OsB2 single crystals show Type-I superconductivity below about 2.1 K in AC susceptibility and specific heat
Authors of an arXiv preprint report that osmium diboride, long classed as Type-II, behaves as a rare binary Type-I superconductor. The data come from single crystals; the work is not peer reviewed.
A preprint on arXiv, not yet peer reviewed, reports experimental evidence that osmium diboride (OsB2) is a Type-I superconductor. The compound has so far been classified as a low-κ Type-II superconductor with a critical temperature of about 2.1 K. The authors say it is a rare nonelemental, binary Type-I superconductor. arxiv.org
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OsB2 crystallizes in an orthorhombic structure (space group Pmmn, No. 59) with corrugated hexagonal boron layers between osmium planes. Earlier work had noted anomalies: a reduced specific heat jump at the transition, Type-I-like magnetization curves and an unusual concave curvature of the phase boundary. arxiv.org
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Samples and methods
The single crystals were grown by arc melting and from solution in a Cu-B flux. They are thin hexagonal plates, and phase purity was checked by powder X-ray diffraction. AC susceptibility was measured in a PPMS-ECII with an AC field of 0.1 Oe at 111 Hz and static DC fields applied parallel to the plates. Specific heat was recorded by a thermal relaxation technique on two crystals from different growth batches. arxiv.org
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Three signatures of Type-I behavior
First, in zero field the AC susceptibility shows a sharp diamagnetic drop at the transition. With a modest DC field applied, a prominent positive peak appears just below the critical temperature. According to the authors, this differential paramagnetic effect marks entry into the intermediate state. arxiv.org
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Second, the zero-field specific heat shows a step-like anomaly at the transition, consistent with earlier reports. In an applied field, applied along the c-axis, the transition sharpens into a peak that far exceeds the zero-field value. The authors interpret this as latent heat absorption in a field-induced first-order transition. arxiv.org
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Third, field-ramped heat capacity cycles at two temperatures below the critical temperature show hysteresis between field-up and field-down sweeps. The authors take this as supercooling at the superconducting-to-normal boundary. arxiv.org
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Context
The authors compare the results with recent reports of Type-I superconductivity in other binary intermetallics. They also note that earlier electronic structure calculations gave high Fermi velocities and large coherence lengths for OsB2, which lower the Ginzburg-Landau parameter into the Type-I range. arxiv.org
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