Prof Arzhang Ardavan

Abstract:

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, with Y=C6H5NO2 and C6H5CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field Bc2 33T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature Tcâ 7K is observed when M=Ga and Y=C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to 18T for M=Cr and Y=C6H5NO2 and M=Fe and Y=C6H5CN. Prominent Shubnikov-de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising 6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic (Fe or Cr). The low effective masses and quasiparticle densities, and the systematic variation of the properties of the β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie [Phys. Rev. Lett. 87, 237002 (2001)], rather than the spin-fluctuation mechanism appropriate for the κ-(BEDT-TTF)2X salts. © 2005 The American Physical Society.

Abstract:

Two different structural isomers of Nd@C82 have been isolated using two-stage high-performance liquid chromatography. Their molecular orbital structures have been studied by a combination of scanning tunneling microscopy (STM) and density functional theory (DFT). Matching filled and empty-states STM images to DFT calculations allowed us to distinguish directly between the two isomers on the surface. © 2005 Elsevier B.V. All rights reserved.

Abstract:

For the first time, two types of the metallofullerene Nd@C82 have been isolated and characterized. HPLC was used to isolate Nd@C 82(I, II). The two isomers were characterized by mass spectrometry and UV-Vis-NIR absorption spectroscopy. Nd@C82(I) was found to be similar in structure to the main isomer of other lanthanofullerenes such as La@C82, as was previously reported. We assign Nd@C82(I) to have a C2v cage symmetry. Nd@C82(II) showed a markedly different UV-Vis-NIR absorption spectrum to Nd@C82(I). Its spectrum is in good agreement with that of the minor isomer of metallofullerenes such as Pr@C82. We therefore assign Nd@C82(II) to have a C s cage symmetry. In contrast to other metallofullerenes, both isomers appear to be equally abundant. © 2005 American Institute of Physics.