Critical points that can be suppressed to zero temperature are interesting because quantum fluctuations have been shown to dramatically alter electron gas properties. Here, the metal formed by Co doping the paramagnetic insulator FeS2, Fe1-xCoxS2 is demonstrated to order ferromagnetically at x > xc = 0.01+/-0.005, where we observe unusual transport, magnetic, and thermodynamic properties. We show that this magnetic semiconductor undergoes a percolative magnetic transition with distinct similarities to the Griffiths phase, including singular behavior at xc and zero temperature.

The magnetotransport properties of single crystals of the highly anisotropic layered metal LaSb2 are reported in magnetic fields up to 45 T with fields oriented both parallel and perpendicular to the layers. Below 10 K the perpendicular magnetoresistance of LaSb2 becomes temperature independent and is characterized by a 100-fold linear increase in resistance between 0 and 45 T with no evidence of quantum oscillations down to 50 mK. The Hall resistivity is hole-like and gives a high field carrier density of n ∼ 3x10 20 cm −3 . The feasibility of using LaSb2 for magnetic field sensors is discussed.One of the most successful strategies for producing technologically relevant magnetoresistive materials is to enhance the effects of field-dependent magnetic scattering processes through the creation of magnetic superlattices [1] or by doping magnetic insulators such that a magnetic and metal-insulator transition coincide [2]. Unexpectedly, there have been several recent discoveries of a large, non-saturating magnetoresistance (MR) in low carrier density nonmagnetic metals [3,4,5,6,7] and semiconductors [8]. One class of these systems, the slightly off-stoichiometric silver chalcogenides, Ag 2+δ Te and Ag 2+δ Se, have shown significant promise as the basis of ultra-high magnetic field sensors by virtue of the fact that they exhibit a multi-fold, quasi-linear MR that remains unsaturated up to 60 T [8]. In this Letter we present magntotransport data on the highly layered non-magnetic metal LaSb 2 which displays a 100-fold, linear MR with no sign of saturation up to 45 T. We show that in many respects, including sensitivity, linearity, synthesis characteristics and intrinsic anisotropy, LaSb 2 is a compelling candidate for high-field sensor development.LaSb 2 is a member of the RSb 2 (R=La-Nd, Sm) family of compounds that all form in the orthorhombic SmSb 2 structure [9, 10]. LaSb 2 , in particular, is comprised of alternating La/Sb layers and two-dimensional rectangular sheets of Sb atoms stacked along the c−axis [11]. Similar structural characteristics give rise to the anisotropic physical properties observed in all the compounds in the RSb 2 series [12]. Since LaSb 2 is non-magnetic, its low-temperature transport properties are not complicated by magnetic phase transitions which occur in the other members of this series [12].Single crystals of LaSb 2 were grown from high purity La and Sb by the metallic flux method [13]. The orthorhombic SmSb 2 -structure type was confirmed by single crystal X-ray diffraction. The crystals grow as large flat layered plates which are malleable and easily cleaved. Typically flux grown samples had dimensions of 5mm x 5mm x 0.2mm. Electrical contact was made using Epotek [14] silver epoxy and 1 mil gold wire. Transport properties were measured using a 27 Hz 4-probe AC technique at temperatures from 0.03 -300 K and in magnetic fields up to 45 T. In all of the measurements presented probe currents of 1 -5 mA where used with corresponding power levels less than 10 nW. Hall effect measurements we...

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We have measured the resistivity, optical conductivity, and magnetic susceptibility of LaSb2 to search for clues as to the cause of the extraordinarily large linear magnetoresistance and to explore the properties of the superconducting state. We find no evidence in the optical conductivity for the formation of a charge density wave state above 20 K despite the highly layered crystal structure. In addition, only small changes to the optical reflectivity with magnetic field are observed indicating that the MR is due to scattering rate, not charge density, variations with field. Although a superconducting ground state was previously reported below a critical temperature of 0.4 K, we observe, at ambient pressure, a fragile superconducting transition with an onset at 2.5 K. In crystalline samples, we find a high degree of variability with a minority of samples displaying a full Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The application of pressure stabilizes the superconducting transition and reduces the anisotropy of the superconducting phase.

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