Plane Optical Conductivity Research Articles

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

The temperature dependence of $ab$-plane optical conductivity of ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$ has been measured below and above its superconducting transition temperature ${T}_{c}\ensuremath{\simeq}35.5$ K. In the normal state, analysis with the two-Drude model reveals a $T$-linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 ${\mathrm{cm}}^{\ensuremath{-}1}$ vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\simeq}9$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\simeq}14$ meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 ${\mathrm{cm}}^{\ensuremath{-}1}$, we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.

Optical conductivity ofURu2Si2in the Kondo liquid and hidden-order phases

We measured the polarized optical conductivity of URu$_2$Si$_2$ from room temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid regime, and the hidden-order phase. The normal state is characterized by an anisotropic behavior between the ab plane and c axis responses. The ab plane optical conductivity is strongly influenced by the formation of the coherent Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV leads to a spectral weight transfer to mid-infrared energies. The c axis conductivity has a different behavior: the Drude peak already exists at 300 K and no particular anomaly or gap signature appears in the coherent Kondo liquid regime. When entering the hidden-order state, both polarizations see a dramatic decrease in the Drude spectral weight and scattering rate, compatible with a loss of about 50 % of the carriers at the Fermi level. At the same time a density-wave like gap appears along both polarizations at about 6.5 meV at 5 K. This gap closes respecting a mean field thermal evolution in the ab plane. Along the c axis it remains roughly constant and it "fills up" rather than closing.

Pseudogap in underdoped Ba1−xKxFe2As2as seen via optical conductivity

We report the observation of a pseudogap in the \emph{ab}-plane optical conductivity of underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($x = 0.2$ and 0.12) single crystals. Both samples show prominent gaps opened by a spin density wave (SDW) order and superconductivity at the transition temperatures $T_{\it SDW}$ and $T_c$, respectively. In addition, we observe an evident pseudogap below $T^{\ast} \sim$ 75 K, a temperature much lower than $T_{\it SDW}$ but much higher than $T_{c}$. A spectral weight analysis shows that the pseudogap is closely connected to the superconducting gap, indicating the possibility of its being a precursor of superconductivity. The doping dependence of the gaps is also supportive of such a scenario.

A−bplane optical conductivity inYBa2Cu3O7−δabove and belowT*

Analysis of the a-b plane optical conductivity $\sigma_{ab}$ for both twinned and untwinned YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ as a function of temperature and doping shows that below a well defined temperature T*, a dip in the spectrum systematically appears separating the infrared charge excitation spectrum into two components with distinct energy scales. The change from monotonic behaviour in $\sigma_{ab}$ is found to be concurrent with the onset of phonon anomalies in Raman and infrared spectra below T*. The optical data are suggested to be evidence for the appearance of an inhomogeneous distribution of carriers rather than the opening of a simple gap for charge excitations below T*, an interpretation wich is consistent with recent angle-resoved photoemission and electronic Raman spectra. We find that the behaviour below T* and the absence of any anomalies at Tc can be interpreted assuming a Bose-Einstein condensation of preformed pairs.

Low-temperature softening of the polaron band in electron-doped cuprates

The a- b plane optical conductivity of three, semiconducting, Nd 2− x Ce x CuO 4± y single crystals with different values of x and y has been measured between 70 and 18000 cm −1 and between 300 and 20 K. In all samples, a polaron peak is observed at room temperature at ∼ 1000 cm −1. As T decreases, the peak remains at the same energy in two low-doping samples with different chemical compositions, while in the most strongly electron-doped crystal it moves to the far-infrared due to a transfer of spectral weight within the polaron band. This unexpected behavior indicates a delocalization of the polaron in this cuprate, which could provide coherent transport at low temperature.

Optical properties of single-crystalLa2CuO4+δ

The $c$-axis and $\mathrm{ab}$-plane optical reflectance of an electrochemically oxidized single crystal of ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4+\ensuremath{\delta}}$ (${T}_{c}\ensuremath{\sim}40$ K; $\ensuremath{\delta}\ensuremath{\sim}0.12$) have been measured over a wide frequency range (30-40 000 ${\mathrm{cm}}^{\ensuremath{-}1}$) and at temperatures between 10 and 300 K. The $\mathrm{ab}$-plane optical conductivity shows metallic behavior at low frequency with weak phonon features whereas the $c$-axis spectrum is typical of an insulator, showing only phonons. Estimates of the carrier density and the oxygen concentration suggest that each excess oxygen contributes just over one mobile hole. The $\mathrm{ab}$-plane midinfrared conductivity obtained when the wave vector q of the light is perpendicular to the Cu${\mathrm{O}}_{2}$ planes in the sample exhibits structure that is absent when q is parallel to the basal planes. The $\mathrm{ab}$-plane conductivity shows a strong departure from Drude metallic response; fits to midinfrared models with frequency-dependent damping require also an additional bound-carrier contribution. A superconducting condensate is evident in the low-frequency $\mathrm{ab}$-plane data; the oscillator strength of this condensate contains about 18% of the doping induced or 80% of the free carrier spectral weight in the normal state. Finally, an anomalous temperature dependence is found in the visible/ultraviolet region of the spectrum.

Optical anisotropy of UNi 2Si 2

The optical anisotropy of UNi 2Si 2 has been systematically studied by reflectance spectroscopy in the range of 50 to 37 000 cm -1. A temperature-dependent absorption peak was found in the c-axis optical conductivity spectra centered at 280 cm -1 in the incommensurate longitudinal spin-density-wave phase (103 K < T < 130 K), which, we believe, arises from the opening up of a SDW pseudo-gap over a portion of the Fermi surface. A narrow absorption peak centered at zero frequency appears in both the c-axis and basal plane optical conductivity spectra at low temperatures. This peak results from the suppression in the scattering rate and the increase in the renormalization of the quasiparticle effective mass at low frequencies. Above the Néel temperature ( T N), the optical conductivity decreases or saturates at low frequencies along the c-axis, while in the basal plane it develops a Drude-like absorption peak centered at ω = 0.

Optical conductivity of single crystals of La2-xSrxNiO4.

Reflectance data (50--50 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) on single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4}$ (x=0,0.2) are used to determine the a-b plane optical conductivity (${\mathrm{\ensuremath{\sigma}}}_{1}$). Similar to the La cuprates, Sr doping introduces a strong mid-ir band in ${\mathrm{\ensuremath{\sigma}}}_{1}$ with a peak at 0.6 eV. The ${\mathrm{\ensuremath{\sigma}}}_{1}$ data are not in agreement with the marginal Fermi-liquid model proposed recently, but are rather well described by a modified Drude-Lorentz model. The hole plasma is overdamped and exhibits a large effective mass m\ensuremath{\sim}30${\mathit{m}}_{0}$. The mid-ir band is well fit by a broad Lorentz oscillator, suggesting a collective mode; its origin cannot be ascribed to interband transitions involving Sr states, or to charge-transfer absorption.