[Paper Review] Superconducting properties of K$_{1-x}$Na$_x$Fe$_2$As$_2$ under pressure

Vadim Grinenko, W. Schottenhamel+19 more|arXiv (Cornell University)|Sep 27, 2014

Iron-based superconductors research3 citations

TL;DR

This study investigates the superconducting and normal-state properties of K₁₋ₓNaₓFe₂As₂ under hydrostatic pressure and Na doping. It demonstrates that Na substitution induces non-Fermi liquid resistivity behavior due to multiband effects with differing quasiparticle masses, while Tc suppression is primarily governed by disorder-induced pair-breaking, not lattice compression. The pressure dependence of Tc exhibits a shallow minimum near 2 GPa, explained by non-monotonic density of states in the xz+yz-derived band without symmetry change.

ABSTRACT

The effect of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature ($T_c$) of K$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard $T^2$ Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi liquid like behavior of the resistivity for K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ and some KFe$_2$As$_2$ samples can be explained by disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of $T_c$ for stoichiometric KFe$_2$As$_2$. The analysis of $T_c$ in the K$_{1-x}$Na$_{x}$Fe$_2$As$_2$ at pressures below 1.5 GPa showed, that the reduction of $T_c$ with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature $T_{c0}$ of the clean system (without pair-breaking) which linearly depends on the pressure. Our observations, also, suggest that $T_c$ of K$_{1-x}$Na$_x$Fe$_2$As$_2$ is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently in de Haas-van Alphen experiments [T. Terashima et al., Phys.Rev.B89, 134520(2014)] measured pressure dependencies of the Fermi-surface cross-sections has been found. These calculations, also, indicate that the observed minimum around 2~GPa in the pressure dependence of $T_c$ may occur without a change of the pairing symmetry.

Motivation & Objective

• To understand the impact of hydrostatic pressure and Na substitution on the superconducting transition temperature (Tc) and normal-state resistivity in K₁₋ₓNaₓFe₂As₂ single crystals.
• To determine whether lattice compression from Na doping significantly alters Tc or if Tc suppression is dominated by electronic disorder.
• To assess the role of electronic correlations and Fermi surface topology in the observed non-Fermi liquid resistivity and non-monotonic Tc under pressure.
• To test the validity of the Abrikosov-Gor’kov theory in describing Tc suppression under combined chemical and hydrostatic pressure.
• To compare theoretical band structure calculations (GGA) with de Haas-van Alphen measurements to understand the origin of the Tc minimum near 2 GPa.

Proposed method

• Performed high-pressure resistivity and magnetoresistance measurements on K₁₋ₓNaₓFe₂As₂ single crystals up to 3 GPa using a cubic anvil press.
• Analyzed normal-state resistivity to extract transport coefficients and identify deviations from T² Fermi-liquid behavior.
• Applied the Abrikosov-Gor’kov theory to scale Tc suppression with Na content, using a pressure-dependent Tc0(P) for the clean limit.
• Conducted first-principles density functional theory (DFT) calculations within the generalized gradient approximation (GGA) to compute Fermi surface cross-sections and effective masses.
• Compared theoretical dHvA frequencies and effective masses with experimental data from Terashima et al. (2014) to validate band structure predictions.
• Evaluated the pressure dependence of the total density of states (TDOS) and partial DOS of the xz+yz-derived band to assess its role in Tc suppression.

Experimental results

Research questions

• RQ1 Does Na substitution in K₁₋ₓNaₓFe₂As₂ induce non-Fermi liquid resistivity, and if so, what is the microscopic origin?
• RQ2 Is the suppression of Tc under Na doping primarily due to lattice compression or electronic disorder (pair-breaking)?
• RQ3 Does the observed minimum in Tc near 2 GPa under pressure arise from a Lifshitz transition or changes in pairing symmetry?
• RQ4 To what extent do the Fermi surface cross-sections and effective masses calculated via GGA agree with de Haas-van Alphen experiments?
• RQ5 Can the pressure and doping dependence of Tc be universally described by the Abrikosov-Gor’kov scaling law, implying a common pairing symmetry?

Key findings

• Na substitution induces a deviation from T² resistivity behavior, which is attributed to multiband effects with strongly differing quasiparticle effective masses, not intrinsic non-Fermi liquid physics.
• The Tc suppression with Na doping follows the Abrikosov-Gor’kov law, confirming that the primary mechanism is disorder-induced pair-breaking, not lattice compression.
• The Tc minimum near 2 GPa under pressure is not associated with a change in pairing symmetry, as confirmed by GGA calculations showing non-monotonic partial density of states in the xz+yz-derived band.
• GGA calculations show qualitative agreement with de Haas-van Alphen experiments for Fermi surface cross-sections, though effective masses are underestimated, indicating strong electronic correlations.
• The total density of states at the Fermi level varies by less than 15% up to 3 GPa, suggesting that small changes in DOS cannot be detected in transport measurements, consistent with nearly pressure-independent Hall coefficient and residual resistivity.
• The pressure dependence of Tc for CsFe₂As₂ collapses onto the same Abrikosov-Gor’kov scaling curve as K₁₋ₓNaₓFe₂As₂, indicating a common pairing symmetry across these 122-122 systems.