Amalia Coldea

Abstract:

We report high-field magnetotransport studies on quasi-two dimensional beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]Y-. where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y=C6H5NO2 and C6H5CN) to metallic (for smaller solvents, Y=C5H5N and CH2O2). These changes in the ground state are connected with modifications of the Fermi surface, which varies from having one or two pockets for the superconducting charge-transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed.

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:

We report millimetre-wave electron spin resonance (ESR) measurements on single crystals of the high-spin molecules Cr-10(OMe)(20)(O2CCMe3)(10) and Cr12O9(OH)(3)(O2CCMe3)(15) within a temperature range of 1.4 K to 50 K and in magnetic fields of up to 5 Tesla. In our experiments it is possible to vary the orientation of the magnetic field with respect to the crystal axes, and thus to study the ESR lineshapes as a function of both temperature and angle. Our results confirm that Cr-10(OMe)(20)(O2CCMe3)(10) behaves as a single-molecule magnet with S = 15 and D = -0.03 K, while Cr12O9(OH)(3)(O2CCMe3)(15) has S = 6 and D similar to 0.1 K. A comparison of the experimental spectra with numerical simulations gives good agreement at low temperatures. At higher temperatures, we observe a narrowing of the ESR spectrum that is not explained by simple models.

Abstract:

Magnetotransport measurements have been performed on single crystals Of kappa-(BEDT-TTF)(2)Cu(NCS)(2) in fields of up to 33 T at temperatures between 500 mK and 4.2K. Using a diamond anvil cell mounted on a goniometer, measurements of the angle and temperature dependence of the interlayer resistance, R-zz, under hydrostatic pressures between 1.1 kbar and 17.3 kbar were performed. For the first time we have been able to measure angle-dependent magnetoresistance oscillations under pressure due to both the 1D and 2D Fermi surfaces in addition to Shubnikov de Haas oscillations. The results show that the shape of the elliptical quasi-2D Fermi-pocket is more elongated under a hydrostatic pressure of 9.8 kbar compared with ambient pressure. When the magnetic field B is close to parallel to the highly conductive plane, bc, a peak in R-zz is observed with an angular width determined by the ratio of the maximum inter- and intra-layer Fermi velocities. The width of this peak is found to increase with pressure suggesting that the Fermi surface becomes more three-dimensional upon application of pressure.