Inverse Isotope Effect in the Ternary Perovskite Hydride SrPdH/D$_{2.9}$: A Signature of Quantum Zero-Point Fluctuations

Abstract

Guided by first-principles calculations, we demonstrate superconductivity in the ternary perovskite hydride SrPdH$_{3-x}$, synthesized at low pressure. Structural characterization via neutron diffraction reveals the near-stoichiometric composition SrPdD$_{2.9(2)}$ with 96\% deuterium site occupancy. Subsequent transport and magnetic susceptibility measurements establish onset superconducting transitions at $T_\text{c} = \SI{2.1}{K} $ (H) and $T_\text{c} = \SI{2.2}{K} $ (D), exhibiting an inverse isotope effect that our first-principles calculations attribute predominantly to quantum zero-point motion. The excellent agreement between theory and experiment with respect to thermodynamic stability and superconducting properties provides important validation for theory-guided superconductor discovery. This work establishes superconductivity in the perovskite hydride structural prototype -- expanding the limited family of experimentally realized ternary hydride superconductors -- and demonstrates the importance of quantum nuclear motion on the accurate theoretical treatment of low-pressure hydride superconductors.

Figures (4)

• Figure 1: a Crystal structure of perovskite hydrides, where the A-site, B-site and H atoms are shown as orange, red, and white spheres. In SrPdH3, Pd and Sr occupy the B-site and A-site, respectively. b Calculated ternary phase diagram of the Sr--Pd--H system at ambient pressure. Structures are represented by rhombuses and colored according to their distance from the convex hull; thermodynamically stable structures are indicated by red solid circles.
• Figure 2: a XRD pattern (black points) for hydrogenated samples (measured at room temperature) with Rietveld refinement (R$_{wp}$ = 4.89%) of the predicted Pm$\Bar{3}$mSrPdH3 structure (blue line). The red line shows the Rietveld residual and vertical ticks marks indicate allowed Bragg reflections. Impurity peaks from SrO and Sr2PdH4 are represented by green and grey ticks, respectively. Inset: photographs of the sample after hydrogenation. b TOF-neutron diffraction pattern for deuterated sample at 10K (black points) and Rietveld refinement (blue line) with R$_{wp}$ of 8.17%. The red line shows the Rietveld residual and the vertical ticks mark the positions of nuclear peaks for SrPdD$_{2.9(2)}$ (blue), SrO (green) and Sr2PdD4 (grey).
• Figure 3: a Electrical transport measurements of the synthesized hydride and deuteride under ambient pressure. Inset show the T$_\text{c}^{\mkern2mu\text{onset}}$ by extrapolating the linear portions of the resistance curves. b Temperature dependence of the electrical resistance for deuteride under applied magnetic fields up to 500Oe, measured in steps of 50Oe. The inset shows the GL fit and estimation of the upper critical field.
• Figure 4: a Anharmonic phonon dispersion, phonon DOS, Eliashberg spectral function $\alpha^2F(\omega)$, and cumulative el--ph coupling $\lambda(\omega)$. b Critical temperatures determined via Migdal-Eliashberg theory as a function of screened Coulomb pseudopotential $\mu^*$.