[Paper Review] Two new parent compounds for FeSe-based superconducting phases

Shifeng Jin, Xiaozhi Wu+7 more|arXiv (Cornell University)|Jul 5, 2016

Iron-based superconductors research25 references3 citations

TL;DR

This study reports two new non-superconducting parent phases in ethylenediamine-intercalated FeSe: a tetragonal phase (Tc = 38 K after Na doping) and an orthorhombic phase (Tc = 46 K after Na doping), both stabilized by neutral spacer layers. The absence of long-range magnetic order down to 2 K and the reversible switch between superconducting and non-superconducting states indicate that undoped iron selenides are quantum paramagnetic ground states, distinct from iron pnictides in their pairing mechanism.

ABSTRACT

It is well established that the occurrence of superconductivity in iron pnictides is closely related to the tetragonal to orthorhombic structural and antiferromagnetic (AFM) phase transitions. It, however, has not been clear whether the same scenario is appropriate for iron chalcogenide counterparts due to the absence of parent compounds for the latter family of superconductors. Here, we report the synthesis and structure determination of two novel phases in ethylenediamine intercalated FeSe, one is tetragonal and the other orthorhombic in room temperature, which can be stabilized with neutral spacer layers. Both phases can be regarded as the parent compounds for superconductivity as they are non-superconducting (non-SC) in pristine form and superconducting (SC) with Tc up to 38K and 46K, respectively after Na doping, and the switch between SC and no-SC is reversible. Moreover, the two non-SC parent compounds show no evidence of long-range magnetic ordering down to 2K, only with dynamic spin fluctuations at low temperatures, suggesting that no competition between SC and AFM ordering. Our results reveal that undoped iron selenides are quantum paramagnetic in ground state, implying that they are distinct from their pnictide counterparts in pairing mechanism.

Motivation & Objective

• To identify stable parent compounds for FeSe-based superconductors, which were previously unknown.
• To investigate the structural and electronic properties of ethylenediamine-intercalated FeSe phases.
• To determine whether long-range antiferromagnetic order competes with superconductivity in iron chalcogenides, as seen in iron pnictides.
• To clarify the role of magnetic order and spin fluctuations in the superconducting mechanism of FeSe-based systems.
• To establish a reversible transition between superconducting and non-superconducting states in undoped phases.

Proposed method

• Synthesis of ethylenediamine-intercalated FeSe phases under high-pressure and high-temperature conditions.
• Structure determination using X-ray and neutron diffraction to identify tetragonal and orthorhombic phases at room temperature.
• Measurement of electrical resistivity and magnetic susceptibility to probe superconducting transition temperatures (Tc).
• Low-temperature magnetic characterization down to 2 K to detect long-range magnetic order or spin fluctuations.
• Systematic Na doping of the non-superconducting parent phases to induce superconductivity and assess reversibility.
• Analysis of spin dynamics via magnetic susceptibility and comparison with iron pnictide systems.

Experimental results

Research questions

• RQ1 Can stable parent compounds for FeSe-based superconductors be synthesized and structurally characterized?
• RQ2 Do the non-superconducting parent phases exhibit long-range antiferromagnetic order at low temperatures?
• RQ3 Is the superconducting transition in FeSe-based systems reversible upon doping and undoping?
• RQ4 How do the magnetic properties of FeSe-based systems compare to those of iron pnictides?
• RQ5 What is the nature of spin fluctuations in the ground state of undoped FeSe-based compounds?

Key findings

• Two new parent phases were synthesized: one tetragonal and one orthorhombic, both non-superconducting in pristine form.
• Na doping induces superconductivity with Tc values of 38 K and 46 K in the tetragonal and orthorhombic phases, respectively.
• No long-range magnetic order was observed down to 2 K in either parent compound, indicating a quantum paramagnetic ground state.
• Dynamic spin fluctuations were detected at low temperatures, suggesting short-range spin correlations without long-range order.
• The transition between superconducting and non-superconducting states is reversible upon Na doping and undoping.
• The absence of competing antiferromagnetic order implies a distinct pairing mechanism in FeSe compared to iron pnictides.