Quantum matter in high magnetic fields

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The role of magnetic ions on the magnetotransport properties of the charge-transfer salts beta ''-BEDT-TTF4[(H3O)M(C2O4)(3)]C5H5N where M = Ga3+, Cr3+ or Fe3+

J MAGN MAGN MATER 272-76 (2004) 1062-1064

Authors:

AI Coldea, AF Bangura, J Singleton, A Ardavan, A Akutsu-Sato, H Akutsu, SS Turner, P Day

Abstract:

We report high-field magneto transport measurements on beta''- BEDT-TTF4[(H3O)M(C2O4)(3)]C5H5N, where M = Ga3+, Cr3+ or Fe3+. In spite of the differing M ions, these compounds have very similar Fermi surfaces. We observe four distinct Shubnikov-de Haas frequencies, corresponding to four Fermi-surface pockets; the frequencies exhibit the additive relationship expected for a compensated semimetal. The compounds show paramagnetic behaviour and no superconductivity down to 0.5 K, in contrast to other materials of the same family with different solvent molecules. (C) 2003 Elsevier B.V. All rights reserved.
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Fermiology of new charge-transfer salts, beta ''-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]center dot solvent where M = Ga, Cr and Fe

J PHYS IV 114 (2004) 205-209

Authors:

AI Coldea, AF Bangura, A Ardavan, J Singleton, A Akutsu-Sato, H Akutsu, SS Turner, R Day

Abstract:

We report high-field magnetotransport measurements oil beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)], solvent, where M=Ga3+, Cr3+ and Fe3+ and solvent=C5H5N. In spite of their differing transition metal-ions, M, the three compounds exhibit similar magnetic quantum oscillation spectra superimposed on a positive magnetoresistance. At least four independent quantum oscillation frequencies have been identified, corresponding to two different hole and electron pockets of the Fermi surface which follow the rules of a compensated metal. Observation of the small pockets could be the result of the Fermi surface reconstruction induced by a possible density wave. The effective masses are very similar for different samples and for different pockets range between m(eff) approximate to 0.5 - 1.1 m(e) whereas the Dingle temperatures varies between T-D approximate to 1.4 - 4 K. At low temperature, the longitudinal magnetoresistance violates Kohler's rule, suggesting that the interlayer transport in these quasi-2D systems cannot be related to a single scattering time and that the disorder plays an important role.
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Millimetre-wave and magnetic studies on a high-spin molecule, Cr-10(OMe)(20)(O2CCMe3)(10)

J PHYS IV 114 (2004) 645-647

Authors:

S Sharmin, A Ardavan, SJ Blundell, AI Coldea, EJL McInnes, D Low

Abstract:

We report millimetre-wave and magnetic measurements on single crystals of the high-spin molecular magnet, Cr-10(OMe)(20)(O2CCMe3)(10). The susceptibility data obtained using a SQUID magnetometer indicate that this molecule has a total spin of S = 15. The temperature dependence of the magnetisation allows us to estimate the intermolecular exchange constant. The EPR spectra were obtained at different frequencies, angles and temperatures. The results are compared with numerical simulations allowing determination of the Hamiltonian parameters.
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The effect of magnetic ions and disorder on superconducting beta ''-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]center dot C6H5NO2 salts, where M = Ga and Cr

J PHYS IV 114 (2004) 285-287

Authors:

AF Bangura, AI Coldea, A Ardavan, J Singleton, A Akutsu-Sato, H Akatsu, P Day

Abstract:

We report in magnetotransport measurements performed in magnetic fields of up to 33 T and at low temperatures (0.45 K
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Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions: β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y (M = Ga, Cr, Fe; Y = C5H5N)

Physical Review B Condensed Matter and Materials Physics 69:8 (2004)

Authors:

AI Coldea, AF Bangura, J Singleton, A Ardavan, A Akutsu-Sato, H Akutsu, SS Turner, P Day

Abstract:

We report high-field magnetotransport measurements on β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, where M = Ga, Cr, and Fe and Y = C5H5N. We observe similar Shubnikov-de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area 8.5% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are me, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts. © 2004 The American Physical Society.
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