The distinction between the liquid and solid states of substances is evident in our everyday experiences. Water flows as a liquid, while ice remains rigid as a solid, reflecting the regular arrangement of atoms and molecules in crystalline solids that is disrupted upon melting. However, the nature of “liquid crystals,” intriguing states that exhibit a combination of order and disorder, has remained less clear. These unique states have significant applications, such as in liquid crystal displays (LCDs).
In an exciting development, researchers from the Max Planck Institute (MPI) for Multidisciplinary Sciences in Göttingen, in collaboration with colleagues from Kiel University (CAU), Deutsches Elektronen Sychrotron DESY, and the University of Göttingen, have achieved the creation of a state in a crystalline material that defies easy categorization as either liquid or crystalline.
The studied layered crystal, cultivated by Kai Rossnagel’s team at CAU and DESY, exhibits minimal distortion in its crystal structure at room temperature. This unique characteristic arises from the arrangement of thin layers of metal and sulfur atoms that are weakly bonded to each other.
When these layers are subjected to ultrashort laser pulses, the orientation of the distortion rapidly changes within a trillionth of a second, resulting in a sudden increase in the material’s electrical conductivity. During this transition, an intriguing highly disordered state emerges, despite both types of distortions maintaining an ordered structure with associated crystalline properties.
Short snapshot: State disappeared after nanosecond fraction
“After being stimulated by light, the atoms within the crystal structure have not yet settled into their slightly altered positions,” explains Till Domröse, a Ph.D. student at MPI and the study’s lead author, as published in the journal Nature Materials.
This transformation leads to an intriguingly disordered state known as a hexatic state, typically observed in liquid crystals. However, in this experiment, the hexatic state is remarkably volatile and dissipates within nanoseconds. Visualizing the hexatic state presented significant challenges for the measurement technology employed. It necessitated both an extremely high temporal resolution to capture a fleeting moment and a remarkable sensitivity to atomic positions to detect the subtle structural changes. While electron microscopes offer the required spatial resolution in principle, they are typically not fast enough.
In recent years, the team from Göttingen, led by Claus Ropers, a Max Planck Director, has bridged this gap by developing an “ultrafast” electron microscope capable of imaging exceedingly rapid processes in the nanoworld. “We utilized this microscope in our experiments, which allowed us to capture the unusually ordered phase and its temporal evolution through a series of images,” explains Ropers. “Simultaneously, we developed a novel high-resolution diffraction mode that will prove essential for investigating numerous other functional nanostructures.”
Unique layered crystals
“The layered crystals exhibit highly intricate dynamics, presenting a plethora of scientific inquiries and potential applications,” remarks Rossnagel, a member of the speaker group for the research focus KiNSIS (Kiel Nano, Surface and Interface Science) at CAU. “These captivating network-like structures can only be developed and comprehensively studied through close collaboration with cutting-edge research infrastructures like the MPI in Göttingen and DESY in Hamburg. This synergy enables outstanding research on quantum materials in northern Germany.”
Since the early 1980s, these remarkable crystals have been cultivated in Kiel, with a longstanding partnership between CAU and DESY, which is now formalized in the Ruprecht-Haensel Laboratory. “DESY’s highly precise nanoanalytics utilizing the PETRA III and FLASH facilities have played a vital role in ensuring the exceptional quality of our crystals and attracting inquiries from around the globe,” adds Rossnagel. Studies such as this collaborative effort with the MPI in Göttingen, uncovering a novel state within a quantum material, also pave the way for future collaborations with DESY research groups to enhance our understanding of emerging quantum materials.
Source: Kiel University