Conceptual Design for SuperCDMS SNOLAB

Beyond the present dark matter direct detection experiment at the Soudan underground laboratory, the SuperCDMS Collaboration is engaged in R&D activities for a 100-kg scale germanium dark matter experiment nominally sited at SNOLAB (2070 m overburden of rock). The expected sensitivity after 3 years of running is 3×10^?46?cm² for the spin-independent cross section, an order of magnitude improvement over present exclusion limits for WIMP masses ～80?GeV/c². At this depth, and appropriate design of shielding and cryostat, neutron backgrounds will be negligible. The baseline design is an expanded version of CDMS II with Ge substrates (100×33?mm discs) instrumented with the iZIP phonon sensor layout to achieve the electron surface-event rejection power required.     

The temperature dependence of stresses in aluminum films on oxidized silicon substrates

In situ measurements were carried out of stress at the AlSiO₂ interface at various temperatures (25–500 °C) and for various film thicknesses (0.2–1.6 microm). These data are complemented with microstructural studies by transmission electron microscopy.For the aluminum films studied, the intrinsic structural component was very small (less than 2 × 10⁸ dyn cm^?2). On heating, thermal mismatch led to a compressive stress, with dσ/dT ≈ ?2 × 10⁷ dyn cm^?2 °C; these films yielded at 6σ6 ; ? 5 × 10⁸ dyn cm^?2, primarily through diffusion creep and grain growth. On cooling from about 450–500 °C, thermal mismatch led to a tensile stress which was limited mainly by dislocation slip. The final room temperature value after thermal cycling ranged from 0.5 × 10⁹ dyn cm^?2 for a 1.5 microm film to 8 × 10⁹ dyn cm^?2 for a 0.2 microm film; these values are believed to represent the critical stress for the generation of dislocation loops within the grains.The grain size of cold-deposited aluminum films ranged from about 0.2 microm for films 0.45 microm thick to about 2 microm for films 1.5 microm thick. Thermal cycling led to an order-of-magnitude increase in the grain size together with the formation of dislocation networks within the grains.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, ΔT _c =0.3 K). A sharp (FWHM 7.2 cm^?1 at 70 K and 10.0 cm^?1 at 110 K) 340 cm^?1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^?1 in theB _1g polarization extends down to 250 cm^?1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^?1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^?1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^?1 mode anomalies have been explained by the electronic spectral function renormalization.     

Characterization of ohmic contacts to InP

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as ohmic contacts are presented. The layers were heat treated at temperatures up to 550°C and were examined with Auger electron spectroscopy. For contact to n-type InP three thin film systems were investigated: gold, nickel and a composite Ni/Au/Ge layer. Nickel was found to produce ohmic behavior in the Ni/Au/Ge/InP system with a minimum specific contact resistance r_c of 3×10^?5 Ω cm² for a net doping of 3×10¹⁶ cm^?3. For contact to p-type InP a film consisting of Au/Mg was investigated. For heat treatment of the Au/Mg/InP system above 350°C, r_c decreased as the temperature of the heat treatment increased and the surface morphology exhibited increasing signs of alloying at higher temperatures. The smoothest surface was obtained at 446°C for 50 min with r_c≈1×10^?4Ω cm² for a net doping of 6×10¹⁷ cm^?3.     

Crystallographic, thermoelectric, and mechanical properties of polycrystalline type-I Ba8Al16Si30-based clathrates

