20 research outputs found
Relativistic +BSE study of the optical properties of Ruddlesden-Popper iridates
We study the optical properties of the Ruddlesden-Popper series of iridates
SrIrO (=1, 2 and ) by solving the
Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and
screened interactions are obtained by the approximation including
spin-orbit coupling. The computed optical conductivity spectra show strong
excitonic effects and reproduce very well the experimentally observed
double-peak structure, in particular for the spin-orbital Mott insulators
SrIrO and SrIrO. However, does not account well for
the correlated metallic state of SrIrO owing to a much too small band
renormalization, and this affects the overall quality of the optical
conductivity. Our analysis describes well the progressive redshift of the main
optical peaks as a function of dimensionality (), which is correlated with
the gradual decrease of the electronic correlation (quantified by the
constrained random phase approximation) towards the metallic limit.
We have also assessed the quality of a computationally cheaper BSE approach
that is based on a model dielectric function and conducted on top of DFT+
one-electron energies. Unfortunately, this model BSE approach does not
accurately reproduce the outcome of the full +BSE method and leads to
larger deviations to the measured spectra.Comment: 13 pages, 8 figure
Anisotropic Magnetic Couplings and Structure-Driven Canted to Collinear Transitions in Spin-orbit Coupled Sr2IrO4
We put forward a scheme to study the anisotropic magnetic couplings in
Sr2IrO4 by mapping fully relativistic constrained noncollinear density
functional theory including an on-site Hubbard U correction onto a general spin
model Hamiltonian. This procedure allows for the simultaneous account and
direct control of the lattice, spin and orbital interactions within a fully ab
initio scheme. We compute the isotropic, single site anisotropy and
Dzyaloshinskii-Moriya (DM) coupling parameters, and clarify that the origin of
the canted magnetic state in Sr2IrO4 arises from the interplay between
structural distortions and the competition between isotropic exchange and DM
interactions. A complete magnetic phase diagram with respect to the tetragonal
distortion and the rotation of IrO6 octahedra is constructed, revealing the
presence of two types of canted to collinear magnetic transitions: a spin-flop
transition with increasing tetragonal distortion and a complete quenching of
the basal weak ferromagnetic moment below a critical octahedral rotation
Symmetry breaking induced insulating electronic state in PbCu(PO)O
The recent experimental claim of room-temperature ambient-pressure
superconductivity in a Cu-doped lead-apatite (LK-99) has ignited substantial
research interest in both experimental and theoretical domains. Previous
density functional theory (DFT) calculations with the inclusion of an on-site
Hubbard interaction consistently predict the presence of flat bands
crossing the Fermi level. This is in contrast to DFT plus dynamical mean field
theory calculations, which reveal the Mott insulating behavior for the
stoichiometric PbCu(PO)O compound. Nevertheless, the existing
calculations are all based on the structure, which is argued to be not
the ground-state structure. Here, we revisit the electronic structure of
PbCu(PO)O with the energetically more favorable
structure, fully taking into account electronic symmetry breaking. We examine
all possible configurations for Cu substituting the Pb sites. Our results show
that the doped Cu atoms exhibit a preference for substituting the Pb2 sites
than the Pb1 sites. In both cases, the calculated substitutional formation
energies are large, indicating the difficulty in incorporating Cu at the Pb
sites. We find that most of structures with Cu at the Pb2 site tend to be
insulating, while the structures with both two Cu atoms at the Pb1 sites
(except one configuration) are predicted to be metallic by DFT+
calculations. However, when accounting for the electronic symmetry breaking,
some Cu-doped configurations previously predicted to be metallic (including the
structure studied in previous DFT+ calculations) become insulating. Our work
highlights the importance of symmetry breaking in obtaining correct electronic
state for PbCu(PO)O, thereby reconciling previous DFT+ and
DFT+DMFT calculations.Comment: 19 pages, 9 figures (including Supplementary Material
Comprehensive ab initio study of effects of alloying elements on generalized stacking fault energies of Ni and NiAl
Excellent high-temperature mechanical properties of Ni-based single crystal
superalloys (NSCSs) are attributed to the yield strength anomaly of NiAl
that is intimately related to generalized stacking fault energies (GSFEs).
