[Paper Review] `17-15' Superconductivity - Rh17S15 and Pd17Se15

H. R. Naren, A. Thamizhavel+3 more|arXiv (Cornell University)|Nov 22, 2010

Rare-earth and actinide compounds22 references7 citations

TL;DR

This study reports the discovery of superconductivity in Pd17Se15 below 2.2 K, establishing it as a superconductor iso-structural to Rh17S15, which exhibits strongly correlated superconductivity. While Rh17S15 shows strong electron correlations and multiband superconductivity with a high upper critical field and enhanced Sommerfeld coefficient (108.41 mJ/mol·K²), Pd17Se15 displays weaker correlations due to larger Pd–Pd distances and a lower density of states at the Fermi level, indicating a conventional BCS-like mechanism.

ABSTRACT

The presence of strongly correlated superconductivity in Rh17S15 has been recently established. In this work, we compare the normal and superconducting parameters of a single crystal of Rh17S15 with those of a polycrystalline sample of Pd17Se15 which is reported here for the first time to be a superconductor below 2.2 K. Pd17Se15, which is iso-structural to Rh17S15 (space group Pm3m), has very different properties and provides for an interesting study in contrast with Rh17S15. We see that large unit volume of Pd17Se15 and the large separation of Pd-Pd atoms in its structure as compared those in a pure Pd metal lead to the absence of strong correlations in this compound. Finally, we report band structure calculations on these two compounds and compare the density of states with estimates from heat capacity data. Upper critical field, Heat capacity (down to 500 mK), Hall effect and band structure studies suggest that Rh17S15 is a multiband superconductor.

Motivation & Objective

• To investigate the superconducting properties of Pd17Se15, a new superconductor iso-structural to Rh17S15.
• To compare the normal and superconducting state parameters of Rh17S15 and Pd17Se15 to understand the role of electron correlations.
• To determine whether the superconductivity in Rh17S15 is unconventional and driven by strong electron correlations.
• To evaluate the accuracy of density functional theory (LDA and LDA+U) in predicting the density of states and electron-phonon coupling in these complex chalcogenide systems.
• To correlate experimental measurements (specific heat, upper critical field, Hall effect) with band structure calculations and theoretical models.

Proposed method

• Synthesis of single-crystalline Rh17S15 and polycrystalline Pd17Se15 via solid-state reaction at 1150 °C, followed by controlled cooling and annealing.
• Structural characterization using powder X-ray diffraction (XRD) and Reitveld refinement to confirm single-phase cubic Pm3m structure for both compounds.
• Electron Probe Micro Analysis (EPMA) to verify stoichiometric composition and rule out significant off-stoichiometry.
• Specific heat capacity measurements down to 500 mK to extract the electronic Sommerfeld coefficient (γ) and estimate Tc via equal entropy method.
• Magnetization, resistivity, and Hall effect measurements to probe superconducting transition, upper critical field (Hc2), and carrier density.
• Density functional theory (DFT) calculations using LDA and LDA+U to compute band structures and density of states (DOS), with comparison to experimental γ values.

Experimental results

Research questions

• RQ1 Is Pd17Se15 a bulk superconductor, and what is its critical temperature?
• RQ2 How do the superconducting parameters (Tc, Hc2, γ, ΔCp/γTc) of Pd17Se15 compare with those of Rh17S15, and what do these differences imply about electron correlation strength?
• RQ3 Why does Rh17S15 exhibit strong electron correlations and unconventional superconductivity, while Pd17Se15 does not?
• RQ4 To what extent do standard LDA calculations accurately predict the density of states at the Fermi level in these systems?
• RQ5 Can LDA+U corrections resolve the discrepancy between experimental and theoretical estimates of the electron-phonon coupling constant (λ) in Rh17S15 and Pd17Se15?

Key findings

• Pd17Se15 is a bulk superconductor with a critical temperature of 2.2 K, confirmed by zero-resistivity, diamagnetic shielding, and a sharp drop in specific heat.
• The Sommerfeld coefficient (γ) for Rh17S15 is 108.41 mJ/mol·K², indicating strong electron correlations, while Pd17Se15 has a significantly lower γ, consistent with weaker correlations.
• The upper critical field (Hc2) of Rh17S15 is ~13 T at 3 K, exceeding the Pauli limit, suggesting spin-triplet or strong-coupling pairing, unlike Pd17Se15 which shows conventional behavior.
• The specific heat jump at Tc for Rh17S15 gives ΔCp/γTc ≈ 2.68, well above the BCS value of 1.43, indicating strong electron-electron interactions beyond phonon-mediated pairing.
• LDA+U calculations reduce the DOS at the Fermi level to 11.4 states/eV per formula unit for Rh17S15 and 6.0 states/eV for Pd17Se15, bringing theoretical λ values into better agreement with McMillan’s estimate for Pd17Se15 but not for Rh17S15.
• The DOS at the Fermi level in Rh17S15 is dominated by Rh 24m atoms (contributing over 80%), and the Fermi level lies on a peak in the DOS, supporting strong correlations and multiband superconductivity.