Crystallographic, thermoelectric, and mechanical properties of polycrystalline Ba₈Al₁₆Si₃₀-based samples with type-I clathrate structure prepared by combining arc melting and spark plasma sintering methods were investigated. The major phase of the samples was a type-I clathrate with an actual Al/Si ratio of ~15/31, strongly suggesting that framework deficiency was absent or was present in very low concentration in the samples. The Hall carrier concentration n of the samples was approximately 1 × 10²¹ cm^?3, which is lower than the values reported so far for the Ba₈Al₁₆Si₃₀ system. Other important material parameters of the samples were as follows: the density-of-states effective mass m* = 2.3m ₀, Hall mobility μ = 7.4 cm² V^?1 s^?1, and the lattice thermal conductivity κ _L = 1.2 W m^?1 K^?1. The thermoelectric figure of merit ZT reached approximately 0.4 (900 K) for a sample with n = 9.7 × 10²⁰ cm^?3. Simulation using the experimentally determined values of material parameters showed that ZT reached values >0.5 if the carrier concentration is optimized at about 3 × 10²⁰ cm^?3. Young’s, shear, and bulk moduli were estimated to be approximately 98, 39, and 117 GPa, respectively, and Poisson’s ratio was found to be 0.25 from the longitudinal and transverse velocities of sound, v _L = 6038 m/s and v _T = 3503 m/s, respectively, for a sample with ZT = 0.4. The coefficient of thermal expansion (CTE) ranged from approximately 8 × 10^?6 K^?1 to 10 × 10^?6 K^?1 (330–690 K), which is smaller than the values reported for Ba₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ clathrates.     

Performance of a scintillating glass calorimeter for electromagnetic showers

A scintillating glass electromagnetic calorimeter consisting of 3 × 3 moduls of 8 × 8 × 66 cm³ each has been studied with electrons in the energy interval 14.7 MeV < E < 6000 MeV. An energy resolution of σ_E/E[%] = √1.6²/E[GeV] + 1.0² was achie spatial resolution turns out to be of the order σ = 4 to 8 m depending on the impact point and the angle of incidence; it improves with increasing energy. The observations are in excellent agreement with the result of an EGS Monte Carlo simulation of the detector including optical effects and photoelectron statistics.     

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB₂ bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB₂ subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB₂ powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600?°C under pressure of 3?C5 GPa for 10?C30 min. The nanocrystalline MgB₂ bulks exhibit the lower onset critical transition temperatures (T _{c onset}) of 32?C33?K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J _c ) are obtained in nanocrystalline bulks. J _c is as high as 10⁵?A/cm² in 8?T at 10?K and 2.7×10³?A/cm² in 4?T at 20?K.     

Local structure and electron spin resonance of copper-doped SrTiO3 ceramics

We report X-ray diffraction and electron spin resonance (ESR) measurements of the effect of SrTiO₃ ceramics doping using Cu²⁺ ions. ESR measurements reveal two kinds of Cu²⁺ centers in weakly (0.2–0.5 mol% Cu) doped SrTiO₃. Both kinds of centers have been attributed to Cu²⁺ at octahedral Ti sites and possibly associated either with a nearest-neighboring oxygen vacancy (center #1) or some other positively charged defect (center #2). The ESR spectra of the above centers are described by the following spin Hamiltonian parameters: g _‖ = 2.263(1), g _⊥ = 2.041(1), A _‖ = 170(1) × 10^?4 cm^?1, A _⊥ = 27(1) × 10^?4 cm^?1 (center #1) and g _‖ = 2.334(1), g _⊥ = 2.059(1), A _‖ = 137(1) × 10^?4 cm^?1, A _⊥ ≈ 0(1) × 10^?4 cm^?1 (center #2). For copper concentration larger than 2 mol%, the antiferromagnetic SrCu₃Ti₄O₁₂ (SCTO) phase has been detected by both X-ray diffraction and ESR. Its volume increases with increase of Cu concentration reaching about 17 % at Cu doping of 20 mol%. The composite SrTiO₃–SCTO ceramics exhibits substantial magnetocapacitance effect, which could be enhanced by electrostriction of SrTiO₃.     

The Magnetic and Structural Properties of SiC-Doped MgB2 Bulks Prepared by the Standard Ceramic Processing

According to general formula MgB_2?x SiC _x (x=0,0.05,0.1,0.2), MgB₂ and SiC-doped bulk superconductors were prepared by the standard ceramic processing. The mixtures of the corresponding powders were sintered at 750?°C for 0.5 h under pressure of 8 bar Argon. X-ray diffraction patterns show that all the samples have MgB₂ as the main phase with a very small amount of MgO; further, with SiC-doped, the presence of Mg₂Si is also noted. The magnetization-temperature measurements showed a transition temperature of 37.5 K for the undoped sample which indicates the typical transition temperature of MgB₂. When the content of SiC increased in the sample, the transition temperatures decreased to the lower temperatures systematically. The M?CH loops measured using a VSM showed very large magnetization value at low temperature for SiC doped samples. The largest M?CH loops were taken from the sample contains 5% SiC. The critical current density of samples calculated from M?CH loops indicated a value of around 4×10⁵ A/cm², which is in good agreement with the literature.     

