Superconductivity and normal-state properties ofThU alloys

Anomalies in the normal-state electrical resistivity, thermoelectric power, and magnetic susceptibility have been observed in dilute alloys of uranium dissolved in thorium. These anomalies appear to be due to localized spin fluctuations with a characteristic temperatureT ₀ 100K which are associated with5f virtual bound states on the U atoms. The depression of the superconducting transition temperature of Th by U impurities is large and deviates markedly from the form given by the Abrikosov-Gor'kov theory.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR-631-67-A.     

Free surface of a rotating superfluid

The free-surface profile near a perpendicular rectilinear vortex is studied with classical hydrodynamics. Numerical values and approximate analytic expressions are derived. For pure⁴He atT=0 K, the dimple has a depth 69.5Å and a radius 4.26×10 ⁴ Å at half depth. The addition of³He impurities increases the central depth. Possible detection of vortex lines with phasecontrast techniques is discussed.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR Contract No. F44620-71-C-0044.     

Light scattering in liquid helium

A quantum-mechanical formulation of light scattering in a dielectric fluid is applied to liquid helium. To lowest order in the (small) dielectric susceptibilityn, the theory reproduces the usual expression for Brillouin scattering. Two-roton processes occur in both first and second order inn, and the relevant ground-state matrix elements are evaluated with Feynman wave functions in the convolution approximation. The resulting two-roton differential cross section is significantly smaller than Stephen's prediction, in good agreement with the experiments of Greytak and Yan. The depolarization is somewhat reduced from that of Stephen; present techniques should be able to detect this difference.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR Contract No. F44620-71-C-0044.Alfred P. Sloan Foundation Research Fellow.     

Thermal resistance between powdered cerium magnesium nitrate and liquid helium at very low temperatures

Experiments have been performed from magnetic temperatures of 2–20 mK on the effect of minute⁴He impurities and a magnetic field on the thermal resistance between powdered cerium magnesium nitrate (CMN) and liquid³He. The thermal resistance decreases with decreasingT and increasing small field but is increased dramatically at a givenT by the addition of roughly a monolayer of⁴He. The resistance is interpreted as resulting from a surface magnetic coupling between CMN and liquid³He.This work has been supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A. 143, and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR-631-67A.     

Thermal contact between cerium magnesium nitrate and liquid3He at very low temperatures

A new experimental arrangement has been used to measure the times for equilibrium between cerium magnesium nitrate (CMN) and liquid³He. Measurements have been made on a single crystal of CMN from temperatures of 5.5–15 mK and on a powdered specimen from temperatures of 2.5–35 mK. The thermal resistance deduced from the single-crystal data is proportional toT ^–2. The thermal resistance obtained from the powder data may be separated into three contributions: a resistance which is proportional toT ^–2in series with a parallel combination of a phonon resistance (with resistivity _P =3×10 ^–4 T ^–3sec·cm²·K ⁴ ·erg ^–1 ) and a magnetic resistance (with resistivity _M =9×10 ² T sec·cm ² ·erg ^–1 in zero applied magnetic field). A reduction in the magnetic resistance was observed when the powdered CMN was placed in a magnetic field, the magnitude of this reduction being comparable to that expected from a Redfield-like theory. TheT ^–2resistance is discussed in terms of two-dimensional energy transport within the CMN. The 17-Hz susceptibility of the powdered CMN was measured in magnetic fields of 0, 18.9, 30.2, and 37.8 G. The magnetic temperatureT* (H=37.8 G) was found to differ by +0.5 mK from the magnetic temperature in zero field whenT* (H=0)=3 mK.Work supported by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A.143.Supported by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

The sensitivity of the superconducting transition temperature to residual stress in high-T c intermetallic compounds

Residual-stress-induced superconductivity is reported to occur only in the high-T _c members of the C14 and C16 crystal systems. Second transitions are also seen in ThIr₂(C15) and MoRe₃(A12). The change inT _c , as a result of introducing large residual stresses, increases with the bulkT _c .Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR Grant No. AF-AFOSR-631-67-A.     

