Does the Definition of Resistance Change in Electricity?

Quantum Hall Effect
Quantum Hall Effect

Electrical resistance is a physical quantity that indicates how much a material opposes the flow of electric current. It is measured in Ohms (Ω) and since 2019 the International System of Units (SI) base units were last revised, the Ohm Von Klitzing constant h/e2 are defined in terms of Planck's constant and the charge on an electron, respectively, where h and e are.

Researchers in Japan have proposed a new way of describing the standard unit of electrical resistance that would eliminate the need for strong magnetic fields.

The new proposal, which will set a standard based on the quantum anomalous Hall effect instead of the ordinary quantum Hall effect, will significantly simplify the experimental setup required to measure a single quantum resistance. Now, let's briefly explain what the Hall Effect is.

What is the Hall Effect?

The Hall effect is the production of a voltage difference (Hall voltage) across an electrical conductor diagonal to an electric current in the conductor and an applied magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879.

If we go back to our article;

To measure this resistance with high precision, scientists use the fact that the von Klitzing constant relates to the quantized change in the Hall resistance of a two-dimensional system of electrons in the presence of a strong magnetic field.

This quantized change in resistance is known as the quantum Hall effect (QHE) and manifests itself in fields of about 10 Tesla in a material such as GaAs or AlGaAs. However, generating such high fields typically requires a superconducting electromagnet, and the stray fields associated with such magnets make it difficult to integrate a QHE-based resistance standard with a voltage standard. .

No Superconducting Magnet Required

A team led by Japanese researchers Yuma Okazaki, along with Minoru Kawamura and colleagues at Tohoku University in Sendai, performed high-precision measurements of quantized resistance without using a superconducting magnet.

They do this by using their measurements as a result of electron transport phenomena recently described in a family of materials known as ferromagnetic topological insulators. They did so by basing it on the quantum anomalous Hall effect (QHAE), a variant of QHE.

Using a small commercially available permanent magnet, the researchers were able to obtain high-precision measurements of the change in resistance.

In a study published in Nature Physics, the team improved Hall resistance to 10.-8 Ω-1 reported that they measured their sensitivity.

This means that the measurement uncertainty is not accurate enough for QAHE-based measurements to serve as the primary resistance standard.7 Contrasting with previous reports, there are more than a few parts in .

"Our result is an important milestone towards the quantum resistance standard without superconducting electromagnets," says Okazaki, lead author of the study.

He explains that the key to obtaining such high-precision measurements is an increase in the critical current at which QAHE breaks down.

This current is the upper limit of the electric current required to sustain the quantized resistance, and previous work by QAHE revealed that it depends on the quality of the ferromagnetic topological insulating film.

“We have optimized various parameters of film quality to improve its quality,” Okazaki said. These are its chemical composition and the temperature at which it grows,” he said.

“As a result, we get a critical current of about 1 microampere, which is one or two orders of magnitude higher than previously reported values.”

The researchers note that with their current device, they were only able to observe QAHE at temperatures below 0.1 K.

They concede that this is far from ideal, as achieving such low operating temperatures requires an expensive cryogenic system such as a 3He/4He dilution refrigerator.

Increasing this value above 0,3 K would be beneficial, Okazaki says, because such temperatures can be achieved with a more compact and less costly 3He sorption refrigerator.

The use of other host materials, along with further optimizing the growth conditions of the ferromagnetic topological insulator film, could enable such a temperature increase, he says.

Source: Physicsworld

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