Researchers Use a Novel Approach to Create Stable “Supercrystals”

A research team, headed by The Pennsylvania State University (Penn State) and Argonne National Laboratory, along with the University of California, Berkeley, and two other national labs, has developed a stable “supercrystal” using a combination of “frustration” technique and a pulse of laser light.

This is one among the first instances of a novel state of matter with extended stability transformed by the energy emitted from a sub-picosecond laser pulse. Supported by the Department of Energy, the researchers are aiming to identify fascinating states of matter having extraordinary properties—properties that no longer exist in equilibrium in nature.

We are looking for hidden states of matter by taking the matter out of its comfortable state, which we call the ground state. We do this by exciting the electrons into a higher state using a photon, and then watching as the material falls back to its normal state. The idea is that in the excited state, or in a state it passes through for the blink of an eye on the way to the ground state, we will find properties that we would desire to have, such as new forms of polar, magnetic and electronic states.

Venkatraman Gopalan, Team Lead and Professor, Department of Materials Science, Penn State.

These states are discovered through a pump-probe method, in which a photon is fired by a laser at the sample for 100 femtoseconds at a wavelength of 400 nm—that is, blue light. After the electrons pump light activates the electrons into a higher energy, a probe light follows immediately; this probe light is a gentler pulse of light which reads the material’s state.

Finding a method to sustain the intermediate state of matter represented a significant challenge for the researchers, because the state may exist for just a fraction of a second and would subsequently disappear. Nevertheless, the research team found that, at room temperature, the supercrystal remains in that state basically forever.

According to Gopalan, this challenge can be compared to pushing a ball to roll down a mountain side, and unless something gets in its course, for example, a ledge, the ball will not come to rest until it gets to the base of the mountain. The researchers attained this by “frustrating the system”—that is, not permitting the material to do what it wishes to do, which is to enable it to reduce its energy completely without limitations.

The investigators achieved this feat by applying strontium titanate and lead titanate—which are single atomic layers of two materials—arranged in alternating layers on top of one another to construct a 3D structure. Lead titanate is known to be a ferroelectric, a type of polar material that possesses electrical polarization resulting in negative and positive electric poles within the material. On the other hand, strontium titanate is a non-ferroelectric material. As a result of this mismatch, the electric polarization vectors are forced to adopt an unusual path, curving back on themselves to create vortices, similar to water swirling down a drain.

The research team from Berkeley developed these kinds of layers on top of a crystal substrate, whose crystals had an intermediate size between the pair of layered materials. This offered a second level of “frustration,” as the layer of strontium titanate attempted to expand to conform with the substrate’s crystal structure, and in turn, caused the lead titanate to compress to conform to it. This puts the entire system into a fragile yet “frustrated” state with various phases arbitrarily spread in the volume.

At this stage, the team used a laser pulse to zap the material; laser pulse adds free charges to the material, introducing more electrical energy to the system and pushing it into a novel state of matter called a supercrystal. Such supercrystals possess a unit cell—the most basic repeating unit present in a crystal—relatively larger than all other normal inorganic crystals, with a million times larger volume compared to the unit cells of the normal two materials. This unique material locates this state on its own.

The supercrystal is different from transient states, and at room temperature, remains around essentially forever—at least a single year in this analysis—unless it is heated to approximately 350 °F where it is erased. It is possible to repeat the process by striking the material with a light pulse and then erasing through heat. Only ultra-short laser pulses, which have a certain minimum amount of threshold energy, can be used to create this state, which otherwise cannot be created by simply spreading out that energy across long pulses.

Vlad Stoica, the lead author and a post-doctoral scholar shared mutually between Argonne National Laboratory and Penn State, applied high-energy X-ray diffraction to analyze the supercrystal before and after it forms, distinctly demonstrating the change from chaotic matter into a supercrystal. The study results have been reported online in Nature Materials, on March 18^th, 2019.

By virtue of its short pulse duration, an ultrafast laser imprints excitations in materials faster than their intrinsic response time. While such dynamical transformations were already explored for decades to stimulate the ordering of materials, a strategy for their steady state stabilization seemed out of reach until now.

Vlad Stoica Study Lead Author and Post-Doctoral Scholar, Penn State and Argonne National Laboratory.

The Argonne research team used high-resolution X-ray diffraction along with imaging at the nanoscale level to view the evolution of permanent structural reordering.

“For the first time, we observed that a single ultrafast laser pulse irradiation of artificially layered polar material can induce long-range structural perfection when starting from relative disorder,” they stated. “This experimental demonstration has already stimulated theoretical developments and has important implications toward future realization of artificial nanomaterials that are not achievable by traditional fabrication.”

“The combination of X-rays and ultrafast optical sources at the Advanced Photon Source gave us the best opportunity to explore the supercrystal’s nanoscale structure, along with the ability to understand why the material could be repeatedly changed from ordered to disordered states,” stated John Freeland, a staff scientist at Argonne National Laboratory and corresponding author on “Optical Creation of a Supercrystal with Three-Dimensional Nanoscale Periodicity”. “This information, together with the modeling, gave us very deep insight into the physics behind the creation of this new phase.”

At Penn State, Long-Qing Chen’s theory group carried out computer calculations with the help of a phase-field software package mu-PRO that closely replicated the experimental outcomes.

It’s quite remarkable that our phase-field simulations were able to predict the three-dimensional real-space images of a supercrystal whose diffraction patterns generally match the experimental patterns, and to identify a range of thermodynamic conditions for the stability of the supercrystal. Such integrated experimental and computational studies are extremely useful and productive.

Long-Qing Chen, Hamer Professor, Department of Materials Science and Engineering, Penn State.

Chen is also a Professor of Engineering Science and Mechanics, and a Professor of Mathematics.

The work was contributed by other team members from Lawrence Berkeley National Lab and Oak Ridge National Lab.

Optical creation of a super crystal with 3D nanoscale periodicity (Video credit: L-Q Chen Group/Penn State)