Low-temperature magnetic properties of HoRh4B4

The low-temperature magnetic properties of HoRh₄B₄ have been studied by means of measurements of the magnetic susceptibility, magnetization, specific heat, thermal expansion, and magnetostriction. The ferromagnetic phase transition at T _M= 6.7 K shows almost ideal S = 1/2 mean field behavior in the specific heat. Crystal field effects due to the splitting of the J = 8 Hund's rule ground state of the Ho³⁺ ions result in Schottky anomalies in the specific heat and the thermal expansion and are also revealed in the low-field magnetic susceptibility and the magnetostriction. Information on the ground state doublet of the 4f electrons has been obtained from the nuclear contribution to the specific heat below 1 K and the high-field magnetization below T_M.This research was supported by the Schweizerische Nationalfonds zur Forderung der wissenschaftlichen Forschung (HRO), by the Department of Energy under Contract No. EY-76-S-03-0034-PA227-3 (LDW, MBM), and by the National Science Foundation under Grant No. NSF/DMR77-08469 (DCJ)     

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.     

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 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.     

Energies of line singularities in 3He-A

We investigate a class of topologically equivalent line singularities in ³He-A to determine which of this class has the lowest free energy density. Most of our results concern the Landau-Ginzburg regime near T _c ; we offer a speculation on what might occur further below T _c .This research was supported in part by the National Science Foundation under Grant No. 77-18329 and through the Materials Science Center of Cornell University, Technical Report No. 4043.     

Cuprate Superconducting Characteristics on the Doping Scale. A Multiband Approach

Some results on the doping dependence of cuprate superconductivity characteristics by an multiband approach are presented. A nonmonotonic dependence of the critical coherence length agrees with the experiment. Periodic spatial structures of T _c , etc. are expected at underdoping by Leggett noncritical mode fluctuations. The paired carrier effective mass diminishes moderately with doping. The inverse plane penetration depth shows a dependence with a maximum dominated by the supercarrier density. For the latter, the isotope effect is predicted and calculated. It is decisive for the isotope effect in the superfluid stiffness. This work was supported by the Estonian Science Foundation Grant No. 6540.     

Low temperature thermal expansion behavior in the Kondo system La1−x Ce x Al2

The thermal expansion coefficient for La_1–x Ce _x Al₂ has been measured from 4 to 50 K and compared to recent calculations based on a valence fluctuation model. The experimental results were found to be inconsistent with the predictions assuming valence fluctuations, even though the predictions of this model for the specific heat and magnetic susceptibility are in striking agreement with the measured quantities. The thermal expansion coefficient was not determined with sufficient precision to allow a quantitative comparison with prediction assuming resonant scattering-Kondo behavior.Work Supported by National Science Foundation Grant No. DMR-76-16433.     

Density of dilute 3He-4He solutions

The number density of dilute ³He-⁴He solutions under saturated vapor pressure is calculated within the framework of quantum hydrodynamics. For temperatures T small compared to the ³He Fermi temperature T _F, results are given for ³He concentrations large enough to manifest ³He quasiparticle interactions. The nondegenerate regime (TT _F) is also considered in the limit of small ³He concentration. Relevance to experimental determination of quasiparticle energies and interactions is emphasized.Supported by the National Science Foundation under Grant No. GP-40831.     

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.     

q-Dependent phonon renormalization: Possible test for the symmetry of the superconducting gap by neutron scattering?

Theq dependence of the phonon renormalization belowT _c is studied for superconducting gaps with different symmetries. It is shown that for small to intermediate values ofq the angular dependence of the renormalization is characteristic of the gap anisotropy. However, we demonstrate that this dependence is not solely due to the symmetry of the gap, but also to the specific functional form of _k. We compare our calculations with recent neutron scattering experiments done on YBa₂Cu₃O₇. These data are well described in terms of ans-wave anisotropic gap resulting from the nontotally screened interaction of holes with long-wave, optical phonons.The authors are grateful to A. Furrer, J. Mesot, K. A. Müller, and M. V. Eremin for discussions. A. B. acknowledges financial support from the Swiss National Science Foundation.     

Dissipation in the superfluid helium film

We have measured the rate of energy dissipation in superfluid helium film flow in an attempt to test a recent theory due to Harris-Lowe, which predicts that for superfluid stream velocitiesv _sthat just exceed the critical velocityv _c0, the rate of dissipation is given by an equation of the form Q=C(v_s–v_c0)^3/2. Our experiments at 1.33 K show that the exponent, predicted to be 3/2, is 1.491±0.021.The research for this paper was supported by the Defence Research Board of Canada, Grant number 9550-57.     

