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Terahertz pulse drives strontium titanate into hidden ferroelectric phase

By Kendra Redmond July 31, 2019
Induced ferroelectric phase
Left: The initial configuration of STO, in which a central titanium ion (Ti4+) is surrounded by oxygen ions (red) and strontium ions (gray, Sr2+); there is no dipole moment and the crystal is paraelectric. Center: A THz pulse drives the soft lattice vibrational mode, causing positive and negative ions to move in different directions as shown by the arrows. Right: The resulting ferroelectric crystalline phase with lower-symmetry geometry and a dipole moment (μ). Credit: Science

With an intense pulse of terahertz radiation, researchers from the Massachusetts Institute of Technology (MIT) and the University of Pennsylvania (Penn) have induced an ultrafast phase transition in a metal oxide. As reported in a recent issue of Science, the transition reveals a hidden phase in which the ferroelectric crystal structure displays different properties than in other phases of the material, suggesting a path toward the collective, coherent control of materials structures. 

The physical properties of a solid are largely controlled by the collective vibrations (phonons) of its crystal lattice. Ferroelectric structural phase transitions are usually associated with the so-called “soft” vibrational mode, which induces a new lattice structure that has reduced crystal symmetry and leads to electric polarization.  

A decade ago, a collaboration led by Keith Nelson at MIT and Andrew Rappe at Penn published a theoretical study suggesting that in some materials, terahertz-frequency (THz) radiation could excite a resonance in the soft mode of the crystal that would move the ions in the lattice from their position in one ferroelectric domain structure to their position in another, changing the direction of the ferroelectric polarization. 

In this new research, Nelson, Rappe, and their colleagues studied whether they could directly drive the soft mode in strontium titanate (SrTiO3, STO) to activate a hidden ferroelectric phase. Hidden phases are metastable collective states of matter not usually accessible on a material’s equilibrium phase diagram. They are of special interest because hidden phases in conventional materials sometimes give rise to exotic physical properties. 

STO is a dielectric material with a cubic perovskite structure at room temperature. Unlike many perovskites, STO does not transition to a ferroelectric material at any point on its equilibrium phase diagram. Even at its critical temperature of 36 K, the material is paraelectric because quantum fluctuations prevent long-range ordering. As a result, the researchers call STO “a textbook example” of a material in the quantum paraelectric (QPE) phase. 

In an experiment at MIT led by then-graduate student Xian Li, the research team sent a single-cycle THz pump pulse through STO, followed by an optical pulse that probed the material’s response. They repeated this process for several different temperatures and THz field strengths. Spectroscopic analyses revealed characteristic signals of lower crystal symmetry, as well as nonlinear increases in dipole ordering and phonon amplitude as a function of field strength. These results demonstrate a QPE-to-ferroelectric phase transition. The new phase was maintained for about 10 ps. 

A complementary theoretical investigation by the Penn colleagues explored whether a single pulse could induce ferroelectricity in STO. The researchers ran a molecular dynamics simulation in which a rapid electric field pulse was applied to a supercell consisting of many units of the lattice. After running the simulation over a range of field strengths, they found that a THz pulse on the order of 200 kV/cm or greater can stimulate a soft mode response that drives ions to new positions in a ferroelectric lattice structure. This work also revealed how other vibrational modes adapt to the soft mode change, stabilizing the hidden polar phase. 

This research “highlights the unique capability of light to selectively deform a material lattice through vibrational resonances,” says Andrea Cavalleri, director of the Max Planck Institute (MPI) for the Structure and Dynamics of Matter and a professor at the University of Oxford. Cavalleri is familiar with this approach, as he recently led a separate research effort at MPI to influence the electric polarization of STO. The MPI team irradiated STO with a mid-infrared pulse over a range of temperatures and frequencies. They also saw signs of lattice deformation, signs that were most pronounced when the pulse was resonant with the highest-frequency vibrational mode of STO. Follow-up experiments suggested that the IR pulse induced a transition to a metastable ferroelectric phase that persisted for several hours. These results were published alongside the MIT-Penn results in Science

“[B]y driving a specific lattice deformation with a single-cycle terahertz pulse, Li et al. have shown that a ferroelectric order forms on ultrafast timescales. Gaining control of technologically relevant properties such as ferroelectricity on short timescales could open up new strategies for next-generation high-speed devices,” says Cavalleri. Rappe agrees. “These studies launch the age of ultrafast reconfiguration of nonlinear optics, paving the way for rapidly reconfigurable optical devices,” he says. 

The MIT-Penn team plans to explore applied facets of this research going forward. “We’ve dreamed for many years about coherent control over collective material structure,” says Nelson. “It is becoming possible to use THz fields to control crystal lattice structure, ferroelectric order, magnetic order, and electronic state (such as insulating or metallic). Next must be control over combinations of these properties in complex materials like high-temperature superconductors, in which changes in the properties are strongly coupled to each other,” he says. 

Read the abstract in Science.