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“Strange” cuprate superconducting behavior motivates need for new models

By Rachel Berkowitz August 29, 2018
strange cuprate
The 100 T magnet at the National Magnetic Field Laboratory made the experiments possible. Credit: Los Alamos National Laboratory

The phenomenon of superconductivity, wherein a material has zero resistance to the flow of electrical current, was discovered in 1911 in mercury cooled to below 4 K (-269°C). Ever since, researchers have attempted to search for or develop materials that behave as superconductors at higher temperatures, with the ultimate goal of finding a room-temperature superconductor. In the 1980s, it was discovered that certain copper oxides, or cuprates, achieve superconductivity at the record high temperature of 138 K (-135°C). But the mechanism for their superconductivity is still a subject of considerable debate.

Now, researchers led by Arkady Shekhter at the National Magnetic Field Laboratory at Florida State University have discovered a new property of cuprates above the superconducting temperature, and characterized how the material’s behavior differs from that of a conventional metal as it approaches superconductivity. The work could help researchers develop a new quantum mechanical model for the inner workings of cuprates yielding high-temperature superconductivity.

Researchers describe the behavior of conventional metals, like copper and aluminium, in terms of a dense collection of strongly interacting electrons. The “standard model” for that behavior is known as the Fermi liquid theory. It captures the major properties of conventional metals, including good electrical and thermal conductivity, low heat capacity, and weak magnetic susceptibility.

Until recently, researchers thought that all metals must behave according to the Fermi liquid theory. But above their superconducting temperature, cuprates were found to become “strange” metals that violated certain behaviors that the standard model predicts. For example, their electrical resistance, or resistivity, changes in step (linearly) with temperature over a temperature range from the superconducting temperature to twice the room temperature. That is in contrast to conventional metals, whose resistivity changes with the square of temperature and does not feature such regularity over the same temperature range.

Shekhter and colleagues wanted to learn about other anomalies in cuprates. In particular, magnetic fields offer insights into metallic behavior because the fields couple directly to the charge carriers in the material. The researchers placed a strontium-doped lanthanum cuprate thin film (La2–xSrxCuO4) in magnetic fields up to 80 T.  They measured the electrical voltage when current flowed in the material at a range of field strengths. The researchers showed that when the magnetic field was increased, resistivity increased linearly with the field. They repeated their experiments over a range of temperatures and found the same magnetoresistance behavior for magnetic field dependence of resistivity.

“The way we learn about the inner workings of metals is by studying their response to external influences, such as elevated temperatures, magnetic fields, and pressure,” Shekhter says. He argues that the cuprate resistivity’s linear response to temperature could have convinced theorists simply to develop a more complex version of standard Fermi-liquid theory to explain superconductivity. But the linear magnetoresistance response to the magnetic field as well confirms that the standard model is not enough to describe high-temperature superconductors.

“It is an interesting set of experimental results and they underline that the equivalence of a Fermi-liquid theory for low-dimensional strongly correlated materials [thin-film strange metals] is still lacking after decades of research,” says physicist Mikael Fogelstroem, head of the Department of Microtechnology and Nanoscience at Chalmers University of Technology in Sweden.

Cuprates require a small amount of chemical doping to become superconductors. Most pure cuprates are actually insulators. Shekhter and colleagues are investigating the material’s behavior with different chemical dopings in order to pinpoint the doping that leads to the highest superconducting temperature.

“Our future understanding of metallic behavior in cuprates most probably [will] not include conventional metals as an approximation,” Shekhter says. Rather, he looks forward to explaining their behavior using “entirely new ideas and new language.”

Read the abstract in Science.