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We have measured the reflectivity of the optimally doped ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ single crystal $({T}_{c}=32.8\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ over the broad frequency range from $40{\text{cm}}^{\ensuremath{-}1}$ to 12 $000{\text{cm}}^{\ensuremath{-}1}$ and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region $(l1000\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1})$ in the normal state (80 K $lT\ensuremath{\le}300$ K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around $100\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below ${T}_{c}$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above ${T}_{c}$ into the SC gap structure below ${T}_{c}$. The extracted two SC gap sizes are ${\mathrm{\ensuremath{\Delta}}}_{S}=4.9$ $\text{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{L}=14.2$ $\text{meV}$, suggesting ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the ${d}_{yz}$ hole band at the $Z$ point and the ${d}_{xz/yz}$ electron band at the $A$ point shift to a higher binding energy side. However, at the high-symmetry points $Z,\phantom{\rule{0.16em}{0ex}}A,\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Gamma}}$, and $M$, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

We report a study of the lattice dynamics in superconducting NaFeAs (${T}_{c}$ = 8 K) and doped ${\text{NaFe}}_{0.97}$${\text{Co}}_{0.03}$As (${T}_{c}$ = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on isostructural and isoelectronic LiFeAs, anomalous broadening of ${E}_{g}$(As) and ${A}_{1g}$(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the ${B}_{1g}$(Fe) vibration is observed. This renormalization cannot be understood within a single band and simple multiband approaches. A theoretical model that includes the effects of spin-density wave correlations along with sign-changing $s$-wave pairing state and interband scattering has been developed to explain the observed behavior of the ${B}_{1g}$(Fe) mode.

Reflectivity spectra of ${\mathrm{CaMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were measured in a wide photon energy region between 5 meV and 30 eV at room temperature. Using the conductivity spectra obtained from the Kramers-Kronig analysis, electronic structures of the manganese oxides were investigated. In particular, the states near the Fermi energy ${\mathrm{E}}_{\mathrm{F}}$, O 2p and Mn 3d (${\mathrm{e}}_{\mathrm{g}}$,${\mathrm{t}}_{2\mathrm{g}}$), were studied in detail. It was found that the O 2p band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than the Mn ${\mathrm{t}}_{2\mathrm{g}}$ band. For ${\mathrm{LaMnO}}_{3}$, the filled Mn ${\mathrm{e}}_{\mathrm{g}\mathrm{\ensuremath{\uparrow}}}^{1}$ band is located closer to ${\mathrm{E}}_{\mathrm{F}}$ than any other valence band. The ${\mathrm{t}}_{2\mathrm{g}}$-${\mathrm{e}}_{\mathrm{g}}$ Hund exchange energy was found to be about 3.4 eV and the Jahn-Teller stabilization energy ${\mathrm{E}}_{0}$ was estimated to be less than 0.5 eV.

We report the pressure response of charge-density-wave (CDW) and ferromagnetic (FM) phases of the rare-earth intermetallic SmNiC${}_{2}$ up to 5.5 GPa. The CDW transition temperature (${T}_{\text{CDW}}$), which is reflected as a sharp inflection in the electrical resistivity, is almost independent of pressure up to 2.18 GPa but is strongly enhanced at higher pressures, increasing from 155.7 K at 2.2 GPa to 279.3 K at 5.5 GPa. Commensurate with the sharp increase in ${T}_{\text{CDW}}$, the first-order FM phase transition, which decreases with applied pressure, bifurcates into the upper (${T}_{M1}$) and lower (${T}_{c}$) phase transitions and the lower transition changes its nature to second order above 2.18 GPa. Enhancement both in the residual resistivity and the Fermi-liquid ${T}^{2}$ coefficient $A$ near 3.8 GPa suggests abundant magnetic quantum fluctuations that arise from the possible presence of a FM quantum critical point.

The large single crystals of stoichiometric and nonstoichiometric Gd monopnictides GdX (X=P, As, Sb, and Bi) are grown by the mineralization method (for X=P and As) and Bridgman method (for X=Sb and Bi). A systematic investigation of the transport properties of GdX single crystals is presented. We report on measurements of the electric resistivity \ensuremath{\rho}(T), magnetoresistance \ensuremath{\rho}(H), and Hall effect performed on the stoichiometric and nonstoichiometric samples at temperatures between 1.6 and 300 K in magnetic fields up to 10 T. The stoichiometric samples behaved as the well-compensated semimetals that order antiferromagnetically at N\'eel temperatures ${\mathit{T}}_{\mathit{N}}$=15.9 K for GdP, 18.7 K for GdAs, 23.4 K for GdSb, and 25.8 K for GdBi. The transverse magnetoresistance measured at low temperature follows a \ensuremath{\rho}(H)\ensuremath{\propto}${\mathit{H}}^{2}$ law, and a larger positive ratio MRR=[\ensuremath{\rho}(H)-\ensuremath{\rho}(0)]/\ensuremath{\rho}(0) is observed at 10 T for the four stoichiometric samples. The temperature dependence of the resistivity can be explained by the d-f Coulomb exchange interaction at lower temperatures. The Hall-effect measurements yield a carrier concentration n=2.1\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ for GdAs and n=4.2\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ for GdSb, which are in a good agreement with the de Haas--van Alphen effect measurements. The nonstoichiometric samples showed some anomalies that could be explained qualitatively by the model of trapped magnetic polaron. \textcopyright{} 1996 The American Physical Society.

