X-Ray Diffraction Studies of the Recrystallization of C60-Based Polymers

Abstract

A time-dependent x-ray scattering study is presented of the kinetics of the C₆₀-based star polymer containing 6 urethane-connected polyether and the star model polymers containing 4, 3, and 2 urethane-connected polyether arms. By varying the temperature from 40°C to -40°C, the order-disorder phase transition (recrystallization) of the polymers has been observed. Two crystalline domains with the periodicities of 4.5±0.6 Å and 3.7±0.4 Å were detected. The domain sizes ranging from 70 Å to 250 Å, the transition temperatures from -15°C to 25°C and the maximum percentage of recrystallization from 13% to 34% increase as the number of polyether arms increases. The x-ray scattering peaks of the order phase grow up as I₁ +I₂(l - e^?t/t0), where I₁ and I₂ are the initial intensity and the proportional intensity modulus, respectively. The relaxation time t₀ of the order phase is varying as a power law with respect to the reduced temperature T_R(=(T - T_C)/T_C): t₀ = (35±10)·|T_R|^?1.42±0.07 seconds for the C₆₀-based star polymer containing 6 urethane-connected polyether; t₀ = (315 ± 64)·|T_R|^?1.68±0.08 seconds for the star model polymer containing 4 urethane-connected polyether arms.     

Measurements of the relaxation of quasiparticle branch imbalance in superconductors

An imbalanceQ in the quasiparticle populations of thek>k _F andk _F branches of a superconductor may be generated by the injection of a current through a tunnel barrier. This imbalance relaxes with a characteristic timeτ _Q . The steady-state value ofQ gives rise to a quasiparticle potentialV that differs from the pair chemical potential, and which may be measured by a second tunnel junction. Tinkham's theory of these effects is briefly reviewed. Detailed experimental results are presented for tin. Most of the data are for injection voltages much greater than the energy gapΔ(T). Close to the transition temperatureT _c ,Q relaxes by inelastic phonon scattering, andτ _Q ^ph =(1.0±0.2)×10^?10 Δ(0)/Δ(T) sec. This time is about one-half that estimated by Tinkham. At temperatures somewhat belowT _c, elastic scattering also contributes to theQ relaxation, provided the superconducting energy gap is anisotropic. With a size-effect-limited mean free path of about 2800 Å, we findτ _Q ^el ≈1.4×10^?10 sec forT/T _c <0.6, a value that is also in good agreement with Tinkham's theory. In a sample with a mean free path of 420 Å, and a greatly reduced anisotropy,τ _Q ^el was increased to about 10^?9 sec. Data were also taken for injection voltages close toΔ(T)/e. The degree of imbalance per unit injection current was greatly reduced, as predicted by the theory. Preliminary measurements were made on lead. At 4.2K (T/T _c =0.58) the characteristic time was about 3×10^?12 sec. The quasiparticle potential apparently increased steadily as the temperature was lowered, probably indicating that the characteristic time also increased. This result is not well understood. However, it is possible that recombination processes play a role in theQ relaxation in lead.     

Molecular Orientational Ordering in Solid C60

Abstract

Single-crystal X-ray diffraction studies of the orientational ordering in solid C₆₀ are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition at T_o = 259K, the unusual behavior of both fundamental and superstructure reflections below T_o and the freezing in of the C₆₀ reorientations at T_g ≈ 85K. The diffuse intensity due to orientational disorder of the C₆₀ molecules at room temperature is found to be strongly modulated, both radially and azimuthally. showing that the molecular orientations are indeed correlated. The corresponding intensity distribution has been calculated within a mean-field theory for different microscopic models of inter molecular interactions. It should allow a better understanding of these interactions.     

Correlation Between Charge, Spin and Lattice in La–Eu–Sr Manganites

To understand the structural, electrical, magnetic, elastic and anelastic properties of La_0.67?x Eu _x Sr_0.33MnO₃ (0.30≤x≤0.39) manganites, a series of samples was prepared by citrate gel route. X-ray diffraction studies along with Rietveld analysis indicate the samples crystallize in single phase with Pnma space group. Studies on the variations of magnetization with temperature indicate that the Curie transition temperature (T _C) decreases with increasing Eu content. Furthermore, Eu substitution is found to increase the electrical resistivity and significantly enhances the colossal magnetoresistance effect, while it is found to decrease the characteristic metal–insulator transition temperature (T _P). On analyzing the electrical resistivity data, it has been concluded that the resistivity below T _P can be explained based on electron–electron, electron–phonon and two magnon scattering processes, while that above T _P, the adiabatic small polaron model is found to explain the observed behavior. Finally, the longitudinal modulus (L) and internal friction (Q ^?1) have been measured and a dramatic change in L (T) is observed at T _C, accompanied by a sharp peak in Q ^?1 (T). Simultaneous occurrence of magnetic, transport and lattice anomalous behavior at T _C indicate the presence of strong electron–phonon and spin–phonon interactions in these manganites.     

