Amazing stuff!
What makes me wonder is that this study was done on atoms instead of electrons. What difference does it make going from electrons to atoms when it comes to superconductivity?
"For the first time, scientists have directly imaged the quantum process underlying superconductivity, a phenomenon in which paired electrons cause electric current to flow without resistance at sufficiently low temperatures. ...
the scientists directly imaged individual atoms pairing up in a special gas cooled nearly to absolute zero — the unreachable limit to how cold things can get. The type of gas, called a Fermi gas, allows scientists to substitute electrons with atoms and probe the physics of superconductors in a controlled way.
Surprisingly, the scientists found that after pairing up, the atoms moved in a synchronized dance, with their positions dependent on those of other pairs — a phenomenon not predicted by the 70-year-old, Nobel Prize-winning theory of superconductivity. ...
Using a newly developed imaging method, the experimental physicists captured snapshots of the relative positions of the pairs. The scientists used a special gas mixture made of lithium atoms, cooled to just a few billionths of a degree Celsius above absolute zero. At these temperatures, the atoms act as fermions, a fundamental class of particles that includes electrons. Since these fermions all follow the same physics of pairing, the atoms are suitable substitutes for studying electron behavior in superconductors.
The imaging revealed that the positions of paired atoms became influenced by those of other pairs. The paired atoms maintained a separation from other paired atoms, just as dancing couples keep their distance from other dancers in a ballroom ... This finding adds a new understanding of these systems that was missing from the historic BCS theory. ..."
"... From these observations, theorists have developed models—notably the Bardeen-Cooper-Schrieffer (BCS) theory, which assumes that the zero-resistance flow in a superconductor arises from electrons forming so-called Cooper pairs. This theory has been successful in explaining a large class of superconductors, but ... colleagues have now observed behavior that contradicts BCS predictions. Using a recently developed technique called atom-resolved continuum quantum gas microscopy, the researchers directly observed spatial correlations in cold atoms that mimic superconducting electrons. These high-precision measurements revealed an unexpected anticorrelation between opposite-spin atoms, implying deficiencies in the BCS theory. This and other surprising results demonstrate once again how new observational lenses can put long-standing theoretical models into question. ...
Predicting the collective behavior of electrons within materials is a formidable challenge. The many-body problem for classical particles is already difficult, but it is made unbelievably more complex for electrons and other fermions by the infamous “sign problem”: The wave function of fermionic particles changes sign upon particle exchange. This antisymmetric behavior gives rise to Pauli’s exclusion principle and makes modeling fermionic many-body systems incredibly challenging. ..."
From the abstract:
"In this Letter, we explore two-dimensional attractive Fermi gases at the microscopic level by probing spatial charge and spin correlations in situ.
Using atom-resolved continuum quantum gas microscopy, we directly observe fermion pairing and study the evolution of two- and three-point correlation functions as interspin attraction is increased.
The precision of our measurement allows us to reveal nonlocal anticorrelations in the pair correlation function, fundamentally forbidden by the mean-field result based on BCS theory but whose existence we confirm in exact auxiliary-field quantum Monte Carlo calculations.
We demonstrate that the BCS prediction is critically deficient not only in the superfluid crossover regime but also deep in the weakly attractive side.
Guided by our measurements, we find a remarkable relation between two- and three-point correlations that establishes the dominant role of pair correlations. Finally, leveraging local single-pair losses, we independently characterize the short-range behavior of pair correlations, via the measurement of Tan’s contact, and find excellent agreement with numerical predictions.
Our measurements provide a novel microscopic view into strongly correlated two-dimensional Fermi gases in the continuum."
Scientists Capture Superconductivity’s ‘Dancing Pairs’ for First Time, Filling Gap in Decades-Old Theory (original news release) "Analysis of a first-of-its-kind experiment reveals missing pieces in the decades-old theory of superconductivity."
Superconductor Theory Under Cold-Atom Scrutiny "Snapshot measurements of cold-atom gases reveal hidden spin correlations that could force an update of some superconductivity theories."
Figure 1: A continuum quantum gas microscope can image a 2D collection of cold atoms (left). In the case of a fermionic gas, the technique can differentiate between spin-up and spin-down atoms. Using the microscope data, researchers can compute the correlation function (right). The observations (solid orange line) disagree with the Bardeen-Cooper-Schrieffer theory (dashed yellow line) in that they show an anticorrelation “dip” for opposite spin atoms at a particular interparticle distance.
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