Origin of Metamagnetic Transition (MMT) in the Spin-Triplet Superconductivity in UTe₂

The metamagnetism realized in heavy fermion systems has been well studied to date. At the metamagnetic transition/crossover fields, magnetization increases discontinuously/non-lineally, and fluctuations develop. Consequently, the effective mass increases. Thus, the metamagnetism may induce a new quantum state, such as a superconducting transition. The uranium-based ferromagnetic superconductors have been good playgrounds to study the relationship between metamagnetism and superconductivity. Quite recently, another related system, UTe₂, was discovered.

The superconductivity in a paramagnetic heavy fermion system, UTe₂, became a hot topic in condensed matter physics community since the realization of the spin-triplet state, which is expected for use in quantum computing. The external stimuli, such as pressure, magnetic field, and its applied direction, make UTe₂ more attractive. The most remarkable feature is the magnetic-field-direction controlled superconductivity. For H || b axis, superconductivity is reinforced by increasing the fields but is killed at the metamagnetic field H_m ~ 35 T, where a first-order metamagnetic transition takes place. However, the field applied near the orthorhombic [011] direction induces another superconducting phase above H_m. Because experimental probes usable under such extreme conditions were limited, the role of metamagnetic transition in the appearance/disappearance of superconductivity is far from complete understanding. Moreover, the metamagnetism in UTe₂ must be more clarified. These still remain a big challenge for experimentalists.

Magnetostriction, that is, the length/volume change with magnetic field, should offer hints at the aforementioned issues. The authors have developed magnetostriction measurements combining the fiber Bragg grating fiber optics and optical filter method that cover a wide range of ultra-high-magnetic fields beyond 100 T. A new attempt demonstrated in this article is simultaneous magnetostriction measurements up to 55 T. We obtained the linear magnetostriction along the two principal axes within a one-field scan; thus, we could perform measurements efficiently. This improvement also enables us to confirm the reliability and reproducibility of the magnetostriction measurements. Using a single crystal of UTe₂, we revealed the magnetostrictions along all principal axes and the volume change at H_m.

Across H_m approaching from the lower fields, the volume shrinks discontinuously. This volume discontinuity well explains the pressure dependence of H_m and magnetic susceptibility reported earlier. Furthermore, we revealed that the metamagnetic transition in UTe₂ involves anisotropic lattice deformation. Considering the localized and itinerant duality nature of uranium ions, we proposed that the uranium valence transition is the origin of metamagnetism. We also discussed the singular behavior in UTe₂ by comparing it with the other heavy fermion systems. Our findings deepen the understanding of the fascinated physics realized in UTe₂.

(The figure has been replaced as of July 19, 2022)

(Written by A. Miyake on behalf of all authors)