Superconducting and normal state properties of Li1+x Ti2−x O4 spinel compounds. II. Low-temperature heat capacity

Heat capacity data are reported which confirm as a bulk effect the previously reported superconductivity in LiTi ₂ O ₄ . These data also establish LiTi₂O₄ as a weak couplingd-band superconductor with superconducting state properties well described by the Bardeen—Cooper—Schrieffer theory of superconductivity. The properties of LiTi ₂ O ₄ are compared with those of other superconducting spinel compounds, and the composition dependence ofT _c for Li _1+x Ti _2–x O ₄ is discussed. The disappearance of superconductivity forx0.1 was found to be correlated with a rapid decrease in the normal-state linear heat capacity coefficient.Research sponsored by the U.S. Energy Research and Development Administration under Contract No. ERDA E(04-3)-34 PA227.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract Number AFOSR/F-44620-72-C-0017.     

The structure,modes of deformation and failure,and mechanical properties of diaminodiphenyl sulphone-cured tetraglycidyl 4,4′diaminodiphenyl methane epoxy

The tensile mechanical properties of diaminodiphenyl sulphone (DDS) — cured tetraglycidyl 4,4diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (12 to 35 wt% DDS)] are reported as a function of temperature and strain rate. TGDDM-DDS (20 to 35 wt% DDS) epoxies, which exhibit broadT _gs near 250° C, are not highly cross-linked glasses because diffusional and steric restrictions limit their cross-link density. TGDDM-DDS (10 to 20wt% DDS) epoxies are more brittle with lowerT _gs as a result of lower molecular weights and/or lower cross-link densities. Electron diffraction and X-ray emission spectroscopy studies indicate that TGDDM-DDS (>25wt% DDS) epoxies contain crystalline regions of unreacted DDS which can be eliminated from these epoxies during cure resulting in microvoids. TGDDM-DDS (12 to 35wt% DDS) epoxies predominantly deform and fail in tension by crazing, as indicated by fracture topography studies. These glasses also deform by shear banding as indicated by right-angle steps in the fracture topography initiation region and mixed modes of deformation that involve both crazing and shear banding. No evidence was found for heterogeneous cross-link density distributions in TGDDM-DDS (15 to 35wt% DDS) epoxies on straining films in the electron microscope.Research sponsored by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract No. F44620-76-0075. The United States Government is authorized to reproduce and distribute preprints for governmental purposes notwithstanding any copyright notation hereon.National Science Foundation Faculty Research Participant: NSF Grant No. SER 76-04721.     

Clear evidence for the spin-wave energy gap in terbium

Our electrical resistivity measurements on single-crystal terbium oriented along the c-axis clearly show the spin-wave energy gap at 21.5°K. The total resistivity fits the equation =₀+2.43×10^–7 T ⁵+2.544×10^–3 T ² exp(21.5/T) µ-cm in the temperature region below 21.5°K.This project was supported by a National Research Council of Canada grant.Killam Postdoctoral Fellow.     

Demagnetization of Ce impurities in superconducting (La,Th)Ce alloys: Specific heat results

Detailed measurements of the specific heat jump C at the superconducting transition temperatureT _c as a function ofT _c are reported for several (La, Th)Ce systems. The measurements document the continuous demagnetization of the Ce impurity ions which proceeds with increasing Th concentration, and provide a critical test of a theory recently developed by Müller-Hartmann and Zittartz for the superconducting behavior of matrix-impurity systems which simultaneously exhibit both superconductivity and the Kondo effect.Research supported by the U.S. Air Force Office of Scientific Research, Grant No. AF-AFOSR-71-2073.Research supported by the University of Chile-University of California Cooperative Program sponsored by the Ford Foundation.     

