A series of Co-doped EuFe2-xCoxAs2 compounds were prepared in both of single crystalline and polycrystalline forms.The Co-doping effects on the crystal structure,electrical resistivity and magnetic susceptibility were systematically studied.Superconductivity was found in polycrystalline Co-doped samples from zero resistivity effects,with the highest onset superconducting transition temperature at 26 K in the optimum doped EuFe1.84Co0.16As2 compound.While due to the stronger competition between the superconducting order and the Eu2+ magnetic order,the zero resistivity effect is absent in the Co-doped single crystal samples.
The vortex domains, structural properties and ferroelectric polarization in Y1-xInxMn O3 with 0 B x B 0.6have been extensively investigated in well-characterized samples. X-ray diffraction measurements demonstrated that the lattice parameters change continuously following the substitution of In for Y. Measurements of magnetic susceptibilities revealed that In substitution could visibly affect the magnetic transition and low-temperature magnetic properties. Transmission electron microscopy study showed that In substitution could result in notable decrease of the size of ferroelectric vortex domains. Cs-corrected scanning transmission electron microscopy observations and our careful analysis on atomic-poling configurations demonstrate that the ferroelectric polarizations of Y1-xInxMn O3 are suppressed with the increase of In content.
Li WangFu-Kuo ChiangJun LiChao MaHuai-Xin YangJian-Qi Li
Crystal structures and microstructural features, such as structural phase transitions, defect structures, and chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, many studies on the structural properties of Fe-based high-Tc superconductors have been published. This review article will mainly focus on the typical microstructural features in samples that have been well characterized by physical measurements. （i） Certain common structural features are discussed, in particular, the crystal structural features for different superconducting families, the local structural distortions in the Fe2Pn2 （Pn = P, As, Sb） or FeeCh2 （Ch = S, Se, Te） blocks, and the structural transformations in the 122 system. （ii） In FeTe（Se） （11 family）, the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. （iii） In the Ko.sFe1.6＋xSe2 system, we focus on the typical compounds with emphasis on the Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.
Electric transport and scanning tunneling spectrum(STS)have been investigated on polycrystalline samples of the new superconductor Bi4O4S3.A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field.Interestingly,a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed.This kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation.From the tunneling spectra,a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kB TC^16.6.This value is much larger than the one predicted by the BCS theory in the weak coupling regime(2Δ/kB TC^3.53),which suggests the strong coupling superconductivity in the present system.Furthermore,the gapped feature persists on the spectra until 14 K in the STS measurement,which suggests a prominent fluctuation region of superconductivity.Such a superconducting fluctuation can survive at very high magnetic fields,which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.
LI ShengYANG HuanFANG DeLongWANG ZhenYuTAO JianDING XiaXinWEN HaiHu
Using self-flux method,we have successfully grown the parent phase of the single crystals of CaFeAsF1-x.The X-ray di?raction indicates good crystallinity.In-plane resistivity shows a bad metallic behavior with a sharp drop of resistivity at about T SDW=119K.This anomaly is associated with the possible spin density wave(SDW)order.Interestingly near T SDW,the resistivity exhibits a cusp-like feature,which may be understood as the strong coupling effect between the electrons and the antiferromagnetic(AF)spin fluctuations.A reduction of fluorine or application of a high pressure will suppress the SDW feature and induce superconductivity.Hall effect measurements reveal a positive Hall coefficient below T SDW indicating a dominant role of the hole-like charge carriers in the parent phase.Strong magnetoresistance has been observed below T SDW suggesting multiple conduction channels of the charge carriers.
Transmission electron microscopy （TEM） study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave （CDW） state below TCDW ≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1=0.62a＊ on the a＊-b＊ plane and q2 = 0.23a＊ on the a＊-c＊ plane. The atomic displacements occur along the b-axis direction for q1 and along the c-axis direction for q2, respectively. Moreover, the correlation between ql and q2 can be generally written as q1 = （q2 ＋ a＊）/2 in the CDW state, suggesting the presence of essential coupling between q1 and q2. A small fraction of Ir doping on the Pt site in Sr（Pt1-xIrx）2As2 （x ≤ 0.06） could moderately change these CDW modulations and also affect their superconductivities.
