Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides (1711.00468v1)

Abstract: Systems that simultaneously exhibit superconductivity and spin-orbit coupling are predicted to provide a route toward topological superconductivity and unconventional electron pairing, driving significant contemporary interest in these materials. Monolayer transition-metal dichalcogenide (TMD) superconductors in particular lack inversion symmetry, enforcing a spin-triplet component of the superconducting wavefunction that increases with the strength of spin-orbit coupling. In this work, we present an experimental and theoretical study of two intrinsic TMD superconductors with large spin-orbit coupling in the atomic layer limit, metallic 2H-TaS$_2$ and 2H-NbSe$_2$. For the first time in TaS$_2$, we investigate the superconducting properties as the material is reduced to a monolayer and show that high-field measurements point to the largest upper critical field thus reported for an intrinsic TMD superconductor. In few-layer samples, we find that the enhancement of the upper critical field is sustained by the dominance of spin-orbit coupling over weak interlayer coupling, providing additional platforms for unconventional superconducting states in two dimensions.

• The paper establishes that monolayer 2H-TaS2 exhibits an upper critical field exceeding 34.5T, far above the conventional Pauli limit.
• The authors combine experimental evidence and DFT calculations to reveal SOC splitting values of 156 meV for TaS2 and 62.6 meV for NbSe2.
• This research paves the way for advanced spintronic and quantum devices through precise control of superconducting states in 2D TMDs.

Tuning Ising Superconductivity in Two-Dimensional Transition-Metal Dichalcogenides

The paper entitled "Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides" provides an intricate paper of the unique interplay of superconductivity and spin-orbit coupling (SOC) in two-dimensional (2D) transition-metal dichalcogenides (TMDs). Specifically, the research focuses on metallic 2H-TaS2 and 2H-NbSe2, offering an experimental and theoretical perspective on these materials as they are thinned to the atomic layer limit.

Key Findings and Numerical Evidence

One of the most significant outcomes of this paper is the characterization of enhanced upper critical field $H_{c2}$ in thin TMD superconductors, especially noticeable in monolayer forms. For monolayer 2H-TaS2, the $H_{c2}$ exceeds the Pauli limit significantly, reaching fields larger than 34.5 T in parallel orientation, corresponding to more than ten times the conventional Pauli limit. Such enhancement is attributed to Ising SOC, a particular kind of SOC responsible for out-of-plane spin pairing—an effect previously inadequately described by orbital-limited models.

For 2H-NbSe2 and 2H-TaS2, theoretical calculations, bolstered by density functional theory (DFT), provide a comprehensive account of the band structures. The calculated values of SOC splitting — 156 meV for TaS2 and 62.6 meV for NbSe2 — account for the different degrees of $H_{c2}$ enhancement observed experimentally.

Theoretical and Practical Implications

From a theoretical standpoint, the versatile manipulation of $H_{c2}$ and superconducting transition temperatures ($T_{c0}$) by tuning SOC introduces new domains for investigating unconventional superconducting states, especially with strong spin-dependent interactions. This material system allows for the induction of different superconducting states, potentially exhibiting exotic properties such as spin-triplet pairing and topological superconductivity, previously unattainable in bulk analogs.

Practically, these findings suggest potential pathways for novel device applications in spintronics and quantum computing, where control over electronic spin and superconductivity at reduced dimensions can be instrumental. These 2D TMDs, in particular, provide promising avenues for developing ultra-fast and energy-efficient electronic devices that leverage their SOC-enhanced properties.

Future Developments in AI and Computational Materials Science

The advancement in materials modeling, particularly through AI-integrated computational frameworks and enhanced DFT methodologies, might offer predictive insights and enable fiction-free design of superconductive materials with tailored properties. Future explorations may involve deep learning models trained on topological insulators and superconductors to precisely predict phase diagrams and gauge the impact of mechanical strain, electrostatic doping, or magnetic proximity effect on $T_{c0}$ and $H_{c2}$.

Conclusion

In conclusion, this research underscores the importance of SOC in governing isotropic superconductivity across atomic limits in 2D TMDs. By bridging experimental findings with robust theoretical underpinnings, it opens the door to refined control over 2D superconductive states through layer manipulation and SOC, thus charting a course for future explorations in quantum materials science.