A quantum phase transition from triangular to stripe charge order in NbSe$_{2}$

TL;DR

This study uncovers a quantum phase transition between two types of charge density waves in NbSe$_2$, driven by local strain, providing insights into competing electronic phases in correlated materials.

Contribution

The paper reports the discovery of a strain-tuned quantum phase transition between triangular and stripe charge density waves in NbSe$_2$, clarifying longstanding debates about its electronic properties.

Findings

Identification of a unidirectional stripe CDW interfacing with a triangular CDW

Local strain as the tuning parameter for the phase transition

Resolution of debates on the spectroscopic gap and Fermi surface nesting in NbSe$_2$

Abstract

The competition between proximate electronic phases produces a complex phenomenology in strongly correlated systems. In particular, fluctuations associated with periodic charge or spin modulations, known as density waves, may lead to exotic superconductivity in several correlated materials. However, density waves have been difficult to isolate in the presence of chemical disorder, and the suspected causal link between competing density wave orders and high temperature superconductivity is not understood. Here we use scanning tunneling microscopy to image a previously unknown unidirectional (stripe) charge density wave (CDW) smoothly interfacing with the familiar tri-directional (triangular) CDW on the surface of the stoichiometric superconductor NbSe$_2$. Our low temperature measurements rule out thermal fluctuations, and point to local strain as the tuning parameter for this quantum phase transition. We use this discovery to resolve two longstanding debates about the anomalous spectroscopic gap and the role of Fermi surface nesting in the CDW phase of NbSe$_2$. Our results highlight the importance of local strain in governing phase transitions and competing phenomena, and suggest a new direction of inquiry for resolving similarly longstanding debates in cuprate superconductors and other strongly correlated materials.