Researchers capture an image of negative capacitance in action

Researchers capture an image of negative capacitance in action
This image shows negative capacitance in action. Within the ferroelectric layer at the top of the image, the core region has a higher energy than other regions. This is where the local permitivity is negative. Credit: Pablo Garcia Fernandez & Javier Junquera, Universidad de Cantabria

For the first time ever, an international team of researchers imaged the microscopic state of negative capacitance. This novel result provides researchers with fundamental, atomistic insight into the physics of negative capacitance, which could have far-reaching consequences for energy-efficient electronics.

The team, led by scientists at the University of California, Berkeley, describes their results in a paper published in the January 14 issue of Nature.

Capacitors are simple devices that can store an . Their capacitance, or ability to store , is determined by how much the capacitor's charge changes when it is connected to a source, like a battery. Negative capacitance occurs when a change in charge causes the net voltage across a material to change in the opposite direction; so that a decrease in voltage leads to an increase in charge.

"The upshot is that the opposite relation between charge and voltage could locally enhance the voltage across the common dielectric material," said Sayeef Salahuddin, professor of electrical engineering and computer sciences, who led the overall effort. "The voltage 'amplification' gained could be used to reduce the supply voltage requirement in a transistor, thus making computers and other electronic devices more energy-efficient."

As we increasingly rely on computers for daily tasks, the energy needed to run these systems is becoming substantial. Studies show that the total electricity consumption by the world's data centers is equivalent to 10 percent of all electricity used in the United States. "This is where a new physical phenomenon such as negative capacitance could provide a completely new set of tools to improve the energy efficiency of our computers," said Salahuddin.

In 2008, Salahuddin theoretically predicted that the state of negative capacitance can be locally stabilized in a ferroelectric material by placing it together with another common dielectric, or insulating material. But until recently, this phenomenon could only be detected indirectly.

The work in this paper directly captured negative capacitance in an atomically perfect superlattice of ferroelectric-dielectric heterostructure, synthesized by the group of Ramamoorthy Ramesh, professor of physics and of material science and engineering. Using state-of-the-art imaging techniques, the researchers mapped out the polarization as well as the electric field with atomic resolution. This allowed them to estimate the local energy density, which clearly showed regions where the curvature of the energy density is negative, indicating stabilization of the steady-state negative capacitance.

The same results were also obtained from state-of-the-art modeling techniques. Salahuddin notes that the confluence of experimental observation and theoretical calculation provides a concrete validation of the negative capacitance concept as well as an atomistic picture of a material in this state.

"We believe that the microscopic insight of negative capacitance obtained in this work will allow researchers to design highly -efficient transistors that can exploit the negative capacitance in the most optimum manner," said Salahuddin. "The implication of our work, however, goes well beyond transistors. Negative could find use in batteries, super capacitors and non-conventional electromagnetic applications."

Explore further

Negative capacitance detected

More information: Ajay K. Yadav et al. Spatially resolved steady-state negative capacitance, Nature (2019). DOI: 10.1038/s41586-018-0855-y
Journal information: Nature

Citation: Researchers capture an image of negative capacitance in action (2019, January 21) retrieved 18 October 2019 from
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Jan 21, 2019
Ferroelectrics switch their polarization at a certain critical voltage
causes accumulation of bound charge at the material's surface
momentarily exceeds free charge supplied to the electrodes
a resistance placed between the electrodes
charge supplied by the external voltage reduced
decrease in voltage across the ferroelectric detected while the charge is still increasing
capacitance charge divided by voltage has negative value.

Pure Obfuscation at its finest
at critical polarisation voltage is achieved, while the charge is still increasing
in the same instant a resistance inserted between the electrodes as the supplied voltage is reduced
Remember how this started
as the charge is increasing the critical voltage of polarisation is reached
In those micro seconds, the charge is still increasing
We reduce the voltage in those micro seconds, as the charge is still increasing
This is not negative capacitance, it is time delay

Jan 21, 2019
We could equally at a voltage of our own choosing
switch the voltage
while the charge is still increasing
place a resistor across the electrodes as we reduce the voltage
Achieving the same result

Jan 21, 2019

if you short circuit such a capacitor that was pre-charged to a voltage of U0. This means it starts out with a charge of Q0=-|C|*U0. Short circuiting the two plates would allow a current flow driven by the voltage, which normally would discharge the capacitor. But here, instead of discharging, the capacitor would charge up more and more; ad infinitum. This is non-physical. There is no such thing as negative capacitance.

The concept of negative capacitance is often equated with inductance as long as a single AC frequency is concerned. More accurately, capacitance should be called inverse inductance. The negative sign comes in because of the property of the imaginary unit that the multiplicative and additive inverses are identical: 1/i=−i.

In other words, it a phenomenon where a capacitor appears to act like an inductor for an AC signal.

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