106 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
Pressure cycle of superconducting Cs0.8Fe2Se2: a transport study
We report measurements of the temperature and pressure dependence of the
electrical resistivity of single crystalline iron-based chalcogenide
Cs0.8Fe2Se2. In this material superconductivity Tc~30K develops from a normal
state with extremely large resistivity. At ambient pressure a large "hump" in
the resistivity is observed around 200K. Under pressure, the resistivity
decreases by two orders of magnitude, concomitant with a sudden Tc suppression
around p~8GPa. Even at 9GPa a metallic resistivity state is not recovered, and
the {\rho}(T) "hump" is still detected. A comparison of the data measured upon
increasing and decreasing the external pressure leads us to suggest that
superconductivity is not related to this hump
Phase co-existence and structured diffuse scattering seen by X-ray 3D mapping of reciprocal space for Cs0.8Fe1.6Se2
AxFe2-ySe2 (A=K, Rb, Cs) superconductors may show a complex mixture of
structural phases even in a single crystal form. A full sphere of the
diffraction intensity has been collected, with the help of synchrotron
radiation, for Cs0.8Fe1.6Se2 . In addition to the expected pattern for the
tetragonal phase with ordered Fe vacancies, a diffuse scattering from Cs
occupational disorder has been observed, together with an extra Bragg
contribution from a minor phase. The minor phase, in agreement with previous
findings, is compressed in the tetragonal a-b plane and expanded along the
c-direction; a set of modulated Bragg rods evidences a planar disorder.
Fourfold splitting of the rods as well as the main Bragg peaks for L\neq0 imply
that symmetry of the minor phase is not higher than monoclinic. The monoclinic
distortion was estimated to be 90.7 degrees. Structured diffuse scattering,
observed on top of the Bragg component, relates to the major phase and is
attributed to a correlated distribution of Cs ions. Elastic nature of the
observed scattering was confirmed by additional inelastic X-ray scattering
experiment. Diffuse scattering forms 3D objects that have topological
similarity with what one would expect, under assumptions discussed in the text,
for a Fermi surface nesting.Comment: 17 pages, 6 figure
- …