Anisotropic magnetism, superconductivity, and the phase diagram of Rb(1-x)Fe(2-y)Se2

TL;DR

This study explores the anisotropic magnetic and superconducting properties of Rb(1-x)Fe(2-y)Se2 crystals, revealing how stoichiometry influences phase behavior, with superconductivity emerging near Rb2Fe4Se5 and coexistence with antiferromagnetism.

Contribution

It provides detailed phase diagrams and critical parameters for Rb(1-x)Fe(2-y)Se2, highlighting the relationship between composition, magnetism, and superconductivity, which is novel for this material system.

Findings

Superconductivity occurs for 1.53 < 2-y < 1.6 with T_c up to 32.4 K.

Coexistence of superconductivity and static antiferromagnetic order is observed.

Critical current density reaches 1.6×10^4 A/cm^2 at 2 K.

Abstract

We report the crystal growth and structural, magnetic, conductivity, and specific heat investigations of Rb(1-x)Fe(2-y)Se2 single crystals with varying stoichiometry prepared by self-flux and Bridgman methods. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400 K. For 1.53 <2-y< 1.6 superconductivity is found, whereas for Fe concentrations 2-y< 1.5 and 2-y> 1.6 insulating and semiconducting behavior is observed, respectively. The sharpest transition to the superconducting state and the highest transition temperature T_c of 32.4 K is found for compositions close to Rb2Fe4Se5. A critical current density j_c of 1.6*10^4 A/cm^2 (at 2 K) is obtained in the superconducting samples with the highest diamagnetic response. In these samples additional intense x-ray reflexes appear, which are incompatible with the tetragonal symmetry. Upper critical fields H_{c2} of ~250 kOe for the in-plane and 630 kOe for the inter-plane configurations are estimated from resistivity in magnetic fields parallel and perpendicular to the c-axis, respectively. In the non-superconducting samples with the Fe concentration below 1.45 both specific heat and susceptibility revealed an anomaly at 220 K which is not related to antiferromagnetic or structural transformations. Comparison with the magnetic behavior of non-superconducting samples provides evidence for the coexistence of superconductivity and static antiferromagnetic order.