Deborah Van Vechten

United States Navy · Office of Naval Research

The Armed Forces has a long standing requirement for a robust global communications infrastructure to support real time operations. The bandwidth elasticity continues to be tested as demand continues to grow in support of multiple theater events. The US Army CERDEC Space and Terrestrial Communications Directorate, coupled with the US Navy is doing...

Thin films of amorphous SixNl-x alloys were produced by nitrogen ion beam assisted deposition of electron beam evaporated silicon. Infrared reflection spectra were measured in the range 600 to 10000 cm−1. Fringes were observed due to interference between light multiply-reflected from the front surface and film-substrate (single crystal silicon) int...

The annealing behavior of the optical properties of silicon nitride films (Si 1-x N x ) is described for films fabricated by ion beam assisted deposition. The data are needed for the precise manufacture of optical filters, where the index of refraction must be predicted from deposition parameters and film annealing history. The reflection of homoge...

Superconductor analog-to-digital converters (ADC) offer high sensitivity and large dynamic range. We report recent advances in the low-pass phase-modulation-demodulation (PMD) oversampled analog-to-digital converter (ADC). The oversampled data from the PMD front-end are passed along with sampling clock to an on-chip digital filter. In the past, the...

To confirm previously reported evidence of high-temperature superconductivity in laser processed Sr-Ru-O, we performed simultaneous two-probe and four-probe resistive measurements using bar-geometry samples. A superconducting-type transition with an onset at about 250K was recorded in one of the samples, consistent with our previously reported meas...

We have discovered that samples of a new material produced by special processing of crystals of Sr2RuO4 (which is known to be a triplet superconductor with Tc values ~1.0-1.5K) exhibit signatures of superconductivity (zero DC resistance and expulsion of magnetic flux) at temperatures exceeding 200K. The special processing includes deposition of a s...

This Superconducting Technology Assessment (STA) has been conducted by the National Security Agency to address the fundamental question of a potential replacement for silicon complementary metal oxide semiconductor (CMOS) in very high-end computing (HEC) environments. Recent industry trends clearly establish that design tradeoffs between power, clo...

''QVD'' detectors are based on thermoelectric heat-to-voltage (Q --> V) conversion and digital (V --> D) readout. We have devised and analyzed the performance of QVD detectors with several different sensor designs that enable use of high thermoelectric figure of merit samples, be they of thin film, bulk crystal, or whisker form. Our first QVD devic...

Practical implementation of thermoelectric (QVD) detectors requires cryogenic thermoelectric sensors with a high figure of merit. For this requirement there can be different solutions: thin films, bulk materials and whiskers. Initial prototype QVD devices used Au–Fe thin film sensors. Another class of materials, lanthanum (cerium) hexaborides La(Ce...

‘QVD’ detectors are based on thermoelectric heat-to-voltage (Q→V) conversion and digital (V→D) readout. In theory, they are competitive with superconducting tunnel junction detectors and transition edge sensor devices. We analyze the performance of the QVD detectors with different design architectures. It is concluded that the detectors with lantha...

As we reported earlier [1], thermoelectric detectors can be competitive as nondispersive energy resolving focal-plane instruments in X-ray/UV spectrum. The first generations of prototype devices demonstrated the viability of detector design and provided good agreement between theoretical expectations and experimental data. These devices exploited s...

Development of single photon sensors capable of determining the energy (“color”) of each photon incident on a pixelated focal plane in real time is a central activity of many groups worldwide developing state-of-the-art hardware for the space astrophysics community. Terrestrial applications in materials analysis are also being targeted. The most su...

A feasibility study of megapixel microcalorimeter arrays, based on thermoelectric energy to voltage conversion and digital superconducting readout, is presented. The design concept originated from the philosophy of employing the simplest principles at the single-pixel level to enable large arrays without sacrificing energy resolution, fast operatio...

A novel hot-electron microbolometer concept suitable for hyperspectral imaging at high photon counting rates is described. When each photon is absorbed, its energy is thermalized and homogenized within the normal-metal absorber to provide a highly reproducible temperature gradient across a thin film thermoelectric sensor which separates the absorbe...