Therefore, clarifying the effects of alloying elements on the GSFEs is of great
significance for alloys design. Here, by means of ab initio density functional
theory calculations, we systematically calculated the GSFEs of different slip
systems of Ni and NiAl without and with alloying elements using the alias
shear method. We obtained that for Ni, except for magnetic elements Mn, Fe, and
Co, most of alloying elements decrease the unstable stacking fault energy
() of the and slip systems
and also decrease the stable stacking fault energy () of the
slip system. For NiAl, most of alloying elements in
groups IIIB-VIIB show a strong Al site preference. Except for Mn and Fe, the
elements in groups VB-VIIB and the first column of group VIII increase the
values of of different slip systems of NiAl. On the other
hand, the elements in groups IIIB-VIIB also increase the value of
. We found that Re is an excellent strengthening alloying element
that significantly increases the slip barrier of the tailing slip process for
Ni, and also enhances the slip barrier of the leading slip process of three
slip systems for NiAl. W and Mo exhibit similar effects as Re. We
predicted that Os, Ru, and Ir are good strengthening alloying elements as well,
since they show the strengthening effects on both the leading and tailing slip
process for Ni and NiAl
Anisotropic linear and nonlinear charge-spin conversion in topological semimetal SrIrO3
Over the past decade, utilizing spin currents in the linear response of
electric field to manipulate magnetization states via spin-orbit torques (SOTs)
is one of the core concepts for realizing a multitude of spintronic devices.
Besides the linear regime, recently, nonlinear charge-spin conversion under the
square of electric field has been recognized in a wide variety of materials
with nontrivial spin textures, opening an emerging field of nonlinear
spintronics. Here, we report the investigation of both linear and nonlinear
charge-spin conversion in one single topological semimetal SrIrO3(110) thin
film that hosts strong spin-orbit coupling and nontrivial spin textures in the
momentum space. In the nonlinear regime, the observation of crystalline
direction dependent response indicates the presence of anisotropic surface
states induced spin-momentum locking near the Fermi level. Such anisotropic
spin textures also give rise to spin currents in the linear response regime,
which mainly contributes to the fieldlike SOT component. Our work demonstrates
the power of combination of linear and nonlinear approaches in understanding
and utilizing charge-spin conversion in topological materials.Comment: 18 pages, 5 figure
Effect of Axial Force on the Performance of Micromachined Vibratory Rate Gyroscopes
It is reported in the published literature that the resonant frequency of a silicon micromachined gyroscope decreases linearly with increasing temperature. However, when the axial force is considerable, the resonant frequency might increase as the temperature increases. The axial force is mainly induced by thermal stress due to the mismatch between the thermal expansion coefficients of the structure and substrate. In this paper, two types of micromachined suspended vibratory gyroscopes with slanted beams were proposed to evaluate the effect of the axial force. One type was suspended with a clamped-free (C-F) beam and the other one was suspended with a clamped-clamped (C-C) beam. Their drive modes are the bending of the slanted beam, and their sense modes are the torsion of the slanted beam. The relationships between the resonant frequencies of the two types were developed. The prototypes were packaged by vacuum under 0.1 mbar and an analytical solution for the axial force effect on the resonant frequency was obtained. The temperature dependent performances of the operated mode responses of the micromachined gyroscopes were measured. The experimental values of the temperature coefficients of resonant frequencies (TCF) due to axial force were 101.5 ppm/°C for the drive mode and 21.6 ppm/°C for the sense mode. The axial force has a great influence on the modal frequency of the micromachined gyroscopes suspended with a C-C beam, especially for the flexure mode. The quality factors of the operated modes decreased with increasing temperature, and changed drastically when the micromachined gyroscopes worked at higher temperatures
Particle Size Distribution of Cemented Rockfill Effects on Strata Stability in Filling Mining
It is of great significance for engineering safety, economic benefits, environmental protection, and sustainable development to investigate the strata stability in filling mining with cemented rockfill. Consequently, this paper is based on a specific coal mine where we applied the fully-mechanized longwall mining and filling and designed a cemented rockfill material for which the particles satisfied the Talbot gradation. Uniaxial and triaxial compression experiments were carried out on the cemented rockfill specimen, which obtained the relations between the mechanical parameters (Poisson ratio, elastic modulus, compressive strength, cohesive force, internal friction angle, and tensile strength) and the particle size distribution of the aggregate. The excavation and filling processes in the coal seam were simulated based on the numerical software FLAC3D. The characteristics of the displacement and stress fields of the strata when the goaf was filled by cemented rockfill with different granule gradations were discussed. The influences of the particle size distribution and mining distance on the maximum subsidence displacement of the coal seam roof, internal stress of the backfill, and the stress of the rock mass in the coalface were analyzed. The feasibility and effectiveness of the filling mining with cemented rockfill to protect the integrity of the overlying strata were discussed. The results showed that optimizing the particle size distribution of the aggregate in cemented rockfill could increase the loading capacity of the backfill to improve the filling effect, effectively control the strata movement, and decrease the stress of rock mass in the coalface to reduce the potential danger