[Paper Review] Creating superconductivity in WB2 through pressure-induced metastable planar defects

Jinhyuk Lim, Ajinkya C. Hire+12 more|arXiv (Cornell University)|Sep 23, 2021

Superconductivity in MgB2 and Alloys4 citations

TL;DR

This study demonstrates that superconductivity in WB2 can be induced not through structural phase transitions, but via pressure-induced metastable planar defects—specifically stacking faults and twin boundaries—resembling the MgB2 structure. Superconductivity emerges abruptly above 50 GPa with a maximum critical temperature of 17 K at 91 GPa, despite the host hP12 phase remaining structurally intact, as confirmed by synchrotron XRD and theoretical calculations showing electron-phonon coupling in the defective hP3-like regions.

ABSTRACT

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tc of 17 K at 90 GPa. Upon further compression up to 190 GPa, the Tc gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2} (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa} show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

Motivation & Objective

• To investigate the origin of superconductivity in WB2 under high pressure, particularly given conflicting prior reports of low Tc at ambient pressure.
• To resolve the paradox of high Tc (17 K) observed under pressure despite the thermodynamically stable hP12 phase having negligible Tc.
• To determine whether structural phase transitions or defect-mediated mechanisms are responsible for the onset of superconductivity.
• To identify the role of metastable planar defects such as stacking faults and twin boundaries in enabling electron-phonon mediated superconductivity.
• To establish a new design principle for engineering superconductivity in otherwise non-superconducting materials via defect engineering under pressure.

Proposed method

• High-pressure electrical resistivity measurements were performed using a diamond anvil cell (DAC) with a four-point probe configuration to measure resistivity down to 1.8 K at pressures up to 187 GPa.
• Synchrotron X-ray diffraction (XRD) was used to monitor structural evolution up to 145 GPa, confirming persistence of the ambient-pressure hP12 phase (P6₃/mmc) with increasing c/a ratio.
• Density-functional theory (DFT) calculations were employed to compute enthalpies and electronic structures of competing phases (hP12, hR6, hP3), including metastable stacking fault and twin boundary configurations.
• Electron-phonon coupling was calculated using the linear-response method in Quantum Espresso, with isotropic Eliashberg equations solved to predict Tc for the hP3-like defective structures.
• Van der Pauw method was applied to estimate resistivity from measured resistance, assuming isotropic in-plane behavior and using sample thickness (~10 µm) and geometry.
• Ruby fluorescence pressure calibration was used to determine pressure at low temperatures, with corrections applied for small pressure shifts during cooling.

Experimental results

Research questions

• RQ1 Why does WB2 exhibit a sharp onset of superconductivity above 50 GPa despite no observed structural phase transition?
• RQ2 What is the origin of the high Tc (~17 K) observed at 91 GPa, given that the ambient hP12 phase is predicted to have Tc < 1 K?
• RQ3 Can metastable planar defects such as stacking faults and twin boundaries in WB2 host superconductivity similar to the MgB2 structure?
• RQ4 How do the electronic and vibrational properties of defective hP3-like regions compare to the stable hP12 phase under high pressure?
• RQ5 Is the observed superconductivity driven by a true phase transition or by percolating defect networks?

Key findings

• Superconductivity in WB2 emerges abruptly above 50 GPa with a maximum Tc of 17 K at 91 GPa, followed by a gradual decrease up to 187 GPa.
• Synchrotron XRD shows that the ambient-pressure hP12 phase (P6₃/mmc) persists up to 145 GPa, with a monotonic increase in the c/a ratio, indicating no structural phase transition.
• DFT calculations confirm that the stable hP12 phase has Tc < 1 K across all pressures studied, while the metastable hP3 phase (AlB2-type) is predicted to have Tc between 25–40 K.
• Theoretical analysis identifies stacking faults and twin boundaries as the key defects that locally stabilize hP3-like structures, enabling electron-phonon coupling and superconductivity.
• The onset of superconductivity correlates with the percolation of these planar defects rather than a bulk structural change, explaining the abrupt transition.
• The results establish a new route to superconductivity: engineering metastable defects in non-superconducting materials under pressure, bypassing the need for stable phase transitions.