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Specific heat measurements on Ce4SB3 were carried out in various magnetic fields and the phase diagram was determined. A large linear specific heat coefficient γ = 160 mJ⧸K2 mol Ce was obtained at H = 0. The entropy at the phase transition temperature was only 40% of ln2 and largely suppressed by applied magnetic field. Similarity between Ce4Bi3 and Ce4Sb3 was found. Various anomalous properties suggest competition between Kondo and ferromagnetic interactions in both compounds.

Magnetic, transport and specific heat measurements were carried out in Yb 4 As 2.7 Sb 0.3 between 4.2 and 300 K. Contrary to our expectations this sample showed enhanced heavy fermion behavior compared to Yb 4 As 3 , and the carrier concentration remained extremely low.

The optical reflectivity spectra were measured on single crystals of the Kondo substance CeSb and its reference LaSb. Absorptions due to a Kondo peak and relating to the virtual f-d excitons are observed in far infrared and infrared regions, respectively. Absorption due to a p-d transition is observed around 6 eV.

The optical reflectivity spectra of clathrates Sr 8 Ga 16 Ge 30 and β -Eu 8 Ga 16 Ge 30 have been measured to investigate their electronic structure. A clear Drude shape was observed in the spectrum of Sr 8 Ga 16 Ge 30 . On the other hand, a broad Drude shape due to magnetic scattering by Eu 4f electrons and a peak originated from Eu 4f states were observed in the spectrum of β -Eu 8 Ga 16 Ge 30 . No clear change of optical conductivity due to the magnetic order was observed in β -Eu 8 Ga 16 Ge 30 and no evidence of the rattling in Sr 8 Ga 16 Ge 30 observed in contrast to α -Eu 8 Ga 16 Ge 30 . These results essentially originated from the large carrier density of these materials.

We have performed the high-resolution photoemission spectroscopy on the isostructural heavy fermion compounds, CeCoGe 2 and CeNiGe 2 , as the representative samples of CeNi 1 - x Co x Ge 2 to investigate the origin of the wide Ce 4f characters from the magnetic ( x = 0 ) to non-magnetic regime ( x = 1 ) via the quantum critical point ( x = 0.3 ). We directly observed a Kondo peak, whose width corresponds to the Kondo temperature, in both materials but the crystal field splitting only in CeNiGe 2 . Both the Kondo temperature and the crystal field splitting are concluded to be affected by the hybridization between Ce 4f and Ni/Co 3d states.

We have used angle-resolved photoemission spectroscopy as a function of excitation energy to elucidate the three dimensionality at the electronic structure of quasi-two-dimensional strongly correlated Ce 4f electron compound CeTe 2 . We observed that there are two types of Te 5p bands along Γ Z symmetry line at the valence band, one is non-dispersive bands and the other is dispersive one. While the former is ascribed to the Te 5p bands at two-dimensional Te layer relating to the CDW formation, the latter to one at CeTe double layers.

We report the orbital character of the Fermi surfaces for the Fe-based superconductor LiFeAs using the polarization-dependent three-dimensional angle-resolved photoemission spectroscopy and the angular-dependent dipole selection rule analysis. The evaluated orbital character is well reproduced by the band calculation. The results demonstrate that the polarization-dependent ARPES combined with the selection rule analysis is useful to identify the orbital character of the Fe-based unconventional superconductivity.

We have measured the electrical resistivity and Seebeck coefficient of CeTe2−xSbx (x = 0.0, 0.05, 0.1, 0.25, and 0.5) single crystals from 100 K to 300 K along the ab-plane, and we calculated their electronic structures and Fermi surfaces by using the density functional theory approach. The band structures of CeTe2 show the 2-dimensional (2D) Fermi surface nesting behavior, which induce the charge density wave (CDW). In addition, there is a 3-dimensional (3D) electron Fermi surface hindering the perfect CDW gap opening. By hole doping with the substitution of Sb at the Te-site, the 3D-like Fermi surface disappears and the 2D perfect CDW gap opening enhances the power factor up to x = 0.1. With further hole doping, the Fermi surfaces become 3-dimensional structure with heavy hole bands. The enhancement of the power factor is observed near the dimensional crossover of CDW, at x = 0.1, where the CDW gap is maximized. Here we show the strong relationship between the dimensionality of CDW and high thermoelectr...

We report the observation of pseudogap (PG) behavior in optical conductivity in an underdoped Ba0.6+δK0.4−δFe2As2 (Tonsetc ≈ 36 K) single crystal far above Tc, up to 100 K (≈3Tc). Wide separation of the energy scales of the magnetic and superconducting (SC) correlations in the Ba0.6+δK0.4−δFe2As2 sample enabled us to establish that the PG structures observed in optical conductivity in the range of 50–150 cm−1 are uniquely driven by the SC correlation, and the magnetic correlation coexists in the far separated range of about 700 cm−1. Theoretical calculations, based on the preformed Cooper pair model, provided an excellent description of the temperature evolution of the SC correlation in optical conductivity data from below to above Tc.

