Ferroelectricity and multiferroic quantum critical behaviors in Co2V2O7

We present isothermal ferroelectric (FE) and quasiadiabatic phase diagrams for single-crystal ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ with magnetic fields ($H$) along the $c$ axis, obtained through magnetization, polarization ($P$), and magnetocaloric effect measurements under pulsed magnetic fields up to 30 T. The FE diagram reveals two FE phases, denoted as FE-I and FE-II, with overlapping boundaries at low temperatures ($T$). At 2 K, the $P$ signal along $c$ exhibits a positive value in FE-I, transitioning to a negative value in FE-II with increasing $H$. During the descending-$H$ process, $P$ reverses, with FE-II displaying positive polarization and FE-I displaying negative, indicative of dynamic ferroelectric behavior. Above 3 K, a paraelectric (PE) phase emerges, effectively separating the two FE phases. This PE phase correlates with the 1/2 plateau observed in the magnetization curve. Simultaneously, as $T$ increases, the negative $P$ signals of FE-I and FE-II sequentially reverse to positive, leading to the gradual diminishment of the signal reversal of $P$ along the $c$ axis due to the history of $H$ sweeping. The magnetic Gr\"uneisen parameter diverges near the field on both sides of the 1/2 plateau, providing evidence of multiferroic quantum criticality. Employing symmetry analysis, we propose a chiral ``water-strider'' type spin configuration to elucidate the origin of ferroelectricity and the magnetization process. The high-field FE phase may be associated with the quintuplet excitation of the Co1 dimer in the presence of magnetic fields.