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We revisit our recent investigations of the optical properties in the underdoped regime of the title compounds with respect to their anisotropic behavior as a function of both temperature and uniaxial stress across the ferro-elastic tetragonal-to-orthorhombic transition. By exploiting a dedicated pressure device, we can tune and control uniaxial stress in situ thus changing the degree of detwinning of the samples in the orthorhombic SDW state as well as pressure-inducing an orthorhombicity in the paramagnetic tetragonal phase. We discover a hysteretic behavior of the optical anisotropy; its stress versus temperature dependence across the structural transition bears testimony to the analogy with the magnetic-field versus temperature dependence of the magnetization in a ferromagnet when crossing the Curie temperature. In this context, we find furthermore an intriguing scaling of the stress and temperature dependence of the optical anisotropy in Ba(Fe1−x Co x )2As2.

We investigate the anisotropy of the optical response (i.e., dichroism) in the representative ‘parent’ ferropnictide BaFe2As2. The dichroism is achieved through measurements of the optical reflectivity in a broad spectral range, across the tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions, from which we extract the real part of the optical conductivity. The collection of data on the specimen with variable degree of detwinning is made possible via our novel pressure device which allows tuning in situ uniaxial stress. Thedichroism bears testimony to an important electronic polarization, reflecting the impact of the nematic phase and its fluctuations on the electronic structure.

The emergence of superconductivity upon progressively suppressing the long-range, charge-density-wave (CDW) order characterizes the phase diagram of several materials of interest in the on-going solid-state physics research. Se-doped ${\mathrm{ZrTe}}_{3}$ compounds provide the most recent, suitable arena in order to investigate the interplay of otherwise competing orders in layeredlike two-dimensional systems. We present an optical study of the CDW state in ${\mathrm{ZrTe}}_{3\ensuremath{-}x}{\mathrm{Se}}_{x}$ at selected Se dopings, based on the measurement of the reflectivity from the far-infrared up to the ultraviolet, as a function of temperature. We particularly focus our attention to the redistribution of the spectral weight, which images the impact of the CDW state within the optical conductivity across the phase diagram of the title compounds. The electrodynamic response is consistent with a scenario based on a long-range CDW condensate at low Se doping. Upon increasing the Se content, this then gives way to local, short-range order CDW segments. Our spectral weight analysis reveals the presence of a pseudogap phase, as fingerprint of the CDW precursor effects and thus shaping the charge dynamics of the title compounds in their normal state, preceding the onset of superconductivity.

We perform, as a function of uniaxial stress, a temperature-dependent optical-reflectivity investigation of the parent Fe-arsenide compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ over a broad spectral range, from the far infrared up to the ultraviolet, across the coincident structural tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions at ${T}_{s,N}=135$ K. Our results provide knowledge to the complete electrodynamic response of the title compound over a wide energy range as a function of both tunable variables. For temperatures below ${T}_{s,N}$, varying the uniaxial stress in situ affects the twin domain population and yields hysteretic behavior of the optical properties as the stress is first increased and then decreased, whereas for temperatures above ${T}_{s,N}$ the stress-induced optical anisotropy is reversible, as anticipated. In particular, by analyzing the low-frequency infrared response, we obtain detailed insight to the effects determining the intrinsic anisotropy of the (metallic) charge dynamics in the orthorhombic state, and similarly the induced one due to applied uniaxial stress at higher temperatures in the tetragonal phase. The low-frequency optical conductivity thus allows establishing a link to the $dc$ transport properties and reveals that they are determined almost exclusively by changes in the Drude weight, therefore by the anisotropy in the Fermi surface parameters. Finally, we show that the spectral weight distribution in the SDW state occurs for energies below approximately 1 eV, and therefore points towards a correlation mechanism due to Hund's coupling rather than on-site Coulomb interactions.

We perform, as a function of uniaxial stress, an optical-reflectivity investigation of the representative "parent" ferropnictide BaFe(2)As(2) in a broad spectral range, across the tetragonal-to-orthorhombic phase transition and the onset of the long-range antiferromagnetic (AFM) order. The infrared response reveals that the dc transport anisotropy in the orthorhombic AFM state is determined by the interplay between the Drude spectral weight and the scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than in the scattering rate, definitively establishing the anisotropy of the Fermi surface parameters as the primary effect driving the dc transport properties in the electronic nematic state.

