Compact and power-efficient plastic electronic synapses are of fundamental importance to overcoming the bottlenecks of developing a neuromorphic chip. Memristor is a strong contender among the various electronic synapses in existence today. However, the speeds of synaptic events are relatively slow in most attempts at emulating synapses due to the material-related mechanism. Here we revealed the intrinsic memristance of stoichiometric crystalline Ge₂Sb₂Te₅ that originates from the charge trapping and releasing by the defects. The device resistance states, representing synaptic weights, were precisely modulated by 30 ns potentiating/depressing electrical pulses. We demonstrated four spike-timing-dependent plasticity (STDP) forms by applying programmed pre- and postsynaptic spiking pulse pairs in different time windows ranging from 50 ms down to 500 ns, the latter of which is 10⁵ times faster than the speed of STDP in human brain. This study provides new opportunities for building ultrafast neuromorphic computing systems and surpassing Von Neumann architecture.

Thermal conductivity of chalcogenide material with superlatticelike (SLL) structure is investigated using the $3\omega$ method and the molecular dynamics method. Both the measured and calculated results show that the thermal conductivity of SLL is lower than those of conventional chalcogenide materials and will decrease to a minimum as the number of interfaces increases. The Raman spectrum is introduced to study the phonon behavior of SLL and the “phonon mode vanishing” is proposed to explain its lower thermal conductivity. Finite-element analysis and phase change memory cell testing confirm the enhancement of cell performance for SLL with minimum thermal conductivity.

The bipolar memristive switching of stoichiometric crystalline Ge₂Sb₂Te₅ (GST) thin film has been demonstrated. In contrast to the filamentary switching reported previously for a GST memristor, the intrinsic memristance is interpreted as arising from a trap-associated space-charge limited current mechanism, which is confirmed by the frequency-dependent resistance and capacitance. The contributions of charge trapping in grain defects and grain boundary defects are analyzed, and the latter ones may dominate the resistance variation. Unraveling the intrinsic memristance of GST will help us further understand the conduction mechanism of chalcogenides and promote the design of future nonvolatile memory and neuromorphic devices.

X-ray diffraction and photoelectron spectroscopy of different Co-doped concentration GeTe have been made to identify the vacancies in rock-salt GeTe. The results show that Co occupies germanium vacancy and forms Co–Te bond, but the alloy retains a rock-salt structure when the concentration of doped Co is less than the vacancy ratio in crystalline GeTe. If we construct 8% germanium vacancy and relative distortion in rock-salt GeTe, the magnetism calculation based on spin-polarized density functional theory of Co-doped GeTe agrees well with superconducting quantum interference device magnetometer result. From experiment and calculation results, we can confirm the existence of vacancies in rock-salt GeTe and the ratio is 8%.