Evidence for failed room-temperature superconductivity in beta-tantalum*

“Bad” metals such as beta-Ta – one of the most efficient known spin Hall effect (SHE) materials - exhibit electronic properties inconsistent with single-particle Fermi liquid theory, but the underlying correlations are debated. In superconductors, correlations manifested as Cooper pairing are detected as doubling of the shot noise in metallic and tunnel junctions [1]. We utilize cryogenic shot noise measurements to detect the charge q of the charge carriers in non-superconducting beta-Ta. For tunnel junctions, the Fano factor F is substantially larger than F=1 expected for q=e, consistent with electron pairing. The value of F is almost temperature-independent, suggesting that pairing may persist at high temperatures. For ultrashort metallic junctions, the Fano factor becomes larger than 1/3 expected for q=e, in agreement with the results for tunneling junctions.  

Room-temperature Fourier transform infrared (FTIR) spectroscopy and ultrafast IR pump-probe measurements exhibit signatures of anomalous IR absorbance and relaxation of low-energy electrons in beta-Ta consistent with the possibility that pairing persists at room temperature. Our results suggest that electron pairing may be common to many correlated systems even in the absence of superconductivity. 

[1] F. Lefloch, C. Hoffmann, M. Sanquer, and D. Quirion, Phys. Rev. Lett. 90, 067002 (2003)