Abstract
Synaptic strengths between neurons in brain networks are highly adaptive due to synaptic plasticity. Spike-timing-dependent plasticity (STDP) is a form of synaptic plasticity induced by temporal correlations between the firing activity of neurons. The development of experimental techniques in recent years enabled the realization of brain-inspired neuromorphic devices. Particularly, magnetic tunnel junctions (MTJs) provide a suitable means for the implementation of learning processes in molecular junctions. Here, we first considered a two-neuron motif subjected to STDP. By employing theoretical analysis and computer simulations we showed that the dynamics and emergent structure of the motif can be predicted by introducing an effective two-neuron synaptic conductance. Then, we considered a phenyl-based single-molecule MTJ connected to two ferromagnetic (FM) cobalt electrodes and investigated its electrical properties using the non-equilibrium Green’s function (NEGF) formalism. Similar to the two-neuron motif, we introduced an effective spin-polarized conductance in the MTJ. Depending on the polarity, frequency and strength of the bias voltage applied to the MTJ, the system can learn input signals by adaptive changes of the effective conductance. Interestingly, this voltage-dependent plasticity is an intrinsic property of the MTJ where its behavior is reminiscent of the classical temporally asymmetric STDP. Furthermore, the shape of voltage-dependent plasticity in the MTJ is determined by the molecule-electrode coupling strength or the length of the molecule. Our results may be relevant for the development of single-molecule devices that capture the adaptive properties of synapses in the brain.
Highlights
Synaptic plasticity is a fundamental brain mechanism for learning and information storage [1, 2]
In the second part we considered a single-molecule magnetic tunnel junctions (MTJs) comprising a phenyl dithiol (PDT) molecule or a biphenyl dithiol (BPDT) molecule connected to two ferromagnetic (FM) cobalt electrodes mediated by a sulfur anchoring unit and inspected current-voltage characteristics and conductance using the non-equilibrium Green’s function (NEGF) formalism
We first considered a two-neuron motif schematically shown in Fig 1A comprising two neuronal phase oscillators described by Eq (1) connected to each other via plastic synapses where their strength is modified by the classical Spike-timing-dependent plasticity (STDP) introduced in Eq (5)
Summary
Synaptic plasticity is a fundamental brain mechanism for learning and information storage [1, 2]. By activity-dependent modification of the synaptic strengths, synaptic plasticity plays a key role in shaping neural circuits during development [3] and its dysfunction may be involved in several neuropsychiatric disorders [4]. Voltage-dependent plasticity in magnetic tunnel junctions postsynaptic neurons is strengthened by their correlated firing activity, later termed as the Hebbian learning [6]. Hebb’s postulate was extended to incorporate the weakening of synapses between neurons with uncorrelated firing activity [7]. In this context, the persistent strengthening of the synaptic efficacy is called long-term potentiation (LTP), whereas its weakening is termed as long-term depression (LTD). Inspired by the Hebb’s idea, numerous experiments were devoted to validate long-term activity-dependent synaptic changes
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