Paul Goddard

Abstract:

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 κ- (BEDT-TTF)2 Cu (NCS)2 using hydrostatic pressures of up to 9.8 kbar and magnetic fields of up to 33 T. In addition, we show that similar oscillations are revealed in ambient-pressure measurements, provided that the Shubnikov-de Haas oscillations are suppressed either by elevated temperatures or filtering of the data. These results provide a compelling validation of Pippard's semiclassical picture of magnetic breakdown. © 2007 The American Physical Society.

Abstract:

Comprehensive magnetic-field-orientation dependent studies of the susceptibility and de Haas-van Alphen effect have been carried out on single crystals of the filled skutterudites PrOs4AS12 and LaOs4AS12 using magnetic fields of up to 40 T. Several peaks are observed in the low-field susceptibility of PrOs4AS 12, corresponding to cascades of metamagnetic transitions separating the low-field antiferromagnetic and high-field paramagnetic metal (PMM) phases. The de Haas-van Alphen experiments show that the Fermi-surface topologies of PrOs4AS12 in its PMM phase and LaOs4AS 12 are very similar. In addition, they are in reasonable agreement with the predictions of bandstructure calculations for LaOs4AS 12 on the PrOs4AS12 lattice. Both observations suggest that the Pr 4f electrons contribute little to the number of itinerant quasiparticles in the PMM phase. However, while the properties of LaOs 4AS12 suggest a conventional nonmagnetic Fermi liquid, the effects of direct exchange and electron correlations are detected in the PMM phase of PrOs4AS12. For example, the quasiparticle effective masses in PrOs4AS12 are found to decrease with increasing field, probably reflecting the gradual suppression of magnetic fluctuations associated with proximity to the low-temperature, low-field antiferromagnetic state. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Abstract:

The interlayer magnetoresistance rho(zz) of the organic metal kappa-(BEDT-TTF)(2)Cu(NCS)(2) is studied in fields of up to 45 T and at temperatures T from 0.5 to 30 K. The peak in rho(zz) seen in in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor approximately 30. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the T dependence of the scattering rate tau(-1). The results suggest that tau(-1) does not vary strongly over the Fermi surface, and that it has a T(2) dependence due to electron-electron scattering.

Abstract:

TmAgGe is an antiferromagnet in which the spins are confined to distorted kagome-like planes at low temperatures. We report angle-dependent measurements of the magnetization M in fields of up to 55 T that show that there are two distinct and separate energy scales present in TmAgGe, each responsible for a set of step-like metamagnetic transitions: weak exchange interactions and strong crystalline-electric-field (CEF) interactions. Simulations of M using a three-dimensional, free-energy minimization technique allow us to specify the physical origin of the metamagnetic transitions in low, in-plane fields. We also show that the transitions observed with the field perpendicular to the kagome planes are associated with the CEF-split multiplet of Tm. © 2007 The American Physical Society.

Abstract:

The interlayer magnetoresistance ρzz of the organic metal κ-(BEDTTTF)2Cu(NCS)2 has been studied in fields B of up to 45 T and at temperatures T from 0.5 K to 50 K. The peak in ρzz seen in exactly in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor ∼ 25. Angle-dependent magnetoresistance oscillations (AMROs) due to Fermi-surface orbits are suppressed by rising T, with a T2dependence suggesting electron-electron scattering. © 2006 IOP Publishing Ltd.