Supercrystal Exhibits a Stable State Over Time

A supercrystal created by researchers from Pennsylvania State University and Argonne National Laboratory represents a new state of matter with long-term stability. The goal of the research team, which also includes researchers from the University of California, Berkeley; Oak Ridge National Laboratory; and Lawrence Berkeley National Laboratory, is to discover states of matter with unusual properties that do not 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,” said professor Venkatraman Gopalan. “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.”

To find hidden states, the researchers use a pump-probe technique, irradiating a sample for 100 fs at a wavelength of 400 nm (blue light). The pump light excites the electrons into a higher energy state. It is followed by a probe light, which is a gentler pulse that reads the state of the material.

Stimulation with ultrafast light pulses can realize and manipulate states of matter with emergent structural, electronic, and magnetic phenomena. However, these nonequilibrium phases are often transient, and the challenge is to stabilize them as persistent states.

To maintain the intermediate state of matter, the team found a way to block the system, by preventing the material from doing what it wanted to do, which was to minimize its energy fully without constraints.

A 3D image of a supercrystal from phase-field simulations using the software μ-PRO. Courtesy of L-Q Chen Group, Penn State.

The researchers stacked alternate layers of lead titanate and strontium titanate to build a 3D structure. Lead titanate is a ferroelectric material; strontium titanate is not. This mismatch forced the electric polarization vectors to take an unnatural path, curving back on themselves to make vortexes, like water swirling down a drain.

The Berkeley team grew these layers on top of a crystal substrate between the two layered materials. This provided a second level of obstruction, as the strontium titanate layer tried to stretch to conform with the crystal structure of the substrate, while the lead titanate had to compress to conform to it; putting the system into a state with multiple phases randomly distributed in the volume. At this point, the researchers irradiated the material, dumping free charges in the material to add extra electrical energy to the system. This drove the system into a new state of matter, a supercrystal.

These supercrystals have a unit cell much larger than any ordinary inorganic crystal, with a volume one million times larger than the unit cells of the original two materials. The material finds this state on its own, the researchers said.

Unlike transient states, this supercrystal state can persist potentially forever at room temperature. When it is heated to about 350 °F, it is erased. The process can be repeated by hitting the material with a light pulse and erasing it using heat. The supercrystal state can only be created by ultrashort laser pulses with a specific minimum of threshold energy, and cannot be created by spreading out the energy over long pulses.

The researchers used high-energy x-ray diffraction to examine the supercrystal before and after it formed. Their examination clearly showed the transformation from disordered matter into a supercrystal state.

“By virtue of its short pulse duration, an ultrafast laser imprints excitations in materials faster than their intrinsic response time,” researcher Vlad Stoica said. “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.”

The researchers used high-resolution x-ray diffraction combined with imaging at the nanoscale level to observe the evolution of irreversible structural reordering. They said that for the first time, they were able to observe that a single ultrafast laser pulse irradiation of artificially layered polar material can induce long-range structural perfection when starting from relative disorder. The team reported that this experimental demonstration has already stimulated theoretical developments and could have implications toward the future realization of artificial nanomaterials that cannot be developed through 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,” said John Freeland, staff scientist at Argonne National Lab. “This information, together with the modeling, gave us very deep insight into the physics behind the creation of this new phase.”

Researchers at Penn State performed computer calculations using a phase-field software package that closely simulated the experimental results. “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,” professor Long-Qing Chen said. “Such integrated experimental and computational studies are extremely useful and productive.”

The research was published in Nature Materials (https://doi.org/10.1038/s41563-019-0311-x).

Optical creation of a super crystal with 3D nanoscale periodicity. Courtesy of L-Q Chen Group, Penn State.