Closing In On High-Temperature Superconductivity

The quest to know the mysterious recipe for high-temperature superconductivity, which could enable revolutionary advances in technologies that make or use electricity, just took a big leap forward thanks to new research by an international team of experimental and theoretical physicists.

The research paper appears in the journal Science on Sept. 16, 2016. The research is focused on revealing the mysterious ingredients required for high-temperature superconductivity — the ability of a material’s electrons to pair up and travel without friction at relatively high temperatures, enabling them to lose no energy — to be super efficient — while conducting electricity.

The research team’s achievements are an important step in recent efforts to improve today’s superconducting materials, which have superconducting powers only if they are cooled below a critical temperature, hundreds of degrees below the freezing point of water — temperatures at which helium is a liquid — making them impractical for use in most electronic devices.

“We want to understand exactly which ingredients are necessary for high-temperature superconductivity, a beautiful quantum phenomenon with potentially important uses,” said Marcos Rigol, professor of physics at Penn State University and a theorist on the research team led by Martin Zwierlein, professor of physics and principal investigator at the NSF Center for Ultracold Atoms and the Research Laboratory of Electronics at the Massachusetts Institute of Technology (MIT).

For the first time, experimenters on the team have made hundreds of observations of individual potassium atoms, cooled to just slightly above absolute zero, trapped by lasers in a two-dimensional grid, and interacting with each other in intriguing ways that could help to reveal the behaviors of superconducting electrons. The team’s scientists suspect that they have observed one of the important dynamics that contribute to producing high-temperature superconductivity; that is, that electrons start forming pairs that “bunch” with empty spaces in the lattice.

An important contribution of the theorists on the team is their demonstration that the mathematical model developed to understand real materials (the so-called Hubbard model) could reproduce the behaviors of the atoms in the team’s 2-D experiments within a certain temperature range.

“If we can discover all the essential ingredients for superconductivity, we will have the opportunity to design recipes — theoretical models — for making high-temperature superconducting materials that can have a wide range of practical and innovative uses,” Rigol said.

Zwierlein led the team in building the experimental setup to help identify the ideal conditions for inducing superconductivity. This “quantum simulator” experiment uses atoms in a 2-D gas as stand-ins for electrons in a superconducting solid in order “to understand what’s really going on in these superconductors, and what one should do to make higher-temperature superconductors, approaching hopefully room temperature,” Zwierlein said.

Because of strong interactions, which are thought to be essential for high-temperature superconductivity to occur, not even the most powerful computers in the world have been able to solve the Hubbard model at the temperatures at which electrons are expected to become superconducting. A challenge for physicists, then, is to come up with computational techniques that can solve this model at the lowest possible temperatures in the current supercomputers. Rigol and collaborators developed one such technique, which was able to describe the experimental results.

“Our theoretical results precisely describe how the atoms in our team’s 2-D experiments actually behaved within the accessible temperature range,” Rigol said. “If future experiments are able to demonstrate at lower temperatures that the atoms in the experimental quantum simulator become superconducting — at temperatures at which our equations are just too difficult to solve — then we will know for sure that our theoretical model of high-temperature superconductivity is a good one.” The approach is a bit like one used in the field of astrophysics, where the experimental detection of gravitational waves recently served as a test and a validation of Einstein’s theory of general relativity.

The team’s results are important because, if superconductivity is observed at lower experimental temperatures, “we will know for sure that strong repulsive interactions between the electrons can produce high-temperature superconductivity,” Rigol said.

“Achieving this understanding could have a profound impact in technology, as well, because knowing the features of a material that are necessary for producing high-temperature superconductivity could lead to the engineering of more advanced superconducting materials.”

In addition to Rigol at Penn State and Zwierlein at MIT, the research team includes Lawrence W. Cheuk, Matthew A. Nichols, Katherine R. Lawrence, Melih Okan, and Hao Zhang at the MIT-Harvard Center for Ultracold Atoms; Ehsan Khatami (a former postdoctoral researcher in Rigol’s group) at San José State University; Nandini Trivedi at Ohio State University; and Thereza Paiva at the Universidade Federal do Rio de Janiero.

This research was supported, in part, by the National Science Foundation, the Air Force Office of Scientific Research, the Army Research Office, and the David and Lucile Packard Foundation.

Superconductivity research reveals potential new state of matter

Carefully aligned microstructured devices of CeRhIn5 enabled high field transport measurements that reveal an in-plane symmetry breaking for magnetic fields of approximately 30 Tesla along the tetragonal c-axis. The anomaly size and direction is determined by a small in-plane component of the magnetic field.
Credit: Los Alamos National Laboratory

A potential new state of matter is being reported in the journal Nature, with research showing that among superconducting materials in high magnetic fields, the phenomenon of electronic symmetry breaking is common. The ability to find similarities and differences among classes of materials with phenomena such as this helps researchers establish the essential ingredients that cause novel functionalities such as superconductivity.

The high-magnetic-field state of the heavy fermion superconductor CeRhIn5 revealed a so-called electronic nematic state, in which the material’s electrons aligned in a way to reduce the symmetry of the original crystal, something that now appears to be universal among unconventional superconductors. Unconventional superconductivity develops near a phase boundary separating magnetically ordered and magnetically disordered phases of a material.

