New Evidence Supports the Existence of Pair Density Waves in Cuprate Superconductors

April 10, 2020 by Tyler Charboneau

Roughly 50 years ago, researchers first proposed the existence of pair density waves—a unique state of matter previously shrouded in mystery.

Scientists from Brookhaven National Laboratory have now definitively confirmed their existence within specialized superconductor applications. 

Almost all semiconductors and superconductors perform optimally under specific conditions. Engineers have been researching ways to reduce or eliminate any resulting inefficiencies. Superconductor flows don't inherently generate a lot of heat—superconducting does kickstart at lower critical temperatures, however, typically requiring supercooling. The team at Brookhaven is investigating ways to raise this critical temperature while seeking improved compatibility between phase states and materials. 

The Relationship Between Phases and Materials

Achieving superconductivity isn’t always simple. Researchers experiment with phases and conductive materials to find optimal matches. Engineers are hoping to find a superconducting tandem that requires less supercooling. We can hopefully unlock 100% power retention thereafter. 

The U.S. Department of Energy’s physicists has had a breakthrough: uncovering spectroscopic evidence of pair density waves at the microscopic level. This phase interacts with bismuth-based copper oxide to coexist with superconductivity. Superconductivity occurs when like-charged electrons pair and move freely, despite their polarity. Pair density waves exist despite the absence of applied magnetic fields within these materials.


A diagram mapping out the superconducting energy of individual electrons.

A diagram mapping out the superconducting energy gap of individual electrons, measured by a sensitive microscope scanning the surface. Image credited to Brookhaven National Laboratory.


The Brookhaven Experiment

Brookhaven’s OASIS lab, the epicenter of this discovery, used an advanced tunneling microscope to perform spectroscopic image scanning. Scientists made their determinations by measuring the tunneling behavior of individual electrons. What do they specifically look for?

  • Electron tunneling from a sample surface to the microscope’s superconducting, electrode tip

  • Tunneling behavior as voltage changes

  • Electron density differences between states

  • Quantities of electrons at given locations, indicating flow (or lack thereof)

Brookhaven’s team mapped out lattices and electronic structures from these observations. With bismuth copper oxide, electrons are spread across a continuous spectrum when superconductivity is inactive. This superconductivity ramps up as temperature decreases. 


Key Takeaways

Electron pairing jumpstarts and varies as voltages change. Kazuhiro Fujita—the project’s lead—shared that energy gaps form at certain points along this spectrum. These points show a marked absence of electrons. Fujita states that such gaps coincide with the voltage needed to split those electron pairs. 

Significantly, the research team found modulation in these energy gaps. Modulation essentially tells us that the strength of electron bonds varies, from a measurable minimum to a maximum. This occurs in a pattern across the lattice—an observation supporting the existence of pair density waves. Bonding patterns repeat themselves every eight atoms across the structure. 

Tunneling currents also varied in the same way. This observation provided evidence for the existence of pair density waves. Gapping sizes are crucial indicators. Previous insights into Cooper pairs gave credence to this idea four years ago, though physicists finally affirmed these suspicions four years later. 


Brookhaven research team.

The team of researchers behind the study. Kazuhiro Fujita (top), and from left to right: Genda Gu, Sang Hyun Joo, Zengyi Du, Peter Johnson, and Hui Li. Image credited to Brookhaven Laboratory.

Unraveling Mysteries Behind Superconducting

Fujita’s team has given fellow researchers hope. The complexities behind superconducting are immense, yet the Department’s insights will help us determine how conducting patterns develop across different materials. Temperature, magnetism, and charge density all contribute to variations. 

It appears that pair density waves are major building blocks in superconducting. This conduction isn’t perfect. Half-vortices are also present, which can disrupt consistent conducting behavior across a system. Pair density wave patterns mirror “stripes”—other modulating patterns that emerge under different electromagnetic conditions. 

Fujita team has advanced our understanding of high-temperature cuprate superconductors. Those developments will certainly fuel other engineering breakthroughs in the near future.