Elusive State of Electronic Matter Was Finally Discovered

April 15, 2016 | Joanne Kennell

A schematic image representing a periodic variation in the density of Cooper pairs
Photo credit: Brookhaven National Laboratory

It was first predicted by theorists back in 1964.

Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory, Cornell University, as well as several collaborators have captured the first direct evidence of a state of electronic matter first predicted by theorists back in 1964. The discovery, described in a paper published in Nature, may provide insights into the inner workings of high-temperature superconductors.

The over 50-year-old prediction was that “Cooper pairs” of electrons in a superconductor could exist in two possible states. They could form a “superfluid” where all the particles are in the same quantum state and all move as one, carrying a current with zero resistance — aka a superconductor — or the Cooper pairs could periodically vary in density across space, a so-called “Cooper pair density wave.”

However, this latter state has been out of reach to researchers for decades, possibly because no instrument has been capable of observing its existence. Until now, that is.

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The research team, led by J.C. Séamus Davis, a physicist at Brookhaven Lab and the James Gilbert White Distinguished Professor in the Physical Sciences at Cornell, and Andrew P. Mackenzie, Director of the Max-Planck Institute CPMS in Dresden, Germany, developed an innovative way of using a scanning tunneling microscope (STM) to directly image Cooper pairs.

Before diving into how the researchers finally managed to “snap a photo” of the elusive pairs, here is a little background information. Superconductivity was first identified in metals cooled almost to absolute zero (-273.15 degrees Celsius or -459.67 degrees Fahrenheit), but recently developed materials known as cuprates — copper oxides laced with other atoms — superconduct at temperatures as “high” as 148 degrees above absolute zero.

At these frigid temperatures, electrons join in pairs that are magnetically neutral — meaning they do not interact with atoms and can move without resistance.

Mohammed Hamidian, a research associate now at Harvard, and graduate student Stephen Edkins at St. Andrews University in Scotland, while working as members of Davis’ research team, investigated a cuprate that incorporated bismuth, strontium, and calcium (Bi2Sr2CaCu2O8), using an extremely sensitive STM. The sample cuprate was refrigerated to within a few thousandths of a degree above absolute zero, a temperature that allows Cooper pairs to hop across short distances from one superconductor to another — a phenomenon known as Josephson tunneling.

To witness the Cooper pairs, the researchers briefly lowered the tip of a superconducting STM to touch the surface and pick up a flake of the cuprate material. This step allowed the Cooper pairs to tunnel between the superconductor surface and the microscope. The instrument became “the world’s first scanning Josephson tunneling microscope,” Davis said in the Brookhaven National Laboratory media release.

So, what did the team observe? A Cooper pair density wave, confirming the 52-year-old prediction.

The researchers noted that this technique could be used to search for Cooper pair density waves in other cuprates or the recently discovered iron-based superconductors.

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