Lattice Disorder Unlocks New Understanding of Superconductivity in Materials Like H3S

Unconventional superconductivity remains a central problem in condensed matter physics. Yu-Cheng Zhu, Jia-Xi Zeng, and Xin-Zheng Li, from the State Key Laboratory for Artificial Microstructure and Mesoscopic Physics at Peking University, demonstrate that lattice quantum disorder plays a crucial role in this phenomenon. Their research incorporates nuclear many-body effects into first-principles calculations, revealing a lattice disordered phase in superconductors H3S and La3Ni2O7. This discovery is significant because the team found a strong correlation between this disordered phase and the maximum superconducting transition temperature, suggesting it underpins the pairing mechanism and offers a unifying framework for understanding and predicting high-temperature superconductivity in a wider range of materials.

Prevailing theories of superconductivity have largely focused on electronic behaviour, often overlooking the significant role of the lattice structure itself.

This work incorporates the complex quantum behaviour of atomic nuclei within first-principles calculations, demonstrating that a lattice quantum disordered phase exists within a specific region of the pressure-temperature phase diagram. The boundaries of this phase precisely align with the onset of superconductivity on the left side of the superconducting dome, suggesting a fundamental connection between lattice disorder and the superconducting state.
Researchers employed path-integral molecular dynamics to accurately model the quantum many-body effects of the nuclei, going beyond traditional molecular dynamics simulations. By mapping the free-energy surface, they determined the structural phase boundaries and identified the lattice quantum disordered phase, where quantum fluctuations stabilize a disordered atomic arrangement.

This disordered phase occupies a triangular region on the pressure-temperature diagram, extending to approximately 220 K for H3S, 160 K for D3S, and 77 K for La3Ni2O7. The maximum superconducting transition temperature for each material coincides with the maximum extent of this lattice quantum disordered phase, indicating that superconductivity originates within this structurally disordered regime.

This discovery challenges conventional understandings of superconductivity and establishes a unifying framework for predicting new superconducting materials. The research demonstrates that the left flank of the superconducting dome, the region crucial for understanding high-temperature superconductivity, is intimately linked to this lattice quantum disorder.

By accurately accounting for nuclear quantum effects, the study provides a more complete picture of structural phase transitions and their influence on superconducting properties, potentially paving the way for advancements in materials science and condensed matter physics. The findings suggest that lattice dynamics, beyond the conventional phonon picture, are essential for understanding and ultimately enhancing superconductivity.

Path-integral simulations define free-energy surfaces and lattice dynamics in compressed hydrogen sulfide at high pressures and low temperatures

Path-integral molecular dynamics (PIMD) constitutes the core methodology employed to investigate nuclear quantum many-body effects from first principles. This technique accurately models the behaviour of nuclei, accounting for their quantum nature and interactions, unlike classical molecular dynamics which treats them as classical particles.

Structural phase boundaries were determined by constructing the free-energy surface using the centroid potential of mean force derived from PIMD simulations. This approach enabled precise identification of the lattice quantum disordered (LQD) phase, a higher-symmetry disordered state stabilized by quantum fluctuations beyond the conventional phonon picture.

The research team meticulously calculated the dispersion relation of lattice dynamics for Im3m H3S at 200 K and 141 GPa using PIMD, comparing the results with harmonic phonon spectra. This comparison revealed suppression of structural instability indicated by a soft phonon mode, demonstrating the impact of quantum effects on lattice vibrations. Further analysis involved tracking the frequency of this soft mode at the Γ point as a function of temperature and pressure.

Lattice quantum disorder defines the left flank of the high-temperature superconductivity dome and its associated pseudogap phase

Researchers have discovered a lattice quantum disordered (LQD) phase in the superconductors H3S and La3Ni2O7, revealing a crucial link to the origin of high-temperature superconductivity. This phase occupies a triangular region on the pressure-temperature phase diagram, directly correlating with the superconducting dome’s left flank.

The boundary of this LQD phase, determined through path-integral molecular dynamics, precisely aligns with the superconducting transition temperature on the left side of the dome. Calculations demonstrate that the maximum superconducting transition temperature coincides with the maximum temperature within the LQD phase, suggesting this disordered state is fundamental to the pairing mechanism driving superconductivity.

The research employed path-integral molecular dynamics to accurately model nuclear quantum many-body effects, constructing a free-energy surface to define the structural phase boundary. This approach revealed a structural instability suppressed within the Im 3m phase of H3S, as evidenced by the dispersion relation of lattice dynamics at 200 Kelvin and 141 GigaPascals. Analysis of the soft mode frequency at the Γ point, as a function of temperature and pressure, confirms its disappearance near the structural transition.

Lattice disorder correlates with maximal superconductivity in hydrogen sulfide and lanthanum nickelate, suggesting a common mechanism

Researchers have identified a lattice disordered phase in the superconductors H3S and La3Ni2O7, offering new insights into the mechanisms driving high-temperature superconductivity. By incorporating nuclear many-body effects into first-principles calculations, the study reveals this disordered phase occupies a specific region within the pressure-temperature phase diagram, aligning with the onset of superconductivity on the left side of the superconducting dome.

The maximum superconducting temperature consistently coincides with the maximum temperature of the lattice disordered phase, suggesting a fundamental connection between the two phenomena. This finding establishes a unifying framework for understanding superconductivity in these materials and potentially others.

The lattice disordered phase appears to be a key ingredient in the pairing mechanism responsible for superconductivity, challenging previous interpretations that relied heavily on electronic degrees of freedom. Investigations into both H3S and La3Ni2O7 demonstrate this correlation, strengthening the generality of the proposed model.

The authors acknowledge limitations related to the computational methods employed, specifically noting the avoidance of the PBE+U approach in La3Ni2O7 due to its inaccuracies in predicting structural stability. Future research should focus on experimentally verifying the predicted changes in lattice parameters near the identified phase boundaries and exploring the implications of this lattice disordered phase for a wider range of condensed matter systems.

The identification of a tricritical point within the phase diagram also warrants further investigation, potentially revealing new avenues for manipulating and enhancing superconducting properties. This work provides a basis for predicting new superconductors and for a more comprehensive understanding of complex material behaviour.