427 research outputs found
On the self-consistency of DFT-1/2
DFT-1/2 is an efficient band gap rectification method for density functional
theory (DFT) under local density approximation (LDA) or generalized gradient
approximation. It was suggested that non-self-consistent DFT-1/2 should be used
for highly ionic insulators like LiF, while self-consistent DFT-1/2 should
still be used for other compounds. Nevertheless, there is no quantitative
criterion prescribed for which implementation should work for an arbitrary
insulator, which leads to severe ambiguity in this method. In this work we
analyze the impact of self-consistency in DFT-1/2 and shell DFT-1/2
calculations in insulators or semiconductors with ionic bonds, covalent bonds
and intermediate cases, and show that self-consistency is required even for
highly ionic insulators for globally better electronic structure details. The
self-energy correction renders electrons more localized around the anions in
self-consistent LDA-1/2. The well-known delocalization error of LDA is
rectified, but with strong overcorrection due to the presence of additional
self-energy potential. However, in non-self-consistent LDA-1/2 calculations,
the electron wavefunctions indicate that such localization is much more severe
and beyond a reasonable range, because the strong Coulomb repulsion is not
counted in the Hamiltonian. Another common drawback of non-self-consistent
LDA-1/2 lies in that the ionicity of the bonding gets substantially enhanced,
and the band gap can be enormously high in mixed ionic-covalent compounds like
. The impact of LDA-1/2-induced stress is also discussed
comprehensively.Comment: 31 pages, 16 figure
Generative Dense Retrieval: Memory Can Be a Burden
Generative Retrieval (GR), autoregressively decoding relevant document
identifiers given a query, has been shown to perform well under the setting of
small-scale corpora. By memorizing the document corpus with model parameters,
GR implicitly achieves deep interaction between query and document. However,
such a memorizing mechanism faces three drawbacks: (1) Poor memory accuracy for
fine-grained features of documents; (2) Memory confusion gets worse as the
corpus size increases; (3) Huge memory update costs for new documents. To
alleviate these problems, we propose the Generative Dense Retrieval (GDR)
paradigm. Specifically, GDR first uses the limited memory volume to achieve
inter-cluster matching from query to relevant document clusters.
Memorizing-free matching mechanism from Dense Retrieval (DR) is then introduced
to conduct fine-grained intra-cluster matching from clusters to relevant
documents. The coarse-to-fine process maximizes the advantages of GR's deep
interaction and DR's scalability. Besides, we design a cluster identifier
constructing strategy to facilitate corpus memory and a cluster-adaptive
negative sampling strategy to enhance the intra-cluster mapping ability.
Empirical results show that GDR obtains an average of 3.0 R@100 improvement on
NQ dataset under multiple settings and has better scalability.Comment: EACL 2024 mai
Impact of Zr substitution on the electronic structure of ferroelectric hafnia
-based dielectrics are promising for nanoscale ferroelectric
applications, and the most favorable material within the family is
Zr-substituted hafnia, i.e., (HZO). The extent of Zr
substitution can be great, and x is commonly set to 0.5. However, the band gap
of is lower than , thus it is uncertain how
the Zr content should influence the electronic band structure of HZO. A reduced
band gap is detrimental to the cycling endurance as charge injection and
dielectric breakdown would become easier. Another issue is regarding the
comparison on the band gaps between /
superlattices and HZO solid-state solutions. In this work we systematically
investigated the electronic structures of ,
and HZO using self-energy corrected density functional theory. In particular,
the conduction band minimum of - is found to lie at an
ordinary k-point on the Brillouin zone border, not related to any interlines
between high-symmetry k-points. Moreover, the rule of HZO band gap variation
with respect to x has been extracted. The physical mechanisms for the
exponential reduction regime and linear decay regime have been revealed. The
band gaps of / ferroelectric superlattices are
investigated in a systematic manner, and the reason why the superlattice could
possess a band gap lower than that of is revealed through
comprehensive analysis.Comment: 23 pages, 9 figure
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