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Curated by RSF Research Staff

New understanding of the high temperature superconductor mechanism

Since the 1986 discovery of high-temperature superconductivity in copper-oxide compounds called cuprates, scientists have been trying to understand how these materials can conduct electricity without resistance at temperatures hundreds of degrees above the ultra-low temperatures required by conventional superconductors. These properties, beyond being a real scientific challenge, are also of huge interest for industrial purposes allowing potentially huge savings in generating and transporting electricity.

One of the best material candidates for a high-temperature superconductor is still the cuprates. Recent results are suggesting the cause of the phenomenon is linked to the fluctuating stripes. Experiments have established charge stripes as universal in underdoped cuprate superconductors. However, alternative interpretations based on itinerant electrons exist, and conclusive experimental evidence for fluctuating stripes remains elusive.

Crystal structures of cuprates HgBa2CuO4+ and YBa2Cu3O6+

In 2017, a team from Standford showed numerical evidence of these fluctuating stripes in high-temperature superconductors cuprates leading the way to a better understanding of the phenomenon. Their main idea was that thermal and quantum fluctuations cause static stripes to melt into a fluctuating state with dynamic correlations. Suspecting this fluctuating form, they used a numerical quantum Monte Carlo simulation to demonstrate the dynamic of the system. They based their theory on the three-band Hubbard model, which represents the local electronic structure of the copper-oxygen plane.

The Hubbard model is a highly simplified model for spin-1/2 fermions moving in a periodic lattice. The particles can hop from site-to-site with rate t and they can have on-site interactions U with particles of opposite spin, as shown in the figure below.

Their results showed robustness to varying parameters, cluster size, and boundary conditions. It showed that in the disordered phase, stripes maintain their characteristics and periodicity in a fluctuating form, while being robust to variations. The fluctuating stripe order observed up to such high temperatures is a strong piece of corroborating evidence that these phenomena are strong enough to affect all electronic properties.

There's reason to think that stripes of charges and spin may be intimately tied to the emergence of high-temperature superconductivity in these materials, which was discovered 30 years ago but so far is not understood or explained.

Edwin Huang, Stanford Institute for Materials and Energy Sciences (SIMES)

The next step will be to answer the question of whether or how the fluctuating stripes figure into superconductivity.

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