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 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.     

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.     

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 conductance of layer structure dichalcogenides

We have measured the thermal conductance in the temperature range 0.5–7 K of the layer structure compounds TaS_1.6Se_0.4 and TaS_1.6Se_0.4 (pyridine)_1/2 parallel to the plane of the layers in both the normal and superconducting states. Our results indicate that the phonon contribution to the thermal conductivity, which varies approximately asT ^1.6 at low temperatures, dominates the electronic contribution in both the normal and superconducting phases.     

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.     

Low-field magnetization measurements on small TaS2(aniline)3/4 crystals

Sharp magnetic transitions were obtained in carefully selected superconducting TaS₂(aniline)_3/4 crystals which are believed to represent the intrinsic material. The temperature dependence of H _c 1 was determined and compared with other data and found to be much stronger than expected.Research supported by the National Research Council of Canada.     

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.     

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.     

Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature

The interconnect half‐pitch size will reach ≈20 nm in the coming sub‐5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross‐section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back‐end‐of‐line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaS_x ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaS_x ultrathin film is industry‐friendly, BEOL‐compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaS_x compared to the TaN/Ta bilayer. This single‐stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node.     

Superconducting and normal state properties of Li1+x Ti2−x O4 spinel compounds. I. Preparation,crystallography, superconducting properties,electrical resistivity,dielectric behavior,and magnetic susceptibility

LiTi ₂ O ₄ is one end member of the homogeneity range of the spinel phase Li _1+x Ti _2–x O ₄ (0x1/3) and is superconducting at temperatures up to 13.7 K. Various measurements were carried out in order to characterize the superconducting and normal state properties of LiTi ₂ O ₄ and of other compositions within the homogeneity range of the spinel phase. These measurements establish LiTi ₂ O ₄ as ad-band superconductor and show thatT _c decreases to <1.5 K forx0.1. This disappearance of superconductivity with increasingx was found to be correlated with anomalous changes in the lattice parameter with composition, and, from electrical resistivity measurements, is tentatively attributed to the occurrence of a composition-induced metal-semiconductor transition atx0.1. The metallic character of LiTi ₂ O ₄ and the composition dependence of the observed electrical properties are shown to follow from crystallographic considerations.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract Number AFOSR/F-44620-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.     

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.     

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.     

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.     

NMR studies on HxTaS2 and DxTaS2

H_0.33TaS₂ and D_0.33TaS₂ have been prepared electrochemically and their magnetic resonance properties investigated. The ²D quadrupole coupling constant and ¹H spin-lattice relaxation rates have been measured and are similar to the same quantities measured in transition metal hydrides. Spin-spin relaxation rate data show that the hydrogen is immobile on an NMR time scale. These results are consistent with earlier structural conclusions from neutron diffraction studies, which locate the H within the TaS₂ layers.     

Flux pinning by thin chromium layers

The flux pinning strength and upper critical field have been measured in multilayered evaporated films composed of thick layers of polycrystalline Pb-18% Bi alloy periodically interleaved with much thinner layers of chromium. The flux pinning strength of a single Cr layer with the applied magnetic field parallel to the layers is estimated to be at least 180 N/m² at a reduced field ofh=H/H _c2=0.7, and is independent of Cr layer thickness. The parallel flux pinning strength drops sharply, however, if the layer spacingd ₁ is reduced to less than 60 nm. The upper critical field with the applied field normal to the layers is slightly elevated for larged ₁ , but is strongly depressed ford ₁ <100 nm; the parallel upper critical field is independent ofd ₁ .This work was supported financially by the U.S. Air Force Office of Scientific Research through grants AFSOR-77-3107 and F44620-74-C-0019. This work benefited from the use of the facilities of the Materials Science Center at Cornell, which is funded by the U.S. National Science Foundation.     

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.