Atomic ‘Swiss Army knife’ precisely measures quantum materials

Technology News | July 7, 2020

By Rich Pell

Test and Measurement Electromechanical Analog Sensing / Conditioning Materials & processes Data Acquisition

Designed to to study quantum materials, the instrument images single atoms, maps atomic-scale “hills and valleys” on metal and insulating surfaces, and records the flow of current across atom-thin materials subject to giant magnetic fields. Together, say the researchers, these measurements can uncover new knowledge about a wide range of special materials that are crucial for developing the next generation of quantum computers, communications, and a host of other applications.

For example, the instrument could be used to measure properties such as the resistance-less flow of electric current, quantum jumps in electrical resistance that could serve as novel electrical switches, and new methods to design quantum bits, which could lead to solid-state-based quantum computers. In a paper on the device, the researchers present a detailed recipe for building the instrument, which they describe as “a kind of Swiss Army knife for atom-scale measurements.”

“We describe a blueprint for other people to copy,” says NIST researcher Joseph Stroscio. “They can modify the instruments they have; they don’t have to buy new equipment.”

The instrument combines a trio of precision measuring devices. Two of the devices – an atomic force microscope (AFM) and a scanning tunneling microscope (STM) – examine microscopic properties of solids, while the third tool records the macroscopic property of magnetic transport — the flow of current in the presence of a magnetic field.

“No single type of measurement provides all the answers for understanding quantum materials,” says NIST researcher Nikolai Zhitenev. “This device, with multiple measuring tools, provides a more comprehensive picture of these materials.”

To build the instrument, the researchers designed an AFM and a magnetic-transport-measuring device that were more compact and had fewer moving parts than previous versions. They then integrated the tools with an existing STM.

Both an STM and an AFM use a needle-sharp tip to examine the atomic-scale structure of surfaces. An STM maps the topography of metal surfaces by placing the tip within a fraction of a nanometer (billionth of a meter) of the material under study. By measuring the flow of electrons that tunnels out of the metal surface as the sharp tip hovers just above the material, the STM reveals the sample’s atomic-scale hills and valleys.

In contrast, an AFM measures forces by changes in the frequency at which its tip – which is mounted on a miniature cantilever – oscillates as it hovers over a surface. The oscillation frequency shifts as the sharp probe senses forces, such as the attraction between molecules, or the electrostatic forces with the material’s surface.

To measure magnetic transport, a current is applied across a surface immersed in a known magnetic field. A voltmeter records the voltage at different places on the device, revealing the electrical resistance of the material.

The ensemble is mounted inside a cryostat – a device that chills the system to one-hundredth of a degree above absolute zero. At that temperature, the random quantum jitter of atomic particles is minimized and large-scale quantum effects become more pronounced and easier to measure. The three-in-one device, which is shielded from external electrical noise, is also five to 10 times more sensitive than any previous set of similar instruments, say the researchers, approaching the fundamental quantum noise limit that can be achieved at low temperatures.

Although using three entirely independent instruments to make the same measurements is possible, say the researchers, inserting and then retracting each tool can disturb the sample and diminish the accuracy of the analysis. Separate instruments can also make it difficult to replicate the exact conditions, such as the temperature and rotation angle between each ultrathin layer of the quantum material, under which previous measurements were made.

“By connecting the atomic with the large scale, we can characterize materials in a way that we couldn’t before,” says Stroscio. “We have now achieved the ultimate resolution given by thermal and quantum limits in this new instrument.”

For more, see “Achieving µeV tunneling resolution in an in-operando scanning tunneling microscopy, atomic force microscopy, and magnetotransport system for quantum materials research .”