The thermally activated flux flow (TAFF) of a LiFeAs single crystal was studied in magnetic fields up to 9 T. The thermally activated energy (TAE) was analyzed using a conventional Arrhenius relation and a modified TAFF theory. The modified TAFF method results are in better agreement with experimental data than the Arrhenius ones. Through the analysis of the modified TAFF method, we found that the LiFeAs superconductor is regarded as a three-dimensional (3D) system in the TAFF region. In addition, we obtained the vortex-glass temperature Tg from the linear region of the (dln ρ/dT)− 1. And the fluctuation conductivity was analyzed for the LiFeAs superconductor using Aslamazov-Larkin (AL) theory and the lowest Landau level (LLL) scaling, in zero and nonzero magnetic fields. With the determined values of Tc(H), the fluctuation conductivity was scaled within a 3D-LLL approach. Compared with the NMR result of the 2D antiferromagnetic spin fluctuation, these results imply that LiFeAs exhibited a 2D-3D crossover of the fluctuation conductivity (FC) with decreasing temperature.

The reflectivity of CeSbNi0.15 is measured at several temperatures in the low energy regions. The optical conductivity is obtained from the measured reflectivity. The carrier concentration and the relaxation time are evaluated by the Drude model. It is found that the p–f mixing collapse takes place in CeSbNi0.15 and that the gap formation occurs at 7 K.

We measured the temperature-dependent thermal conductivity κ(T), Seebeck coefficient S(T), and electrical resistivity ρ(T) of the polycrystalline Ce1−xCuxSe2 (x=0.0, 0.1, 0.2, and 0.3) series compounds. The high temperature thermoelectric property measurements of Ce0.9Cu0.1Se2 have shown that the maximum thermoelectric figure-of-merit (ZT) reached up to 0.18 at 800 K due to the large Seebeck coefficient (S≈344 μV/K) and relatively low thermal conductivity (κ=0.71 W m−1 K−1). By increasing the Cu-doping concentration (x≥0.2), the ZT is lowered mainly due to increasing the thermal conductivity. From the band structure calculation, the high Seebeck coefficient for the Ce0.9Cu0.1Se2 compound is attributed to the localized Ce f-band near the Fermi level due to Cu d- and Se p-orbital hybridization. The localized f-band has been shifted to a higher energy from the Fermi level with increased Cu-doping concentration, which is consistent with the decrease in the Seebeck coefficient. This research proposes that the ...

We report the high thermoelectric figure-of-merit (ZT) on the Se-deficient polycrystalline compounds of In4Se3−x (0.02≤x≤0.5) and the anisotropic electronic band structure. The Se-deficiency (x) has the effect of decreasing the semiconducting band gap and increasing the power factor. The band structure calculation for In4Se3−x (x=0.25) exhibits localized hole bands at the Γ-point and Y-S symmetry line, whereas the significant electronic band dispersion is observed along the c-axis. Here, we propose that the high ZT values on those compounds are originated from the anisotropic electronic band structure as well as Peierls distortion.

We have investigated magnetic properties of the heavy fermion system Ce1−xGdxCoSi3(0≤x≤1) via specific heat and electrical resistivity measurements. The specific heat shows antiferromagnetism at 9 K for GdCoSi3(x=1.0). With increasing Ce concentration, TN linearly decreases and is suppressed at x=0.1. An anomalous hump in specific heat was found below the ordering temperature for x≥0.4. For x=0.1, non-Fermi-liquid behavior was found in electrical resistivity and −ln T divergence in C/T below 3 K. Our results indicate that x=0.1 is a quantum critical point.

The thermal and electronic transport properties on rare-earth dichalcogenide compounds of CeTe2−xSnx (x≤0.1) were investigated by measuring electrical resistivity ρ, thermal conductivity κ, and Seebeck coefficient S. Compound of CeTe2 exhibited very low κ (1.25 W m−1 K−1) at 300 K, which was understood by the charge density wave lattice distortion. From the ρ(T) measurements, the metallic property of CeTe2 was significantly enhanced by Sn substitution. The temperature-dependent behavior of S(T) for CeTe2−xSnx (x=0.05 and 0.1) was well described by the inelastic Umklapp process and the phonon drag. The low Seebeck coefficient of CeTe2 (S≈2.5 μV/K at T≈300 K) was increased by Sn doping. From the band structure calculation, the semimetallic band character of CeTe2 was revealed with symmetric electron-hole band dispersion near the Fermi level. The symmetric electron-hole band dispersion of CeTe2 is responsible for the low Seebeck coefficient. The enhancement of the metallic property and Seebeck coefficient be...

