Akihiro Mitsuda

Pressure-Induced Valence Transition in Antiferromagnet EuRh2Si2

Considering the unique properties of EuRh 2 Si 2 from the viewpoint of the Eu valence, we have examined its physical properties under external pressure. At ambient pressure, EuRh 2 Si 2 is an antiferromagnet with a Neel temperature T N of 25 K, and the Eu ion is in the divalent state. The application of pressure up to 0.84 GPa slightly shifts T N toward higher values. Under pressures higher than 1.00 GPa, an abrupt first-order valence transition emerges simultaneously with the disappearance of antiferromagnetism. For P =1.17 GPa, the valence change associated with valence transition is roughly estimated to be ∼0.19 from the thermal expansion anomaly. The valence transition temperature T v increases rapidly with increasing pressure. The temperature–pressure phase diagram of EuRh 2 Si 2 is very similar to those of the other systems showing pressure-induced valence transition.

Molecular Beam Epitaxy Growth of Superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2

We report on the molecular beam epitaxy growth of nonsuperconducting SrFe2As2 and BaFe2As2 and superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2 thin films. SrFe2As2 and BaFe2As2 films were obtained rather easily at the growth temperatures of 540–600 °C, which are not much different from those for GaAs growth. However, superconducting Sr1-xKxFe2As2 and Ba1-xKxFe2As2 films cannot be obtained at the same growth temperatures as elemental K is highly volatile. The key to incorporating K into films is low-temperature growth (≤350 °C) in reduced As flux. The resultant films showed good superconducting properties: Tcon (Tcend) = 33.2 K (30.0 K) and 38.3 K (35.5 K) for Sr1-xKxFe2As2 and Ba1-xKxFe2As2 thin films respectively.

High-Field Magnetization and Neutron Diffraction Studies of One-Dimensional Compound Ca3CoRhO6

High-field magnetization and neutron diffraction measurements were performed on the one-dimensional compound Ca 3 CoRhO 6 . The long-range ordered state was first observed below 90 K. A plateau at 1/3 of the saturation magnetization was observed in the magnetization curve at 70 K, indicating ferrimagnetic alignment of the ferromagnetic Ising chains. We also observed a large hysteresis originating from a spin freezing of ferromagnetic chains in the magnetization curve at 4.2 K. Based on the results of high-field magnetization and neutron diffraction measurements, a possible magnetic structure is discussed in terms of the spin frustration on the triangular lattice.

Application of Doniach Diagram on Valence Transition in EuCu2(SixGe1-x)2

To investigate the valence transition of Eu in the pseudo-binary compounds series, EuCu 2 (Si x Ge 1- x ) 2 , measurements of the lattice constants, the X-ray absorption spectroscopy (XAS), the mag...

First-Order Valence Transition of EuPd2Si2 Induced by High Magnetic Fields.

In order to study the magnetic field effects on the intermediate valence of the Eu system, magnetization was measured for EuPd 2 Si 2 and Eu(Pd 0.95 Pt 0.05 ) 2 Si 2 up to 100 T. A first-order metamagnetic transition was observed for both samples under high magnetic fields. The transition field is 93T for EuPd 2 Si 2 at 6 K. The saturation magnetization close to 7µ B indicates that this metamagnetism is a field-induced valence transition from an intermediate valence state to a divalent state of the Eu ion. Based on the interconfigurational fluctuation (ICF) model, the excitation energy to convert an Eu 3+ ion into Eu 2+ was estimated. Temperature dependence of the metamagnetic transition field of EuPd 2 Si 2 was also studied.

High-magnetic-field X-ray absorption spectroscopy of field-induced valence transition in EuNi2(Si1-xGex)2

The magnetic-field-induced valence transition in EuNi 2 (Si 1- x Ge x ) 2 ( x =0.82, 0.85) has been studied by X-ray absorption spectroscopy at low temperatures. The field dependence of the Eu valence is directly observed for the first time. We find a significant decrease in the valences at high magnetic fields above 40 T. The valence of Eu changes from v * =2.75 at a zero field to v * =2.33 at 40 T for x =0.82. The results are analyzed using a theoretical model. The hybridization parameter V between the Eu 2+ and Eu 3+ states and the energy separation Δ E are determined, which are crucial for quantum mechanical mixing.

