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 Hf_0.5Zr_0.5O₂. (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 Hf_0.5Zr_0.5O₂ for 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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"Quantum Materials"

This news report highlights the discover of a semimetal known as they Weyl-Kondo semimetals. I've mentioned something similar in a previous post.

However, it should be noted that there are already a lot of material whose properties "... cannot be explained by classical physics...", and many of them are now considered to be common materials, mostly used in our modern electronics.

In fact, early on in the development of quantum mechanics, superconductivity was discovered. We now know that, as stated by Carver Mead, superconductivity is the clearest manifestation of quantum mechanics. People at that time just didn't realize it back then because they don't have the QM tools yet at their disposal.

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Graphene Closer To Commercial Use

When an article related to physics makes it to the CNN website, you know it is a major news.

This article covers the recent "breakthrough" in graphene that may make it even more viable for commercial use. I'm highlighting it here in case you or someone else needs more evidence of the "application" of physics, and if anyone who thinks that something that got awarded the Nobel Prize in is usually esoteric and useless.

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Genius Materials on the ISS

Advances in material science on the International Space Station.



Pay attention, kids. These are physics applications that have direct impacts on your lives. You are using, at this very moment, things that were first studied as part of physics/material science.

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How To Destroy A Magnet

Besides learning how to destroy a magnet, you get to learn why certain things are magnetic, or can be magnetized.



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Molecular Wires With 2000% Change In Magnetoresistance!

Holy magnetic wires, Batman! This thing blows away those colossal magnetoresistance material! What's bigger than "colossal" for us to give a name to this thing?

A new paper appearing in Science today is reporting the properties of a molecular wire that has the capability of having a 2000% increase in its magnetoresistance, and all at room temperature!

Ironically, the new molecular wires aren't made with magnetic materials at all. Rather, their MR effect relies on the conductivity of nonmagnetic organic dye molecules called DXP, which the Italian automaker Ferrari once used to give their roadsters their trademark red color. Unlike conventional inorganic metals in which electrons zip through a crystalline lattice, in organics electrons must hop from one molecule to another, like pails of water being passed by a bucket brigade. To create a MR, material researchers need to switch off that bucket brigade in the presence of a magnetic field.

In organic materials researchers do this with a little help from quantum mechanics. A tenet of quantum mechanics called the Pauli Exclusion Principle states that no two fermions (particles in a family that includes electrons) can occupy the same quantum state. If two electrons with the same quantum state try to hop onto the same DXP, they can't. The bucket brigade turns off and resistance skyrockets.

Of course, we all know (don't we?) that this is the field that has been responsible for invention of computer magnetic storage disks, etc. So kids, this is another example of real-world, practical application of physics, and quantum mechanics in particular.

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Kelvin Probe

Hey, I'm an experimentalist. It may be a revelation to some because I see this blog linked to many different sites, and some of them referred to it as ".. written by a high energy theorist..."! Me? High energy theorist? :)

Anyhow, I think it is always good to highlight certain experimental techniques, especially the ones that I'm quite familiar with. So here's one.

Often, for most of us, our first encounter to measuring the work function of something is via the photoelectric effect. Later on, if some of us go into photoemission spectroscopy, we deal with the work function there as well. However, that is not the only means to measure the work function. A technique that does not require the use of any light source to perform such measurement is called the Kelvin probe.

If you understand the physics behind a pn junction in semiconductors, then you've already understood the physics used in a Kelvin probe. You have a metal (the probe) with a particular Fermi energy, in close proximity (or in contact) with another material that has a different Fermi energy. This creates a contact potential difference. The Kelvin probe measures this contact potential difference, which is the difference between the two Fermi energies.

The link that I gave above gives you more details on such a measurement. If you know the value of the Fermi energy of the probe, you get the value of the work function of the material being studied. The only caveat here is that if the sample you are looking at is a semiconductor, you do not get the "work function", but rather, the value of the Fermi energy with respect to the vacuum state. In many cases, this is called the work function (energy between the Fermi level and the vacuum state) of the semiconductor, but one should not confuse this with the photoemission threshold, because the Fermi level resides in the gap. The photoemission threshold is the energy between the top of the valence band and the vacuum level.

Confused? Let's go on.

A significant improvement in the Kelvin probe technique was introduced with the invention of the Atomic Force Microscopy (AFM). It turns out that one can adapt an AFM system to work in the Kelvin probe mode. This technique is now called Kelvin Probe Force Microscopy (KPFM). I've linked to a very nice review paper on the physics and capabilities of KPFM technique. This technique allows for the mapping of the surface potential (or work function if you know the work function of the tip) of a surface with spatial resolution of the order of 100 nm.

