The AC Josephson effect refers to work that Brian Josephson published in 1962 regarding the flow of an electrical current between superconductors. In this work, for which he shared a 1973 Nobel Prize, Josephson predicted that when a constant voltage difference is maintained across two weakly linked superconductors separated by a thin insulating barrier (an arrangement now known as a Josephson junction), an alternating electrical current would flow through the junction (imagine turning on a water faucet and having the water start flowing up as well as down once it leaves the spigot). The frequency of the current oscillations is directly related to the applied voltage.
These predictions were fully confirmed by an immense number of experiments, and the standard volt is now defined in terms of the frequency of the Josephson AC current. The Josephson effect has numerous applications in physics, computing and sensing technologies. It can be used for ultra high sensitive detection of electromagnetic radiation, extremely weak magnetic fields and in superconducting quantum computing bits.
Now, experimental physicist Alexei Marchenkov and theoretician Uzi Landman at Georgia Tech have discovered that the AC Josephson effect can be used to detect mechanical motion of atoms placed in the Josephson junction.
"We show here that in addition to being able to detect the effects of electromagnetic radiation on the AC Josephson current, one can also use it to probe mechanical motions of atoms or molecules placed in the junction,” said Landman, director of the Center for Computational Materials Science, Regents and Institute professor, and Callaway Chair of Physics at Georgia Tech. “The prospect of being able to explore, and perhaps utilize, atomic-scale phenomena using this effect is very exciting.”
In January 2007, Marchenkov and Landman published a paper in Physical Review Letters detailing their discovery that fluctuations in the conductance of ultra-thin niobium nanowires are caused by a pair of atoms, known as a dimer, shuttling back and forth between the bulk electrical leads.
In this latest research, Marchenkov and Landman, along with their collaborators Zhenting Dai, Brandon Donehoo and Robert Barnett, report that when a microfabricated junction assembly is held below its superconducting transition temperature, unusual features are found in traces of the electrical conductance measured as a function of the applied voltage.
“In our experiments, only nanowires - which we know now to contain a single dimer have consistently shown a series of additional peaks in the conductance versus voltage curves. Since a peak in such measurements signifies a resonance and knowing that we have intrinsic high-frequency Josephson current oscillations, we started looking into the possible physical mechanisms,” said Marchenkov, assistant professor in the School of Physics.
The team hypothesized that the new measured peaks likely originate from mechanical motions of the dimer, which causes enhancement of the electrical current at particular values of the applied voltage. At each of the peak voltages, the frequency of the AC Josephson current would resonate with the vibrational frequency of the nanostructure in the junction.
Subsequent first principles calculations by Landman’s team predicted that such peaks would occur at three different frequencies, or voltages, and their integer multiples. One corresponds to a back and forth vibration of the dimer suspended between the two niobium electrode tips, a second corresponds to motion in the direction perpendicular to the axis connecting the two tips, and the remaining corresponds to a wagging, or rocking, vibration of the dimer about the inter-tip axis. Ensuing targeted experiments demonstrated that the resonance peaks disappear gradually as one approaches the superconducting transition temperature from below, while their positions do not change. These observations, exhaustive qualitative and quantitative agreement between experimental measurements and theoretical predictions confirm that vibrational motions of the nanowire atoms are indeed the cause for the newly observed conductance peaks.
Marchenkov and Landman plan to further explore vibrational effects in weak link junctions, using the information obtained through these studies for determining vibrational characteristics, atomic arrangements, and transport mechanisms in metallic, organic and biomolecular nanostructures.
“One of our aims is the development of devices and sensing methodologies that utilize the insights gained from our research,” said Landman.
David Terraso | EurekAlert!
Unraveling the nature of 'whistlers' from space in the lab
15.08.2018 | American Institute of Physics
Early opaque universe linked to galaxy scarcity
15.08.2018 | University of California - Riverside
Scientists at the University of California, Los Angeles present new research on a curious cosmic phenomenon known as "whistlers" -- very low frequency packets...
Scientists develop first tool to use machine learning methods to compute flow around interactively designable 3D objects. Tool will be presented at this year’s prestigious SIGGRAPH conference.
When engineers or designers want to test the aerodynamic properties of the newly designed shape of a car, airplane, or other object, they would normally model...
Researchers from TU Graz and their industry partners have unveiled a world first: the prototype of a robot-controlled, high-speed combined charging system (CCS) for electric vehicles that enables series charging of cars in various parking positions.
Global demand for electric vehicles is forecast to rise sharply: by 2025, the number of new vehicle registrations is expected to reach 25 million per year....
Proteins must be folded correctly to fulfill their molecular functions in cells. Molecular assistants called chaperones help proteins exploit their inbuilt folding potential and reach the correct three-dimensional structure. Researchers at the Max Planck Institute of Biochemistry (MPIB) have demonstrated that actin, the most abundant protein in higher developed cells, does not have the inbuilt potential to fold and instead requires special assistance to fold into its active state. The chaperone TRiC uses a previously undescribed mechanism to perform actin folding. The study was recently published in the journal Cell.
Actin is the most abundant protein in highly developed cells and has diverse functions in processes like cell stabilization, cell division and muscle...
Scientists have discovered that the electrical resistance of a copper-oxide compound depends on the magnetic field in a very unusual way -- a finding that could help direct the search for materials that can perfectly conduct electricity at room temperatur
What happens when really powerful magnets--capable of producing magnetic fields nearly two million times stronger than Earth's--are applied to materials that...
08.08.2018 | Event News
27.07.2018 | Event News
25.07.2018 | Event News
15.08.2018 | Physics and Astronomy
15.08.2018 | Earth Sciences
15.08.2018 | Physics and Astronomy