[Paper Review] The distinct in-plane resistivity anisotropy in the nematic states of detwinned NaFeAs and FeTe single crystals: evidences for Hund's rule metal

Juan Jiang, C. He+7 more|arXiv (Cornell University)|Oct 1, 2012

Iron-based superconductors research3 citations

TL;DR

This study reveals distinct in-plane resistivity anisotropy in detwinned NaFeAs and FeTe single crystals, with NaFeAs showing lower resistivity along the antiferromagnetic (AFM) direction and FeTe showing higher resistivity along the AFM direction. The opposite behavior is attributed to Hund’s rule coupling: in NaFeAs (itinerant regime), $J_H$ drives electronic nematicity and band reconstruction, while in FeTe (localized regime), $J_H$ creates an effective potential barrier hindering electron hopping along the AFM direction, analogous to colossal magnetoresistance in manganites. The findings establish Hund’s rule coupling as the dominant correlation mechanism in iron-based superconductors.

ABSTRACT

The in-plane resistivity anisotropy has been studied with the Montgomery method on two detwinned parent compounds of the iron-based superconductors, NaFeAs and FeTe. For NaFeAs, the resistivity in the antiferromagnetic (AFM) direction is smaller than that in the ferromagnetic (FM) direction, similar to that observed in BaFe2As2 before. While for FeTe, the resistivity in the AFM direction is larger than that in the FM direction. We show that these two opposite resistivity anisotropy behaviors could be attributed to the strong Hund's rule coupling effects: while the iron pnictides are in the itinerant regime, where the Hund's rule coupling causes strong reconstruction and nematicity of the electronic structure; the FeTe is in the localized regime, where Hund's rule coupling makes hopping along the FM direction easier than along the AFMdirection, similar to the colossal magnetoresistance observed in some manganites.

Motivation & Objective

• To investigate the in-plane resistivity anisotropy in detwinned parent compounds of iron-based superconductors, specifically NaFeAs and FeTe.
• To understand the origin of contrasting resistivity anisotropy behaviors between NaFeAs (lower resistivity in AFM direction) and FeTe (higher resistivity in AFM direction).
• To determine whether Hund’s rule coupling ($J_H$) is the dominant local correlation mechanism driving electronic nematicity and magnetic order in iron-based superconductors.
• To correlate the energy scale of band reconstruction ($\Delta_{H0}$), ordered magnetic moments, and Néel temperatures ($T_N$) across multiple iron-based compounds to assess the universality of Hund’s metal physics.

Proposed method

• Detwinning of NaFeAs and FeTe single crystals using uniaxial strain applied via a custom-designed mechanical device with insulating interlayers to prevent electrical shorting.
• Measurement of in-plane resistivity using the Montgomery method, which enables precise determination of resistivity along different crystallographic axes by applying current and measuring voltage in two orthogonal configurations.
• Use of angle-resolved photoemission spectroscopy (ARPES) to extract electronic band structures and identify nematic electronic reconstruction in both materials.
• Analysis of resistivity anisotropy as a function of temperature to identify the onset of nematic order ($T_A$) and compare it with magnetic transition temperatures ($T_N$).
• Correlation of $\Delta_{H0}$, the energy scale of band reconstruction, with $T_N$ and magnetic moments across multiple iron-based compounds to assess the role of $J_H$ in electronic nematicity.

Experimental results

Research questions

• RQ1 Why does NaFeAs exhibit lower resistivity along the AFM direction, while FeTe shows higher resistivity along the same direction, despite both being in nematic states?
• RQ2 How does Hund’s rule coupling ($J_H$) differentially influence electronic structure and transport anisotropy in the itinerant (NaFeAs) versus localized (FeTe) regimes?
• RQ3 To what extent does $J_H$ dominate local electronic correlations in iron-based superconductors, even when the on-site Coulomb repulsion $U$ is not strong?
• RQ4 Can the observed resistivity anisotropy be explained by a unified Hund’s metal framework across different iron-based compounds?
• RQ5 What is the relationship between the energy scale of band reconstruction ($\Delta_{H0}$), magnetic order ($T_N$), and ordered moments across iron-based superconductors?

Key findings

• In NaFeAs, the resistivity along the AFM direction is lower than along the FM direction, consistent with the behavior observed in BaFe2As2 and indicating nematic electronic reconstruction driven by Hund’s rule coupling.
• In FeTe, the resistivity along the AFM direction is higher than along the FM direction, opposite to NaFeAs, indicating that $J_H$ suppresses electron hopping along the AFM direction.
• The opposite resistivity anisotropy in NaFeAs and FeTe arises from distinct manifestations of $J_H$: in NaFeAs, $J_H$ induces nematic band reconstruction in the itinerant regime; in FeTe, $J_H$ creates an effective potential barrier in the localized regime, analogous to colossal magnetoresistance.
• The energy scale of band reconstruction ($\Delta_{H0}$) correlates monotonically with $T_N$ across iron-based compounds, supporting the role of $J_H$ as a universal driver of nematic order.
• The ordered moment in FeTe (~2 $\mu_B$) is significantly larger than in iron pnictides with similar $T_N$, indicating a stronger $J_H$-driven effect in FeTe, consistent with its localized character.
• ARPES measurements show that band reconstruction in FeTe occurs over a broad energy range and exhibits polaronic character, further supporting the localized nature of electrons and the role of $J_H$ in forming self-trapped states.