Measurement ofP-V-T relations and critical indices of3He

By measuring the capacitance of a parallel-plate capacitor filled with³He as a function of temperature and pressure, we have determined the dielectric constant of³He along 28 isotherms between 3.00 and 3.45 K at pressures up to 10³ Torr, and along the coexistence curve between 3.20 K and the critical temperature for both saturated liquid and vapor. The density along these same trajectories can then be calculated assuming the Clausius-Mossotti relation holds. We have evaluated the critical temperatureT _c , pressureP _c , and density _c, as well as the critical indices , , , and .Work of this author supported by the Air Force Office of Scientific Research.Presently supported by the National Science Foundation.Supported in part by Grant No. AF-AFOSR 1117-66, Air Force Office of Scientific Research.     

Ultrasonic attenuation of longitudinal and shear waves in the normal and superconducting states of niobium

Ultrasonic attenuation measurements of both longitudinal and shear waves in the normal and superconducting states were made on niobium single crystals. Frequencies between 50 and 450 M H z were employed. Independent of the mode, frequency, and crystal studied, a deviation of the attenuation ratio _s/_n from BCS behavior was observed. The dependence of _s/_n on frequency and crystal purity are interpreted in terms of a model which assumes a difference in electron mean-free path due to phonon scattering in the normal and superconducting states.Research sponsored in part by NASA Grant 36-006-003 and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under ASOSR Grant No. 68-1479B.     

Superconductivity in the high-pressure phases of barium

The superconducting transition temperature of barium was redetermined as a function of pressure. The results are at variance with the work of Il'ina and Itskevich. Apparently mixed phase samples occurred rather generally. We trust that at least the overall shape of the pressure dependence ofT _c has been correctly determined.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR contract number AFOSR/F-44620-72-C-0017.     

The effect of nonmagnetic 4f resonances on the superconducting transition temperature:ThCe alloys

It is shown that a recent theory due to Kaiser concerning the effect of nonmagnetic resonant states on superconductivity appropriately and accurately describes the dependence of the transition temperatureT _c ofThCe alloys on Ce concentration. The parameters describing thef-resonant states in the Anderson model are determined by fitting Kaiser's theory to the data, and reasonable values are obtained. With the assumption that the Hartree-Fock level width remains constant under pressure, it is shown that thef level moves further above the Fermi level as the pressure is increased. This is discussed in relation to recent pressure experiments on La and La compounds containing Ce impurities.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under Grant No. AF-AFOSR-631-67-A.     

Influence of Fermi surface topology on the pressure dependence ofT c for indium and dilute indium alloys

The pressure dependence of the superconducting transition temperature for In, In-Cd (to 3.5 at.%), and In-Pb (to 12 at.%) has been determined to 24 kbar. A small deviation from a linear dependence ofT _c on volume compression is found for indium. Alloying with Cd and Pb modified this nonlinear contribution, which is interpreted as arising from changes in the Fermi surface topology. In all, four transitions are identified and their possible origins, based upon the known Fermi surface of indium, are discussed.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR contract number AFOSR-F44620-72-C-0017.     

Effect of annealing on the anomalous electrical resistivity of dilute copper-iron alloys at low temperatures

Electrical resistivity measurements in the temperature range 1.5–35 K on two copper alloys containing 115 and 380 atomic ppm iron are reported, in their unannealed state and also after annealing for 16 and 66 h in fore-vacuum at 530–550°C. Below the temperature of the resistivity minimum the impurity resistivity has the Kondo lnT behavior. However, in the liquid helium region the resistivity drops from its value atT=0, in proportion toT ², conforming to Nagaoka's theory forT<T _K/5. The Kondo temperatureT _K is evaluated from the versusT ² plots using Nagaoka's equation and is found to decrease with increasing concentration. Annealing is found to reduce the effective iron concentration and alsoT _K. The impurity resistivity per atomic percent in our samples can be expressed as a universal function ofT/T _K at the lowest temperatures underT _K/4.     

