Steven Ruggiero

Cooling of bulk material by electron-tunneling refrigerators

Improved refrigeration techniques have lead to scientific discoveries such as superconductivity and Bose-Einstein condensation. Improved refrigeration techniques also enhance our quality of life. Semiconductor processing equipment and magnetic-resonance imaging machines incorporate mechanical coolers operating below 10 K. There is a pressing need for refrigeration techniques to reach even lower temperatures because many next-generation analytical and astronomical instruments will rely on sensors cooled to temperatures near 100 mK. Here we demonstrate a solid-state, on-chip refrigerator capable of reaching 100 mK based on the quantum-mechanical tunneling of electrons through normal metal-insulator-superconductor junctions. The cooling power and temperature reduction of our refrigerator are sufficient for practical applications and we have used it to cool bulk material that has no electrical connection to the refrigerating elements.

Rapid Molecular Detection of Invasive Species in Ballast and Harbor Water by Integrating Environmental DNA and Light Transmission Spectroscopy

Invasive species introduced via the ballast water of commercial ships cause enormous environmental and economic damage worldwide. Accurate monitoring for these often microscopic and morphologically indistinguishable species is challenging but critical for mitigating damages. We apply eDNA sampling, which involves the filtering and subsequent DNA extraction of microscopic bits of tissue suspended in water, to ballast and harbor water sampled during a commercial ships 1400 km voyage through the North American Great Lakes. Using a lab-based gel electrophoresis assay and a rapid, field-ready light transmission spectroscopy (LTS) assay, we test for the presence of two invasive species: quagga (Dreissena bugensis) and zebra (D. polymorpha) mussels. Furthermore, we spiked a set of uninfested ballast and harbor samples with zebra mussel tissue to further test each assays detection capabilities. In unmanipulated samples, zebra mussel was not detected, while quagga mussel was detected in all samples at a rate of 85% for the gel assay and 100% for the LTS assay. In the spiked experimental samples, both assays detected zebra mussel in 94% of spiked samples and 0% of negative controls. Overall, these results demonstrate that eDNA sampling is effective for monitoring ballast-mediated invasions and that LTS has the potential for rapid, field-based detection.

High-precision sizing of nanoparticles by laser transmission spectroscopy

We describe the implementation of precision laser transmission spectroscopy for sizing and counting nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system that measures light transmission over a wide (210-2300 nm) wavelength range with high precision and sensitivity. Spectral inversion is employed to determine both the particle size distribution and absolute particle density. In this paper we discuss results for particles with sizes (diameters) in the range from 5 to 3000 nm. For polystyrene particles 404 to 1025 nm in size, uncertainties of ±0.5% in size and ±4% in density were obtained. For polystyrene particles from 46 to 3000 nm in size, the dynamic range of the system spans densities from ~10(3)/ml to ~10(10)/ml (5 × 10(-8) to 0.5 vol. %), implying a sensitivity 5 orders of magnitude higher than dynamic light scattering.

Quantitative and Rapid DNA Detection by Laser Transmission Spectroscopy

Laser transmission spectroscopy (LTS) is a quantitative and rapid in vitro technique for measuring the size, shape, and number of nanoparticles in suspension. Here we report on the application of LTS as a novel detection method for species-specific DNA where the presence of one invasive species was differentiated from a closely related invasive sister species. The method employs carboxylated polystyrene nanoparticles functionalized with short DNA fragments that are complimentary to a specific target DNA sequence. In solution, the DNA strands containing targets bind to the tags resulting in a sizable increase in the nanoparticle diameter, which is rapidly and quantitatively measured using LTS. DNA strands that do not contain the target sequence do not bind and produce no size change of the carboxylated beads. The results show that LTS has the potential to become a quantitative and rapid DNA detection method suitable for many real-world applications.

Mixing in TlCaBaCuO superconducting films at 61 GHz

The results of mixing at 61 GHz using weak-link, grain-boundary junctions in Tl/sub 2/CaBa/sub 2/Cu/sub 2/O/sub 8/ thin films are discussed. Films were prepared using laser deposition of precursor material on MgO substrates followed by high-temperature postprocessing. Mixing experiments were conducted at 75 K, at an IF frequency of 1 GHz, using a bow-tie antenna configuration. The signal-to-noise ratio for the system at the IF frequency was approximately 10/sup 4/.

DNA-based species detection capabilities using laser transmission spectroscopy.

