 10 pA), consistent with phase diffusion in this device. The observed modulation in the SQUID thus demonstrates that the quantum mechanical uncertainty in the phase difference across the small junction was significantly reduced by coupling to the large junction through the loop inductance. We discuss the sensitivity of this technique for determining the gauge-invariant phase difference of small Josephson junctions, and the implications for imaging the phase of superconductors at the atomic scale using scanning tunneling microscopy.

Tu1742 Human Small Intestinal Barrier Function and Tight Junction Integrity During Ischemia Reperfusion

interacts with the E-cadherin mRNA and regulates its expression posttranscriptionally, thus implicating in control of the epithelial barrier function. Methods: Caco-2 cells were used in these studies. The binding of HuR with E-Cadherin mRNA was examined by biotin pulldown assays and ribonucleoprotein/IP analysis. E-cadherin mRNA stability was examined by measuring its half-life with real-time PCR analysis, and its translation was examined by chimeric luciferaseE-cadherin 3'UTR reporter assays. HuR function was investigated by its gene silencing and overexpression. Barrier function was detected by paracellular tracer flux assay and trans epithelial electric resistant. Results: HuR bound to the E-cadherin mRNA by directly interacting with the E-cadherin 3'UTR rather than its 5'UTR and coding region. Ectopic overexpression of HuR by transient transfection increased [HuR/ E-cadherin mRNA] complex, enhanced E-cadherin translation (by~3 times), and up-regulated its protein level (by~3 times), but it failed to alter E-cadherin mRNA stability and total E-cadherin mRNA level. In contrast, silencing HuR decreased HuR/ E-cadherin mRNA association, inhibited E-cadherin translation, and down-regulated its protein abundance without affecting the stability and level of E-cadherin mRNA. Ectopic overexpression of HuR also promoted the epithelial barrier function as indicated by a decrease in paracellular permeability. Decreased E-cadherin translation by silencing HuR disrupted the barrier function and increased paracellular permeability. Conclusions: These results indicate: 1) E-cadherin mRNA is a target of HuR; 2) increased HuR/ E-cadherin mRNA association enhances E-cadherin mRNA translation; and 3) HuR plays an important role in the regulation of intestinal epithelial barrier function.

Abstract 519: Attenuation of mammary tumor growth by gap junction enhancer.

Abstract Normal cells have functional gap junctional intercellular communication (GJIC), while cancer cells exhibit many defects in cellular communication that contribute to the loss of tissue homeostasis. It is believed that restoring GJIC is linked to drug sensitivity and reduction of tumorigenicity. Previous studies show that GJIC and neoplastic cellular growth are inversely related and small molecule targeting gap junction can alter tumor growth. A dose of 25 mg/kg gap junction enhancer (PQ) can cause a 77% decrease of xenograft T47D breast tumors. In addition these small molecules can induce a decrease of tumor growth by 110%, 129%, and 73% in the pre, early, and late stages of tumor development of the transgenic mouse strain, FVB/N-Tg(MMTV-PyVT)634Mul/J. Western blot analysis shows that the gap junction protein, Cx43, was highly expressed in the PQ-treated tumors compared to the controls. Data suggests that enhancement of GJIC attenuates tumor growth; specifically Cx43 plays a significant role in tumorigenesis. Establishment of a continuous line of mammary tumor cells from the malignant neoplasm of a female transgenic mouse demonstrates that PQs can cause an increase of Cx43 expression and subsequently trigger a decrease of cell viability. Overall, targeting gap junction by small molecule PQ has shed light to the dynamic of connexins and gap junctions in mammary tumors. Citation Format: Stephanie N. Shishido, Thu A. Nguyen. Attenuation of mammary tumor growth by gap junction enhancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 519. doi:10.1158/1538-7445.AM2013-519

A Centrifugal Microfluidics Platform for Potential Application on Immobilization-Free Bead-Based Immunoassays

In this work, we demonstrate a simple yet convenient centrifugal microfluidics, or lab-on-a-disc (LOAD) platform for bead-based immunoassays. The disc contains a network of passive valves for sample manipulation and a bead-aggregating barrier structure to instead of commonly used immobilization method. The narrow-channel structure not only provides a geometric barrier for retaining the beads, but also significantly increased fluidic resistance which results in lower flow rate and longer reaction time for each reagent in the reaction chamber. Centrifuge-forced beads densification in the small junction area and signal concentrating allows high signal sensing capability. The acceleration speed is optimized for largest signal-to-noise ratio. The device uses different rotation speed to trigger a cascade of flow events. The integrated platform contains all the necessary functional elements for conducting one-step parallel assays through a sequential protocol. For our test device, the total time required to complete an immunoassay experiment is typically under 6 minutes. Through this work we have also shown that the use of a geometric barrier for the aggregation of signaling beads is an effective approach for the enhancement of optical signals in common immunoassays. This flexible design is suitable for a broad range of bead-based immunoassays with low-cost and high-speed operation.

