A. Jánossy

Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data.

We observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi(s) = 2.0 x 10(-5) emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T(c) at several frequencies suggests that MgB2 is a strongly anisotropic superconductor with the upper critical field, H(c2), ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB2 is an anisotropic superconductor with H(ab)(c2)/H(c)(c2) approximately 6-9.

Magnetic Fullerenes inside Single-Wall Carbon Nanotubes

C(59)N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C(59)N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C(59)N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of the spin-lattice relaxation time T(1) of C(59)N indicates a reversible charge transfer toward the host nanotubes above approximately 350 K. Bound C(59)N-C(60) heterodimers are formed at lower temperatures when C(60) is coencapsulated with the functionalized C(59)N. In the 10-300 K range, T(1) of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

Distribution of Iodine in Doped Polyacetylene Filme

Abstract The distribution of iodine in polyacetylene doped under conditons typical for most published work is investigated by energy dispersive X-ray analysis under an electron microscope. The 9.1 GHz dielectric constant of a series of iodine doped (CH)x is measured as a function of temperature. From the X-ray analysis we conclude that the reaction takes place in two steps: a fast reaction at the fiber surfaces and a slow reaction with the interior of (CH)x fibers. The dielectric dconstant of lightly doped (CH)x in the interior of the fibers. The temperature dependence of e is believed to be related to the intrinsic temperatue dependence of the conductivity of the highly doped fiber surface layers.

Spin-stretching modes in anisotropic magnets: Spin-wave excitations in the multiferroic ba2coge2o7

We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba(2)CoGe(2)O(7) in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or noncentrosymmetric lattice structure.

Influence of Nd on the magnetic properties of Nd1-xCaxMnO3: An ESR study

The role played by the Nd ions in the magnetic properties of Nd0.5Ca0.5MnO3 and Nd0.7Ca0.3MnO3 is studied using static magnetization, neutron diffraction and high frequency (9.4-475GHz) Electron Spin Resonance. We show that the Nd ions are weakly coupled to the Mn ions via ferromagnetic exchange and are responsible for the peculiar ferromagnetic resonance observed in the FM phase of both compounds (ground state below 120K for x=0.3, high field state for x=0.5). We then use ESR to look for magnetic phase separation in the low field, CO phase of Nd0.5Ca0.5MnO3. We show that there is no trace of the FM phase imbedded in the CO phase, contrary to what is observed in La0.5Ca0.5MnO3 or Pr0.5Sr0.5MnO3.

Azafullerene C59N, a stable free radical substituent in crystalline C60

Abstract Solid solutions of C 59 N azafullerene in C 60 with concentrations of 10 −5 to 10 −4 were produced in large quantities in an electric gas discharge tube. C 59 N is a stable monomeric substituent molecule in crystalline C 60 . The isotropic 14 N and 13 C hyperfine coupling constants measured by electron spin resonance (ESR) are characteristic of the extent of delocalization of the charge over the cage and are a sensitive test of electronic structure calculations. The C 59 N reorientational activation energy measured below the face centered cubic (fcc) to simple cubic (sc) transition is 2300 K. This value is similar to that of the matrix C 60 molecules, indicating that C 59 N–C 60 intermolecular interactions are weak.

Polymeric alkali fullerides are stable in air

Infrared transmission, electron spin resonance, and x‐ray diffraction measurements show unambiguously that RbC60 and KC60 are stable in air, in contrast to Rb6C60 which decomposes rapidly upon exposure. The specimens studied transform to pure C60 and other by‐products when heated above 100 °C, approximately the temperature of the orthorhombic‐fcc phase transition. The stability of these compounds raises the possibility of applying them as protective layers for the superconducting fullerides.

Spin diffusion and magnetic eigenoscillations confined to single molecular layers in the organic conductors κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Cl,Br)

The layered organic compounds, kappa-(BEDT-TTF)2Cu[N(CN)2]X X=Cl, Br) are metals at ambient temperatures. At low temperatures, the Cl compound is a weakly ferromagnetic Mott insulator while the isostructural Br compound is a superconductor. We find by conduction electron spin resonance and antiferromagnetic resonance (AFMR) an extreme anisotropy of spin transport and magnetic interactions in these materials. In the metallic state spin diffusion is confined to single molecular layers within the spin lifetime of 10(-9) s. Electrons diffuse several hundreds of nm without interlayer hopping. In the magnetically ordered insulating phase of the Cl compound we observe and calculate the four AFMR modes of the weakly coupled single molecular layers. The interplane exchange field is comparable or less than the typically 1 mT dipolar field and almost 10(6) times less than the intralayer exchange field.

Pressure and temperature dependence of interlayer spin diffusion and electrical conductivity in the layered organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Cl, Br)

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