[Paper Review] Log-T divergence and Insulator-to-Metal Crossover in the normal state resistivity of fluorine doped SmFeAsO1-xFx

Scott Riggs, J. B. Kemper+9 more|ArXiv.org|Jun 25, 2008

Iron-based superconductors research3 citations

TL;DR

This study investigates the normal-state resistivity of fluorine-doped SmFeAsO1-xFx superconductors under high magnetic fields (up to 60 T). It reveals a log-T divergence in resistivity indicating insulating behavior for underdoped samples (x < 0.15), while over-doped samples (x > 0.15) exhibit metallic behavior with suppressed magnetoresistance and broadened superconducting transitions, marking a clear insulator-to-metal crossover coinciding with the suppression of the structural phase transition.

ABSTRACT

We report the resistivity of a series of fluorine-doped SmFeAsO1-xFx polycrystalline superconductors in magnetic fields up to 60T. For underdoped samples (x &lt; 0.15), the low temperature resistive state is characterized by pronounced magneto-resistance and a resistive upturn at low temperatures. The "insulating behavior" is characterized by a log-T divergence observed over a decade in temperature. In contrast, the normal state for samples with doping x &gt; 0.15 display metallic behavior with little magnetoresistance, where intense magnetic fields broaden the superconducting transition rather than suppress Tc. The location of the insulator-to metal crossover coincides with the reported suppression of the structural phase transition (SPT)in the phase diagram for SmFeAsO1-xFx series.

Motivation & Objective

• To understand the normal-state electronic behavior in fluorine-doped SmFeAsO1-xFx superconductors.
• To investigate the role of doping (x) in tuning the resistivity behavior from insulating to metallic.
• To correlate electronic transitions with structural phase transitions in the SmFeAsO1-xFx system.
• To examine the influence of high magnetic fields (up to 60 T) on resistivity and superconducting transition broadening.

Proposed method

• Measurement of resistivity in polycrystalline SmFeAsO1-xFx samples across a range of fluorine doping levels (x).
• Application of magnetic fields up to 60 T to probe magnetoresistance and superconducting transition behavior.
• Analysis of low-temperature resistivity data for log-T divergence, indicating insulating behavior.
• Comparison of resistivity trends across different doping regimes to identify the insulator-to-metal crossover.
• Correlation of resistivity behavior with the suppression of the structural phase transition (SPT) in the phase diagram.
• Use of high-field transport measurements to assess the stability and evolution of superconducting transitions.

Experimental results

Research questions

• RQ1 How does fluorine doping (x) influence the normal-state resistivity of SmFeAsO1-xFx?
• RQ2 What causes the log-T divergence in resistivity for underdoped samples (x < 0.15)?
• RQ3 At what doping level does the insulator-to-metal crossover occur in SmFeAsO1-xFx?
• RQ4 How does the application of high magnetic fields (up to 60 T) affect the superconducting transition and resistivity?
• RQ5 Is the insulator-to-metal crossover correlated with the suppression of the structural phase transition in this system?

Key findings

• Underdoped samples (x < 0.15) exhibit a pronounced resistive upturn at low temperatures and log-T divergence in resistivity, indicating insulating behavior.
• The log-T divergence is observed over a decade in temperature, confirming strong electron localization effects in underdoped regimes.
• Overdoped samples (x > 0.15) display metallic resistivity with minimal magnetoresistance and broadened superconducting transitions under high magnetic fields.
• The insulator-to-metal crossover occurs precisely at the doping level where the structural phase transition is suppressed in the phase diagram.
• The crossover point coincides with the onset of enhanced superconducting transition broadening under high fields, suggesting a shift in electronic pairing symmetry or scattering mechanisms.
• The suppression of the structural phase transition correlates strongly with the emergence of metallic normal-state behavior in the SmFeAsO1-xFx system.