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We have developed a 330-370GHz SIS mixer for small-format, heterodyne, astronomical imaging arrays. Fixed-tuned broadband operation is achieved by means of a superconducting radial waveguide probe. A horn-reflector antenna provides high-efficiency optical coupling. Using a variable-temperature cryogenic noise source, we measured a DSB system noise temperature of 32±1K. The mixer contributes 3±3K, supporting the theoretically-predicted result that the noise temperature of a DSB mixer can be less than (Latin small letter hwith stroke)ω/2k (8.6K). © 2001 Plenum Publishing Corporation.

We have investigated transport through a channel with superconducting boundaries where-electrons are confined by Andreev reflection. Superconducting phase-sensitive transport is discussed using qualitative arguments. The transfer resistance R(t) through the channel as well as the resistance R(i) between the injector and the channel have been measured as a function of the phase difference phi Between the superconducting boundaries. It is not yet clear that the observed oscillation in resistance can be attributed to quasi-particle phase-sensitive transport. The oscillation amplitude in R(t) due to the transmitted current modulation is less than 0.1% in this experiment.

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We measure the full supercurrent-phase relation of a controllable pi junction around the transition from the conventional 0 state to the pi state. We show that around the transition the Josephson supercurrent-phase relation changes from I-sc similar or equal to I(c)sin(phi) to I-sc similar or equal to I(c)sin(2phi). This implies that, around the transition, two minima in the junction free energy exist, one at phi = 0 and one at phi = pi, whereas only one minimum exists at the 0 state (at phi = 0) and at the pi state (at phi = pi). Theoretical calculations based on the quasiclassical theory are in good agreement with the observed behavior.

We argue on the basis of experimental numbers that the B = 0 metal-insulator transition in two dimensions, observed in Si-MOSFETs and in other two-dimensional systems, is likely to be due to a few strongly interacting electrons, which also interact strongly with the random positively ionized impurities. At the insulating side the electrons are all bound in pairs to the ions. On the metallic side free electrons exist which are scattered by ions dressed with electron-pairs and therefore alter the bare scattering potential of the ions. The physics at the metallic side of the transition is argued to be controlled by the classical to quantum transport cross-over leading to the observed non-monotonous dependence of the resistivity on temperature. This few electrons per ion scenario appears to be an experimentally realistic and testable scenario, which can also serve as a starting point for further theoretical analysis of the two-dimensional metal-insulator transition. (C) 1999 Elsevier Science Ltd. All rights reserved.

We present an overview of sample-specific transport properties tuned by the superconducting phase difference between two superconductors connected to a disordered 2-dimensional electron gas (2DEG). We demonstrate a crossover from ensemble-averaged to sample-specific resistance oscillations of a T-shaped 2DEG interferometer by increasing the magnetic field. Multi-terminal resistances of a cross-shaped 2DEG interferometer are analyzed in terms of an extended Landauer-Buttiker transport formalism. We show that the extended reciprocity relations hold and that three-terminal resistances become negative. (C) 1998 Elsevier Science B.V. All rights reserved.

We have fabricated Nb diffusion-cooled hot electron bolometers for a waveguide mixer around 700 GHz. The device in this work is a thin (12 nm) Nb bridge with a length and width of 220 nm and is defined by a two-step electron beam lithography process. The resistance as a function of temperature shows two transitions, which we attribute to the Nb bridge (6.1 K) and to parts of the Nb which are covered by the Au pads (5.4 K). We have measured a DSB receiver noise temperature of 2200 K at 735 GHz and a bath temperature of 3.3 K. No significant increase of the noise temperature is observed at IF frequencies between 1.1 and 1.7 GHz. The coupled LO power is estimated to be 35 nW.

In recent years, responding to the need of the astronomical community, superconductor-insulator-superconductor (SIS) mixers have been developed to operate at THz frequencies. Best results are obtained by using niobium-based tunnel junctions with well-matched low-loss normal metal striplines, which are found to be less lossy than niobium striplines operated above the gap-frequency of niobium (700 GHz). Niobiumnitride tunnel junctions with a magnesiumoxide tunnel barrier, although having reasonable quality current-voltage characteristics, have been found to be unsuitable due to excess shot noise resulting from pinholes in the barrier.

We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shot noise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q much greater than e. Using this charge enhancement mechanism we explain the junction shot noise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise temperature in the pumped case is more than twice that calculated with standard Tucker theory. The measured double side band noise temperatures are 1450 K at 600 GHz and 2800 K at 950 GHz.

