A Ferroelectric on the Edge

Physics 13, s49
A 2D electron gas sandwiched between a metal and a metal oxide exhibits “impossible” ferroelectric-like behavior.

The material strontium titanate, or STO for short, is a jack-of-all-trades: depending on how it is doped, it behaves as an insulator, a metal, a superconductor, or a ferroelectric. Moreover, when undoped (insulating) STO is layered with other materials, it can host a thin conductive interface, called a two-dimensional electron gas (2DEG). Julien Bréhin, from the French National Center for Scientific Research (CNRS) in Paris, and his colleagues have now induced a 2DEG using a ferroelectric form of STO. Surprisingly, the thin conductor exhibits a kind of ferroelectric memory effect in the way that it switches between two different resistance states.

As previous work has shown, doping STO with small amounts of calcium can transform it into a ferroelectric, which is a material that can retain a permanent electric polarization—like the permanent magnetization in a ferromagnet. Earlier research also demonstrated that depositing aluminum on pure STO can induce a 2DEG at the interface between the two materials.

For their experiment, Bréhin and colleagues combined both techniques, depositing an aluminum layer on top of calcium-doped STO. When they applied a variable electric field to the system, they witnessed the 2DEG at the interface cycling between two states—one with high resistance, the other with low resistance. Like in ferroelectrics, it was the electric-field history that determined which state the 2DEG occupied. This memory effect is a signature of electrical polarization in the 2DEG, which seems impossible given that free electrons in the two-dimensional conductor should move around and cancel any internal polarization. The team says that further study is needed to understand how polarization may be occurring in this two-dimensional system.

This research is published in Physical Review Materials.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

Subject Areas

Materials ScienceCondensed Matter Physics

Related Articles

Stuffing Water into Bird Feathers
Biological Physics

Stuffing Water into Bird Feathers

Researchers have uncovered the details of how the feathers of a desert sandgrouse absorb water, a finding that could aid in the design of water-storing artificial materials. Read More »

A Textbook Dirac Semimetal
Condensed Matter Physics

A Textbook Dirac Semimetal

Researchers have synthesized a chalcogenide compound that has the electronic structure of an ideal Dirac semimetal—which could facilitate the study of this exotic class of materials. Read More »

Bond Density Not Strength Controls Polymer Stickiness
Materials Science

Bond Density Not Strength Controls Polymer Stickiness

Experiments show that the sticky behavior of so-called associative polymers is controlled by the density of bonding structures, contradicting theoretical predictions. Read More »

More Articles