We report on a study of the electromagnetic response of three different families of high-T c superconductors that in combination allowed us to cover the whole doping range from under-to overdoped. The discussion is focused on the ab-plane charge dynamics in the pseudogap state which is realized in underdoped materials below a characteristic temperature T * , a temperature that can significantly exceed the superconducting transition temperature T c . We explore the evolution of the pseudogap response by changing the doping level, by varying the temperature from above to below T * , or by introducing impurities in the underdoped compounds. We employ a memory function analysis of the ab-plane optical data that allows us to observe the effect of the pseudogap most clearly. We compare the infrared data with other experimental results, including the c-axis optical response, dc transport, and angle-resolved photoemission.

In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy 1 . The hightransition-temperature (T c ) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-T c superconductivity in the iron pnictides 2-4 puts a spotlight on the relevance of correlation effects in these materials 5 . Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.The recent discovery of superconductivity in the iron pnictides promises to be an important milestone in condensed-matter physics 2,3 . Here is a new class of materials with a layered structure and relatively high superconducting T c values 3,4 rivalling the doped cuprates. Electronic conduction is believed to occur in the ironpnictogen layers 6 , similar to the cuprates where the charge carriers are delocalized in the copper-oxygen planes. Two decades of research on the cuprates have established that a proper account of the exotic normal-state properties is a prerequisite for the understanding of the superconducting instability 7 .The correlated metallic state of the superconducting cuprates is derived through chemical doping of the parent compounds, which are strongly correlated (Mott) insulators. However, the parent compounds of the iron pnictides are metallic, albeit highly dissipative, bad metals 5 . Magnetic ordering in the parent iron arsenides at low temperatures leads to partial gapping of the Fermi surface but does not initiate an insulating state. Moreover, there is no evidence of long-range magnetic ordering in the 1111-iron phosphide LaFePO (ref. 8). Recent theoretical work on the normal state of the iron pnictides suggests that despite the apparent itinerant behaviour, Mott physics is relevant to charge dynamics and magnetic properties 5,9,10 . Thus motivated, we investigated the normal state of the 1111-iron phosphide superconductor LaFePO with infrared and optical spectroscopy, focusing on charge dynamics in the conducting planes.An optical experiment measures the dynamical response of the electron subjected to an external electromagnetic field and facilitates monitoring of many-body effects experienced by the electron in a material. These many-body effects include the interaction of the electron with other electrons, phonons as well as ordered or fluctuating spins. Figure 1a shows the real part of the ab-plane optical conductivity σ 1 (ω) of LaFePO over a broad frequency range. Sample growth and characterization procedures, and...

Hexagonal boron nitride (h-BN) is a natural hyperbolic material, in which the dielectric constants are the same in the basal plane (ε(t) ≡ ε(x) = ε(y)) but have opposite signs (ε(t)ε(z) < 0) in the normal plane (ε(z)). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons--collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN, so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon-phonon polaritons. The hyperbolic plasmon-phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5-2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon-phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone.

The superfluid response of single-crystal YBa 2 Cu 3 O 72x and YBa 2 Cu 4 O 8 shows a strong anisotropy in the a-b plane as revealed by far infrared spectroscopy. The value of the London penetration depth in both systems is considerably smaller in the direction along the Cu-O chains, suggesting that a substantial portion of the superconducting condensate is on the chains.

We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi2Se3. Far-infrared and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal c axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity σ1(ω). With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as the increase in the absorption edge due to direct electronic transitions. Magnetic fields H c dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum freecarrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field. arXiv:0912.2769v2 [cond-mat.str-el]