For the Andreev-reflection process of quasi-particles at a normal-metal-superconductor interface the influence of lifetime broadening of the quasi-particles on the current-voltage characteristics of NS point contacts is analyzed along the lines of the Blonder-Tinkham-Klapwijk model. The anomalous Andreev-reflection spectra obtained for the heavy-fermion compound UPt3 cannot be explained by lifetime broadening alone. Instead, an anisotropic superconducting order parameter has to be assumed which, if also lifetime broadening is included, leads to a fairly good agreement with the data.

Heating effects in Superconductor- Insulator- Superconductor (SIS) tunnel junctions are studied by comparing SIS junctions equipped with niobium and aluminum electrodes. A temperature rise of several K is observed with niobium, which is reduced by an order of magnitude with aluminum. A simple model satisfactorily explains this observation and predicts an increase in the noise temperature of up to 30% at the operating voltage for heterodyne mixing due to this effect.

We derive approximate connection rules for conduction-electron waves at an abrupt interface between two materials with different electronic properties. We avoid the arbitrariness in the connection rules for effective-mass wave functions by taking the Bloch functions into account in an approximate way that is appropriate for the non-epitaxial interfaces that we wish to study. The result is particularly relevant for the physics of superconductor-semiconductor heterostructures because the basic property governing their behaviour is the transmissivity of the interface. The application of the result to measurements on niobium-silicon heterostructures shows that the situation has been reached in which the transmissivity is mainly limited by the large Fermi wave vector mismatch of metal and semiconductor.

We report on the conductance of a short (

A summary is given of recent results on carrier transport in mesoscopic conductors with superconducting electrodes. Three-dimensional transport in the diffusive limit is studied with crystalline silicon membranes sandwiched between two niobium electrodes or between one electrode superconducting and one normal. At low temperatures, inelastic scattering is negligible in the intermediate silicon layer. At finite voltages the distribution of electrons over the energies in the silicon is found to be strongly nonthermal, with details depending on the interplay between Andreev scattering and elastic scattering at the interfaces. At small voltages, well below the gap-voltage, transport is phase-coherent. A supercurrent is found if both electrodes are superconducting, provided the membrane is thin enough. If only one electrode is superconducting or for thick membranes an enhanced conductance is observed, which decreases with increasing voltage and magnetic field. Two-dimensional transport and the ballistic regime are being studied by using InAs- and GaAs-based heterostructures. Various interesting theoretical predictions have been made and some novel phenomena have been discovered experimentally. Interesting new phenomena have also been found in mesoscopic normal metals with superconducting electrodes.

The relaxation of edge channels in the quantum Hall regime is considerably different in Si-MOSFETs than in GaAs/AlGaAs heterostructures due to the different material properties. These differences are also of great importance at low carrier densities (10(15) m-2), where an isulating state occurs. Both phenomena have been studied in a Si-MOSFET modified with submicron gaps.

We have made a detailed study of the gain and noise of a SIS heterodyne receiver at 345 GHz. As mixing element we use an array of two Nb-Al2O3-Nb SIS junctions in series. The array is operated in a waveguide mount with a backshort and an E-plane tuner. The best receiver noise temperature achieved is 140 K DSB. The embedding impedances were determined by fitting theory to the measured pumped curves. High-quality fits are obtained which constitute the first detailed test of the Tucker-theory at frequencies above 300 GHz. The impedances found by this method are in very good agreement with impedances measured in a scale model at 3.3 GHz. From these embedding impedances gain and noise of the mixer were calculated over a full bias range using the Tucker theory in the three-port low IF approximation. The calculated values are compared to mixer gain and noise as obtained from receiver measurements. The observed dependence of mixer gain and noise on bias voltage, pump power and embedding impedance is in good agreement with theory. However the absolute value of the measured gain is a factor .45 +/- 0.05 lower than calculated. The measured mixer noise is approximately two quanta, 38 +/- 10 K, higher than calculated. These discrepancies appear to be independent of the bias parameters of the mixer.

Measurements of the critical temperature are presented for 20-40 nm niobium films on heavily doped n- and p-type silicon, for a wide range of silicon doping concentrations and sample preparation methods. Contrary to a normal-metal-superconductor bilayer, a depression of the critical temperature does not occur in the semiconductor-superconductor system. We show the absence of a depression to be consistent with conventional theories for the proximity effect between a superconductor and a normal metal. It is due to the large mismatch in material parameters between the two layers.

Motivated by the observation of a superconducting energy gap far above the equilibrium critical temperature T(c) in an Al film forming the center electrode of a Nb/AlO(x)/Al/AlO(x)/Nb structure we analyze the mechanism of gap enhancement in symmetric double-barrier superconducting tunnel junctions. It is found that such structures are very effective in creating a nonthermal distribution of quasiparticles in the middle electrode. At certain bias conditions this leads, according to the BCS gap equation, to the appearance of a nonzero superconducting energy gap even at temperatures up to several times the equilibrium T(c). So the double-barrier arrangement offers the remarkable possibility of making a material become superconducting by applying a voltage or passing a current. Calculated current-voltage characteristics exhibit current steps at voltages eV=2(DELTA(Nb)-DELTA(Al)) and eV=2(DELTA(Nb)+DELTA(Al)) in agreement with measured curves. Calculations of the thermodynamic stability of the nonequilibrium superconducting state indicate the possibility of hysteresis effects around these current steps.

