Deborah Van Vechten

STRAIN ENERGY OF THREE-MEMBERED RINGS : A NEW ULTRADIAGONAL DEFINITION AS APPLIED TO SILICON- AND CARBON-CONTAINING SPECIES

Strain energies (SEs) in the 3-membered rings (CH2)2X and (SiH2)2X, where X is CH2, NH, O and SiH2, have been calculated by ab initio methods using the homodesmic reactions cyclo-(CH2)2X → 12cyclo-(CH2)4X2 and cyclo-(SiH2)2X → 12cyclo-(SiH2)4X2. All species involved were fully geometry optimized by HF6–31G∗∗ calculations, and energies were calculated at the MP2//SCF level. Thermal energy contributions were included. For the (CH2)2X series, we find that the SEs decrease on going from X being CH2 to NH to O, but increase substantially when X is SiH2. For the (SiH2)2X series we find a slight increase from X = CH2 to X = O, but all of these values are higher than for X = SiH2. The predicted SEs are in accordance with the limited experimental information available.

Effect of additive noise and bandpass filter on the performance of a Josephson junction noise thermometer

A Josephson junction may be used to convert the voltage fluctuations generated by Johnson (i.e., thermal) noise in a resistor into frequency fluctuations. The variance of the frequency fluctuations is thus a measurement of the Johnson noise and, therefore, of the temperature. This particular type of noise thermometer has been used at the National Bureau of Standards as part of a program to define a cryogenic temperature scale. We have studied the detailed influence of two types of post‐detection bandpass filters (square and one‐pole Butterworth) on the measured variance. We report here on these measurements and the circuit model used to fit them. The conclusion is that small but well understood corrections must be applied to the measured variance whenever the measurement time (i.e., gate time) of the frequency becomes comparable with the response time (defined as the inverse of the bandwidth) of the bandpass filter.

Superconducting detectors for photon and particle spectroscopy: Criteria for transitions to normal state

Abstract The nonequilibrium state produced when a high energy (>1 keV) quanta interacts with a superconducting solid is the physical basis for many new detectors. The most common approaches magnetically monitor the phase of small “grains” (SGDs) or monitor transients in the quasiparticle tunneling current in voltage-biased Josephson junctions (TJDs). While the former requires transitions to the normal state for its signal, the latters energy resolution is degraded by excursions in the order parameter. To facilitate detector design, this article quantifies how the choices of sample material, bath temperature and field bias combine to influence the magnitude and spatial extent of the thermal excursion. Extensive tables of materials parameters are included. A wide range of inherent effects including the magnetic field dependent latent heat of the transition are discussed in the context of the limitations they produce on the operating energy range and size of proposed SGD detectors. Superheated initial conditions are not considered because the need for automatic resetting in astrophysical X-ray spectroscopy. Of the 23 elemental superconductors, we show that as SGDs only 5 have maximum grain sizes larger than 10 μm and only 3 are usable for photons with energies above 100 keV. Alpha particle detectors will be restricted by conceptually equivalent physics. Only by using inhomogeneous initial states or local, rather than global, transitions may these restrictions on bolometric detectors be lessened.

A self-calibrating rhodium-iron resistive SQUID thermometer for the range below 0.5 K

We report on experiments with a prototype resistive SQUID device which show that it can serve both as a primary noise thermometer and as a secondary resistance thermometer in the range 0.01–0.52 K. The resistor in the circuit was made from an alloy of Rh with 0.5% Fe whose resistivity has an appreciable temperature dependence in this range. The high sensitivity of the SQUID allowed the resistance to be measured very accurately with negligible dissipation of heat. Since values of absolute temperature could be obtained by noise thermometry, the device was in effect a self-calibrating resistance thermometer. This combination of features is a rarity in thermometry in general, and may be unique in this temperature range. A version of this new thermometer has been fabricated and tested in the range 0.01–0.52 K. The results of experiments with this prototype are described, its limitations are examined, and ways of improving it are outlined.

