Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe(2)O(4). Using resonant X-ray scattering measurements, we determine the ordering of the Fe(2+) and Fe(3+) ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.

The crystal structure of LnFeAsO 1Ày (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO 1Ày is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs 4 -tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs 4 -tetrahedrons.

We have synthesized a series of oxygen-deficient, fluorine-free polycrystalline samples with nominal chemical formula NdFeAsO 1Ày using high-pressure technique. The ZrCuSiAs type crystal structure, as expected for NdFeAsO, is formed as the main phase for the composition 0 y 0:6. Sharp superconducting (SC) transition with their transition temperature (T c ) as high as 54 K is observed for 0:3 y 0:8 samples, which is higher than that of F-doped NdFeAsO. While T c does not depend on y, SC volume fraction shows maximum at y ¼ 0:4. Around the same y, the lattice parameters show discontinuous shrinkage. These experimental results suggest the existence of a stable superconducting phase with a fixed atomic ratio close to Nd : Fe : As : O ¼ 1 : 1 : 1 : 0:6, presumably associated with a regular array of oxygen deficiency.

High-density bulk materials of a newly discovered 40K intermetallic MgB2 superconductor were prepared by high pressure sintering. Superconducting transition with the onset temperature of 39K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M-H) curve shows the behavior of a typical Type II superconductor and the lower critical field Hc1(0) estimated from M-H curve is 0.032T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 2x10 4 A/cm 2 at 20K and 1T. Upper critical field Hc2(0) determined by extrapolating the onset of resistive transition and assuming a dirty limit is 18T

We present57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0:6 K 0:4 Fe 2 As 2 (T c ¼ 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1=T 1 ) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q ¼ 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1=T 1 results are totally consistent with a s AE -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1=T 1 data for Ba 0:6 K 0:4 Fe 2 As 2 and LaFeAsO 0:7 , which seemingly follow a T 5 -and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap. The recent discovery of superconductivity in the iron (Fe)-based oxypnictide LaFeAsO 1Àx F x at the superconducting (SC) transition temperature T c ¼ 26 K has provided a new route toward the realization of high-T c superconductivity.1) The mother material, LaFeAsO, exhibits a structural phase transition from tetragonal (P4=nmm) to orthorhombic (Cmma) form at T $ 155 K and then exhibits a striped antiferromagnetic (AFM) order with Q ¼ ð0; Þ or ð; 0Þ and T N $ 140 K.2) The calculated Fermi surfaces (FSs) for undoped LaFeAsO consist of two small electron cylinders around the tetragonal M point and two hole cylinders, plus a heavy 3D hole pocket, around the À point.3) Measurements of the nuclear spin-lattice relaxation rate (1=T 1 ) for the LaFeAsO system in the SC state revealed the lack of a coherence peak below T c and the presence of T 3 -like behavior, suggesting an unconventional SC nature. [4][5][6]