[Paper Review] Ising Superconductivity in Transition Metal Dichalcogenides

Noah F. Q. Yuan, Benjamin T. Zhou+2 more|arXiv (Cornell University)|May 6, 2016

2D Materials and Applications7 references3 citations

TL;DR

This paper investigates Ising superconductivity in monolayer transition metal dichalcogenides (TMDs), demonstrating that strong spin-orbit coupling locks electron spins perpendicular to the plane, enabling unusually high in-plane upper critical fields. The authors show this mechanism enables engineering of topological superconductors hosting Majorana fermions, with key implications for fault-tolerant quantum computing.

ABSTRACT

In this work, we review the results of several recent works on the experimental and theoretical studies of monolayer superconducting transition metal dichalcogenides (TMD) such as superconducting MoS2 and NbSe2. We show how the strong Ising spin-orbit coupling (SOC), a special type of SOC which pins electron spins to out-of-plane directions, can affect the superconducting properties of the materials. Particularly, we discuss how the in-plane upper critical fields of the materials can be strongly enhanced by Ising SOC and how TMD materials can be used to engineer topological superconductors and nodal topological superconductors which support Majorana fermions.

Motivation & Objective

• To understand the role of Ising spin-orbit coupling in monolayer TMD superconductors like MoS2 and NbSe2.
• To explain the experimentally observed enhancement of in-plane upper critical fields beyond the Pauli limit.
• To explore the potential of TMDs for realizing topological superconductivity and hosting Majorana fermions.
• To provide a theoretical framework linking strong spin-orbit coupling to unconventional superconducting properties in 2D materials.

Proposed method

• Analyzes the electronic band structure of monolayer TMDs to identify the origin of Ising spin-orbit coupling.
• Models the superconducting state using a spin-singlet pairing Hamiltonian modified by strong spin-orbit coupling.
• Calculates the upper critical field (Hc2) in the presence of in-plane magnetic fields, showing enhancement due to spin locking.
• Evaluates the conditions under which topological superconducting phases can emerge, particularly in proximity to spin-triplet pairing channels.
• Uses symmetry analysis and effective field theory to identify the emergence of nodal topological superconducting states.
• Considers the role of interface effects and proximity to magnetic substrates in stabilizing Majorana zero modes.

Experimental results

Research questions

• RQ1 How does Ising spin-orbit coupling in monolayer TMDs lead to enhanced in-plane upper critical fields?
• RQ2 What is the mechanism by which strong spin-orbit coupling stabilizes superconductivity under in-plane magnetic fields?
• RQ3 Under what conditions can TMD-based heterostructures host topological superconducting phases?
• RQ4 Can nodal topological superconductivity be engineered in TMDs to support Majorana fermions?
• RQ5 What are the symmetry-protected topological invariants that characterize the superconducting phases in these systems?

Key findings

• The in-plane upper critical field in monolayer TMDs exceeds the Pauli limit due to Ising spin-orbit coupling, which locks electron spins perpendicular to the plane and suppresses spin-flip scattering.
• The strong Ising spin-orbit coupling leads to a spin-splitting that protects the superconducting state against in-plane magnetic fields.
• TMD materials provide a platform for engineering topological superconductors with robust Majorana zero modes at vortex cores.
• Nodal topological superconducting phases can emerge in TMD heterostructures when spin-orbit coupling and superconducting pairing are tuned appropriately.
• Theoretical analysis confirms that the combination of spin-orbit locking and s-wave pairing can lead to topological superconducting phases with nontrivial Chern numbers.
• The model predicts that proximity to magnetic insulators or ferromagnetic layers can stabilize chiral topological superconductivity in TMDs.