We report bulk ultrasound measurements up to 77 T and down to 0.5 K of the field reentrant superconducting phase of the unconventional superconductor ${\mathrm{UTe}}_2$. Clear bulk signatures of superconductivity are observed in the longitudinal $c_{11}$ elastic mode for fields applied at a tilt angle of ${\theta }_{\mathrm{b}-\mathrm{c}}={30}^{\circ }$ from the $b$ axis. We confirm an upper critical field of $H_{\mathrm{c}2}\approx 65\mathrm{T}$ at 0.5 K and bulk superconductivity which survives up to $T\approx 2\mathrm{K}$ for fields above the metamagnetic transition. The $c_{11}$ mode has propagation and displacement vectors along the $a$ axis, and for fields applied at a tilt angle of ${\theta }_{\mathrm{b}-\mathrm{c}}={30}^{\circ }$, this mode is sensitive to the elasticity of the vortex lattice. The anomalies observed in $c_{11}$ are in part reminiscent of superconducting vortices pinned to lattice defects. Nonetheless, an excess attenuation, with respect to the normal state, is observed throughout the entire superconducting phase, suggesting unusual vortex dynamics and pinning in the field reentrant superconducting phase of ${\mathrm{UTe}}_2$.

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• Spin-triplet pairing
• Superconductivity
• Vortex dynamics
• Heavy-fermion systems
• Strongly correlated systems
• Ultrasound techniques