We study Na2IrO3 by angle-resolved photoemission spectroscopy, optics, and band structure calculations in the local-density approximation (LDA). The weak dispersion of the Ir 5d-t(2g) manifold highlights the importance of structural distortions and spin-orbit (SO) coupling in driving the system closer to a Mott transition. We detect an insulating gap Δ(gap)≃340  meV which, at variance with a Slater-type description, is already open at 300 K and does not show significant temperature dependence even across T(N)≃15  K. An LDA analysis with the inclusion of SO and Coulomb repulsion U reveals that, while the prodromes of an underlying insulating state are already found in LDA+SO, the correct gap magnitude can only be reproduced by LDA+SO+U, with U=3  eV. This establishes Na2IrO3 as a novel type of Mott-like correlated insulator in which Coulomb and relativistic effects have to be treated on an equal footing.

Cuprate high-T c superconductors exhibit enigmatic behavior in the nonsuperconducting state. For carrier concentrations near "optimal doping" (with respect to the highest T c s) the transport and spectroscopic properties are unlike those of a Landau-Fermi liquid. On the Mott-insulating side of the optimal carrier concentration, which corresponds to underdoping, a pseudogap removes quasiparticle spectral weight from parts of the Fermi surface and causes a breakup of the Fermi surface into disconnected nodal and antinodal sectors. Here, we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. The lifetime τ(ω, T) of a quasi-particle depends on its energy ω as well as on the temperature T. For a Fermi liquid, 1/τ(ω, T) is expected to collapse on a universal function proportional to (h ω) 2 + (pπk B T) 2 . Magnetotransport experiments, which probe the properties in the limit ω = 0, have provided indications for the presence of a T 2 dependence of the dc (ω = 0) resistivity of different cuprate materials. However, Fermi liquid behavior is very much about the energy dependence of the lifetime, and this can only be addressed by spectroscopic techniques. Our optical experiments confirm the aforementioned universal ω-and T dependence of 1/τ(ω, T), with p ∼ 1.5. Our data thus provide a piece of evidence in favor of a Fermi liquid-like scenario of the pseudogap phase of the cuprates.optical spectroscopy | superconductivity | mass renormalization | self energy T he compound HgBa 2 CuO 4+δ (Hg1201) is the single-layer cuprate that exhibits the highest T c (97 K). We therefore measured the optical conductivity of strongly underdoped single crystals of Hg1201 ðT c = 67 KÞ. Here we are interested in the optical conductivity of the CuO 2 layers. We therefore express the optical conductivity as a 2D sheet conductance GðωÞ = d c σðωÞ, where d c is the interlayer spacing. The real part of the sheet conductance normalized by the conduction quantum G 0 = 2e 2 =h is shown in Fig. 1. As seen in the figure, a gap-like suppression below 140 meV is clearly observable for temperatures below T c and remains visible in the normal state up to ∼250 K. This is a clear optical signature of the pseudogap. We also observe the zero-energy mode due to the free charge carrier response, which progressively narrows upon lowering the temperature. In materials where the charge carrier relaxation is dominated by impurity scattering, the width of this "Drude" peak corresponds to the relaxation rate of the charge carriers. Relaxation processes arising from interactions have the effect of replacing the constant (frequency-independent) relaxation rate with a frequencydependent one. The general expression for the optical conductivity of interacting electrons is then Gðω; TÞ = iπK Zω + Mðω; TÞ G 0 :[1]The spectral weight K corresponds to minus the kinetic energy if the frequency integration of the experimental data is restricted to intraband transitions. The effect of electron-electron interactions and coupling to collective mo...

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...

International audienceWe performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu 2 Si 2 as a function of temperature. The light's electric-field was applied along the a-axis or the c-axis of the tetragonal structure. We show that in addition to a pronounced anisotropy, the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K. We attribute these observations to a change in the bandstructure below 30 K. However, since these changes have no noticeable impact on the entropy nor on the DC transport properties, we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. 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 the temperature is lowered below a crossover temperature T * ∼ 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 along both the a and c axes, 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 ∼6.5 meV, which closes at T HO .