arXiv subjects

Shin-ichi Kimura

Publications and source records attributed to Shin-ichi Kimura.

46 records · Page 3Linked to original sources

arXiv2010

Pseudogap of Superconducting correlation origin in iron-pnictide superconductor Ba0.6+δK0.4-δFe2As2

Pseudogap (PG), a phenomenon of opening of gap like features above superconducting (SC) transition temperature (Tc), has been universally observed in the high-Tc cuprates (HTC) (1,2), some heavy fermion superconductors (3), and iron pnictides (4-7). Here, we report the observation of PG behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (Tconset =36 K) single crystal far above Tc, up to 100 K (~ 3Tc). Unique scale separation in Ba0.6+δK0.4-δFe2As2 compound - magnetic and SC correlation energy scales being widely separated - enabled us to establish that the PG structures observed in the range of 50 cm-1 - 150 cm-1 in optical conductivity is caused by the SC correlation and the magnetic correlation origin is ruled out. Theoretical calculations, based on the preformed Cooper pair model (8), provided an excellent description to the temperature evolution of the optical conductivity data from below to above Tc.

cond-mat.supr-con↗

arXiv2010

Polaronic behavior of photoelectron spectra of Fe3O4 revealed by both hard X-ray and extremely low energy photons

Hard X-ray and extremely low energy bulk-sensitive photoelectron spectroscopy has been performed in the temperature range of 100-330 K for Fe3O4. In the high temperature phase just above the Verwey transition, the intensity at the Fermi level (EF) is still negligible, but it increases gradually with further increasing the temperature (250 K, 330 K) in consistence with the temperature dependence of the conductivity. The spectral behaviors near EF with temperature are well explained by the model, which takes the polaron effect into account.

cond-mat.str-el↗

arXiv2009

Kondo-like behaviors in magnetic and thermal properties of single crystal Tm5Si2Ge2

We grew the single crystal of stoichiometric Tm5Si2.0Ge2.0 using a Bridgeman method and performed XRD, EDS, magnetization, ac and dc magnetic susceptibilities, specific heat, electrical resistivity and XPS experiments. It crystallizes in orthorhombic Sm5Ge4-type structure. The mean valence of Tm ions in Tm5Si2.0Ge2.0 is almost trivalent. The 4f states is split by the crystalline electric field. The ground state exhibits the long range antiferromagnetic order with the ferromagnetically coupled magnetic moments in the ac plane below 8.01 K, while the exited states exhibit the reduction of magnetic moment and magnetic entropy and -log T-behaviors observed in Kondo materials.

cond-mat.str-el↗

arXiv2009

Origin of infrared peaks in the optical conductivity of ytterbium compounds

We have calculated optical conductivity [$σ(ω)$] spectra of ytterbium compounds (YbAl$_3$, YbAl$_2$, YbCu$_2$Si$_2$, YbNi$_2$Ge$_2$, YbInCu$_4$, YbRh$_2$Si$_2$, YbIr$_2$Si$_2$, and YbB$_{12}$) based on the direct interband transition derived from first-principle band calculation and compared the results with the experimentally obtained $σ(ω)$ spectra. The spectral feature of a peak in the middle-infrared region (mid-IR peak) and a shoulder structure in the far-infrared region (far-IR shoulder) in the experimental $σ(ω)$ spectra can be described by the band calculation with a common renormalization factor. This result indicates that the infrared spectra of Yb compounds originate from the interband transition from the Yb $4f$ state but that the Yb $4f$ state shifts to the Fermi level with strong electron correlation.

cond-mat.str-el↗

arXiv2008

Origin of Middle-Infrared Peaks in Cerium Compounds

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.

