Moessbauer spectroscopy evidence for the lack of iron magnetic moment in superconducting FeSe

TL;DR

This study uses Moessbauer spectroscopy and magnetic susceptibility measurements to show that superconducting FeSe lacks a magnetic moment on iron atoms, indicating Pauli paramagnetism or diamagnetism.

Contribution

It provides direct spectroscopic evidence that iron in FeSe does not develop a magnetic moment in the superconducting state, challenging previous assumptions about magnetism in iron-based superconductors.

Findings

No magnetic moment detected on iron atoms in FeSe.

Electron density on iron decreases sharply at 105 K.

Transition from P4/nmm to Cmma structure correlates with electron density change.

Abstract

Superconducting FeSe has been investigated by measurements of the magnetic susceptibility versus temperature and Moessbauer spectroscopy at various temperatures including strong external magnetic fields applied to the absorber. It was found that isomer shift exhibits sharply defined increase at about 105 K leading to the lowering of the electron density on iron nucleus by 0.02 electron/a.u.^3. Above jump in the electron density is correlated with the transition from the P4/nmm to the Cmma structure, while decreasing temperature. Moessbauer measurements in the external magnetic field and for temperatures below transition to the superconducting state revealed null magnetic moment on iron atoms. Hence, the compound exhibits either Pauli paramagnetism or diamagnetic behavior. The principal component of the electric field gradient on the iron nucleus was found as negative on the iron site.