Anomalous Paramagnetic Meissner-like AC Response in EuRbFe₄As₄ Superconductor

Area of Science:

• Condensed Matter Physics
• Materials Science
• Magnetism and Superconductivity

Background:

• EuRbFe4As4 exhibits coexistence of superconductivity and magnetic ordering.
• Interactions between Abrikosov vortices and Eu2+ spins are crucial but poorly understood dynamically.
• Previous investigations relied primarily on static (DC) magnetization measurements.

Purpose of the Study:

• To investigate the dynamic interactions between superconducting and magnetic sub-systems in EuRbFe4As4.
• To probe the system's response using AC susceptibility measurements across a range of temperatures and DC fields.
• To understand the interplay of superconductivity, magnetism, and vortex dynamics.

Main Methods:

• AC susceptibility measurements were performed over a wide temperature range.
• Superimposed DC magnetic fields were applied in various orientations (perpendicular and parallel to superconducting planes).
• Measurements were conducted under different cooling regimes to assess thermal history effects.

Main Results:

• A clear magnetic transition at 15 K was observed in low DC fields.
• An anomalous AC susceptibility dependence was found just below the critical temperature for perpendicular field orientation, absent in parallel orientation.
• A paramagnetic AC response was observed in the normal phase, indicating interactions between Eu2+ spins and flux lines.

Conclusions:

• The observed anomaly is attributed to sample dimensions and the temperature-dependent London penetration depth, potentially revealing a paramagnetic Meissner-like response.
• The phenomenon represents an AC susceptibility manifestation of a field-stabilized critical state, not a thermodynamic phase.
• Eu2+ spin and flux line interactions are confirmed through paramagnetic AC response in the normal phase.