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Papers by Steven Ruggiero

Review of Scientific Instruments, 1986

A circuit has been developed for the measurement of the electrical characteristics of devices sen... more A circuit has been developed for the measurement of the electrical characteristics of devices sensitive to extrinsic noise and intrinsic power-supply transients. The circuit comprises two active current sources running in opposition. The current can be ramped smoothly through current excursions of a maximum of ±1 A. Provision is also made for the modulation of the dc current output by an external ac source.

We describe the implementation of precision laser transmission spectroscopy (LTS) for sizing nano... more We describe the implementation of precision laser transmission spectroscopy (LTS) for sizing nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system which measures light transmission over a wide (210 -- 2300 nm) wavelength range with high precision and sensitivity. Spectral inversion was employed to determine both the particle size distribution and absolute density of particles with diameters over a total range of 5 to 3000 nm. LTS has a dynamic range of ~ 103 particles/mL to ~ 1010 particles/mL (5 x10-8 vol.% to 0.5 vol.%). Currently, LTS is being applied as a tool to investigate the behavior of liposomes, dipalmitoylphosphatidycholine (DPPC) and dipalmitoylphosphatidylserine (DPPS), under the presence of fusing and de-aggregating agents. Our measurements indicate a maximum diameter of 400 nm for liposomes suspended in solution after fusion.

We calculate the quasiparticle current for two mesoscopic superconducting tunnel junctions connec... more We calculate the quasiparticle current for two mesoscopic superconducting tunnel junctions connected in series using the semiclassical model. The I-V characteristics exhibit a sequence of steps whose spacing is determined by the superconducting gap energy and the electrostatic charging energy of the junctions. These steps are reminiscent of those observed in point-contact tunneling experiments performed on granular high-Tc superconductors. We propose that the ``multiple-gap structure'' observed in such experiments arises from charging effects in a material with a single gap, and suggest experiments to verify this.

$Research paper thumbnail of Charging effects in coupled superconducting tunnel junctions and their implications for tunneling measurements of high-T_{c} superconductors$

Phys Rev B, 1988

We report on work with Au/Graphene/Nb FET-type structures. Samples are created by e-beam lithogra... more We report on work with Au/Graphene/Nb FET-type structures. Samples are created by e-beam lithography on electronic-grade oxidized Si substrates, using commercially prepared graphene flakes and epitaxial films on SiC. Raman scattering is used to verify the single-layer nature of samples under study. We discuss noise studies in these systems and the status of experiments designed to observe predicted oscillations in the tunnel conductance of samples, associated with Klein tunneling in the graphene films and Andreev reflections at graphene/lead interfaces.

We describe a new approach for characterizing nanoparticles in suspension using laser transmissio... more We describe a new approach for characterizing nanoparticles in suspension using laser transmission spectroscopy. Our apparatus precisely measures light transmission as a function of wavelength from the near UV to the near IR. The transmittance versus wavelength is inverted using a theoretical model to obtain particle size, geometry, and density information. The precision of our measurements allows us to determine both the particle size distribution and the absolute number of particles with diameters ranging of ˜5 nm to ˜3000 nm with ˜1 nm resolution. The size range of applicability is comparable to that of dynamic light scattering, but with approximately six orders of magnitude higher sensitivity (down to ˜1000 particles/mL). The technique also allows us to determine the length and width of rod shaped particles including biological objects. We will present results for a variety of systems including metal, polystyrene, and metal-oxide particles, and organisms including viruses and bacteria.

We report on progress with lateral tunneling in N/G/S (Normal-metal/Graphene/Superconductor) tunn... more We report on progress with lateral tunneling in N/G/S (Normal-metal/Graphene/Superconductor) tunnel junctions. Our primary approach is the study of systems of the form: Au/Graphene/Nb where lateral tunneling occurs across graphene bridges. Gates are also provided for Fermi-level adjustments in the standard ``FET'' format. Samples are created by e-beam lithography on electronic-grade oxidized Si substrates, using commercially prepared graphene flakes. Raman scattering is used to verify the single-layer nature of samples under study. We discuss the current status of studies designed to observe predicted oscillations in the tunnel conductance of samples with the above-described geometry, associated with Klein tunneling in the graphene films and Andreev reflections at graphene interfaces.

We report on the low-frequency electronic noise in graphene-based FET structures. Samples were cr... more We report on the low-frequency electronic noise in graphene-based FET structures. Samples were created using standard e-beam lithography and exfoliated, epitaxially-grown, and CVD-grown single-layer graphene films. The lowest overall noise was observed in epitaxially-grown films on SiC. We also investigated the gate dependence of the noise amplitude. Previous studies have suggested that the noise dependence should be either Lambda\ Lambda Lambda-shaped in keeping with the Hooge model, or M-shaped as described by the ...

