89 research outputs found
From electronic structure to magnetism and skyrmions (Topical review)
Solid state theory, density functional theory and its generalizations for
correlated systems together with numerical simulations on supercomputers allow
nowadays to model magnetic systems realistically and in detail and can be even
used to predict new materials, paving the way for more rapid material
development for applications in energy storage and conversion, information
technologies, sensors, actuators etc. Modelling magnets on different length
scales (between a few \r{A}ngstr\"om and several micrometers) requires,
however, approaches with very different mathematical formulations. Parameters
defining the material in each formulation can be determined either by fitting
experimental data or from theoretical calculations and there exists a
well-established approach for obtaining model parameters for each length scale
using the information from the smaller length scale. In this review, this
approach will be explained step-by-step in textbook style with examples of
successful multiscale modelling of different classes of magnetic materials from
the research literature as well as based on results newly obtained for this
review.Comment: 43 pages, 14 figure
Tunable topological magnetism in superlattices of nonmagnetic B20 systems
We predict topological magnetic properties of B20 systems, that are organized
in atomically thin multilayers. In particular we focus on FeSi/CoSi and
FeSi/FeGe superlattices with different number of layers and interface
structure. We demonstrate that absence of long range magnetic order, previously
observed in bulk FeSi and CoSi, is broken near the FeSi/CoSi interface, where a
magnetic state with non-trivial topology appears. Using electronic structure
calculations in combination with the magnetic force theorem, we calculate the
Heisenberg and Dzyaloshinskii-Moriya (DM) interactions in these systems. With
this information, we perform atomistic spin dynamics simulations at finite
temperature and applied magnetic field for large supercells with up to
spins to capture the complexity of non-collinear textures induced
by the DM interaction. The spin dynamics simulations predict the formation of
antiskyrmions in a [001]-oriented FeSi/CoSi multilayer, intermediate skyrmions
in a [111]-oriented FeSi/CoSi system and Bloch skyrmions in the FeSi/FeGe (001)
system. The size of different types of skyrmions is found to vary between 7 nm
and 37 nm. The varying topological magnetic texture in these systems can be
attributed to the complex asymmetric structure of the DM micromagnetic matrix,
which is different from previously known topological magnets. Furthermore,
through structural engineering, we demonstrate that both FM and AFM skyrmions
can be stabilized, where the latter are especially appealing for applications
due to the zero skyrmion Hall effect. The proposed B20 multilayers show
potential for further exploration and call for experimental confirmation.Comment: 13 pages, 10 figure
Trends in pressure-induced layer-selective half-collapsed tetragonal phases in the iron-based superconductor family AeAFe(4)As(4)
By performing pressure simulations within density functional theory for the family of iron-based superconductors AeAFe4As4 with Ae=Ca, Sr, Ba and A=K, Rb, Cs we predict in these systems the appearance of two consecutive half-collapsed tetragonal transitions at pressures Pc1 and Pc2, which have a different character in terms of their effect on the electronic structure. We find that, similar to previous studies for CaKFe4As4, spin-vortex magnetic fluctuations on the Fe sublattice play a key role for an accurate structure prediction in these materials at zero pressure. We identify clear trends of critical pressures and discuss the relevance of the collapsed phases in connection to magnetism and superconductivity. Finally, the intriguing cases of EuRbFe4As4 and EuCsFe4As4, where Eu magnetism coexists with superconductivity, are discussed as well in the context of half-collapsed phases
For optical flickering in symbiotic star MWC 560
This study is based on observations of MWC560 during the last two
observational seasons (2020/2021 and 2021/2022). Other than looking for
flickering we were interested in following the variability of brightness in the
same period. Looking for similarities in the spectra with other types of stars
is also of great interest to us because it could help clarify the stellar
configuration of such objects. Our observations during the last two
observational seasons of MWC560 confirm the absence of flickering. From the
similarities of the gathered spectra of XX Oph and MWC560 we assume that the
components in XX Oph are a red giant and a white dwarf, which are also
surrounded by a common shell
Pressure induced half-collapsed-tetragonal phase in CaKFeAs
We report the temperature-pressure phase diagram of CaKFeAs
established using high pressure electrical resistivity, magnetization and high
energy x-ray diffraction measurements up to 6 GPa. With increasing pressure,
both resistivity and magnetization data show that the bulk superconducting
transition of CaKFeAs is suppressed and then disappears at
4 GPa. High pressure x-ray data clearly indicate a phase transition
to a collapsed tetragonal phase in CaKFeAs under pressure that
coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray
data, combined with resistivity data, indicate that the collapsed tetragonal
transition line is essentially vertical, occuring at 4.0(5) GPa for
temperatures below 150 K. Band structure calculations also find a sudden
transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes
place across the Ca-layer. Bonding across the K-layer only occurs for
12 GPa. These findings demonstrate a new type of collapsed tetragonal
phase in CaKFeAs: a half-collapsed-tetragonal phase
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