Sunday, January 20, 2019

Negative Capacitance in Ferroelectric Material Finally Found

I love this sort of reports, because it is based on a material that has been discovered for a long time and rather common, it is based on a consequence of a theory, it has both direct applications and a rich physics, and finally, it has an amazing resemblance to what many physics students have seen in textbooks.

A group of researchers have finally confirmed the existence of negative capacitance in ferroelectric material haffnium zirconium oxide Hf0.5Zr0.5O2. (You may access the Nature paper here or from that news article).

Researchers led by Michael Hoffmann have now measured the double-well energy landscape in a thin layer of ferroelectric Hf0.5Zr0.5Ofor the first time and so confirmed that the material indeed has negative capacitance. To do this, they first fabricated capacitors with a thin dielectric layer on top of the ferroelectric. They then applied very short voltage pulses to the electrodes of the capacitor, while measuring both the voltage and the charge on it with an oscilloscope.

“Since we already knew the capacitance of the dielectric layer from separate experiments, we were then able to calculate the polarization and electric field in the ferroelectric layer,” Hoffmann tells Physics World. “We then calculated the double-well energy landscape by integrating the electric field with respect to the polarization.”

Of course, there are plenty of potential applications for something like this.

One of the most promising applications utilising negative capacitance are electronic circuits with much lower power dissipation that could be used to build more energy efficient devices than any that are possible today, he adds. “We are working on making such devices, but it will also be very important to design further experiments to probe the negative capacitance region in the structures we made so far to help improve our understanding of the fundamental physics of ferroelectrics.”

But the most interesting part for me is that, if you look at Fig. 1 of the Nature paper, the double-well structure is something that many of us former and current physics students may have seen. I know that I remember solving this double-well problem in my graduate level QM class. Of course, we were solving it energy-versus-space dimension, instead of the energy-versus-polarization dimension as shown in the figure.


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