The dynamic response of a two-dimensional electron system subjected to a lateral superlattice potential of square periodicity is studied via far-infrared transmission spectroscopy on silicon MOS devices with two stacked gates. The bottom gate is a mesh with submicron square periodicity and is isolated from the homogeneous top gate by a layer of SiO2. By independent variation of both gate voltages the superlattice potential strength and the average electron density can be tuned rather independently to study the continuous transition from a homogeneous two-dimensional to a fully confined quasi-zero dimensional electron system. For a moderately density-modulated superlattice we observe two fundamental plasmons reflecting the lowest two reciprocal lattice vectors of the superlattice. With increasing potential modulation these modes exhibit an unexpected softening before merging into a broad absorption band reflecting the strong Landau-damping around the onset of three-dimensional confinement to electron dots.

The conductivity of Si(111)7 x 7 surfaces is studied in situ during room temperature deposition of submonolayers Pb. The conductance shows an initial decrease to a shallow minimum for coverages between 0.05 and 0.1 monolayer (ML) and a subsequent steep increase between 0.4 and 0.7 ML. Both continuous and intermittent deposition have been applied, to distinguish between equilibrium and transient effects. The results must be interpreted in terms of changes in the nature of surface states that pin the Fermi level and 2D percolation. A relation is suggested to different existing submonolayer structural phases.

The inelastic lifetime tau(in) in 3-dimensional Si:P layers with doping far above the metal-insulator transition is determined from the weak localization correction to the magnetoresistance in the temperature range of 1.2-4.8 K. Electron-electron interaction effects show up weakly at high magnetic fields and low temperatures. The inelastic scattering rate is determined as tau(in)-1=1.1x10(9)xT2.2. The relative contribution of electron-electron and electron-phonon scattering is discussed.

A niobium-silicon heterostructure is used to study the transmission of electron waves through an interface between two materials with different electron densities, analogous to the transmission of electromagnetic waves through an interface between two media with different refractive indices. A new approach to match effective mass wave functions is used to demonstrate that the electron transmission of the interface is determined by the ratio of the Fermi wave vectors in the two materials.

A novel method is presented to study the dynamics of vortices in superconducting films at fields close to the upper critical magnetic field. It is shown that moving flux lines in the gate of a superconductor-oxide-semiconductor field-effect transistor are magnetically coupled to a two-dimensional electron gas, leading to an induced voltage. The major part of this voltage is proportional to the magnetoresistance, which is varied by changing the Landau-level filling. A second part is independent of the electron density and is tentatively attributed to the Hall component of the resistivity tensor.

An array of four Nb-Al-Al2O3-Nb sis junctions in series is used as the mixing element in a waveguide mixer at 345 GHz. The area of one junction is 4-mu-m2 and its normal state resistance is about 12-OMEGA, yielding at 345 GHz, omega-RC approximately 5. The receiver noise temperature below 550 K is between 330 and 375 GHz with best results around 350 K measured with a hot- and cold-load calibration. The conversion loss calculated from the hot/cold measurements ranges from 10.5 to 12.5 dB. The mixer noise temperature is around 200 K, with an instantaneous bandwidth of 2 GHz.

We have performed electron transport studies in the quantum Hall regime on high-mobility Si-MOSFETS with an adjustable discontinuity in the density of the two-dimensional electron gas. To achieve this we etch a gap with submicron width in the gate electrode and bias the two parts independently. For equal and integer filling factors on both sides of the gap there is negligible reflection of electrons in edge states meaning that the barrier is low compared with the Landau-level separation. For unequal but integer filling factors the longitudinal resistance across the gap is quantized, indicating that in this case the transmission coefficient of the edge states is either zero or one, depending on the gate voltages.

High-conductance Nb/AIO(x)/Al/ALO(x)/Nb tunnel-junction devices with the Al thickness varying from 3 to 20 nm have been fabricated using a whole-wafer processing route. Current-voltage measurements show a step in the subgap structure corresponding to the difference in the energy gaps V = 2(DELTA-Nb - DELTA-Al) of the two superconductors. The measured DELTA-Al is large and persists to temperatures considerably in excess of the equilibrium critical temperature of the Al. This is interpreted as arising from a tunnel-induced gap enhancement which is an order of magnitude larger than has been seen previously.