cond-mat.str-el↗

arXiv2007

Low-Energy Electrodynamics of Heavy Quasiparticles in ZrZn2

The temperature dependence of the optical conductivity spectrum of an itinerant ferromagnetic material, ZrZn$_2$, was obtained and the dynamical effective mass and the scattering rate were derived. A renormalized Drude peak with a heavy effective mass developed at low temperatures. The effective mass rapidly increased below $\hbarω$ = 10 meV and at 10 K it was observed to be 3 times heavier than that at 300 K. The scattering rate also rapidly decreased below 10 meV at 10 K. These results indicate the creation of heavy quasiparticles at low temperatures due to spin fluctuations.

cond-mat.str-el↗

arXiv2007

Infrared study on the electronic structure of alkaline-earth-filled skutterudites AT4Sb12 (A = Sr, Ba; M = Fe, Ru, Os)

The optical conductivity [$σ(ω)$] spectra of alkaline-earth-filled skutterudites with the chemical formula $A^{2+}M_{4}$Sb$_{12}$ ($A$ = Sr, Ba, $M$ = Fe, Ru, Os) and a reference material La$^{3+}$Fe$_{4}$Sb$_{12}$ were obtained and compared with the corresponding band structure calculations and with calculated $σ(ω)$ spectra to investigate their electronic structures. At high temperatures, the energy of the plasma edge decreased with the increasing valence of the guest atoms $A$ in the Fe$_{4}$Sb$_{12}$ cage indicating hole-type conduction. A narrow peak with a pseudogap of 25 meV was observed in SrFe$_{4}$Sb$_{12}$, while the corresponding peak were located at 200 and 100 meV in the Ru- and Os-counterparts, respectively. The order of the peak energy in these compounds is consistent with the thermodynamical properties in which the Os-compound is located between the Fe- and Ru-compounds. This indicated that the electronic structure observed in the infrared $σ(ω)$ spectra directly affects the thermodynamical properties. The band structure calculation implies the different electronic structure among these compounds originates from the different $d$ states of the $M$ ions.

cond-mat.str-el↗

arXiv2006

Magnetic Field-Induced Superconductor-Insulator-Metal Transition in an Organic Conductor: An Infrared Magneto-Optical Imaging Spectroscopy

The magnetic field-induced superconductor-insulator-metal transition (SIMT) in partially deuterated $κ$-(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.

cond-mat.str-el↗

arXiv2006

Iron-Based Heavy Quasiparticles in SrFe$_{4}$Sb$_{12}$: An Infrared Spectroscopic Study

Temperature-dependent infrared reflectivity spectra of SrFe$_{4}$Sb$_{12}$ has been measured. A renormalized Drude peak with a heavy effective mass and a pronounced pseudogap of 10 meV develops in the optical conductivity spectra at low temperatures. As the temperature decreases below 100 K, the effective mass ($m^{*}$) rapidly increases, and the scattering rate ($1/τ$) is quenched. The temperature dependence of $m^{*}$ and $1/τ$ indicates that the hybridization between the Fe 3d spins and the charge carriers plays an important role in determining the physical properties of SrFe$_{4}$Sb$_{12}$ at low temperatures. This result is the clear evidence of the iron-based heavy quasiparticles.

cond-mat.str-el↗

arXiv2004

Change of Electronic Structure Induced by Magnetic Transitions in CeBi

The temperature dependence of the electronic structure of CeBi arising from two types of antiferromagnetic transitions based on optical conductivity ($σ(ω)$) was observed. The $σ(ω)$ spectrum continuously and discontinuously changes at 25 and 11 K, respectively. Between these temperatures, two peaks in the spectrum rapidly shift to the opposite energy sides as the temperature changes. Through a comparison with the band calculation as well as with the theoretical $σ(ω)$ spectrum, this peak shift was explained by the energy shift of the Bi $6p$ band due to the mixing effect between the Ce $4f Γ_8$ and Bi $6p$ states. The single-layer antiferromagnetic ($+-$) transition from the paramagnetic state was concluded to be of the second order. The marked changes in the $σ(ω)$ spectrum at 11 K, however, indicated the change in the electronic structure was due to a first-order-like magnetic transition from a single-layer to a double-layer ($++--$) antiferromagnetic phase.

cond-mat.str-el↗