Susannah Speller

Large Single Crystals of Graphene on Melted Copper Using Chemical Vapor Deposition

Summary form only given. A simple one-step method is presented for synthesizing large single crystal graphene domains on melted copper using atmospheric pressure chemical vapour deposition (CVD). This is achieved by performing the reaction above the melting point of copper (1090 °C) and using a molybdenum support to prevent balling of the copper from dewetting. By controlling the amount of hydrogen during growth, individual single crystal domains of monolayer graphene greater than 200 μm are produced, determined by electron diffraction mapping. Angular resolved photoemission spectroscopy is used to show the graphene grown on copper exhibits a linear dispersion relationship and has no sign of doping.

Study of the structural, electric and magnetic properties of Mn-doped Bi2Te3 single crystals

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ~ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.

Microstructural analysis of phase separation in iron chalcogenide superconductors

The interplay between superconductivity, magnetism and crystal structure in iron-based superconductors is a topic of great interest amongst the condensed matter physics community as it is thought to be the key to understanding the mechanisms responsible for high temperature superconductivity. Alkali metal doped iron chalcogenide superconductors exhibit several unique characteristics which are not found in other iron-based superconducting materials such as antiferromagnetic ordering at room temperature, the presence of ordered iron vacancies and high resistivity normal state properties. Detailed microstructural analysis is essential in order to understand the origin of these unusual properties. Here we have used a range of complementary scanning electron microscope based techniques, including high-resolution electron backscatter diffraction mapping, to assess local variations in composition and lattice parameter with high precision and sub-micron spatial resolution. Phase separation is observed in the CsxFe2?ySe2?crystals, with the minor phase distributed in a plate-like morphology throughout the crystal. Our results are consistent with superconductivity occurring only in the minority phase.

Persistent current joints between technological superconductors

Persistent current joints are crucial components of superconducting magnets—enabling the production of the high and ultra-stable magnetic fields required, for instance, for magnetic resonance measurements. At this critical juncture when persistent mode magnets containing commercial high temperature superconductors may soon become a reality, it is of value to take stock and evaluate current challenges faced in the field of jointing. This paper provides a review of progress made to date on the production and characterization of joints between the five major technological superconductors—NbTi, Nb3Sn, MgB2, BiSCCO and REBCO, including the materials that are used to make these joints.

Analysis of local chemical and structural inhomogeneities in FeySe1−xTex single crystals

Co-existence of superconductivity and magnetism has been reported in certain regions of the Fe(Se,Te) phase diagram. Here, we address the key question of whether these different properties occur simultaneously within a homogeneous crystal or whether a “two-phase” description is more appropriate. Extensive microstructural analysis suggests that a Te-rich sample exhibiting both bulk superconductivity and magnetic order contains significant chemical and structural inhomogeneity, suggesting that the “two-phase” explanation may be valid in this sample. The high-resolution electron backscatter diffraction (HR-EBSD) technique has been used to map local changes in lattice parameter very precisely in these compounds.

The processing and properties of single grain Y?Ba?Cu?O fabricated from graded precursor powders

The preparation of single grain, Y?Ba?Cu?O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7?? (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40?wt% Y-211 in the vicinity of the seed, which decreased to 30?wt% and then 20?wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30?wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Analysis of FeySe1 − xTex thin films grown by radio frequency sputtering

Superconducting thin films of FeySe1 − xTex have been successfully grown on a variety of single-crystal substrate materials by radio frequency sputtering, and the effect of processing conditions on the microstructural development and superconducting properties has been investigated. The degree of unit cell anisotropy ( ratio), which is known to be linked to the superconducting transition temperature, is found to be sensitive to both Te and Fe content. However, the Te and Fe content cannot be varied independently, as the stable concentration of excess Fe is found to increase with Te content. Large scale compositional variations are found to exist in some samples, and it is believed that sub-micron scale chemical inhomogeneities are present in all samples. Post-deposition heat treatments carried out on the thin films indicate that the ternary Fe(Se, Te) phase is not very thermodynamically stable, partially transforming into Fe(Se, Te)2 upon heating at modest temperatures for short times.

High-resolution characterization of microstructural evolution inRbxFe2−ySe2crystals on annealing

The superconducting and magnetic properties of phase-separated A

Individual grain boundary properties and overall performance of metal-organic deposition coated conductors

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Buffer layers for Tl-2212 thin films on MgO and sapphire substrates

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