61 research outputs found
Charge Trap Memory Based on Few-Layered Black Phosphorus
Atomically thin layered two-dimensional materials, including transition-metal
dichacolgenide (TMDC) and black phosphorus (BP), (1) have been receiving much
attention, because of their promising physical properties and potential
applications in flexible and transparent electronic devices . Here, for the
first time we show non-volatile chargetrap memory devices, based on
field-effect transistors with large hysteresis, consisting of a few-layer black
phosphorus channel and a three dimensional (3D) Al2O3 /HfO2 /Al2O3 charge-trap
gate stack. An unprecedented memory window exceeding 12 V is observed, due to
the extraordinary trapping ability of HfO2. The device shows a high endurance
and a stable retention of ?25% charge loss after 10 years, even drastically
lower than reported MoS2 flash memory. The high program/erase current ratio,
large memory window, stable retention and high on/off current ratio, provide a
promising route towards the flexible and transparent memory devices utilising
atomically thin two-dimensional materials. The combination of 2D materials with
traditional high-k charge-trap gate stacks opens up an exciting field of
nonvolatile memory devices.Comment: 16 pages, 10 figures, 1 table. arXiv admin note: substantial text
overlap with arXiv:1407.7432 by other authors; text overlap with
arXiv:1505.04859 by other authors without attributio
Tuning a binary ferromagnet into a multi-state synapse with spin-orbit torque induced plasticity
Inspired by ion-dominated synaptic plasticity in human brain, artificial
synapses for neuromorphic computing adopt charge-related quantities as their
weights. Despite the existing charge derived synaptic emulations, schemes of
controlling electron spins in ferromagnetic devices have also attracted
considerable interest due to their advantages of low energy consumption,
unlimited endurance, and favorable CMOS compatibility. However, a generally
applicable method of tuning a binary ferromagnet into a multi-state memory with
pure spin-dominated synaptic plasticity in the absence of an external magnetic
field is still missing. Here, we show how synaptic plasticity of a
perpendicular ferromagnetic FM1 layer can be obtained when it is
interlayer-exchange-coupled by another in-plane ferromagnetic FM2 layer, where
a magnetic-field-free current-driven multi-state magnetization switching of FM1
in the Pt/FM1/Ta/FM2 structure is induced by spin-orbit torque. We use current
pulses to set the perpendicular magnetization state which acts as the synapse
weight, and demonstrate spintronic implementation of the excitatory/inhibitory
postsynaptic potentials and spike timing-dependent plasticity. This
functionality is made possible by the action of the in-plane interlayer
exchange coupling field which leads to broadened, multi-state magnetic reversal
characteristics. Numerical simulations, combined with investigations of a
reference sample with a single perpendicular magnetized Pt/FM1/Ta structure,
reveal that the broadening is due to the in-plane field component tuning the
efficiency of the spin-orbit-torque to drive domain walls across a landscape of
varying pinning potentials. The conventionally binary FM1 inside our
Pt/FM1/Ta/FM2 structure with inherent in-plane coupling field is therefore
tuned into a multi-state perpendicular ferromagnet and represents a synaptic
emulator for neuromorphic computing.Comment: 37 pages with 11 figures, including 20 pages for manuscript and 17
pages for supplementary informatio
DropPos: Pre-Training Vision Transformers by Reconstructing Dropped Positions
As it is empirically observed that Vision Transformers (ViTs) are quite
insensitive to the order of input tokens, the need for an appropriate
self-supervised pretext task that enhances the location awareness of ViTs is
becoming evident. To address this, we present DropPos, a novel pretext task
designed to reconstruct Dropped Positions. The formulation of DropPos is
simple: we first drop a large random subset of positional embeddings and then
the model classifies the actual position for each non-overlapping patch among
all possible positions solely based on their visual appearance. To avoid
trivial solutions, we increase the difficulty of this task by keeping only a
subset of patches visible. Additionally, considering there may be different
patches with similar visual appearances, we propose position smoothing and
attentive reconstruction strategies to relax this classification problem, since
it is not necessary to reconstruct their exact positions in these cases.
