We present a detailed temperature and frequency dependence of the optical conductivity measured on clean high quality single crystals of URu$_{2}$Si$_{2}$ of $ac$- and $ab$-plane surfaces. Our data demonstrate the itinerant character of the narrow 5f
bands, becoming progressively coherent as temperature is lowered below a cross-over temperature $T^*{sim}75~K$. $T^*$ is higher than in previous reports as a result of a different sample preparation, which minimizes residual strain. We furthermore present the density-response (energy-loss) function of this compound, and determine the energies of the heavy fermion plasmons with $a$-and $c$-axis polarization. Our observation of a suppression of optical conductivity below 50~meV both along $a$ and $c$-axis, along with a heavy fermion plasmon at 18~meV, points toward the emergence of a band of coherent charge carriers crossing the Fermi energy and the emergence of a hybridization gap on part of the Fermi surface. The evolution towards coherent itinerant states is accelerated below the hidden order temperature $T_{HO}=17.5$~K. In the hidden order phase the low frequency optical conductivity shows a single gap at $sim 6.5$meV, which closes at $T_{HO}$.
Switched capacitor arrays (SCA) ASICs are becoming more and more popular for the readout of detector signals, since the sampling frequency of typically several gigasamples per second allows excellent pile-up rejection and time measurements. They suff
er however from the fact that their sampling bins are not equidistant in time, given by limitations of the chip process. In the past, this limited time measurements of optimal signals to standard deviations ({sigma}) of about 4-25 ps in accuracy for the split pulse test, depending on the specific chip. This paper introduces a novel time calibration, which determines the true sampling speed of an SCA. Additionally, for two independently running SCA chips, the achieved time resolution improved to less than 3 ps ({sigma}) independently from the delay for the split pulse test, when simply applying a linear interpolation. When using a more advanced analyzing technique for the split pulse test with a single SCA, this limit is pushed below 1 ps ({sigma}) for delays up to 8 ns. Various test measurements with different boards based on the DRS4 ASIC indicate that the new calibration is stable over time but not over larger temperature variations.
We report optical measurements demonstrating that the low-energy relaxation rate ($1/tau$) of the conduction electrons in Sr$_2$RuO$_4$ obeys scaling relations for its frequency ($omega$) and temperature ($T$) dependence in accordance with Fermi-liqu
id theory. In the thermal relaxation regime, $1/taupropto (hbaromega)^2 + (ppikB T)^2$ with $p=2$, and $omega/T$ scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling, and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing resilient quasiparticle excitations above the Fermi energy.
Cuprate high-T_c superconductors on the Mott-insulating side of optimal doping (with respect to the highest T_cs) exhibit enigmatic behavior in the non-superconducting state. Near optimal doping the transport and spectroscopic properties are unlike t
hose of a Landau-Fermi liquid. For carrier concentrations below optimal doping a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface, and causes a break-up of the Fermi surface into disconnected nodal and anti-nodal sectors. Here we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. Our optical measurements reveal that the dynamical relaxation rate 1/tau(omega,T) collapses on a universal function proportional to (hbar omega)^2+(1.5 pi k_B T)^2. Hints at possible Fermi liquid behavior came from the recent discovery of quantum oscillations at low temperature and high magnetic field in underdoped YBa2Cu3O6+d and YBa2Cu4O8, from the observed T^2-dependence of the DC ({omega}=0) resistivity for both overdoped and underdoped cuprates, and from the two-fluid analysis of nuclear magnetic resonance data. However, the direct spectroscopic determination of the energy dependence of the life-time of the excitations -provided by our measurements- has been elusive up to now. This observation defies the standard lore of non-Fermi liquid physics in high T_c cuprates on the underdoped side of the phase diagram.
