Left to right: Kamal Joshi, Ruslan Prozorov and Naufer Nusran. Photo: Ames Laboratory.
Left to right: Kamal Joshi, Ruslan Prozorov and Naufer Nusran. Photo: Ames Laboratory.

The US Department of Energy's Ames Laboratory has successfully demonstrated that a new type of optical magnetometer, the NV magnetoscope, can map a unique feature of superconductive materials that, along with zero resistance, defines superconductivity itself. That unique feature is the Meissner effect, which is the expulsion of the magnetic field during a material's transition to a superconducting state.

"The Meissner effect is the hallmark signature of a true superconductor, which separates it from a hypothetical perfect metal with zero resistance," said Ruslan Prozorov, an Ames Laboratory physicist who is an expert in superconductivity and magnetism at low temperatures. "That is fine in textbooks and in principle, but in real superconducting materials the Meissner effect is quite complicated. Robust screening of a magnetic field by a superconducting sample and Meissner expulsion upon cooling in a magnetic field can be confused. This effect is actually very weak and fragile and difficult to observe."

Until now, physicists have been able to observe the Meissner effect, but were unable to visualize its spatial distribution in a material and how that might vary between different superconducting compounds. With the new magnetoscope, which takes advantage of the quantum state of a particular kind of atomic defect, called nitrogen-vacancy (NV) centers, in diamond, it is now possible to map unique and distinguishing features of the Meissner effect.

While the science behind using NV centers as sensors is well known, scientists at Ames Laboratory wanted to know if the technology could be harnessed for probing magnetic fields with unprecedented sensitivity and good spatial resolution in various magnetic and superconducting materials.

"This technique, which is minimally invasive and extremely sensitive, is implemented in an optical device that operates successfully while samples are at the low temperatures (four degrees above absolute zero), which is necessary for quantum materials exploration. This was no trivial undertaking," said Prozorov.

A member of Prozorov's group, Ames Laboratory scientist Naufer Nusran, led the development of the NV magnetoscope, which comprises a diamond film with NV-centers implanted right beneath the surface. The group then used the NV magnetoscope to measure the spatial distribution of the Meissner effect, proving that the technique works and is ready to be deployed to study even more complex problems. The scientists report their work in a paper in the New Journal of Physics.

Nusran also partnered with the Center for Nanoscale Materials, a DOE Office of Science user facility at Argonne National Laboratory, to design and fabricate the nanoscale pillars of diamond, each with a single NV center, required for the construction of the magnetoscope, which took three years. Deployment of these sensors, now housed in Ames Laboratory's ultra-low-noise Sensitive Instrumentation Facility (SIF), is the next step in research for the Prozorov group.

The work has already led to some big surprises. Although iron-based superconductors are considered some of the most robust, they showed practically none of that ‘hallmark’ Meissner effect. "This is a big puzzle and we have no explanation," said Prozorov. "It will be an exciting new avenue in research to understand why this happens."

This story is adapted from material from Ames Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.