[en] Simultaneous high-pressure X-ray diffraction and electrical resistance measurements have been carried out on a PbO-type α-FeSe_0.92 compound to a pressure of 44 GPa and temperatures down to 4 K using designer diamond anvils at synchrotron source. A ambient temperature, a structural phase transition from a tetragonal (P4/nmm) phase to an orthorhombic (Pbnm) phase is observed at 11 GPa and the Pbnm phase persists up to 74 GPa. The superconducting transition temperature (T_c) increases rapidly with pressure reaching a maximum of ∼28 K at ∼6 GPa and decreases at higher pressures, disappearing completely at 14.6 GPa. Simultaneous pressure-dependent X-ray diffraction and resistance measurements at low temperatures show superconductivity only in a low-pressure orthorhombic (Cmma) phase of the α-FeSe_0.92. Upon increasing pressure at 10 K near T_c, crystalline phases change from a mixture of orthorhombic (Cmma) and hexagonal (P63/mmc) phases to a high-pressure orthorhombic (Pbnm) phase near 6.4 GPa where T_c is maximum.

[en] It has been more than four years since the discovery of β-FeSe_1−x superconductors. Through the efforts of many outstanding research groups, unprecedented advances in the field have been achieved. High-quality single crystals of β-FeSe_1−x and related compounds have been prepared by various techniques, allowing us to explore in detail the physical properties of this class of materials. Detailed characterizations of the structure and properties of these crystals have helped us to understand the origin of superconductivity in β-FeSe_1−x. The occurrence of superconductivity is associated with the low-temperature structure distortion, which is accompanied by several anomalies. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in β-FeSe_1−x suggest the opening of an energy gap below 130–140 K, accompanied by a coincident transfer of optical spectral weight in the visible range and alterations in transport properties. These observations provide convincing evidence that the modification of the electronic structure occurs prior to the lattice distortion. They further suggest that the high-temperature gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders. (topical reviews)

[en] Our previous studies indicate that small additions of nano-scale RE₂BaCuO₅ (RE211, RE: rare elements) form nano-sized compositional fluctuations during the melt-growth process and thus act as field induced pinning centres. The solubility, peritectic temperature, and atomic size of these nmRE211 additives alter the J_c-H performance. To further reveal the effect of size and amount of RE211 additives, in this study, Y₂Ba₁Cu₁O₅ (Y211) particles of two different sizes, micro-scale (μm) and nano-scale (nm), were used. The superconducting results indicate that the contribution of J_c in high field regions is larger in nmY211 than μmY 211. The microstructure results imply that μmY 211 particles tend to form Y-rich regions in the core shell, while nmY211 particles tend to produce regions with Y fluctuations and high defect density. These different results are attributed to the reactions between the Y211 additions and the 123 matrix during the melt-textured growth process

[en] From magnetic susceptibility, dielectric permittivity, electric polarization and specific heat measurements we discover spin-induced ferroelectricity and magnetoelectric coupling in Mn_3TeO_6 and observe two successive magnetic transitions at low temperatures. A non-ferroelectric intermediate magnetic state occurs below 23 K and a multiferroic ground state emerges below 21 K. Moreover, Mn_3TeO_6 is a candidate for a multiferroic material where two types of incommensurate spin structures, cycloidal and helical, coexist. Theoretically, both spin substructures may contribute to the macro electric polarization via different mechanisms. This could open new ways of manipulating the ferroelectric polarization in a multiferroic material. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] In this study, we report low temperature x-ray diffraction studies combined with electrical resistance measurements on single crystals of iron-based layered superconductor FeSe to a temperature of 10 K and a pressure of 44 GPa. The low temperature high pressure x-ray diffraction studies were performed using a synchrotron source and superconductivity at high pressure was studied using designer diamond anvils. At ambient temperature, the FeSe sample shows a phase transformation from a PbO-type tetragonal phase to a NiAs-type hexagonal phase at 10 ± 2 GPa. On cooling, a structural distortion from a PbO-type tetragonal phase to an orthorhombic Cmma phase is observed below 100 K. At a low temperature of 10 K, compression of the orthorhombic Cmma phase results in a gradual transformation to an amorphous phase above 15 GPa. The transformation to the amorphous phase is completed by 40 GPa at 10 K. A loss of superconductivity is observed in the amorphous phase and a dramatic change in the temperature behavior of electrical resistance indicates formation of a semiconducting state at high pressures and low temperatures. The formation of the amorphous phase is attributed to a kinetic hindrance to the growth of a hexagonal NiAs phase under high pressures and low temperatures.

