A negative Poisson's ratio refers to the phenomenon of a material expanding along the transverse direction upon uniaxial tension. Here, we show a negative Poisson's ratio observed directly by experiments in a single-crystal Heusler-type Cu–16.9Al–11.6Mn (at%) shape memory alloy, in addition to the alloy's superelasticity. When stretched along the vicinity of the  direction, the alloy expands along the [11¯0] direction, showing a significant negative Poisson's ratio of −0.51, while shrinking along the  direction with a large positive Poisson's ratio of 1.36. These experimental results agree well with the values calculated from the elastic constants and elastic anisotropy.
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