Electronic Transport Behaviors of Insulating Icosahedral Al–Pd–Re Bulk Samples

Electronic transport measurements are summarised on two insulating icosahedral quasicrystalline (IQC) Al_69.0 Pd_22.8 Re_8.2 samples. Data were taken between 0.020 K ≤ T ≤ 292 K and in magnetic fields Bs up to 18 T. For a weakly insulating IQC, the electronic conductivity followed the expression σ(T) = σ₀T^zexp(T/ T₀) over four decades of temperature. For a strongly insulating sample, the conductivity above 0.3 K followed simple temperature power laws, σ (T) = σ₀ T^z. Below 0.2 K, the conductivity displayed an activated variable-range hopping (VRH) law. These fits included a conductivity contribution arising from the presence of a second metallic phase, which caused “weak saturation” of the measured resistances below 1 K.     

Analysis of the Normal-State Magnetotransport in CeIrIn5

We present an analysis of the normal-state magnetotransport in the heavy-fermion superconductor CeIrIn₅. The Hall effect and the transverse magnetoresistance in this material do not appear to be uniquely correlated, as inferred from the field dependence of the current ratio (R _σ =σ _xy /σ _xx ² H). The Hall coefficient is seen to satisfy a scaling equation of the form R _H=f[H/(a+bT ^c )]. These results are compared to those observed earlier in CeCoIn₅, and are discussed in terms of the contrasting phase diagram which the CeIrIn₅ system exhibits in relation to its Co counterpart.     

Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties

An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10⁻⁵). The drops relax exponentially towards equilibrium. The typical relaxation time t_c decreases with contact angle. t_c is proportional to the drop size R, thus defining a characteristic velocity U^* = R/t_c. The corresponding U* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = σ /η, with σ the gas–liquid surface tension and η the viscosity). The dynamics of receding (coalescence) and spreading motion is found to be of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensation-induced coalescence. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Anomaly in the Complex Conductivity of Overdoped Y1?xCaxBa2Cu3O7?δ Thin Films from THz Spectroscopy

We measured the complex conductivity of Ca-doped YBCO thin films in the THz frequency range. The films were measured using both Time domain and Frequency domain methods for THz spectroscopy. We show that a subgap exists in the overdoped samples of 5% and 10% Ca doping. The subgap appears as a sharp decrease in the real part of conductivity at frequencies equivalent to gap energy of 1 meV and is more prominent with increased doping. We suggest that this decrease in conductivity is related to a d_x²-y²d_{x^{2}-y^{2}}-wave pairing symmetry with an imaginary part of is or id _xy . The imaginary part of the conductivity shows the well-known 1/ω behavior, but its ωσ ₂ product shows a dip in the spectrum at about ∼1 meV.     

Equivalence of the Effects on the Complex Conductivity of Superconductor due to Temperature Change and External Pair Breaking

The responsivity of microwave kinetic inductance detector (MKID) is related to the derivative of the complex conductivity σ with respect to the quasiparticle density n _qp. For thermal quasiparticles, this derivative can be evaluated from σ(T) and n _qp(T) given by the Marttis-Bardeen theory (Phys. Rev. 14:412, [1958]). For excess quasiparticles due to external pair breaking, d σ/dn _qp can be evaluated by introducing an effective chemical potential μ ^* into the Marttis-Bardeens theory, as did by Owen and Scalapino (Phys. Rev. Lett. 28:1559, [1972]). In this paper, we derive the expressions of for both thermal and excess quasiparticles and show that they are approximately equal within the temperature and frequency range MKIDs operate. This equivalence validates the method of using bath temperature sweep to calibrate the responsivity of MKIDs. Response curves of an antenna-coupled submm MKID to bath temperature sweep and blackbody source temperature sweep are compared which demonstrates this equivalence.      

Elastic properties of ZnF2-PbO-TeO2 glasses doped with certain rare earth ions

The elastic moduli (Y, η, σ andH) and some thermodynamical parameters, such as Debye temperatureθ _D, diffusion constantD _i and latent heat of melting ΔH _m, of ZnF₂-PbO-TeO₂ glasses doped with some rare earth (Pr³⁺, Nd³⁺, Eu³⁺ and Tb³⁺) ions are determined as functions of temperature using piezo electric composite oscillator technique. All the parameters are found to decrease with increase in the atomic number Z of the rare earth ions and with increase in the temperature of measurement. The results are explained on the basis of the density of localized bonding defect states within the band gap of the material.     