Visualization of STIM1 and Orai1 by Electron Microscopy

STIM1 and Orai1 constitute the core machinery of store-operated calcium entry. We used transmission and freeze-fracture electron microscopy to visualize these proteins at endoplasmic reticulum (ER)-plasma membrane (PM) junctions. In thin sections, ER formed frequent extensive cortical junctions in HEK-293 cells overexpressing STIM1 and Orai1 and exposed to thapsigargin (TG), but small and rare junctions in control cells. Extended molecules, most likely STIM1, decorated the ER and bridged the ER-PM gap. Freeze-fracture replicas of cells treated in the same manner showed elevated PM domains (“puncta”) packed with particles at a higher density than on the surface of control cells (2564±443/µm2 versus 1863±331/µm2). Puncta varied in size (∼0.01 to 0.45 µm2) and frequency. Clusters of particles in puncta typically exhibited a clear sharp border, but particle density sometimes declined more gradually to background. Since the puncta most likely correspond to sites of STIM1-Orai1 co-clustering at ER-PM junctions, targeting to puncta was used to guide the classification of particles potentially representing Orai1 channels based upon structural parameters. A high percentage of particles in puncta exhibited a common signature; “large and tall” particles were present at a significantly higher density within puncta than in untransfected cells (average 1300±309 versus 225±84/µm2). Three characteristics of these particles are similar to the tetrameric CaV1.1 channels of skeletal muscle: 1) diameter, only slightly smaller than that of CaV1.1; 2) unusual height indicated by the platinum free “shadow”; 3) square, slightly scalloped outline of the same shadow in some images. It is reasonable to postulate that the “large and tall” particles represent tetrameric Orai1 channels clustered in correspondence with cortical Stim-bearing ER junctions. These observations introduce a new approach to visualize individual unlabeled Stim and Orai molecules in situ. Funded by NIH RO1 HL-48093 (CFA) and NS-14609 (MDC).

Increased plasma zonulin in patients with sepsis

Introduction:Zonulin is a eukaryotic protein structurally similar to Vibrio cholerae’s zonula occludens toxin. It plays an important role in the opening of small intestine tight junctions. The loss of gut wall integrity during sepsis might be pivotal and has been described in various experimental as well as human studies. Increased levels of zonulin could be demonstrated in diseases associated with increased intestinal inflammation, such as celiac disease and type 1 diabetes. We therefore investigated the role of plasma levels of zonulin in patients with sepsis as a non-invasive marker of gut wall integrity.Materials and methods:Plasma level of zonulin was measured in 25 patients with sepsis, severe sepsis or septic shock according to ACCP/SCCM criteria at the first day of diagnosed sepsis. 18 non-septic post-surgical ICU-patients and 20 healthy volunteers served as control. Plasma levels were determined by using commercially available ELISA kit. Data are given as median and interquartile range (IQR).Results:Significantly higher plasma concentration of zonulin were found in the sepsis group: 6.61 ng/mL (IQR 3.51–9.46), as compared to the to the post-surgical control group: 3.40 ng/mL (IQR 2.14–5.70) (P = 0.025), as well as to the healthy group: 3.55 ng/mL (IQR 3.14–4.14) (P = 0.008).Conclusion:We were able demonstrate elevated levels of plasma zonulin, a potential marker of intestinal permeability in septic patients. Increased zonulin may serve as an additional mechanism for the observed increased intestinal permeability during sepsis and SIRS.

Strong-coupling BCS models of Josephson qubits

The strong-coupling version of the BCS theory for superconductors is used to derive microscopic models for all types of small Josephson junctions—charge qubit, flux qubit and phase qubit. Applied to Josephson qubits it yields a more complicated structure of the lowest-lying energy levels than that obtained from phenomenological models based on quantization of the Kirchhoff equations. In particular, highly degenerate levels emerge, which act as probability sinks for the qubit. The alternative formulae concerning spectra of superconducting qubits are presented and compared with the experimental data. In contrast to the existing theories those formulae contain microscopic parameters of the model. In particular, for the first time, the density of Cooper pairs at zero temperature is estimated for an Al-based flux qubit. Finally, the question whether small Josephson junctions can be treated as macroscopic quantum systems is briefly discussed.

Photon shot noise dephasing in the strong-dispersive limit of circuit QED

We study the photon shot noise dephasing of a superconducting transmon qubit in the strong-dispersive limit, due to the coupling of the qubit to its readout cavity. As each random arrival or departure of a photon is expected to completely dephase the qubit, we can control the rate at which the qubit experiences dephasing events by varying in situ the cavity mode population and decay rate. This allows us to verify a pure dephasing mechanism that matches theoretical predictions, and in fact explains the increased dephasing seen in recent transmon experiments as a function of cryostat temperature. We observe large increases in coherence times as the cavity is decoupled from the environment, and after implementing filtering find that the intrinsic coherence of small Josephson junctions when corrected with a single Hahn echo is greater than several hundred microseconds. Similar filtering and thermalization may be important for other qubit designs in order to prevent photon shot noise from becoming the dominant source of dephasing.

