Recent advancements in the field of electronic materials have brought researchers closer to controlling electronic states with remarkable precision. Addressing this complex challenge, a team from Peking University, led by Professor Nanlin Wang and in collaboration with Professor Qiaomei Liu and Associate Research Scientist Dong Wu, has discovered a novel method for manipulating the electronic polar states of certain materials using ultrafast laser pulses. Their study, published in Nature Communications on October 16, 2024, reveals how these lasers can achieve reversible, non-volatile control over the electronic polar order in a specific charge-density-wave (CDW) material called EuTe₄ at room temperature.
The significance of this discovery lies in its potential implications for the development of electronic devices that require precise control over electronic states, such as data storage, neuromorphic computing, and other advanced technologies. Traditionally, manipulating electronic states has been challenging due to factors like thermal instability and the need for extremely low temperatures. However, the findings of this study suggest a promising route for achieving stable control at ambient conditions.
The material at the heart of this research, EuTe₄, is a rare and unique CDW semiconductor that was first discovered by Dong Wu in 2019. EuTe₄ has a complex crystalline structure, characterized by the presence of in-plane polarization due to the regular arrangement of tellurium (Te) trimers. Its crystal structure is made up of tellurium sheets interspersed with europium-tellurium slabs, forming a superlattice that exhibits polar order. This polar arrangement breaks the spatial inversion symmetry along one axis of the crystal (the a-axis), resulting in intriguing electronic properties that are sensitive to external stimuli.
The research team focused on understanding how ultrafast laser pulses can induce reversible changes in the electronic properties of EuTe₄, particularly its second harmonic generation (SHG) signal and electrical resistance. SHG is a nonlinear optical process that is highly sensitive to changes in the material’s polar order, making it an ideal tool for probing the effects of laser excitation on the material. By employing 800 nm wavelength ultrafast laser pulses, the researchers were able to manipulate the SHG response and resistance of nm-thick samples of EuTe₄ at room temperature.
One of the key findings of this study is the discovery of two distinct regimes of laser excitation, each producing different effects on the material. In the “weak regime,” where the laser pulse fluence ranged between 1.5 to 2.5 mJ/cm², the team observed a hysteresis effect. This effect allowed them to induce non-volatile changes in both the SHG signal and electrical resistance. Essentially, after the laser pulse, the material retained its modified state without needing continuous external energy input, indicating its potential for data storage applications where non-volatility is crucial.
In the “strong regime,” with higher pulse fluences of 4 to 5 mJ/cm², the researchers were able to push the material into a completely new non-volatile state. This state was characterized by the absence of an SHG signal but a significant increase in electrical resistance. Further experimentation revealed that applying a sequence of laser pulses in this regime could induce a new polar state with lower resistance and a distinct SHG pattern. This suggests that EuTe₄ can be toggled between multiple stable electronic phases using targeted laser excitations, offering a potential pathway for creating reconfigurable electronic circuits.
The underlying mechanism behind these photo-induced changes in EuTe₄ appears to be linked to the reversible inversion of polarization within its layered structure. According to the study, the material behaves like an anharmonic two-level system, where higher laser fluences weaken the material’s polarization, while lower fluences help restore it by overcoming shallow energy barriers. The researchers propose that lattice distortions within the tellurium layers play a crucial role in this process, particularly under strong excitation conditions. However, more research is required to fully understand how these lattice dynamics influence the polar phases.
Another fascinating aspect of EuTe₄’s behavior is its potential to exhibit multiple polar phases due to its quasi-two-dimensional layered structure. The overall polarity of the material is determined by the relative stacking order of the tellurium layers, meaning that altering this order could result in different polar configurations. The study even hints at the possibility of an antipolar phase with inversion symmetry, a theoretical phase predicted by crystallographic models but not yet confirmed experimentally.
The implications of these findings are profound. By demonstrating that EuTe₄’s electronic polar states can be controlled with ultrafast lasers at room temperature, the research opens new possibilities for developing next-generation electronic devices. These could include non-volatile memory systems, where data is stored in the form of reversible changes in material states, or neuromorphic computing systems that mimic the functionality of biological neural networks. Furthermore, the ability to switch between multiple electronic phases in a controlled manner could lead to advancements in reconfigurable electronics, where a single device can perform multiple functions depending on its current state.
Source: Peking University