Terahertz-Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO$_3$ (1812.10785v1)

Abstract: "Hidden phases" are metastable collective states of matter that are typically not accessible on equilibrium phase diagrams. These phases can host exotic properties in otherwise conventional materials and hence may enable novel functionality and applications, but their discovery and access are still in early stages. Using intense terahertz electric field excitation, we show that an ultrafast phase transition into a hidden ferroelectric phase can be dynamically induced in the quantum paraelectric SrTiO$_3$. The induced lowering in crystal symmetry yields dramatic changes in the phonon excitation spectra. Our results demonstrate collective coherent control over material structure, in which a single-cycle field drives ions along the microscopic pathway leading directly to their locations in a new crystalline phase on an ultrafast timescale.

• The paper demonstrates how intense THz fields induce ferroelectricity in SrTiO3 by triggering nonlinear phonon responses, particularly above 340 kV/cm.
• It employs TFISH and TKE spectroscopy alongside MD simulations to track temperature-dependent soft mode transitions starting above 30 K.
• Findings indicate that controlling nonequilibrium phase transitions in SrTiO3 could pave the way for advanced memory devices and heterostructures.

Terahertz-Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO₃

The paper titled "Terahertz-Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO₃" presents significant advancements in the field of material science, focused on inducing and stabilizing hidden ferroelectric phases in a quantum paraelectric, strontium titanate (SrTiO₃ or STO). The research delineates an innovative approach employing intense terahertz (THz) electric field excitation to drive a controlled ultrafast phase transition in STO, originally known for its robust dielectric properties and cubic perovskite structure at room temperature.

Background and Methodology

The underlying phenomena prior to this paper reveal that hidden phases, while not observable in equilibrium phase diagrams, harbor unique properties with potential functional and application-driven implications. SrTiO₃ occupies a prominent position in studies of quantum paraelectricity, wherein the interplay of quantum fluctuations, antiphased structural distortions, and FE ordering prevents the classical long-range ferroelectric state at low temperatures.

To probe these aspects, the authors utilized a distinctive challenge: directly exciting soft phonon modes via a single-cycle terahertz field. This method allows dynamic generation of dipole moments, culminating in a macroscopically ferroelectric phase. Experiments involve using time-resolved techniques like terahertz-field-induced second-harmonic (TFISH) spectroscopy and the terahertz Kerr effect (TKE) spectroscopy to observe changes in STO crystal symmetry.

Key Findings

Experimental results demonstrate a threshold-dependent induction of a ferroelectric phase, effectively overcoming quantum fluctuations inherent to STO at low temperatures.

• Nonlinear Phonon Responses: The authors report highly nonlinear phonon responses occurring beyond certain THz field strengths, notably a marked rise in ferroelectric ordering when exposed to fields above 340 kV/cm.
• Temperature-Dependent Behavior: TFISH data indicate that the ferroelectric soft mode emerges distinctly above 30 K, with nonlinear phononic behavior becoming evident at lower temperatures and higher THz field amplitudes.
• Soft Mode Displacement and Ionic Displacement: A collective transition occurs via displacements of ions through soft mode amplitudes into a FE structure, an observation supported by molecular dynamics simulations.

Further insights from classical molecular dynamics (MD) simulations corroborate the experimental data, emphasizing the threshold field approximately at 1 MV/cm, essential for consistent ferroelectric polarization in STO. This theoretical model employing a supercell approach verifies that THz fields effectively induce phase transitions correlating well with observed macroscopic polarization.

Implications for Future Research

The reported findings extend the understanding of nonequilibrium material dynamics in solid-state physics, particularly for ferroelectric materials like STO. The implications for practical applications are notable, with potential pathways for developing novel memory devices, enhancive heterostructures, or superconductor innovations. Future work may include exploring other quantum paraelectrics and dynamic structure transformations, potentially widening the scope for coherent control methodologies. Additionally, understanding the coupling of THz fields with various phononic states could unlock new paradigms in material phase transitions.

In conclusion, this paper exemplifies the manipulation of hidden phases, aligned with advancing modern material science for both theoretical exploration and impactful technological deployment. Such coherent, terahertz-driven transformations reflect not merely on novel ferroelectric functionalities but also hint at broader possibilities across electronic and photonic domains.