The recently discovered superconductor UTe2 with a Tc between 1.5 K and 2 K, has attracted much attention due to indications of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36 T along the hard b axis in order to follow the superconducting transition up to the metamagnetic transition at Hm = 34.75 T. They demonstrate the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, possibly a spin-triplet to spin-singlet transition.

Organometallic liquids provide good properties for ionization detectors. TriMethyl Bismuth (TMBi) has been proposed as a detector medium with charge and Cherenkov photon readout for Positron Emission Tomography. In this work, we present studies for the handling of TMBi at different electric fields and under different environmental conditions to find applicable configurations for the suppression of electrical breakdowns in TMBi at room temperature. A simple glass cell with two electrodes filled with TMBi was constructed and tested under different operation conditions. Working at the vapour pressure of TMBi at room temperature of about 40 mbar and electric fields of up to 20 kV/cm in presence of a small oxygen contamination we found the formation of a discharge channel in the liquid and a steady increase in the current. Further reduction of pressure by pumping caused the TMBi to boil and a spontaneous combustion. Eliminating the oxygen contamination led the TMBi under the same condition to only decompose. When operating the setup under an argon atmosphere of 1 bar we did not observe breakdowns of the electrical potential up to field strengths of 20 kV/cm. Still, in presence of a small oxygen contamination fluctuating currents in the nA range were observed, but no decomposition or combustion. We conclude from our experiments that TMBi at room temperature in a pure argon atmosphere of 1 bar remains stable against electrical breakdown at least up to electric field strengths of 20 kV/cm, presumably because the formation of gaseous TMBi was prevented.

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A new type of detector for positron-emission tomography (PET) has been proposed recently, using a heavy organo-metallic liquid — TriMethyl Bismuth (TMBi) — as target material. TMBi is a transparent liquid with the high Z element Bismuth contributing 82% of its mass. 511keV annihilation photons are converted efficiently into photo-electrons within the detector material producing both Cherenkov light and free charge carriers in the liquid. While the optical component enables a fast timing, a charge readout using a segmented anode can provide an accurate position reconstruction and energy determination. The charge measurement requires a high level of purification, as any electronegative contaminants cause signal degradation. In addition to the purity requirements, the reactive nature of TMBi poses many challenges that need to be met until a fully functioning detector for PET applications can be realized. The paper presents an experimental setup that aims to remove electronegative impurities by electrostatic filtering and to characterise the properties of TMBi, e.g. the relative permittivity, for its application as a detector medium for charge read out.