Periodic Reporting for period 1 - MottSwitch (Highly Energy-Efficient Resistive Switching in Defect- and Strain- Engineered Mott Insulators for Neuromorphic Computing Applications)
Reporting period: 2022-09-01 to 2025-02-28
The pressing need for beyond-von-Neumann computing paradigms has triggered intensive efforts into understanding and controlling various resistive switching mechanisms. This switching, in which the two-terminal resistance of a device is controlled by current is at the heart of emerging technologies such as resistive random access memory and neuromorphic computation. These technologies promise to revolutionize artificial neural networks and mimic the behavior of biological brains, triggering a race the for optimal resistive switching materials.
In Mott insulators electrical currents can change resistance by orders-of-magnitude due to a phase transition from insulating to metallic states. The volatility of switching in Mott insulators can be adjusted by several tuning parameters, enabling both memory devices and neuron-like functionalities. Moreover, Mott insulators have potential for extremely fast switching timescales and energy efficiency. These unique properties have made Mott insulators prominent candidate materials for resistive switching applications. However, the physical mechanisms behind resistive switching in these materials are poorly understood and not easily controllable, hampering advancement in this field.
We suggest two main routes towards Mott-based resistive switching with ultrahigh energy efficiency. The first is by switching purely in the electronic sector while minimizing structural distortions. The low heat capacity of electrons may enable switching with a fraction of the energy required in an insulator-metal transition coupled to structural transition. The second is absorption of latent heat and/or elastic energy from the surroundings of the switching element, thus reducing the externally supplied power consumption. Our aim is to shed light on the basic mechanisms governing the phase transition in Mott insulators and use defects, doping and strain engineering to understand and tune the switching mechanisms. This may allow for novel functionalities and ultralow energy consumption.
In Mott insulators electrical currents can change resistance by orders-of-magnitude due to a phase transition from insulating to metallic states. The volatility of switching in Mott insulators can be adjusted by several tuning parameters, enabling both memory devices and neuron-like functionalities. Moreover, Mott insulators have potential for extremely fast switching timescales and energy efficiency. These unique properties have made Mott insulators prominent candidate materials for resistive switching applications. However, the physical mechanisms behind resistive switching in these materials are poorly understood and not easily controllable, hampering advancement in this field.
We suggest two main routes towards Mott-based resistive switching with ultrahigh energy efficiency. The first is by switching purely in the electronic sector while minimizing structural distortions. The low heat capacity of electrons may enable switching with a fraction of the energy required in an insulator-metal transition coupled to structural transition. The second is absorption of latent heat and/or elastic energy from the surroundings of the switching element, thus reducing the externally supplied power consumption. Our aim is to shed light on the basic mechanisms governing the phase transition in Mott insulators and use defects, doping and strain engineering to understand and tune the switching mechanisms. This may allow for novel functionalities and ultralow energy consumption.
The metal-insulator phase transition in the prototypical Mott insulator V2O3 is a classic example of Mott physics. While the effects of doping, pressure, and anisotropic strain have been studied for years, their precise effects remain debated. We investigated pure V2O3 films with anisotropic strains to understand these influences. We found that films with certain distortions entered a “negative pressure” regime—similar to one observed in Cr-doped V2O3. In these conditions the V2O3 films exhibited a mix of paramagnetic insulating (PI) and metallic (PM) phases between 180–500 K (see Figure 1). Modeling via density functional theory aligned well with experimental findings. These results elucidate the most important structural degrees of freedom which dominate the phase diagram of V2O3, paving the way for manipulating the insulator-metal transition in V2O3 above room temperature and expanding its potential applications in advanced electronics. (see https://onlinelibrary.wiley.com/doi/epdf/10.1002/adfm.202211801(opens in new window))
Another important aspect of insulator-metal transitions in Mott insulators is the coupling between structural, electronic and magnetic degrees of freedom across the transition. This coupling makes it hard to determine the main driving mechanism behind the transition. Specifically, in V2O3 the role of magnetism has been debated and its interplay with the other transitions has not been established. To address this issue, we used a combination of several experimental techniques: muon spin relaxation/rotation measured (at the Paul Scherrer Institute), electrical transport and x-ray based reciprocal space mapping which allows to correlate magnetic, electronic and structural degrees of freedom in strain-engineered V2O3 thin films. Evidence is found for a magnetic instability in the vicinity of the structural transition in both the insulating and metallic phases (see Figure 2). Our results reveal the importance of an AF instability in the paramagnetic phase in triggering the metal-insulator transition and the crucial role of the structural transition in allowing for the formation of an ordered antiferromagnetic state. (see https://arxiv.org/abs/2410.23030(opens in new window))
At the heart of our project is the understanding of non-thermal switching effects in Mott insulators. We have recently shown that, surprisingly, V2O3 can be made to transition from the metallic state to the insulating state by optical excitation. By following a non-thermal trajectory the metastable metallic state can be excited and driven through an out-of-equilibrium process into the insulating state. This counterintuitive effect has important implications on our understanding of dynamic processes in Mott insulators and for Mott-memory based applications requiring a “reset” process. (see https://journals.aps.org/prb/abstract/10.1103/PhysRevB.110.L081108(opens in new window))
Another important aspect of insulator-metal transitions in Mott insulators is the coupling between structural, electronic and magnetic degrees of freedom across the transition. This coupling makes it hard to determine the main driving mechanism behind the transition. Specifically, in V2O3 the role of magnetism has been debated and its interplay with the other transitions has not been established. To address this issue, we used a combination of several experimental techniques: muon spin relaxation/rotation measured (at the Paul Scherrer Institute), electrical transport and x-ray based reciprocal space mapping which allows to correlate magnetic, electronic and structural degrees of freedom in strain-engineered V2O3 thin films. Evidence is found for a magnetic instability in the vicinity of the structural transition in both the insulating and metallic phases (see Figure 2). Our results reveal the importance of an AF instability in the paramagnetic phase in triggering the metal-insulator transition and the crucial role of the structural transition in allowing for the formation of an ordered antiferromagnetic state. (see https://arxiv.org/abs/2410.23030(opens in new window))
At the heart of our project is the understanding of non-thermal switching effects in Mott insulators. We have recently shown that, surprisingly, V2O3 can be made to transition from the metallic state to the insulating state by optical excitation. By following a non-thermal trajectory the metastable metallic state can be excited and driven through an out-of-equilibrium process into the insulating state. This counterintuitive effect has important implications on our understanding of dynamic processes in Mott insulators and for Mott-memory based applications requiring a “reset” process. (see https://journals.aps.org/prb/abstract/10.1103/PhysRevB.110.L081108(opens in new window))
Understanding the means to tune the trajectory of phase transitions in Mott insulators is at the heart of this project. To this end it is crucial to understand the basic mechanisms governing the stability of the various phases. We have shown the how to control phase transitions by strain engineering in the prototypical Mott insulator V2O3 and through this, elucidated the intricate interplay between structural and magnetic degrees of freedom. We have also examined how to induce non-thermal transition trajectories using optical excitation, resulting in an unexpected optically induced metal-to-insulator transition. Our findings enable us to obtain a room-temperature transition in strained V2O3 thin films and to perform an optical reset to a higher resistance state. These effects, which may also apply to other Mott insulators, have potential in enabling new and improved functionalities in the field of resistive switching. To further develop these capabilities and shed light on the switching mechanisms, we are exploring new materials, fabrication methods and dopant combinations in conjunction with cutting-edge synchrotron and lab-based measurement techniques.