A new leap in understanding nickel oxide superconductors

Researchers discover they contain a phase of quantum matter, known as charge density waves, that’s common in other unconventional superconductors. In other ways, though, they’re surprisingly unique.

BY GLENNDA CHUI

A new study shows that nickel oxide superconductors, which conduct electricity with no loss at higher temperatures than conventional superconductors do, contain a type of quantum matter called charge density waves, or CDWs, that can accompany superconductivity.

The presence of CDWs shows that these recently discovered materials, also known as nickelates, are capable of forming correlated states – “electron soups” that can host a variety of quantum phases, including superconductivity, researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University reported in Nature Physics today.

“Unlike in any other superconductor we know about, CDWs appear even before we dope the material by replacing some atoms with others to change the number of electrons that are free to move around,” said Wei-Sheng Lee, a SLAC lead scientist and  investigator with the Stanford Institute for Materials and Energy Science (SIMES) who led the study.

“This makes the nickelates a very interesting new system – a new playground for studying unconventional superconductors.”

Nickelates and cuprates

In the 35 years since the first unconventional “high-temperature” superconductors were discovered, researchers have been racing to find one that could carry electricity with no loss at close to room temperature. This would be a revolutionary development, allowing things like perfectly efficient power lines, maglev trains and a host of other futuristic, energy-saving technologies.

But while a vigorous global research effort has pinned down many aspects of their nature and behavior, people still don’t know exactly how these materials become superconducting.

So the discovery of nickelate’s superconducting powers by SIMES investigators three years ago was exciting because it gave scientists a fresh perspective on the problem. 

Since then, SIMES researchers have explored the nickelates’ electronic structure – basically the way their electrons behave – and magnetic behavior. These studies turned up important similarities and subtle differences between nickelates and the copper oxides or cuprates – the first high-temperature superconductors ever discovered and still the world record holders for high-temperature operation at everyday pressures.

Since nickel and copper sit right next to each other on the periodic table of the elements, scientists were not surprised to see a kinship there, and in fact had suspected that nickelates might make good superconductors. But it turned out to be extraordinarily difficult to construct materials with just the right characteristics.

“This is still very new,” Lee said. “People are still struggling to synthesize thin films of these materials and understand how different conditions can affect the underlying microscopic mechanisms related to superconductivity.”

Read more on the SLAC website

Image: An illustration shows a type of quantum matter called charge density waves, or CDWs, superimposed on the atomic structure of a nickel oxide superconductor. (Bottom) The nickel oxide material, with nickel atoms in orange and oxygen atoms in red. (Top left) CDWs appear as a pattern of frozen electron ripples, with a higher density of electrons in the peaks of the ripples and a lower density of electrons in the troughs. (Top right) This area depicts another quantum state, superconductivity, which can also emerge in the nickel oxide. The presence of CDWs shows that nickel oxides are capable of forming correlated states – “electron soups” that can host a variety of quantum phases, including superconductivity.

Credit: Greg Stewart/SLAC National Accelerator Laboratory

Spin keeps electrons in line in iron-based superconductor

Researchers from PSI’s Spectroscopy of Quantum Materials group together with scientists from Beijing Normal University have solved a puzzle at the forefront of research into iron-based superconductors: the origin of FeSe’s electronic nematicity. Using Resonant inelastic X-ray scattering (RIXS) at the Swiss Light Source (SLS), they discovered that, surprisingly, this electronic phenomenon is primarily spin driven. Electronic nematicity is believed to be an important ingredient in high-temperature superconductivity, but whether it helps or hinders it is still unknown. Their findings are published in Nature Physics.

Near PSI, where the Swiss forest is ever present in people’s lives, you often see log piles: incredibly neat log piles. Wedge shaped logs for firewood are stacked carefully lengthways but with little thought to their rotation. When particles in a material spontaneously line up, like the logs in these log piles, such that they break rotational symmetry but preserve translational symmetry, a material is said to be in a nematic state. In a liquid crystal, this means that the rod shaped molecules are able to flow like a liquid in the direction of their alignment, but not in other directions. Electronic nematicity occurs when the electron orbitals in a material align in this way. Typically, this electronic nematicity manifests itself as anisotropic electronic properties: for example, resistivity or conductivity exhibiting vastly different magnitudes when measured along different axes.

Since their discovery in 2008, the past decade has seen enormous interest in the family of iron based superconductors. Alongside the well-studied cuprate superconductors, these materials exhibit the mysterious phenomenon of high temperature superconductivity. The electronic nematic state is a ubiquitous feature of iron-based superconductors. Yet, until now, the physical origin of this electronic nematicity is a puzzle; in fact, arguably one of the most important puzzles in the study of iron-based superconductors.