Cyclotron Resonance in ZnO Heterostructures at High Magnetic Fields

The carrier-density dependence of the cyclotron resonance was studied in ZnMgO/ZnO heterostructures between n=5.4×10¹² cm^?2 and n=7.5×10¹² cm^?2. The effective mass was obtained as m ^?=0.32m ₀, and the significant carrier-density dependence of the effective mass was not observed in the present samples. This is partially due to the suppression of the resonant polaron effect in the dense carrier system. Oscillatory behavior appears in the cyclotron resonance spectra at the higher magnetic field side of the resonant field. The oscillatory period is perfectly coincident with the one of the Shubnikov-de Hass oscillation.     

Dependence of Superconductivity and Its Weakly Linked Behavior in Bulk LaO1?x F x FeAs on F Doping

Samples of oxypnictide compound LaO_1?x F _x FeAs, with x=0.15 and 0.2 corresponding to over- and highly over-doped compositions, respectively, were prepared by solid-state reaction. We present their characterization by XRD and HRTEM, as well as resistivity ??(T), magnetization M(B) and microwave modulated absorption (MMA) response between 4.2?C300?K and applied fields B=0?C8?T. With change in?x, both the superconducting and magnetic behavior of the samples shows an interesting pattern. The ??magnetic anomaly?? at T??130?K, observed in M(T) for x=0, instead of getting totally suppressed shows a tendency to reappear in x=0.2 sample. Both samples typically show ??(300?K)>2.8×10^?3????cm and critical current density J _c(5?K, 1?T)<2×10⁷?A/m². The superconducting transitions as measured by ??(T) at B=0 are found broad for both x=0.15 and 0.2 samples with transition widths ??2.5 and 6?K, respectively. The slope |dB _c2/dT| (where B _c2 is upper critical field) determined by resistive onsets, for the x=0.15 and 0.2 samples, has values ??7.5 and 3.5?T/K, respectively. The superconducting state characteristics as reflected by ??(T,B), M(T), magnetic J _c(B) and MMA response are typical of the presence of weakly linked inter-grain regions in both the samples. Our HRTEM images of the x=0.15 sample show the presence of high angle (??43^°) grain boundaries, which are well known to limit the J _c in cuprate-based high T _c bulk materials.     

Structural relaxation and electrical conductivity of molybdovanadate glass

⁵⁷Fe Mössbauer spectrum of conductive barium iron vanadate glass with a composition of 20BaO·10Fe₂O₃·70V₂O₅ (in mol%) showed paramagnetic doublet peak due to distorted Fe^IIIO₄ tetrahedra with isomer shift (δ) value of 0.37 (±?0.01) mm s^?1. Mössbauer spectra of 20BaO·10Fe₂O₃·xMoO₃·(70???x)V₂O₅ glasses (x?=?20–50) showed paramagnetic doublet peaks due to distorted Fe^IIIO₆ octahedra with δ’s of 0.40–0.41 (±?0.01) mm s^?1. These results evidently show a composition-dependent change of the 3D-skeleton structure from “vanadate glass” phase, composed of distorted VO₄ tetrahedra and VO₅ pyramids, to “molybdate glass” composed of distorted MoO₆ octahedra. After isothermal annealing at 500 °C for 60 min, Mössbauer spectra also showed a marked decrease in the quadrupole splitting (Δ) of Fe^III from 0.70 to 0.77 to 0.58–0.62 (±?0.02) mm s^?1, which proved “structural relaxation” of distorted VO₄ tetrahedra which were randomly connected to FeO₄, VO₅, MoO₆, FeO₆ and MoO₄ units by sharing corner oxygen atoms or edges. DC-conductivity (σ) of barium iron vanadate glass (x?=?0) measured at room temperature was 3.2?×?10^?6 S cm^?1, which increased to 3.4?×?10^?1 S cm^?1 after the annealing at 500 °C for 60 min. The σ’s of as-cast molybdovanadate glasses with x’s of 20–50 were ca. 1.1?×?10^?7 or 1.2?×?10^?7S cm^?1, which increased to 2.1?×?10^?2 (x?=?20), 6.7?×?10^?3 (x?=?35) and 1.9?×?10^?4 S cm^?1 (x?=?50) after the annealing at 500 °C for 60 min. It was concluded that the structural relaxation of distorted VO₄ tetrahedra was directly related to the marked increase in the σ, as generally observed in several vanadate glasses.     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.     

