Field-Reentrant Superconductivity Close to a Metamagnetic Transition in the Heavy-Fermion Superconductor UTe₂

Abstract

We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe 2 by magnetoresistivity measurements in pulsed magnetic fields up to 60 T and static magnetic fields up to 35 T. We show that superconductivity survives up to the metamagnetic transition at H m ≈ 35 T at low temperature. Above H m superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of H c2 . By studying the angular dependence of the upper critical field close to the b axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching H m appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction. 1 arXiv:1905.05181v1 [cond-mat.str-el]

Insulator-Metal Transition and Topological Superconductivity in UTe2 from a First-Principles Calculation

“…Uniform magnetic susceptibility, magnetization, NMR Knight shift, and 1/T 1 T support the proximity of metallic ferromagnetic quantum criticality [1,5,6]. An extremely large upper critical field and re-entrant superconductivity have been observed by high-field experiments [1,7,8]. These observations strongly suggest odd-parity superconductivity induced by ferromagnetic fluctuations.…”

Tuning magnetic confinement of spin-triplet superconductivity

“…In addition, at least two forms of re-entrant superconductivity have been observed in high magnetic fields, including one that extends the low-field superconducting phase upon precise field alignment along the crystallographic b axis 10 , and an extreme high-field phase that onsets in pulsed magnetic fields above the paramagnetic normal state at angles tilted away from the b axis 11 .…”

“…Upon further pressure increase, evidence of suppression of the Kondo energy scale leads to an abrupt disappearance of superconductivity and a transition to a ferromagnetic phase 12 . Together with the ambient pressure magnetic field-induced phenomena 10,11,14,15 , the axes of the magnetic field, temperature, and pressure provide for a very rich and interesting phase space in this system. One of the key questions is in regard to the field-polarized (FP) phase that appears to truncate superconductivity at 34.5 T under proper b-axis field alignment 10,11 , in particular regarding the nature of the coupling of the two phases and whether superconductivity could persist to even higher fields in the absence of the competing FP phase.…”

“…Together with the ambient pressure magnetic field-induced phenomena 10,11,14,15 , the axes of the magnetic field, temperature, and pressure provide for a very rich and interesting phase space in this system. One of the key questions is in regard to the field-polarized (FP) phase that appears to truncate superconductivity at 34.5 T under proper b-axis field alignment 10,11 , in particular regarding the nature of the coupling of the two phases and whether superconductivity could persist to even higher fields in the absence of the competing FP phase. The relation between the FP phase and the pressureinduced magnetic phase, which also competes with superconductivity 11 , is similarly not yet fully understood.…”

Theory of Ferromagnetic Superconductors —Analogue of Superfluid 3He A Phase—

“…Thus the superconducting state survives under a strong internal field coming through an exchange interaction. The H c2 curve for a field applied parallel to the magnetic hard axis far exceeds the Pauli paramagnetic limitation, and exhibits a re-entrant behavior in URhGe, or an Sshape and L-shape curves in UCoGe and UTe 2 [4] respectively. Those facts clearly point to a triplet pairing rather than a singlet one.…”