Unabridged phase diagram for single-phased FeSexTe1-x thin films

A complete phase diagram and its corresponding physical properties are essential prerequisites to understand the underlying mechanism of iron based superconductivity. For the structurally simplest 11 (FeSeTe) system, earlier attempts using bulk samples have not been able to do so due to the fabrication difficulties. Here, thin FeSexTe1-x films with the Se content covering the full range were fabricated by using pulsed laser deposition method. Crystal structure analysis shows that all films retain the tetragonal structure in room temperature. Significantly, the highest superconducting transition temperature (TC = 20 K) occurs in the newly discovered domain, 0.6 - 0.8. The single-phased superconducting dome for the full Se doping range is the first of its kind in iron chalcogenide superconductors. Our results present a new avenue to explore novel physics as well as to optimize superconductors

Investigation of Electron-Phonon Coupling in Epitaxial Silicene by In-situ Raman Spectroscopy

In this letter, we report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on Ag(111) surface was probed by an in-situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at edge sites in the silicene were identified. Detecting relationship between EPC and Dirac fermion through the Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.Comment: 15 pages, 4 figure

Reaction method control of impurity scattering in C-doped MgB2: proving the role of defects besides C substitution level

In this study, Si and C were incorporated into polycrystalline MgB 2 via in situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The electrical transport and magnetic properties of the two series of samples were compared. The corrected resistivity at 40 K, ρA(40 K), is higher for the samples reacted with SiC regardless of the carbon (C) substitution level, indicating larger intragrain scattering because of the simultaneous reaction between Mg and SiC and carbon substitution during the formation of MgB2. In addition, because of the cleaner reaction route for the samples reacted with SiC, the calculated active area that carries current, AF, is twice that of the (Si+C) samples. On the other hand, the upper critical field, Hc2, was similar for both sets of samples despite their different C substitution levels, which proves the importance of defect scattering in addition to C substitution level. Hence, the form of the precursor reactants is critical for tuning the form of Hc2(T)

Magnetic field processing to enhance critical current densities of MgB2 superconductors

A magnetic field of up to 12T was applied during the sintering process of pure MgB2 and carbon nanotube(CNT)dopedMgB2wires. The authors have demonstrated that magnetic field processing results in grain refinement, homogeneity, and enhancement in Jc(H) and Hirr. The extent of improvement in Jc increases with increasing field. The Jc for a 10T field processed CNTdoped sample increases by a factor of 3 at 10K and 8T and at 20K and 5T, respectively. Hirr for the 10T field processed CNTdoped sample reached 9T at 20K, which exceeded the best value of SiC dopedMgB2 at 20K. Magnetic field processing reduces the resistivity in CNTdopedMgB2, straightens the entangled CNTs, and improves the adherence between CNTs and the MgB2 matrix

Processing and characterisation of nano carbon doped MgB2 form of wire and bulk

The objective of this work is to further enhance the critical current density of the MgB2 superconductor by doping with the two carbon sources: multiwalled carbon nanotube (CNT) and nano carbon. The work in this thesis concentrates on the fabrication and characterization on the CNT and nano C doped MgB2 with main objective being the enhancement of the critical current density in the high magnetic field. Consequently, introducing effective pinning centres in the form of dopants to enhance the flux pinning will be the main task of this project. In this project, the effect of carbon doping MgB2 with carbon nanotubes and nano C on transition temperature, lattice parameters, critical current density and flux pinning for MgB2-xCx with x = 0, 0.05, 0.1, 0.2 and 0.3 under the various condition was studied. Both types of doping showed excellent Jc compared to the pure MgB2, with significant enhancement observed at higher temperature. Magnetic Jc(H) was enhanced by a factor of 72 at 5K for a field 8T and a factor of 33 at 20K for a field of 5T for nano C bulk samples, respectively. On the other hand, Jc(H) of CNT samples was enhanced by a factor of 26 and 13 under the equivalent conditions. In high field, transport Jc of magnitude 2122 A/cm2 and 3821 A/cm2 was observed at 4.2K and 12T for CNT and nano C doped MgB2. These results indicate that flux pinning was enhanced by the boron substitution for carbon with increasing processing temperature. However, it was found that the lattice distortion and optimum doping level is different in the CNT and nano C samples which is due to the reactivity of the carbon source, resulting in different carbon substitution rate. Due to better reactivity and homogenous mixing of nano C, nano C doped MgB2 resulted in better improvement in magnetic and transport Jc(H), as compared to CNT doped MgB2. This is mainly because CNT fibres with high aspect ratio tend to entangle, which suppressed the reactivity. The depression of Tc, which is caused by the boron substitution for carbon, increases with increasing the doping level, processing temperature and duration for both types of carbon doping. By controlling the extent of the substitution and inclusion of carbon, we can achieve the optimal improvement of critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. In addition, the values of Hc2 and Hirr are higher for CNT doped samples than for the pure MgB2 at the same value of T/Tc. The morphology of the CNT doped MgB2 is similar to that of nano C doped MgB2, but different from the pure MgB2. The microstructure exhibits noticeable nanoparticles with size around 10-20nm, which are believed to be MgO and MgB2. Magnetization measurements indicate a change in the critical current density with the length of nanotube and not with its outside diameter. This is due to longer nanotubes tending to entangle with each other, preventing their homogenous mixing with MgB2 and dispersion. Low intensity ultrasonication, as a method of dispersion of CNT’s into precursor magnesium and boron powder, was introduced to improve homogeneity of mixing of CNT’s with the MgB2 matrix. Ultrasonication of CNT doped MgB2 resulted in a significant enhancement in the field dependence of critical current density, while avoiding the side-effects that would occur at higher processing temperatures. Carbon nanotubes (CNT’s) have unusual electrical, mechanical and thermal properties. The elongated CNT’s induce anisotropy in Jc in relation to the direction of applied field in MgB2/Fe wires and the value of Jc for the carbon nanotube-doped wires is insensitive to heating rates. We believe that by taking the extraordinary electrical, mechanical and thermal properties of CNT’s, the mechanical properties and thermal stability of CNT doped wire will be substantially improved. Studies on these properties are underway

