Following the recent discovery of superconductivity in the heavy-fermion paramagnet UTe2, which is suspected to lie on the verge of ferromagnetism [S. Ran et al., arXiv:1811.11808v3, D. Aoki et al., J. Phys. Soc. Jpn. 88 (2019) 043702], complementary and almost simultaneous series of magnetoresistivity experiments on UTe2 were performed in steady fields up to 35 T at LNCMI Grenoble and in pulsed fields up to 70 T at LNCMI Toulouse.
As step one, low-temperature metamagnetism was evidenced in a magnetic field applied along the hard magnetic b axis at µ0Hm = 35 T (Figure, left). When the temperature is raised, a signature of first-order metamagnetism was observed up to a critical endpoint at 7 K. A sharp maximum in the quadratic Fermi-liquid coefficient of the resistivity found at Hm indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi-surface instability.
As step two, we have discovered that low-temperature superconductivity is reinforced when Hm is approached in a field applied along b, and suddenly disappears above Hm (Figure, right). A home-made dilution refrigerator specially developed for the pulsed-field experiments at LNCMI Toulouse was used. By studying the angular dependence of the superconducting upper critical field in field directions close to the b axis, the maximum of the reentrant superconducting temperature was found to be rapidly depinned from the metamagnetic field. A key ingredient for the field reinforcement of superconductivity on approaching Hm appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction. Further studies of the field-induced superconductivity in UTe2 are now planned at LNCMI.
Figure: Signatures of (left) high-temperature metamagnetism and (right) low-temperature superconductivity in the electrical resistivity of UTe2 in a magnetic field applied along the crystallographic b axis.
- Magnetic-field-induced phenomena in the paramagnetic superconductor UTe2, W. Knafo, M. Valiska, D. Braithwaite, G. Lapertot, G. Knebel, A. Pourret, J.-P. Brison, J. Flouquet, and D. Aoki, J. Phys. Soc. Jpn. 88, 063705 (2019). Link
Field-reentrant superconductivity close to a metamagnetic transition in the heavy-fermion superconductor UTe2, G. Knebel, W. Knafo, A. Pourret, Q. Niu, M. Valiska, D. Braithwaite, G. Lapertot, M. Nardone, A. Zitouni, S. Mishra, I. Sheikin, G. Seyfarth, -P. Brison, D. Aoki, and J. Flouquet, J. Phys. Soc. Jpn. 88, 063707 (2019). Link