Hall Effect Evolution Across a Field-Induced Quantum Critical Point in U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2}$Si$_{2}$

The heavy fermion compound U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2} $Si$_{2}$ has been recently identified as a unique system in which a magnetic field-induced quantum critical point $($QCP$)$ is avoided by the creation of a single magnetic phase [1]. To further understand the nature of phase formation across the putative QCP, we have measured the Hall effect of U$($Ru$_{0.96}$Rh$_{0.04}$$)$$_{2}$Si$_{2}$ between 0.6 and 30K using a 50T mid-pulse magnet. We find that the Hall coefficient $R_{\rm H}$ abruptly increases inside the new phase formed between 28 and 38T, around $B_{\rm QCP}$ $\sim$ 34T, evidencing formation of a gap in the Fermi surface. Furthermore, low temperature $R_{\rm H}$ at $B$ $>$ 38T is much smaller than that at $B$ $<$ 28T, indicating the different Fermi surface area in each Fermi liquid states. While the field-induced phase disappeared above 9.5K, $R_{\rm H}$ still exhibits a broad maximum near $B_{\rm QCP}$, signaling a finite temperature crossover of the Fermi surface change. All of above findings are consistent with an electronic structure evolution from a heavy fermion to spin-polarized Fermi liquids across $B_{\rm QCP}$. [1] K. H. Kim ${\it et~al.} $, Phys. Rev. Lett. ${\bf 93}$, 206402 $($2004$)$