Quantum Phase Transitions in Ba(1-x)CaxFe12O19

TL;DR

This study investigates how chemical pressure via Ca2+ substitution affects the quantum critical behavior of BaFe12O19, revealing a transition from quantum criticality to a quantum electric dipolar glass state.

Contribution

It demonstrates that Ca2+ substitution stabilizes a quantum electric dipolar glass state and characterizes the quantum phase transition through dielectric and specific heat measurements.

Findings

Ca2+ substitution generates positive chemical pressure.

Ca2+ drives BFO away from quantum critical point.

Pure BFO is near but slightly away from QCP with Tc ~ 2.91 K.

Abstract

The ground state of BaFe12O19 (BFO) is controversial as three different quantum states, namely quantum paraelectric, frustrated antiferroelectric and quantum electric dipole liquid (QEDL), have been proposed. We have investigated the quantum critical behavior of BFO as a function of chemical pressure (a non-thermal variable) generated by smaller isovalent ion Ca2+ at the Ba2+ site. Analysis of synchrotron x-ray diffraction data confirms that Ca2+ substitution generates positive chemical pressure. Our dielectric measurements reveal that Ca2+ substitution drives BFO away from its quantum critical point (QCP) and stabilizes a quantum electric dipolar glass state whose dielectric peak temperature (Tc) increases with increasing Ca2+ content as Tc ~ (x-xc)1/2, a canonical signature of quantum phase transitions. Our dielectric measurements reveal that pure BFO is slightly away from its QCP with a Tc of 2.91 K. Specific heat measurements reveal excess specific heat of non-Debye and non-magnetic origin with linear temperature dependence below Tc which could be due to QEDL state of BFO.