Improved Transport J c in MgB2 Tapes by Graphene Doping

Graphene is a special form of carbon which can effectively enhance the critical current density J _c of MgB₂. In this work, a systematic investigation on the impact of sintering conditions and doping level was carried out for graphene-doped MgB₂ tapes. It is found that an appropriate addition level, i.e., 8 at% in this work, is very critical to obtain a high J _c in graphene-doped samples. The critical field and pinning force are improved obviously due to the graphene doping. The magnetic J _c of samples sintered at 800 °C with 8 at% graphene doping reached 1.78 × 10⁴ A/cm², at 5 T, 20 K. At the same time, the transport J _c was up to 2.38 × 10⁴ A/cm² at 10 T, 4.2 K. The lattice distortion caused by C substitution and residual C at the grain boundaries were thought to be the major factors affecting the J _c of graphene-doped MgB₂ samples.     

Superconducting and metallurgical properties of bronze processed Nb3Sn wires with Ni and Zn additions and comparison with Ta and Ti additions

Critical current density measurements up to 23 T of nineteen core Nb₃Sn wires with simultaneous addition of Ni to the core and Zn to the Cu-Sn bronze matrix have revealed a considerable increase of J_c at fields above 11 T. For a wire with the composition Nb-0.6 wt% Ni/Cu-10 wt% Sn-3 wt% Zn, reacted at 750°C for 64 h, J_c in the layer was determined to 1.3 × 10⁵A cm^?2 at 14 T and to 4 × 10⁴A cm^?2 at 19 T. Comparison with Ta and Ti core-alloyed Nb₃Sn wires, also performed in the present study, shows very similar results in J_c and J_c vs. ε up to the highest fields.Composition profiles for Sn and for Ni, Ta and Ti additives in the A15 layers were studied by Auger spectroscopy. For Sn a concentration gradient across the layer (from ≈ 25 at% to ≈ 22 at% Sn) was observed, with the highest Sn content occurring at the interface with the bronze. The presence of the additives in the layer was detected as well by Auger analysis as by X-ray diffractometry.     

The solid state proportional counter of an organic insulator

The conduction of holes in a fluorocarbon polymer film was studied under the condition of irradiation of alpha-particles emitted from ²⁴¹Am. Pulsed currents were observed from the film under an electric field above 8 × 10⁴ V/cm in the presence of He atoms in the film. The dependence of the current on applied voltage was reproduced by the formula exp[a(V ? V_t)] as the multiplication factor for a parallel-plate proportional counter. The hole mobility was determined to be 1.0 × 10^?2 cm²/V·s.     

