Yusuke Amakai

X-ray Absorption Near Edge Structure of Amorphous CexRu100-x

We report the x-ray absorption near edge structure at the Ce L 3 edge for amorphous alloys Ce x Ru 100- x ( x =9, 43, and 80) and a C15 Laves phase compound CeRu 2 , which consists of a couple of peaks characteristic of 4 f 1 and 4 f 0 configurations. This indicates that Ce atoms of these materials are in a mixed valence state. The spectra are fitted to one arctangent and two Lorentzian functions. The Ce valence estimated from relative areas of the Lorentzian peaks varies linearly with Ce content. The transition into the continuum along with both peaks shifts to lower energy with increasing Ce concentration. We find a linear relation between the transition energy and the valence.

La substitution effect to the heavy-fermion state in structure-disordered Ce-Ru alloys

We have measured the magnetic susceptibility χ, specific heat Cp and resisitivity ρ on sputtered structure-disordered Ce80−yLayRu20 alloys. The χ for y ≤ 70 follows the Curie-Weiss law in the measurement temperature region. The paramagnetic effective magnetic moment peff is about 1.8 μB/Ce-atom for y = 0 and decreases with increasing the La-concentration (0.6 μB/Ce-atom for y = 70). The Cp follows linear relation of temperature above 7 K for all the samples. The specific heat coefficient γ shows about 200 mJ/molK2 for y = 0 and decreases rapidly with increasing La-concentration. We found the -log T dependence of the ρ in the La-substituted samples. Magnitude and temperature range of the -log T contribution to the whole resistivity increase with increasing the La concentration. On the other hand, we have observed superconducting behaviors of the resistivity ρ = 0 and a large diamagnetization at low temperature (T < 3 K) for y = 80.

Heavy fermion like behavior and superconductivity in amorphous CexRu100-xCexRu100-x alloy

The structure of disordered (amorphous) CexRu100-xCexRu100-x alloys have some characteristic features which are inherent in the heavy fermion (dense Kondo) nature. In the Ru rich region (15⩽x⩽40)(15⩽x⩽40), the superconducting behavior appears. With increasing Ru-concentration TcTc increases monotonically and simultaneously the paramagnetic moment decreases due to increase in the valence fluctuation. The superconductivity in these alloys is thought to be correlated to the 4f-electron state. In the Ce rich region (x⩾47)(x⩾47) the electric resistivity at low temperature exhibits the ρ=AT2ρ=AT2 relation with a large coefficient A. This implies that the coherent Kondo state is realized in the low temperature region. Thus, the amorphous Ce–Ru alloy is recognized to be one of the dense Kondo alloys.

Dense Kondo behavior in the low-temperature resistivity and specific heat for amorphous Ce50Al50 alloy

We measured the low-temperature specific heat Cp, resistivity ρ, and magnetoresistance Δρ(H)/ρ(0) for amorphous Ce50Al50 synthesized by a DC high-rate sputter method. The low-temperature Cp (T < 2 K) decreases rapidly toward 0 K and has no indication of phase transition. The value of γ0 which is extrapolated down to 0 K of the Cp/T is 117 mJ/molK2. The temperature dependence of ρ increases with decreasing temperature down to 0.6 K. We found that both a weak localization effect and a coherent Kondo state might be realized in the low-temperature region for the present alloy from the conductivity analysis. Furthermore, in the low-temperature ρ, a T2 term with a very large coefficient A was observed. The ratio of A/γ02 is 0.63 ×10-5 (μΩcm/K2)/(mJ/molK2)2 and is near the value of typical Ce-based heavy-fermion compounds. The magnetoresistances Δρ(H)/ρ(0) at 0.5 K and 2 K are almost constant in the magnetic field region of H < 10 kOe. We considered that the negative magnetoresistance effect is due to the weak localization and the positive magnetoresistance to the heavy-fermion state in the present alloy.We measured the low-temperature specific heat Cp, resistivity ρ, and magnetoresistance Δρ(H)/ρ(0) for amorphous Ce50Al50 synthesized by a DC high-rate sputter method. The low-temperature Cp (T < 2 K) decreases rapidly toward 0 K and has no indication of phase transition. The value of γ0 which is extrapolated down to 0 K of the Cp/T is 117 mJ/molK2. The temperature dependence of ρ increases with decreasing temperature down to 0.6 K. We found that both a weak localization effect and a coherent Kondo state might be realized in the low-temperature region for the present alloy from the conductivity analysis. Furthermore, in the low-temperature ρ, a T2 term with a very large coefficient A was observed. The ratio of A/γ02 is 0.63 ×10-5 (μΩcm/K2)/(mJ/molK2)2 and is near the value of typical Ce-based heavy-fermion compounds. The magnetoresistances Δρ(H)/ρ(0) at 0.5 K and 2 K are almost constant in the magnetic field region of H < 10 kOe. We considered that the negative magnetoresistance effect is due to the weak l...

