Quasi-One-Dimensional Uniform Spin-1/2 Heisenberg Antiferromagnet Knacup2o7 Probed By P-31 And Na-23 Nmr

We present the structural and magnetic properties of KNaCuP2O7 investigated via x-ray diffraction, magnetization, specific heat, and (31)p and Na-23 NMR measurements and complementary electronic structure calculations. The temperature-dependent magnetic susceptibility and P-31 NMR shift could be modeled very well by the uniform spin-1/2 Heisenberg antiferromagnetic chain model with a nearest-neighbor interaction J/k(B) similar or equal to 58.7 K. The corresponding mapping using first-principles electronic structure calculations leads to J(DFT)/k(B) similar or equal to 59 K with negligibly small interchain couplings, further confirming that the system is indeed a one-dimensional uniform spin-1/2 Heisenberg antiferromagnet. The diverging trend of NMR spin-lattice relaxation rates ((31)1/T-1 and (23)1/T-1) implies the onset of a magnetic long-range ordering at around T-N similar or equal to 1 K. From the value of T-N, the average interchain coupling is estimated to be J'/k(B) similar or equal to 0.28 K. Moreover, the NMR spin-lattice relaxation rates show the dominant contributions from uniform (q = 0) and staggered (q = +/-pi/a) spin fluctuations in the high- and low-temperature regimes, respectively, mimicking one-dimensionality of the spin lattice. We have also demonstrated that (31)1/T-1 in high temperatures varies linearly with 1/root H, reflecting the effect of spin diffusion on the dynamic susceptibility. The temperature-dependent unit cell volume could be described well using the Debye approximation with a Debye temperature of Theta(D) similar or equal to 294 K, consistent with the heat capacity data.