We studied the electrical, thermal, and thermoelectric properties of the polycrystalline compound of SmNiC${}_{2}$. The electrical resistivity and magnetization measurement show the interplay between the charge density wave at ${T}_{\mathrm{CDW}}=150$ K and the ferromagnetic ordering of ${T}_{c}=18$ K. Below the ferromagnetic transition temperature, we observed the magnon gap formation of $\ensuremath{\Delta}\ensuremath{\simeq}4.3$--4.4 meV by $\ensuremath{\rho}(T)$ and ${C}_{p}(T)$ measurements. The charge density wave is attributed to the increase of the Seebeck coefficient resulting in the increase of the power factor ${S}^{2}\ensuremath{\sigma}$. The thermal conductivity anomalously increases with increasing temperature along the whole measured temperature range, which implies the weak attribution of Umklapp phonon scattering. The thermoelectric figure of merit $ZT$ significantly increases due to the increase of the power factor at ${T}_{\mathrm{CDW}}=150$ K. Here we argue that the competing interaction between electron-phonon and electron-magnon couplings exhibits the unconventional behavior of electrical and thermal properties.

We report specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 T. A small specific heat jump at ${T}_{c}$ and finite residual density of states at $T=0$ K in the superconducting (SC) state indicate that there are strong unitary scatterers that lead to states within the SC gap. A sublinear magnetic field dependence of the Sommerfeld coefficient $\ensuremath{\gamma}(H)$ at $T=0$ K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat ${C}_{\text{el}}/T$ at low temperatures argues against a nodal d-wave superconducting gap. We conclude that the SC state of LiFeAs is most compatible with the multiband ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave SC state with the gap values ${\ensuremath{\Delta}}_{\text{small}}=0.46{\ensuremath{\Delta}}_{\text{large}}$.

We report the three-dimensional electronic structure of iron pnictide superconductor LiFeAs obtained by polarization-dependent angle-resolved photoemission spectroscopy. The obtained orbital characters of each Fermi surface (FS) as well as the band dispersions were qualitatively consistent with those derived from local density approximation band calculations. It was found that FS nesting appears between a two-dimensional hole band at the zone center and an electron band at the zone corner with the same ${d}_{xy}$ orbital character. A shadow band attributed to $(\ensuremath{\pi},\ensuremath{\pi},\ensuremath{\pi})$ band folding was also observed without the spin-density-wave transition. This result suggests that FS nesting between bands with the same orbital ${d}_{xy}$ character due to spin fluctuation plays an important role in LiFeAs.

Electronic structures of a charge-density wave (CDW) system CeTe${}_{2\ensuremath{-}x}$Sb${}_{x}$ ($x=0$, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES) and the first-principles electronic and phonon band-structure methods. The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe${}_{2}$ both in shapes and sizes. The metallic states crossing the Fermi level (${E}_{F}$) are observed in ARPES even in the CDW state. The carriers near ${E}_{F}$ have mainly the Te(1) $5p$ character, with a negligible contribution from Ce $4f$ states. The supercell (shadow) bands and the corresponding very weak FSs are found to arise from band folding due to the interaction of Te(1) layers with Ce-Te(2) layers. We found that the CDW modulation vector is along $\ensuremath{\Gamma}$-$X$ (${\mathbf{Q}}_{\text{CDW}}\ensuremath{\approx}X$), which is not coincident with the most prominent FS nesting vector.

The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isotropic superconducting gap. There is no evidence of multi-band character, whereby the gap would be different on different Fermi-surface sheets. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc)., 4 pages, 3 figures

The variations of in- and inter-plane London penetration depths, $\ensuremath{\Delta}\ensuremath{\lambda}(T)$, were measured using a tunnel diode resonator in single crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears to be in the clean limit with a residual resistivity of 4 ($T\ensuremath{\rightarrow}0$) to 8 (${T}_{c}$) $\ensuremath{\mu}\ensuremath{\Omega} $cm and a residual resistivity ratio of 65 to 35, respectively. The superfluid density ${\ensuremath{\rho}}_{s}(T)={\ensuremath{\lambda}}^{2}(0)/{\ensuremath{\lambda}}^{2}(T)$ is well described by the self-consistent two-gap $\ensuremath{\gamma}$ model. Together with the previous data, our results support the universal evolution of the superconducting gap from nodeless to nodal upon departure from optimal doping even in clean systems. We also conclude that pair-breaking scattering plays an important role in the deviation of the low-temperature behavior of $\ensuremath{\lambda}(T)$ from the exponential in Fe-based compounds.

