Milinda Abeykoon

Phase Diagram of KxFe2-ySe2-zSz and the Suppression of its Superconducting State by an Fe2-Se/S Tetrahedron Distortion

We report structurally tuned superconductivity in a K(x)Fe(2-y)Se(2-z)S(z) (0 ≤ z ≤ 2) phase diagram. Superconducting T(c) is suppressed as S is incorporated into the lattice, eventually vanishing at 80% of S. The magnetic and conductivity properties can be related to stoichiometry on a poorly occupied Fe1 site and the local environment of a nearly fully occupied Fe2 site. The decreasing T(c) coincides with the increasing Fe1 occupancy and the overall increase in Fe stoichiometry from z = 0 to z = 2. Our results indicate that the irregularity of the Fe2-Se/S tetrahedron is an important controlling parameter that can be used to tune the ground state in the new superconductor family.

Iron chalcogenide superconductors at high magnetic fields

Abstract Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.

Quantitative nanostructure characterization using atomic pair distribution functions obtained from laboratory electron microscopes

Abstract Quantitatively reliable atomic pair distribution functions (PDFs) have been obtained from nanomaterials in a straightforward way from a standard laboratory transmission electron microscope (TEM). The approach looks very promising for making electron derived PDFs (ePDFs) a routine step in the characterization of nanomaterials because of the ubiquity of such TEMs in chemistry and materials laboratories. No special attachments such as energy filters were required on the microscope. The methodology for obtaining the ePDFs is described as well as some opportunities and limitations of the method.

Synthesis, Crystal Structure, and Magnetism of β-Fe1.00(2)Se1.00(3) Single Crystals

Understanding iron-based superconductors requires high-quality impurity-free single crystals. So far they have been elusive for {beta}-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic-impurity phases. Here, we report synchrotron-clean {beta}-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect-induced weak ferromagnetism that coexists with superconductivity below T{sub c}. In contrast to Fe{sub 1+y}Te-based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).

New In-Situ and Operando Facilities for Catalysis Science at NSLS-II: The Deployment of Real-Time, Chemical, and Structure-Sensitive X-ray Probes

The start of operations at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory heralded a new beginning for photon-science-based research capabilities in catalysis. This new facility builds on many years of pioneering work that was conducted at the NSLS synergistically by many scientists from academia, government labs, and industry. Over several decades, numerous discoveries in catalysis were driven through the emergence of an arsenal of tools at the NSLS that exploited the power of emerging X-ray methods encompassing scattering, spectroscopy, and imaging. In-situ and operando methodologies that coupled reactor environments directly with advanced analytical techniques paved a rapid path towards realizing an improved fundamental understanding at the frontiers of chemical science challenges of the day.

Spin-glass Behavior of Semiconducting KxFe2−yS2

We report the discovery of K{sub x}Fe{sub 2-y}S{sub 2} single crystals, isostructural to K{sub x}Fe{sub 2-y}S{sub 2} superconductors. The sulfide compound is a small gap semiconductor and shows spin-glass behavior below 32 K. Our results indicate that stoichiometry, defects, and the local environment of FeCh (Ch = S,Se) tetrahedra have important effects on the physical properties of isostructural and isoelectronic K{sub x}Fe{sub 2-y}S{sub 2} compounds.

Physical properties of KxNi2−ySe2 single crystals

We have synthesized K0.95(1)Ni1.86(2)Se2 single crystals. The single crystals contain K and Ni deficiencies not observed in KNi2Se2 polycrystals. Unlike KNi2Se2 polycrystals, the superconductivity is absent in single crystals. The detailed physical property study indicates that the K0.95Ni1.86Se2 single crystals exhibit heavy-fermion-like characteristics. The transition to a heavy fermion state below T ~ 30 K results in an enhancement of the electron-like carrier density whereas the magnetic susceptibility shows little anisotropy and suggests the presence of both itinerant and localized Ni orbitals.

Synthesis, crystal structure, and magnetism of β -Fe 1.00(2) Se 1.00(3) single crystals

Understanding iron-based superconductors requires high-quality impurity-free single crystals. So far they have been elusive for {beta}-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic-impurity phases. Here, we report synchrotron-clean {beta}-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect-induced weak ferromagnetism that coexists with superconductivity below T{sub c}. In contrast to Fe{sub 1+y}Te-based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).

Spin glass behavior of insulating K0.8Fe2-xS2

We report the discovery of K{sub x}Fe{sub 2-y}S{sub 2} single crystals, isostructural to K{sub x}Fe{sub 2-y}S{sub 2} superconductors. The sulfide compound is a small gap semiconductor and shows spin-glass behavior below 32 K. Our results indicate that stoichiometry, defects, and the local environment of FeCh (Ch = S,Se) tetrahedra have important effects on the physical properties of isostructural and isoelectronic K{sub x}Fe{sub 2-y}S{sub 2} compounds.

Thermoelectric studies of Ir1−xRhxTe2 (0≤x≤0.3)

Yu Liu, Hechang Lei, Kefeng Wang, Milinda Abeykoon, J. B. Warren, Emil Bozin, and C. Petrovic Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA Photon Science Division, National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA Instrumentation Division, Brookhaven National Laboratory, Upton, New York 11973, USA (Dated: September 27, 2018)