While the phenomenon of superconductivity — in which some materials lose all resistance to electric currents at extremely low temperatures — has been known for more than a century, the temperature at which it occurs has remained too low for any practical applications.
The discovery of “high-temperature” superconductors in the 1980s — materials that could lose resistance at temperatures of up to negative 140 degrees Celsius — led to speculation that a surge of new discoveries might quickly lead to room-temperature superconductors. Despite intense research, these materials have remained poorly understood.
There is still no agreement on a single theory to account for high-temperature superconductivity. Recently, however, researchers at MIT and elsewhere have found a new way to study fluctuating charge-density waves, which are the basis for one of the leading theories. The researchers say this could open the door to a better understanding of high-temperature superconductivity, and perhaps prompt new discoveries of higher-temperature superconductors.
The findings were published this week in the journal Nature Materials by assistant professor of physics Nuh Gedik; graduate student Fahad Mahmood; Darius Torchinsky, a former MIT postdoc who is now at the California Institute of Technology; and two researchers at Brookhaven National Laboratory.
Explaining the basis for high-temperature superconductivity remains “the hardest problem in condensed-matter physics,” Gedik says. But one way of getting a handle on this exotic state of matter is to study what happens to these materials near their “transition temperature,” the point below which they become superconductors.
Previous experiments have shown that above the transition temperature, there is a peculiar state where, Gedik says, “the material starts to behave very weirdly”: Its electrons act in unusual ways, which some physicists believe is caused by a phenomenon called charge-density waves. While the electron density in most conductors is uniform, Gedik explains, in materials with charge-density waves the density is distributed in a sinusoidal pattern, somewhat like ripples on a pond. But so far, such charge-density waves have only been detected in high-temperature superconductors under special circumstances, such as a particular level of doping (the introduction of atoms of another element onto its surface).
Some researchers have proposed that these waves are elusive in high-temperature superconductors because they fluctuate very rapidly, at speeds measured in picoseconds (trillionths of a second). “You can’t see it with conventional techniques,” Gedik says.
That’s where Gedik’s new approach comes in: His team has spent years perfecting methods for studying the movement of electrons by zapping them with laser pulses lasting just a few femtoseconds (or quadrillionths of a second), and then detecting the results with a separate laser beam.
Using that method, the researchers have now detected these fluctuating waves. To do this, they have selectively generated and observed two different collective motions of electrons in these waves: variation in amplitude (the magnitude of modulation of the waves) and in phase (the position of the troughs and peaks of the waves). These measurements show that charge density waves are fluctuating at an interval of only about 2 picoseconds.
“It’s not surprising that static techniques didn’t see them,” Gedik says, but “this settles the question: The fluctuating charge-density waves do exist” — at least in one of the cuprate compounds, the first high-temperature superconducting materials discovered in the 1980s.
Another question: What role, if any, do these charge-density waves play in superconductivity? “Are they helping, or are they interfering?” Gedik asks. To answer this question, the researchers studied the same material, with optimal doping, in which the superconducting transition temperature is maximized. “We see no evidence of charge-density waves in this sample,” Gedik says. This suggests that charge-density waves are probably competing with superconductivity.
In addition, it remains to be seen whether the same phenomenon will be observed in other high-temperature superconducting materials. The new technique should make it possible to find out.
In any case, detecting these fluctuations could help in understanding high-temperature superconductors, Gedik says — which, in turn, could “help in finding other [superconducting materials] that actually work at room temperature.” That elusive goal could enable significant new applications, such as electric transmission lines that eliminate the losses that now waste as much as 30 percent of all electricity produced.
David Hsieh, an assistant professor of physics at Caltech, says the phenomena detected by this research “are known to be very difficult to detect,” so this work “is a great technical achievement and a high-quality piece of research.” By showing for the first time that the fluctuating charge-density waves seem to compete with superconductivity, he says, “It provides the insight that finding a way to suppress this fluctuating charge-density wave order may simultaneously increase” the temperature limits of superconductivity.
The work, which also included researchers Anthony Bollinger and Ivan Bozovic of Brookhaven National Laboratory, was supported by grants from the National Science Foundation and the U.S. Department of Energy.
Sarah McDonnell | EurekAlert!
Electron tomography technique leads to 3-D reconstructions at the nanoscale
24.05.2018 | The Optical Society
These could revolutionize the world
24.05.2018 | Vanderbilt University
The more electronics steer, accelerate and brake cars, the more important it is to protect them against cyber-attacks. That is why 15 partners from industry and academia will work together over the next three years on new approaches to IT security in self-driving cars. The joint project goes by the name Security For Connected, Autonomous Cars (SecForCARs) and has funding of €7.2 million from the German Federal Ministry of Education and Research. Infineon is leading the project.
Vehicles already offer diverse communication interfaces and more and more automated functions, such as distance and lane-keeping assist systems. At the same...
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
25.05.2018 | Event News
02.05.2018 | Event News
13.04.2018 | Event News
25.05.2018 | Event News
25.05.2018 | Machine Engineering
25.05.2018 | Life Sciences