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
ACu(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
∼ 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
2O
8 dimer configuration and effective ferromagnetic interdimer interaction,
ACu(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.
Motivated by recent experiments on
Ba3Cr2O8, a new spin-dimer compound with spin-1/2 moments of
Cr5+ ions, we theoretically investigate the field-induced magnetic ordering in this material in view of the Bose-Einstein condensation (BEC) of triplet excitations (triplons). We apply the self-consistent Hartree-Fock-Popov (HFP) approach to a microscopic Hamiltonian, using the realistic triplon dispersion measured in an inelastic neutron-scattering experiment. In particular, we ask to what extent the BEC of dilute triplons near the critical field can explain the magnetic ordering in this material. For example, we investigate the temperature range where the BEC picture of triplons can be applied via the HFP approach. We also determine the temperature regime where a quadratic approximation of the triplon dispersion works. It is found that the strength of the effective repulsive interaction between triplons is much weaker in
Ba3Cr2O8 than in the canonical spin-dimer compound
TlCuCl3. Small effective repulsive interaction in combination with the narrow band of triplons leads to higher density of triplons
ncr at the critical point. The combined effect points to a bigger HFP correction
Uncr in
Ba3Cr2O8 than in
TlCuCl3. Nonetheless, the HFP approach provides a reasonable explanation of the transverse magnetization and the specific-heat data of
Ba3Cr2O8.