103 research outputs found
Quenched Fe Moment in the Collapsed Tetragonal Phase of CaPrFeAs
We report As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor CaPrFeAs (=0.075 and
0.15). The As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.Comment: 5 pages, 5 figure
The suppression of magnetism and the development of superconductivity within the collapsed tetragonal phase of Ca0.67Sr0.33Fe2As2 at high pressure
Structural and electronic characterization of (Ca0.67Sr0.33)Fe2As2 has been
performed as a func- tion of pressure up to 12 GPa using conventional and
designer diamond anvil cells. The compound (Ca0.67Sr0.33)Fe2As2 behaves
intermediate between its end members-CaFe2As2 and SrFe2As2- displaying a
suppression of magnetism and the onset of superconductivity. Like other members
of the AEFe2As2 family, (Ca0.67Sr0.33)Fe2As2 undergoes a pressure-induced
isostructural volume collapse, which we associate with the development of As-As
bonding across the mirror plane of the structure. This collapsed tetragonal
phase abruptly cuts off the magnetic state, giving rise to superconductivity
with a maximum Tc=22.2 K. The maximum Tc of the superconducting phase is not
strongly correlated with any structural parameter, but its proximity to the
abrupt suppression of magnetism as well as the volume collapse transition
suggests that magnetic interactions and structural inhomogeneity may play a
role in its development. The pressure-dependent evolution of the ordered states
and crystal structures in (Ca,Sr)Fe2As2 provides an avenue to understand the
generic behavior of the other members of the AEFe2As2 family.Comment: 9 pages, 9 figure
Structural collapse and superconductivity in rare earth-doped CaFe2As2
Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2
single-crystals is used to fine-tune structural, magnetic and electronic
properties of this iron-based superconducting system. Neutron and single
crystal x-ray scattering experiments indicate that an isostructural collapse of
the tetragonal unit cell can be controllably induced at ambient pressures by
choice of substituent ion size. This instability is driven by the interlayer
As-As anion separation, resulting in an unprecedented thermal expansion
coefficient of K. Electrical transport and magnetic
susceptibility measurements reveal abrupt changes in the physical properties
through the collapse as a function of temperature, including a reconstruction
of the electronic structure. Superconductivity with onset transition
temperatures as high as 47 K is stabilized by the suppression of
antiferromagnetic order via chemical pressure, electron doping or a combination
of both. Extensive investigations are performed to understand the observations
of partial volume-fraction diamagnetic screening, ruling out extrinsic sources
such as strain mechanisms, surface states or foreign phases as the cause of
this superconducting phase that appears to be stable in both collapsed and
uncollapsed structures.Comment: 15 pages, 18 figure
Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K
In this paper we report the superconductivity above 40 K in the electron
doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction
patterns indicate high crystalline quality and c-axis orientation. the
resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by
partial replacement of Ca by rare earth and a superconducting transition
reaches as high as 43 K, which is higher than the value in electron doping
FeAs-122 compounds by substituting Fe ions with transition metal, even
surpasses the highest values observed in hole doping systems with a transition
temperature up to 38 K. The upper critical field has been determined with the
magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3.
Hall-effect measurements indicate that the conduction in this material is
dominated by electron like charge carriers. Our results explicitly demonstrate
the feasibility of inducing superconductivity in Ca122 compounds via electron
doping using aliovalent rare earth substitution into the alkaline earth site,
which should add more ingredients to the underlying physics of the iron-based
superconductors.Comment: 21 pages, 7 figure
FeAs-based superconductivity: a case study of the effects of transition metal doping on BaFe2As2
The recently discovered FeAs-based superconductors are a new, promising set
of materials for both technological as well as basic research. They offer
transition temperatures as high as 55 K as well as essentially isotropic and
extremely large upper, superconducting critical fields in excess of 40 T at 20
K. In addition they may well provide insight into exotic superconductivity that
extends beyond just FeAs-based superconductivity, perhaps even shedding light
on the still perplexing CuO-based high-Tc materials. Whereas superconductivity
can be induced in the RFeAsO (R = rare earth) and AEFe2As2 (AE = Ba, Sr, Ca))
families by a number of means, transition metal doping of BaFe2As2, e.g.
Ba(Fe1-xTMx)2As2, offers the easiest experimental access to a wide set of
materials. In this review we present an overview and summary of the effect of
TM doping (TM = Co, Ni, Cu, Pd, and Rh) on BaFe2As2. The resulting phase
diagrams reveal the nature of the interaction between the structural, magnetic
and superconducting phase transitions in these compounds and delineate a region
of phase space that allows for the stabilization of superconductivity.Comment: edited and shortened version is accepted to AR:Condensed Matter
Physic
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