The Hg1223 Paradox: Repulsion Creating Superconductivity express
Superconductors conduct current without loss, but usually require extreme cold. Hg1223 — a mixture of mercury, barium, calcium, copper and oxygen — breaks the rule: it stops resisting current already at –139°C. The secret is that the strong electrical repulsion between electrons forces them to pair up. It’s like a crowded subway car: everyone tries to keep their distance, but in the crush people grab onto each other to avoid falling — and so electrons form stable pairs.
Pressure helps: electrons start hopping between atoms more often, and repulsion weakens, raising the working temperature to –113°C. Magnetism, usually an enemy of superconductivity, here does not harm but strengthens the pairs. Understanding this leads to the dream of room-temperature superconductors, which would revolutionize energy.
Original abstract
as written by the authors arXiv:2606.08181
Triple-layer cuprate superconductor $$\mathrm{HgBa_2Ca_2Cu_3O_8}$$ (Hg1223) keeps the record of the highest superconducting (SC) critical temperature $$T_{c}\sim 134$$K among all the existing materials at ambient pressure. $$T_{c}$$ further increases under pressure up to $$T_{c}\sim 160$$K. However, its microscopic mechanism remains to be elucidated. We solve {\it ab initio} Hamiltonians for Hg1223 using a variational solver supplemented by a neural network. The pressure dependence of the $$d$$-wave SC order parameter and estimated $$T_{c}$$ show a dome-like structure in essential agreement with the experimental indications. The origin of the strong SC amplitude at ambient pressure is identified as strong local Coulomb repulsion $$U$$ attributed to poor screening. Further increase in $$T_{c}$$ under pressure is understood from interplay of three elements, namely increased electron hopping $$t$$, decreased $$U$$ and more importantly, strongly reduced offsite Coulomb repulsion $$V$$ with increasing pressure. Pairing mechanism is identified as the emergent local attraction counterintuitively generated from the originally strong local repulsion $$U$$. The emergent attraction is interpreted from ``attraction from reduced repulsion'', originating from the release of the fluctuating doubly-occupied sites characterized from the ``false vacuum'' in the Mott insulator to the double-occupation-free $$d$$-wave SC states upon carrier doping. This instantaneous attraction is in contrast with the conventional BCS SC mediated by bosonic glues. The local attraction is consistent with the electron fractionalization supported in experimental analyses. The coexistence of the SC and antiferromagnetic order is also demonstrated as a characteristic feature of the multi-layer system. The microscopic understanding of Hg1223 offers a new route explicitly using this emergent attraction to design and optimize SC materials.
A superconducting coil can carry current for years without an external source — once started, the current circulates almost forever.