[en] In this study, large single grain Y-Ba-Cu-O (YBCO) superconductors with CeO₂ addition were fabricated by the infiltration and growth (IG) technique. It is noticeable that the CeO₂ doped IG-YBCO sample showed a superior critical current density J_c(H, T) with a peak effect. The J_c (3 T, 65 K) of the IG-YBCO sample with CeO₂ addition reached 10⁵ A cm^-2, which was two times higher than that of the CeO₂ doped top-seeded melt texture (TSMT)-YBCO sample. Microstructure analysis indicated that the concentration of Sm in the CeO₂ doped IG-YBCO sample was 0.2 mol%. The Sm which dissolved from the SmBCO seed diffused into the bulk to form compositional fluctuations of (Y, Sm)BCO and was correlated to the effective pinning in high field regions (or peak effect) to improve the J_c(H, T) in high fields. In addition, the maximum trapped field value of 0.23 T and trapped field profile of the CeO₂ doped sample grown by IG are larger than that of 0.13 T grown by TSMT using the same diameter of precursor pellets.

[en] In order to confirm the superconductivity observed in hexagonal ϵ-NbN reported recently, we have deposited NbN film on (0001) 4H-SiC substrate, with good lattice match, intentionally to grow hexagonal NbN phase. The detailed structural analysis show that the deposited films are (111)-orientated δ-NbN phase with dense rotational (60°) twins. Double diffraction effect in selected area electron diffraction and Moiré fringes in high resolution transmission electron microscope images confirm the existence of rotational twins in nanometer scale. The growth of highly twined film results from no energy preference for the two rotational twins of (111)-orientated δ-NbN to bond with the Si-faced (0001) 4H-SiC substrate. These highly twined δ-NbN epitaxial films exhibit high normal state resistivity, lower T _C, but good spatial uniformity of superconducting energy gap. No ϵ-NbN phase was observed in this study which is might due to a low growth temperature of thin film. (paper)

[en] The study investigated the chemical doping effect of Li in the Fe(Se,Te) system. With the addition of Li, the NiAs phase was precipitated, which resulted in worse superconductivity in FeSe samples. The XRD results indicated that chemical stress was found in the FeTe system which showed no superconductivity by doping Li. However, it was noticeable that the T_c was increased (1–1.5 K) and the enhancement of diamagnetic force was found in FeSe_1−xTe_x (x = 0.5–1) with 1 wt% Li doping. The lattice constant of the c-axis increased with increasing content of Li, implying that the Li interstitial sites are along the c-axis. Furthermore, the XRD results showed that the enhanced superconductivity of FeSe_1−xTe_x resulted from the large stress fields in FeSe_1−xTe_x. The stress field effect was retarded by the large quantity of Li additives. (paper)

[en] A Scanning SQUID Microscope (SSM), developed by using our own Nb-AlO_x-Nb technology, is used to study the distribution of vortices in a superconducting thin film and their movements induced by applied current. The SQUID sensor is a first-order gradiometer with an overall equivalent flux noise of about 5x10^-6Φ₀Hz^-1/2. Using this SSM system, we obtain the vortex images before and after a pulse current with a variable amplitude and duration time is applied to the sample. The changes of a series of vortex images with increasing amplitude of the current pulse provide us with an estimate of the vortex pinning force at various pinning sites and their distribution.

[en] Superconducting Fe_1_+_ySe_1_−_xTe _x nanowires with considerable differences in the diameter and aspect ratio were synthesized via annealing thin films with particular composition prepared by pulsed laser deposition. Using transmission electron microscopy and related techniques, we found they were all crystallized in a PbO-type tetragonal structure with a growth direction of [010] (equivalent to [100]). All the nanowires were covered by a thin amorphous oxide layer (∼5 nm) determined by electron energy loss spectroscopy. Chemical analysis and the calculated lattice constants reveal the fact that these nanowires are different from chemical compositions. High-resolution electron microscopy images show that the two thick nanowires (with diameters ∼80 nm) are composed of multiple phases, while the thin nanowires (with diameters of 14 and 22 nm) are single phase. Digital moiré analysis reveals that the thin nanowires contain surface defects, as well as dislocations, inside the nanowires. (papers)