Pressure-induced collapse of large-moment magnetic order and localized-to-itinerant electronic transition in the host-guest compound [Cs6Cl][Fe24Se26]

Peter Adler,Moaz A Elghazali,Pavel G Naumov,Rudolf Rüffer,Sergey A Medvedev,Martin Valldor

doi:10.1103/physrevb.101.094433

Abstract

The magnetic properties of iron chalcogenides and pnictides have found much interest as magnetic fluctuations are suggested to drive the formation of Cooper pairs in iron-based superconductors. Here, we have studied the pressure dependence of the magnetic and electrical properties of the iron-selenide compound $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ by energy-domain synchrotron M\ossbauer spectroscopy up to 17 GPa and by resistivity measurements up to 45 GPa. Similar to the binary superconductor $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Se}$, the host-guest-type crystal structure of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ contains edge-sharing $\mathrm{FeS}{\mathrm{e}}_{4}$ units, but its ground state is antiferromagnetically ordered. A complex hyperfine pattern suggests a nontrivial spin structure like a spin spiral with large magnetic moments in the range 2 to $3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ at ambient pressure. High pressure drastically suppresses the N\'eel temperature from 220 K at ambient pressure to below 30 K at 12 GPa. Between 5 and 10 GPa the complex magnetic hyperfine pattern collapses and a low-moment magnetic ground state emerges at higher pressures. A concomitant decrease of the resistivity by several orders of magnitude indicates that the electronic system becomes more itinerant. Full metallization occurs near 25 GPa, but no sign of superconductivity down to 1.5 K was found at any pressure. It is suggested that the insulating ground state of $[\mathrm{C}{\mathrm{s}}_{6}\mathrm{Cl}][\mathrm{F}{\mathrm{e}}_{24}\mathrm{S}{\mathrm{e}}_{26}]$ corresponds to an orbitally selective Mott phase, which due to an increased bandwidth, is transformed into a Hund's metal state under pressure.

Highlights

The efforts to understand the origin of superconductivity in iron-based pnictides and chalcogenides [1] has led to a large number of experimental and theoretical studies aiming to unravel their basic electronic and magnetic properties [2,3,4,5]
In agreement with our previous study [22], the spectrum at 230 K can be described by a single quadrupole doublet, which arises from the two crystallographically distinct sites that are, similar in their local coordination environment and not resolved in the spectrum
As in Ref. [22], a complicated hyperfine pattern is found at 5 K, the shape of which was approximated by assuming a superposition of seven sextets differing in their hyperfine fields Bhf and quadrupole splitting parameters QS’, but having a single IS = 0.70 mm/s

Summary

Introduction

The efforts to understand the origin of superconductivity in iron-based pnictides and chalcogenides [1] has led to a large number of experimental and theoretical studies aiming to unravel their basic electronic and magnetic properties [2,3,4,5]. The electronic properties of this class of compounds are often described by a multiorbital electron system featuring moderately strong Mott-Hubbard-type Coulomb repulsion U , sizable electronic bandwidth W , as well as pronounced Hund coupling JH. Depending on the relative strength of these interactions, either Mott-type localized systems (“Mottness”) or strongly correlated Hund metal systems (“Hundness”) may emerge, where in the latter case strong electron correlations are rather driven by JH than by U.

Results

Discussion

Conclusion