Early detection of invasive species is critical for effective biocontrol to mitigate potential ecological and economic damage. Laser transmission spectroscopy (LTS) is a powerful solution offering real-time, DNA-based species detection in the field. LTS can measure the size, shape and number of nanoparticles in a solution and was used here to detect size shifts resulting from hybridization of the polymerase chain reaction product to nanoparticles functionalized with species-specific oligonucleotide probes or with the species-specific oligonucleotide probes alone. We carried out a series of DNA detection experiments using the invasive freshwater quagga mussel (Dreissena bugensis) to evaluate the capability of the LTS platform for invasive species detection. Specifically, we tested LTS sensitivity to (i) DNA concentrations of a single target species, (ii) the presence of a target species within a mixed sample of other closely related species, (iii) species-specific functionalized nanoparticles versus species-specific oligonucleotide probes alone, and (iv) amplified DNA fragments versus unamplified genomic DNA. We demonstrate that LTS is a highly sensitive technique for rapid target species detection, with detection limits in the picomolar range, capable of successful identification in multispecies samples containing target and non-target species DNA. These results indicate that the LTS DNA detection platform will be useful for field application of target species. Additionally, we find that LTS detection is effective with species-specific oligonucleotide tags alone or when they are attached to polystyrene nanobeads and with both amplified and unamplified DNA, indicating that the technique may also have versatility for broader applications.

Ion-beam-deposited films for refractory-metal tunnel junctions

We report on the application of a Kaufman ion source to the deposition of Nb and Ta thin films. We find that high quality Nb films (T c = 9.1 K) can be produced by this technique under tolerant deposition conditions. In addition, substantial, systematic improvement in the I-V characteristics of Nb tunnel junctions has been realized by depositing, in situ, thin (≥10A) Ta layers on the Nb film surface.

Dilute Al-Mn Alloys for Low-Temperature Device Applications*

We discuss results on the superconducting, electron-transport, and tunneling properties of Al doped with Mn in the range of 1000 to 3000 ppm. We demonstrate that the critical temperature of Al can be systematically reduced to below 50 mK. Films are prepared by sputter deposition, and show values of d(ln R)/d(ln T) of ∼500, indicating sharp superconducting transitions. Al-Mn/I/Al-Mn tunnel junctions show low sub-gap conductance and BCS-like characteristics. Our results in general suggest that the material is of interest for transition-edge sensors operating in the 100 mK regime and superconductor/insulator/superconductor (S/I/S) and normal/insulator/ superconductor (N/I/S) devices, in the latter case where heavily doped Al-Mn can replace the normal metal.

The role of lateral tension in calcium-induced DPPS vesicle rupture.

We assess the role of lateral tension in rupturing anionic dipalmitoylphosphatidyserine (DPPS), neutral dipalmitoylphosphatidylcholine (DPPC), and mixed DPPS-DPPC vesicles. Binding of Ca(2+) is known to have a significant impact on the effective size of DPPS lipids and little effect on the size of DPPC lipids in bilayer structures. In the present work we utilized laser transmission spectroscopy (LTS) to assess the effect of Ca(2+)-induced stress on the stability of the DPPS and DPPC vesicles. The high sensitivity and resolution of LTS has permitted the determination of the size and shape of liposomes in solution. The results indicate a critical size after which DPPS single shell vesicles are no longer stable. Our measurements indicate Ca(2+) promotes bilayer fusion up to a maximum diameter of ca. 320 nm. These observations are consistent with a straightforward free-energy-based model of vesicle rupture involving lateral tension between lipids regulated by the binding of Ca(2+). Our results support a critical role of lateral interactions within lipid bilayers for controlling such processes as the formation of supported bilayer membranes and pore formation in vesicle fusion. Using this free energy model we are able to infer a lower bound for the area dilation modulus for DPPS (252 pN/nm) and demonstrate a substantial free energy increase associated with vesicle rupture.

Measurements and modeling of phonon cooling by electron-tunneling refrigerators

We demonstrate cooling of the electrons and phonons of a silicon nitride (Si-N) membrane by solid-state refrigerators based on normal metal/insulator/superconductor (NIS) tunnel junctions. We report a temperature reduction of the Si-N membrane from 260 mK to 240 mK, while the electrons in the normal metal of the refrigerator are cooled to 145 mK. We explain the mechanism for cooling an isolated membrane and make quantitative comparisons between experiment and a finite-element thermal model. The model indicates that increasing the thermal conductivity of the cold-fingers, improving the transparency of the tunnel junctions, and reducing the power load through the membrane will make it possible to cool the membrane from 260 mK to below 170 mK. The refrigeration of a membrane makes it possible to integrate other cryogenic devices that require sub-Kelvin temperatures for optimal performance, such as thin-film sensors. We demonstrate this integration by combining NIS refrigerators with an x-ray Transition-Edge-Sensor (TES).