132 research outputs found
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 di raction mapping, to assess local variations in composition and
lattice parameter with high precision and sub-micron spatial resolution. Phase
separation is observed in the Csx Fe2-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.Comment: Accepted for publication in a special edition of Supercond. Sci.
Techno
Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2
We report on the magnetic properties of CsCoSe with ThCrSi
structure, which we have characterized through a series of magnetization and
neutron diffraction measurements. We find that CsCoSe2 undergoes a
phase transition to an antiferromagnetically ordered state with a N\'eel
temperature of 66 K. The nearest neighbour interactions are
ferromagnetic as observed by the positive Curie-Weiss temperature of 51.0 K. We find that the magnetic structure of CsCoSe consists
of ferromagnetic sheets, which are stacked antiferromagnetically along the
tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic
order. The observed magnitude of the ordered magnetic moment at = 1.5 K is
found to be only 0.20(1)/Co. Already in comparably small
magnetic fields of (5K) 0.3 T, we observe a
metamagnetic transition that can be attributed to spin-rearrangements of
CsCoSe, with the moments fully ferromagnetically saturated in a
magnetic field of (5K) 6.4 T. We discuss the entire
experimentally deduced magnetic phase diagram for CsCoSe with respect
to its unconventionally weak magnetic coupling. Our study characterizes
CsCoSe, which is chemically and electronically posed closely to the
superconductors, as a host of versatile magnetic
interactions
Superconductivity in a new layered bismuth oxyselenide: LaO0.5F0.5BiSe2
We report superconductivity in a new layered bismuth oxyselenide
LaO0.5F0.5BiSe2 with the ZrCuSiAs-type structure composed of alternating
superconducting BiSe2 and blocking LaO layers. The superconducting transition
temperature is TC = 2.6K, as revealed from DC magnetization, resistivity and
muon spin rotation (muSR) experiments. DC magnetization measurements indicate a
superconducting volume fraction of approximately 80%, which is at least twice
higher in comparison to that found in corresponding sulphide LaO0.5F0.5BiS2.
Importantly, the bulk character of superconductivity in LaO0.5F0.5BiSe2 was
confirmed by muSR
High resolution characterisation of microstructural evolution in RbFeSe crystals on annealing
The superconducting and magnetic properties of phase-separated
AFeSe compounds are known to depend on post-growth heat
treatments and cooling profiles. This paper focusses on the evolution of
microstructure on annealing, and how this influences the superconducting
properties of RbFeSe crystals. We find that the minority phase in
the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb
content and increased Fe content compared to the matrix. The microstructure is
rather complex, with two-phase mesoscopic plate-shaped features aligned along
{113} habit planes. The minority phase are strongly facetted on the {113}
planes, which we have shown to be driven by minimising the volume strain energy
introduced as a result of the phase transformation. Annealing at 488K results
in coarsening of the mesoscopic plate-shaped features and the formation of a
third distinct phase. The subtle differences in structure and chemistry of the
minority phase(s) in the crystals are thought to be responsible for changes in
the superconducting transition temperature. In addition, scanning photoemission
microscopy has clearly shown that the electronic structure of the minority
phase has a higher occupied density of states of the low binding energy Fe3d
orbitals, characteristic of crystals that exhibit superconductivity. This
demonstrates a clear correlation between the Fe-vacancy-free phase with high
c/a ratio and the electronic structure characteristics of the superconducting
phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the
abstract have been correcte
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