Plasmonics offers an enticing platform to manipulate light at the subwavelength scale. Currently, loss represents the most serious challenge impeding its progress and broad impact towards practical technology. In this regard, silver (Ag) is by far th
e preferred plasmonic material at optical frequencies, having the lowest loss among all metals in this frequency range. However, large discrepancies exist among widely quoted values of optical loss in Ag due to variations in sample preparation procedures that produce uncontrollable grain boundaries and defects associated with additional loss. A natural question arises: what are the intrinsic fundamental optical properties of Ag and its ultimate possibilities in the field of plasmonics? Using atomically-smooth epitaxial Ag films, we extracted new optical constants that reflect significantly reduced loss and measured greatly enhanced propagation distance of surface plasmon polaritons (SPPs) beyond what was previously considered possible. By establishing a new benchmark in the ultimate optical properties of Ag, these results will have a broad impact for metamaterials and plasmonic applications.
Spin excitations are one of the top candidates for mediating electron pairing in unconventional superconductors. Their coupling to superconductivity is evident in a large number of systems, by the observation of an abrupt redistribution of magnetic s
pectral weight at the superconducting transition temperature, Tc, for energies comparable to the superconducting gap. Here we report inelastic neutron scattering measurements on Fe-based superconductors, Fe1-x (Ni/Cu)x Te0.5 Se0.5, that emphasize an additional signature. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T >> Tc to a distinctly different U-shaped dispersion at low temperature. Importantly, this spectral reconstruction is apparent for temperature up to ~3Tc. If the magnetic excitations are involved in the pairing mechanism, their surprising modification on the approach to Tc demonstrates that strong interactions are involved.
Resistivity, Hall effect and magnetoresistance have been investigated systematically on single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ranging from undoped to optimally doped regions. A systematic evolution of the quasiparticle scattering has been ob
served. It is found that the resistivity in the normal state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is insensitive to the potassium doping concentration, which is very different from the electron doped counterpart Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, where the resistivity at 300 K reduces to half value of the undoped one when the system is optimally doped. In stark contrast, the Hall coefficient R$_H$ changes suddenly from a negative value in the undoped sample to a positive one with slight K-doping, and it keeps lowering with further doping. We interpret this dichotomy due to the asymmetric scattering rate in the hole and the electron pockets with much higher mobility of the latter. The magnetoresistivity shows also a non-monotonic doping dependence indicating an anomalous feature at about 80 K to 100 K, even in the optimally doped sample, which is associated with a possible pseudogap feature. In the low temperature region, it seems that the resistivity has the similar values when superconductivity sets in disregarding the different T$_c$ values, which indicates a novel mechanism of the superconductivity. A linear feature of resistivity $rho_{ab}$ vs. $T$ was observed just above $T_c$ for the optimally doped sample, suggesting a quantum criticality.
Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_c=14 K). Low energy spin fluctuations are found with a characteristic wave vector $(0.5,0.5,L)$ that corresponds to Fermi surface nesting and differs from Q_m=(delta,0,
0.5) for magnetic ordering in Fe_{1+y}Te. A spin resonance with hbarOmega_0=6.5 meV approx 5.3 k_BT_c and hbarGamma=1.25 meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s+/- superconductivity and imperfect quasi-2D Fermi surface nesting.
In recent years, many efforts have been addressed on collision avoidance of collectively moving agents. In this paper, we propose a modified version of the Vicsek model with adaptive speed, which can guarantee the absence of collisions. However, this
strategy leads to an aggregated state with slowly moving agents. We therefore further introduce a certain repulsion, which results in both faster consensus and longer safe distance among agents, and thus provides a powerful mechanism for collective motions in biological and technological multi-agent systems.
