F. Pfuner

Anisotropic charge dynamics in detwinned Ba(Fe1-xCox)2As2

We investigate the optical conductivity with light polarized along the in-plane orthorhombic a and b axes of Ba(Fe1−xCox)2As2 for x=0 and 2.5% under uniaxial pressure across their structural and magnetic transitions. The charge dynamics at low frequencies and temperatures on these detwinned, single-domain samples reveals an enhancement of both the scattering rate and Drude weight of the charge carriers along the antiferromagnetic a-axis with respect to the ferromagnetic b-axis. Our findings also allow us to estimate the dichroism, which extends to high frequencies and temperatures. These results reveal a nematic susceptibility as well as the electronic nature of the structural transition.

Charge dynamics of Co-doped BaFe 2 As 2

We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(CoxFe1 x)2As2 compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for x = 0.025, we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051 6 x 6 0.11, we detect the superconducting gap, while at x = 0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the combination of a coherent and an incoherent component, and extract the respective scattering rates. We establish that the dc transport properties in the normal phase are dominated by the coherent Drude term for 0 6 x 6 0.051 and by the incoherent one for 0.061 6 x 6 0.18, respectively. Finally, through spectral weight arguments, we give clear-cut evidence of moderate electronic correlations for 0 6 x 6 0.061, which then cross over to values appropriate for a regime of weak interacting and nearly-free electron metals for x > 0.11.

Temperature dependence of the excitation spectrum in the charge-density-wave ErTe3 and HoTe3 systems

We provide optical reflectivity data collected over a broad spectral range and as a function of temperature on the ErTe3 and HoTe3 materials, which undergo two consecutive charge-density-wave (CDW) phase transitions at TCDW1= 265 and 288 K and at TCDW2= 157 and 110 K, respectively. We observe the temperature dependence of both the Drude component, due to the itinerant charge carriers, and the single-particle peak, ascribed to the charge-density-wave gap excitation. The CDW gap progressively opens while the metallic component gets narrow with decreasing temperature. An important fraction of the whole Fermi surface seems to be affected by the CDW phase transitions. It turns out that the temperature and the previously investigated pressure dependence of the most relevant CDW parameters share several common features and behaviors. Particularly, the order parameter of the CDW state is in general agreement with the predictions of the BCS theory.

Optical properties of TiN thin films close to the superconductor-insulator transition

We present the intrinsic optical properties over a broad spectral range of TiN thin films deposited on an Si/SiO2 substrate. We analyze the measured reflectivity spectra of the film-substrate multilayer structure within a well-establish procedure based on the Fresnel equation and extract the real part of the optical conductivity of TiN. We identify the metallic contribution as well as the finite energy excitations and disentangle the spectral weight distribution among them. The absorption spectrum of TiN bears some similarities with the electrodynamic response observed in the normal state of the high-temperature superconductors. Particularly, a mid-infrared feature in the optical conductivity is quite reminiscent of a pseudogap-like excitation.

Optical properties of the charge-density-wave polychalcogenide compounds R 2 Te 5 (R=Nd, Sm and Gd)

Abstract.We investigate the rare-earth polychalcogenide R2Te5 (R=Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical reflectivity measurements. We obtain the optical conductivity through Kramers-Kronig transformation of the reflectivity spectra. From the real part of the optical conductivity we then extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTen (n=2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.

Pressure-induced quenching of the charge-density-wave state in rare-earth tritellurides observed by x-ray diffraction

We report an x-ray diffraction study on the charge-density-wave (CDW) LaTe{sub 3} and CeTe{sub 3} compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave-vector, and provide direct evidence for a pressure-induced quenching of the CDW phase. We observe subtle differences between the chemical and mechanical compression of the lattice. We account for these with a scenario where the effective dimensionality in these CDW systems is dependent on the type of lattice compression and has a direct impact on the degree of Fermi surface nesting and on the strength of fluctuation effects.

Incommensurate magnetic order in TbTe3

We report a neutron diffraction study of the magnetic phase transitions in the charge-density wave (CDW) TbTe(3) compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe(3) at T(mag1) = 5.78 K as a result of continuous phase transitions. We observe that near the temperature T(mag1) the magnetic Bragg peaks appear around the position (0, 0, 0.24) (or its rational multiples), that is fairly close to the propagation vector (0,0,0.29) associated with the CDW phase transition in TbTe(3). This suggests that correlations leading to the long-range magnetic order in TbTe(3) are linked to the modulations that occur in the CDW state.