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Physical Review Letters 113 (2014) 107001

Discovery of a superconducting high-entropy alloy

P Koželj, S Vrtnik, A Jelen, S Jazbec, Z Jagličić, S Maiti, M Feuerbacher, W Steurer, and J Dolinšek

High-entropy alloys (HEAs) are multicomponent mixtures of elements in similar concentrations, where the high entropy of mixing can stabilise disordered solid-solution phases with simple structures like a body-centered cubic or a face-centred cubic, in competition with ordered crystalline intermetallic phases. We have synthesised an HEA with the composition Ta_{34}Nb_{33}Hf_{8}Zr_{14}Ti_{11} (in at. %), which possesses an average body-centred cubic structure of lattice parameter a = 3.36 Å. The measurements of the electrical resistivity, the magnetisation and magnetic susceptibility, and the specific heat revealed that the a_{34}Nb_{33}Hf_{8}Zr_{14}Ti_{11} HEA is a type II superconductor with a transition temperature Tc ≈ 7.3 K, an upper critical field μ_{0}H_{c2} ≈ 8.2 T, a lower critical field μ_{0}H_{c1} ≈ 32 mT, and an energy gap in the electronic density of states (DOS) at the Fermi level of 2Δ ≈ 2.2 meV.

The investigated HEA is close to a BCS-type phonon-mediated superconductor in the weak electron-phonon coupling limit, classifying it as a “dirty” superconductor. It is shown that the lattice degrees of freedom obey Vegard’s rule of mixtures, indicating completely random mixing of the elements on the HEA lattice, whereas the electronic degrees of freedom do not obey this rule even approximately so that the electronic properties of a HEA are not a “cocktail” of properties of the constituent elements. The formation of a superconducting gap contributes to the electronic stabilisation of the HEA state at low temperatures, where the entropic stabilisation is ineffective, but the electronic energy gain due to the superconducting transition is too small for the global stabilisation of the disordered state, which remains metastable.

Further reading:

P Koželj, S Vrtnik, A Jelen, S Jazbec, Z Jagličić, S Maiti, M Feuerbacher, W Steurer, and J Dolinšek: Discovery of a superconducting high-entropy alloy,
Physical Review Letters 113 (2014) 107001.

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