Read more on the PSI website

Image: Resonant inelastic x-ray scattering reveals high-energy nematic spin correlations in the nematic state of the iron-based superconductor, FeSe

Credit: Beijing Normal University/Qi Tang and Xingye Lu

Physicists uncover secrets of world’s thinnest superconductor

Physicists report the first experimental evidence to explain the unusual electronic behaviour behind the world’s thinnest superconductor, a material with myriad applications because it conducts electricity extremely efficiently. In this case the superconductor is only an atomic layer thick. 

The research, led by Massachusetts Institute of Technology and Brookhaven National Laboratory, was possible thanks to new instrumentation available at Diamond.  

Diamond is one of only a few facilities in the world to use the new experimental technique, Resonant Inelastic X-ray Scattering (RIXS), which is a combination of X-ray Absorption Spectroscopy (XAS) and X-ray Emission Spectroscopy (XES), where both the incident and emitted energies are scanned. This state-of-the-art facility is where the team from three continents conducted their experiment.  

Read more on the Diamond website

Image: Members of the RIXS team at Diamond. Left to right: Jaewon Choi (Postdoc), Abhishek Nag (Postdoc), Mirian Garcia Fernandez (Beamline Scientist), Charles Tam (joint PhD student), Thomas Rice (Beamline technician), Ke-Jin Zhou (Principal Beamline Scientist), Stefano Agrestini (Beamline Scientist).

Scientists glimpse signs of a puzzling state of matter in a superconductor

Known as “pair-density waves,” it may be key to understanding how superconductivity can exist at relatively high temperatures.

Unconventional superconductors contain a number of exotic phases of matter that are thought to play a role, for better or worse, in their ability to conduct electricity with 100% efficiency at much higher temperatures than scientists had thought possible – although still far short of the temperatures that would allow their wide deployment in perfectly efficient power lines, maglev trains and so on.

Now scientists at the Department of Energy’s SLAC National Accelerator Laboratory have glimpsed the signature of one of those phases, known as pair-density waves or PDW, and confirmed that it’s intertwined with another phase known as charge density wave (CDW) stripes – wavelike patterns of higher and lower electron density in the material.

Observing and understanding PDW and its correlations with other phases may be essential for understanding how superconductivity emerges in these materials, allowing electrons to pair up and travel with no resistance, said Jun-Sik Lee, a SLAC staff scientist who led the research at the lab’s Stanford Synchrotron Radiation Lightsource (SSRL).

Read more on the SLAC website

Image: SLAC scientists used an improved X-ray technique to explore exotic states of matter in an unconventional superconductor that conducts electricity with 100% efficiency at relatively high temperatures. They glimpsed the signature of a state known as pair density waves (PDW), and confirmed that it intertwines with another phase known as charge density wave (CDW) stripes – wavelike patterns of higher and lower electron density in the material. CDWs, in turn, are created when spin density waves (SDWs) emerge and intertwine.

Credit: Jun-Sik Lee/SLAC National Accelerator Laboratory

A new approach for studying electric charge arrangements in a superconductor

X-ray scattering yields new information on “charge density waves”

High-temperature superconductors are a class of materials that can conduct electricity with almost zero resistance at temperatures that are relatively high compared to their standard counterparts, which must be chilled to nearly absolute zero—the coldest temperature possible. The high-temperature materials are exciting because they hold the possibility of revolutionizing modern life, such as by facilitating ultra-efficient energy transmission or being used to create cutting-edge quantum computers.

One particular group of high-temperature superconductors, the cuprates, has been studied for 30 years, yet scientists still cannot fully explain how they work: What goes on inside a “typical” cuprate?

Piecing together a complete picture of their electronic behavior is vital to engineering the “holy grail” of cuprates: a versatile, robust material that can superconduct at room temperature and ambient pressure.

Read more on the NSLS-II website

Image: Brookhaven Lab scientist Mark Dean used the Soft Inelastic X-Ray (SIX) beamline at the National Synchrotron Light Source II (NSLS-II) to unveil new insights about a cuperates, a particular group of high-temperature superconductors. Credit: BNL

Observation of flat bands in twisted bilayer graphene

Magic-angle materials represent a surprising recent physics discovery in double layers of graphene, the two-dimensional material made of carbon atoms in a hexagonal pattern. 

