Heat propagation models for superconducting nanobridges at millikelvin temperatures

Nanoscale superconducting quantum interference devices (nanoSQUIDs) most commonly use Dayem bridges as Josephson elements to reduce the loop size and achieve high spin sensitivity. Except at temperatures close to the critical temperature T c, the electrical characteristics of these bridges exhibit undesirable thermal hysteresis which complicates device operation. This makes proper thermal analysis an essential design consideration for optimising nanoSQUID performance at ultralow temperatures. However the existing theoretical models for this hysteresis were developed for micron-scale devices operating close to liquid helium temperatures, and are not fully applicable to a new generation of much smaller devices operating at significantly lower temperatures. We have therefore developed a new analytic heat model which enables a more accurate prediction of the thermal behaviour in such circumstances. We demonstrate that this model is in good agreement with experimental results measured down to 100 mK and discuss its validity for different nanoSQUID geometries

Modelling of a Two-Signal SFQ Detection Scheme for the Readout of Superconducting Nanowire Single Photon Detectors

We present a two-signal single flux quantum (SFQ) detection scheme for the purpose of reading out two pixels of a superconducting nanowire single photon detector (SNSPD). The circuit is based on a coincidence buffer element which is able to output a signal when both of its input lines are triggered. The circuit model for the SNSPD element is simulated in SPICE and optimized to match the experimental SNSPD response data. The two-signal detection scheme is simulated using JSIM which allows for the simulation of Josephson junction elements in a circuit. We demonstrate a model of the two-signal circuit operating with two simulated SNSPD pixel inputs and investigate the response of the scheme when a phase shift is applied to one of the inputs. The scheme shows potential as a useful coincidence detector of single photons. We also present preliminary experimental results of nanobridge-based Josephson junctions to be used in the realization of the coincidence detector circuit. Evidence of the nanobridges exhibiting Josephson behavior (SQUID modulation) are presented

Scalable, Tunable Josephson Junctions and DC SQUIDs Based on CVD Graphene

Since the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions with graphene as the weak link can have their I-V properties easily tuned by the gate voltage. Most of the previous work on superconductor-graphene-superconductor (SGS) junctions and superconducting quantum interference devices (SQUIDs) were based on mechanically exfoliated graphene, which is not compatible with large scale production. Here, we show that SGS junctions and dc SQUIDs can be easily fabricated from chemical vapor deposition (CVD) graphene and exhibit good electronic properties. The SGS junctions can work without any hysteresis in their electrical characteristics from 1.5 K down to a base temperature of 320 mK, and the critical current can be effectively tuned by the gate voltage by up to an order of magnitude. As a result, dc SQUIDs made up of these junctions can have their critical current tuned by both the magnetic field and the gate voltage

A Key Marine Diazotroph in a Changing Ocean: The Interacting Effects of Temperature, CO2 and Light on the Growth of Trichodesmium erythraeum IMS101

Trichodesmium is a globally important marine diazotroph that accounts for approximately 60-80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt

Growth mechanism, microstructure, EPMA and Raman studies of pulsed laser deposited Nd1-xBa2+xCu3O7-delta thin films

Superconducting thin films of Nd1-xBa2+xCu3O7-delta have been grown on single crystal SrTiO3 substrates by pulsed laser deposition (PLD) using an off-stoichiometric Nd0.97Ba2.03Cu3O6 (Ba-rich NBCO) target. The statistical methods of Experimental design and regression analysis were used to understand and optimise the growth mechanism. The structural properties of both the target and the films were investigated by X-ray diffraction (XRD). The surface morphology of the films was examined by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Electron probe microanalysis (EPMA) using a scanning electron microscope equipped with wavelength-dispersive X-ray (WDX) spectrometers was used to carry out qualitative and quantitative analysis of both the target and films. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of both the target and the films. (c) 2005 Elsevier B.V. All rights reserved

Weak link nanobridges as single flux quantum elements

This paper investigates the feasibility of using weak link nanobridges as Josephson junction elements for the purpose of creating Josephson circuits. We demonstrate the development of a single-step electron beam lithography procedure to fabricate niobium nanobridges with dimensions down to $40\,\mathrm{nm}\times 100\,\mathrm{nm}$. The single-step process facilitates fabrication that is scalable to complex circuits that require many junctions. We measure the IV-characteristics (IVC) of the nanobridges between temperatures of $4.2$ and $9\,{\rm{K}}$ and find agreement with numerical simulations and the analytical resistively shunted junction (RSJ) model. Furthermore, we investigate the behaviour of the nanobridges under rf irradiation and observe the characteristic microwave-induced Shapiro steps. Our simulated IVC under rf irradiation using both the RSJ model and circuit simulator JSIM are in agreement with the experimental data. As a potential use of nanobridges in circuits requiring many junctions, we investigate the theoretical performance of a nanobridge-based Josephson comparator circuit using JSIM

Studies of growth, microstructure, IMP Raman spectroscopy and annealing effect of pulsed laser deposited Ca-doped NBCO thin films

Superconducting thin films of Ndl-xCaxBa2Cu3O7-delta (x = 0.03 and 0.08) have been grown on single crystal SrTiO3 substrates by pulsed laser deposition. The statistical methods of Experimental Design and regression analysis were used to optimize the film properties and to understand the correlation between the growth parameters and film properties. The orientation of the films was investigated by x-ray diffraction. The surface morphology of the films was examined by atomic force microscopy and scanning tunnelling microscopy. Qualitative and quantitative elemental analyses of the films were carried out using electron probe microanalysis. Micro-Raman spectroscopy was used to study the oxygen sublattice vibrations of the films. The effect of annealing oil the superconducting transition temperature of the patterned films was also studied

Focused Ion Beam NanoSQUIDs as Novel NEMS Resonator Readouts

Nano electromechanical systems (NEMS) represent an important new class of device with wide ranging applications. In this paper we report proposals and calculations for novel methods for excitation and readout of cantilever-style NEMS resonators which are applicable over a wide temperature range. We suggest a Lorentz force-based excitation method and discuss an ultrasensitive readout provided by a nanoSQUID, where the cantilever vibration modulates the inductance of the nanoSQUID loop, allowing potentially sub-picometer amplitude sensitivity to be achieved