[Paper Review] The intrinsic electronic phase diagram of iron-oxypnictide superconductors

C. Heß, A. Kondrat+6 more|ArXiv.org|Nov 10, 2008

Iron-based superconductors research4 citations

TL;DR

This study presents the first comprehensive intrinsic electronic phase diagram of iron-oxypnictide superconductors using electrical resistivity measurements on LaFeAsO₁₋ₓFₓ and SmFeAsO₁₋ₓFₓ across a wide doping range. It reveals that high-temperature superconductivity emerges in a regime with anomalous normal-state transport behavior—reminiscent of pseudogap and spin-density wave signatures—while Fermi liquid-like behavior appears at higher doping, with stronger anomalies correlating to enhanced Tc in Sm-based compounds.

ABSTRACT

We present the first comprehensive derivation of the intrinsic electronic phase diagram of the iron-oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity $ρ$ of both LaFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$ for a wide range of doping. Our data give clear-cut evidence for unusual normal state properties in these new materials. In particular, the emergence of superconductivity at low doping levels is accompanied by distinct anomalous transport behavior in $ρ$ of the normal state which is reminiscent of the spin density wave (SDW) signature in the parent material. At higher doping levels $ρ$ of LaFeAsO$_{1-x}$F$_x$ shows a clear transition from this pseudogap-like behavior to Fermi liquid-like behavior, mimicking the phase diagram of the cuprates. Moreover, our data reveal a correlation between the strength of the anomalous features and the stability of the superconducting phase. The pseudogap-like features become stronger in SmFeAsO$_{1-x}$F$_x$ where superconductivity is enhanced and vanish when superconductivity is reduced in the doping region with Fermi liquid-like behavior.

Motivation & Objective

• To derive the intrinsic electronic phase diagram of iron-oxypnictide superconductors in the normal state using resistivity measurements.
• To investigate the connection between anomalous normal-state transport behavior and the emergence of superconductivity.
• To compare the doping evolution of electronic properties in LaFeAsO₁₋ₓFₓ and SmFeAsO₁₋ₓFₓ to understand the role of magnetic and structural instabilities.
• To determine how the strength of pseudogap-like features correlates with superconducting transition temperature Tc.
• To clarify whether the transition from non-superconducting to superconducting state is continuous or first-order, especially in the context of magnetic inhomogeneity.

Proposed method

• Measured electrical resistivity ρ(T) using the four-probe method on single crystals of LaFeAsO₁₋ₓFₓ (0 ≤ x ≤ 0.2) and SmFeAsO₁₋ₓFₓ (0 ≤ x ≤ 0.1).
• Normalized ρ(T) data to ρ(296 K) and used linear and quadratic fits (ρ(T) = ρ₀ + AT²) to extract low-temperature behavior and identify Fermi liquid vs. anomalous regimes.
• Identified characteristic temperatures T_max and T_drop from resistivity anomalies, linked to structural and spin-density wave (SDW) transitions via comparison with XRD and μSR data.
• Correlated doping evolution of resistivity features with superconducting Tc and magnetic order, using WDX to confirm actual F-doping levels.
• Constructed phase diagrams by plotting T_max, T_drop, T_S (structural), and T_c as functions of x, comparing La- and Sm-based systems.
• Used low-temperature resistivity upturn as a signature of carrier localization and SDW gap formation, particularly in LaFeAsO₁₋ₓFₓ.

Experimental results

Research questions

• RQ1 How does the normal-state resistivity ρ(T) evolve with doping in iron-oxypnictide superconductors, and what does it reveal about electronic instabilities?
• RQ2 To what extent do pseudogap-like anomalies in ρ(T) persist in the superconducting state, and how are they related to the parent compound's spin-density wave (SDW) state?
• RQ3 Why is Tc enhanced in SmFeAsO₁₋ₓFₓ despite stronger resistivity anomalies, and how does this compare to LaFeAsO₁₋ₓFₓ?
• RQ4 Is the transition from the non-superconducting magnetic state to superconductivity continuous or first-order, and what role does inhomogeneity play?
• RQ5 How does the emergence of Fermi liquid-like behavior at high doping correlate with the suppression of superconductivity and anomalous transport?

Key findings

• The resistivity of LaFeAsO₁₋ₓFₓ shows a clear maximum at T_max ≈ 160 K and a sharp drop at T_drop ≈ 137 K in the parent compound, corresponding to structural and SDW transitions.
• In underdoped LaFeAsO₁₋ₓFₓ (x ≤ 0.04), resistivity anomalies persist and resemble pseudogap behavior, with a low-T upturn indicating carrier localization and SDW gap formation.
• At higher doping (x = 0.15), ρ(T) exhibits Fermi liquid-like behavior (ρ ∝ T²) below ~150 K, indicating a crossover from anomalous to conventional metallic state.
• In SmFeAsO₁₋ₓFₓ, pseudogap-like anomalies are stronger than in La-based compounds, and no low-T resistivity upturn is observed, suggesting reduced SDW gap and enhanced electronic screening.
• Tc is significantly enhanced in SmFeAsO₁₋ₓFₓ despite stronger resistivity anomalies, indicating a direct correlation between anomalous normal-state behavior and high-Tc superconductivity.
• The phase diagram shows a striking resemblance to cuprates, with a superconducting dome flanked by pseudogap-like anomalies at low doping and Fermi liquid-like behavior at high doping, though the transition to superconductivity appears first-order or inhomogeneous.