Transport properties of antiferromagnetic superconductors

Transport properties of antiferromagnetic superconductors with T _N<T _chave been investigated. Detailed numerical results are given for SmRh₄B₄ by using the following model. The paramagnetic phase (T _NT _c)is described by using Abrikosov-Gorkov theory of magnetic ions in an ordinary superconductor. In the AF phase (TT _N)the effects of the molecular field H _Q (T) and the elastic scattering of conduction electrons from spin fluctuations are included. Expressions for H _Q (T) and the scattering rate from spin fluctuations have been derived. The aim has been to see if properties are enhanced or depressed by the AF ordering occurring below T _N.It is found that whereas the electronic thermal conductivity K _s,nuclear spin relaxation rate R _s,and the longitudinal ultrasonic attenuation _s,are depressed by the AF ordering, the inverse of the magnetic penetration depth, [(T)]^–1 is enhanced below T _N.The effect of all types of impurities is included in the study. Theoretical results for K _sand [(T)]^–1 agree with the experimental values for SmRh₄B₄ (experimental data for other properties are not available in literature). In the above, T _Nis the Néel temperature and T _cis superconducting transition temperature.     

The possibility of increasingT c in oxide high-T c superconductor films: Interaction with elastic fields of defects

The influence onT _c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-T_c superconductors is considered. The distribution function ofT _c in this case was obtained. This is the Lorentzian with center shifted to largerT _c values. The half width of theT _c distribution βn (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT _c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT ^(c) near the surface appears. The inequalityβn>T ^(c)>T _c ^initial is fulfilled. ForT>T ^(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT _c ^local >T ^(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions.     

Mechanical loss and Young's modulus associated with phase transitions in barium titanate based ceramics

The mechanical loss (Q ^–1) and Young's modulus (E) were measured as a function of temperature in the range 100 K3 driven electrostatically in flexural vibration mode at resonance frequencies between 2–4 kHz. Q ^–1(T) and E(T) curves show the following three phase transitions in BaTiO₃: rhombohedral to orthorhombic (T _R-O), orthorhombic to tetragonal (T _O-T), and tetragonal to cubic (T _Curie). Each phase transition induces a very narrow mechanical loss peak of Q ^–1 and a very sharp anomaly in the elastic modulus. Moreover, three other mechanical loss peaks are located below the phase transition temperatures in coarse grained specimens and these peaks seem to be associated with the motion of domain walls. These secondary peaks are not observed in fine grained undoped ceramics. The use of dopants such as Co or Nb produces an attenuation of the mechanical loss, a smoothing of elastic modulus anomalies, and a shifting of the phase transition temperatures.     

Electrical properties of Y-modified Pb(SnTi)O<Subscript>3</Subscript> ferroelectric ceramics

Y-modified Pb(SnTi)O₃ polycrystalline materials with a general formula (Pb (Sn (PYST) (x = 0, 0.05, 0.07, 0.1) have been synthesized using the standard mixed oxide method. Crystallographic and surface morphology studies are carried out by the powder X-ray diffraction and scanning electron microscopy techniques. X-ray diffraction studies of the samples showed the tetragonal structure of the unit cell. Uniform grain distributions with porosity throughout the surface of the sample pellets are observed. The T _C (transition temperature) increases with increase in the mole fraction of the substituent (x). ɛ^/ _max is found to decrease with increase in x. The slope of lnσ_ac∼ 1/T shows distinct variations for temperature below 410 K, 410 K to T _c and for T > T _c. The room temperature ac conductivity at 10 kHz lies in the range of 10⁻⁵–10⁻⁶ (ohm⁻¹ m⁻¹).     

The possibility of increasingTc in oxide high-Tc superconductor films: Interaction with elastic fields of defects

The influence onT _c of dilatation centers (DC) as elastic defects near the surface or in thin films of oxide high-T_c superconductors is considered. The distribution function ofT _c in this case was obtained. This is the Lorentzian with center shifted to largerT _c values. The half width of theT _c distribution n (n is the concentration of DC) is larger than its center shift and corresponds to the increase ofT _c fluctuations near the surface. As a result, a continuum percolation behavior with increasing critical temperature percolation levelT ^(c) near the surface appears. The inequalityn>T ^(c)>T _c ^initial is fulfilled. ForT>T ^(c) the quasi-2D Josephson media takes place where a finite superconductive domain withT _c ^local >T ^(c) exists. The influence of DC considerably increases for strong DC such as off-center impurity ions.     