A possible explanation of meta-magnetic transition in CeRu2Si2 and magnetic behavior in Gd-poor (Ce-Gd)Ru2Si2

Aiming at getting any clue to the mechanism of characteristic behavior of meta-magnetic transition in CeRu2Si2 and the Ce 4f electronic states in Gd-poor contents, the (Ce-Gd)Ru2Si2 crystal mixed system was studied through magnetic measurements. A characteristic behavior of meta-magnetic transition observed in CeRu2Si2 was found to be reproduced in GdRu2Si2 in the temperature range between 30 K and 40 K. The meta-magnetic transition field was increased to higher field by the substitution of small amount of Gd contents of 0.01 and 0.02 at%. Furthermore, the reciprocal susceptibility in CeRu2Si2 (Gd=0) in temperature range between 15 K and 300 K revealed that the electronic state of Ce 4f was dominantly Ce3+, which was in accordance with a previous work, and the electronic states of Ce 4f in CeRu2Si2 could be described by the Van Vleck paramagnetic picture.Aiming at getting any clue to the mechanism of characteristic behavior of meta-magnetic transition in CeRu2Si2 and the Ce 4f electronic states in Gd-poor contents, the (Ce-Gd)Ru2Si2 crystal mixed system was studied through magnetic measurements. A characteristic behavior of meta-magnetic transition observed in CeRu2Si2 was found to be reproduced in GdRu2Si2 in the temperature range between 30 K and 40 K. The meta-magnetic transition field was increased to higher field by the substitution of small amount of Gd contents of 0.01 and 0.02 at%. Furthermore, the reciprocal susceptibility in CeRu2Si2 (Gd=0) in temperature range between 15 K and 300 K revealed that the electronic state of Ce 4f was dominantly Ce3+, which was in accordance with a previous work, and the electronic states of Ce 4f in CeRu2Si2 could be described by the Van Vleck paramagnetic picture.

Amorphization Effect for Kondo Semiconductor CeRu2Al10

We measured the magnetic susceptibility , electrical resistivity , and specific heat of a sputtered amorphous (a-)CeRu2Al10 alloy. value for a-CeRu2Al10 alloy follows a Curie-Weiss paramagnetic behavior in the high-temperature region, and magnetic transition was not observed down to 2 K. The effective paramagnetic moment is 1.19 /Ce-atom. The resistivity shows a typical disordered alloy behavior, that is, small temperature dependence for the whole temperature range. We observed an enhancement of and in the low-temperature region of  K. The enhancement in is suppressed by applying a magnetic field. It is suggested that this behavior is caused by the Kondo effect.

Influence of the Magnetic Moment on the Atomic Distance in Amorphous CexRu100–x*

We perform the extended x-ray absorption fine structure (EXAFS) measurement to investigate the local structure of amorphous alloys CexRu100–x (x = 9, 43 and 80). The interatomic distances of the nearest Ru-Ce and Ru-Ru pairs derived from EXAFS are fully independent of Ce concentration. On the other hand the distance between neighboring Ce atoms increases sharply with the Ce content, which is exactly proportional to the Ce effective magnetic moment. We discuss the relation between the atomic distance and the effective magnetic moment from the point of view of the magneto-volume effect.

Relation between the atomic distance and the Ce valence of amorphous CexRu100?x

We report the local structure of amorphous alloys CexRu100?x (x=9, 43 and 80) and a C15 Laves phase compound CeRu2 determined from an extended x-ray absorption fine structure measurements near Ru K edge and Ce K edge. The structure analyses indicate that the Ce-Ru and Ru-Ru interatomic distances of amorphous CexRu100?x are constant for all of the Ce concentration, while the Ce-Ce distance increases with increasing the Ce content. The interatomic distance of Ru-Ru in the amorphous alloy system is as same as that of the CeRu2 compound. The Ce-Ce interatomic distance of amorphous CexRu100?x is closely connected with the Ce valence.

Stripe order and superconductivity in the mechanically milled La1.6−xNd0.4SrxCuO4

We examine the effect of mechanical milling on the superconducting transition temperature Tc in stripe-ordered La1.6−xNd0.4SrxCuO4 with x~1/8. The X-ray diffraction patterns of samples milled for 24h show that the orthorhombicity is slightly reduced, in addition to the reduction of crystallite size and the increase of lattice strain, by mechanical milling. The Tc is enhanced by the mechanical milling, although the magnitude of the diamagnetic signal is largely suppressed. The enhancement of Tc suggests that the static stripe is suppressed in the mechanically milled sample.

Site random superconductivity of A15 compound Nb3Sn induced by mechanical milling

We performed mechanical milling of the A15-type superconductor Nb3Sn with transition temperature Tc = 18K. Tc decreased with increasing milling time, a result similar to that of Di, Loeff and Bakker, except that Tc seemed to decrease discontinuously with milling time. X-ray diffraction (XRD) patterns were taken using a conventional X-ray diffractometer with CuKa radiation at room temperature. The height of the Bragg peaks lowered and their width broadened after milling, but the patterns of the A15 structure persisted for up to 48 h of milling. We compared the XRD patterns of Nb3Sn with simulations using REATAN-2000, and obtained evidence that the site random disorder of Nb and Sn atoms in the A15 structure caused the decrease in Tc.