[Paper Review] Uniaxial Strain Tuning of Superconductivity in 2$H$-NbSe$_{2}$

Andrew Wieteska, Ben Foutty+8 more|arXiv (Cornell University)|Mar 12, 2019

Organic and Molecular Conductors Research4 citations

TL;DR

This study demonstrates that uniaxial strain strongly suppresses the superconducting transition temperature (T_sc) in 2H-NbSe₂ beyond 0.2% tensile or 0.1% compressive strain, with T_sc decreasing at rates of 1.3 K/% and 2.5 K/%, respectively. The suppression is attributed to a strain-driven transition from a 3Q to a 1Q charge density wave (CDW) phase, which enhances CDW gap formation on Fermi surface pockets hosting the superconducting order parameter, thereby competing with superconductivity and reducing T_sc.

ABSTRACT

We explore the effect of lattice anisotropy on the charge-ordered superconductor 2H-NbSe$_{2}$. Using a novel strain apparatus, we measure the superconducting transition temperature $T_{sc}$ as a function of uniaxial strain. It is found that $T_{sc}$ is independent of tensile(compressive) strain below a threshold of 0.2\% (0.1\%), but decreases strongly with larger strains with an average rate of $1.3\,$K/\% ($2.5\,$K/\%). Transport signatures of charge order are largely unaffected as a function of strain. Theoretical considerations show that the change in the behavior of $T_{sc}$ with strain coincides with a phase transition from 3Q to 1Q charge order in the material. The spectral weight on one of the Fermi surface bands is found to change strongly as a consequence of this phase transition, providing a pathway to tune superconducting order.

Motivation & Objective

• To investigate the effect of large, continuously tunable uniaxial strain on the superconducting transition temperature (T_sc) in 2H-NbSe₂.
• To determine how lattice anisotropy and strain-induced structural changes influence the interplay between superconductivity and charge order.
• To explore whether a phase transition from 3Q to 1Q charge density wave (CDW) order under strain drives the observed suppression of T_sc.
• To link changes in Fermi surface spectral weight and CDW gap formation to the measured T_sc suppression using theoretical modeling.
• To establish a strain-tunable platform for probing symmetry-breaking phases and their competition with superconductivity in correlated materials.

Proposed method

• Employed a custom piezoelectric uniaxial strain device with shear actuators to apply continuous, reversible strain to high-quality 2H-NbSe₂ single crystals.
• Measured longitudinal resistivity in a four-probe configuration to extract T_sc as a function of strain and temperature, with Hall resistivity used to confirm carrier type and mobility.
• Used capacitative sensors to calibrate the temperature-dependent displacement of piezoelectric stacks, correcting for thermal contraction effects.
• Applied density functional theory (DFT) calculations to model electronic band structure and phonon spectra under varying uniaxial strain.
• Calculated ARPES spectral density at the Fermi level (A(E_F, k)) for unstrained, 3Q-distorted, and 1Q-distorted phases at 0.6% tensile and compressive strains to assess Fermi surface gapping.
• Used random phase approximation (RPA) with momentum-dependent electron-phonon coupling to model CDW formation and its strain dependence, consistent with prior STM and ARPES data.

Experimental results

Research questions

• RQ1 How does uniaxial strain affect the superconducting transition temperature T_sc in 2H-NbSe₂?
• RQ2 Does strain induce a transition from a 3Q to a 1Q charge density wave (CDW) phase, and if so, how does this affect superconductivity?
• RQ3 To what extent does the Fermi surface spectral weight redistribution due to CDW gaps correlate with the suppression of T_sc?
• RQ4 Why is the strain dependence of T_sc asymmetric between tensile and compressive strains?
• RQ5 Can the observed suppression of T_sc be explained by competition between superconductivity and CDW order on specific Fermi surface pockets?

Key findings

• T_sc remains nearly constant for uniaxial tensile strain below 0.2% and compressive strain below 0.1%, indicating a strain-insensitive regime.
• Beyond these thresholds, T_sc decreases linearly with strain at a rate of 1.3 K/% under tensile strain and 2.5 K/% under compressive strain.
• The suppression of T_sc is strongly correlated with a strain-induced transition from a 3Q to a 1Q charge density wave (CDW) phase, as confirmed by theoretical modeling.
• In the 1Q CDW phase, the CDW gap opens on a significantly larger portion of the Fermi surface pockets—particularly the inner K-pocket—where the superconducting gap is dominant in the unstrained state.
• The two-fold asymmetry in the strain dependence of T_sc is naturally explained by the model assuming approximately constant unit cell area under strain.
• Theoretical calculations show that the spectral weight on the inner K-pocket is strongly suppressed in the 1Q phase, directly linking Fermi surface gapping to T_sc suppression.