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Electronic mechanism for resistance drift in phase-change memory materials: Link to persistent photoconductivity
‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory
Bridging the gap between nanowires and Josephson junctions: a superconducting device based on controlled fluxon transfer across nanowires
The basis for superconducting electronics can broadly be divided between two
technologies: the Josephson junction and the superconducting nanowire. While
the Josephson junction (JJ) remains the dominant technology due to its high
speed and low power dissipation, recently proposed nanowire devices offer
improvements such as gain, high fanout, and compatibility with CMOS circuits.
Despite these benefits, nanowire-based electronics have largely been limited to
binary operations, with devices switching between the superconducting state and
a high-impedance resistive state dominated by uncontrolled hotspot dynamics.
Unlike the JJ, they cannot increment an output through successive switching,
and their operation speeds are limited by their slow thermal reset times. Thus,
there is a need for an intermediate device with the interfacing capabilities of
a nanowire but a faster, moderated response allowing for modulation of the
output. Here, we present a nanowire device based on controlled fluxon
transport. We show that the device is capable of responding proportionally to
the strength of its input, unlike other nanowire technologies. The device can
be operated to produce a multilevel output with distinguishable states, which
can be tuned by circuit parameters. Agreement between experimental results and
electrothermal circuit simulations demonstrates that the device is classical
and may be readily engineered for applications including use as a multilevel
memory
Route Planning in Transportation Networks
We survey recent advances in algorithms for route planning in transportation
networks. For road networks, we show that one can compute driving directions in
milliseconds or less even at continental scale. A variety of techniques provide
different trade-offs between preprocessing effort, space requirements, and
query time. Some algorithms can answer queries in a fraction of a microsecond,
while others can deal efficiently with real-time traffic. Journey planning on
public transportation systems, although conceptually similar, is a
significantly harder problem due to its inherent time-dependent and
multicriteria nature. Although exact algorithms are fast enough for interactive
queries on metropolitan transit systems, dealing with continent-sized instances
requires simplifications or heavy preprocessing. The multimodal route planning
problem, which seeks journeys combining schedule-based transportation (buses,
trains) with unrestricted modes (walking, driving), is even harder, relying on
approximate solutions even for metropolitan inputs.Comment: This is an updated version of the technical report MSR-TR-2014-4,
previously published by Microsoft Research. This work was mostly done while
the authors Daniel Delling, Andrew Goldberg, and Renato F. Werneck were at
Microsoft Research Silicon Valle
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