Triplon Bose-Einstein condensation and proximate magnetism in dimerized antiferromagnets

TL;DR

This study investigates triplon Bose-Einstein condensation and magnetic states in dimerized antiferromagnets, revealing a quantum critical point and unique magnetic properties due to their crystal structure and interactions.

Contribution

The paper combines experimental measurements and theoretical modeling to understand the magnetic ground states and quantum phase transitions in two related dimerized antiferromagnets.

Findings

Demonstrated a singlet ground state with a 7.9 K triplon gap in HgCu(SeO$_3$)$_2$

Observed Bose-Einstein condensation with antiferromagnetic order below 4.4 K in CdCu(SeO$_3$)$_2$

Identified the role of crystal structure and interactions in distinguishing these materials from other dimerized magnets

Abstract

Dimerized quantum magnets provide a useful arena for novel quantum states and phases transitions with the singlet-triplet type of triplon excitations. Here we study the triplon physics and the Bose-Einstein condensation in two isostructural dimerized antiferromagnets $A$Cu(SeO$_3$)$_2$ ($A$ = Hg, Cd). With the systematic measurements, we demonstrate a dimer singlet ground state in HgCu(SeO$_3$)$_2$ with a triplon gap $\sim$ 7.9 K and a triplon Bose-Einstein condensation with an antiferromagnetic order in CdCu(SeO$_3$)$_2$ below 4.4 K. We further adopt the bond-operator technique and show that the elemental replacement preserves the Hamiltonian and allows the study in a unified theoretical framework with tunable interdimer and intradimer interactions on the opposite sides of the quantum critical point. With the peculiar Cu$_2$O$_8$ dimer configuration and effective ferromagnetic interdimer interaction, $A$Cu(SeO$_3$)$_2$ is distinguished from other $S$ = 1/2 dimerized antiferromagnets. Our results represent a global understanding of the magnetic ground states as well as the magnetic transitions in the dimerized magnets of this unusual crystal structure.