When the upper layer of two stacked layers of graphene is rotated by about 1 degree, the material suddenly turns into a superconductor. At a temperature of 3 Kelvin, this so-called twisted bilayer graphene (tbg) conducts electricity without resistance.

Now, an international team of scientists from Geneva, Barcelona, and Leiden have finally confirmed the mechanism behind this new type of superconductors. In Nature Physics, they show that the slight twist causes the electrons in the material to slow down enough to sense each other. This enables them to form the electron pairs which are necessary for superconductivity.

How can such a small twist make such a big difference? This is connected with moiré patterns, a phenomenon also seen in the everyday world. When two patterned fences are in front of another, one observes additional dark and bright spots, caused by the varying overlap between the patterns. Such moiré patterns (derived from the the French name of textile patterns made in a similar way) generally appear where periodical structures overlap imperfectly.

Read more on the Elettra website

Image: Angle resolved photoemission spectrum revealing flat non-dispersing electronic band filled with slow electrons separated by mini gaps from the rest of electronic structure in twisted bilayer graphene device.

Investigating high temperature superconductors

Researchers from the ARC Centre of Excellence in Future Low Energy Electronic Technologies (FLEET) used the Soft X-ray Spectroscopy beamline at the Australian Synchrotron to investigate the structure of a promising high-temperature superconductor, a calcium-doped graphene material.

The FLEET Centre has provided a detailed description of the research, published in The Chemistry of Materials, on their website.

In characterising the material, the investigators wanted to clarify where the calcium went after it was added to a sample consisting of a single layer of graphene on a silicon carbide substrate.

Measurements at the Australian Synchrotron were able to pinpoint that the calcium atoms were located, unexpectedly, near the silicon carbide surface.

Read more on the ANSTO website

Image: Dr Anton Tadich (far right) with SXR beamline team members and researchers from the FLEET Centre.

Scientists use pressure to make liquid magnetism breakthrough

It sounds like a riddle: What do you get if you take two small diamonds, put a small magneticcrystal between them and squeeze them together very slowly?

The answer is a magnetic liquid, which seems counterintuitive. Liquids become solids under pressure, but not generally the other way around. But this unusual pivotal discovery, unveiled by a team of researchers working at the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Argonne National Laboratory, may provide scientists with new insight into high-temperature superconductivity and quantum computing.

Though scientists and engineers have been making use of superconducting materials for decades, the exact process by which high-temperature superconductors conduct electricity without resistance remains a quantum mechanical mystery. The telltale signs of a superconductor are a loss of resistance and a loss of magnetism. High-temperature superconductors can operate at temperatures above those of liquid nitrogen (−320 degrees Fahrenheit), making them attractive for lossless transmission lines in power grids and other applications in the energy sector.

Read more on the APS website

Image: APS

A closer look at superconductors

A new measuring method helps understand the physics of high-temperature superconductivity

From sustainable energy to quantum computers: high-temperature superconductors have the potential to revolutionize today’s technologies. Despite intensive research, however, we still lack the necessary basic understanding to develop these complex materials for widespread application. “Higgs spectroscopy” could bring about a watershed as it reveals the dynamics of paired electrons in superconductors. An international research consortium centered around the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Max Planck Institute for Solid State Research (MPI-FKF) is now presenting the new measuring method in the journal Nature Communications (DOI: 10.1038/s41467-020-15613-1). Remarkably, the dynamics also reveal typical precursors of superconductivity even above the critical temperature at which the materials investigated attain superconductivity.

Read more on the TELBE at HZDR website

Image: Deciphering previously invisible dynamics in superconductors – Higgs spectroscopy could make this possible: Using cuprates, a high-temperature superconductor, as an example, an international team of researchers has been able to demonstrate the potential of the new measurement method. By applying a strong terahertz pulse (frequency ω), they stimulated and continuously maintained Higgs oscillations in the material (2ω). Driving the system resonant to the Eigenfrequency of the Higgs oscillations in turn leads to the generation of characteristic terahertz light with tripled frequency (3ω).

Preparation and characterization of mesoscale single crystals

What did the Scientists Discover?
Single crystals are materials with periodic structure that extends across macroscopic distances as a coherent lattice free of grain boundaries. By isolating and studying their properties, bulk single crystals have revolutionized our fundamental understanding of materials from semiconductors to biomacromolecules, fueling innovations from microelectronic devices to pharmaceutical compounds. In contrast, our understanding of many mesostructured materials is still in its infancy in part due to the lack of available single crystals. Block copolymer self-assembly of mesostructured systems presented here is a promising method to prepare periodic 10–100 nm structures with coherent orientation over macroscopic lengths enabling their study.