Enhancement of Hc2 and Jc by carbon-based chemical doping

In the past 5 years, various kinds of doping of MgB2, including single elements (metal and non-metal), silicates, various carbon sources, and other compounds have been investigated and reported. Most nanoparticle doping leads to improvement of critical current density, Jc(H), and performance, but some types show a negative effect. In this paper, the effect of carbon doping on Jc and the upper critical field, Hc2, of MgB2 is reviewed. Carbon substitution effects make two distinguishable contributions to the enhancement of Jc field performance: increase of Hc2 and improvement of flux pinning, both because carbon substitutes for boron in the MgB2 lattice. Among all the carbon sources so far, nano-SiC has been confirmed to be the most effective dopant to enhance the Jc in magnetic fields and Hc2. An irreversibility field, Hirr, of 10 T has been achieved with nano-SiC doping at 20 K, exceeding Hirr of NbTi at 4.2 K. Besides that, Hc2 of carbon alloyed MgB2 film has reached the value of 71 T. The significant enhancement in Jc(H) and Hc2 via carbon substitution has provided great potential for practical applications of MgB2. The dual reaction model proposed by the authors group provides a comprehensive understanding of the mechanism of enhancement in Jc and Hc2 by chemical doping. Further improvement in self-field Jc performance while maintaining the already achieved in-field performance remains as a major challenge in the development of MgB2

Improved Jc of MgB2 superconductor by ball milling using different media

In this paper, the effects of ball milling B powders using different media, such as acetone, ethanol, and toluene on the superconducting properties of MgB2 have been studied. It was observed that toluene medium was the most effective of them all for enhancing Jc. Jc was estimated to be 5 x 103 A/cm2 at 8 T and 5 K. This value is much higher than that of pure MgB2 that was not ball milled, by a factor of 20. It was considered that ball-milled B using toluene leads to smaller MgB2 grains, resulting in enhanced Jc at low operating temperature and high field

Specific heat and magnetic relaxation analysis of MgB2 bulk samples with and without additives

The effects of SiC and Carbon doping on the superconducting properties of MgB2 polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of Birr and Jc. However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB2. It follows that doping acts mainly introducing disorder into the superconductor and thus raising Bc2. In the case of MgB1.9C0.1, specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of Tc determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C

Enhancement of critical current density and irreversibility field by nano carbon substitution in MgB2

We have investigated the effect of nano-carbon doping on phase formation, critical current density, flux pinning and irreversibility field in MgB2-x bulk sample where x = 0, 0.05, 0.1, 0.2 and 0.3. XRD results showed a systematic shift of lattice parameter a, which is attributed to carbon substitution, while the lattice parameter c almost did not change as the carbon content increased. Enhancement of magnetic J(c) by two orders at 5 K and 8 T was observed for the sample of x = 0.1, as the J(c)(H) reached the value of \u3e 10(4) at 8 T. In addition, the H-irr of the 5% doped carbon was enhanced from 4.9 T to 5.8 T as compared to the undoped sample. Boron substitution for carbon is proposed to be responsible for the enhancement of flux pinning. (c) 2007 Elsevier B.V. All rights reserved