Abstract
A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe
2
As
2
, which exhibits twin-domains without uniaxial ...strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature
T
s
. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature
T
N
( <
T
s
) and create in-plane resistivity anisotropy above
T
s
. Here we use neutron polarization analysis to show that such a strain on BaFe
2
As
2
also induces a static or quasi-static out-of-plane (
c
-axis) AF order and its associated critical spin fluctuations near
T
N
/
T
s
. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe
2
As
2
actually rotates the easy axis of the collinear AF order near
T
N
/
T
s
, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.
We use inelastic neutron scattering to study spin waves in the antiferromagnetic ordered phase of iron pnictide NaFeAs throughout the Brillouin zone. Comparing with the well-studied AFe sub(2)As ...sub(2)(A = Ca, Sr, Ba) family, spin waves in NaFeAs have considerably lower zone boundary energies and more isotropic effective in-plane magnetic exchange couplings. These results are consistent with calculations from a combined density functional theory and dynamical mean field theory and provide strong evidence that pnictogen height controls the strength of electron-electron correlations and consequently the effective bandwidth of magnetic excitations.
The parent compounds of iron pnictide superconductors are bad metals with a collinear antiferromagnetic structure and Néel temperatures below 220 K. Although alkaline iron selenide Ay Fe1.6+x Se2 ...(A=K, Rb, Cs) superconductors are isostructural with iron pnictides, in the vicinity of the undoped limit they are insulators, forming a block antiferromagnetic order and having Néel temperatures of roughly 500 K. Here we show that the spin waves of the insulating antiferromagnet Rb0.89 Fe1.58 Se2 can be accurately described by a local moment Heisenberg Hamiltonian. A fitting analysis of the spin wave spectra reveals that the next-nearest neighbour couplings in Rb0.89 Fe1.58 Se2 , (Ba,Ca,Sr)Fe2 As2 , and Fe1.05 Te are of similar magnitude. Our results suggest a common origin for the magnetism of all the Fe-based superconductors, despite having different ground states and antiferromagnetic orderings.
Topological vortices with swirling ferroelectric, magnetic and structural anti-phase relationship in hexagonal RMnO3 (R = Ho to Lu, Y, and Sc) have attracted much attention because of their ...intriguing behaviors. Herein, we report the structure of multiferroic vortex domains in YMnO3 at atomic scale using state-of-the-art aberration-corrected scanning transmission electron microscopy (STEM). Two types of displacements were identified among six domain walls (DWs); six translation-ferroelectric domains denoted by α+, γ-, β+, α-, γ+ and β-, respectively, were recognized, demonstrating the interlocking nature of the anti-vortex domain. We found that the anti-vortex core is about four unit cells wide. In addition, we reconstructed the vortex model with three swirling pairs of DWs along the 001 direction. These results are very critical for the understanding of topological behaviors and unusual properties of the multiferroic vortex.
We use neutron scattering to study Vanadium (hole)-doped LiFe1−xVxAs. In the undoped state, LiFeAs exhibits superconductivity at Tc = 18 K and transverse incommensurate spin excitations similar to ...electron overdoped iron pnictides. Upon Vanadium doping to form LiFe0.955V0.045, the transverse incommensurate spin excitations in LiFeAs transform into longitudinally elongated ones in a similar fashion to that of potassium (hole)-doped Ba0.7K0.3Fe2As2 but with dramatically enhanced magnetic scattering and elimination of superconductivity. This is different from the suppression of the overall magnetic excitations in hole-doped BaFe2As2 and the enhancement of superconductivity near optimal hole doping. These results are consistent with density function theory plus dynamic mean field theory calculations, suggesting that Vanadium doping in LiFeAs may induce an enlarged effective magnetic moment Seff with a spin crossover ground state arising from the inter-orbital scattering of itinerant electrons.