We perform, as a function of uniaxial stress, an optical-reflectivity investigation of the representative "parent" ferropnictide BaFe(2)As(2) in a broad spectral range, across the tetragonal-to-orthorhombic phase transition and the onset of the long-range antiferromagnetic (AFM) order. The infrared response reveals that the dc transport anisotropy in the orthorhombic AFM state is determined by the interplay between the Drude spectral weight and the scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than in the scattering rate, definitively establishing the anisotropy of the Fermi surface parameters as the primary effect driving the dc transport properties in the electronic nematic state.

We use a new technique that allows in situ variation of uniaxial stress to probe the polarization dependence of the optical reflectivity of the representative underdoped iron-arsenide $\text{Ba(}{\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}{\text{)}}_{2}{\text{As}}_{2}$ through the tetragonal-to-orthorhombic structural transition and with respect to their electronic nematic phase. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress in the orthorhombic state associated with twin boundary motion, whereas the pressure-induced anisotropy of the optical response is reversible in the tetragonal state.

ZeTe${}_{3}$ belongs to the ample class of chainlike chalcogenide charge-density-wave (CDW) materials. Its peculiarity consists in the formation of the CDW condensate at ${T}_{\mathrm{CDW}}$ along the crystallographic directions perpendicular to the $b$-axis, Zr-Zr chains. Ni and Cu intercalation of ZrTe${}_{3}$ leads to the onset of bulk superconductivity at ${T}_{c}l{T}_{\mathrm{CDW}}$, raising questions about the possible coexistence as well as competition between both broken-symmetry ground states. Here, we report on the optical investigation of Ni${}_{0.05}$ZrTe${}_{3}$ and Cu${}_{0.05}$ZrTe${}_{3}$ single crystals performed with electromagnetic radiation polarized along the in-plane crystallographic $a$ and $b$ axes, over a broad spectral range and as a function of temperature. The charge dynamics of the CDW state displays a polarization dependence within the $ab$ plane and gives evidence for a partial gapping of the Fermi surface, which affects almost exclusively the crystallographic direction parallel to the $a$ axis. We provide a complementary analysis of the spectral weight distribution, disentangling its reshuffling in the excitation spectrum across the symmetry-breaking CDW transition. While the superconducting energy gap lies outside our measurable spectral range, we observe nonetheless that the CDW-gap feature persists at $Tl{T}_{c}$. We propose that CDW and superconductivity coexist along the $a$ axis.

The optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ of Sr${}_{4}$Ru${}_{3}$O${}_{10}$ has been studied by means of reflectivity measurements between 12 and 300 K both in the $ab$ planes (from 40 to 28 000 cm${}^{\ensuremath{-}1}$) and along the $c$ axis (from 40 to 19 000 cm${}^{\ensuremath{-}1}$). In the far and mid infrared, ${\ensuremath{\sigma}}_{ab}(\ensuremath{\omega})$ results from a superposition of a Drude term, with a plasma frequency which increases below the ferromagnetic transition at ${T}_{c}=105$ K, and of two bands in the infrared. One is peaked at about 400 cm${}^{\ensuremath{-}1}$ for $Tl$ 300 K, but softens to 300 cm${}^{\ensuremath{-}1}$ at room temperature. The other one is peaked at 1300 cm${}^{\ensuremath{-}1}$ and, below ${T}_{c}$, provides spectral weight to the Drude term, suggesting that it has a magnetic origin. Along the $c$ axis, ${\ensuremath{\sigma}}_{c}(\ensuremath{\omega})$ is instead that of a good insulator, with phonon peaks whose frequencies ${\ensuremath{\omega}}_{\mathrm{ph}}$ regularly increase as $T$ decreases. For the mode at the highest frequency, however, which can be assigned to the apical oxygens of the RuO${}_{6}$ octahedra, both ${\ensuremath{\omega}}_{\mathrm{ph}}$ and the intensity ${I}_{\mathrm{ph}}$ have an anomalous behavior below ${T}_{c}$.

