Fragile antiferromagnetism in the heavy-fermion compound YbBiPt

The discovery of YbBiPt [1] generated strong interest due to its extraordinary Sommerfield coefficient ($\gamma \approx $ 8 J/mol-K$^{2})$ and the fact that all of its relevant energy scales including the Kondo temperature, Weiss temperature, crystal field splitting, and a proposed antiferromagnetic (AFM) ordering below $T_{\textrm{N}} =$ 0.4 K are small and comparable, suggesting a complex interplay of competing interactions at low temperature. Much of the recent attention on YbBiPt has focused on the possibility of a magnetic-field-tuned AFM quantum critical point occurring at a low critical magnetic field of $\mu_{0}H_{\textrm{c}}$ = 0.4 T [2]. Although thermodynamic and transport measurements in ambient fields suggested that YbBiPt manifests AFM order below $T_{\textrm{N}}$, scattering measurements over the past 22 years failed to identify magnetic ordering in powder or single-crystal samples. In this talk, I will present recent elastic and inelastic neutron scattering experiments on single crystals of YbBiPt that demonstrated clear scattering signatures of unusual AFM order at low temperature [3]. The ambient field elastic scattering consists of two components: a narrower component that appears below $T_{\textrm{N}} \approx $ 0.4 K, which can be identified with features observed in the bulk transport measurements; and a broad scattering component that persists up to $T^{\ast} \approx $ 0.7 K corresponding to AFM correlations extending over $\approx $ 20 {\AA}.\\[4pt] [1] P. C. Canfield \textit{et al}., J. Appl. Phys. \textbf{70}, 5800 (1991); Z. Fisk \textit{et al}., Phys. Rev. Lett. \textbf{67}, 3310 (1991).\\[0pt] [2] E. D. Mun \textit{et al}., Phys. Rev. B \textbf{87}, 075120 (2013).\\[0pt] [3] B. G. Ueland\textit{ et al}., Phys. Rev. B \textbf{89}, 180403(R) (2014).