  Negative thermal expansion materials are important and desirable in science
and engineering applications. However, natural materials with isotropic
negative thermal expansion are rare and usually unsatisfied in performance.
Here, we propose a novel method to achieve negative thermal expansion via a
metamaterial approach. The metamaterial is constructed with unit cells that
combine bi-material strips and anti-chiral structures. Both experimental and
simulation results display isotropic negative thermal expansion properties. The
coefficient of negative thermal expansion of the metamaterials is demonstrated
to be dependent on the difference between the thermal expansion coefficients of
two component materials in the bi-material strips, as well as on the circular
node radius and the ligament length in the anti-chiral structures. The measured
value of the linear negative thermal expansion coefficient reaches -68.1X10-6
1/K in an operating temperature range from 303.15 K to 773.15 K, which is among
the largest achieved in experiments to date. Our findings provide a novel and
practical approach to obtaining materials with tunable isotropic negative
thermal expansion on any scale.