Empirical evaluations of DropPos show strong capabilities. DropPos outperforms
supervised pre-training and achieves competitive results compared with
state-of-the-art self-supervised alternatives on a wide range of downstream
benchmarks. This suggests that explicitly encouraging spatial reasoning
abilities, as DropPos does, indeed contributes to the improved location
awareness of ViTs. The code is publicly available at
https://github.com/Haochen-Wang409/DropPos.Comment: Accepted by NeurIPS 202
Tuning a binary ferromagnet into a multi-state synapse with spin-orbit-torque-induced plasticity
Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchangeâcoupled by another inâplane ferromagnetic layer (FM2), where a magnetic fieldâfree currentâdriven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spinâorbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timingâdependent plasticity are demonstrated. This functionality is made possible by the action of the inâplane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the inâplane field component tuning the efficiency of the spinâorbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent inâplane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics
Semantics-Consistent Feature Search for Self-Supervised Visual Representation Learning
In contrastive self-supervised learning, the common way to learn
discriminative representation is to pull different augmented "views" of the
same image closer while pushing all other images further apart, which has been
proven to be effective. However, it is unavoidable to construct undesirable
views containing different semantic concepts during the augmentation procedure.
It would damage the semantic consistency of representation to pull these
augmentations closer in the feature space indiscriminately. In this study, we
introduce feature-level augmentation and propose a novel semantics-consistent
feature search (SCFS) method to mitigate this negative effect. The main idea of
SCFS is to adaptively search semantics-consistent features to enhance the
contrast between semantics-consistent regions in different augmentations. Thus,
the trained model can learn to focus on meaningful object regions, improving
the semantic representation ability. Extensive experiments conducted on
different datasets and tasks demonstrate that SCFS effectively improves the
performance of self-supervised learning and achieves state-of-the-art
performance on different downstream tasks
Enhanced photoresponse in MoTe2 photodetectors with asymmetric graphene contacts
Atomically thin two dimensional (2D) materials are promising candidates for miniaturized high-performance optoelectronic devices. Here, we report on multilayer MoTe2 photodetectors contacted with asymmetric electrodes based on n- and p-type graphene layers. The asymmetry in the graphene contacts creates a large (Ebi ~100 kV cm-1) built-in electric field across the short (l = 15 nm) MoTe2 channel, causing a high and broad (? = 400 to 1400 nm) photoresponse even without any externally applied voltage. Spatially resolved photovoltage maps reveal an enhanced photoresponse and larger built-in electric field in regions of the MoTe2 layer between the two graphene contacts. Furthermore, a fast (~10 ?s) photoresponse is achieved in both the photovoltaic and photoconductive operation modes of the junction. Our findings could be extended to other 2D materials and offer prospects for the implementation of asymmetric graphene contacts in future low-power optoelectronic applications
Adjustable current-induced magnetization switching utilizing interlayer exchange coupling
Electrical current-induced deterministic magnetization switching in a magnetic multilayer structure without external magnetic field is realized by utilizing interlayer exchange coupling. Two ferromagnetic Co layers, with in-plane and out-of-plane anisotropy respectively, are separated by a spacer Ta layer, which plays a dual role of inducing antiferromagnetic interlayer coupling, and contributing to the current-induced effective magnetic field through the spin Hall effect. The current-induced magnetization switching behavior can be tuned by pre-magnetizing the in-plane Co layer. The antiferromagnetic exchange coupling field increases with decreasing thickness of the Ta layer, reaching 630 Âą5 Oe for a Ta thickness of 1.5nm. The magnitude of the current-induced perpendicular effective magnetic field from spin-orbit torque is 9.2 Oe/(107Acm-2). The large spin Hall angle of Ta, opposite in sign to that of Pt, results in a low critical current density of 9Ă106A/cm2. This approach is promising for the electrical switching of magnetic memory elements without external magnetic field
High-detectivity ultraviolet photodetectors based on laterally mesoporous GaN
Photodetectors for the ultraviolet (UV) range of the electromagnetic spectrum are in great demand for several technologies, but require the development of novel device structures and materials. Here we report on the high detectivity of UV photodetectors based on well-ordered laterally mesoporous GaN. The specific detectivity of our devices under UV-illumination reaches values of up to 5.3Ă1014 Jones. We attribute this high specific detectivity to the properties of the mesoporous GaN/metal contact interface: the trapping of photo-generated holes at the interface lowers the Schottky barrier height thus causing a large internal gain. The high detectivity along with the simple fabrication process make these laterally mesoporous GaN photodetectors of great potential for applications that require selective detection of weak optical signals in the UV range
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