Temperature hysteretic effect and its influence on colossal magnetoresistance of La0.33Nd0.33Ca0.33MnO3

Electrical resistance (R) measurements of a bulk La_0.33Nd_0.33Ca_0.33MnO₃ perovskite in magnetic fields up to 40 kOe have revealed anomalous temperature hysteretic effects both in 0 Oe and 20 kOe magnetic fields. The sharp peak observed in the R vs. T plot indicates the occurrence of metal-to-insulator (M-I) transition at a temperature of T _MI=110 K and 140 K, for cooling and warming paths, respectively. An applied magnetic field of 20 kOe reduces the resistance and shifts T _MI to 160 K and 185 K for cooling and warming, respectively. We have observed a much higher resistance in the cooling path than in the warming path leading to the hysteretic resistance ratio (R _cool/R _warm) of 200 at 110 K and 1.8 at 160 K for 0 Oe and 20 kOe, respectively. Record values of colossal magnetoresistance (CMR) have been achieved. The CMR value reaches nearly 99% in the temperature ranges of 90 K to 140 K and 90 K to 170 K for 20 kOe and 40 kOe magnetic fields in the cooling mode, respectively. The observed unusual behavior is attributed to the co-existence of La-rich and Nd-rich domains assumed to be distributed randomly in the compound.     

The microwave spectrum of piperidine: Equatorial and axial ground states

The microwave spectra of piperidine and N-deuterated piperidine were investigated between 8 and 40 GHz. The ground states of both equatorial and axial conformers have been identified by both type-A and type-C transitions, and the substitution coordinates of the imino hydrogen have been determined for both conformers. Dipole-moment components for the equatorial conformer are μ_a = 0.178 D, μ_c = 0.80 D, μ = 0.82 D, and for the axial conformer are μ_a = 1.07 D, μ_c = 0.521 D, μ = 1.19 D. The quadrupole coupling constants for the axial conformer are: χ_aa = ?3.80 MHz, χ_bb = 2.91 MHz, χ_cc = 0.83 MHz and for the equatorial conformer χ_cc = ?4.83 MHz. The rotational constants indicate a significant flattening of the ring in axial piperidine compared with equatorial piperidine. The equatorial conformer is the more abundant; intensity measurements on several sets of lines indicate the excess energy of the axial conformer to be 3.1 ± 0.3 kJ mole^?1. This represents a significant change from our earlier reported value and is now more in line with measurements obtained by other methods.     

ac Conductivity of mixed spinel NiAl0.7Cr0.7Fe0.6O4

ac Conductivity measurements are carried out across the metal to insulator transition in NiAl_0.7Cr_0.7Fe_0.6O₄. The low frequency data is analyzed using Summerfield scaling theory for hopping conductivity. The exponent of the scaling behavior has significantly different values in the conducting and insulating regimes. The hopping frequency and the zero frequency conductivity are found to increase with temperature, slowly in the metallic regime and rapidly in the insulating regime.     

Wormlike micelles from a cage amine metallosurfactant

We have shown that copper and cobalt metallosurfactants derived from Cu(II) and Co(III) complexes of a macrobicyclic hexamine ("cage") can form wormlike micelles in aqueous solution that may coexist with or easily interconvert with vesicle structures. The cylindrical micelle structures are unusual for triple-chain surfactants with a single headgroup and are not easily accounted for using geometrical packing arguments. The solution behavior has been characterized by cryo-TEM and SAXS measurements. Both the Cu and Co compounds display viscoelastic solutions at 1 wt %, indicating that such behavior may be anticipated for the full variety of stable metal complexes formed by the cage headgroup, auguring applications based on the incorporation of metallo aggregates into mesoporous silica structures.     

Comparison of strong anion-exchangers for the purification of a PEGylated protein

We have studied the effect of protein PEGylation on ion-exchange adsorption using bovine serum albumin as a model system. The free sulfhydryl group of BSA, located on cysteine 34, was PEGylated using the maleimido-PEG chemistry. Several different BSA preparations were screened for extent of reaction using a 30 kDa PEG reagent. The highest yielding BSA preparation was PEGylated using linear 12 kDa and 30 kDa PEG reagents at the 1 liter scale. The PEGylated reaction mixture was purified by anion-exchange gradient elution chromatography to remove native protein and aggregates. Purity following anion-exchange chromatography was >90% as determined by analytical size exclusion chromatography. The elution salt concentration decreased with increasing PEG chain length. Breakthrough studies on six commercially available anion-exchange stationary phases with purified PEG-BSA conjugates confirm a very large decrease in dynamic binding capacity compared to the native protein. The decrease in dynamic binding capacity is likely due to modulation of electrostatic interactions caused by the neutral PEG chain and increased mass transfer resistance associated with the large size of the molecule. Of the stationary phases evaluated, the open porous structure of the agarose based ion-exchangers resulted in the highest dynamic binding capacities for the PEG-BSA conjugates. Frontal analysis experiments demonstrate use of this technique for purification of PEGylated proteins. A stationary phase that tended to exclude the large PEG-BSA conjugate was very efficient in removing native protein from a crude reaction mixture by frontal analysis.     

