60 research outputs found
Reduction of Activation Energy Barrier of Stone-Wales Transformation in Endohedral Metallofullerenes
We examine effects of encapsulated metal atoms inside a C molecule on
the activation energy barrier to the Stone-Wales transformation using {\it ab
initio} calculations. The encapsulated metal atoms we study are K, Ca and La
which nominally donate one, two and three electrons to the C cage,
respectively. We find that isomerization of the endohedral metallofullerene via
the Stone-Wales transformation can occur more easily than that of the empty
fullerene owing to the charge transfer. When K, Ca and La atoms are
encapsulated inside the fullerene, the activation energy barriers are lowered
by 0.30, 0.55 and 0.80 eV, respectively compared with that of the empty
C (7.16 eV). The lower activation energy barrier of the Stone-Wales
transformation implies the higher probability of isomerization and coalescence
of metallofullerenes, which require a series of Stone-Wales transformations.Comment: 13 pages, 3 figures, 1 tabl
Accelerated identification of equilibrium structures of multicomponent inorganic crystals using machine learning potentials
The discovery of new multicomponent inorganic compounds can provide direct
solutions to many scientific and engineering challenges, yet the vast size of
the uncharted material space dwarfs current synthesis throughput. While the
computational crystal structure prediction is expected to mitigate this
frustration, the NP-hardness and steep costs of density functional theory (DFT)
calculations prohibit material exploration at scale. Herein, we introduce
SPINNER, a highly efficient and reliable structure-prediction framework based
on exhaustive random searches and evolutionary algorithms, which is completely
free from empiricism. Empowered by accurate neural network potentials, the
program can navigate the configuration space faster than DFT by more than
10-fold. In blind tests on 60 ternary compositions diversely selected
from the experimental database, SPINNER successfully identifies experimental
(or theoretically more stable) phases for ~80% of materials within 5000
generations, entailing up to half a million structure evaluations for each
composition. When benchmarked against previous data mining or DFT-based
evolutionary predictions, SPINNER identifies more stable phases in the majority
of cases. By developing a reliable and fast structure-prediction framework,
this work opens the door to large-scale, unbounded computational exploration of
undiscovered inorganic crystals.Comment: 3 figure
Disorder-dependent Li diffusion in investigated by machine learning potential
Solid-state electrolytes with argyrodite structures, such as
, have attracted considerable attention due to their
superior safety compared to liquid electrolytes and higher ionic conductivity
than other solid electrolytes. Although experimental efforts have been made to
enhance conductivity by controlling the degree of disorder, the underlying
diffusion mechanism is not yet fully understood. Moreover, existing theoretical
analyses based on ab initio MD simulations have limitations in addressing
various types of disorder at room temperature. In this study, we directly
investigate Li-ion diffusion in at 300 K using
large-scale, long-term MD simulations empowered by machine learning potentials
(MLPs). To ensure the convergence of conductivity values within an error range
of 10%, we employ a 25 ns simulation using a supercell
containing 6500 atoms. The computed Li-ion conductivity, activation energies,
and equilibrium site occupancies align well with experimental observations.
Notably, Li-ion conductivity peaks when Cl ions occupy 25% of the 4c sites,
rather than at 50% where the disorder is maximized. This phenomenon is
explained by the interplay between inter-cage and intra-cage jumps. By
elucidating the key factors affecting Li-ion diffusion in
, this work paves the way for optimizing ionic
conductivity in the argyrodite family.Comment: 34 pages, 6 figure
GW Calculations on post-transition-metal oxides
In order to establish the reliable GW scheme that can be consistently applied to post-transition-metal oxides (post-TMOs), we carry out comprehensive GW calculations on electronic structures of ZnO, Ga2O3, In2O3, and SnO2, the four representative post-TMOs. Various levels of self-consistency (G0W0, GW0, and QPGW0) and different starting functionals (GGA, GGA + U, and hybrid functional) are tested and their influence on the resulting electronic structure is closely analyzed. It is found that the GW0 scheme with GGA + U as the initial functional turns out to give the best agreement with experiment, implying that describing the position of metal-d level precisely in the ground state plays a critical role for the accurate dielectric property and quasiparticle band gap. Nevertheless, the computation on ZnO still suffers from the shallow Zn-d level and we propose a modified approach (GW0+Ud) that additionally considers an effective Hubbard U term during GW0 iterations and thereby significantly improves the band gap. It is also shown that a GGA + U-based GW0(+Ud) scheme produces an accurate energy gap of crystalline InGaZnO4, implying that this can serve as a standard scheme that can be applied to general structures of post-TMOs. © 2014 American Physical Society.1991sciescopu
Investigation of the mechanism of the anomalous Hall effects in Cr2Te3/(BiSb)2(TeSe)3 heterostructure
The interplay between ferromagnetism and the non-trivial topology has
unveiled intriguing phases in the transport of charges and spins. For example,
it is consistently observed the so-called topological Hall effect (THE)
featuring a hump structure in the curve of the Hall resistance (Rxy) vs. a
magnetic field (H) of a heterostructure consisting of a ferromagnet (FM) and a
topological insulator (TI). The origin of the hump structure is still
controversial between the topological Hall effect model and the multi-component
anomalous Hall effect (AHE) model. In this work, we have investigated a
heterostructure consisting of BixSb2-xTeySe3-y (BSTS) and Cr2Te3 (CT), which
are well-known TI and two-dimensional FM, respectively. By using the so-called
minor-loop measurement, we have found that the hump structure observed in the
CT/BSTS is more likely to originate from two AHE channels. Moreover, by
analyzing the scaling behavior of each amplitude of two AHE with the
longitudinal resistivities of CT and BSTS, we have found that one AHE is
attributed to the extrinsic contribution of CT while the other is due to the
intrinsic contribution of BSTS. It implies that the proximity-induced
ferromagnetic layer inside BSTS serves as a source of the intrinsic AHE,
resulting in the hump structure explained by the two AHE model
Interaction and ordering of vacancy defects in NiO
By using a first-principles method employing the local density approximation plus Hubbard parameter approach, we study point defects in NiO and interactions between them. The defect states associated with nickel or oxygen vacancies are identified within the energy gap. It is found that nickel vacancies introduce shallow levels in the density of states for the spin direction opposite to that of the removed Ni atom, while the oxygen vacancy creates more localized in-gap states. The interaction profiles between vacancies indicate that specific defect arrangements are strongly favored for both nickel and oxygen vacancies. In the case of nickel vacancies, defect ordering in a simple-cubic style is found to be most stable, leading to a half-metallic behavior. The ionized oxygen vacancies also show a tendency toward clustering, more strongly than neutral pairs. The microscopic origin of vacancy clustering is understood based on overlap integrals between defect states. © 2008 The American Physical Society.open343
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