Interplay between magnetism and superconductivity in UTe2

TL;DR

This study investigates the interplay between magnetism and superconductivity in UTe2, revealing a critical state of ferromagnetic vortices and the role of ferromagnetic fluctuations in its non-unitary order parameter.

Contribution

It provides new insights into the magnetic-field-trained Kerr effect and demonstrates the critical state of ferromagnetic vortices in UTe2.

Findings

Identification of a remanent Kerr signal indicating TRSB response.

Determination of the maximum field screened by the critical state.

Observation of shielding currents opposing vortex-induced magnetization.

Abstract

Time-reversal symmetry breaking (TRSB) in UTe2 was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the non-unitary composite order-parameter of this material. Here we present an extensive study of the magnetic-field-trained Kerr effect, which unveils a unique critical state of pinned ferromagnetic vortices. We show that a remanent Kerr signal that appears following the removal of a training magnetic field, which reflects the response of the TRSB order parameter and the external magnetic field through the paramagnetic susceptibility. This unambiguously demonstrate the importance of the ferromagnetic fluctuations and their intimate relation to the composite order parameter. Focusing the measurement to the center of the sample, we are able to accurately determine the maximum field that is screened by the critical state and the respective critical current. Measurements in the presence of magnetic field show the tendency of the superconductor to produce shielding currents that oppose the increase in vortex-induced magnetization due to the diverging paramagnetic susceptibility.