Unconventional superconductivity from lattice quantum disorder

Unconventional superconductivity presents a defining and enduring challenge in condensed matter physics. Prevailing theoretical frameworks have predominantly emphasized electronic degrees of freedom, largely neglecting the rich physics inherent in the lattice. Although conventional phonon theory offers an elegant description of structural phase diagrams and lattice dynamics, its omission of nuclear quantum many-body effects results in misleading phase diagram interpretations and, consequently, an unsound foundation for superconducting theory. Here, by incorporating nuclear quantum many-body effects within first-principles calculations, we discover a lattice quantum disordered phase in superconductors H3S and La3Ni2O7. This phase occupies a triangular region in the pressure-temperature phase diagram, whose left boundary aligns precisely with Tc of the left flank of the superconducting dome. The Tcmax of this quantum disordered phase coincides with the maximum of superconducting Tc, indicating this phase as both the origin of superconductivity on the dome’s left flank and a key ingredient of its pairing mechanism. Our findings advance the understanding of high-temperature superconductivity and establish the lattice quantum disordered phase as a unifying framework, both for predicting new superconductors and for elucidating phenomena in a broader context of condensed matter physics.

💡 Research Summary

The manuscript tackles a long‑standing puzzle in unconventional superconductivity by shifting the focus from purely electronic degrees of freedom to the quantum dynamics of the lattice itself. Using first‑principles density‑functional theory (DFT) combined with path‑integral molecular dynamics (PIMD), the authors explicitly include nuclear quantum many‑body effects—i.e., the collective tunnelling and zero‑point motion of the nuclei—in the construction of the free‑energy surface (FES). By extracting the centroid potential of mean force from the PIMD trajectories, they locate the pressure‑temperature (P‑T) points where a soft phonon mode at the Γ point changes sign. This sign change defines a quantum structural phase boundary that is distinct from the classical boundary obtained with conventional molecular dynamics (MD).

Applying this methodology to two representative high‑pressure superconductors—hydrogen sulfide (H₃S/D₃S) and the nickel‑based oxide La₃Ni₂O₇—the authors discover a previously unrecognized “lattice quantum disordered” (LQD) phase. In the LQD regime, quantum fluctuations stabilize a higher‑symmetry structure (Im‾3m for H₃S) even when the underlying potential energy surface exhibits a double‑well instability. The region between the quantum (PIMD) and classical (MD) boundaries forms a triangular domain in the P‑T diagram. Crucially, the left edge of this triangle aligns precisely with the low‑pressure flank of the experimentally observed superconducting dome, and the temperature at which the two boundaries intersect (Tₘₐₓᶜ,ᴸᴼᴰ) coincides with the maximum superconducting transition temperature (Tₘₐₓᶜ,ˢ궤) for both materials: ~220 K for H₃S, ~160 K for D₃S, and ~77 K for La₃Ni₂O₇.

These observations lead to several key insights. First, superconductivity in these systems emerges entirely within the high‑symmetry LQD phase, contradicting earlier two‑phase interpretations that placed the low‑pressure side of the dome in a low‑symmetry structure. Second, the LQD phase is not a static high‑symmetry crystal; its lattice dynamics are governed by strong quantum fluctuations that suppress the soft phonon instability predicted by harmonic phonon theory. This “quantum order–disorder” transition provides a new type of bosonic excitation that can mediate electron pairing, supplementing or even supplanting conventional electron‑phonon mechanisms. Third, the intersection of the quantum and classical phase lines constitutes a tricritical point, which the authors argue determines the maximal Tc achievable in a given material.

From a practical standpoint, the authors propose a predictive workflow: (i) identify candidate materials whose first‑principles PIMD simulations reveal a broad LQD region; (ii) fine‑tune carrier concentration (via doping or pressure) to place the system near the tricritical point; (iii) thereby maximize Tc. They suggest that this strategy should be applicable not only to high‑pressure hydrides and nickelates but also to other families such as cuprates, where lattice quantum effects may be significant.

In summary, the paper provides compelling computational evidence that lattice quantum disorder is a unifying framework for understanding unconventional superconductivity on the low‑pressure side of the dome, and that the quantum order‑disorder transition itself may be the key to achieving higher transition temperatures. The work bridges the gap between structural quantum fluctuations and electronic pairing, opening a new avenue for both theoretical exploration and materials discovery.