Multiferroic LuFe${}_{2}$O${}_{4}$ (LFO) exhibits three-dimensional (3D) charge order below ${T}_{\text{CO}}\ensuremath{\sim}$ 350 K and strong electroresistance (ER) above a static threshold field ${E}_{\mathrm{th}}$. By measuring simultaneously, in LFO single crystals, the dc current and the far-infrared reflectivity along different axes, we do not detect any insulator-to-metal transition above ${E}_{\mathrm{th}}$. Combined current-temperature measurements confirm that the ER is due to self-heating of LFO, as recently reported. The data can be fit by the standard activation law for an intrinsic semiconductor, with a gap value $\ensuremath{\Delta}$ = 0.57 eV. This value is consistent with that of the optical gap reported for LFO in the literature.

We show how synchrotron radiation (SR) in the terahertz (THz) region provides the possibility to measure the properties of conventional and exotic superconductors in their superconducting state. Indeed, through the coupling of SR and a conventional Michelson interferometer, one can obtain in the THz range a signal-to-noise ratio up to 103. We review the application of this technique to superconductors with a different degree of complexity: the single-gap boron-doped diamond BCS isotropic material; CaAlSi, a superconductor isostructural to MgB2 with a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis; and isotropic V3Si, where superconductivity opens two gaps at the Fermi energy.

We review the results obtained on novel superconductors and on materials showing Charge Density Waves (CDW), which have in common the application of the Bardeen-Cooper-Schrieffer theory, by using Coherent synchrotron Radiation as a source for sub-Terahertz spectroscopy. We also present new data on the low-energy excitations of the CDW pahse of multiferroic LuFe(2)O(4).

We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an increase of the metallic character on cooling consistent with electron doping, even though at room temperature values of all the properties nearly overlaps with those of SmFeAsO. However, with temperature decrease all anomalies related to the SDW instability are missed and the superconducting transition occurs. This suggests that doping breaks abruptly the symmetries of the Fermi surface inhibiting the SDW formation in favor of the superconducting transition, with no substantial changes in the density of states or in the effective mass., Comment: Submitted to SUST

We report an optical study of CaAlSi, a superconductor which displays both the crystal structure of MgB2 and the electronic band structure of intercalated graphites. The reflectivity of a CaAlSi single crystal was measured down to subterahertz frequencies and to 3.3 K, with the use of coherent synchrotron radiation. A single superconducting gap in the hexagonal planes and two gaps along the c axis were found and measured consistently with the structure of the CaAlSi Fermi surface. The normal-state optical conductivity is also anisotropic: in the ab plane, the plasma frequency is larger by more than a factor of 2 than along the c axis. An analysis of the ab-plane spectral weight in comparison with the corresponding quantity in a cuprate such as La2-xSrxCuO4 shows that in CaAlSi the correlation effects are negligible.

We show how coherent synchrotron radiation (CSR) allows one to measure the slight increase in the reflectivity of a BCS superconductor as it is cooled below T-c. In fact, by CSR coupled to a conventional interferometric apparatus, one can obtain a signal-to-noise ratio similar to 10(3) in the sub-THz range. We apply this technique to the measurement of the optical gap in boron-doped diamond and in CaAlSi, a superconductor isostructural to MgB2. In the latter compound we are also able to determine a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis. (c) 2007 Elsevier B.V. All rights reserved.

We measured the Raman and the Infrared phonon spectrum of SmFeAsO polycrystalline samples. We also performed Density Functional Theory calculations within the pseudopotential approximation to obtain the structural and dynamical lattice properties of both the SmFeAsO and the prototype LaFeAsO compounds. The measured Raman and Infrared phonon frequencies are well predicted by the optical phonon frequencies computed at the Gamma point, showing the capability of the employed ab-initio methods to describe the dynamical properties of these materials. A comparison among the phonon frequencies of different oxypnictides suggests a possible role of the high frequency phonons in the pairing mechanism leading to superconductivity in these materials., Comment: text file (.tex) and 2 figures (.eps)