Electron-dependent thermoelectric properties in Si/Si_(1_x)Ge_(x) heterostructures and Si_(1-x)Ge_(x) alloys from first-principles

Unlike phononic thermal conductivity (which is shown in the literature to be reduced due to alloying and has a nearly constant value over a range of compositional variations), electron-dependent thermoelectric properties are shown here, from first-principles, to vary nonlinearly with composition. Of the Si/Si_(1_x)Ge_(x) systems considered, the maximum thermopower observed, which is 10% higher than that of crystalline Si, is obtained for a Si_(0.875)Ge_(0.125) alloy. Also, heterostructuring is shown to reduce thermopower, electrical conductivity, and electron thermal conductivity. Additionally, neither Lorenz number nor Seebeck coefficient shows oscillations for heterostructures, regardless of electron/hole energies, contradicting the conclusions obtained with miniband approximations

Effect of diazinon 60 EC on Anabas testudineus, Channa punctatus and Barbodes gonionotus

Anabas testudineus, Channa punctatus and Barbodes gonionotus were exposed to 5.62, 6.25, 6.87, 7.50, 8.12 and 8.75 ppm; 1.13, 2.26, 3.39, 4.52, 5.65 and 6.78 ppm; and 2.00, 2.50, 3.00, 3.50, 4.00 and 4.50 ppm of Diazinon 60 EC, respectively. The median lethal concentration (LC50) values of Diazinon 60 EC on A. testudineus, C. punctatus and B. gonionotus were 6.55, 3.09 and 2.72 ppm for 96 hrs of exposure. The fish species showed several abnormal behaviors which included restlessness, arena movements, loss of equilibrium, increased opercular activities, strong spasm, paralysis and sudden quick movements during the exposure. For histopathological studies, A. testudineus, C. punctatus and B. gonionotus were exposed for 7 days to sublethal concentrations of 1.13 and 3.75 ppm; 1.13 and 2.26 ppm; and 1.13 and 2.26 ppm of Diazinon 60 EC, respectively. Hypertrophy, necrosis and pyknosis of hepatocytes, pyknosis and degenerative changes such as necrosis of tubular and haematopoietic cells of kidney were the major histopathological effects

Itinerant and local-moment magnetism in EuCr2As2 single crystals

We report on the crystal structure, physical properties, and electronic structure calculations for the ternary pnictide compound EuCr2As2. X-ray diffraction studies confirmed that EuCr2As2 crystalizes in the ThCr2Si2-type tetragonal structure (space group I4/mmm). The Eu ions are in a stable divalent state in this compound. Eu moments in EuCr2As2 order magnetically below Tm = 21 K. A sharp increase in the magnetic susceptibility below Tm and the positive value of the paramagnetic Curie temperature obtained from the Curie-Weiss fit suggest dominant ferromagnetic interactions. The heat capacity exhibits a sharp {\lambda}-shape anomaly at Tm, confirming the bulk nature of the magnetic transition. The extracted magnetic entropy at the magnetic transition temperature is consistent with the theoretical value Rln(2S+1) for S = 7/2 of the Eu2+ ion. The temperature dependence of the electrical resistivity \r{ho}(T) shows metallic behavior along with an anomaly at 21 K. In addition, we observe a reasonably large negative magneto-resistance (~ -24%) at lower temperature. Electronic structure calculations for EuCr2As2 reveal a moderately high density of states of Cr-3d orbitals at the Fermi energy, indicating that the nonmagnetic state of Cr is unstable against magnetic order. Our density functional calculations for EuCr2As2 predict a G-type AFM order in the Cr sublattice. The electronic structure calculations suggest a weak interlayer coupling of the Eu moments.Comment: 9 pages, 7 figure

Interface driven reentrant superconductivity in HoNi5_5-NbN-HoNi5_5 nanostructures

Superconductivity (S) and ferromagnetism (F) are probed through transport and magnetization measurements in nanometer scale HoNi$_5$-NbN (F-S) bilayers and HoNi$_5$-NbN-HoNi$_5$ (F-S-F) trilayers. The choice of materials has been made on the basis of their comparable ordering temperatures and strong magnetic anisotropy in HoNi$_5$. We observe the normal state reentrant behavior in resistance vs. temperature plots of the F-S-F structures just below the superconducting transition in the limited range of HoNi$_5$ layer thickness d$_{HN}$ (20 nm $<$ d$_{HN}$ $<$ 80 nm) when d$_{NbN}$ is fixed at $\simeq$ 10 nm. The reentrance is quenched by increasing the out-of-plane (H$_{\perp}$) magnetic field and transport current where as in-plane (H$_{\parallel}$) field of $\leq$ 1500 Oe has no effect on the reentrance. The thermally activated flux flow characteristics of the S, F-S and F-S-F layers reveal a transition from collective pinning to single vortex pinning as we place F layers on both sides of the S film. The origin of the reentrant behavior seen here in the range of 0.74 $\leq$ T$_{Curie}$/T$_C$ $\leq$ 0.92 is attribute to a delicate balance between the magnetic exchange energy and the condensation energy in the interfacial regions of the trilayer.Comment: 13 pages and 5 figure

