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.     

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.     

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.     

Electromagnetic wave attenuation by superconducting fluctuations in a thin,granular aluminum film

A sharp local minimum has been observed atT _c in the microwave attenuation by a thin, granular aluminum film which is believed to result from the imaginary component of the fluctuation conductivity and for which the particular geometry of the specimen appears to enhance the observation of the fluctuation effects.Research supported by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 71-2079.     

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.     

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.     

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.     

The superconductivity and magnetic susceptibility of some zirconium-transition-metal compounds; evidence for an anticorrelation

The properties of the superconducting transition temperatureT _cof the CuAl ₂ -type compounds of Zr ₂ Co, Zr ₂ Ni, Zr ₂ Rh, Zr ₂ Fe, and Zr ₂ Ir are discussed with respect to effects due to alloying and heat treatment. Dilute pseudobinary alloys of transition-metal elements with Zr ₂ Rh (T _c=11.3 K) produced lower transition temperatures suggesting that the valence-electron concentration of 5.67 electrons/atom gives a maximum inT _c for this crystal type. Results for Zr ₂ Co (T _c=5.0 K) and Zr ₂ Ir (T _c=7.3 K) show that their transition temperatures are raised somewhat when the electron concentration is increased by alloying. A peak is seen near 5.72 electrons/atom. A peak in the room-temperature magnetic susceptibility in the Zr ₂ Co-Zr ₂ Ni system occurs near the same electron concentration as the peak inT _c. Similar susceptibility behavior takes place in the Zr ₂ Rh-Zr ₂ Ni system, however, with no peak inT _c. Results of susceptibility measurements on Zr ₂ Ir and its isomorphic alloys indicate correlation betweenT _c and susceptibility. An anticorrelation occurs for Zr ₂ Co alloys suggesting the presence of Coulomb interactions. A sharp symmetric drop inT _c near the stoichiometric composition as well as a marked decrease of the transition temperature with a low-temperature (600 C) anneal indicate that the coulomb interactions are sensitive to crystalline order. The lattice parameters of Zr ₂ Ir are reported as a=6.508 Å andc=5.721 Å. The superconductivity of Zr ₃ Co (T _c=3.9 K) and Zr ₃ Ir (T _c=2.13 K) is reported.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR grant number AF-AFOSR-631-67-A.     

Low-temperature specific heat of layered compounds

Specific heat measurements in the temperature range from 1 to 10 K and in magnetic fields up to 1.2 T have been made on a number of layered transition metal dichalcogenide crystal complexes. The thermodynamic critical fieldH _c and the Ginzburg-Landau coherence length and penetration depth have been calculated. For TaS₂ (pyridine)_1/2 and TaS_1.6Se_0.4 (pyridine)_1/2, the coherence length perpendicular to the layers is slightly less than the interlayer separation.Research at Stanford supported by Air Force Office of Scientific Research, Air Force Systems Command, USAF, under grant AFOSR 73-2435B.     

Transverse and longitudinal ultrasonic attenuation measurements of the superconducting energy gap in high-purity rhenium

The attenuation of transverse and longitudinal waves propagating along thec axis in high-purity rhenium has been measured in the normal and superconducting states. In the superconducting state the attenuation was compared with the simple BCS theory using as adjustable parameters the gap parameter and the residual attenuation atT _c (for transverse waves). Using transverse-wave attenuation a gap parameter of 3.50±0.10 was found, while the longitudinal attenuation data yielded a gap parameter of 2.90±0.10. At present, the only apparent explanation for this difference is that the integrals for transverse-and longitudinal wave energy loss in an ultrasonic wave contain different azimuthal angle dependencies and thus average an anisotropic energy gap differently.Research sponsored by the Air Force Office of Scientific Research under AFOSR Grant No. 71-2079.     

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.     

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.     

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.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

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.     

Superconducting and magnetic properties of ErRh4B4

The thermal expansion, magnetization, magnetostriction, and ac electrical resistance of the compound ErRh₄B₄ have been measured at low temperatures. The influence of magnetic fields on the superconducting and magnetic properties of this compound is emphasized, and the qualitative behavior of the upper critical field as a function of temperature has been determined from the experiments.Research supported by the Schweizerische Nationalfonds zur Förderung der wissen-schaftlichen Forschung.Research sponsored by the U.S. Energy Research and Development Administration under Contract No. ERDA E(04-3)-34PA227.Research in La Jolla sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, U.S. Air Force, under AFOSR Contract No. F49620-77-C-0009.     

Size effects and the anisotropy of the electron relaxation processes in single crystal films of gallium at low temperatures

The variation of electrical resistivity of thin rectangular, single crystal films of 99.9999% gallium has been investigated from 1.1° to 20°K, as a function of crystalline orientation, for thicknesses between 40 and 250 µ. As long as the ratio of the thickness to the average bulk mean free path 1, the ideal resistivity is proportional to a power which depends on the orientation of the film and which varies betweenT ³ andT ^1.5 . It is argued that the strongT ² term, found in the bulk resistivity at low temperatures, is not a consequence of electron-electron collisions, but is caused by an averaging, over the entire Fermi surface, of temperature dependences which are different for different regions because of Umklapp scattering. For the thinnest specimens size effects are appreciable up to 15°K, but there is both quantitative and qualitative disagreement between our results and the theoretical calculations of Fuchs, which are for an idealized metal. Attempts are made to show that a change in the nature of the bulk scattering mechanisms can be deduced by comparing our data with Fuch's predictions.Work supported by Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR Grant No. AF-AFOSR 1051-66.     

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.     

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.     

Symmetry, mechanism of the pairing, and the isotope effect in the cuprates

This paper is concerned mainly with the problem of the origin of highT _c .s-Wave vs.d-wave pairing, the isotope effect, and the strong coupling of the carriers to low-frequency optical phonon modes are discussed. This latter strong coupling is sufficient to provide highT _c .The work of VZK was sponsored by the U.S. Office of Naval Research Contract No. N00014-92-F0006.