The Tomonaga-Luttinger liquid (TTL) is a universal low-temperature state of gapless, one-dimensional (1D) quantum systems. 1D antiferromagnets having an energy gap are expected to become a TLL, when a strong magnetic field is applied to collapse the energy gap. To date, however, all experimental evidence in these antiferromagnets has been either controversial or circumstantial. In the spin-1 bond-alternating antiferromagnet NTENP, we have observed an unambiguous signature of a TLL: a linear temperature (T) dependence of the magnetic specific heat (Cmag). The linear Cmag appears only in magnetic fields above the critical value Hc = 9.3 T and the Sommerfeld constant Cmag/T increases as the field is reduced toward Hc. This field dependence agrees well with the results of our calculation, providing a conclusive evidence for a TLL.
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