6,181 research outputs found
The super-super exchange mechanism in iron-based antiperovskite chalco-halides
By using the first-principles electronic structure calculations, we have
systematically studied the magnetism in three recently synthesized iron-based
antiperovskite chalco-halides: Ba(FeS)Cl, Ba(FeS)Br, and
Ba(FeSe)Br. These compounds consist of edge-sharing Ba (=Cl or
Br) octahedra intercalated with isolated Fe (=S or Se) tetrahedra. We
find that even though the shortest distances between the nearest-neighboring Fe
atoms in these three compounds already exceed 6 \AA, much larger than the bond
length of a chemical bonding, they all remarkably show antiferromagnetic (AFM)
coupling along axis with very weak spin-spin correlation along axis.
Our study shows that the mechanism underlying this novel AFM coupling is such a
new type of exchange interaction between the nearest-neighboring Fe-based
super-moments mediated by Ba cations, which we call the super-super exchange
interaction, in which each magnetic Fe atom partially polarizes its four
nearest-neighboring atoms to form a super-moment through - orbital
hybridization and the atoms in neighboring Fe tetrahedra along
axis antiferromagnetically couple with each others through the intermediate Ba
cations. Different from the conventional superexchange, here it is cations
rather than anions that mediate two neighboring super-moments. According to the
calculated strength of the AFM coupling, we predict that among these compounds
the highest AFM phase transition temperature may reach 110 K in
Ba(FeSe)Br, in comparison with the observed s of 84 K in
Ba(FeS)Br and 95 K in Ba(FeS)Cl.Comment: 6 pages, 6 figures, 2 tables. Comments are welcom
Nematic antiferromagnetic states in bulk FeSe
We revisit bulk FeSe through the systematic first-principles electronic
structure calculations. We find that there are a series of staggered -mer
antiferromagnetic (AFM) states with corresponding energies below that of the
collinear AFM state which is the ground state for the parent compounds of most
iron-based superconductors. Here the staggered -mer ( any integer )
means that a set of adjacent spins parallel on a line along -axis with
spins in antiparallel between -mers and along -axis. Among them, the
lowest energy states are quasi-degenerate staggered dimer and staggered trimer
AFM states as well as their any staggered combinations. Thus, to have the
largest entropy to minimize the free energy at low temperature, the most
favorable state is such a quasi-one-dimensional antiferromagnet in which along
-axis a variety of -mers, mostly dimers and trimers, are randomly
antiparallel aligned while along -axis spins are antiparallel aligned, i.e.
actually a nematic paramagnet. This finding accounts well for the absence of
long-range magnetic order in bulk FeSe and meanwhile indicates the dominant
stripe spin fluctuation and the nematicity as spin-driven.Comment: 6 pages and 3 figures with Supplementary Material
Electronic structures of quasi-one-dimensional cuprate superconductors BaCuO
An intact CuO plane is widely believed to be a prerequisite for the
high- superconductivity in cuprate superconductors. However, an exception
may exist in the superconducting BaCuO materials where CuO
chains play a more important role. From first-principles density functional
theory calculations, we have studied the electronic and magnetic structures of
BaCuO. The stoichiometric BaCuO and BaCuO
contain quasi-one-dimensional CuO chains and intact two-dimensional CuO
planes, respectively. In comparison with the nonmagnetic metal BaCuO,
BaCuO is found to be an antiferromagnetic (AFM) Mott insulator. It
possesses a nearest-neighbor intra-chain antiferromagnetic (AFM) coupling and a
weak inter-chain interaction, and its lowest unoccupied band and highest
occupied band are contributed by Cu 3-orbital (or
-orbital if we denote the -plane as the -plane) and O
2-orbitals, respectively. Total energy calculations indicate that the oxygen
vacancies in BaCuO prefer to reside in the planar sites rather
than the apical oxygens in the CuO chains, in agreement with the experimental
observation. Furthermore, we find that the magnetic frustrations or spin
fluctuations can be effectively induced by moderate charge doping. This
suggests that the superconducting pairing in oxygen-enriched
BaCuO or oxygen-deficient BaCuO is likely to
be mainly driven by the AFM fluctuations within CuO chains.Comment: 7 pages, 7 figures, 3 table
First-principles study of magnetic frustration in FeSe epitaxial films on SrTiO
The effects of electron doping and phonon vibrations on the magnetic
properties of monolayer and bilayer FeSe epitaxial films on SrTiO have been
studied, respectively, using first-principles calculations with van der Waals
correction. For monolayer FeSe epitaxial film, the combined effect of electron
doping and phonon vibrations readily leads to magnetic frustration between the
collinear antiferromagnetic state and the checkerboard antiferromagnetic N\'eel
state. For bilayer FeSe epitaxial film, such magnetic frustration is much more
easily induced by electron doping in its bottom layer than its top layer. The
underlying physics is that the doped electrons are accumulated at the interface
between the FeSe layers and the substrate. These results are consistent with
existing experimental studies
Tuning the magnetism of the top-layer FeAs on BaFeAs(001): First-principles study
The magnetic properties of BaFeAs(001) surface have been studied
by using first-principles electronic structure calculations. We find that for
As-terminated surface the magnetic ground state of the top-layer FeAs is in the
staggered dimer antiferromagnetic (AFM) order, while for Ba-terminated surface
the collinear (single stripe) AFM order is the most stable. When a certain
coverage of Ba or K atoms are deposited onto the As-terminated surface, the
calculated energy differences among different AFM orders for the top-layer FeAs