We performed angle-resolved photoelectron spectroscopy (ARPES) studies on mechanically detwinned BaFe${}_{2}$${\mathrm{As}}_{2}$. We observe clear band dispersions, and the shapes and characters of the Fermi surfaces are identified. Shapes of two-hole pockets around the $\ensuremath{\Gamma}$ point are found to be consistent with the Fermi surface topology predicted in the orbital ordered states. Dirac-cone-like band dispersions near the $\ensuremath{\Gamma}$ point are clearly identified as theoretically predicted. At the$X$ point, split bands remain intactdespite detwinning, barring a twinning origin of the bands. Observed band dispersions are compared with calculated band structures. With a magnetic moment of 0.2 $\ensuremath{\mu}$${}_{B}$ per iron atom, there is good agreement between the calculation and the experiment.

Ce 4d-4f resonant angle-resolved photoemission spectroscopy was carried out to study the electronic structure of strongly correlated Ce 4f electrons in a quasi-two-dimensional nonmagnetic heavy-fermion system CeCoGe1.2Si0.8. For the first time, dispersive coherent peaks of an f state crossing the Fermi level, the so-called Kondo resonance, are directly observed together with the hybridized conduction band. Moreover, the experimental band dispersion is quantitatively in good agreement with a simple hybridization-band picture based on the periodic Anderson model. The obtained physical quantities, i.e., coherent temperature, Kondo temperature, and mass enhancement, are comparable to the results of thermodynamic measurements. These results manifest an itinerant nature of Ce 4f electrons in heavy-fermion systems and clarify their microscopic hybridization mechanism.

We present the first direct study of charge density wave (CDW) formation in quasi-2D single layer LaTe_2 using high-resolution angle resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED). CDW formation is driven by Fermi surface (FS) nesting, however characterized by a surprisingly smaller gap (50meV) than seen in the double layer RTe_3 compounds, extending over the entire FS. This establishes LaTe_2 as the first reported semiconducting 2D CDW system where the CDW phase is FS nesting driven. In addition, the layer dependence of this phase in the tellurides and the possible transition from a stripe to a checkerboard phase is discussed., Comment: 4 pages, 4 figures, Published in Phys. Rev. Lett

The magnetic field-induced superconductor-insulator-metal transition (SIMT) in partially deuterated $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which is just on the Mott boundary, has been observed using the infrared magneto-optical imaging spectroscopy. The infrared reflectivity image on the sample surface revealed that the metallic (or superconducting) and insulating phases coexist and they have different magnetic field dependences. One of the magnetic field dependence is SIMT that appeared on part of the sample surface. The SIMT was concluded to originate from the balance of the inhomogenity in the sample itself and the disorder of the ethylene end groups resulting from fast cooling., Comment: 5 pages, 5 figures, to appear in Phys. Rev. B

The electronic structure of an $f$-electron charge-density-wave (CDW) system, $\mathrm{Ce}{\mathrm{Te}}_{2}$, has been investigated using high-resolution angle-resolved photoemission spectroscopy (ARPES). Two gaps of $\ensuremath{\sim}100$ and $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ have been observed both along $\ensuremath{\Gamma}X$ and $\ensuremath{\Gamma}Y$, implying the $2\ifmmode\times\else\texttimes\fi{}2$ lattice deformation in the Te(1) sheets with the CDW gap of $\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. By employing the fine-structure (FS) resonance, the experimental Ce $4f$ ARPES spectra have been measured. The FS resonance ARPES shows a typical two-peak structure with the peaks at $\ensuremath{\sim}\ensuremath{-}4$ and $\ensuremath{\sim}\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and the weight shift in the latter peak in contrast to the traditional giant resonance. The Ce $4f$ states have the negligibly small intensity near ${E}_{\mathrm{F}}$, reflecting the minor contribution from Ce $4f$ electrons to the metallic ground state of $\mathrm{Ce}{\mathrm{Te}}_{2}$. This study reveals that the carriers near ${E}_{\mathrm{F}}$ should have mainly the Te(1) $5p$ and Ce $5d$ character, but that only the Ce $5d$ bands cross ${E}_{\mathrm{F}}$.

Ce $3d\text{\ensuremath{-}}4f$ and $4d\text{\ensuremath{-}}4f$ resonant photoemission spectroscopies have been performed on the heavy-fermion compound $\mathrm{Ce}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$, where the ground-state properties systematically change from the magnetic $(0\ensuremath{\leqslant}xl0.3)$ to nonmagnetic $(0.3lx\ensuremath{\leqslant}1.0)$ regime via the quantum critical point (QCP, $x=0.3$). Co-substitution dependence of the bulk Ce $4f$ electronic structure shows gradual evolution of Kondo resonance at the Fermi level together with the reduction of the Ce $4{f}^{0}$ final state in agreement with the single impurity Anderson model (SIAM). The SIAM analysis shows that the Kondo temperature and specific-heat coefficient change continuously from the weakly hybridized $\mathrm{Ce}\mathrm{Ni}{\mathrm{Ge}}_{2}$ to strongly hybridized $\mathrm{Ce}\mathrm{Co}{\mathrm{Ge}}_{2}$. These indicate that the Ce $4f$ electronic structure of $\mathrm{Ce}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$ changes continuously through the QCP.