Electrical properties of exchange-enhanced systems: Fe in (Pd95Rh5)

The electrical resistivity of the giant moment system Pd₉₅Rh₅ containing between 0.8 and 1.85 at % Fe has been measured from 1.4 to 300 K. The incremental resistivity in alloys containing more than 1.25 at % Fe is found to exhibit aT ² limiting low-temperature form; however for the 0.8 and 1.1 at % Fe alloys such aT ² form is not clearly discernible, with (T) exhibiting a temperature dependence intermediate betweenT ² andT ^3/2. The former result is predicted from conduction electron-magnon scattering for which wave vector conservation holds, from which it is inferred that the criterion for wave vector conservation in this type of alloy isnot determined by mean free path effects. Estimates of the acoustic spin-wave stiffnessD are derived from the measuredT ² coefficients. These resistivity data also enable estimates of the exchange coupling parameterJ _s-local to be made. The magnetic ordering temperatureT _c is considerably less discernible than in single-component hosts but approximate values have been derived for the various alloys, from which estimates of the exchange coupling parameterJ _d-local have been made. In the disordered phase the measured incremental resistivity is found to contain a term which decreases approximately linearly with increasing temperature, at a rate of –(1.1±0.45)10^–3 µ cm/K at % Fe. Using existing pressure data on both Pd- and PdRh-based alloys, it is shown that both the sign and magnitude of this term can be accounted for in terms of the volume dependence of the potential and exchange terms, in conjunction with a large coefficient of thermal expansion.This work has been supported in part by the Defence Research Board under Grant No. 9510-100.     

Mössbauer and magnetic studies of dicalcium ferrite (Ca2Fe2O5)

In the antiferromagnetic compound Ca₂Fe₂O₅, the ferric ions occupy two non-equivalent sites. By means of Mössbauer studies in the temperature range 80 to 800 ° K, the variation of the magnetic hyperfine fields at both sites, as a function of temperature, is determined. It is found that all ferric ions order antiferromagnetically at a single Néel temperature, T _N=(725±2) ° K.From previous studies, the magnetic point group of this compound is known, and leads one to anticipate the existence of a ferromagnetic component in the ordered state. Magnetic measurements do, in fact, indicate the existence of such a component.The experimental values obtained for the quadrupole splittings at both ferric sites are compared with point charge calculations. This leads to a complete disagreement, both in the ordered region and in the paramagnetic region, showing the inadequacy of the point charge model for the present case.All the results are compared with the results of previous investigations on the same compound.Sponsored in part by the Air Force Materials Laboratory Research and Technology Division AFSC through the European Office of Aerospace Research, United States Air Force Contract F61052-67 C-0040.on leave from the Weizmann Institute.     

Superconductivity and phase transitions in single-crystal and polycrystal α-U at high pressure

The superconducting transition temperature of single-crystal and polycrystal uranium has been determined as a function of hydrostatic pressure to 24 kbar. Although the general form of theT _c (P) variation is similar for the two types of material, features are present in the detailed behavior for the single-crystal samples which are not found for polycrystal samples. These features are presumed to be associated with the first-order transitions observed at zero pressure. A low-temperature phase diagram has been constructed and the electronic nature of the various phases is discussed.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR contract no. AFOSR-F44620-72-C-0017.Work performed under the auspices of the U.S. Atomic Energy Commission.     

Surface acoustic wave determination of the superconducting fraction of a Nb3Ge film

Surface acoustic waves at 1200 MHz were used to investigate the properties of a 0.5-µm-thick film of Nb₃Ge. This film was rf-sputtered onto a substrate consisting of a piezoelectrically active 3.5-µm AlN layer which was chemically vapor-deposited over a sapphire substrate. The attenuation coefficient of the surface acoustic waves was measured from 0.8 to 30 K. The raw data are analyzed to eliminate interference effects due to splitting of the wave into two components. One of these is at the surface of the film and the other may be a surface skimming bulk mode which is at the film-substrate interface. The resultant curve of attenuation versus temperature in the superconducting region is then used to determine the distribution function of the superconducting transition temperature of the film. Although the film starts to become superconducting at 21 K and the majority of the film appears to become superconducting at 18 K, it is also found that a significant amount of the film does not become superconducting until 10 K. The difference between the attenuation measured in the normal state and the superconducting state is used to obtain the electron mean free path in the film. This is compared to values obtained from electrical measurements.Research supported by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 81-0002.Research supported by the U.S. Air Force Office of Scientific Research under Contract No. AFOSR F49620-78-C-0031.