[Paper Review] High-temperature superconductivity in A15-type La4H23 below 100 GPa

Sam Y. Cross, Jonathan Buhot+5 more|arXiv (Cornell University)|Aug 6, 2023

Advanced Chemical Physics Studies4 citations

TL;DR

This study demonstrates high-temperature superconductivity in A15-type La4H23 at 95 GPa, with a critical transition temperature of ~90 K, achieved via laser heating of a Pd-capped La film in ammonia borane. The superconducting phase is confirmed by XRD and resistance measurements, reinforcing hydrogen cage structures as key to high-Tc superconductivity in hydrides at reduced pressures.

ABSTRACT

High-temperature superconductivity has been observed in binary hydrides such as LaH10 at pressures above 150 GPa. Hydrogen cage structures have been identified as a common motif beneficial for high critical temperatures Tc. Efforts are now focused on finding hydride high-temperature superconductors at lower pressures. We present evidence for high-temperature superconductivity in binary La4H23 with A15-type structure featuring hydrogen cages at a pressure of P = 95 GPa. We synthesise La4H23 from a lanthanum film capped with a palladium catalyst promoting the dissociation of hydrogen. In resistance measurements, we observe superconductivity with a transition temperature Tc = 90 K. In X-ray diffraction on the same sample, we identify the A15-type cubic body centred structure of the lanthanum sublattice. From comparison with earlier XRD and structure prediction studies, we identify this phase with La4H23. Our study reinforces the concept of hydrogen cages for high-temperature superconductivity.

Motivation & Objective

• To experimentally verify high-temperature superconductivity in A15-type La4H23 at pressures below 100 GPa.
• To investigate the role of hydrogen cage structures in stabilizing high-Tc superconductivity in binary lanthanum hydrides.
• To develop a synthesis route using a Pd-capped La film and ammonia borane to enable hydrogen diffusion and phase formation under high pressure.
• To confirm the A15-type cubic body-centered structure of La4H23 using in situ X-ray diffraction.
• To measure the superconducting transition temperature and magnetic field dependence of resistivity in the same sample.

Proposed method

• Synthesis of La4H23 via laser heating of a 260 nm thick Pd-capped lanthanum film in a diamond anvil cell with ammonia borane as a hydrogen donor.
• Use of a 1064 nm YAG laser in 300 ms pulses to achieve temperatures up to ~1000 K, promoting hydrogen diffusion and reaction.
• In situ X-ray diffraction at the I15 beamline of Diamond Light Source using a 0.425 Å wavelength beam and Pilatus3 X CdTe 2M detector.
• Rietveld and Le Bail refinements of XRD patterns to identify the A15-type structure and rule out competing phases.
• Four-point resistance measurements with 10 µA excitation current to detect superconducting transitions and magnetic field suppression up to 12 T.
• Pressure calibration using the Raman shift of the diamond anvil edge via the Akahama scale.

Figure 1: Detection of the A-15 structure in X-Ray diffraction. (a) Rietveld and Le Bail refinements of La 4 H 23 to the observed XRD pattern shown by the solid red and grey lines respectively. Data between $11\text{\,}\mathrm{\SIUnitSymbolDegree}13\text{\,}\mathrm{\SIUnitSymbolDegree}$ are dominate

Experimental results

Research questions

• RQ1 Can high-temperature superconductivity be observed in A15-type La4H23 at pressures below 100 GPa?
• RQ2 Does the A15-type structure in La4H23, featuring hydrogen cages, support a critical temperature above 90 K?
• RQ3 Can a Pd-capped lanthanum film enable efficient hydrogenation and phase formation of La4H23 under high pressure?
• RQ4 What is the structural identity of the superconducting phase in the La-H system at 95 GPa, and how does it compare to predicted and previously observed phases?
• RQ5 How does the superconducting transition in La4H23 respond to applied magnetic fields, and what does this imply about its coherence length and upper critical field?

Key findings

• A superconducting transition with Tc ≈ 90 K was observed in La4H23 at 95 GPa via four-point resistance measurements.
• X-ray diffraction confirmed the A15-type cubic body-centered structure (space group Pm3̄n) of La4H23, with characteristic reflections at 2θ ≈ 7.5–10° and 13.5–16°.
• The A15 structure was identified through Rietveld and Le Bail refinements, with unit cell parameters consistent with La4H23 and not other known hydrides.
• Residual elemental lanthanum (Fmmm) and gold (Fm3̄m) were detected from the electrodes and sample preparation, but no evidence of competing phases like LaH4, LaH9, or LaCH2 was found.
• Magnetic field studies showed suppression of resistance below Tc up to 12 T, confirming the superconducting nature of the transition.
• The residual resistance ratio (RRR) of the initial La film was ~2.5, and the superconducting transition of pure La at 95 GPa was observed at Tc = 8.6 K, slightly higher than previous reports, possibly due to pressure gradients.

Figure 2: Detection of superconductivity in La 4 H 23 and resistance measurements on subsequent cooling runs. In each case, cooling and warming measurements are indicated. The value of $T_{\text{c}}$ was determined at the onset of superconductivity from the intersection of linear fits to the normal