arXiv subjects

F. Iga

Publications and source records attributed to F. Iga.

25 records · Page 2Linked to original sources

arXiv2004

High-resolution soft x-ray photoemission study of a Kondo semiconductor and related compounds

We have performed the bulk-sensitive high-resolution soft x-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces shows that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Ce 4d core-level spectra show clear differences in the electronic states among the materials.

cond-mat.str-el↗

arXiv2004

Structural, transport, and thermal properties of single crystalline type-VIII clathrate Ba8Ga16Sn30

We report the electrical resistivity, Hall coefficient, thermoelectric power, specific heat, and thermal conductivity on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Sn-flux. Negative S and R_H over a wide temperature range indicate that electrons dominate electrical transport properties. Both rho(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 uV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7x10^19 cm^(-3). Hall mobility u_H shows a maximum of 62 cm^2/Vs around 70 K. The analysis of temperature dependence of u_H suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.

cond-mat.str-el↗

arXiv2002

Paramagnetic Meissner Effect in Superconducting Single Crystals of Ba_{1-x}K_xBiO_3

The paramagnetic Meissner effect (PME) in the field cooled susceptibility has been observed in superconducting single crystals of Ba_{1-x}K_xBiO_3 using magnetometers of Quantum-Design Co.. The PME is similar to that observed in cuprates and Nb superconductors. For the Ba_{0.63}K_{0.37}BiO_3 crystal, the PME is observed in cooling process under the field up to 750 Oe. The PME was not found in the virgin-charged superconducting magnet. The PME is found in a superconductor with trapped flux and may arise from a remnant field, a temperature inhomogeneity near T_c, and a potassium-concentration inhomogeneity.

cond-mat.supr-con↗

arXiv2002

The Metal-Insulator Transition in Y1-xCaxTiO3 Caused by Phase Separation

In order to explore the origin of the Metal-Insulator (M-I) transition, the precise crystal structures of the hole-doped Mott insulator system, Y1-XCaXTiO3 (x=0.37, 0.39 and 0.41), are studied for the temperature range between 20K and 300K by the synchrotron radiation (SR) X-ray powder diffraction. For both Y0.63Ca0.37TiO3 and Y0.61Ca0.39TiO3 compositions, the orthorhombic (Pbnm) - monoclinic (P21/n) structural phase transition occurs at around 230K, which is much higher than their own M-I transition temperatures, i.e. 60K and 130K, respectively. For these compositions, the significant phase separation (low-temperature orthorhombic + monoclinic) is also found in the vicinity of the M-I transition temperature. On the other hand, Y0.59Ca0.41TiO3, which dose not exhibit M-I transition and preserve a metallic behavior down to 1.5K, is in two phases state from 20K to 300K. It is concluded that the existence of the phase separation causes the M-I transition in Y1-xCaxTiO3, and the low-temperature orthorhombic phase contributes to the metallic property of this system.

cond-mat.str-el↗

arXiv2000

Electronic structure of the strongly hybridized ferromagnet CeFe2

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare earth-iron Laves phase compounds. Our experimental results indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.

cond-mat.mtrl-sci↗

arXiv1998

Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB_12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong supression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ~ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ~40 meV in σ(ω), which is attributed to optical transitions between the Yb 4f-derived states across the gap. In addition, a broad peak is observed at ~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.

cond-mat.str-el↗

arXiv1996

Low frequency spectroscopy of the correlated metallic system $Ca_xSr_{1-x}VO_3$

We study the photoemission and optical conductivity response of the strongly correlated metallic system $Ca_xSr_{1-x}VO_3$. We find that the basic features of the transfer of spectral weight in photoemission experiments and the unusual lineshape of the optical response can be understood by modeling the system with a one band Hubbard model close to the Mott-Hubbard transition. We present a detailed comparison between the low frequency experimental data and the corresponding theoretical predictions obtained within the LISA method that is exact in the limit of large lattice connectivity.

cond-mat↗