Transient voltage pulses spontaneously occur in anisotropic films following the absorption of photons. Experiments on normal state YBCO unambiguously confirm that the pulses sensed between the sample ends arise from the thermoelectric response to the heat flux propagating through the film, perpendicular to the voltage measurement axis. Good agreeme...

We have deposited and investigated thin films of lanthanum hexaborate with 1% of the lanthanum replaced by cerium. In bulk single-crystalline form, this material has, due to the Kondo-mechanism, the highest known Seebeck coefficient at sub-K temperatures. Thus it is a good candidate for several thermoelectric applications at very low temperatures....

The inherent properties of naturally layered materials such as the high-temperature superconductors cause a transient voltage when a light pulse is absorbed. The amplitude is proportional to the absorbed energy. Observable pulses arise in the HTS materials even at room temperature. A phenomenological model of the effects consistent with our observa...

Transient longitudinal voltage pulses occur in the normal state of thin film samples of high-temperature superconductors following the absorption of light pulses. Our experiments unambiguously confirm that the pulses arise from the thermoelectric response of these layered oxide materials to the heat fluxes propagating within the film in the directi...

Energy transfer from hot-electrons to phonons at low temperatures may take a longer time than the duration of transient thermoelectric effect. In this time interval phonons are dynamically decoupled from hot-electron heat flux so that an effective enhancement of thermoelectric figure of merit takes place. Based on this conclusion, we developed a co...

The inherent properties of naturally layered materials such as the high temperature superconductors cause a transient voltage when a light pulse is absorbed. The amplitude is proportional to the absorbed energy. Observable pulses arise in the HTS materials even at room temperature. A phenomenological model of the effects consistent with our observa...

The direct transformation of the energy of an incident high-energy photon into a measurable potential difference within an absorbing metal is investigated. Experimental evidence is presented that the effect arises from the inherent energy dependence of the electronic density of states, rather than from a simple temperature excursion. The similariti...

A non-zero, gauge invariant potential μ has been recognized since 1972 to occur in superconductors in various non-equilibrium circumstances. It is central to the description of the time dependent properties. Non-zero values of this potential cause gap suppression. Thus if the local value of the potential μ can be made sufficiently large, the materi...

UV and X-ray astronomy need hyperspectral imaging devices, i.e. large format (1000×1000) arrays of pixels in which each single-photon event is tagged with spectral information. Such devices permit multicolor imagery and surveys to be obtained with a single camera. Here we discuss the first-stage of development of a new generation of UV/X-ray detect...

When a superconductor is disturbed by a high energy deposition, an asymmetry in the distributions of electron-like and hole-like excitations results. This manifests itself as a voltage output, which can be measured in different ways. Here we study the behavior of this quantity using the kinetic equations to model the energy cascade. Special attenti...

The concept of the electrochemical potential in thermodynamic equilibrium is clearly understood. Moreover, a prescription exists for determining its value from the temperature dependent distribution function. However, when the material absorbs a high energy quanta, the local distribution function is strongly disturbed and in general does not have a...

The creation of quasiparticle branch imbalance in superconductors, when pumped by high-energy quanta, is considered. The physical basis for imbalance creation lies in the geometrical difference between the electron- and hole-like excitation densities of states in momentum space. Simple analytical solutions for distribution functions of electron- an...

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., , 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 Jun...

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 calculat...

The mechanism for creation of electron-hole branch imbalance in superconductors subjected to high-energy particle/radiation bombardment is considered. The physical basis for the production of imbalance lies in the geometrical difference between the densities of states in phase space for electron- and hole-like excitations in metals. The requirement...

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 t...

A new design for high‐energy radiation/particle detectors is presented. The nonequilibrium response of a superconductor to the absorption of the incident quanta is sensed by electromagnetic measurements of the altered dynamic conductivity. Microwave absorption may be used to amplify the signal. Such a detector will provide better energy resolution...

Superconducting tunnel junctions have been shown to have potential as high resolution X-ray detectors. Recent reductions in the noise environment of our experiment have yielded a substantial increase in the resolution of our Nb/Ta junctions. The junctions used in the experiment are epitaxial base layer devices including a thick Ta absorber and have...

