We report optical measurements demonstrating that the low-energy relaxation rate (1/τ) of the conduction electrons in Sr 2 RuO 4 obeys scaling relations for its frequency (ω) and temperature (T ) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, 1/τ ∝ (ħ hω) 2 + (pπk B T ) 2 with p = 2, and ω/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. PACS numbers: 78.47.db, 71.10.Ay, 72.15.Lh, 74.70.Pq Liquids of interacting fermions yield a number of different emergent states of quantum matter. The strong correlations between their constituent particles pose a formidable theoretical challenge. It is therefore remarkable that a simple description of low-energy excitations of fermionic quantum liquids could be established early on by Landau [1], in terms of a dilute gas of "quasiparticles" with a renormalized effective mass, of which 3 He is the best documented case [2, 3].Breakdown of the quasiparticle concept can be observed in the transport of metals tuned onto a quantum phase transition, but Fermi-liquid (FL) behavior is retrieved away from the quantum-critical region [4, 5]. The relevance of FL theory to electrons in solids is documented by a number of materials, such as transition metals [6], heavy-fermion compounds [7], and doped semiconductors [8]. Among transition-metal oxides, Sr 2 RuO 4 is a remarkable example which has been heralded as the solid-state analogue of 3 He [9] for at least three reasons: remarkably large and clean monocrystalline samples can be prepared, transport properties display low-temperature FL characteristics [10], and there is evidence for p-wave symmetry of its superconducting phase [11], as in superfluid 3 He.FL theory makes a specific prediction for the universal energy and temperature dependence of the inelastic lifetime of quasiparticles: Because of phase-space constraints imposed by the Pauli principle as well as momentum and energy conservation, it diverges as 1/ω 2 or 1/T 2 [1, 5]. More precisely, the inelastic optical relaxation rate is predicted to vanish according to the scaling law 1/τ ∝ (ħ hω) 2 + (pπk B T ) 2 , with p = 2 [12][13][14]. This leads to universal ω/T scaling of the optical conductivity σ(ω) in the thermal regime ħ hω ∼ k B T [14]. Surprisingly however, despite almost 60 years of research on Fermi liquids, this universal behavior of the optical response, and especially the specific statistical factor p = 2 relating the energy and temperature dependence have not yet been established experimentally [13][14][15][16][17].Here, we report optical measurements of Sr 2 RuO 4 with 0.1 meV resolution [18,19] which reveal this universal FL scaling law [20]. We establish experiment...

Creation of patterns and structures on surfaces at the micro- and nano-scale is a field of growing interest. Direct femtosecond laser surface structuring with a Gaussian-like beam intensity profile has already distinguished itself as a versatile method to fabricate surface structures on metals and semiconductors. Here we present an approach for direct femtosecond laser surface structuring based on optical vortex beams with different spatial distributions of the state of polarization, which are easily generated by means of a q-plate. The different states of an optical vortex beam carrying an orbital angular momentum ℓ = ±1 are used to demonstrate the fabrication of various regular surface patterns on silicon. The spatial features of the regular rippled and grooved surface structures are correlated with the state of polarization of the optical vortex beam. Moreover, scattered surface wave theory approach is used to rationalize the dependence of the surface structures on the local state of the laser beam characteristics (polarization and fluence). The present approach can be further extended to fabricate even more complex and unconventional surface structures by exploiting the possibilities offered by femtosecond optical vector fields.

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the dxy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.

The evolution of the antiferromagnetic order parameter in CeFeAsO1−xFx as a function of the fluorine content x was investigated primarily via zero-field muon-spin spectroscopy. The long-range magnetic order observed in the undoped compound gradually turns into a short-range order at x=0.04, seemingly accompanied or induced by a drastic reduction of the magnetic moment of the iron ions. Superconductivity appears upon a further increase in doping (x>0.04) when, unlike in the cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidences the presence of a crossover region, where the superconducting and the magnetic order parameters coexist on a nanoscopic range