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John O | May 2017

New research details composition of complex nanomaterials in superconductors

A new study conducted by scientists at Yale University, Harvard University, and the U.S. Department of Energy’s Brookhaven National Laboratory (Upton, N.Y.) has demonstrated a novel technique combining atomic-force microscopy with near-field spectroscopy showing that even subtle forces can impart extensive damage at the nanoscale.


The new instrument, developed at Brookhaven and in use at Yale, combines atomic
force microscopy (AFM) and scanning near-field optical microscopy to provide
unprecedented insight into these complex nanomaterials. (Adrian Gozar)


According to a report on the Brookhaven Lab website, the study was conducted in part to determine why helium-ion beam lithography was unsuccessful at developing the high-performing superconducting nanowires that had been theorized and calculated in simulations.


The article explained, “In previous work, heavy ion beams were used to carve 10-nm-wide channels—some 10,000 times thinner than a human hair—through custom-made materials. However, the new study revealed beam-induced damage rippling out over 50 times that distance. At this scale, that difference was both imperceptible and functionally catastrophic.”


As noted in the article, this research could provide a significant benefit to the development of quantum computing, which has been undertaken by large companies such as IBM and Google.


In previous work, the researchers tried to use alternating superconductor-insulator-supercondcutor (SIS) interfaces (known as Josephson interfaces) that should have been easily built through helium-ion beam lithography (HIB), which focuses a particle beam to less than a nanometer. The nanowires were being implanted onto precise microchannels in LSCO (lanthanum, strontium, copper, and oxygen) thin films.


Despite simulations and calculations predicting it would work, superconductivity was suppressed when a current was pushed through nanowires that were narrower than a couple hundred nanometers, according to the article.


“The scientists turned to scanning near-field optical microscopy (SNOM) to examine the spectroscopic sheen on the HIB pathways,” the article continued. “But this technique, which funnels light through a gilded glass capillary, has a resolution limit of about 100 nanometers—much too large to examine the nanoscale superconducting interfaces.”


A special device was created that combined spectroscopy with atomic-field microscopy and worked at cryogenic temperatures, which was a necessity for testing the materials.


The article noted, “The novel technique revealed the unexpected and widespread damage left in the wake of the helium ions. Despite the 0.5-nanometer focus of the beam, its effects rattled atoms across a 500-nanometer spread and altered the structure enough to prevent superconductivity. For nanomaterial construction, this so-called lateral straggle is utterly untenable.”


While it was of course disappointing to the researchers to see that the superconductivity was not possible with the current design, they were excited at the prospect of being able to observe the reactions and get a better understanding of the mechanics of the nanoscale conductivity.


The work was recently published in Nano Letters. The abstract stated:


“Helium ion beams (HIB) focused to subnanometer scales have emerged as powerful tools for high-resolution imaging as well as nanoscale lithography, ion milling, or deposition. Quantifying irradiation effects is an essential step toward reliable device fabrication, but most of the depth profiling information is provided by computer simulations rather than the experiment.


“Here, we demonstrate the use of atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB.


“By imaging the infrared dielectric response obtained from light demodulation at multiple harmonics of the AFM tapping frequency, we find that amorphization caused by the nominally 0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film and by ∼500 nm laterally.


“This unexpectedly widespread damage in morphology and electronic structure can be attributed to a helium depth distribution substantially modified by the internal device interfaces.


“Our study introduces AFM-SNOM as a quantitative tomographic technique for noninvasive 3D characterization of irradiation damage in a wide variety of nanoscale devices.”

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