A. Bangura

Topological Change of the Fermi Surface in Ternary Iron Pnictides with Reduced c/a Ratio: A de Haas-van Alphen Study of CaFe2P2

We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2 using low-temperature torque magnetometry up to 45 T. This system is a close structural analog of the collapsed tetragonal nonmagnetic phase of CaFe2As2. We find the Fermi surface of CaFe2P2 to differ from other related ternary phosphides in that its topology is highly dispersive in the c axis, being three dimensional in character and with identical mass enhancement on both electron and hole pockets ( approximately 1.5). This suggests that when the bonding between pnictogen layers becomes important nesting conditions are not fulfilled.

Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions, \beta''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y

We report high-field magnetotransport measurements on β″ -(BEDT-TTF)[(HO)M(CO) ]Y, where M =Ga, Cr, and Fe and Y=CHN. 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 ∼m , 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) [(HO)M(CO)]Y salts.

Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits

We present experimental evidence for a hitherto unconfirmed type of angle-dependent magnetoresistance oscillation caused by magnetic breakdown. The effect was observed in the organic superconductor

A new quantum fluid at high magnetic fields in the marginal charge-density-wave system

\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cu}{(\mathrm{NCS})}_{2}

\alpha

using hydrostatic pressures of up to

-(BEDT-TTF)

9.8\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}

_2M

and magnetic fields of up to

Hg(SCN)

33\phantom{\rule{0.3em}{0ex}}\mathrm{T}

_4

. In addition, we show that similar oscillations are revealed in ambient-pressure measurements, provided that the Shubnikov\char21{}de Haas oscillations are suppressed either by elevated temperatures or filtering of the data. These results provide a compelling validation of Pippards semiclassical picture of magnetic breakdown [Philos. Trans. R. Soc. London, Ser. A 270, 1 (1962)].

(where

Single crystals of the organic charge-transfer salts α-(BEDT-TTF) 2 M Hg(SCN) 4 have been studied using Hall-potential measurements (M=K) and magnetization experiments (M=K, Rb). The data show that two types of screening currents occur within the high-magnetic-field, low-temperature charge-density wave (CDW x ) phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (j f r e e ), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (j m a g ), in that it is a microscopic, quasiorbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.