Showing papers by "László Forró published in 1994"

High mobility n‐type charge carriers in large single crystals of anatase (TiO2)

Abstract: Resistivity, thermopower, and Hall‐effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n‐type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.

Polymeric fullerene chains in RbC60 and KC60

Abstract: NEARLY all of the molecular crystals containing C60, formed at ambient pressure1,2 have inter-fullerene separations of the order of 10 A — the expected distance based on the molecular van der Waals radii. The sole exceptions are the room-temperature phases of AC60 (where A denotes K, Rb or Cs), which are formed by reversible solid-state transformation from high-temperature (>150 °C) phases3. These phases have lattice parameters about 9% shorter in one direction, and in addition RbC60 has magnetic properties suggestive of a one-dimensional metal4. We suggested in ref. 4 that this short distance may be due to covalent bonding between neighbouring C60 molecules. Here we provide direct evidence for such bonding from powder X-ray diffraction studies of RbC60 and KC60 . The linkage is through a [2+2] cycloaddition, which has been hypothesized to take place during photopolymerization of solid C60 (ref. 5), and which has also been proposed6 for RbC60. Such inter-fullerene linkages are calculated7,8 to be the preferred mode of dimerization of C60. The AC60 phases thus provide an example of a thermal phase transition driven by the reversible formation and breaking of covalent bonds.

Quasi-one-dimensional electronic structure in orthorhombic RbC60.

Abstract: X-ray diffraction studies show that the stable phase of the alkali fullerene RbC[sub 60] is orthorhombic ([ital o]-RbC[sub 60]) below 350 K. C[sub 60] molecules form chains along [bold a] with an unusually short spacing of 9.12 A and magnetic properties suggest that [ital o]-RbC[sub 60] is a quasi-1D metal with a transition to a spin density wave ground state at 50 K. The high temperature fcc phase of RbC[sub 60] may be stabilized below 300 K by quenching from 500 K; it is paramagnetic above 300 K and transforms into a nonmagnetic ground state beween 300 and 250 K.

Hopping in disordered conducting polymers

Abstract: This paper addresses some important questions concerning hopping conduction in doped conducting polymers, i.e., dimensionality, homogeneity, adiabaticity, Coulomb interactions, and multiphonon character. It will be shown that electron transport in these materials is due to correlated hopping between polaronic clusters. We propose a model which describes quantitatively this process. Conductivity and magnetoresistance measurements performed on disordered polypyrroles doped with polyelectrolytes are presented in support of these ideas. This model emphasizes the role of the dopant as an attractive potential and as a tunneling bridge between neighboring chains. The dopant ions are the source of both the polaronic clusters stability and of the transverse conduction.

Single-crystalline (KC60)n: A conducting linear alkali fulleride polymer

Abstract: Single crystals of a linear cycloadduct conducting polymer, (KC60) n , have been grown that are a few tenths of a millimeter in length. Partial oxidation under toluene transformed these crystals into bundles of fibers. The degree of polymerization exceeded 100,000.

Thermal conductivity of insulating Bi 2 Sr 2 YCu 2 O 8 and superconducting Bi 2 Sr 2 CaCu 2 O 8 : Failure of the phonon-gas picture

Abstract: The ab-plane thermal conductivity \ensuremath{\kappa}(T) of insulating ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{YCu}}_{2}$${\mathrm{O}}_{8}$ and superconducting ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ is measured from T=10 to 300 K on single-crystal samples. Metallic ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ has a significantly higher \ensuremath{\kappa} than ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{YCu}}_{2}$${\mathrm{O}}_{8}$; the difference \ensuremath{\Delta}\ensuremath{\kappa} agrees well in magnitude with a Wiedemann-Franz estimate of the electronic contribution to \ensuremath{\kappa} in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. The shape of \ensuremath{\kappa}(T) in insulating ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{YCu}}_{2}$${\mathrm{O}}_{8}$ differs from normal insulators described by the Peierls-Boltzmann theory. Assuming that atomic vibrations are the main heat carrier, and noting that \ensuremath{\kappa} is more similar to that of silica glass than to a normal insulating crystal like CuO, we suggest that the ``phonon'' mean free path is sufficiently short that the Peierls-Boltzmann theory is not applicable. This is consistent with evidence from neutron scattering that phonons are poorly defined. Our data support the idea that the peak in \ensuremath{\kappa}(T) observed below ${\mathit{T}}_{\mathit{c}}$ in superconducting samples originates from electronic rather than vibrational heat currents.

NMR study on Rb3C60

Abstract: 85,87Rb-NMR has been done on Rb3C60 in the normal state. A new broad peak has been found below ∼150K. Comparison of the NMR spectra of the two isotopes reveals the existence of a quadrupole interaction at the site associated with the new peak. It is also suggested that the nuclear relaxation at the O-site is significantly contributed by phonons.

Hopping in high {Tc} cuprates

Abstract: High temperature superconductors based on copper oxide are fascinating materials not only because of their high transition temperatures which reach values as high as 130K, but also because of the possibility of studying other phenomena such as low dimensional magnetism, two dimensional localization, weak localization and hopping. Many oxide families are now known with different maximum {Tc} values. All these systems have CuO{sub 2} planes as conducting units. At low carrier density the system is electrically insulating with a long range antiferromagnetic order below a Neel temperature of the order of 300 K, which falls strongly with increased carrier concentration. Above a critical carrier density superconductivity appears, {Tc} reaches maximum at an optimal carrier density and decreases on further doping. For the superconducting compositions the system is generally metallic above {Tc} but there is a common opinion that it is an unusual metal with strong electron-electron correlations. Studies of hopping can be made in the low carrier density range, where the samples are non-metallic and one hopes to gain insight into the density of states, electron correlations, dimensionality of the charge transport etc. by resistivity measurements. It is less commonly known that there is also hopping-like transport even for highermore » carrier concentrations. Namely this is the case for the transverse charge transport between the CuO{sub 2} planes in the metallic phase.« less

Rb NMR study of the electronic properties of Rb1C60

Abstract: 87 Rb (and 85 Rb) NMR spectra in the orthorhombic phase ( (o) phase) of Rb 1 C 60 show up broadening, with respect to the high-T cubic phase and to the quenched phase, which signals change of symmetry of the Rb site. The spin lattice relaxation rate follows a linear T dependence in the (o) phase above 100 K, suggesting that this phase is metallic. The increase of 1/T 1 T at low T seems to be correlated with the transition seen by E.S.R.