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Optical conductivity of nodal metals
Fermi liquid theory is remarkably successful in describing the transport and
optical properties of metals; at frequencies higher than the scattering rate,
the optical conductivity adopts the well-known power law behavior
. We have observed an unusual non-Fermi
liquid response in the ground
states of several cuprate and iron-based materials which undergo electronic or
magnetic phase transitions resulting in dramatically reduced or nodal Fermi
surfaces. The identification of an inverse (or fractional) power-law behavior
in the residual optical conductivity now permits the removal of this
contribution, revealing the direct transitions across the gap and allowing the
nature of the electron-boson coupling to be probed. The non-Fermi liquid
behavior in these systems may be the result of a common Fermi surface topology
of Dirac cone-like features in the electronic dispersion.Comment: 8 pages including supplemental informatio
Eigenstructure Assignment Based Controllers Applied to Flexible Spacecraft
The objective of this paper is to evaluate the behaviour of a controller designed using a parametric Eigenstructure Assignment method and to evaluate its suitability for use in flexible spacecraft. The challenge of this objective lies in obtaining a suitable controller that is specifically designated to alleviate the deflections and vibrations suffered by external appendages in flexible spacecraft while performing attitude manoeuvres. One of the main problems in these vehicles is the mechanical cross-coupling that exists between the rigid and flexible parts of the spacecraft. Spacecraft with fine attitude pointing requirements need precise control of the mechanical coupling to avoid undesired attitude misalignment. In designing an attitude controller, it is necessary to consider the possible vibration of the solar panels and how it may influence the performance of the rest of the vehicle. The nonlinear mathematical model of a flexible spacecraft is considered a close approximation to the real system. During the process of controller evaluation, the design process has also been taken into account as a factor in assessing the robustness of the system
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