Determinantal representations of smooth cubic surfaces

For every smooth (irreducible) cubic surface S we give an explicit construction of a representative for each of the 72 equivalence classes of determinantal representations. Equivalence classes (under GL₃ × GL₃ action by left and right multiplication) of determinantal representations are in one to one correspondence with the sets of six mutually skew lines on S and with the 72 (two-dimensional) linear systems of twisted cubic curves on S. Moreover, if a determinantal representation M corresponds to lines (a ₁,...,a ₆) then its transpose M ^t corresponds to lines (b ₁,...,b ₆) which together form a Schläfli’s double-six \(a_1\ldots a_6 \choose b_1\ldots b_6\) . We also discuss the existence of self-adjoint and definite determinantal representation for smooth real cubic surfaces. The number of these representations depends on the Segre type F _i . We show that a surface of type F _i , i = 1,2,3,4 has exactly 2(i?1) nonequivalent self-adjoint determinantal representations none of which is definite, while a surface of type F ₅ has 24 nonequivalent self-adjoint determinantal representations, 16 of which are definite.     

The temperature dependence of dislocation charge in calcium doped single crystals of potassium chloride

Abstract

Using the ultrasonic piezoelectric composite oscillator technique we have measured the temperature dependence of dislocation charge in bent KCl single crystals as a function of Ca impurity concentration. In each case the charge changes sign at an extrinsic isoelectric temperature, T_e , and the shift in Te with impurity concentration allows the calculation of the individual thermodynamic vacancy formation parameters. We obtain h₊ = 1.244±0.014 eV, s₊ = 4.50±0.19 k, h- = 1.346? 0.014 eV and s- = 5.11?0.19K.     

Single-phase Bi2+x(Ca,Sr) n+1 Cu n O2n+4+δ, n = 0, 1, 2, 3 AND ∞, and some substitutional DERIVATIVES

The preparation, structure and properties of five members of the homologous series Bi_2+x(Ca,Sr) _n+1 Cu _n O_2n+4+δ are discussed, namely for n = 0, 1, 2, 3 and ∞. The two end members are insulating phases with a fixed oxygen stoichiometry but the members n = 1, 2 and 3 are superconducting phases with T_c depending on the calcium to strontium ratio and on oxygen stoichiometry as determined by annealing temperature and oxygen partial pressure. Maximum zero resistance T_c 's obtained are n = 1: 76 K, n = 2: 91 K and n = 3: 106 K. The effects of Pb-substitution in n = 2 and n = 3 are discussed with particular reference to the stabilisation of the latter phase. Rare-earth substitution is shown to raise T_c for n = 2 to at least 101 K without the introduction of the n = 3 phase.     

On the theory of slowing down gracefully

We discuss the transport of a tracer particle through the Bose?CEinstein condensate of a Bose gas. The particle interacts with the atoms in the Bose gas through two-body interactions. In the limiting regime where the particle is very heavy and the Bose gas is very dense, but very weakly interacting (??mean-field limit??), the dynamics of this system corresponds to classical Hamiltonian dynamics. We show that, in this limit, the particle is decelerated by emission of gapless modes into the condensate (Cerenkov radiation). For an ideal gas, the particle eventually comes to rest. In an interacting Bose gas, the particle is decelerated until its speed equals the propagation speed of the Goldstone modes of the condensate. This is a model of ??Hamiltonian friction??. It is also of interest in connection with the phenomenon of ??decoherence?? in quantum mechanics. This note is based on work we have carried out in collaboration with D Egli, I M Sigal and A Soffer.     

Regenerating a symmetry in asymmetric dark matter

Asymmetric dark matter theories generically allow for mass terms that lead to particle-antiparticle mixing. Over the age of the Universe, dark matter can thus oscillate from a purely asymmetric configuration into a symmetric mix of particles and antiparticles, allowing for pair-annihilation processes. Additionally, requiring efficient depletion of the primordial thermal (symmetric) component generically entails large annihilation rates. We show that unless some symmetry completely forbids dark matter particle-antiparticle mixing, asymmetric dark matter is effectively ruled out for a large range of masses, for almost any oscillation time scale shorter than the age of the Universe.