Nuclear magnetic resonance study of the S = 1/2 Heisenberg ladder Cu2(C5H12N2)2Cl4:: Quantum phase transition and critical dynamics

We present an extensive NMR study of the spin-1/2 antiferromagnetic Heisenberg ladder Cu-2(C5H12N2)(2)Cl-4 in a magnetic field range 4.5-16.7 T. By measuring the proton NMR relaxation rate 1/T-1 and varying the magnetic field around the critical field H-c1 = Delta/g mu(B) approximate to 7.5 T, we have studied the transition from a gapped spin liquid ground state to a gapless magnetic regime which can be described as a Luttinger liquid. We identify an intermediate regime T greater than or equal to \H - H-c1\, where the spin dynamics is (possibly) only controlled by the T = O critical point H-c1.