Origin of Weak Ferromagnetism in YbxFe4Sb12, Relationship between Weak Ferromagnetism and Filling Ratio x

The magnetization and specific heat of single-crystalline samples of filled-skutterudite compounds Yb x Fe 4 Sb 12 were measured. Six single crystals with the filling ratio of Yb, x , from 0.875 to 0.910 were prepared. The weak ferromagnetism of Yb x Fe 4 Sb 12 appearing at low temperatures was found to strongly correlate with x ; with increasing x from 0.875 to 0.910, the transition temperature, T C , decreases from 17 to 5 K and the weak-ferromagnetic moment, m 0 , decreases from 0.066 to 0.005 µ B /Fe. From these trends, both T C and m 0 were expected to reach zero at the critical concentration, x c =0.93. In the temperature dependence of the specific heat, no anomaly at T C was observed, reflecting that m 0 is extremely small compared with the effective Bohr magneton number. With the application of pressure, the T C of the sample with x =0.882 increases rapidly, with the ratio of d T C /d P =+5.27 K/GPa. This weak ferromagnetism of Yb x Fe 4 Sb 12 originates from the itinerant 3d electrons of Fe in an...

Spectroscopy Studies of Temperature-Induced Valence Transition on EuNi2(Si1-xGex)2 around Eu 3d–4f, 4d–4f and Ni 2p–3d Excitation Regions

The resonant photoemission spectra and X-ray absorption spectra of temperature-induced valence transition material EuNi 2 (Si 1- x Ge x ) 2 around Eu 3 d –4 f , 4 d –4 f and Ni 2 p –3 d resonant excitation regions have been measured. From the comparison between the resonant photoemission spectra around the 3 d –4 f (bulk sensitive) and 4 d –4 f (surface sensitive) excitation regions, the Eu divalent component originating from the surface is separated from the bulk one. The divalent and the trivalent features of 4 f electrons are obviously distinguished depending on the excitation conditions. It was confirmed that the intensity ratio of the 4 f electronic structures between the Eu divalent and trivalent ions changes as a function of the temperature. The mean valence values estimated from the Eu M -edge X-ray absorption spectra and from the photoemission spectra were rather smaller than those obtained previously from the L -edge X-ray absorption spectra. The reason of the discrepancy is discussed. The spect...

Charge density wave and superconductivity of RPt2Si2 (R = Y, La, Nd, and Lu)

The temperature dependence of electrical resistivity and magnetic susceptibility was studied for RPt2Si2 compounds with R = Y, La, Nd, and Lu. LaPt2Si2 and NdPt2Si2 show steps in the resistivity vs temperature curve at \(T^{*} = 112\) and 77 K, respectively. We performed X-ray diffraction and selected-area electron diffraction (SAED) measurements for LaPt2Si2 at low temperatures. It was found that LaPt2Si2 undergoes a structural transition from tetragonal to orthorhombic at \(T^{*}\). Moreover, \((n/3, 0, 0)\) superlattice reflections with \(n = 1\) and 2 were observed at 10 K in the SAED patterns. These results strongly suggest the formation of a charge density wave (CDW) in LaPt2Si2 below \(T^{*}\). On the other hand, the compounds R = Y, La, and Lu were found to show superconductivity below 1.5–1.9 K. The origin of a CDW and its coexistence with superconductivity in RPt2Si2 are discussed.

Thermal expansion and electrical resistivity of EuNi2(Si1-xGex)2

The thermal expansion and temperature dependence of electrical resistivity were measured for EuNi 2 (Si 1- x Ge x ) 2 , which shows a valence transition against temperature in 0.5≤ x ≤0.82. A first...