Aps March Meeting Abstracts, Mar 1, 2007

Superconductors with tunable transition temperatures and energy gaps are useful for a variety of ... more Superconductors with tunable transition temperatures and energy gaps are useful for a variety of device applications. For instance, transition-edge μ-calorimeter sensors and electron-tunneling μ-refrigerators have been made based on Al with various levels of Mn doping. The transition temperature of AlMn can be continuously tuned from about ˜1.35 K to below 10 mK by Mn concentrations up to several thousand ppm. Here, we present detailed measurements of the superconducting density of states of AlMn made with both normal metal-insulator-superconductor and superconductor-insulator-superconductor tunnel junctions. We compare the data to theoretical expectations for magnetic and non-magnetic impurities in superconductors. In particular we show that the density of states of AlMn is not gapless as we would expect from magnetic impurities. Rather, present evidence indicates that the density of states is essentially BCS-like with an increased Dynes parameter. The increased Dynes parameter corresponds to a broadened peak at the gap and an increase in subgap states. We discuss the implications of this behavior for tunnel junction devices with AlMn electrodes.

$Research paper thumbnail of Ion-beam deposited films for refractory-metal tunnel junctions$

Bulletin of the American Physical Society, Mar 1, 2007

2012 Asia Pacific Microwave Conference Proceedings, 2012

ABSTRACT Sinuous antennas can provide ultra-broadband operation, frequency-independent embedding ... more ABSTRACT Sinuous antennas can provide ultra-broadband operation, frequency-independent embedding impedances and isolated dual polarizations, making them suitable for many applications in the terahertz frequency regime. In this paper, a lens-coupled planar sinuous antenna has been designed and fabricated for terahertz balanced mixers. Simulation results show that the antenna exhibits a nearly frequency-independent embedding impedance of ~100 Ω over a broad frequency range. To verify the aforementioned antenna properties at THz frequencies, a Schottky diode was mounted to the center of the antenna to realize a direct detector. The radiation patterns of the lens-coupled sinuous antenna were studied at 196 GHz and 585 GHz. A responsivity of 300-1000 V/W was measured up to 900 GHz demonstrating the ultra-broadband operation of the antenna. In addition, an isolation of larger than 15 dB between the two polarizations has been demonstrated for both the frequency ranges of 190-210 GHz and 570-630 GHz.

Appl Phys Lett, 2004

We demonstrate a thin-film, solid-state refrigerator based on the removal of hot electrons from a... more We demonstrate a thin-film, solid-state refrigerator based on the removal of hot electrons from a metal by quantum-mechanical tunneling. We have reduced the electronic temperature in a metal film from 260 to ˜130 mK. The base temperature of the device is predicted to increase to near 140 mK under a power load of 10 pW. Both the cooling power and temperature reduction of the refrigerator are well matched to practical applications. This refrigerator will make high-performance cryogenic photon sensors more accessible to the astronomical and analytical communities.

We present results for the analysis of particle size, geometry, and density based on laser spectr... more We present results for the analysis of particle size, geometry, and density based on laser spectroscopy. The range of applicability of the technique is comparable to dynamic light scattering, but with approximately six orders of magnitude higher sensitivity (down to 1000 particles/mL). We discuss results for a variety of particle types including metal, polystyrene, and metal-oxide particles, and organisms including viruses and bacteria.

We report on the photoresponse of YBa_2Cu_3O_7-delta thin film systems. Our present work has focu... more We report on the photoresponse of YBa_2Cu_3O_7-delta thin film systems. Our present work has focused on wavelength-dependent effects in underdoped, superconducting YBCO films. In our experiments, we use an optically chopped monochromator source which provides light in the of 0.4-1.0 mum range. While a significant portion of the observed response is consistent with bolometric effects, there appear to be residual elements of the response which are nonbolometric. Residual nonbolometric effects are examined in the context of the gap structure of underdoped YBCO.

We evaluate the performance of the Single Particle Soot Photometer (SP2) and the Integrating Sphe... more We evaluate the performance of the Single Particle Soot Photometer (SP2) and the Integrating Sphere/Integrating Sandwich Spectrophotometer (ISSW) in quantifying the concentration of refractory black carbon (BC) in snow samples. We find that the SP2 can be used to measure BC mass concentration in snow with substantially larger uncertainty (60%) than for atmospheric sampling (<30%). Achieving this level of accuracy requires careful assessment of nebulizer performance and SP2 calibration with consideration of the fact that BC in snow tends to larger sizes than typically observed in the atmosphere. Once these issues are addressed, the SP2 is able to measure the size distribution and mass concentration of BC in the snow. Laboratory comparison of the SP2 and the Integrating Sphere/Integrating Sandwich Spectrophotometer (ISSW) revealed significant biases in the estimate of BC concentration from the ISSW when test samples contained dust or non-absorbing particulates. These results sugges...