Atomic ‘Swiss Army Knife’ accurately measures materials for quantum computers

Images of individual atoms. Maps hills and valleys on an atomic scale over metal and insulating surfaces. And it records the flow of current through thin materials like atoms subject to giant magnetic fields. Scientists at the National Institute of Standards and Technology (NIST) have developed a novel instrument that can perform three types of atom-scale measurements simultaneously. Together, these measurements can uncover new insights into a wide range of special materials that are crucial to developing the next generation of quantum computers, communications, and a host of other applications.

From smartphones to multicookers, devices that perform multiple functions are often more convenient and potentially less expensive than the single-purpose tools they replace, and their multiple functions often work better in concert than separately. The new three-in-one instrument is a kind of Swiss Army knife for atomic scale measurements. NIST researcher Joseph Stroscio and his colleagues, including Johannes Schwenk and Sungmin Kim, present a detailed recipe for building the device on the Review of scientific instruments.

“We describe a plan for other people to copy,” said Stroscio. “They can modify the instruments they have; they don’t have to buy new equipment.”

By simultaneously taking measurements on scales ranging from nanometers to millimeters, the instrument can help researchers focus on the atomic origins of various unusual material properties that may be invaluable to a new generation of computers and communication devices. These properties include the flow of electrical current without resistance, quantum leaps in electrical resistance that could serve as new electrical switches, and new methods for designing quantum bits, which could lead to solid-state based quantum computers.

“By connecting the atomic with the large scale, we can characterize the materials in a way that we couldn’t before,” Stroscio said.

Although the properties of all substances have their roots in quantum mechanics, the physical laws that govern the Lilliputian realm of atoms and electrons, quantum effects can often be ignored on a large scale, like the macroscopic world that we all experience. days. But for a very promising class of materials known as quantum materials, which generally consist of one or more atomically thin layers, strong quantum effects persist between groups of electrons at great distances, and the rules of quantum theory can dominate even on macroscopic length scales. . These effects lead to remarkable properties that can be exploited for new technologies.

To study these properties more precisely, Stroscio and his colleagues combined a trio of precision measuring devices into a single instrument. Two of the devices, an atomic force microscope (AFM) and a scanning tunnel microscope (STM), examine the microscopic properties of solids, while the third tool records the macroscopic property of magnetic transport: current flow in presence of a magnetic field. .

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

To build the instrument, the NIST team designed an AFM and magnetic transport measurement 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 sharp point to examine the structure of surfaces on an atomic scale. 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 electron flux leaving the metal’s surface as the sharp tip floats just above the material, the STM reveals the atomic-scale hills and valleys of the sample.

In contrast, an AFM measures forces by changing the frequency at which its tip oscillates when it travels over a surface. (The tip is mounted on a miniature overhang, allowing the probe to oscillate freely.) The frequency of oscillation changes as the sharp probe detects forces, such as attraction between molecules or electrostatic forces with the surface of the material. To measure magnetic transport, a current is applied through a surface immersed in a known magnetic field. A voltmeter records the voltage at different locations on the device, revealing the electrical resistance of the material.

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

Although it is possible for three completely independent instruments, an STM, an AFM, and a magnetic transport setup, to perform the same measurements, inserting and then retracting each tool can alter the sample and decrease the accuracy of the analysis. Separate instruments can also make it difficult to replicate exact conditions, such as temperature and angle of rotation between each ultrathin layer of quantum material, under which previous measurements were made.

To achieve the goal of a highly sensitive three-in-one instrument, the NIST team partnered with an international team of experts, including Franz Giessibl from the University of Regensburg, Germany, who invented a highly effective AFM known as the qPlus AFM. The team chose a compact design that increased the stiffness of the microscope and equipped the system with a series of filters to detect radio frequency noise. The STM’s atomically thin needle was doubled as a force sensor for the AFM, which was based on a new force sensor design created by Giessibl for the three-in-one instrument.

For Stroscio, a pioneer in the construction of increasingly sophisticated STMs, the new device is something of a pinnacle in a career of more than three decades in scanning microscopy. His team, he noted, had been struggling for several years to dramatically reduce electrical noise in his measurements. “We have now achieved the maximum resolution given by the thermal and quantum limits in this new instrument,” said Stroscio.

“This feels like I have climbed the highest peak in the Rocky Mountains,” he added. “It is a good synthesis of everything I have learned in the last 30 years.”

This story is republished courtesy of NIST. Read the original story here.