Michal Vališka

Multiphase superconductors—materials that host two or more distinct superconductive phases—are exceptionally rare. Examples include heavy-fermion CeRh ₂ As ₂ alongside some uranium compounds such as UPt ₃ and URhGe (refs. ^1,2,3 ). In the multiphase p -wave superfluid ³ He, complex vortex dynamics can occur at the phase boundary between the A and B phases ^4,5 . Here we study the p -wave superconductor candidate UTe ₂ (refs. ^6–8 ). On applying a magnetic field to access an intermediate regime straddling two distinct superconducting phases ^9,10 , we find that direct current pulses can push the material in and out of a metastable state that has an enhanced critical current density J _c . This switching is controllable by the strength and duration of the stimuli, with the system ‘remembering’ whether it is in the high or low J _c state for extended periods. We interpret this phenomenology to be due to the quenching of a disordered out-of-equilibrium glassy vortex state under perturbation, which has stronger pinning forces and thus higher J _c . The equilibrium vortex lattice is reattained by annealing the system with a gradual current ramp, returning it to the original state. Rather than requiring proximate magnetic or semiconducting interfaces ^11–14 , this memory functionality seems to be an intrinsic property of UTe ₂ rooted in the superconducting order itself. Our findings underscore the rich complexity of multiphase quantum vortex matter.

The recently discovered heavy-fermion spin-triplet superconductor candidate UTe₂ provides a rich platform for unconventional pairing and topological phenomena. However, limited has been known about its superconducting order parameters and their evolution with control parameters, largely due to the lack of appropriate symmetry-sensitive detection methods. Here, we report comprehensive point-contact spectroscopy measurements of pressurized UTe₂ on the (0 0 1) surface. The observation of Andreev bound states strongly suggests the presence of a p_z component in the superconducting order parameters. Quantitative analysis based on an extended Blonder-Tinkham-Klapwijk model unveils the superconducting order parameters with a finite odd-k_z component (e.g., B_2u or B_3u) for both ambient and pressurized UTe₂. Remarkably, the multiple superconducting phases can be distinguished by a single parameter 〈Δ_z〉/〈Δ_x(y)〉, the relative weight between the p_z-wave and p_x(y)-wave pairings. These findings place stringent constraints on the pairing symmetry and provide essential spectroscopic signatures for distinguishing pressure-induced multiple superconducting phases in UTe₂.

The strongly correlated spin-triplet superconductor UTe₂ hosts an unusual magnetic-field-sensitive charge density wave (CDW) phase, positioning it as a compelling system for studying intertwined electronic orders. A central challenge is determining whether the observed charge modulations arise from a triplet pair density wave (PDW) order and, if so, how the anisotropic magnetic field response of triplet superconductivity is manifested in the CDW response. Here, using a scanning tunneling microscope equipped with a vector magnetic field, we systematically investigate the evolution and interrelation of distinct CDW orders. Complementing the previously identified incommensurate CDW peaks (q_i=1,2,3), we resolve an additional set of nondispersive modulations (p_i=1,2,3 and h_1,2) with distinct temperature and magnetic field dependencies. The p_i CDW peaks vanish near T_c, while the q_i peaks survive well above T_c but are progressively suppressed by magnetic field in an anisotropic manner. The critical fields of the q_i peaks mirror the directional hierarchy of H_c2, which suggests a PDW is present above the bulk T_c. This is consistent with a Landau free-energy picture where PDWs with wavevectors p_i form above the bulk T_c, leading to composite CDW orders with wavevector q_i. Below T_c, the coupling of PDWs and uniform superconductivity leads to the p_i CDWs. Together, these findings establish UTe₂ as a rare platform where both the parent PDW and descendant orders are directly resolved, enabling access to both the fundamental and emergent manifestations of PDW physics.

The phase landscape of UTe ₂ features a remarkable diversity of superconducting phases under applied pressure and magnetic field. Recent quantum oscillation studies at ambient pressure have revealed the quasi-2D Fermi surface of this material. However, the pressure–dependence of the Fermi surface remains an open question. Here we track the evolution of the UTe ₂ Fermi surface as a function of pressure up to 19.5 kbar by measuring quantum interference oscillations. We find that in sufficient magnetic field to suppress both superconductivity at low pressures and incommensurate antiferromagnetism at higher pressures, the quasi-2D Fermi surface found at ambient pressure smoothly connects to that at 19.5 kbar, with no signs of a reconstruction over this pressure interval. We observe a smooth increase in oscillatory frequency with increasing pressure, indicating that the warping of the cylindrical Fermi sheets continuously increases with pressure. By computing a tight-binding model, we show that this enhanced warping indicates increased f -orbital contribution at the Fermi level – up to and beyond the critical pressure at which superconductivity is truncated. These findings highlight the value of high-pressure quantum interference measurements as a sensitive probe of the electronic structure in heavy fermion materials.

The phase diagram of the heavy fermion compound UTe 2 contains multiple superconducting phases, several of which show characteristics of odd-parity pairing. We have investigated the pressure dependence of the superconducting transition in high-quality crystals of UTe 2 by tracking its signature in the magnetic susceptibility χ ( T ) . A single, sharp superconducting transition is observed at low pressures < 0.3 GPa . At higher pressure, a second feature emerges in χ ( T ) , which is located at the lower-temperature superconducting phase boundary previously identified in specific heat measurements. This second transition anomaly in χ ( T ) can be attributed to a step change in the London penetration depth, providing direct evidence for a change in the superconducting order parameter of UTe 2 . Thermodynamic constraints suggest that the low temperature, high pressure superconducting state is distinct from zero pressure superconductivity as well as from the high pressure, high temperature superconducting state, raising the possibility of multicomponent superconductivity in high pressure UTe 2 . Published by the American Physical Society 2025

Quantum critical phenomena are widely studied across various materials families, from high-temperature superconductors to magnetic insulators. They occur when a thermodynamic phase transition is suppressed to zero temperature as a function of some tuning parameter such as pressure or magnetic field. This generally yields a point of instability—a so-called quantum critical point—at which the phase transition is driven exclusively by quantum fluctuations. Here, we show that the heavy fermion metamagnet UTe 2 possesses a quantum phase transition at extreme magnetic field strengths of over 70 T. Rather than terminating at one singular point, we find that the phase boundary is sensitive to magnetic field components in each of the three Cartesian axes of magnetic field space. This results in the transition surface being bounded by a continuous ring of quantum critical points, the locus of which forms an extended line of quantum criticality—a novel form of quantum critical phase boundary. Within this quantum critical line sits a magnetic field-induced superconducting state in a toroidal shape, which persists to fields over 70 T. We model our data by a phenomenological free energy expansion and show how a quantum critical line—rather than a more conventional singular point of instability—anchors the remarkable high magnetic field phase landscape of UTe 2 .