Hideaki Kitazawa

Anisotropy of superconductivity from MgB2 single crystals

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the research of its superconducting mechanism and its applications.

A new antiferromagnetic heavy Fermion compound : CePd2Al3

We report the first measurements of the electrical resistivity, the magnetic susceptibility and the specific heat of the new hexagonal compound CePd 2 Al 3 . The results indicate that CePd 2 Al 3 h is a new heavy fermion system with an electronic specific heat coefficient (γ) of 380 mJ/mol·K 2 and antiferromagnetic ordering below T N =2.8 K.

Geometrically frustrated magnetic structures of the heavy-fermion compound CePdAl studied by powder neutron diffraction

The heavy-fermion compound CePdAl with ZrNiAl-type crystal structure (hexagonal space group ) was investigated by powder neutron diffraction. The triangular coordination symmetry of magnetic Ce atoms on site 3f gives rise to geometrical frustration. CePdAl orders below with an incommensurate antiferromagnetic propagation vector , and a longitudinal sine-wave (LSW) modulated spin arrangement. Magnetically ordered moments at Ce(1) and Ce(3) coexist with frustrated disordered moments at Ce(2). The experimentally determined magnetic structure is in agreement with group theoretical symmetry analysis considerations, calculated by the program MODY, which confirm that for Ce(2) an ordered magnetic moment parallel to the magnetically easy c-axis is forbidden by symmetry. Further low-temperature experiments give evidence for a second magnetic phase transition in CePdAl between 0.6 and 1.3 K. Magnetic structures of CePdAl are compared with those of the isostructural compound TbNiAl, where a non-zero ordered magnetic moment for the geometrically frustrated Tb(2) atoms is allowed by symmetry.

Conduction band caused by oxygen vacancies in aluminum oxide for resistance random access memory

As a next-generation memory, we have developed a rare-metal-free memory using Al oxide with a high-density of oxygen vacancies (Vos). The electronic structure has been simulated using first-principles calculations. In this paper, we report the electronic structure of the band gap, analyzed using thermally stimulated current measurements, to evaluate the simulated results. We observed electronic states corresponding to resistance changes for the first time. These results show that Vo+2 (electron empty Vo) changes to Vo+1 by electron injection; the overlapped Vo+1 electron changes into a “Vo conduction band” (VoCB), and the changed structure is stabilized by structural relaxation of Al ions around Vo. VoCB is considered as a kind of mid-gap impurity band. The origin of the on/off switching is considered to be generation/degeneration of the VoCB caused by increasing/decreasing numbers of Vo electrons. Based on knowledge of the electronic mechanism, we have changed metal/insulator/metal structure to a metal/i...

Confirmation of a one-dimensional spin-1/2 Heisenberg system with ferromagnetic first-nearest-neighbor and antiferromagnetic second-nearest-neighbor interactions in Rb

We have investigated magnetic properties of Rb

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Cu

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Mo

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