A possible explanation for the isotope effect in high-Tc and Rb3C60 superconductors

The role of pair breaking effects on the isotope effect coefficient is investigated using the generalized Abrikosov and Gorkov theory of superconductors. It is found that the pair breaking effects enhance the isotope effect coefficient. This provides a plausible explanation for the enhancement of the isotope effect coefficient as a function of impurity concentration in yttrium-, europium-, and lanthanum-based high-T _c superconductors. It may also explain the very large value of the isotope effect coefficient in Rb₃C₆₀ superconductors provided some intrinsic disorder is assumed.     

Lattice instability in the normal state of La2?xSrxCuO4

Novel softening has been found in the transverse elastic constant (C ₁₁–C ₁₂)/2 below 50 K in single-crystalline La_1.86Sr_0.14CuO₄ (LSCO) by high-precision ultrasonic measurements in magnetic fieldsH along thec axis. With decreasing temperature, this lattice softening persists down to the superconducting transition temperatureT _c(H), which is reduced to 14 K by applying fields up to 14 T. BelowT _c(H) the softening turns to rapid hardening. This behavior indicates the presence of lattice instability of the orthorhombic (Bmab) structure in the normal state of LSCO, which disappears in the superconducting state. This is evidence for the intimate interplay between high-T _c superconductivity and the lattice instability in LSCO.     

Anomalous raman scattering in the normal state of uederdoped crystals of YBA2Cu3O7-x

The integrated intensity of the electronic background between 200 and 700 cm^-1 observed in the Raman spectra of the YBa₂Cu₃O_7-x 1-2-3 system exciting at 1.16 eV shows three changes with temperature. This electronic background is due to the scattering between quasi-degenerate bands at the Fermi level with apical oxygen character which have neither CuO₂ nor chain character. One of these transitions coincides with the appearance of superconductivity in the system. This indicates that even if the opening of the gap in the apical oxygen electronic levels is not clearly observed, the interband transition between these two levels is coupled to the superconducting order parameter. The other two changes of the background intensity occur at temperatures greater thanT _c . The one at lower temperature is proportional toT _c and coincides probably with the appearance of self-organized electronic nanostructures already observed by EXAFS measurements in other HT_c material systems (La-Sr-Cu-O, Ba-Sr-Ca-Cu-O). The latter transition is observed at T_D>- T_c (where T_D decreases as T_c increases). This temperature coincides with the opening of a pseudogap that has been detected by several experimental techniques in underdoped High-T,. systems. The possibility to observe the opening of a pseudogap in the density of states by means of Raman scattering is analyzed in terms of different theoretical models that have been postulated to explain superconductivity in cuprates.     

ESR Studies on C60.OX and C60.HMTTEF

Abstract

The electron spin resonance (esr) of C₆₀.O_x and C₆₀.HMTTEF (hexamethylentetratellurafulvalene) has been investigated at 9.36 GHz as a function of temperature. T (298 T 4 K). C₆₀.O_x shows an esr absorption of equal ‘g’ value to that of C₆₀ exposed to O₂ and light but is more intense. The C₆₀ in its pure form is in a singlet state. The impurity sites introduced by O₂ produce the esr absorption. The Curie - Weiss plots of inverse esr absorption intensity versus temperature indicate an antiferromagnetic TH = 50 for air exposed C₆₀ and 90K for C₆₀O_x. From the esr intensity at room temperature, the calculated number of free spins (S = 1/2) is =1/3 per mole of C_60-O_x. In case of C₆₀.HMTTEF, there is a very weak esr absorption at room temperature suggesting that the room temperature form is diamagnetic with very small charge transfer between C₆₀ and HMTTEF. This conclusion is consistent with the structure and magnetic susceptibility of this cocrystal. As the temperature is lowered, the equilibrium: AD <=> A?^? + D?⁺ is displaced towards the formation of free radical species, A?^? and D?⁺.     

Superconductivity in carrier-doped silicon carbide

Abstract

We report growth and characterization of heavily boron-doped 3C-SiC and 6H-SiC and Al-doped 3C-SiC. Both 3C-SiC:B and 6H-SiC:B reveal type-I superconductivity with a critical temperature T_c=1.5 K. On the other hand, Al-doped 3C-SiC (3C-SiC:Al) shows type-II superconductivity with T_c=1.4 K. Both SiC:Al and SiC:B exhibit zero resistivity and diamagnetic susceptibility below T_c with effective hole-carrier concentration n higher than 10²⁰ cm^?3. We interpret the different superconducting behavior in carrier-doped p-type semiconductors SiC:Al, SiC:B, Si:B and C:B in terms of the different ionization energies of their acceptors.     

Mössbauer Study of a New Superconductor GdOFeAs

The ⁵⁷Fe-Mössbauer spectroscopy measurements on the polycrystalline superconductor GdO_0.88F_0.12FeAs with the bulk critical temperature T _C=53 K were carried out at temperatures between 5 and 300 K. Two nonequivalent Fe²⁺ sites were revealed in the Fe-As superconducting planes. Iron ions show clear nonmagnetic behavior in all temperature ranges. There were no observed significant changes in the hyperfine Mössbauer parameters below and above T _C. The magnetic component appearing below 75 K indicates the presence of a small fraction of impurity phase FeAs.     

Experiments on the Self-Organized Critical State of 4He

When a heat flux Q is applied downward through a sample of ⁴He near the lambda transition, the helium self organizes such that the gradient in temperature matches the gravity-induced gradient in T _λ . All the helium in the sample is then at the same reduced temperature and the helium is said to be in the Self-Organized Critical (SOC) state. We have made the first measurements of the ⁴He SOC state specific heat, C _{∇ T} (T(Q)). There is no measurable difference between C _{∇ T} and the static zero-gravity ⁴He specific heat for temperatures between 650 and 250 nK below T _λ . Closer to T _λ , the specific heat is depressed and reaches a maximum value at 50 nK below T _λ . This depression is similar to that predicted theoretically as reported by R. Haussmann (Phys. Rev. B 60, 12349, 1999). Contrary to the expectations of theory, however, we see another depression far below T _λ . In addition, over the heat flux range of 30 nW/cm² to 13 μW/cm², we have made improved measurements of the speed of a recently discovered propagating thermal mode, which travels only upstream against the nominal heat flux of the SOC state. We are able to accurately predict the speed of this wave by treating the helium of SOC state as a traditional fluid with a temperature dependent thermal conductivity.     

Comparison between Torsional Oscillator and Ultrasonic Measurements of 4He Confined in Nano-Porous Media

We have performed torsional oscillator and ultrasonic measurements for ⁴He confined in 2D nano-porous substrate, hectorite. For ⁴He films, the resonance frequency of the torsional oscillator increases rapidly just below a superfluid transition temperature (T _C ), and then becomes constant at the lower temperature. In contrast, the sound velocity below T _C increases monotonically with decreasing temperature. For liquid ⁴He the sound velocity has a small bend at T _b of about 0.9 K. Its increase below T _b is enhanced as the pressure is increased.     

Thermomechanical properties of iron: viscoplasticity of ferrite and of austenite—ferrite mixtures

The available data on secondary creep in creep tests and on maximum stress in hot deformation tests obtained for δ and α ferrite have been reviewed and expressed in the form of the equation ? = K(σ/E)^P_c sinh(Vσ/RT) exp(—Q_c/RT) with p_c = 2·5, V = 0·0013 m³ mol^?1, and K = 2·6 × 10¹⁷ s^?1, E and Q_c being temperature dependent functions. The approximate range of validity is Z = ? exp (Q_c/RT) ≈ 10¹—10¹¹ s^?1. Together with the corresponding equation for austenite the data can be used to predict the viscoplastic behaviour of austenite—ferrite mixtures.

MST/1538     

Superconducting and normal state properties of dilute alloys of LaSn3 containing Nd impurities

The results of an investigation of the superconducting and normal state physical properties of (LaNd)Sn₃ alloys are reported. Superconducting state data are presented describing the depression of the superconducting transition temperature T _cwith Nd impurity concentration, the reduction of the reduced jump in heat capacity C/C ₀ with decreasing reduced superconducting transition temperature T _c/T_c0, and the pressure dependence of T _c. Normal state data for the temperature dependences of the magnetic susceptibility and heat capacity demonstrate the importance of crystalline electric field effects in the physical behavior of (LaNd)Sn₃ alloys and indicate that Nd interimpurity magnetic interactions and possibly a small amount of Nd-Sn antisite disorder are manifested in the experimental results. Theories based on an isotropic exchange depairing model are unable to give a satisfactory account of all of the experimental data, suggesting that the physical properties of(LaNd)Sn₃ alloys may be modified by processes involving the orbital exchange scattering of host conduction electrons by the Nd impurities.Supported by the U.S. Department of Energy under Contract No. Ey-76-S-03-0034-PA227-3.Fellow of the Consejo Nacional de Investigaciones, Cientificas y Tecnicas, de la Republica Argentina; leave from Centro Atomico Bariloche, Bariloche, Argentina.