SIS tunnel junctions (STJD) are predicted to have exceptionally good energy resolution as x-ray detectors. This paper spectra illustrative of the current state of the art. They were obtained when ⁵⁵Fe and ¹⁰⁹Cd x-ray sources bombarded an epitaxial base layer Nb/Ta/Al/AlO_x/Al/Nb junction having a diamond shape and si...

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, heteroly...

The geometrical shape and layer thicknesses of superconducting tunnel junctions (STJ) influence their suitability as x-ray detectors. Examples relating to field biasing are discussed. Two related energy loss mechanisms are also modeled.

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 samp...

The authors consider the consequences of fabricating integral X-ray detectors with thin electrodes, 0.17 μm for Nb and 0.25 μm for Sn, with 6-keV photons incident. A simple geometric argument demonstrates that energetic electron loss during the first 1 ps of each event will cause less than the full energy of the incident photon to be captured in a...

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...

We report on our ongoing work using Nb/Al/AlO(x)/Nb junctions for the detection of X-rays. Detectors based on superconducting tunneling junctions offer the prospect of resolution over an order of magnitude higher than is obtainable with the current generation of semiconductor-based detectors. Results of measurements taken at 1.85 K (a temperature a...

Voltage-biased, superconducting tunnel junctions are investigated as x-ray detectors for applications requiring both high quantum efficiency and better than 1% energy resolution. The nonequilibrium quasiparticles, produced as the energy deposited degrades to the few-meV-per-excitation level, tunnel and are detected before they are lost to recombina...

The response of Nb-Al-Al₂O₃-Al-Nb superconductor-insulator-superconductor (SIS) tunnel junctions to pulsed 650-nm radiation was measured. The rise in the voltage developed across the junction increases linearly for the duration of the applied pulse. The decay time of 300 μs is an intrinsic property of the junction. The respons...

Optimization of the energy resolution of Josephson tunnel junction X-ray detectors requires the minimization of event-to-event variation in the number of nonequilibrium quasiparticles that tunnel and are thereby detected for monochromatic input. This requires a detailed understanding of the determinants of the time scales for the degradation of the...

We consider global transitions from superconducting to normal state of small grains induced by the absorption of energetic (1- 100 keV) photons. Initially the energy is transferred to a single photoelectrons, which loses the energy as it scatters inelastically. To measure the energy of the photon, this collision cascade must be contained within the...

An ion‐beam‐assisted‐deposition (IBAD) system is under development to fabricate Si 1-x N x films for optical devices. Reproducible film composition requires characterization of the relationship between the incorporated nitrogen atom fraction x and the real time experimental measurable quantities. In this paper a simple model is presented which rela...

A method is given to obtain an absolute calibration of the ion and evaporant fluxes in an ion‐beam‐assisted deposition system based upon a Kaufman ion source and an electron beam vapor source. The nitrogen‐ion silicon‐vapor material system is used for the calibration; Rutherford backscattering is used for measurement of composition and thickness of...

We report on the epitaxial growth of superconducting molybdenum films on sapphire substrates. These films are to be etched into arrays of isolated cylinders, each 1–5μm in diameter. When placed in a magnetic field and biased at 0.95 Tc(H), the flux movement associated with their bolometric response to the energy deposited when radiation is absorbed...

The rugate filter employs a sinusoidal refractive index depth profile to produce high reflection in a narrow band of wavelengths. Fabrication relies on a continuously variable index of refraction in the wavelength regime of interest. The near IR refractive index of amorphous silicon-nitrogen films decreases continuously as the composition varies fr...

Elastic backscattering of helium ions accelerated to 6.2 MeV has been used to determine the compositional depth profile of thin films of high Tc superconductors. The higher than normal bombarding energy allows separation of the contributions from Cu, Y and Ba in the measured spectra for film thicknesses up to about 5500 Å, rather than about 1000 Å...

We have produced alloy films of B(1−x)Nx (x:0 to 0.5) via ion beam assisted deposition (IBAD). Rutherford backscattering spectroscopy (RBS) measurements show that the stoichiometric films are achieved with a nitrogen to boron atom arrival rate of N/B = 2.55. The film N/B ratio for all measured compositions is fit with a model that predicts that the...

