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Activated by action potentials and Ca2+ ion migration, neurotransmitters in biological synapses are released from vesicles at the presynaptic membrane to the cleft and bonded to receptors on the postsynaptic membrane. The bonded neurotransmitters modify the electrochemical properties of the postsynaptic membrane and, thereby, the synaptic plasticity, which forms the basis for learning, memory, emotion, cognition and consciousness. Here, oxygen vacancy transport in Au/SrTiO3 (STO)/La0.67Sr0.33MnO3 (LSMO) oxide tunnel junctions (OTJ) is exploited to mimic neurotransmission processes in an artificial ionic electronic device. Using voltage pulses of varying number, amplitude, and polarity, it is demonstrated that reversible oxygen vacancy migration across the STO/LSMO interface provides stable multilevel resistance switching for octal memory devices and resembles the quantal, stochastic, and excitatory or inhibitory nature of neurotransmitter release dynamics. Moreover, fundamental synaptic behaviors including long-term potentiation/depression and various types of spike-timing dependent plasticity characteristics are emulated, opening a promising bio-realistic approach to the design of neuromorphic devices.
Oxide-based electronic devices are expected to have fascinating properties, unlike those made from conventional semiconductors. SrTiO3 (STO) is a key material for this new field of electronics1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ¹¹. Here we report on blue-light emission at room temperature from Ar⁺-irradiated, metallic STO. The irradiation introduces oxygen deficiencies to a depth of ~20 nm from the crystal surface. These deficiencies generate conduction carriers and stabilize a hole level in a self-trapped state. We propose a model by which the doped conduction electrons and the in-gap state produce a radiative process that results in blue-light emission. The emitting region can be patterned into any size and shape with conventional microscopic fabrication techniques.
Memory is believed to occur in the human brain as a result of two types of synaptic plasticity: short-term plasticity (STP) and long-term potentiation (LTP; refs 1-4). In neuromorphic engineering, emulation of known neural behaviour has proven to be difficult to implement in software because of the highly complex interconnected nature of thought processes. Here we report the discovery of a Ag(2)S inorganic synapse, which emulates the synaptic functions of both STP and LTP characteristics through the use of input pulse repetition time. The structure known as an atomic switch, operating at critical voltages, stores information as STP with a spontaneous decay of conductance level in response to intermittent input stimuli, whereas frequent stimulation results in a transition to LTP. The Ag(2)S inorganic synapse has interesting characteristics with analogies to an individual biological synapse, and achieves dynamic memorization in a single device without the need of external preprogramming. A psychological model related to the process of memorizing and forgetting is also demonstrated using the inorganic synapses. Our Ag(2)S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory.
Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity, colossal magnetoresistance or multiferroicity . The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO(3) and SrTiO(3) (ref. 4) represents an important milestone towards exploiting such properties in all-oxide devices. This conducting interface shows a number of appealing properties, including a high electron mobility, superconductivity and large magnetoresistance, and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×10(13) cm(-2), can be formed at the bare SrTiO(3) surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.
OF the various inhibitory actions converging on to spinal motoneurones the `direct' inhibition is the one that has been the most extensively investigated1-6. It is provoked by impulses in large muscle afferents from annulo-spinal endings (Ia fibres)7-9. Monosynaptic excitatory action to motoneurones is also supplied by Ia fibres, and, with any particular group of motoneurones, the inhibitory action is caused by Ia fibres originating from the muscles antagonistic to those supplying excitatory action. The `direct' inhibitory action is caused by a brief hyperpolarization of the motoneuronal membrane, the inhibitory post-synaptic potential10. The inhibitory post-synaptic potentials have a minimal latency of about 1.2-1.4 msec. relative to the time when the Ia volley reaches the spinal cord or approximately 0.8 msec. longer than for the monosynaptic excitatory post-synaptic potential. It has been postulated that this longer latency is due to the existence of an interneurone in the `direct' inhibitory pathway, and experimental evidence has been presented suggesting that these interneurones are located in the intermediate nucleus11.