Why is this important?
The method presented here can prepare macroscopic bulk single crystals with other block copolymer systems, suggesting that the method is broadly applicable to block copolymer materials assembled by solvent evaporation. It is expected that such bulk single crystals will enable fundamental understanding and control of emergent mesostructure-based properties in block-copolymer-directed metal, semiconductor, and superconductor materials.

>Read more on the CHESS website

Image: (extract, full image see here) Representative SAXS patterns with log scale colors from locations as indicated in (c), exhibiting polycrystalline (e), multi-(three) crystalline (f), and single crystal (g) behavior. Diagonal bars across bottom are shadows from photodiode wire.

The role of ‘charge stripes’ in superconducting materials

The studies could lead to a new understanding of how high-temperature superconductors operate.

High-temperature superconductors, which carry electricity with zero resistance at much higher temperatures than conventional superconducting materials, have generated a lot of excitement since their discovery more than 30 years ago because of their potential for revolutionizing technologies such as maglev trains and long-distance power lines. But scientists still don’t understand how they work.
One piece of the puzzle is the fact that charge density waves – static stripes of higher and lower electron density running through a material – have been found in one of the major families of high-temperature superconductors, the copper-based cuprates. But do these charge stripes enhance superconductivity, suppress it or play some other role?
In independent studies, two research teams report important advances in understanding how charge stripes might interact with superconductivity. Both studies were carried out with X-rays at the Department of Energy’s SLAC National Accelerator Laboratory.

>Read more on the LCLS at SLAC website

Image: This cutaway view shows stripes of higher and lower electron density – “charge stripes” – within a copper-based superconducting material. Experiments with SLAC’s X-ray laser directly observed how those stripes fluctuate when hit with a pulse of light, a step toward understanding how they interact with high-temperature superconductivity.
Credit: Greg Stewart/SLAC National Accelerator Laboratory

Superconductor exhibits “glassy” electronic phase

The study provides valuable insight into the nature of collective electron behaviors and how they relate to high-temperature superconductivity.

At extremely low temperatures, superconductors conduct electricity without resistance, a characteristic that’s already being used in cryogenically cooled power lines and quantum-computer prototypes. To apply this characteristic more widely, however, it’s necessary to raise the temperature at which materials become superconducting. Unfortunately, the exact mechanism by which this happens remains unclear.

Recently, scientists found that electrons in cuprate superconductors can self-organize into charge-density waves—periodic modulations in electron density that hinder the flow of electrons. As this effect is antagonistic to superconductivity, tremendous effort has been devoted to fully characterizing this charge-order phase and its interplay with high-temperature superconductivity.

>Read more on the Advanced Light Source at L. Berkeley Lab website

Image: At low doping levels, the charge correlations in the copper–oxide plane possess full rotational symmetry (Cinf) in reciprocal space (left), in marked contrast to all previous reports of bond-oriented charge order in cuprates. In real space (right), this corresponds to a “glassy” state with an apparent tendency to periodic ordering, but without any preference in orientation (scale bar ~5 unit cells).

Exotic properties of iridium compounds

Scientists at DESY’s X-ray source PETRA III and the London Centre for Nanotechnology, at University College London, have developed a new method for examining the astonishing properties of a special class of iridium oxides known as iridates. The team of principal author Pavel Alexeev, from the Dynamics Beamline P01 at PETRA III, is presenting the procedure in the journal Scientific Reports.

Many oxides belonging to certain groups of transition metals (chemical elements with an incomplete d electron shell) are known for their exotic magnetic and electronic properties. These can be attributed qualitatively to a range of interactions between the charge of the electrons, their magnetic moment, their localization within the crystals and their atomic orbitals. The relative strengths of the various interactions determine whether an oxide is magnetic, an insulator, an electrical conductor or even a superconductor. The so-called 4d and 5d transitions metals are particularly interesting in this respect.

The properties of many of these oxides can be specifically adjusted by applying external electric or magnetic fields, or exerting pressure on the material. This makes them interesting for numerous applications in micro- and nanoelectronics, for data storage and information processing. Such behaviour is particularly pronounced in the oxides of 5d transition metals, such as tantalum, tungsten, osmium and iridium. The oxides of iridium are especially remarkable because they lose their magnetisation when subjected to pressure, and even under normal conditions develop unexpected magnetic structures. Although some of their properties have been known for quite a while, efforts to explain this behaviour are still in their infancy. This makes it all the more important to develop methods that provide detailed insights into such materials.

