22 research outputs found
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation
Muon spin rotation and relaxation studies have been performed on a "111"
family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was
characterized by obtaining the temperature and doping dependences of the local
ordered magnetic moment size and the volume fraction of the magnetically
ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long
range magnetically ordered state is observed, while for higher x than 0.4 %
magnetic order becomes more disordered and is completely suppressed for x = 1.5
%. The magnetic volume fraction continuously decreases with increasing x. The
combination of magnetic and superconducting volumes implies that a
spatially-overlapping coexistence of magnetism and superconductivity spans a
large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction
of both the ordered moment size and the volume fraction is observed below the
superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron
pnictides in which one of these two parameters exhibits a reduction below TC,
but not both. The suppression of magnetic order is further enhanced with
increased Ni doping, leading to a reentrant non-magnetic state below T_C for x
= 1.3 %. The reentrant behavior indicates an interplay between
antiferromagnetism and superconductivity involving competition for the same
electrons. These observations are consistent with the sign-changing s-wave
superconducting state, which is expected to appear on the verge of microscopic
coexistence and phase separation with magnetism. We also present a universal
linear relationship between the local ordered moment size and the
antiferromagnetic ordering temperature TN across a variety of iron-based
superconductors. We argue that this linear relationship is consistent with an
itinerant-electron approach, in which Fermi surface nesting drives
antiferromagnetic ordering.Comment: 20 pages, 14 figures, Correspondence should be addressed to Prof.
Yasutomo Uemura: [email protected]
Tunable vortex Majorana zero modes in LiFeAs superconductor
The recent realization of pristine Majorana zero modes (MZMs) in vortices of
iron-based superconductors (FeSCs) provides a promising platform for
long-sought-after fault-tolerant quantum computation. A large topological gap
between the MZMs and the lowest excitations enabled detailed characterization
of vortex MZMs in those materials. Despite those achievements, a practical
implementation of topological quantum computation based on MZM braiding remains
elusive in this new Majorana platform. Among the most pressing issues are the
lack of controllable tuning methods for vortex MZMs and inhomogeneity of the
FeSC Majorana compounds that destroys MZMs during the braiding process. Thus,
the realization of tunable vortex MZMs in a truly homogeneous compound of
stoichiometric composition and with a charge neutral cleavage surface is highly
desirable. Here we demonstrate experimentally that the stoichiometric
superconductor LiFeAs is a good candidate to overcome these two obstacles.
Using scanning tunneling microscopy, we discover that the MZMs, which are
absent on the natural surface, can appear in vortices influenced by native
impurities. Our detailed analysis and model calculations clarify the mechanism
of emergence of MZMs in this material, paving a way towards MZMs tunable by
controllable methods such as electrostatic gating. The tunability of MZMs in
this homogeneous material offers an unprecedented platform to manipulate and
braid MZMs, the essential ingredients for topological quantum computation.Comment: 21 pages, 10 figures. Suggestions and comments are welcom
Two distinct superconducting states controlled by orientation of local wrinkles in LiFeAs
We observe two types of superconducting states controlled by orientations of
local wrinkles on the surface of LiFeAs. Using scanning tunneling
microscopy/spectroscopy, we find type-I wrinkles enlarge the superconducting
gaps and enhance the transition temperature, whereas type-II wrinkles
significantly suppress the superconducting gaps. The vortices on wrinkles show
a C2 symmetry, indicating the strain effects on the wrinkles. A discontinuous
switch of superconductivity occurs at the border between two different
wrinkles. Our results demonstrate that the local strain effect could affect
superconducting order parameter of LiFeAs with a possible Lifshitz transition,
by alternating crystal structure in different directions.Comment: 21 pages, 9 figure
Realized C–H Functionalization of Aryldiazo Compounds via Rhodium Relay Catalysis
An
unprecedented C–H functionalization of aryldiazo compounds
without a preinstallation of directing group has been realized under
mild conditions, which differs from former reports in its use of diazo
compounds as coupling partners in directed C–H activations.
This novel transformation has been realized by a rhodium self-relay
catalysis, a tandem process of the in situ formation of a directing
group and sequential C–H bond activation
Crystal Growth and Investigation of High-Pressure Physical Properties of Fe2As
We reported the growth of Fe2As single crystals and the study of its physical properties via comprehensive measurements, such as transport properties under pressure and high-pressure synchrotron radiation X-ray diffraction. Fe2As is an antiferromagnetic metal with TN ~ 355 K. Within the pressure range of 100 GPa, no superconductivity was observed above 2 K. The abrupt drop in resistance from 21 to 31.7 GPa suggests a high-pressure phase transition happens. The high-pressure X-ray experiments indicate a new high-pressure phase appears, starting from 27.13 GPa. After the refinement of the high-pressure X-ray data, the pressure dependence of lattice constants of Fe2As (P4/nmm phase) was plotted and the bulk modulus B0 was obtained to be 168.6 GPa