Infrared rugate filters were fabricated under computer control by nitrogen beam-assisted silicon e-beam evaporation. Refractive index, composition, deposition parameters, and thermal treatments were correlated.

We have studied normal-state and superconducting transport properties in a granular cermet consisting of B1-structure NbN grains in a boron nitride insulating matrix. By varying the volume fraction of the two components we produced films (20--70 nm in thickness) that exhibited transport behavior in either of two distinct and mutually exclusive clas...

We have measured with high accuracy (100 ppm) and high precision (5–10 ppm) the impedance of a rf-driven resistive superconducting quantum interference device (SQUID) as a function of the amplitude and frequency of the radio-frequency (rf) bias, as a function of the current bias, and as a function of several other circuit parameters. We have develo...

Thin films of substoichiometric silicon nitride of uniform composition have been produced by ion beam assisted deposition (IAD) of electron beam evaporated silicon on amorphized Si substrates. IAD films are found to have improved adhesion, increased density, and markedly lower oxygen contamination than non-IAD films produced simultaneously. The Res...

With the goal to test a new synthetic pathway to the proposed high temp. superconductor MoN, Mo powder has been reacted with manganese(III) acetate and NaN3 in refluxing glacial acetic acid.

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 re...

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Molecular chemiluminescent emission has been observed when the vapors of the transition-metal halides TiCl 4, VCl 4, NbCl 5, and MoCl 5 (as well as VOCl 3) are injected into an active nitrogen stream. The carrier species have all been identified from their respective high-resolution spectra as the metal mononitrides (MN).

Fractal-like structures have been observed in sputter-deposited thin films of NbGe2. These structures exhibit a striking resemblance to those produced by computer simulations of diffusion-limited aggregation using the Witten-Sander model. An effective fractal dimensionality of about 1.9 has been determined from digitized photomicrographs. The resul...

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 th...

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 B...

Recent experiments on noise thermometers made with Josephson junctions [rf SQUID's (superconducting quantum interference devices) shunted by a small resistance R] have shown that the SQUID impedance has an envelope which generally rises to the value of R as the rf power is increased, although the envelope can be distorted when the frequency is just...

In this paper we show that the large number of approaches using apparently unrelated strainless increments for unsubstituted alicyclic hydrocarbons in the literature are neither mathematically nor conceptually unique. We additionally demonstrate that if the strain energy assigned to a compound by any three sets of increments is known, the strain en...

In this paper, we assess the ability of trifluoro-methyl groups to thermodynamically stabilize unsaturated and strained organic species relative to saturated analogues. Theoretical calculations with the STO-3G basis set are utilized in this analysis. The resulting theoretical data, as well as experimental calorimetric data, indicate little stabiliz...

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 hig...

The Aslamasov-Larkin (AL) theory can be derived from a classical treatment of the conductivity due to short-lived statistical fluctuations into the superconducting state if one truncates the Ginzburg-Landau free energy density expression to read F(psi) = ..cap alpha../sub 0/ vertical barpsi vertical bar/sup 2/ + c/sub 0/ vertical bar del psi vertic...

Twenty six American male, post Ph.D. qualifying exam students in the UMCP physics program were interviewed this June in order to probe why they had chosen their research subfield. The results were analyzed in terms of the relative weights given to, and the degree of overlap between, the student's inherent interests and the economic realities of the...

During the cascade that follows an x-ray absorption event, the mean energy of the excitations passes through the Fermi energy 10 eV. After this, the majority of energy is in the form of conductivity band electronic excitations which cascade down in energy through the electron-electron and electron-phonon channels of relaxation, eventually creating...

The authors measured the dc impedance of an rf-biased R-SQUID as a function of applied dc bias current, rf frequency and amplitude, and the critical current of the weak link. They conclude that the inclusion of an rf driving current, as required for parametric upconversion influences, in an as-yet incompletely modeled way, the dc impedance of an R-...

An interpretation of the process of measurement is proposed which can be placed wholly within the quantum theory. The entire system including the apparatus and even the mind of the observer can be considered to develop according to the Schrödinger equation. No separation, in principle, of the observer and the observed is necessary; nor is it necess...