A particularly suitable and extremely sensitive method of studying the electronic and magnetic properties of solids is nuclear resonant scattering (NRS) using synchrotron radiation. This method uses the nuclei of the atoms of certain isotopes as local probes for the material’s properties. In view of its numerous possible applications, specialised measuring stations have been set up for this purpose on the P01 beamline at PETRA III, which are used by many scientists from all over the world every year. Among other things, the method allows the orientation of atomic magnetic moments to be determined with great accuracy. NRS therefore complements other X-ray techniques and – in contrast to neutron techniques – makes it possible to study small samples, for example when used on samples subject to high pressure.

>Read more on the PETRA III at DESY website

Image: Samples of strontium-iridium-trioxid crystals.
Credit: University College London, James Vale/Emily Hunter

Signatures of enhanced superconducting phase coherence in cuprates

The capability to control material properties on short timescales is one of the key challenges of modern condensed matter physics. This possibility becomes even more attractive in the case of intriguing material phases, such as superconductivity. As a matter of fact, despite the evolution of non-equilibrium spectroscopies of the last two decades have increased our understanding of the physics of strongly correlated materials, after more than 30 years from its discovery, High Temperature Superconductivity is still discussed and a clear and unanimous explanation of the origin of the phenomenon is still lacking. Moreover, the understanding of the phenomena at the basis of this effects could affect several technological applications, from the need for fast digital circuits and for speeding up computer performances, to the detection of very low magnetic fields, with implication in geology (mineral exploration and earthquake prediction), medical sciences (neuron activity and magnetic resonance), oil prospecting and, of course, research.
We focused our research on cuprates, a class of materials known for displaying unconventional superconductivity at relatively temperatures, and on which various studies have shown the possibility of turning off (and, to some extent, on) superconductivity by ultrashort light pulses. In our work, we reveal that light pulses characterized by long wavelength (and a peculiar polarization) can induce, for a very short time interval (1-2 ps), a state displaying superconductivity even above the critical temperature, i.e. in conditions where superconductivity is not observed at equilibrium.

>Read more on the FERMI at Elettra Sincrotrone Trieste website

Figure: Difference between the transient reflectivity due to Cu-Cu and Cu-O polarized pump in time and temperature, induced by excitations with (a) 70 and (b) 170 meV pump photon energies. The dashed lines highlight the critical temperature Tc.

Spin-momentum locking in cuprate high-temperature superconductors

The results open a new chapter in the mystery of high-temperature superconductors, suggesting that new, unexplored interactions and mechanisms might be at play.

In the world of superconductors, “high temperature” means that the material can conduct electricity without resistance at temperatures higher than expected, but still far below room temperature. Within this special class of high-temperature superconductors (HTSCs), cuprates—consisting of superconducting CuO2 layers separated by spacer layers—are some of the best performers, generating interest in these materials for potential use in super-efficient electrical wires that can carry power without any loss of electron momentum.

A new spin on cuprate HTSCs

Two kinds of electron interactions have been known to give rise to novel properties in new materials, including superconductors. Scientists who study cuprate superconductors have focused on just one of those interactions: electron correlation—electrons interacting with each other. The other kind of electron interaction found in exotic materials is spin-orbit coupling—the way in which an electron’s magnetic moment interacts with atoms in the material.

>Read more on the Advanced Light Source website

Image: Chris Jozwiak, Alessandra Lanzara, Kenneth Gotlieb, and Chiu-Yun Lin.
Credit: Peter DaSilva/Berkeley Lab

The first observation of near-room-temperature superconductivity

For decades, room-temperature superconductivity has been one of physics’ ultimate goals, a Holy Grail-like objective that seems to keep drifting within realization yet always stubbornly out of reach. Various materials, theories, and techniques have been proposed and explored in search of this objective, but its realization has remained elusive. Yet recent experimental work on hydrogen-rich materials at high pressures is finally opening the pathway to practical superconductivity and its vast potential. Russell Hemley, a materials chemist at George Washington University in Washington, D.C., first announced evidence of superconductivity at 260 K in May, 2018, and then hints of an even higher 280 K transition in August of that year. Now Hemley, along with a team of researchers from The George Washington University and the Carnegie Institution for Science synthesized several lanthanum superhydride materials that demonstrated the first experimental evidence of superconductivity at near room temperature, and with colleagues from Argonne National Laboratory characterized them at the U.S. Department of Energy’s Advanced Photon Source (APS). Read more