Drag Current Reversal in Capacitively Coupled Arrays of Small Josephson Junctions

The current-voltage characteristics of two long capacitively coupled one-dimensional arrays of small Josephson junctions were measured, where the coupling capacitance was comparable with the capacitance of the junction. The current in both arrays was simultaneously measured at various applied magnetic fields with the voltage applied to only one of the arrays. The current was induced in the unbiased array beyond the threshold voltage for the Coulomb blockade of the biased array. The direction of the induced current is the same as that of the current in the other array when the applied magnetic field is small and Cooper-pair tunneling is dominant, while it changes to the opposite direction when the applied magnetic field is large and single-electron tunneling is dominant. This suggests that the current induction mechanism between arrays of small Josephson junctions is substantially different from that in arrays of small normal tunnel junctions.

Multipole Surface Plasmon Resonances in Conductively Coupled Metal Nanowire Dimers

We report on the experimental and the theoretical investigation of multipole surface plasmon resonances in metal nanowires conductively connected by small junctions. The influence of a conductive junction on the resonance energies of nanowire dimers was simulated using the finite element method based software CST Microwave Studio and experimentally measured by electron energy-loss spectroscopy in a transmission electron microscope. We extend the analysis of conductively connected structures to higher order multipole modes up to third order, including dark modes. Our results reveal that an increase in junction size does not shift significantly the antibonding modes, but causes a strong blue shift of the bonding modes, leading to an energetic rearrangement of the modes compared to those of a capacitively coupled dimer with similar dimensions.

Turn-on and turn-off voltages of an avalanche p—n junction

Characteristics of the turn-on and turn-off voltage of avalanche p—n junctions were demonstrated and studied. As opposed to existing reports, the differences between the turn-on and turn-off voltage cannot be neglected when the size of the p—n junction is in the order of microns. The difference increases inversely with the area of a junction, exerting significant influences on characterizing some parameters of devices composed of small avalanche junctions. Theoretical analyses show that the mechanism for the difference lies in the increase effect of the threshold multiplication factor at the turn-on voltage of a junction when the area of a junction decreases. Moreover, the “breakdown voltage" in the formula of the avalanche asymptotic current is, in essence, the avalanche turn-off voltage, and consequently, the traditional expression of the avalanche asymptotic current and the gain of a Geiger mode avalanche photodiode were modified.

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.

Spin transport in ferromagnetic/normal-metal tunnel junction arrays

An array of alternating ferromagnetic and normal-metal islands separated by small tunnel junctions is theoretically investigated in the sequential tunneling regime. A numerical Monte Carlo method is used to calculate the transport properties. The spin-dependent tunneling currents give rise to nonequilibrium spin accumulation on the normal island. The tunneling magneto resistance (TMR) is calculated for a large range of array parameters. The TMR oscillates with bias voltage and can become negative for certain array parameters. We show that the long-range electrostatic interaction in the arrays can significantly increase the TMR; for experimentally accessible parameters the magnitude of the TMR can be as large as 100$%$.

Charge fluctuations in single-electron tunneling oscillations

It has been predicted that in the presence of a sufficiently high-dissipative environment transport in a small tunnel junction can become extremely regular, giving rise to the phenomenon of single-electron tunneling oscillations. Recent progress in detection of high-frequency current fluctuations and the interest in single-electron sources motivate further investigations on the expected accuracy of the charge oscillations as a function of the impedance of the environment. In this paper we study theoretically the charge-fluctuation spectrum at finite frequency for the system at hand, and investigate its evolution as a function of the external impedance. The evolution and the disappearance of the single-electron oscillations peak is described by analytical and numerical methods.

Highly sensitive miniature SQUID magnetometer fabricated with cross-type Josephson tunnel junctions

We report on a new method suitable for the fabrication of highly sensitive SQUID magnetometers exhibiting very small effective areas. In contrast to commonly used approaches, which mostly make use of constrictions in a superconducting material to form the Josephson junctions, we have been able to fabricate such sensors with SIS cross-type Josephson tunnel junctions in an Nb/AlOx/Nb trilayer process. The small junction size and vanishing idle region of our cross-type junctions and a therefore very low capacitance lead to a high sensitivity of the resulting SQUIDs. First results on fabricated miniature SQUID magnetometer within this technology are shown. We found, that even with SQUID loop dimensions of about 10×10μm2 very low flux noise levels could be achieved, which clearly demonstrate the high potential of our approach. Furthermore we estimate the required SQUID parameter which may allow the detection of single electron spin-flips with such devices in the future.

Closing the gap on liver toxicity

U.S. researchers have identified a small molecule gap junction inhibitor that protects mice from drug-induced liver toxicity. The team has founded Heprotech to further characterize and optimize the inhibitor.