18,332 research outputs found
Neural Networks for Combined Control of Capacitor Banks and Voltage Regulators in Distribution Systems
Electronic correlations and unusual superconducting response in the optical properties of the iron-chalcogenide FeTe0.55Se0.45
The in-plane complex optical properties of the iron-chalcogenide
superconductor FeTe0.55Se0.45 have been determined above and below the critical
temperature Tc = 14 K. At room temperature the conductivity is described by a
weakly-interacting Fermi liquid; however, below 100 K the scattering rate
develops a frequency dependence in the terahertz region, signaling the
increasingly correlated nature of this material. We estimate the dc
conductivity just above Tc to be sigma_dc ~ 3500 Ohm-1cm-1 and the superfluid
density rho_s0 ~ 9 x 10^6 cm-2, which places this material close to the scaling
line rho_s0/8 ~ 8.1 sigma_dc Tc for a BCS dirty-limit superconductor. Below Tc
the optical conductivity reveals two gap features at Delta_1,2 ~ 2.5 and ~ 5.1
meV.Comment: Minor revisions, 5 pages, 4 figure
On the Reconstructed Fermi Surface in the Underdoped Cuprates
The Fermi surface topologies of underdoped samples the high-Tc superconductor
Bi2212 have been measured with angle resolved photoemission. By examining
thermally excited states above the Fermi level, we show that the Fermi surfaces
in the pseudogap phase of underdoped samples are actually composed of fully
enclosed hole pockets. The spectral weight of these pockets is vanishingly
small at the anti-ferromagnetic zone boundary, which creates the illusion of
Fermi "arcs" in standard photoemission measurements. The area of the pockets as
measured in this study is consistent with the doping level, and hence carrier
density, of the samples measured. Furthermore, the shape and area of the
pockets is well reproduced by a phenomenological model of the pseudogap phase
as a spin liquid.Comment: 4 pages, 4 figures. Submitted to Physics Review Letter
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