Dimension-Reducing Quantum Transition in Copper Elpasolite

The cubic compound K$_{2}$PbCu(NO$_{2})_{6}$ (elpasolite) undergoes Jahn-Teller-driven orbital ordering near 270K, which results in quasi-1D behavior below 5K. At T$_{N} \quad =$ 0.7K, the Cu$^{2+}$ s $=$ 1/2 spins undergo Neel order, due to an inter-chain interaction approximately 10 times smaller than the intra-chain interaction of 5.4 k$_{B}$. Application of a magnetic field initially increases T$_{N}$ due to spin-dimensionality reduction. We present measurements of the specific heat, susceptibility, and neutron diffraction to characterize both the zero-field ordered state as well as the phase boundary as T approaches zero.