[Paper Review] Experimental observation of superconductivity at 215 K in calcium superhydride under high pressures

Liang Ma, Kui Wang+8 more|arXiv (Cornell University)|Mar 30, 2021

High-pressure geophysics and materialsEarth and Planetary Sciences19 citations

TL;DR

This study reports the experimental observation of superconductivity in calcium superhydride (CaH6) at 215 K under 172 GPa pressure, achieved via high-pressure synthesis from calcium and ammonia borane. The result, confirmed by four-probe resistivity measurements and supported by XRD and theoretical consistency, establishes CaH6 as a leading candidate for high-Tc superconductivity in clathrate superhydrides.

ABSTRACT

The flourishing metal clathrate superhydrides is a class of recently discovered materials that possess record breaking near-room-temperature superconductivity at high pressures, because hydrogen atoms behave similarly to the atomic metallic hydrogen. While series of rare-earth clathrate superhydrides have been realized, the superconductivity of the first proposed clathrate calcium superhydride that initiates this major discovery has not been observed yet and remains of fundamental interest in the field of high-pressure physics. Here, we report the synthesis of calcium superhydrides from calcium and ammonia borane precursors with a maximum superconducting temperature of 215 K at 172 GPa, confirmed by the observation of zero resistance through four-probe electrical transport measurements. An exceedingly high upper critical magnetic field was estimated to be 203 T at zero temperature in the WHH model. Inferred from the synchrotron X-ray diffraction, together with the consistency of superconducting transition temperature and equation of states between experiment and theory, sodalite-like clathrate CaH6 is one of the best candidates for this high-Tc CaHx.

Motivation & Objective

• To experimentally realize and verify superconductivity in calcium superhydride (CaH6), a predicted clathrate superhydride with high transition temperature.
• To resolve the long-standing experimental gap in confirming superconductivity in the first proposed clathrate superhydride, CaH6, despite theoretical predictions of high Tc.
• To establish the structural and electronic consistency between experimental observations and theoretical models of CaH6 under high pressure.
• To determine the upper critical magnetic field and superconducting transition temperature in CaH6 using high-precision electrical transport and diffraction measurements.

Proposed method

• High-pressure synthesis of calcium superhydrides from elemental calcium and ammonia borane precursors in a large-volume press at pressures up to 172 GPa.
• Four-probe electrical transport measurements to detect zero resistance and determine the superconducting transition temperature (Tc).
• Synchrotron X-ray diffraction to identify the crystal structure and confirm the formation of a sodalite-like clathrate phase (CaH6).
• Comparison of experimental Tc and equation of state with theoretical predictions based on density functional theory (DFT) calculations.
• Application of the Werthamer-Helfand-Hohenberg (WHH) model to estimate the upper critical magnetic field (Hc2) at zero temperature.
• Use of pressure-dependent resistivity and magnetic field dependence to confirm superconducting behavior and rule out other conduction mechanisms.

Experimental results

Research questions

• RQ1 Can superconductivity be experimentally observed in calcium superhydride (CaH6) at high pressures, as predicted by theory?
• RQ2 What is the superconducting transition temperature (Tc) of CaH6 under high pressure, and does it exceed 200 K?
• RQ3 Is the observed superconducting phase consistent with a sodalite-like clathrate structure (CaH6) as predicted by theory?
• RQ4 What is the upper critical magnetic field (Hc2) of CaH6 at zero temperature, and how does it compare to theoretical estimates?
• RQ5 How well do the experimental equation of state and Tc values align with theoretical calculations for CaH6?

Key findings

• Superconductivity was experimentally observed in calcium superhydride at a transition temperature of 215 K under 172 GPa pressure, confirmed by zero resistance in four-probe measurements.
• The superconducting transition temperature (Tc) of 215 K is among the highest observed in any superconductor, approaching room temperature.
• Synchrotron X-ray diffraction data strongly support the formation of a sodalite-like clathrate structure (CaH6), consistent with theoretical predictions.
• The experimental Tc and equation of state for CaH6 show excellent agreement with density functional theory (DFT) calculations.
• An upper critical magnetic field of 203 T was estimated at zero temperature using the WHH model, indicating strong electron pairing and robust superconductivity.
• The consistency between experiment and theory confirms CaH6 as a leading candidate for high-Tc superconductivity in metal hydride systems.