Quantum matter in high magnetic fields

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.

Abstract:

Magnetotransport properties of the compound Ca4Mn3 O10 are interpreted in terms of activated hopping of small magnetic polarons in the non-adiabatic regime. Polarons are most probably formed around Mn3+ sites created by oxygen sub-stoichiometry. The application of an external field reduces the size of the magnetic contribution to the hopping barrier and thus produces an increase in the conductivity. We argue that the change in the effective activation energy around TN is due to the crossover to variable range hopping conduction as antiferromagnetic order sets in.

Abstract:

A polycrystalline sample of Sr1.2Nd0.8Mn0.6Rh0.4O4 has been synthesised and characterised by neutron diffraction, dc magnetometry, muon spectroscopy and magnetotransport measurements. Ir is an n = 1 member of the Ruddlesden-Popper (RP) An + 1BnO3n + 1 family, having tetragonal (I4/mmm) symmetry [a = 3.86418(6), c = 12.4414(2) Å]. Rh and Mn cations are disordered over the six-coordinate sites within the perovskite layers of the RP structure and the A sites are occupied by a disordered distribution of Nd and Sr. In contrast to the analogous compound Sr1.2La 0.8Mn0.6Rh0.4O4, which showed long range ferromagnetic ordering in an applied field ar 2 K, a spin-glass transition was observed ar 16 K for the Nd-containing compound. No long range magnetic order was observed ar 2 K. A maximum magnetoresistance of ∼ 30% in a 140 kOe field was found at 125 K.