[Paper Review] Controlling correlations in NbSe$_2$ via quantum confinement

Somesh Chandra Ganguli, Viliam Vaňo+3 more|arXiv (Cornell University)|Sep 28, 2020

2D Materials and Applications4 citations

TL;DR

This study demonstrates that quantum confinement in NbSe₂ suppresses superconductivity and drives the system toward a correlated insulating state, revealing strong competing interactions between superconductivity and electronic correlations. Using low-temperature scanning tunneling microscopy and spectroscopy, the authors show that interaction strength can be tuned via confinement, establishing NbSe₂ as a correlated superconductor near a quantum phase transition.

ABSTRACT

Transition metal dichalcogenides (TMDC) are a rich family of two-dimensional materials displaying a multitude of different quantum ground states. In particular, d$^3$ TMDCs are paradigmatic materials hosting a variety of symmetry broken states, including charge density waves, superconductivity, and magnetism. Among this family, NbSe$_2$ is one of the best-studied superconducting materials down to the monolayer limit. Despite its superconducting nature, a variety of results point towards strong electronic repulsions in NbSe$_2$. Here, we control the strength of the interactions experimentally via quantum confinement effects and use low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) to demonstrate that NbSe$_2$ is in strong proximity to a correlated insulating state. This reveals the coexistence of competing interactions in NbSe$_2$, creating a transition from a superconducting to an insulating quantum correlated state by confinement-controlled interactions. Our results demonstrate the dramatic role of interactions in NbSe$_2$, establishing NbSe$_2$ as a correlated superconductor with competing interactions.

Motivation & Objective

• To investigate the role of electron-electron interactions in NbSe₂ near the monolayer limit.
• To understand how quantum confinement modulates competing quantum phases in a superconducting transition metal dichalcogenide.
• To experimentally tune interaction strength and probe the emergence of correlated insulating states in NbSe₂.

Proposed method

• Low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) were used to probe electronic structure at atomic scale.
• Quantum confinement was achieved by mechanically exfoliating NbSe₂ to few-layer and monolayer thicknesses.
• The strength of electron-electron interactions was probed via local density of states measurements.
• Correlation effects were assessed by analyzing spectral features such as gaps and van Hove singularities.
• The evolution of electronic states with thickness was tracked to identify signatures of insulating behavior.

Experimental results

Research questions

• RQ1 How does quantum confinement affect electron correlation strength in NbSe₂?
• RQ2 What is the nature of the electronic state that emerges when superconductivity is suppressed in NbSe₂?
• RQ3 Can interactions in NbSe₂ be experimentally tuned to reveal proximity to a correlated insulating phase?
• RQ4 What evidence supports the coexistence of competing superconducting and insulating orders in confined NbSe₂?

Key findings

• Quantum confinement in NbSe₂ suppresses superconductivity and drives the system toward a correlated insulating state.
• STM/STS measurements reveal enhanced spectral gaps and suppressed density of states near the Fermi level in monolayer NbSe₂, indicating insulating behavior.
• The system exhibits strong proximity to a quantum phase transition between superconducting and insulating states.
• The results demonstrate that electron correlations in NbSe₂ are dramatically enhanced under confinement, indicating a competing interaction channel.
• The coexistence of superconductivity and correlation-driven insulating order is confirmed through tunable interaction strength via thickness control.