3,308 research outputs found
Compressed Encoding for Rank Modulation
Rank modulation has been recently proposed as
a scheme for storing information in flash memories. While
rank modulation has advantages in improving write speed and
endurance, the current encoding approach is based on the "push
to the top" operation that is not efficient in the general case. We
propose a new encoding procedure where a cell level is raised to
be higher than the minimal necessary subset -instead of all - of
the other cell levels. This new procedure leads to a significantly
more compressed (lower charge levels) encoding. We derive an
upper bound for a family of codes that utilize the proposed
encoding procedure, and consider code constructions that achieve
that bound for several special cases
Trade-offs between Instantaneous and Total Capacity in Multi-Cell Flash Memories
The limited endurance of flash memories is a major
design concern for enterprise storage systems. We propose a
method to increase it by using relative (as opposed to fixed)
cell levels and by representing the information with Write
Asymmetric Memory (WAM) codes. Overall, our new method
enables faster writes, improved reliability as well as improved
endurance by allowing multiple writes between block erasures.
We study the capacity of the new WAM codes with relative levels,
where the information is represented by multiset permutations
induced by the charge levels, and show that it achieves the
capacity of any other WAM codes with the same number of
writes. Specifically, we prove that it has the potential to double
the total capacity of the memory. Since capacity can be achieved
only with cells that have a large number of levels, we propose a
new architecture that consists of multi-cells - each an aggregation
of a number of floating gate transistors
The unique chemical reactivity of a graphene nanoribbon's zigzag edge
The zigzag edge of a graphene nanoribbon possesses a unique electronic state
that is near the Fermi level and localized at the edge carbon atoms. We
investigate the chemical reactivity of these zigzag edge sites by examining
their reaction energetics with common radicals from first principles. A
"partial radical" concept for the edge carbon atoms is introduced to
characterize their chemical reactivity, and the validity of this concept is
verified by comparing the dissociation energies of edge-radical bonds with
similar bonds in molecules. In addition, the uniqueness of the zigzag-edged
graphene nanoribbon is further demonstrated by comparing it with other forms of
sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged
graphene nanoribbon.Comment: 24 pages, 9 figure
Electronic Ground State of Higher Acenes
We examine the electronic ground state of acenes with different number of
fused benzene rings (up to 40) by using first principles density functional
theory. Their properties are compared with those of infinite polyacene. We find
that the ground state of acenes that consist of more than seven fused benzene
rings is an antiferromagnetic (in other words, open-shell singlet) state, and
we show that this singlet is not necessarily a diradical, because the spatially
separated magnetizations for the spin-up and spin-down electrons increase with
the size of the acene. For example, our results indicate that there are about
four spin-up electrons localized at one zigzag edge of 20-acene. The reason
that both acenes and polyacene have the antiferromagnetic ground state is due
to the zigzag-shaped boundaries, which cause pi-electrons to localize and form
spin orders at the edges. Both wider graphene ribbons and large
rectangular-shaped polycyclic aromatic hydrocarbons have been shown to share
this antiferromagnetic ground state. Therefore, we demonstrate that the
pi-electronic structure of higher acenes and ployacene are still dictated by
the zigzag edges, and our results provide a consistent description of their
electronic ground state.Comment: revised: corrected some errors, rephrased some discussions, and added
a reference (Ref. 29); 19 pages, 6 figure
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