The Putative Liquid-Liquid Transition is a Liquid-Solid Transition in Atomistic Models of Water

We use numerical simulation to examine the possibility of a reversible liquid-liquid transition in supercooled water and related systems. In particular, for two atomistic models of water, we have computed free energies as functions of multiple order parameters, where one is density and another distinguishes crystal from liquid. For a range of temperatures and pressures, separate free energy basins for liquid and crystal are found, conditions of phase coexistence between these phases are demonstrated, and time scales for equilibration are determined. We find that at no range of temperatures and pressures is there more than a single liquid basin, even at conditions where amorphous behavior is unstable with respect to the crystal. We find a similar result for a related model of silicon. This result excludes the possibility of the proposed liquid-liquid critical point for the models we have studied. Further, we argue that behaviors others have attributed to a liquid-liquid transition in water and related systems are in fact reflections of transitions between liquid and crystal

Electromagnetic performance comparisons of 0.85 THz integrated bias-tee SIS mixers with twin-junction and end-loaded tuning schemes

We compare the design of two 0.85 THz SIS mixers fed with a radial probe antenna aligned to the E-Plane of the input full-height rectangular waveguide connected to a drilled smooth-walled horn. Both designs employ the same 0.5 µm2 hybrid Nb/AlN/NbN tunnel junction technology, sandwiched between a NbTiN ground and aluminium wiring layer fabricated on top of a 40 µm quartz substrate. The two designs is differed by how we tune out the unwanted junction capacitance for broadband performance. The first design uses the commonly-used twin-junction tuning scheme; whilst the second design utilises an end-loaded scheme. We successfully achieve close to 2× the double sideband quantum noise performance for both schemes, but the twin-junction design is less sensitive to fabrication accuracy of planar circuit components utilised. However, the end-loaded design offers a much better IF bandwidth performance, almost twice wider than the twin-junction design. The need for an ultra-wide IF bandwidth mixer is becoming more pressing and important for the future and up-coming upgrades of various millimetre (mm) and sub-mm astronomical instruments, hence we conclude that the end-loaded design is a better solution for the THz heterodyne mixing applications

Submillimeter superconducting integrated receivers: Fabrication and yield

Fabrication procedure and yield analysis of superconducting integrated receivers is reported. These chip receivers, apart from the quasi-optical SIS mixers, contain internal local oscillators and associated rf and de interfaces. Due to both complexity and design requirements of the integrated circuit, certain restrictions are applied to the standard Nb/Al/AlxOy/Nb SNEAP process. To obtain accurate area for micron-size SIS junctions and thickness for multi-layer SiO2 insulation, a few solutions and modifications were developed. The possibility of transfering this fabrication process worldwide has been proven experimentally

Forward and backward waves in Cherenkov flux-flow oscillators

Josephson flux-flow oscillators (FFOs) have been used as an on-chip local oscillator at frequencies up to 650 GHz. An autonomous FFO linewidth of about 1 MHz was measured in the resonant regime at V-b <950 mu V for niobium-aluminium oxide-niobium tunnel junctions, while considerably larger values were reported at higher voltages. To overcome this fundamental linewidth broadening we propose an on-chip Cherenkov radiation Aux-flow oscillator (CRFFO). It consists of a long Josephson junction and a superconducting slow-wave transmission line that modifies significantly the junction dispersion relation. Two superconductor-insulator-superconductor junction detectors are connected to both the long Josephson junction and the slow-wave line to determine the available microwave power. The power is measured at different CRFFO biasing conditions. Both a forward wave and a backward wave oscillation regime are observed. An FFO and a CRFFO with the same junction parameters are compared

Superconducting chip receivers for imaging application

Experimental details of a unique superconducting imaging array receiver are discussed. Each pixel contains an internally pumped receiver chip mounted on the back of the elliptical microwave lens. Each chip comprises a quasi-optical SIS mixer integrated with a superconducting flux-flow oscillator (FFO) both fabricated from the same Nb/AlOx/Nb trilayer on a silicon substrate. Properties of the integrated lens antenna were studied using an externally pumped reference SIS mixer which showed antenna sidelobes below -17 dB and a receiver double side band noise temperature, T-RX(DSB), below 100 K within the frequency range 460 - 500 GHz that is close to the quantum noise. For the imaging array T-RX(DSB) = 150 K has been measured at 500 GHz using the internal flux-flow oscillator as a local oscillator (LO). A balanced SIS mixer was tested showing T-RX(DSB) <100 K within the range of 480 - 510 GHz using the internal LO. A computer system was developed to control simultaneously the de bias of the SIS mixer and the frequency and power provided by FFO. The system also performs automatic optimization of the receiver noise temperature