So why is this useful. Obviously, knowledge of the surface potential is crucial in the understanding of the behavior of many materials (read some of the references in the KPFM review paper). It is another parameter that is part of the characteristics of the material. It also provides another check to the value of the work function that are often obtained from photoemission experiments. In fact, some material actually react to the exposure of light (especially UV), and the value of the work function obtained may not be as accurate as it should be from photoemission measurement.

There. So now, if you haven't heard about this before, you've learned something new! :)

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Crystal Growth Under Zero Gravity

This is a rather interesting report on the growth of He4 crystal with and without gravity. They even have a video of the growth, which obviously occurs in a very short time.



Here's the synopsis accompanying the video:

There are several limitations to growing crystals on Earth, which is why a group of researchers from Japan have taken to the skies to grow them under zero gravity.

They have successfully grown helium crystals 10 times larger than can be achieved on Earth with ordinary materials and it took just a matter of seconds -- ordinary, classic crystals can take up to a thousand years to reach their final shape.

This was achieved using high pressures, extremely low temperatures (-270oC) and by splashing the crystals with a superfluid.

From 13 December, the paper can be downloaded from http://iopscience.iop.org/1367-2630/14/12/123023/article

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NOVA's "Making Stuff: Stronger"

If you are in the US or have access to PBS, this might be of interest to you.

The first part of a 4-part NOVA "Making Stuff" series will air tomorrow (Sept. 19, 2012). The first installment will be on the strongest material.

What is the strongest material in the world? Is it steel, Kevlar, carbon nanotubes, or something entirely new? NOVA kicks off the four-part series "Making Stuff" with a quest for the world's strongest substances. Host David Pogue takes a look at what defines strength, examining everything from steel cables to mollusk shells to a toucan's beak. Pogue travels from the deck of a U.S. naval aircraft carrier to a demolition derby to the country's top research labs to check in with experts who are re-engineering what nature has given us to create the next generation of strong stuff.

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The Physics Of Ponytails

I kid you not! They even came up with something called the "Rapunzel Number"!

It turns out that there is an important-enough research being done on the physics of ponytails that it warrants a publication in the Physical Review Letters, no less!

He and two other physicists have been trying to determine whether the shape of a ponytail can be deduced from the properties of a single hair. After all, a head with 100,000 strands is a complex physical system, as anyone with a copious coiffure can attest. 

And it turns out that there is a simple theory. The crucial characteristics are elasticity, density and curliness, which essentially tell how springy a piece of hair is, plus the length of the ponytail. The researchers came up with a simple formula that describes the ponytail shape when the hair is bundled together.

They called it the Rapunzel number. “We couldn’t resist,” Dr. Goldstein said. 

Oh dear!

I'll try to find the exact reference to this one when it appears and post it here. I wish they explain why this is important and what other implications this involved. Maybe it's written in the actual paper. The only thing I can find in the news article is the sentence "...  Dr. Goldstein said the findings could also be applied to bundles of other long filaments, including fiberglass and wool...", which doesn't say much.

Another news report on this story mention quite a bit more, but it is still unclear what properties actually that is so important.

Scientists said the work has implications for understanding the structure of materials made up of random fibers, such as wool and fur and will have resonance with the computer graphics and animation industry, where the representation of hair has been a challenging problem.

"Our findings extend some central paradigms in statistical physics and show how they can be used to solve a problem that has puzzled scientists and artists ever since Leonardo da Vinci remarked on the fluid-like streamlines of hair in his notebooks 500 years ago," Goldstein said.

So maybe this won't be nominated for an Ig Nobel after all! :)

Edit: the Ponytail physics is getting a Synopsis coverage! Click that link for the exact reference.

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Chemical Vapour Deposition Synthetic Diamond: Materials, Technology and Applications

This is a humongous review article on CVD diamond.

Abstract: Substantial developments have been achieved in the synthesis of chemical vapour deposition (CVD) diamond in recent years, providing engineers and designers with access to a large range of new diamond materials. CVD diamond has a number of outstanding material properties that can enable exceptional performance in applications as diverse as medical diagnostics, water treatment, radiation detection, high power electronics, consumer audio, magnetometry and novel lasers. Often the material is synthesized in planar form, however non-planar geometries are also possible and enable a number of key applications. This article reviews the material properties and characteristics of single crystal and polycrystalline CVD diamond, and how these can be utilized, focusing particularly on optics, electronics and electrochemistry. It also summarizes how CVD diamond can be tailored for specific applications, based on the ability to synthesize a consistent and engineered high performance product.

Diamond, and in particular these CVD diamonds, are not only strong and hard, but also can have properties that can be tailored to meet various needs simply by different doping. It can be a good field emitter with the proper doping. Not only is it a strong insulator electrically, but it also has a relatively good thermal conductivity for an insulator, which makes it a good heat conductor.

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