We have demonstrated that the middle-infrared (mid-IR) peaks in the optical conductivity spectra of Ce$X_3$ ($X$ = Pd, Sn, In) can be explained by first-principle band structure calculation with the spin-orbit interaction. The mid-IR peak shapes in these materials are not identical to one another: CePd$_3$, CeSn$_3$, and CeIn$_3$ have a triple-peak structure, double-peak structure and broad single-peak structure, respectively. These peaks can be theoretically explained by the optical transition from the occupied state to the spin-orbit splitted Ce $4f$ state. This result indicates that the mid-IR peaks originate from the simple band picture with the Ce $4f$ state near the Fermi level, not from the conventional cf hybridization gap based on the periodic Anderson model., Comment: 5 pages, 6 figures. To be published in J. Phys. Soc. Jpn. 78(1) (2009)

The electroluminescence (EL) of an organic light-emitting diode specially fabricated with a pure thin dye layer 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) of 1-nm thickness inserted at the interface between N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine and tris(8-quinolinolato) aluminum has been investigated. The EL behavior of this device changed dramatically at a driving voltage of about 12.25V, which has been ascribed to the breakage of the thin DCJTB layer. It is suggested that not only such thin layer of DCJTB can transport electrons and holes, but also trap them. A comparison of the EL performance with four other devices confirms this mechanism. Therefore, the high efficiency and long life of the dye doped Alq devices are attributed to the hole and electron trapping effect and the high photoluminescence efficiency of the dye.

GdSb and TmSb are typical ‘model’ materials in rare earth compounds. The former is an exchange interaction only sample and the latter is a CEF only sample. In order to compare the transport properties between the two samples, we have measured the Hall effect and resistivity of pure GdSb and TmSb in various fields. The CEF effect and magnetic scattering have been observed in both resistivity and Hall effect measurements for TmSb and GdSb, respectively. The mobilities of electrons and holes as well as the carrier concentration are calculated using the experimental data.

Magnetic and transport properties of CexLa1−xP were investigated to reveal anomalous properties observed in CeP. The temperature dependence of the magnetic susceptibility in the paramagnetic region was not affected by replacing 10% of Ce by La, while the magnetic behavior in the ordered state changes; the various magnetic phases seen in pure CeP were not observed.

The magnetization process and Shubnikov-de Haas (SdH) effect have been investigated for CeAs single crystals in steady high magnetic fields up to 280 kOe using hybrid magnets. We found that a discontinuity appeared in the magnetization process and the magnetoresistivity had a hysteresis at the antiferro- (AFM) to ferromagnetic (FM) phase transition. An increase of the number of carriers and the structural change of Fermi surface were observed accompanied with the magnetic phase transition.

We could measure the Shubnikov-de Haas effect in good quality single crystal of CeAs obtained by well controlling its stoichiometry. It revealed that carrier concentrations were extremely low to be about 0.001/Ce and showed dense Kondo behavior.

De Haas-van Alphen (dHvA) oscillation is measured at both above and below antiferromagnetic ordering temperature of 2.1K for SmSb which shows Kondo behavior in specific heat and magnetic susceptibility measurement.

The optical reflectivities were measured on the single crystals of the typical Kondo substance CeB6 and its reference LaB6 in the energy range from 2 meV to 5 eV at 300 K and 10 K. Characteristic absorptions are observed around 12 meV from the excited Kondo state and 0.45 eV due to the virtual exiton of f-d transition in CeB6.

Reflectivity spectra of single crystals of CeIn 3 and CeTe show absorption due to the Kondo peak in the far infrared region at low temperature. The reflectivity spectra due to the conduction electron have been analyzed with the frequency dependence of the relaxation time of the conduction electron.

Single crystals of Ce 4 X 3 were prepared and measurements of magnetic and transport properties were done. Ce 4 Bi 3 exhibits the first-order magnetic phase transition and d H / d T of its boundary in the magnetic phase diagram is positive. Ce 4 X 3 indicate the crystal field effects in various properties except the reciprocal susceptibility of Ce 4 Bi 3 , which varies lineraly with the temperature.

We have measured the dHvA effect of CeSb down to 50 mK and discovered the largest hole Fermi surface, β 4 . The relative large electronic specific heat coefficient of CeSb compared with that of LaSb has been observed. These results are interpreted by the band calculation based on the p-f mixing model.

The frequency dependence of the dielectric constant and of the electrical conducitivity were measured by the Time Domain Spectroscopy (TDS). The temperature dependence of ϵ(ω) shows an anomaly at 305.5 K. From the analogy to the ferroelectric phase transition, our result suggests the occurrence of a charge ordered state associated with an electric dipole order.