Phase diagram and optical conductivity of La1.8-xEu0.2SrxCuO4

La1.8-xEu0.2SrxCuO4 (LESCO) is the member of the 214 family which exhibits the largest intervals among the structural, charge ordering (CO), magnetic, and superconducting transition temperatures. By using new dc transport measurements and data in the literature we construct the phase diagram of LESCO between x = 0.8 and 0.20. This phase diagram has been further probed in ac, by measuring the optical conductivity {\sigma}1({\omega}) of three single crystals with x = 0.11, 0.125, and 0.16 between 10 and 300 K in order to associate the extra-Drude peaks often observed in the 214 family with a given phase. The far-infrared peak we detect in underdoped LESCO is the hardest among them, survives up to room temperature and is associated with charge localization rather than with ordering. At the CO transition for the commensurate doping x = 0.125 instead the extra-Drude peak hardens and a pseudogap opens in {\sigma}1({\omega}), approximately as wide as the maximum superconducting gap of LSCO.Comment: 6 pages, 6 figure

Raman Scattering versus Infrared Conductivity: Evidence for one-dimensional Conduction in La_{2-x}Sr_{x}CuO_{4}

Raman and Infrared (IR) spectra of an underdoped La_{1.90}Sr_{0.10}CuO_{4} single crystal have been measured as a function of temperature. Both techniques provide unconventional low-energy spectra. The IR conductivity exhibits features peaked at finite frequencies which do not have a counterpart in the Raman response. Below approximately 100 K a transfer of both Raman and IR spectral weight towards lower energies is found and a new component in the Raman response builds up being characterized by a very long lifetime of electrons propagating along the Cu-O bonds.Comment: 4 pages, 3 eps figure

Low-energy electrodynamics of superconducting diamond

Heavily-boron-doped diamond films become superconducting with critical temperatures $T_c$ well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm$^{-1}$, by also using Coherent Synchrotron Radiation. We thus determine the optical gap, the field penetration depth, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function. We conclude that diamond behaves as a dirty BCS superconductor.Comment: 4 pages including 3 figure

Infrared signatures of charge stripes in La(2-x)Sr(x)CuO(4)

The in-plane optical conductivity of seven La(2-x)Sr(x)CuO(4) single crystals with x between 0 and 0.15 has been studied from 30 to 295 K. All doped samples exhibit strong peaks in the far-infrared, which closely resemble those observed in Cu-O "ladders" with one-dimensional charge-ordering. The behavior with doping and temperature of the peak energy, width, and intensity allows us to conclude that we are observing charge stripes dynamics in La(2-x)Sr(x)CuO(4) on the fast time scale of infrared spectroscopy.Comment: 9 pages including figs. in pdf forma

Optical conductivity of the nonsuperconducting cuprate La(8-x)Sr(x)Cu(8)O(20)

La(8-x)Sr(x)Cu(8)O(20) is a non-superconducting cuprate, which exhibits a doubling of the elementary cell along the c axis. Its optical conductivity sigma (omega) has been first measured here, down to 20 K, in two single crystals with x = 1.56 and x = 2.24. Along c, sigma (omega) shows, in both samples, bands due to strongly bound charges, thus confirming that the cell doubling is due to charge ordering. In the ab plane, in addition to the Drude term one observes an infrared peak at 0.1 eV and a midinfrared band at 0.7 eV. The 0.1 eV peak hardens considerably below 200 K, in correspondence of an anomalous increase in the sample dc resistivity, in agreement with its polaronic origin. This study allows one to establish relevant similarities and differences with respect to the spectrum of the ab plane of the superconducting cuprates.Comment: Revised version submitted to Phys. Rev. B, including the elimination of Fig. 1 and changes to Figs. 4 and

Optical Absorption of an Interacting Many-Polaron Gas

The optical absorption of a many (continuum) polaron gas is derived in the framework of a variational approach at zero temperature and weak or intermediate electron-phonon coupling strength. We derive a compact formula for the optical conductivity of the many-polaron system taking into account many-body effects in the electron or hole system. Within the method presented here, these effects are contained completely in the dynamical structure factor of the electron or hole system. This allows to build on well-established studies of the interacting electron gas. Based on this approach a novel feature in the absorption spectrum of the many-polaron gas, related to the emission of a plasmon together with a phonon, is identified. As an application and illustration of the technique, we compare the theoretical many-polaron optical absorption spectrum as derived in the present work with the `d-band' absorption feature in Nd$_{2}$CuO$_{2}$. Similarities are shown between the theoretically and the experimentally derived first frequency moment of the optical absorption of a family of differently doped Nd$_{2-x}$Ce$_{x}$CuO$_{4-y}$ materials.Comment: 24 pages, 5 figures; revised and expanded versio

Anomalous optical absorption in overdoped cuprates near the charge-ordering instability

We propose an interpretation for the hump observed in the optical conductivity at or below a few hundreds of cm$^{-1}$, in overdoped cuprates like the electron-doped Nd_{2-x}Ce_xCuO_{4-y} at x\gtrsim 0.15 and the hole-doped Bi_2Sr_2CuO_6 and La_{2-x}Sr_xCuO_4. This interpretation is based on the direct excitation of charge collective modes, which become nearly critical in the proximity to a charge-ordering instability. The nearly critical character of these excitations entails a peculiar temperature dependence and a pseudo-scaling form of the lineshapes, which are in agreement with the experimental data.Comment: 5 pages, 3 figure

Pressure dependence of the optical properties of the charge-density-wave compound LaTe2_2

We report the pressure dependence of the optical response of LaTe$_2$, which is deep in the charge-density-wave (CDW) ground state even at 300 K. The reflectivity spectrum is collected in the mid-infrared spectral range at room temperature and at pressures between 0 and 7 GPa. We extract the energy scale due to the single particle excitation across the CDW gap and the Drude weight. We establish that the gap decreases upon compressing the lattice, while the Drude weight increases. This signals a reduction in the quality of nesting upon applying pressure, therefore inducing a lesser impact of the CDW condensate on the electronic properties of LaTe$_2$. The consequent suppression of the CDW gap leads to a release of additional charge carriers, manifested by the shift of weight from the gap feature into the metallic component of the optical response. On the contrary, the power-law behavior, seen in the optical conductivity at energies above the gap excitation and indicating a weakly interacting limit within the Tomonaga-Luttinger liquid scenario, seems to be only moderately dependent on pressure