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Thanks to high Tc of 40 K, high Jc of > 10^7 A.cm^-2, and no weak link behavior across the grain boundary in MgB2 material. This highest Tc among all conventional BCS superconductors, and better material properties of MgB2 compared to high Tc cuprate superconductors makes this material attractive for many applications including, but not limited to, power cables, Josephson junction based electronic devices, SRF cavities, THz sensors and single photon counters. Ultrathin superconducting films are a key element in various detectors utilized in remote sensing over a large part of the entire electromagnetic spectrum. The superconducting hot electron bolometer (HEB) mixer is a crucial detector for high-resolution spectroscopy at THz frequencies. The state-of-the-art NbN phonon-cooled HEB mixers have a relatively narrow (IF) bandwidth ~ 3- 4 GHz as a direct result of the poor acoustic transparency of the film-substrate interface and low sound velocity in NbN reducing the phonon escape time in the film. Alternatively, MgB2 displays a very short τe-ph ~ ps. The phonon escape time is also short due to the high sound velocity in the material (~ 7 Km.s^-2) thus giving rise to a broader IF bandwidth. Also, smaller magnetic penetration depth (λ ≈ 40 nm) of MgB2 makes material of choices for single photon detector application. The response time of an SNSPD is proportional to the square of its magnetic penetration depth λ. Therefore, MgB2 may potentially operate 10-fold faster than the NbN (λ =200 nm) based SNSPD. In this work, I present my effort to fabricate high quality ultrathin superconducting MgB2 films on 6H-SiC (0001) substrates, and study their superconducting and electronic properties. C- epitaxial 10 nm showed Tc of above 36 K, while residual resistivity of up to 26 μΩ.cm was achieved. Critical currents of more than 6 × 10^6 A · cm^−2 at 20 K have been measured for the films with thicknesses iv ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films. Ultrathin film as thin as 1.8 nm (6 unit cells) can be achieved by Hybrid Physical-Chemical Vapor Deposition (HPCVD) followed by low angle Ar ion milling. These post processed films exhibit better superconducting properties compared to directly grown films. The 1.8 nm, showed Tc > 28 K and Jc > 10^6 A/cm^2 4 K. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. These results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit. Finally, I measured the upper critical field of MgB2 films of various thickness and extracted their thickness dependent in-plane intraband diffusivities by using Gurevich model developed for two-band MgB2 superconductor in dirty limit. Results showed that π band diffusivity (Dπ) decreases rapidly from 71.12 cm^2/s for 100 nm film to 4.6 cm^2/s for 5 nm film where as �� band diffusivity (����) decreases much slower from 2.8 cm^2/s for 100 nm film to 0.8 cm^2/s for 5 nm film. This larger Dπ than ���� indicates the cleaner π band. / Physics

Time-lapse electrical resistivity tomography (TL-ERT) enables an accurate characterization of the heterogeneity of flow through the unsaturated zone especially when compared to point measurements taken within the same survey area. The most powerful tool for understanding the unsaturated zone is a combination of several techniques. Many models of unsaturated zone flow assume a uniform wetting front even though the existence of preferential flow paths is well-documented in the literature. TL-ERT surveys were collected perpendicular to a stream at the Stroud Water Research Center in Chester County, PA to provide continuous measurement of unsaturated flow during two natural infiltration events. Dielectric sensors were installed along this transect to collect soil moisture data during these events. Additionally, slug tests and infiltrometer tests were collected along the transect to characterize the subsurface at the study site. TL-ERT successfully located sections with preferential flow, and these results were reproducible three months later. Other methods of measuring soil moisture content or infiltration rates were less successful at identifying preferential flow. The rates determined from point measurements often did not match where the TL-ERT identified zones of preferential flow. This comparison reveals that slow-infiltration points can exist within preferential pathways and exemplifies the importance of large-scale measurements in the unsaturated zone. Any scientific study looking at infiltration should consider utilizing TL-ERT to map where preferential flow may be occurring. / Geology

Concrete construction methods and practices influence the final in-place quality of concrete. A low permeability concrete mixture does not alone ensure quality in-place concrete. If the concrete mixture is not transported, placed and cured properly, it may not exhibit the desired durability and mechanical properties. This study investigates the in-place permeation properties of low permeability concrete bridge decks mixtures used in the Commonwealth of Virginia. Permeation properties were assessed in both the laboratory and in the field using 4-point Wenner array electrical resistivity, surface air flow (SAF), and chloride ion penetrability (ASTM C 1202-97). Laboratory test specimens consisted of two concrete slabs having dimensions of 280 x 280 x 102-mm (11 x 11 x 4-in) and twelve 102 x 204-mm (4 x 8-in) cylinders per concrete mixture. Specimens were tested at 7, 28 and 91-days. Thirteen cylinder specimens per concrete mixture underwent standard curing in a saturated limewater bath. The simulated field-curing regimes used wet burlap and plastic sheeting for 3 (3B) and 7 days (7B) respectively and was applied to both slabs and cylinder specimens. Slab specimen were tested on finished surface using the SAF at 28 and 91 days, and 4-point electrical resistivity measurements at 1, 3, 7, 14, 28 and 91 days. Compressive strength (CS) tests were conducted at 7 and 28 days. Chloride ion penetrability tests were performed at 7, 28, and 91 days. Statistical analyses were performed to assess the significance of the relationships for the following: Total charge passed and initial current (ASTM C 1202-97); 3B resistivity and 7B resistivity; Slab and cylinder resistivity; Slab resistivity and ASTM C-1202-97 (Total Charge and Initial current); and Surface Air Flow and ASTM C-1202-97. Field cast specimens, test slabs and cylinders, were cast on-site during concrete bridge deck construction. The slab dimensions were 30.5 x 40.6 x 10.2-cm (12 x 16 x 4 in.), and the cylinders were 10.2 x 20.4-cm (4 x 8-in). In-situ SAF and resistivity measurements were taken on the bridge deck at 14, 42 and 91 days. In-place SAF and resistivity measurements on laboratory field cast slabs were taken at 7, 14 and 28-days. ASTM C 1202-97 specimens were prepared from field cast cylinders and tested at 7 and 28 and 42-days. The relationship between in-place permeation measures from field specimens was compared to laboratory data. Results indicated no difference in chloride ion penetrability (Figures 7.4 and 7.5) and 28-day compressive strength (Figure 7.2) with regard to differing simulated field curing regimes, for same age testing. There was no significant difference at the 95 % confidence level between 3B resistivity and 7B resistivity specimens tested at the same age (Figures 7.9 and 7.10). A well defined relationship was observed between total charge passed and initial current (Figure 7-6). An inverse power function was found to describe the relationship between charge passed/initial current and electrical resistivity for all laboratory mixtures used in this study (Figure 7.17 – 7.22). Field data was used to validate laboratory established models for charge passed/initial current and electrical resistivity. Laboratory established models were able to predict 30 to 50% of the field data (Figures 7.31 – 7.34). Results indicate that the SAF lacked the sensitivity to classify the range of concretes used in this study (Figure 7.24). / Ph. D.