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Divergent Pressure Response of Superconductivity in Sc$_{6}$MTe$_{2}$ ($M$ = Fe, Ru and Ir)

J. N. Graham, S. S. Islam, K. Yuchi, P. Král, O. Gerguri, S. Huber, J. Chang, R. Khasanov, Y. Okamoto, Z. Guguchia

TL;DR

The paper addresses non-BCS superconductivity in the Sc$_6$MTe$_2$ family by mapping how hydrostatic pressure tunes superconductivity across M = Fe, Ru, Ir. Using μSR and AC susceptibility, they track $T_ ext{C}$ and the superfluid density $σ_ ext{sc}$, finding divergent pressure responses: $T_ ext{C}$ decreases for Fe but increases for Ru and Ir, while $σ_ ext{sc}$ remains flat for Fe and drops for Ru/Ir. These trends imply different microscopic mechanisms, with a crossover from correlation-dominated to SOC-dominated regimes, and resemble cuprate-like $T_ ext{C}$–$σ_ ext{sc}$ relations. The work motivates further high-pressure and spectroscopic studies to develop a unified theory of non-BCS superconductivity in $d$-electron materials.

Abstract

Identifying and understanding non-BCS superconductivity remains a central challenge in condensed-matter physics. Here we focus on the Sc$_{6}$MTe$_{2}$ family (M =d-electron metal), which provides a unique platform of isostructural compounds exhibiting superconductivity across 3d, 4d, and 5d systems. Using hydrostatic pressure as an additional tuning parameter, muon-spin rotation ($μ$SR) and AC susceptibility measurements uncover strongly contrasting pressure responses of superconductivity across the Sc$_{6}$MTe$_{2}$ series. The superconducting transition temperature, $T_{\rm C}$ decreases under pressure in the 3d Fe-based compound but increases for the 4d Ru- and 5d Ir-based systems, with the Ru compound showing the largest enhancement of nearly 50% within 2 GPa. The superfluid density exhibits similarly distinct pressure dependences, remaining nearly pressure independent for Fe while decreasing with increasing pressure for Ru and Ir. This suggests fundamentally different correlations between $T_{\rm C}$ and the superfluid density. Together, these results indicate that superconductivity emerging from strongly correlated and spin-orbit-dominated regimes in Sc$_{6}$MTe$_{2}$ is likely governed by different microscopic mechanisms and offer a useful experimental basis for future microscopic theoretical studies.

Divergent Pressure Response of Superconductivity in Sc$_{6}$MTe$_{2}$ ($M$ = Fe, Ru and Ir)

TL;DR

The paper addresses non-BCS superconductivity in the ScMTe family by mapping how hydrostatic pressure tunes superconductivity across M = Fe, Ru, Ir. Using μSR and AC susceptibility, they track and the superfluid density , finding divergent pressure responses: decreases for Fe but increases for Ru and Ir, while remains flat for Fe and drops for Ru/Ir. These trends imply different microscopic mechanisms, with a crossover from correlation-dominated to SOC-dominated regimes, and resemble cuprate-like – relations. The work motivates further high-pressure and spectroscopic studies to develop a unified theory of non-BCS superconductivity in -electron materials.

Abstract

Identifying and understanding non-BCS superconductivity remains a central challenge in condensed-matter physics. Here we focus on the ScMTe family (M =d-electron metal), which provides a unique platform of isostructural compounds exhibiting superconductivity across 3d, 4d, and 5d systems. Using hydrostatic pressure as an additional tuning parameter, muon-spin rotation (SR) and AC susceptibility measurements uncover strongly contrasting pressure responses of superconductivity across the ScMTe series. The superconducting transition temperature, decreases under pressure in the 3d Fe-based compound but increases for the 4d Ru- and 5d Ir-based systems, with the Ru compound showing the largest enhancement of nearly 50% within 2 GPa. The superfluid density exhibits similarly distinct pressure dependences, remaining nearly pressure independent for Fe while decreasing with increasing pressure for Ru and Ir. This suggests fundamentally different correlations between and the superfluid density. Together, these results indicate that superconductivity emerging from strongly correlated and spin-orbit-dominated regimes in ScMTe is likely governed by different microscopic mechanisms and offer a useful experimental basis for future microscopic theoretical studies.
Paper Structure (8 sections, 5 equations, 5 figures, 1 table)

This paper contains 8 sections, 5 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Superconducting state
  3. Normal state
  4. Discussion
  5. Conclusion and outlook
  6. Methods
  7. Acknowledgments
  8. Author contributions

Figures (5)

  • Figure 1: Summary of AC susceptibility and $\mu$SR measurements under pressure on the Sc$_6M$Te$_2$ ($M =~$Fe, Ru, Ir) compounds. AC susceptibility measurements under various applied pressures up to $\sim 2.1~$GPa for a Sc$_6$FeTe$_2$, b Sc$_6$RuTe$_2$, and c Sc$_6$IrTe$_2$. $\mu$SR measurements under various applied pressures up to 1.8 GPa for d Sc$_6$FeTe$_2$, e Sc$_6$RuTe$_2$, and f Sc$_6$IrTe$_2$. Fits of a nodeless s-wave superconducting gap structure are shown for Sc$_6$FeTe$_2$ by the solid lines in d. $\mu$SR data are shown on a log-scale for Sc$_6$RuTe$_2$ (e) and Sc$_6$IrTe$_2$ (f) due to the limited temperature range.
  • Figure 2: Evolution of $T^*$ transition in the normal state of Sc$_6$FeTe$_2$. The change in relaxation rate, indicating a phase transition ($T^*$), does not change with pressure. The feature is slightly broadened from the transition determined in ambient conditions ($T^*_\mathrm{amb}$graham2025tailoring) which is likely due to the pressure cell subtraction.
  • Figure 3: Summary of pressure response of the Sc$_6M$Te$_2$ ($M =~$Fe, Ru, Ir) compounds. a Relative change of $T_\mathrm{C}$ of each Sc$_6M$Te$_2$ ($M =~$Fe, Ru, Ir) compound with pressure. $T_\mathrm{C}$ is normalised to the ambient pressure value to put all compounds on the same scale. Sc$_6$RuTe$_2$ has overall the largest change with pressure. b Relationship between $T_\mathrm{C}$ and superfluid density, $\sigma_\mathrm{sc}$ where all three compounds show contrasting responses with pressure. Arrows and coloured ovals (light to dark) denote increasing hydrostatic pressure.
  • Figure 4: The temperature dependence of the relaxation rate of the CuBe/MP35N pressure cell with the sample inside is shown. The measurements were performed in a $20~$mT applied transverse field at a muon momentum of 98 MeV/c, for which the majority of muons stop in the pressure cell. The relaxation rate exhibits a non-linear temperature dependence, with only a very weak increase below 2 K attributable to the influence of the superconducting sample.
  • Figure 5: Example fit of zero-field $\mu$SR data in the normal state of Sc$_6$FeTe$_2$ at $5~$K. Data were fit with Eq. \ref{['Eq_GKT']}.