Acoustic investigations on PbO-Al2O3-B2O3 glasses doped with certain rare earth ions

Elastic moduli (Y, η), Poisson’s ratio (σ), microhardness (H) and some thermodynamical parameters such as Debye temperature (θ_D), diffusion constant (D _i),latent heat of melting (ΔH _m) etc of PbO-Al₂O₃-B₂O₃ glasses doped with rare earth ions viz. Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺ and Yb³⁺, are studied as functions of temperatures (in the temperature range 30–200°C) by ultrasonic techniques. All these parameters are found to increase with increasing atomic numberZ of the rare earth ions and found to decrease with increasing temperature of measurement. From these results (together with IR spectra of these glasses), an attempt is made to throw some light on the mechanical strength of these glasses.     

The development of forced convection heat transfer near a forward stagnation point with Newtonian heating

A mathematical model for the unsteady forced convection boundary-layer flow near a forward stagnation point is considered when there is Newtonian heating on the surface whereby the heat transfer is proportional to the local surface temperature. In a previous paper (Salleh et al. J Eng Math 69:101–110, 2011), a critical value γ _c, dependent on the Prandtl number σ, of the heat transfer coefficient γ was identified, with solutions for the corresponding steady problem possible only for γ < γ _c. The unsteady problem considered here shows that these steady states are attained at large times when γ < γ _c. For γ > γ _c, the solution still continues to large time, now growing exponentially with time. This rate of growth is determined by an eigenvalue problem which we solve numerically for general values of γ and σ and asymptotically for large γ and both large and small σ.     

Subsampling weakly dependent time series and application to extremes

This paper provides extensions of the work on subsampling by Bertail et al. in J. Econ. 120:295–326 (2004) for strongly mixing case to weakly dependent case by application of the results of Doukhan and Louhichi in Stoch. Proc. Appl. 84:313–342 (1999). We investigate properties of smooth and rough subsampling estimators for sampling distributions of converging and extreme statistics when the underlying time series is η- or λ-weakly dependent.     

Point-Contact Spectroscopy in Mn-Doped MgB2 Single Crystals: Effects of Magnetic Impurities in a Two-Band Superconductor

We performed point-contact spectroscopy (PCS) measurements in Mg_1−x Mn _x B₂ single crystals, with x≤0.015 and bulk T _c down to 13.3 K. The gaps Δ _σ and Δ _π were obtained by fitting the conductance curves of the point contacts with the two-band Blonder–Tinkham–Klapwijk (BTK) model. Both Δ _σ and Δ _π decrease with the critical temperature of the junctions T _c ^A , but remain clearly distinct down to the lowest critical temperature (T _c ^A ≅9 K). Once analyzed within the Eliashberg theory, the gap trends as a function of T _c ^A can be explained by a doping-induced increase in the pair-breaking scattering within the σ band, with smaller contributions from the π-π or the σ-π channels.     

Melting of Two-Dimensional Electron Crystal on Liquid 3He Induced by Resonance Microwave Absorption

Microwave-resonance induced increase of magneto-resistivity σ _xx ⁻¹ of the surface-state electrons (SSE) on liquid ³He is observed at low temperatures at which SSE crystallize into the Wigner solid (WS). At the resonance, the relative change of σ _xx ⁻¹ of SSE in the solid phase is found to be several times of the corresponding change in the liquid phase measured under similar experimental conditions. We attribute this effect to melting of WS induced by electron heating which accompanies the microwave absorption. σ _xx ⁻¹ is found to increase gradually with microwave power, which is rather different from the sharp resistivity increase induced by direct heating of the substrate in the absence of microwaves. The effect of microwave resonance-induced heating on the sliding of WS from the periodic surface deformation of liquid is also investigated.      

Corner flow in free liquid films

A lubrication-flow model for a free film in a corner is presented. The model, written in the hyperbolic coordinate system ξ = x ² −y ², η = 2xy, applies to films that are thin in the η-direction. The lubrication approximation yields two coupled evolution equations for the film thickness and the velocity field which, to lowest order, describes plug flow in the hyperbolic coordinates. A free film in a corner evolving under surface tension and gravity is investigated. The rate of thinning of a free film is compared to that of a film evolving over a solid substrate. Viscous shear and normal stresses are both captured in the model and are computed for the entire flow domain. It is shown that normal stress dominates over shear stress in the far field, while shear stress dominates close to the corner. This revised version was published online in October 2005 with corrected page numbers.     