“The appearance of the electronic alignment, called nematic behavior, in a prototypical heavy-fermion superconductor highlights the interrelation of nematicity and unconventional superconductivity, suggesting nematicity to be common among correlated superconducting materials,” said Filip Ronning of Los Alamos National Laboratory, lead author on the paper. Heavy fermions are intermetallic compounds, containing rare earth or actinide elements.

“These heavy fermion materials have a different hierarchy of energy scales than is found in transition metal and organic materials, but they often have similar complex and intertwined physics coupling spin, charge and lattice degrees of freedom,” he said.

The work was reported in Nature by staff from the Los Alamos Condensed Matter and Magnet Science group and collaborators.

Using transport measurements near the field-tuned quantum critical point of CeRhIn5 at 50 Tesla, the researchers observed a fluctuating nematic-like state. A nematic state is most well known in liquid crystals, wherein the molecules of the liquid are parallel but not arranged in a periodic array. Nematic-like states have been observed in transition metal systems near magnetic and superconducting phase transitions. The occurrence of this property points to nematicity’s correlation with unconventional superconductivity. The difference, however, of the new nematic state found in CeRhIn5 relative to other systems is that it can be easily rotated by the magnetic field direction.

The use of the National High Magnetic Field Laboratory’s pulsed field magnet facility at Los Alamos was essential, Ronning noted, due to the large magnetic fields required to access this state. In addition, another essential contribution was the fabrication of micron-sized devices using focused ion-beam milling performed in Germany, which enabled the transport measurements in large magnetic fields.

Superconductivity is extensively used in magnetic resonance imaging (MRI) and in particle accelerators, magnetic fusion devices, and RF and microwave filters, among other uses.

Nickel Shows Potential For High-Temperature Superconductivity

A team of researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory has identified a nickel oxide compound as an unconventional but promising candidate material for high-temperature superconductivity.

The team successfully synthesized single crystals of a metallic trilayer nickelate compound, a feat the researchers believe to be a first.

“It’s poised for superconductivity in a way not found in other nickel oxides. We’re very hopeful that all we have to do now is find the right electron concentration.”

This nickel oxide compound does not superconduct, said John Mitchell, an Argonne Distinguished Fellow and associate director of the laboratory’s Materials Science Division, who led the project, which combined crystal growth, X-ray spectroscopy, and computational theory. But, he added, “It’s poised for superconductivity in a way not found in other nickel oxides. We’re very hopeful that all we have to do now is find the right electron concentration.”

Mitchell and seven co-authors announced their results in this week’s issue of Nature Physics.

Superconducting materials are technologically important because electricity flows through them without resistance. High-temperature superconductors could lead to faster, more efficient electronic devices, grids that can transmit power without energy loss and ultra-fast levitating trains that ride frictionless magnets instead of rails.

Only low-temperature superconductivity seemed possible before 1986, but materials that superconduct at low temperatures are impractical because they must first be cooled to hundreds of degrees below zero. In 1986, however, discovery of high-temperature superconductivity in copper oxide compounds called cuprates engendered new technological potential for the phenomenon.

But after three decades of ensuing research, exactly how cuprate superconductivity works remains a defining problem in the field. One approach to solving this problem has been to study compounds that have similar crystal, magnetic and electronic structures to the cuprates.

Nickel-based oxides – nickelates – have long been considered as potential cuprate analogs because the element sits immediately adjacent to copper in the periodic table. Thus far, Mitchell noted, “That’s been an unsuccessful quest.” As he and his co-authors noted in their Nature Physics paper, “None of these analogs have been superconducting, and few are even metallic.”

The nickelate that the Argonne team has created is a quasi-two-dimensional trilayer compound, meaning that it consists of three layers of nickel oxide that are separated by spacer layers of praseodymium oxide.

“Thus it looks more two-dimensional than three-dimensional, structurally and electronically,” Mitchell said.

This nickelate and a compound containing lanthanum rather than praseodymium both share the quasi-two-dimensional trilayer structure. But the lanthanum analog is non-metallic and adopts a so-called “charge-stripe” phase, an electronic property that makes the material an insulator, the opposite of a superconductor.

“For some yet-unknown reason, the praseodymium system does not form these stripes,” Mitchell said. “It remains metallic and so is certainly the more likely candidate for superconductivity.”

Argonne is one of a few laboratories in the world where the compound could be created. The Materials Science Division’s high-pressure optical-image floating zone furnace has special capabilities. It can attain pressures of 150 atmospheres (equivalent to the crushing pressures found at oceanic depths of nearly 5,000 feet) and temperatures of approximately 2,000 degrees Celsius (more than 3,600 degrees Fahrenheit), conditions needed to grow the crystals.

“We didn’t know for sure we could make these materials,” said Argonne postdoctoral researcher Junjie Zhang, the first author on the study. But indeed, they managed to grow the crystals measuring a few millimeters in diameter (a small fraction of an inch).

The research team verified that the electronic structure of the nickelate resembles that of cuprate
materials by taking X-ray absorption spectroscopy measurements at the Advanced Photon Source, a DOE Office of Science User Facility, and by performing density functional theory calculations. Materials scientists use density functional theory to investigate the electronic properties of condensed matter systems.

“I’ve spent my entire career not making high-temperature superconductors,” Mitchell joked. But that could change in the next phase of his team’s research: attempting to induce superconductivity in their nickelate material using a chemical process called electron doping, in which impurities are deliberately added to a material to influence its properties.