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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Magnetic properties of RRh3B2(R=La,Ce, Nd,Gd) compounds

The¹¹Bnmr results on RRh₃B₂(R=La, Ce, Nd and Gd) are reported. For CeRh₃B₂, specific heat and electrical resistivity are reported. From a comparative study of the¹¹Bnmr Knight shifts and the spin lattice relaxation times of these compounds it is shown that in CeRh₃B₂, there is strong hybridization of 4f states with the conduction electrons. A local moment on Ce with admixture of 4f and 5d–6s orbitals is suggested.     

Magnetic free energy of ErRh4B4 in paramagnetic and superconducting states

Magnetization curves for a polycrystalline specimen of ErRh₄B₄ are analyzed to estimate the paramagnetic contribution to the Gibbs free energy as well as the differences in free energy between the superconducting and normal states. These results are compared with the theoretical predictions of a suitably constrained model based on Ginzburg-Landau theory for magnetic superconductors.Work supported by the U.S. Department of Energy.Research at UW—Milwaukee supported by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR 81-0002.     

Low-temperature heat capacity of small Nb3Sn polycrystals by ac calorimetry

It is shown by an ac calorimetry technique that the multiple heat capacity anomalies which occur below the superconducting transition temperature for small polycrystalline Nb₃Sn samples are intrinsic to these samples. The recent suggestions that shear stresses can account for these results are analyzed for their validity. The dependence of the occurrence of these multiple anomalies upon the thermal history of the samples was investigated.Supported in part by U.S. Atomic Energy Commission under the Contract No. USAEC-AT-(04-3)-34 at the Department of APIS, UCSD, La Jolla, California. Present work supported under the auspices of ERDA.Supported by Air Force Office of Scientific Research, Air Force Systems Command USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

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.     

Magnetic properties of RE x Mo6Se8 compounds between 0.7 and 295 K

Magnetization measurements are reported for seven members of the series of compounds RE _x Mo ₆ Se ₈ (x1; RE=La, Ce, Pr, Eu, Gd, Yb, and Lu). These measurements establish that Ce, Pr, and Gd ions are trivalent in these compounds, but that Eu and Yb are primarily divalent, in agreement with crystallographic data. Large deviations of the susceptibilities for CeMo ₆ Se ₈ , PrMo ₆ Se ₈ , and YbMo ₆ Se ₈ from Curie-Weiss behavior above 10 K are attributed predominantly to crystal field effects, and quantitative agreement between crystal field theory and the experimental data was obtained for CeMo ₆ Se ₈ above 25 K. Further, we find for Gd _1.2 Mo ₆ Se ₈ an unusually small spin exchange interaction between the Gd ions and conduction electrons, consistent with previous estimates of this quantity based on superconductivity data. For the Gd, Eu, and Ce compounds, the low-temperature data indicate very small Curie-Weiss temperatures of –1.7, –0.6, and +0.4 K, respectively. No evidence for magnetic ordering of the RE ions in any of the above compounds was observed, with the exception of CeMo ₆ Se ₈ , for which the magnetization data at 0.7–1.3 K suggest the onset of ferromagnetic order.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract No. AFOSR/F-44620-72-C-0017.     

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.     

Superconductivity versus ferromagnetism

The disappearance or reentry of superconductivity at lower temperatures in disordered systems is generally only the superposition of separate and not connected effects.Also at the Institute for Pure and Applied Physical Sciences, University of California—San Diego, La Jolla, California, where the research is sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract AFOSR F4920-77-C-0009, and Bell Laboratories, Murray Hill, New Jersey.     

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.     

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.     

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.     

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.     

Ginzburg-Landau parameters for a vanadium single crystal having a resistivity ratio of 7.8

Magnetization curves for a cylindrical specimen made of a single crystal of vanadium are obtained at several temperatures. The method utilizes electronic integration of the difference in terminal voltages from two identical coils, one of which contains the specimen when the applied field is cycled over the operating range. The integrated voltage is proportional to the magnetic moment of the specimen, and is plotted continuously on anx-y recorder as a function of applied field. The Ginzburg-Landau parametersK ₁andK ₂are obtained from the magnetization curves, and the results are compared with the theoretical calculations of Eilenberger and others.Research supported by Air Force Office of Scientific Research under Grant No. AFSOR 76-2893.