Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable β-Ga with a critical temperature Tc = 6.04(5) K, while stable α-Ga has a much lower Tc < 1.2 K. Here we use a membrane-based nanocalorimeter to initiate the transition from α-Ga to β-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in-situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of α-Ga. After such treatment, the liquid supercools down to 232 K, where β-Ga solidifies. We find that β-Ga is a strong-coupling type-I superconductor with ∆(0)/kBTc = 2.00(5) and a Sommerfeld coefficient γn = 1.53(4) mJ/molK 2 , 2.55 times higher than that in the α phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

Recent advances in electronics and nanofabrication have enabled membrane-based nanocalorimetry for measurements of the specific heat of microgram-sized samples. We have integrated a nanocalorimeter platform into a 4.5 T split-pair vertical-field magnet to allow for the simultaneous measurement of the specific heat and x-ray scattering in magnetic fields and at temperatures as low as 4 K. This multi-modal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the x-ray beam, and fast acquisition of x-ray scans over a wide temperature range without the need for time-consuming re-centering and re-alignment. Furthermore, on an YBaCuO crystal and a copper foil, we demonstrate a novel approach to x-ray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the first-order magneto-elastic phase transition of FeP, a material that is of interest for magnetocaloric applications.

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We report on specific heat measurements on clean overdoped BaFe2(As1−xPx)2 single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature γr = C/T |T →0 is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave α model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of ∆0 ∼ 5.3 meV, corresponding to ∆0/kBTc ∼ 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of ∆0 ∼ 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on γr, all samples however show similar behavior [γr(H) − γr(H = 0) ∝ H n , with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.