We present a direct NMR method to determine whether the interactions in a Tomonaga-Luttinger liquid (TLL) state of a spin- 1/2 Heisenberg antiferromagnetic ladder are attractive or repulsive. For the strong-leg spin ladder compound (C7H10N)2CuBr4 we find that the isothermal magnetic field dependence of the NMR relaxation rate T1−1 (H) displays a concave curve between the two critical fields bounding the TLL regime. This is in sharp contrast to the convex curve previously reported for a strong-rung ladder, (C5H12N)2CuBr4. We show that the concavity and the convexity of T1−1 (H), which is a fingerprint of spin fluctuations, directly reflect the attractive and repulsive fermionic interactions in the TLL, respectively. The interaction sign is alternatively determined from an indirect method combining bulk magnetization and specific heat data.
|Number of pages||6|
|Journal||Physical Review Letters|
|Publication status||Published - 1 Sep 2016|