Synthesis,nonlinear optical properties and cellular imaging of hybrid ZnS nanoparticles capped with conjugated terpyridine derivatives

A novel organic/inorganic nanohybrids which consisted of ligand L (L = 2-[(2-hydroxy-ethyl)-(4-[2,2′;6′,2″] terpyridin-4′-yl-phenyl)-amino]-ethanol), an optical terpyridine derivative and ZnS nanoparticles (NPs), had been prepared through a solution-phase synthesis technique. The intermolecular interactions at the interface between ZnS and the ligand components were analyzed by FT-IR, far-IR, UV–Vis absorption spectroscopy, XRD and TEM. Particular properties had been shown by fluorescence spectra, fluorescence lifetime, Raman spectrum, aggregation emission and non-linear optical response. The results indicated that the nano-composite L–ZnS NPs had an obvious aggregation-induced emission in ethanol/n-hexane mixtures, and had larger two-photon absorption (TPA) when compared to the free ligand L. The data for the TPA cross-section value (σ = 16,247.8 GM), nonlinear refractive index (γ = 4.46 × 10^?13 cm² W^?1) and the third order nonlinear polarizability [Imχ ⁽³⁾ (esu) = 1.13 × 10^?14] were measured and discussed. Meanwhile, due to the laser irradiation induced charge transfer from the ligand to ZnS NPs, the composite could be potentially applied in vitro and in vivo cellular imaging.     

Optical conductivity of single crystals of Ba1−x Mx BiO3 (M=K,Rb, x=0.04, 0.37)

Reflectance data (0.001–4.0 eV) from several large (a typical surface area 3×3 mm²) single crystals of Ba_1?x K_xBiO₃ (x=0.04, 0.37) (BKBO) and Ba_1?x,Rb_xBiO₃ (x=0.37) (BRBO) were obtained by Fourier transform infrared (FTIR) and ellipsometric methods. Normal-state optical conductivities (σ₁) of these samples were obtained from infrared and ellipsometric measurements using a Kramers-Kronig transform. A broad mid-IR band was observed that peaked at 0.3 eV for BKBO and at 0.16 eV for BRBO at room temperature. Each band was fitted with two Lorentz oscillators. The optical mass of the charge carriers was obtained from a Drude fit, and was found to be large (m= 28?33m _e ). These overdamped charge carriers can be viewed as polarons with a large effective mass. An optic phonon mode at 325cm^?1 was also observed in the metallic phase. This mode was identified as a disorder-induced lattice mode, and was strongly enhanced at 8 K, favoring a strong coupling between this phonon and itinerant electronic states. Low-frequency spectra between 10 and 400 cm^?1 observed below the superconducting temperature indicated an energy gap that agreed with the BCS-type mechanism. Interpretations of low-temperature measurements on BKBO and BRBO were complicated due to the change of color of the sample from bluish-green to bronze-red. Upon warming, samples revert to their original bluish-green color.     

Acoustic-wave-stimulated transformations of radiation defects in γ-irradiated n-type silicon crystals

The effect of an ultrasonic treatment (UST) in various regimes (f _UST = 4?30 MHz; W _UST = 0.1?2 W/cm²) on the electrical activity of radiation defects in γ-irradiated (D = 10⁸ and 10⁹ rad) n-type silicon crystals doped with oxygen (～10¹⁸ and <5 × 10¹⁵ cm^?3) has been studied. The energies and concentrations of the electrically active centers have been determined from an analysis of the temperature dependence (100?300 K) of the Hall effect characteristics, assuming a multilevel structure of these centers. The main types of the acoustically active defects, which change the properties of the semiconductor material upon UST, are the A-type centers (E _c ? 0.20 eV) and divacancies (E _c ? 0.26 eV) in Czochralski-grown single crystals and the divacancies and/or P _s?C _i complexes (E _c ? 0.23 eV) in floating-zone-melted samples.     

Investigation of traps in AlGaN/GaN HEMTs by current transient spectroscopy

Deep levels in AlGaN/GaN HEMTs on Si substrate are known to be responsible for trapping processes like: threshold voltage shift, leakage current, degradation in saturation current and hysteresis effect. The related deep levels are directly characterized by Conductance Deep Level Transient Spectroscopy (CDLTS) method. Hereby we have detected four carrier traps with activation energy of 0.83, 0.50, 0.20 and 0.07 eV and capture cross-section respectively of σ = 3.14 × 10^− 14 cm², σ = 2.57 × 10^− 15 cm², σ = 3.03 × 10^− 17 cm² and σ = 2.65 × 10^− 15 cm². All these traps are located between the substrate and the two-dimensional electron gas (2DEG) channel.