on BaFeAs(001) can be much reduced, indicating enhanced spin
fluctuations. To identify the novel staggered dimer AFM order for the As
termination, we have simulated the scanning tunneling microscopy (STM) image
for this state, which shows a different pattern from
the case of half Ba coverage. Our results suggest: i) the magnetic properties
of the top-layer FeAs on BaFeAs(001) can be tuned effectively by
surface doping; ii) both the surface termination and the AFM order in the
top-layer FeAs can affect the STM image of BaFeAs(001).Comment: 6 pages, 5 figures, 1 tabl
Magnetic interactions in a proposed diluted magnetic semiconductor (BaK)(ZnMn)P
By using first-principles electronic structure calculations, we have studied
the magnetic interactions in a proposed BaZnP-based diluted magnetic
semiconductor (DMS). For a typical compound
Ba(ZnMn)P with only spin doping, due to the
superexchange interaction between Mn atoms and the lack of itinerant carriers,
the short-range antiferromagnetic coupling dominates. Partially substituting K
atoms for Ba atoms, which introduces itinerant hole carriers into the
orbitals of P atoms so as to link distant Mn moments with the spin-polarized
hole carriers via the - hybridization between P and Mn atoms, is very
crucial for the appearance of ferromagnetism in the compound. Furthermore,
applying hydrostatic pressure first enhances and then decreases the
ferromagnetic coupling in
(BaK)(ZnMn)P at a turning point
around 15 GPa, which results from the combined effects of the pressure-induced
variations of electron delocalization and - hybridization. Compared with
the BaZnAs-based DMS, the substitution of P for As can modulate the
magnetic coupling effectively. Both the results for BaZnP-based and
BaZnAs-based DMSs demonstrate that the robust antiferromagnetic (AFM)
coupling between the nearest Mn-Mn pairs bridged by anions is harmful to
improving the performance of this II-II-V based DMS materials.Comment: 7 pages, 6 figures, 1 table; Accepted by Chinese Physics B (2018
First-principles study of FeSe epitaxial films on SrTiO3
The discovery of high temperature superconductivity in FeSe films on SrTiO3
substrate has inspired great experimental and theoretical interests.
First-principles density functional theory calculations, which have played an
important role in the study of bulk iron-based superconductors, also
participate in the investigation of interfacial superconductivity. In this
article, we review the calculation results on the electronic and magnetic
structures of FeSe epitaxial films, emphasizing on the interplay between
different degrees of freedom, such as charge, spin, and lattice vibrations.
Furthermore, the comparison between FeSe monolayer and bilayer films on SrTiO3
is discussed.Comment: invited revie
An X-ray periodicity of 1.8 hours in a narrow-line Seyfert 1 galaxy Mrk 766
In the narrow-line Seyfert 1 galaxy Mrk 766, a Quasi-Periodic Oscillation
(QPO) signal with a period of s is detected in the \emph{XMM-Newton}
data collected on 2005 May 31. This QPO signal is highly statistical
significant at the confidence level at with the quality factor of
. The X-ray intensity changed by a factor of 3 with root
mean square fractional variability of . Furthermore, this QPO signal
presents in the data of all three EPIC detectors and two RGS cameras and its
frequency follows the - relation spanning from
stellar-mass to supermassive black holes. Interestingly, a possible QPO signal
with a period of s had been reported in the literature. The
frequency ratio of these two QPO signals is 3:2. Our result is also in
support of the hypothesis that the QPO signals can be just transient. The
spectral analysis reveals that the contribution of the soft excess component
below 1 keV is different between epochs with and without QPO, this
property as well as the former frequency-ratio are well detected in X-ray BH
binaries, which may have shed some lights on the physical origin of our event.Comment: 7 pages, 5 figures, 1 table. Accepted for publication in Ap
Perfect charge compensation in extremely large magnetoresistance materials LaSb and LaBi revealed by the first-principles calculations
By the first-principles electronic structure calculations, we have
systematically studied the electronic structures of recently discovered
extremely large magnetoresistance (XMR) materials LaSb and LaBi. We find that
both LaSb and LaBi are semimetals with the electron and hole carriers in
perfect balance. The calculated carrier densities in the order of
cm are in good agreement with the experimental values, implying long
mean free time of carriers and thus high carrier mobilities. With a
semiclassical two-band model, the perfect charge compensation and high carrier
mobilities naturally explain (i) the XMR observed in LaSb and LaBi; (ii) the
non-saturating quadratic dependence of XMR on external magnetic field; and
(iii) the resistivity plateau in the turn-on temperature behavior at very low
temperatures. The explanation of these features without resorting to the
topological effect indicates that they should be the common characteristics of
all perfectly electron-hole compensated semimetals.Comment: 7 pages, 7 figures, 1 tabl
Pressure-induced topological phase transition in LaSb: First-principles study
By using first-principles electronic structure calculations, we predict that
the extreme magnetoresistance (XMR) material LaSb takes a topological phase
transition without breaking any symmetry under a hydrostatic pressure applied
between 3 and 4 GPa, meanwhile the electron-hole compensation remains in its
electronic band structure. Thus LaSb provides an ideal platform for studying
the individual role of topological property playing in the XMR phenomenon, in
addition to the electron-hole compensation. This has general implication to the
relationship of XMR effect and topological property in topological materials.Comment: 6 pages, 4 figures, 2 table
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