Alternative non-equilibrium superconducting X-ray detectors

Non-equilibrium X-ray detectors based on isotropic superconductor absorbers offer the hope of energy resolution limited by the statistics of excitation production, i.e., δEE ∼ (ΔE)12, where Δ is the superconducting gap energy. This can be numerically equal to that of the best bolometers at significantly higher operating temperatures. Superconducting Tunnel Junction Detectors (STJDs) count the X-ray event induced excitations (quasiparticles (QP)) as an excursion of the subgap tunnel current. While STJDs are presently the best studied QP readout scheme, they have made slow progress toward the limiting energy resolution. Some of the difficulties are enumerated herein and are inherent in tunnel junctions. One alternative is to measure the conductivity excursion of a superconducting film that follows photon absorption. This excursion obeys exactly the same equation as the effective quasiparticle tunneling conductivity in a Josephson junction. Hence the two schemes should produce the same readout waveforms when the same superconducting absorbers are used. Seven specific approaches to realizing the measurement are presented. All versions of the conductivity readout offer simpler sample fabrication than STJD. Any superconducting material may be used. In commercial applications, the obvious benefit in operating temperature of using the HTSC materials may more than compensate for the penalty in energy resolution.

Epitaxial base layer Nb superconducting tunnel junctions with Ta absorbers

Abstract Superconducting tunnel junction detectors have shown theoretical promise in high resolution X-ray spectrometry. Here we present results for epitaxial Nb tunnel junctions with relatively thick Ta absorbers. In these devices the epitaxial Ta absorber also forms the base electrode for the junction. We report on several junction geometries where both the total size of the junction and the size of the absorber is varied. Current results show a maximum resolution of 65 eV at 5.89 keV, limited primarily by external noise in the junction signal. Significant differences in junction performance are observed for the variation in absorber size, although variation in total junction size from 15 × 15 μ m to 40 × 40 μ m shows no consistent effect on the resolution. The results indicate a significant loss of recombination phonons into the substrate.

The energetics of ions, atoms, and salts: Sodium and its chloride

In this study we reconcile three seemingly contradictory assertions regarding sodium chloride. First, gaseous sodium chloride is Coulombically bound and highly ionic. Second, upon pulling the molecule apart atomic sodium and chlorine are produced. This is somewhat surprising; despite the high ionicity of NaCl, since IP(Na) > EA(Cl). Third, heterolytically dissociating NaCl(g) into Na++Cl+ costs more energy than ionizing gaseous Na into Na++e−. Does this violate Coulombs law since the Na-Cl bond distance in NaCl(g) is greater than the average Na nucleus-valance electron distance?

Non-periodic trends in the oxidative dissolution of transition metals by manganese(III) acetate

Abstract As part of a study designed to synthesize the proposed high temperature superconductor MoN, molybdenum powder was reacted with manganese(III) acetate and sodium azide in refluxing glacial acetic acid. No MoN was found. Instead, rapid and complete oxidative dissolution of the metal to soluble forms of Mo(VI) were observed. Removal of the azide or replacement of the azide by chloride had no qualitative effect. Other metals were studied and four groups may be discerned: (1) quantitative dissolution in cold, pure acetic acid - Mn, Fe, Co; (2) quantitative dissolution in refluxing Mn(III) solution  Cu, Mo, In, Sn; (3) partial dissolution in refluxing Mn(III) solution  Ni, V; (4) no reaction  Ti, Zr, Nb, Ta, Cr, W, Re, Rh, Ru, and Pt. Use of preformed metal nitrides, e.g. MoN 0.6 , and variation of the oxidant, e.g. replacing Mn(III) by Fe(III) or by Co(III), resulted in little qualitative change. Some unsuccessful attempts to explain this include literature periodic trends for transition metal chemistry and Pourbaixs electrochemical ‘thermodynamic and practical nobility scales’ for corrosion by aqueous media.

The superconductive energy gaps of thin-film AuAl2 and AuIn2

Thin films of the intermetallic compounds AuAl2 and AuIn2 were prepared and used as electrodes in AuAl2/oxide/Al,Al/oxide/AuIn2, or AuAl2/oxide/AuIn2 tunnel junctions. The tunnel barriers were produced by rf sputter oxidation. The temperature dependence of the energy gaps of the AuAl2 and AuIn2 films was measured and found to agree well with BCS theory.

Nb/Al/AlOx/Nb superconducting tunnel junctions as x-ray detectors

Nb/Al/AlOx/Nb junctions have been shown to be effective x-ray detectors that are robust to thermal cycling. We compare results from two junctions, one with a fine-grained base-layer electrode and counterelectrode and the other with an epitaxial base-layer electrode and a fine-grained counterelectrode. For a 6 keV x-ray, at 0.4 K, the epitaxial sample had a FWHM resolution of 157 eV and the fine-grained junction had a FWHM resolution of 300 eV. The differences between the junctions will be discussed.