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Register a new userHall coefficient and Hc2 in underdoped LaFeAsO0.95F0.05
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The electrical resistivity and Hall coefficient of LaFeAsO0.95F0.05 polycrystalline samples were measured in pulsed magnetic fields up to m0H = 60 T from room temperature to 1.5 K. The resistance of the normal state shows a negative temperature coefficient (dr/dT < 0) below 70 K for this composition, indicating insulating ground state in underdoped LaFeAsO system in contrast to heavily doped compound. The charge carrier density obtained from Hall effect can be described as constant plus a thermally activated term with an energy gap DE = 630 K. Upper critical field, Hc2, estimated from resistivity measurements, exceeds 75 T with zero-field Tc = 26.3 K, suggesting an unconventional nature for superconductivity.
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Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-Tc superconductor, such as YBa2Cu3Oy, the magnetic field value where this melting occurs can be much lower than the mean-field critical field Hc2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and fit the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of Hc2(T =0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.
It has recently been proposed that the Fermi surface of underdoped high Tc copper oxide materials within the charge-ordered regime consists of a diamond-shaped electron pocket constructed from arcs connected at vertices. We show here that on modeling the in-plane magnetotransport of such a Fermi surface using the Shockley-Chambers tube integral approach and a uniform scattering time, several key features of the normal state in-plane transport of the underdoped copper oxide systems can be understood. These include the sign reversal in the Hall coefficient, the positive magnetoresistance and magnetic quantum oscillations in the Hall coefficient.
The use of MgB2 in superconducting applications still awaits for the development of a MgB2-based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured Hc2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.
Temperature dependence of the Hall coefficient, R_H_, has been investigated in charge-spin stripe-ordered La-214 high-T_c_ superconductors. Using the simplest stripe-ordered system of La_2-x_Ba_x_CuO_4_, it has been clarified that both the behavior of R_H_ and its sign exhibit significant dependences on the hole concentration. That is, R_H_ is zero in the ground state of the charge-spin stripe order at x=1/8, while it is negative in the less-stabilized state of the charge stripe for x<1/8. These are interpreted as owing to the delicate balance of the contributions of the hole-like Fermi surface and the possible electron pocket arising from the formation of the charge-spin stripe order.
Although the Hall coefficient R_H is an informative transport property of metals and semiconductors, its meaning in the cuprate superconductors has been ambiguous because of its unusual characteristics. Here we show that a systematic study of R_H in La_{2-x}Sr_{x}CuO_{4} single crystals over a wide doping range establishes a qualitative understanding of its peculiar evolution, which turns out to reflect a two-component nature of the electronic structure caused by an unusual development of the Fermi surface recently uncovered by photoemission experiments.
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