Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe (1512.06951v1)

Abstract: The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (${T_{\rm c}}$) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. More importantly, a pressure-induced fourfold increase of ${T_{\rm c}}$ has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to $\sim9$ GPa, which uncover a hidden magnetic dome superseding the nematic order. Above ${\sim6}$ GPa the sudden enhancement of superconductivity (${T_{\rm c}\le38.3}$ K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed above the high-${T_{\rm c}}$ phase. The obtained phase diagram highlights unique features among iron-based superconductors, but bears some resemblance to that of high-${T_{\rm c}}$ cuprates.

• The paper demonstrates that high-pressure conditions induce a dome-shaped magnetic order in FeSe that competes with superconductivity.
• It employs high-pressure magnetotransport measurements up to 9 GPa to construct a detailed temperature-pressure phase diagram with T_c increasing to 38.3 K.
• The study suggests that the suppression of a spin density wave may enhance superconductivity, shedding light on unconventional high-T_c mechanisms.

Analyzing the Competition between Magnetic Order and Superconductivity in High-Pressure FeSe

The paper "Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe" presents a focused investigation into the interplay between magnetic order and superconductivity in FeSe under high-pressure conditions. FeSe distinguishes itself from other iron-based superconductors due to its unconventional behavior, characterized by nematic ordering without coinciding magnetism. This paper aims to unravel the mysteries surrounding the superconductivity in FeSe, particularly when subjected to elevated pressures resulting in a marked increase in the superconducting transition temperature, $T_{\rm c}$.

Key Findings and Experimental Approach

FeSe typically undergoes a structural transition with the creation of a nematic state without displaying a magnetic order at ambient pressure. However, under high-pressure conditions, the paper showcases the emergence of a dome-shaped magnetic order, peaking at approximately 45 K, which doesn't exist in ambient conditions. Importantly, this magnetic order coincides with alterations in $T_{\rm c}$, which reaches a maximum of 38.3 K when the magnetic order gets suppressed at about 6 GPa.

The researchers employed high-pressure magnetotransport measurements reaching up to 9 GPa using high-quality single crystal samples to construct a temperature-pressure phase diagram of FeSe. This comprehensive diagram maps out distinct phases, indicating how the structural transition temperature ($T_{\rm s}$), the magnetic transition temperature ($T_{\rm m}$), and $T_{\rm c}$ evolve under varying pressure conditions.

Key Results and Interpretations

The pressure-induced emergence of magnetic order suggests a spin density wave (SDW) phase intervening in the superconducting state, which unveils a delicate balance and takes on competitive interactions with the superconductivity. This interplay is evidenced by the observation that $T_{\rm c}$ is enhanced in three plateau stages as pressure increases: the initial low-pressure range shows $T_{\rm c}$ around 10 K, escalating to about 20 K and eventually peaking near 38 K at higher pressures.

The notable result in this paper is the identification of a $T_{\rm m}(P)$ dome that persists within a certain pressure range and reveals a competition between magnetic order and superconductivity. This interplay is inferred from the correlation between the sudden upward change in $T_{\rm c}$ around 6 GPa and the diminishing magnetic order, with both phenomena attributed to comparable energy scales. This behavior contrasts with other iron-based superconductors, where the magnetic transition temperature is significantly higher than their maximum $T_{\rm c}$.

Theoretical Implications and Future Outlook

The findings highlight the unusual coupling between magnetism and superconductivity in FeSe, an aspect that deviates from the conventional scenarios observed in other high-$T_{\rm c}$ superconductors. The results point towards frustration-driven magnetic order and its suppression as critical factors enhancing superconductivity in FeSe.

This paper presents a unique phase diagram for FeSe that, while bearing similarities to the cuprates, stands distinct among iron-based superconductors. It invites further exploration into the fundamental mechanisms governing the interplay of competing orders in the phase diagram. Future research should focus on delineating the precise nature of the SDW phase under high pressure and exploring the implications of quasi-critical behaviors observed in resistivity.

Overall, this paper is a significant contribution to understanding the fine balance between magnetic order and superconductivity under extreme conditions, potentially offering deeper insights into high-$T_{\rm c}$ superconductors' underlying physical principles.