Multiferroic Quantum Criticality in a Proper Ferroelectric Quasi-one-dimensional Ising Antiferromagnet.

Quantum phase transitions are driven at absolute zero by quantum-mechanical fluctuations, offering unique opportunities for the emergence of exotic phenomena ranging from unconventional superconductivity to quantum magnetism. In multiferroics, where magnetic and ferroelectric orders are cross-correlated, this raises an intriguing possibility: a state of quantum criticality in which both fluctuations are intertwined. Here we show that the quasi-one-dimensional Ising antiferromagnet Sr1- xBaxCo2V2O8 hosts multiferroic quantum criticality through the coexistence of ferroelectric and magnetic quantum critical points (QCPs) tuned by independent non-thermal parameters. Structural and dielectric analyses establish this system as a proper displacive-type ferroelectric-an exceptionally rare feature among magnetic materials. Isovalent Ba substitution continuously suppresses the ferroelectric transition to zero temperature, producing a quantum paraelectric state with a signature T2 scaling of the inverse permittivity. In this regime, a transverse magnetic field drives the system to a magnetic QCP. Their resulting coexistence gives rise to a pronounced magnetocapacitance, revealing a strong interplay between fluctuations in the lattice and spin systems. Our findings establish Sr1- xBaxCo2V2O8 as a unique platform for opening new avenues at the intersection of quantum criticality and multiferroicity.