Abstract
Superconductivity in BaFe
2
(As
1−
x
P
x
)
2
iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at
T
s
and ...three-dimensional (3D) collinear antiferromagnetic order at
T
N
(
T
s
=
T
N
) are gradually suppressed with increasing
x
, reaching optimal superconductivity around
x
= 0.30 with
T
c
≈ 30 K. Here we show that a moderate uniaxial pressure along the
c
-axis in BaFe
2
(As
0.70
P
0.30
)
2
spontaneously induces a 3D collinear antiferromagnetic order with
T
N
=
T
s
> 30 K, while only slightly suppresses
T
c
. Although a ~ 400 MPa pressure compresses the
c
-axis lattice while expanding the in-plane lattice and increasing the nearest-neighbor Fe–Fe distance, it barely changes the average iron-pnictogen height in BaFe
2
(As
0.70
P
0.30
)
2
. Therefore, the pressure-induced antiferromagnetic order must arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D nematic phase is a competing state with superconductivity.
Evaluation of transmitting performance of cylindrical polycapillary Xiaoyan, Lin; Yude, Li; Guotai, Tan ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2007, Letnik:
572, Številka:
2
Journal Article
Recenzirano
Based on a detailed ray-tracing code for capillary optics, a MATLAB program for the simulation of X-ray transmission in a cylindrical polycapillary is described. The simulated and experimental ...results for the spatial distributions and power density gain of the X-rays in the beam guided through a cylindrical polycapillary are in good agreement, and the results show that the spatial distribution of the X-rays in the beam guided through a cylindrical polycapillary is uneven.
Zinc nanoplates were grown using thermal evaporation without catalyst or template involved. Tunneling electron microscopy and selected area electron diffraction analyses showed that the plates were ...single crystals with either {0001 } or { 11-20} as basal surfaces. The morphological characteristics were explained in terms of the intrinsic growth anisotropy of zinc, the surface energy of the nano-crystals, the size of the critical nucleus and the migration of the adatoms. Our results suggested a promising low-cost route for synthesis of pure zinc nanoplates which could be used as precursor for further preparing core-shell nanoplate structures.
Triangular lattice of rare-earth ions with interacting effective spin-1/2local moments is an ideal platform to explore the physics of quantum spin liquids (QSLs) in the presence of strong spin-orbit ...coupling, crystal electric fields, and geometrical frustration. The Yb delafossites,NaYbCh2(Ch=O, S, Se) with Yb ions forming a perfect triangular lattice, have been suggested to be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance, and powder-sample neutron scattering measurements onNaYbCh2have supported the suggestion of the QSL ground states. The key signature of a QSL, the spin excitation continuum, arising from the spin quantum number fractionalization, has not been observed. Here we perform both elastic and inelastic neutron scattering measurements as well as detailed thermodynamic measurements on high-quality single-crystalNaYbSe2samples to confirm the absence of long-range magnetic order down to 40 mK, and further reveal a clear signature of magnetic excitation continuum extending from 0.1 to 2.5 meV. The comparison between the structure of the magnetic excitation spectra and the theoretical expectation from the spinon continuum suggests that the ground state ofNaYbSe2is a QSL with a spinon Fermi surface.
High-temperature superconductivity is closely adjacent to a long-range antiferromagnet, which is called a parent compound. In cuprates, all parent compounds are alike and carrier doping leads to ...superconductivity, so a unified phase diagram can be drawn. However, the properties of parent compounds for iron-based superconductors show significant diversity and both carrier and isovalent dopings can cause superconductivity, which casts doubt on the idea that there exists a unified phase diagram for them. Here we show that the ordered moments in a variety of iron pnictides are inversely proportional to the effective Curie constants of their nematic susceptibility. This unexpected scaling behavior suggests that the magnetic ground states of iron pnictides can be achieved by tuning the strength of nematic fluctuations. Therefore, a unified phase diagram can be established where superconductivity emerges from a hypothetical parent compound with a large ordered moment but weak nematic fluctuations, which suggests that iron-based superconductors are strongly correlated electron systems.
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