Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Polymorphism and superconductivity in the V-Nb-Mo-Al-Ga high-entropy
  alloys   High-entropy alloys (HEAs) are at the focus of current research for their
diverse properties, including superconductivity and structural polymorphism.
However, the polymorphic transition has been observed only in
nonsuperconducting HEAs and mostly under high pressure. Here we report the
discovery of superconductivity and temperature-driven polymorphism in the
(V$_{0.5}$Nb$_{0.5}$)$_{3-x}$Mo$_{x}$Al$_{0.5}$Ga$_{0.5}$ (0.2 $\leq$ $x$
$\leq$ 1.4) HEAs. It is found that the as-cast HEA is of a single body-centered
cubic (bcc) structure for $x$ = 0.2 and a mixture of the bcc and A15 structures
for higher $x$ values. Upon annealing, the bcc structure undergoes a
polymorphic transformation to the A15 one and all HEAs exhibits bulk
superconductivity. For $x$ = 0.2, whereas the bcc polymorph is not
superconducting down to 1.8 K, the A15 polymorph has a superconducting
transition temperature $T_{\rm c}$ of 10.2 K and an estimated zero-temperature
upper critical field $B_{\rm c2}$(0) of 20.1 T, both of which are the highest
among HEA superconductors. With increasing Mo content $x$, both $T_{\rm c}$ and
$B_{\rm c2}$(0) of the A15-type HEAs decrease, yet the large ratio of $B_{\rm
c2}$(0)/$T_{\rm c}$ signifies a disorder-induced enhancement of the upper
critical field over a wide $x$ range. The decrease in $T_{\rm c}$ is attributed
to the decrease in both the electronic specific-heat coefficient and
electron-phonon coupling strength. Furthermore, the valence electron count
dependence of $T_{\rm c}$, which is different from both the binary A15 and
other structurally different HEA superconductors, suggests that $T_{\rm c}$ may
be increased further by reducing the number of valence electrons. Our results
not only uncover HEA superconductors of a new structural type, but also provide
the first example of polymorphism dependent superconductivity in HEAs.

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The categories of this paper are: acc-phys
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The categories of this paper are: stat
7
The categories of this paper are: hep-th
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The categories of this paper are: hep-lat
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The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
5
The categories of this paper are: gr-qc
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The categories of this paper are: adap-org
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The categories of this paper are: funct-an
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The categories of this paper are: cmp-lg
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The categories of this paper are: plasm-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: nlin
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The categories of this paper are: bayes-an
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-ex