MIT's Terahertz Microscope Reveals Hidden Quantum Motion of Electrons Inside a Superconductor for the First Time
MIT physicists built the first terahertz near-field microscope and used it to directly image quantum oscillations of superconducting electrons in BSCCO, a result published in Nature.
A team of MIT physicists has built the first microscope capable of imaging quantum motion at terahertz frequencies, using it to directly observe the collective oscillation of superconducting electrons inside a material for the first time. The results, published in Nature, open a new observational window into how superconductors work at the quantum level and could inform future efforts to engineer room-temperature superconducting materials.
The microscope was trained on bismuth strontium calcium copper oxide, known as BSCCO (pronounced “BIS-co”), a copper-oxide compound that becomes superconducting at relatively high temperatures compared with conventional superconductors. When cooled near absolute zero, the material’s electrons form a frictionless “superfluid” that carries electrical current without resistance. Physicists had long predicted that this superfluid should oscillate collectively at terahertz frequencies, but no instrument existed to observe the phenomenon directly.
“This new microscope now allows us to see a new mode of superconducting electrons that nobody has ever seen before,” said Nuh Gedik, the Donner Professor of Physics at MIT and the study’s senior author.
Beating the Diffraction Limit
Terahertz radiation occupies the electromagnetic spectrum between microwaves and infrared light, with wavelengths exceeding 100 microns. That long wavelength normally makes it impossible to resolve microscopic features, because the diffraction limit prevents focusing light to a spot smaller than roughly half its wavelength. The MIT team circumvented this constraint by placing samples in the near field of a spintronic emitter, a stack of ultrathin metallic layers that produces sharp terahertz pulses when struck by a femtosecond laser.
By positioning the BSCCO sample extremely close to the emitter, the researchers trapped the terahertz field before it could spread out, compressing it into a region far smaller than its wavelength. A Bragg mirror, a reflective multilayered film structure, filtered out unwanted wavelengths and shielded the sample from the triggering laser pulse.
Seeing the Superfluid Jiggle
When the team fired terahertz pulses into the superconducting BSCCO sample, the instrument captured a distinctive signal: the terahertz field was “dramatically distorted, with little oscillations following the main pulse,” according to lead author Alexander von Hoegen, a postdoctoral researcher in MIT’s Materials Research Laboratory. Those trailing oscillations represent the superfluid of Cooper-paired electrons sloshing back and forth collectively at terahertz frequencies.
“It’s this superconducting gel that we’re sort of seeing jiggle,” von Hoegen said. The phenomenon, described in the paper as a “terahertz superfluid plasmon,” had been theoretically anticipated but never directly visualized.
The research team included collaborators from Harvard University, the Max Planck Institute for the Structure and Dynamics of Matter, the Max Planck Institute for the Physics of Complex Systems, and Brookhaven National Laboratory. Funding came from the U.S. Department of Energy, the Gordon and Betty Moore Foundation, and MIT’s Research Laboratory of Electronics.
Implications for Superconductor Research and Wireless Communications
The ability to probe superconducting materials at terahertz frequencies with microscopic resolution addresses a longstanding gap in experimental physics. Conventional tools could measure terahertz responses only in bulk, averaging signals across large sample areas and obscuring spatial variations in the superconducting state. The new near-field technique allows researchers to map how the superfluid behaves at different locations within a material, potentially revealing inhomogeneities that influence superconducting performance.
That spatial resolution could prove particularly valuable in the study of high-temperature and unconventional superconductors, where the mechanism behind electron pairing remains one of the major unsolved problems in condensed matter physics. By directly imaging the superfluid plasmon and tracking how it changes with temperature, doping, or crystal structure, researchers may gain new clues about what enables certain materials to superconduct at higher temperatures than others.
Beyond fundamental physics, the microscope also has potential applications in terahertz technology development. The terahertz band is a target for next-generation wireless communications because of its high bandwidth, but building practical terahertz devices requires understanding how the radiation interacts with microscopic structures.
“If you have a terahertz microscope, you could study how terahertz light interacts with microscopically small devices that could serve as future antennas or receivers,” von Hoegen noted.
The study was published in Nature, Volume 650, Issue 8103.