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تسجيل مستخدم جديدSynthesis, magnetization and magneto transport study of RECoPO (RE = La, Nd & Sm)
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We report the electrical, magneto transport and specific heat of the layered polycrystalline RECoPO (RE = La, Nd and Sm) samples. These compounds are iso-structural to recently discovered superconductor LaFeAs(O/F). Bulk polycrystalline samples are synthesized by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Cobalt in these compounds is in itinerant state with its paramagnetic moment above 1.4muB and the same orders ferromagnetically (FM) with saturation moment of around 0.20muB below say 80K. Though, LaCoPO shows single paramagnetic (PM) to ferromagnetic (FM) transition near 35K, the NdCoPO and SmCoPO exhibit successive PM-FM-AFM transitions. Both FM and AFM transition temperatures vary with applied field. Although the itinerant ferromagnetism occurs with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 69K and 14K for Nd and 57K and 45K for Sm. This FM-AFM transition of Co spins in NdCoPO and SmCoPO is both field and temperature dependent. The Magneto-transport of NdCoPO and SmCoPO distinctly follows their successive PM-FM-AFM transitions. It is clear that Sm/Nd (4f) interacts with the Co (3d) in first time synthesized Sm/NdCoPO.
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We synthesized bulk polycrystalline samples of RECoAsO (RE=La, Nd and Sm) by solid state reaction route in an evacuated sealed quartz tube. All these compounds are crystallized in a tetragonal structure with space group P4/nmm. The Co, in these compo
unds is in itinerant ferromagnetic state with its paramagnetic moment above 1.5 microB and the same orders ferromagnetically (FM) with small saturation moment of around 0.20 microB below say 80K. This bulk intrinsic magnetism of Co changes dramatically when nonmagnetic La is changed by magnetic Sm and Nd. Although the itinerant ferromagnetism occurs below 80-100K with small saturation moment, typical anti-ferromagnetic (AFM) transitions (TN1, TN2) are observed at 57K and 45K for Sm and at 69K and 14K for Nd. The transition of Co spins from FM to AFM, for magnetic Sm and Nd in RECoAsO is both field and temperature dependent. For applied fields below 100Oe, both TN1 and TN2 are seen, with intermediate fields below 1-2kOe only TN1 and above say 5kOe the AFM transition is not observed. This is evidenced in isothermal magnetization (MH) plots as well. It is clear that Sm/Nd magnetic moments interact with the ordered Co spins in adjacent layer and thus transforms the FM ordering to AFM. All the studied compounds are metallic in nature, and their magneto-transport R(T)H follows the temperature and field dependent FM-AFM transition of ordered Co spins.
Point-contact (PC) Andreev-reflection (AR) measurements of the superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have been carried out. The value of the gap is distributed in the range 2Delta simeq 5-10 meV (for Re=Sm) with a
maximum in the distribution around 6 meV. Temperature dependence of the gap Delta(T) can be fitted well by BCS curve giving reduced gap ratio 2Delta /kT_c^*simeq 3.5 (here T_c^* is the critical temperature from the BCS fit). At the same time, an expected second larger gap feature was difficult to resolve distinctly on the AR spectra making determination reliability of the second gap detection questionable. Possible reasons for this and the origin of other features like clear-cut asymmetry in the AR spectra and current regime in PCs are discussed.
In order to understand why the inter- and intra-granular current densities of polycrystalline superconducting oxypnictides differ by three orders of magnitude, we have conducted combined magneto-optical and microstructural examinations of representat
ive randomly oriented polycrystalline Nd and Sm single-layer oxypnictides. Magneto optical images show that the highest Jc values are observed within single grains oriented with their c axes perpendicular to the observation plane, implying that the intragranular current is anisotropic. The much lower intergranular Jc is at least partially due to many extrinsic factors, because cracks and a ubiquitous wetting As-Fe phase are found at many grain boundaries. However, some grain boundaries are structurally clean under high resolution transmission electron microscopy examination. Because the whole-sample global Jc(5K) values of the two samples examined are 1000-4000 A/cm2, some 10-40 times that found in random, polycrystalline YBa2Cu3O7-x, it appears that the dominant obstruction to intergranular current flow of many present samples is extrinsic, though some intrinsic limitation of current flow across grain boundaries cannot yet be ruled out.
A new crystal growth technique for single-crystals of REFeAsO (RE = La, Ce, Pr, Nd, Sm, Gd, and Tb) using NaI/KI as flux is presented. Crystals with a size up to 300 $mu$m were isolated for single-crystal X-ray diffraction measurements. Lattice param
eters were determined by LeBail fits of X-ray powder data against LaB6 standard. A consistent set of structural data is obtained and interpreted in a hard-sphere model. Effective radii for the rare-earth metal atoms for REFeAsO are deduced. The relation of the intra- and inter-plane distances of the arsenic atoms is identified as limiter of the phase formation, and its influence on Tc is discussed.
Polycrystalline samples of Sr1-xRExFBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transit
ion temperatures (Tc) within the range of 2.1-2.7 K under ambient pressure, the estimated superconducting volume fraction was small. This indicated the non-bulk nature of superconductivity in these samples under ambient pressure. A dramatic evolution of the bulk superconducting phases was achieved on applying an external hydrostatic pressure. Near pressures below 1 GPa, bulk superconductivity was induced with a Tc of 2.5-2.8 K, which is termed as the low-P phase. Moreover, the high-P phase (Tc = 10.0-10.8 K) featuring bulk characteristics was observed at higher pressures. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Thus, this phase transition indicates a pressure-induced superconducting-superconducting transition similar to the transition in LaO0.5F0.5BiS2. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for evaluating the mechanisms of superconductivity in BiCh2-based superconductors.
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