Magnetic behaviour of PrPd2B2C

We have synthesized a new quaternary borocarbide PrPd$_{2}$B$_{2}$C and measured its magnetization, electrical resistivity and specific heat. The compound crystallizes in the LuNi$_{2}$B$_{2}$C-type tetragonal structure (space group {\it I4/mmm}). Above 100 K the magnetic susceptibility follows Curie-Weiss behavior with effective moment $\mu_{eff}$ = 3.60 $\mu_{B}$, which is very close to the value expected for Pr$^{3+}$ ions. We do not find evidence for magnetic or superconducting transition down to 0.5 K. Specific heat exhibits a broad Schottky type anomaly with a peak at 24 K, very likely related to crystal electric field (CEF) excitation. The magnetic properties suggest the presence of a singlet CEF ground state leading to a Van-Vleck paramagnetic ground state.Comment: 2 pages, 2 figure

Development of Local and Global Corrosion Sensing Technique to Monitor Structural Behavior of Prestressed Concrete Structures

Corrosion of steel rebar in reinforced concrete structures is a concern for highway bridge owners. According to 2002 study by the Federal Highway Administration, ~15% of the highway bridges in the US are structurally deficient due to corrosion and have an estimated annual direct cost of $8.3 billion. Generally, in post-tensioned bridges, ducts filled with poor or incomplete grouting can allow the tendons to come into contact with water, leading to corrosion and fracture. Although new and improved procedures of grouting have been developed to reduce the instances of poor grouting, the problem of how to reliably inspect tendons on existing structures remains. This research aimed to evaluate the feasibility of using currently available local and global corrosion sensing techniques to monitor the performance of concrete structures. This dissertation explores the use of three local corrosion sensors (resistivity sensors, relative humidity sensors, and resistor-inductor- capacity [RLC] sensors). In this research, local sensors (i.e. RH sensors and four-point resistivity sensors) placed at an interval of every three foot were able to detect corrosion conducive environment in PT ducts by measuring the electrical properties and moisture contents of the grout. However, the measured RH was consistently lower in the low point of the specimens compared to the high points for all the specimens. The four-point resistivity sensors were able to detect a clear difference between the grouted ducts with good grout compared to ducts with chloride-rich grout with voids and layered-chloride rich grout without voids. To evaluate destructive testing and monitor the global response, two full-scale prestressed concrete inverted-tee (IT) beams were constructed and instrumented with vibrating wire strain gauges (VWSGs) located in multiple planes along the length. The VWSGs were able to clearly detect the initiation of corrosion in the first prestressing strand in each beam. The local corrosion damage in the first strand resulted in a change in the behavior of the beam, which was detected by the VWSGs (through a change in slope of curvature versus time)

75As NMR local probe study of magnetism in (Eu1-xKx)Fe2As2

75As NMR measurements were performed as a function of temperature and doping in (Eu1-xKx)Fe2As2 (x=0,0.38,0.5,0.7) samples. The large Eu2+ moments and their fluctuations are found to dominate the 75As NMR properties. The 75As nuclei close to the Eu2+ moments likely have a very short spin-spin relaxation time (T2) and are wiped out of our measurement window. The 75As nuclei relatively far from Eu2+ moments are probed in this study. Increasing the Eu content progressively decreases the signal intensity with no signal found for the full-Eu sample (x=0). The large 75As NMR linewidth arises from an inhomogeneous magnetic environment around them. The spin lattice relaxation rate (1/T1) for x=0.5 and 0.7 samples is nearly independent of temperature above 100K and results from a coupling to paramagnetic fluctuations of the Eu2+ moments. The behavior of 1/T1 at lower temperatures has contributions from the antiferromagnetic fluctuations of the Eu2+ moments as also the fluctuations intrinsic to the FeAs planes and from superconductivity.Comment: 6 pages, 6 figures (to appear in EPJB