The Signal
UTe₂ Measurements Establish a Tetracritical Point at 0.20 GPa
Ultrasound measurements of uranium ditelluride reveal a previously missing superconducting phase boundary and establish a tetracritical point near 0.20 GPa. The results map where two superconducting order parameters coexist under pressure a
Quantum.world Team · Written with AI · fact-checked ·
Here we observe an additional phase boundary that is characterized by an upward jump in the sound velocity.
Sahas Kamat, Jared Dans, Shanta Saha, Artem D. Kokovin, Johnpierre Paglione, Jörg Schmalian and B. J. Ramshaw, Nature Physics
Published on 30 October 2026, the study uses ultrasound measurements to resolve a long-standing inconsistency in the pressure–temperature phase diagram of uranium ditelluride, or UTe₂. The researchers identify a missing transition that changes the apparent meeting point of superconducting phases into a tetracritical point. Their results show that two distinct superconducting order parameters coexist in parts of UTe₂’s pressure and magnetic-field phase diagram.
The missing boundary in UTe₂
UTe₂ is a heavy-fermion superconductor studied as a candidate for unconventional, potentially topological superconductivity. Earlier phase diagrams showed two second-order transition boundaries meeting at a triple point. The authors note that such a meeting is thermodynamically forbidden, pointing to a hidden transition or an incomplete picture of the material.
To look for that transition, the team measured sound velocity and elastic properties in pressurized UTe₂. Ultrasound responds to changes in a material’s elastic moduli, which can reveal thermodynamic phase transitions. The researchers tracked these changes as they varied temperature, pressure and magnetic field along the material’s b axis.
An upward jump marks the lost superconducting component
At a pressure of 0.213 gigapascals, the measurements show downward jumps in a compression modulus as the sample enters two superconducting states: SC2 at 1.63 kelvin and SC1 at 1.58 kelvin. Cooling further to 1.0 kelvin produces an upward jump at a third boundary, labelled Tc2*. The authors interpret that feature as the loss of the SC2 component while SC1 remains.
The transition shows no hysteresis between warming and cooling, consistent with a second-order phase transition. That evidence matters because the newly identified boundary supplies the missing line needed to resolve the forbidden triple-point picture. Between about 0.20 and 0.22 gigapascals, the sample can enter SC2, develop a mixed SC1-plus-SC2 state, and then lose SC2 as it cools.
A point near 0.20 GPa extends into a field-dependent line
The researchers place the pressure–temperature tetracritical point near 0.20 gigapascals and 1.6 kelvin. Mapping the transition in magnetic field turns that point into a line of tetracritical points in the broader field–pressure–temperature diagram. At 0.11 gigapascals, the point lies near 7 tesla and 1.25 kelvin; with increasing pressure, its field value falls until it reaches zero near 0.20 gigapascals.
The mixed state also appears at ambient pressure when a magnetic field is applied along the b axis. The paper places it between 12 and 18 tesla. The field-dependent measurements therefore connect pressure-induced SC2 to the field-induced superconducting phase, while tracing where the two components coexist.
Coexisting orders do not settle UTe₂’s topology
The authors’ Ginzburg–Landau analysis describes the two superconducting orders as strongly competing: SC1 grows as the SC2 component is suppressed, until SC2 disappears at the newly identified boundary. Their model also finds that coupling between the orders locks their relative phase, which can suppress superconducting fluctuations. At 0.77 gigapascals, the team observes unusually high sound attenuation inside SC2 and attributes it to low phase stiffness in that state.
The measurements establish a multicomponent superconducting region, but they do not determine whether it breaks time-reversal symmetry or is topological. The paper says those possibilities depend on the coupling between the order parameters; its symmetry-breaking scenario is conditional, not an experimental finding. The concrete result is a revised phase map, including an ambient-pressure mixed state between 12 and 18 tesla along the b axis.