Resistivity and T c in disordered superconductors

The universal depression of the superconducting transition temperature T _cin disordered A-15 compounds is examined. It is found that their anomalous behavior can be explained by a simple model for the density of states, which is enhanced by disorder in some cases. The dramatic drop in T _cin constant-density-of-states A-15 compounds like Nb₃Ge or Nb₃Al at a critical value of the resistivity can be attributed to overdamping of acoustic plasmons, which decreases the electron pairing interaction despite relatively small changes in the density of states. Agreement for T _cand susceptibility with previous calculations is found if the position of the Fermi energy is near a peak. Possible experiments are proposed to check the above models.This work was supported in part by NSF Grant No. DMR77-13167.     

Fermionic Superfluidity: From High T c Superconductors to Ultracold Fermi Gases

We present a pairing fluctuation theory which self-consistently incorporates finite momentum pair excitations in the context of BCS—Bose–Einstein condensation (BEC) crossover, and we apply this theory to high T _c superconductors and ultracold Fermi gases. There are strong similarities between Fermi gases in the unitary regime and high T _c superconductors. Here, we address key issues of common interest, especially the pseudogap. In the Fermi gases, we summarize recent experiments including various phase diagrams (with and without population imbalance), as well as evidence for a pseudogap in thermodynamic and other experiments. This work was supported by NSF PHY-0555325 and NSF-MRSEC Grant No. DMR-0213745.     

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.     

Low-density properties of gaseous,spin-aligned atomic hydrogen

The ground-state properties of bulk atomic hydrogen are calculated for the low-density gas, assuming that the atoms interact via the ³ _u ⁺ pair potential. The Monte Carlo technique is applied to a system consisting of 32 hydrogen atoms in a box, with periodic boundary conditions. These results are compared with those derived from the hard-sphere Bose-Einstein gas model.Work supported by NASA Grant No. NGR 06-002-159.     

Systematics of the thermopower below Tc in Tl2Ba2CaCu2O8+δ

A systematic study of the mixed-state thermopower in Tl₂Ba₂CaCu₂O₈₊ indicates a that there are two contributions to the total signal. The largest contribution is accurately proportional to the resistivity, as previously reported. We have also identified a new, additional contribution to the thermopower. The source of the excess thermopower is not presently determined, and may be related to vortex fluctuations or d-wave superconductivity.This work is supported in part by USAFOSR F49620-93-1-0310, NSF Grant No. DMR 91-22043, ARPA Grant MDA 972-90-J-1001, the Texas Center for Superconductivity at the University of Houston, and the T.L.L. Temple Foundation.     

Frequency dispersion of HTSC-film impedance in the MHz range

The impedance of a circuit including an HTSC film as a function of temperature and frequency has been measured, and the presence of an intrinsic reactance in the HTSC films has been shown. The equivalent scheme of the measuring circuit has been synthesized, and the temperature dependence, type, and value of the HTSC-film reactance has been determined in the transition region nearT _c . It is shown that the change of reactance sign with temperature observed earlier by a number of researchers and attributed to the intrinsic property of HTSC-film reactance was rather the result of the T-independent parasite reactive elements while the film impedance was becoming small.This work was supported by Grant No. 4077292 from the Israel Ministry of Science and Technology.     

Phase diagram for the helical texture in 3He-A

The stability region of the helical textures in³He-A in thev _s-H phase diagram is theoretically determined. Herev _sis the superflow velocity andH (v _s) is the external magnetic field. The pitch of the helixk ^–1 that minimizes the Gibbs free energy and the corresponding inclination angles ofl andd are determined numerically as functions ofv _sandH. The NMR frequencies (two longitudinal and four transverse frequencies) associated with the helical texture are obtained.Work supported by the National Science Foundation under Grant No. DMR 76-21032.     

Calculation of mixed-mode stress field at a sharp notch tip using <Emphasis Type="Italic">M</Emphasis><Subscript>1ɛ</Subscript>-integral

Direct computation of the mixed-mode stress field at a sharp notch tip appears to be difficult in that the mode I and mode II asymptotic stresses are in general governed by different orders of singularity. In this paper, we first present a path-independent integral termed M₁. The relation between M₁ and the generalized stress intensity factors is then derived and expressed as function of the notch angle. Once the M₁-integrals are accurately computed, the generalized SIF's and, consequently, the asymptotic mixed-mode stress field can thus be properly determined. No extra complementary solutions are required in the formulation. Further, no particular singular elements are required when the integration is performed by using finite elements. This work has been partially supported by National Science Council Grant No. NSC90-2211-E-008-040 to National Central University.     

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.