Repulsive Tomonaga-Luttinger liquid in the quasi-one-dimensional alternating spin-¹₂ antiferromagnet NaVOPO₄

We probe the magnetic field -induced Tomonaga-Luttinger liquid (TLL) state in the bond -alternating spin1/2 antiferromagnetic (AFM) chain compound NaVOPO4 using thermodynamic as well as local mu SR and P-31 NMR probes down to mK temperatures in magnetic fields up to 14 T. The mu SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power -law behavior, enabling us to directly determine the interaction parameter K as a function of the magnetic field. These estimates are crosschecked using magnetization and specific heat data. The field -dependent K lies in the range of 0.4 < K < 1 and indicates the repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO4 the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied S = 1/2 bond -alternating AFM-AFM chain compounds.