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Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as EuFe 2 As 2 , is responsible for superconductivity in the doped compounds, such as Ba͑Fe 0.92 Co 0.08 ͒ 2 As 2 and Ba͑Fe 0.95 Ni 0.05 ͒ 2 As 2 . Analyzing the dc resistivity and scattering rate of this contribution, a hidden T 2 dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.

The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap 2∆ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental verification, we measured the complex terahertz transmission and tunneling density of states (DOS) of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite 2∆ and the threshold energy for electromagnetic absorption which vanishes critically towards the SIT. We identify the excess absorption below 2∆ as a strong evidence of the Higgs mode in two dimensional quantum critical superconductors.The Higgs mechanism, which has great implications to recent developments in particle physics [1], originates in Anderson's pioneering work on symmetry breaking with gauge fields in superconductors [2]. A superconductor spontaneously breaks continuous U (1) symmetry and acquires the well-known Mexican hat potential with a degenerate circle of minima described by the order parameter Ψ = Ae iϕ , see Fig. 1a. Excitations from the ground state can be classified as transverse Nambu-Goldstone (phase) modes and massive longitudinal Higgs (amplitude) modes (see blue and red lines in Fig. 1a). In particle physics, the latter manifest themselves as the Higgs boson which was recently discovered at CERN [3]. Indications of a Higgs mode in correlated many-body systems have been found in one-dimensional charge-densitywave systems [4], quantum antiferromagnets [5] and twodimensional superfluid to Mott transition in cold atoms [6]. An amplitude mode, also named Higgs mode, was theoretically predicted for superconductors [7] and recently reported to be measured by pump-probe spectroscopy [8]. This amplitude mode describes pairing fluctuations, which are qualitatively distinct from the purely bosonic mode expected from the O(2) field theory. The Higgs-amplitude mode analogous to the highenergy Higgs Boson has not yet been observed in superconductors. A partial reason is that in homogeneous, BCS superconductors the Higgs mode is short-lived and decays to particle hole (Bogoliubov) pairs [9,10]. Nevertheless, collective modes were recently predicted to be significant in strongly disordered superconductors [11], and, in particular it was shown [12][13][14] that the Higgs mode softens but remains sufficiently sharp near a quantum critical point (QCP) in two dimensions since it is found to be a critical energy scale of the quantum phase transition. Hence, the Higgs mass can be reduced below twice the pairing gap, 2∆, making this mode experimentally visible. Such a critical...

We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline SmB 6 in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures 5 KϽTϽ20 K the electrodynamic response and the dc conductivity of SmB 6 are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of SmB 6 are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

The localization of charge carriers by electronic repulsion was suggested by Mott in the 1930s to explain the insulating state observed in supposedly metallic NiO. The Mott metal-insulator transition has been subject of intense investigations ever since-not least for its relation to high-temperature superconductivity. A detailed comparison to real materials, however, is lacking because the pristine Mott state is commonly obscured by antiferromagnetism and a complicated band structure. Here we study organic quantum spin liquids, prototype realizations of the single-band Hubbard model in the absence of magnetic order. Mapping the Hubbard bands by optical spectroscopy provides an absolute measure of the interaction strength and bandwidth-the crucial parameters that enter calculations. In this way, we advance beyond conventional temperature-pressure plots and quantitatively compose a generic phase diagram for all genuine Mott insulators based on the absolute strength of the electronic correlations. We also identify metallic quantum fluctuations as a precursor of the Mott insulator-metal transition, previously predicted but never observed. Our results suggest that all relevant phenomena in the phase diagram scale with the Coulomb repulsion U, which provides a direct link to unconventional superconductivity in cuprates and other strongly correlated materials.

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The charge response of the ladders in Sr 14−x Ca x Cu 24 O 41 is characterized by dc resistivity, low frequency dielectric, and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x = 0, 3, and 6, in addition to the mode along the legs. For x = 8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.The physics of doped Mott-Hubbard insulators challenges conventional theories of metals and insulators. 1 The effect of strong Coulomb interactions produces a rich variety of exotic ordering phenomena, which have been the focus of intense scientific activity in recent years. The spin-chain and ladder self-doped compound Sr 14−x Ca x Cu 24 O 41 has attracted much attention since it is the first superconducting copper oxide with a nonsquare lattice. 2 Theoretically, in doped two-leg Cuu O ladders, superconductivity ͑SC͒ is tightly associated with the spin gap and in competition with charge-density wave ͑CDW͒. 3 While both the spin gap and CDW were established in the ladders of Sr 14−x Ca x Cu 24 O 41 , 4-6 the relevance of these objects to electronic properties and superconductivity is still subject of intensive discussion. Recently, it was shown, on the basis of dielectric response data, that substitution of Sr 2+ by Ca 2+ gradually suppresses the insulating CDW phase, which eventually vanishes for x Ͼ 9, Ref. 7. In contrast to these results, dynamical Raman response observed above RT for x = 0 was assigned to CDW fluctuations and found to persist in the metallic phase of x = 12, a system which becomes SC under pressure. 8 It is of particular interest to learn more about the nature of CDW order in the spin ladders, which presents a nice experimental system of strongly interacting electrons. Although the ground state for 0 ഛ x ഛ 9 reveals a number of well-known fingerprints of the conventional CDW, such as the pinned phason 9 and the broad dispersion at radio frequencies due to screening of the CDW by free carriers, 5-7 its origin is certainly more complicated, since the system does not undergo a metal-to-insulator but an insulator-to-insulator transition. The role of Ca substitution is another open issue. Suppression of the CDW phase was ascribed 7 to worsened nesting conditions 10 implying that the system becomes more 2D already at ambient pressure for large x. On the other hand, it was suggested that at ambient pressure ͑for all x͒ the charge dynamics is essentially one-dimensional 2,11 ͑1D͒ and that only the application of pressure induces the dimensional crossover from 1 to 2. 2 In this Report, we address these important questions concerning the charge-ordered ground state in the ladders of Sr 14−x Ca x Cu 24 O 41 . Our results give evidence that the CDW is two dimensional with an anisotropic dispersion: the longrange charge order develops only in ladder planes, leading to a screen...

The problem of the reduced dielectric response in thin films of high-permittivity materials is analyzed by studying the soft-mode response in several SrTiO 3 thin films by means of Fourier transform far infrared, monochromatic submillimeter, and micro-Raman spectroscopies. A 300-nm-thick metalorganic chemical vapor deposition film, quasiepitaxially grown on a ͑0001͒ sapphire substrate with a perfect ͗111͘ orientation, displays a ferroelectric transition near 125 K induced by a tensile residual stress, appearing apparently simultaneously with the antiferrodistortive transition. On the other hand, polycrystalline chemical solution deposition films grown on ͑0001͒ sapphire, and also tensile stressed, show a harder soft mode response without the appearance of macroscopic ferroelectricity. This effect, which increases with the film thickness, is explained by a strong depolarizing field induced by the percolated porosity and cracks ͑in the 10-nm scale͒ along the boundaries of columnar grains ͑normal to the probe field direction͒. Brick-wall model calculations showed that 0.2 vol. % of such a porosity type reduces the permittivity from 30000 to less than 1000. The activation of the forbidden IR modes in the Raman spectra in the whole 80-300-K temperature range studied is explained by the effect of polar grain boundaries, in analogy with the bulk ceramics.