We use scanning SQUID microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: they tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors.
Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi_2Se_3 in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9times10^16cm^-3, the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the u =1 Landau level attained by a field of 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.
The three-dimensional Fermi surface morphology of superconducting BaFe_2(As_0.37}P_0.63)_2 with T_c=9K, is determined using the de Haas-van Alphen effect (dHvA). The inner electron pocket has a similar area and k_z interplane warping to the observed hole pocket, revealing that the Fermi surfaces are geometrically well nested in the (pi,pi) direction. These results are in stark contrast to the Fermiology of the non-superconducting phosphides (x=1), and therefore suggests an important role for nesting in pnictide superconductivity.
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 analogue of the collapsed tetragonal non-magnetic 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 (~1.5). The dramatic change in topology of the Fermi surface suggests that in a state with reduced (c/a) ratio, when bonding between pnictogen layers becomes important, the Fermi surface sheets are unlikely to be nested.
We report a detailed magnetotransport study of the highly anisotropic quasi-one-dimensional oxide Li$_{0.9}$Mo$_6$O$_{17}$ whose in-chain electrical resistivity diverges below a temperature $T_{rm min} sim$ 25 K. For $T < T_{rm min}$, a magnetic field applied parallel to the conducting chain induces a large negative magnetoresistance and ultimately, the recovery of a metallic state. We show evidence that this insulator/metal crossover is a consequence of field-induced suppression of a density-wave gap in a highly one-dimensional conductor. At the highest fields studied, there is evidence for the possible emergence of a novel superconducting state with an onset temperature $T_c >$ 10 K.
We report on a thorough optical investigation of BaFe$_2$As$_2$ over a broad spectral range and as a function of temperature, focusing our attention on its spin-density-wave (SDW) phase transition at $T_{SDW}=135$ K. While BaFe$_2$As$_2$ remains metallic at all temperatures, we observe a depletion in the far infrared energy interval of the optical conductivity below $T_{SDW}$, ascribed to the formation of a pseudogap-like feature in the excitation spectrum. This is accompanied by the narrowing of the Drude term consistent with the $dc$ transport results and suggestive of suppression of scattering channels in the SDW state. About 20% of the spectral weight in the far infrared energy interval is affected by the SDW phase transition.