Effect of Uniaxial Pressure on the Isotope Effect on the Spectrum of Hamiltonians with Electron–Phonon Coupling

We have studied the effect of a change in the equilibrium nearest neighbour distances on the dynamics of charge and ions on a three-site cluster, identified with an O–Cu–O cluster present in high temperature superconductors. We consider a model Hamiltonian that contains an electronic part represented by a single band Hubbard model with onsite electronic correlations and a phononic part consisting of non-interacting Raman and infrared active phonons. The electron–phonon coupling is introduced through the change in interatomic distances generated by Coulomb repulsion between charges at neighbouring sites. For intermediate and strong values of the electron–phonon coupling, this model generates correlated electron–ion motion, i.e., polaron formation. In order to simulate the effect of change in the equilibrium nearest neighbour distances, we assume that the main effect such a change is a variation of the intersite electron hopping probability, t. We, therefore, studied the excitation spectrum of this model and the local lattice distortion in the Cu–O bond length as a function of t. We also studied the effect of a change in t in the polaron tunnelling energy when we use different oxygen isotopic masses, i.e., O¹⁶ and O¹⁸. We find that as function of t, the isotopic shift does not show a monotonic behaviour, as it does as a function of the electron–phonon coupling constant. It exhibits a minimum for values of t for which the electron–phonon coupling generates local lattice distortions with magnitudes similar to those observed experimentally in high-temperature superconductors. This observation could be related with the observed maximum on T _c as a function of the microstrain of the Cu–O bonds (Sanna et al. in Int. J. Mod. Phys. B 14(29–31), 2000; Bianconi et al. in J. Phys.: Condens. Matter 12:10655, 2000; Agrestini et al. in J. Phys. A: Math. Gen. 36:9133, 2003).     

Induction time approach to surface induced crystallization in polyethylene/poly (ɛ-caprolactone) melt

The surface induced crystallization of poly (ɛ-caprolactone) (PCL) on an ultra-high modulus polyethylene (PE) fibre was investigated using a new approach based on the induction timet _i. This approach allows estimation of the free energy difference function Δσ as it appears in the theory of heterogeneous nucleation. The classical approach based on the rate of heterogeneous nucleationl is not applicable to transcrystallization because the nucleation density at the fibre surface cannot be measured. A relationship betweenl andt _i is proposed and a theoretical justification is presented. Good agreement between the two approaches is obtained for a verification case where bothl andt _i can be determined. A transcrystalline growth rate study yields an estimate of the parameter σσ_e for PCL of 680 erg²cm⁻⁴. The maximum growth rateg ^* is also obtained. The results obtained indicate the influence of certain parameters on the appearance of transcrystallinity. It is also shown how the interfacial morphology can be controlled by the knowledge of the variations of the induction time with temperature.     

Tricritical Dynamics at the Demixing-λ-Transition in 3He-4He Mixtures

We calculate in two loop order the dynamical critical behavior at the tricritical point occuring in ³He-⁴He mixtures. Model F^′ introduced by Siggia and Nelson (Phys. Rev. B 15:1427, [1977])—already problematic in one loop order—is shown in two loop order to lead to a divergence of the mass diffusion D in contradiction to experiments. Within the complex symmetric version of this model (model E ^⋆′) the dynamical critical exponents of the kinetic coefficients can be calculated exactly leading to a mass diffusion going to zero at the tricritical point as D(t _X )∼t _X ^1/2 (t _X is the relative temperature distance at constant concentration) and the thermal diffusion ratio k _T ∼t _X ⁻¹ diverges proportional to the concentration susceptibility in the experimental region. We also discuss effective dynamical critical exponents and compare them with recent simulations of the vector Blume-Emery-Griffths model.      

