Transport mechanisms in doped LaMnO3:Evidence for polaron formation

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density

Structural and electronic properties of (NH3)xK3C60

We have investigated the synthesis, structure, and electronic properties of (NH3)xK3C60 by exposing preformed K3C60 to NH3 vapor. Measurements were made on bulk powders and films with a variety of techniques including in-situ x-ray diffraction and monitoring of NH3 pressure, Raman scattering and ac susceptibility. The reaction of NH3 with K3C60 is completely reversible and leaves the (C60)3- charge state intact, while allowing the distance between the C60 molecules to vary. We observe two different crystallographic structures at x=1 and x=8-10. The x=1 structure is face-centered orthorhombic with a single NH3 on the octahedral interstitial site. It is single phase at 100°C and NH3 pressures 400 torr and remains single phase at 0 torr if the temperature is reduced to 25°C. Although (NH3)K3C60 exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the x=8-10 structure are uncertain, however it appears that this phase is body-centered tetragonal with NH3 on both the octahedral and tetrahedral sites. It occurs at room temperature and NH3 pressures 500 torr, but is not stable at lower NH3 pressures. Resistivity measurements of NH3-doped K3C60 films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 cm

Superconductivity at 40K in Cesium Doped C60

We report superconductivity in Cs3C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable Cs1C60 and Cs4C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.

Transport mechanisms in doped LaMnO3: Evidence for polaron formation

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density

Structural and electronic properties of (NH3)xK3C60

We have investigated the synthesis, structure, and electronic properties of (NH3)xK3C60 by exposing preformed K3C60 to NH3 vapor. Measurements were made on bulk powders and films with a variety of techniques including in-situ x-ray diffraction and monitoring of NH3 pressure, Raman scattering and ac susceptibility. The reaction of NH3 with K3C60 is completely reversible and leaves the (C60)3– charge state intact, while allowing the distance between the C60 molecules to vary. We observe two different crystallographic structures at x = 1 and x = 8–10. The x = 1 structure is face-centered orthorhombic with a single NH3 on the octahedral interstitial site. It is single phase at 100°C and NH3 pressures ≥400 torr and remains single phase at 0 torr if the temperature is reduced to 25 °C. Although (NH3)K3C60 exhibits no superconductivity at ambient pressure, superconductivity can be induced by hydrostatic pressure with an onset at 28 K. The details of the x = 8–10 structure are uncertain, however it appears that this phase is body-centered tetragonal with NH3 on both the octahedral and tetrahedral sites. It occurs at room temperature and NH3 pressures ≥500 torr, but is not stable at lower NH3 pressures. Resistivity measurements of NH3-doped K3C60 films show that the formation of the new structure results in a rapid increase of the resistivity to values exceeding 1 Ω cm.