Spin- 1 2 chain compound Ba 2 Cu 2 Te 2 P 2 O 13 : Magnetization, specific heat, and local-probe NMR

We present synthesis and characterization of ${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{13}$ (BCTPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements. BCTPO crystallizes in a monoclinic structure with ${\mathrm{Cu}}^{2+}$ ions forming coupled chains. In magnetization $M(T)$ measurements, a broad maximum is observed around 53 K. Upon the application of magnetic fields, a sharp anomaly evolves around 4 K in the heat capacity ${C}_{P}(T)$ data, indicative of long-range order, while it is absent in zero field. The $^{31}\mathrm{P}$ NMR line shift at about 93.954 kOe tracks the spin susceptibility, whereas the line broadening evidences the onset of magnetic order at about 4 K or lower. The inferred isotropic and axial components of the hyperfine coupling constant are ${A}_{hf}^{\mathrm{iso}}\ensuremath{\simeq}$ 3635 $\text{Oe}/{\ensuremath{\mu}}_{B}$ and ${A}_{hf}^{ax}\ensuremath{\simeq}$ 479 $\text{Oe}/{\ensuremath{\mu}}_{B}$, respectively. The $^{31}\mathrm{P}$ NMR spin-lattice relaxation rate $1/{T}_{1}$ shows a pronounced divergence for $T\ensuremath{\longrightarrow}0$. We assign this behavior to critical fluctuations on the verge of order.