Optical absorption and photoconductivity in gamma-irradiated zinc selenide crystals

The possibility of radiation-induced enhancement of photoconductivity (σ_ph) in crystalline zinc selenide (ZnSe) has been studied. The electron concentration in the initial ZnSe:O crystal was N _Eg = 1.26 × 10¹⁶ cm⁻³ as determined from the optical density of the sample at E _g = 2.58 eV. The irradiation of the ZnSe:O crystal by gamma-photons from ⁶⁰Co source to a dose of 10⁶ Gy leads to the creation of five new energy levels (Γ_6v, 5.76 eV; L _1.3v, 4.85 eV; Zn_i, 3.34 eV; O_Se, 3.13 eV; X, 2.72 eV), a decrease in the electron concentration to N _Eg = 0.63 × 10¹⁶ cm⁻³, and a twofold increase in σ_ph. The doping with Te to 0.2 wt % also creates the L _1.3v level and increases N _Eg to 2.02 × 10¹⁶ cm⁻³ and σ_ph to 2.01 × 10⁻¹⁰ Ω⁻¹. The irradiation of Te-doped crystals leads to the creation of an additional resonant level (Zn_i, 3.34 eV) and a two-fold increase in σ_ph. An increase in the content of Te to 0.5 wt % results in a shift of the Zn_i level to 3.39 eV and a growth in the conductivity to σ_ph = 7.64 × 10⁻¹⁰ Ω⁻¹. However, the gamma-irradiation of these crystals leads to decomposition of this center and to a decrease in the photoconductivity.     

Subatmospheric vapor pressures evaluated from internal-energy measurements

Vapor pressures were evaluated from measured internal-energy changes in the vapor+liquid two-phase region, ΔU ⁽²⁾. The method employed a thermodynamic relationship between the derivative quantity (ϖU ⁽²⁾/ϖV) _T and the vapor pressure (p _σ) and its temperature derivative (ϖp/ϖT)_σ. This method was applied at temperatures between the triple point and the normal boiling point of three substances: 1,1,1,2-tetrafluoroethane (R134a), pentafluoroethane (R125), and difluoromethane (R32). Agreement with experimentally measured vapor pressures near the normal boiling point (101.325 kPa) was within the experimental uncertainty of approximately ±0.04 kPa (±0.04%). The method was applied to R134a to test the thermodynamic consistency of a publishedp-p-T equation of state with an equation forp _σ for this substance. It was also applied to evaluate publishedp _σ data which are in disagreement by more than their claimed uncertainty.     

Feshbach Shape Resonance in Multiband Superconductivity in Heterostructures

We report experimental data showing the Feshbach shape resonance in the electron doped MgB₂ where the chemical potential is tuned by Al, Sc, and C substitutions. The scaling of the critical temperature T _c as a function of the Lifshitz parameter z = E _Γ−E _F, where E _F is the chemical potential and E _Γ is the energy of the Γ critical point where the σ Fermi surface changes from the 3D to a 2D topology, is reported. The resonant amplification of T _c(z) driven by the interband pairing is assigned to a Feshbach shape resonance characterized by quantum superposition of pairs in states corresponding to different spatial location and different parity. It is centered at z = 0 where the chemical potential is tuned to a Van Hove-Lifshits feature for the change of Fermi surface dimensionality in the electronic energy spectrum in one of the subbands. In this heterostructure at atomic limit the multiband superconductivity is in the clean limit because the disparity and negligible overlap between electron wavefunctions in different subbands suppresses the single electron interband impurity scattering rate. The emerging scenario from these experimental data suggests that the Feshbach shape resonance could be the mechanism for high T _c in particular nanostructured architectures.     

Notch effect and brittle-ductile transition

We present the fresults of the investigation of the effect of notches on the brittle-ductile transition in the process of fracture of structural materials. It is shown that the local elastoplastic state of a material in the vicinity of the notch can be described by using the corresponding stress intensity factorsK _I ^σ , strain intensity factorsK _i ^ε , and strain-energy intensity factorsK _I ^ω . We prove that the factorsK _I ^σ andK _I ^ε are linear functions of the notch radius ρ on the coordinates (wherei=σ or ε) and propose to characterize the brittle-ductile transition in the process of fracture of the material by the slopes of these functions. Bay Zoltan Institute of Logistics and Production Systems, Miskolc-Tapolca, Hungary. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 5, pp. 27–33, September–October, 1998.     

Effect of Ag on the electrical properties of a-Ge<Subscript>20</Subscript>Se<Subscript>80</Subscript> glasses

This article reports the effect of Ag impurity (low ~4 and ~6 at.% and high ~10 at.%) on the ac conductivity (σ _ac) of a-Ge₂₀Se₈₀ glass. Frequency (ω)-dependent ac conductance and capacitance of the samples over a frequency range ~100 Hz to 50 kHz was taken in the temperature range ~268–358 K. At frequency 2 kHz and temperature 298 K, the value of σ _ac increases at low as well as at higher concentration of Ag. The σ _ac is proportional to ω ^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases.