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Exact Edge Between Superconducting and Magnetic States has been Measured

Scientists at the U.S. Department of Energy’s Ames Laboratory have devised a technique to accurately measure the “exact edge” or onset upon which a magnetic field enters a superconducting material. The knowledge of this threshold known as the lower critical field plays a critical role in overcoming the challenges that have prevented the broader use of superconductivity in new technologies.

In condensed matter physics, scientists differentiate between various superconducting states. When put in a magnetic field, the upper critical field is defined as the strength of the field at which it eliminates superconducting behavior of a material.

The Meissner effect, can be seen as its opposite. The Meissner effect occurs when a material transitions into a superconducting state, completely expelling a magnetic field from its interior. In this state, the magnetic field is reduced to zero at a small (typically less than a micrometer) length called the London penetration depth.

However, what happens in the area between the upper critical field and the London penetration depth?

Virtually all superconductors are categorized as type II, meaning that at larger magnetic fields, they do not show a complete Meissner effect. Instead, they exhibit a mixed state, with quantized magnetic vortices called Abrikosov vortices threading the material and forming a two-dimensional vortex lattice.

These Abrikosov vortices and significantly affect the behavior of superconductors. Most importantly, flowing electrical current can push these vortices around, resulting in a disipation of superconductivity.

The point at which these vortices first begin to penetrate a superconductor is called the lower critical field, one that’s been notoriously challenging to measure because of a distortion of the magnetic field near sample’s edges. However, knowledge of this lower critical field is needed to better understand and control superconductors for applications.

“The boundary line, the temperature-dependent value of the magnetic field at which this happens, is very important; the presence of Abrikosov vortices changes the behavior of the superconductor a great deal,” said Ruslan Prozorov, an Ames Laboratory physicist who is an expert in superconductivity and magnetism. “Many of the applications for which we’d like to use superconductivity, like the transmission of electricity, are hindered by the existence of this vortex phase.”

To validate the novel procedure developed to measure this boundary line, Prozorov and his team probed three already well-studied superconducting materials.

They utilized a recently developed optical magnetometer that takes advantage of the quantum state of a certain kind of an atomic defect, called nitrogen-vacancy (NV) centers, in diamond.

The highly sensitive device allowed the scientists to measure minuscule deviations in the magnetic signal that reside very close to the sample edge, the onset of vortices penetration.

“Our method is non-invasive, very precise and has better spatial resolution than previously used methods,” said Prozorov.

In addition, theoretical calculations performed with another Ames Laboratory scientist, Vladimir Kogan, allow them to find the lower critical field values using the measured onset of vortex penetration.

The work was supported by DOE’s Office of Science. The scientists further discussed the research in the paper, “Measuring the Lower Critical Field of Superconductors Using Nitrogen-Vacancy Centers in Diamond Optical Magnetometry,” authored by K.R. Joshi, N.M. Nusran, M. A. Tanatar, Kyuil Cho, W. R. Meier, S.L. Bud’ko, P.C. Canfield, and R. Prozorov; and published in Physical Review Applied.

Ames Laboratory is a U.S. Department of Energy Office of Science national laboratory operated by Iowa State University.

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