Manufacturing Bits: Jan. 2

World’s coldest chip; thermoelectric materials; hot electrons.


World’s coldest chip
Using a network of nuclear refrigerators, the University of Basel and others claim to have set the record for the world’s coldest chip.

Researchers have cooled a chip to a temperature lower than 3 millikelvin. A millikelvin is one thousandth of a kelvin. Absolute zero is 0 kelvin or minus 273.15 °C. In the experiment, researchers used a chip that includes a Coulomb blockade thermometer (CBT). CBTs are precise electronic thermometers.

For years, researchers have been developing ultra-low temperature devices. However, it is challenging to cool a device below 10 mK, due to heat leaks, vibrations, microwave radiation and noise, according to researchers.

The University of Basel, however, has made a breakthrough in the arena. The idea is to develop devices in ultra-low temperatures close to absolute zero. This helps researchers understand the properties of physics close to absolute zero. This, in turn, could enable quantum states of matter and/or devices like superconducting qubits. Qubits are used in quantum computers.

In the experiment, meanwhile, researchers built a parallel network of nuclear refrigerators. Then, they used a magnetic refrigeration technique to attain low temperatures. In this case, the technique is called adiabatic nuclear demagnetization.

A chip with a Coulomb blockade thermometer on it is prepared for experiments at extremely low temperatures. (University of Basel, Department of Physics)

Then, researchers cooled a chip through its leads. With magnetic cooling, researchers cooled a nanoelectronic chip to a temperature below 2.8 millikelvin. In fact, they cooled the electrical connections down to 150 microkelvin, which is less than a thousandth of a degree away from absolute zero.

“The combination of cooling systems allowed us to cool our chip down to below 3 millikelvin, and we are optimistic than we can use the same method to reach the magic 1 millikelvin limit,” said Dominik Zumbühl, a professor at the University of Basel.

Thermoelectric materials
Osaka University and Hitachi have developed a new thermoelectric (TE) material with an improved power factor at room temperatures.

TE materials enable the so-called thermoelectric effect. If you apply heat on one side of the device, an electric current flows, according to Osaka University and Hitachi. Then, if you run an external current through the device, one side becomes hotter than the other, according to researchers.

TE materials have interesting properties, but they are limited to high-temperature devices. They are used in power generators, refrigerators and other products.

To address the issues, researchers combined silicon with ytterbium to create ytterbium silicide. The advantage of this compound is that the atoms occupy a mixture of valence states. It also has an unusual layered structure, which, in turn, enables it to have metal-like high electrical conductivity at low temperatures.

Figure 1. (a) Three-dimensional crystal structure of YbSi2, (b) view along the a-axis, and (c) along the c-axis. (© 2017 Kurosaki et al.)

The result is a high power factor. “Unfortunately, most TE materials are often based on rare or toxic elements,” said Sora-at Tanusilp, a researcher who co-authored the study. “To address this, we combined silicon – which is common in TE materials – with ytterbium, to create ytterbium silicide [YbSi2]. We chose ytterbium over other metals for several reasons. First, its compounds are good electrical conductors. Second, YbSi2 is non-toxic. Moreover, this compound has a specific property called valence fluctuation that makes it a good TE material at low temperatures.”

Ken Kurosaki, a researcher who co-authored the study, added: “The use of Yb shows we can reconcile the conflicting needs of TE materials through carefully selecting the right metals. Room-temperature TEs, with moderate power, can be seen as complementary to the conventional high-temperature, high-power devices. This could help unlock the benefits of TE in everyday technology.”

Hot electrons
The U.S. Department of Energy’s (DOE) Argonne National Laboratory has found a new way to convert energy from light into energetic electrons.

Argonne has created hybrid nanomaterials, based on silver nanocubes and gold films. These structures are separated by aluminum oxide spacers.

With the technology, researchers created energetic or hot electrons. Hot electrons carry the same amount of energy as a photon. This could one day enable advances in photocatalytic water splitting. This is when special materials convert solar energy into clean and renewable hydrogen fuel. It also enables photovoltaics, which convert solar energy into electricity.

“You want to preserve all that energy in the photon as much as possible, so we’re focusing on what kind of nanostructure you need in order to make a lot of those,” said Gary Wiederrecht, senior scientist and group leader of the Nanophotonics and Biofunctional Structures group at Argonne. “In larger particles, you see very few of these energetic electrons with energies near the photon energy. So you need a smaller particle.

“One of the key advances is our ability to produce energetic electrons over a very broad spectral range — from the ultraviolet through the visible and into the near infrared,” Wiederrecht said, adding that processes for converting sunlight to energetic electrons typically work within smaller bands of wavelength. “That’s less useful for solar energy opportunities than if you could create hot electrons over a much broader spectral range,” he said.