[Paper Review] Superconductivity in SrB3C3 clathrate

Li Zhu, Hanyu Liu+13 more|arXiv (Cornell University)|Aug 11, 2017

Superconductivity in MgB2 and Alloys79 references4 citations

TL;DR

This study predicts and experimentally confirms superconductivity in the carbon-boron clathrate SrB3C3, with a critical temperature (Tc) of approximately 20 K at 40 GPa, using first-principles calculations and in situ high-pressure transport measurements. The material exhibits non-hysteretic resistivity drops and magnetic field suppression consistent with conventional electron-phonon coupling, establishing carbon-based clathrates as a new class of superconductors stable under ambient conditions.

ABSTRACT

We predict superconductivity for the carbon-boron clathrate SrB3C3 at 27-43 K for Coulomb pseudopotential (mu*) values between 0.17 and 0.10 using first-principles calculations with conventional electron-phonon coupling. Electrical transport measurements, facilitated by a novel in situ experimental design compatible with extreme synthesis conditions (>3000 K at 50 GPa), show non-hysteretic resistivity drops that track the calculated magnitude and pressure dependence of superconductivity for mu*=0.15, and transport measurements collected under applied magnetic fields confirm superconductivity with an onset Tc of approximately 20 K at 40 GPa. Carbon-based clathrates thus represent a new class of superconductors similar to other covalent metals like MgB2 and doped fullerenes. Carbon clathrates share structures similar to superconducting superhydrides, but covalent C-B bonds allow metastable persistence at ambient conditions.

Motivation & Objective

• To investigate the superconducting properties of the carbon-boron clathrate SrB3C3 under extreme conditions.
• To determine whether covalent C-B bonded clathrates can exhibit superconductivity despite their metastable nature at ambient pressure.
• To establish a link between theoretical predictions of superconductivity and experimental observation in a novel class of covalent materials.
• To explore the role of electron-phonon coupling in determining Tc in complex borocarbonitride structures.

Proposed method

• First-principles calculations with conventional electron-phonon coupling were used to predict superconducting transition temperatures (Tc) for SrB3C3 across a range of Coulomb pseudopotential (μ*) values.
• A novel in situ high-pressure experimental design enabled electrical transport measurements at pressures up to 50 GPa and temperatures exceeding 3000 K.
• Resistivity measurements under varying pressure and magnetic fields were used to identify superconducting transitions and confirm Tc onset.
• Theoretical Tc values were compared with experimental data using μ* = 0.15 to assess consistency in magnitude and pressure dependence.
• Structural and electronic properties were analyzed to confirm the stability and covalent bonding nature of the clathrate framework.

Experimental results

Research questions

• RQ1 Can the carbon-boron clathrate SrB3C3 exhibit superconductivity under high pressure, and what is its critical temperature?
• RQ2 How does the predicted Tc from first-principles calculations compare with experimentally observed superconducting transitions?
• RQ3 To what extent does electron-phonon coupling explain the superconducting behavior in this covalent clathrate?
• RQ4 Can metastable carbon-boron clathrates remain stable at ambient conditions while exhibiting superconductivity?

Key findings

• Theoretical calculations predict a Tc range of 27–43 K for SrB3C3 at μ* values between 0.17 and 0.10, with Tc decreasing as μ* increases.
• Experimental resistivity measurements show non-hysteretic drops consistent with superconductivity, with an onset Tc of approximately 20 K at 40 GPa.
• The pressure dependence of the observed Tc matches the calculated trend for μ* = 0.15, validating the theoretical model.
• Magnetic field-dependent transport measurements confirm superconductivity, with suppression of resistivity onset under applied fields.
• The material’s metastable structure persists under ambient conditions due to strong covalent C-B bonds, enabling experimental access.