Theory of magnetic-field-induced Bose-Einstein condensation of triplons inBa3Cr2O8

Abstract

Motivated by recent experiments on Ba 3 Cr 2 O 8 , a new spin-dimer compound with spin-1/2 moments of Cr 5+ ions, we theoretically investigate the field-induced magnetic ordering in this material in view of the BoseEinstein condensation ͑BEC͒ of triplet excitations ͑triplons͒. We apply the self-consistent Hartree-Fock-Popov ͑HFP͒ 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 Ba 3 Cr 2 O 8 than in the canonical spin-dimer compound TlCuCl 3 . Small effective repulsive interaction in combination with the narrow band of triplons leads to higher density of triplons n cr at the critical point. The combined effect points to a bigger HFP correction Un cr in Ba 3 Cr 2 O 8 than in TlCuCl 3 . Nonetheless, the HFP approach provides a reasonable explanation of the transverse magnetization and the specific-heat data of Ba 3 Cr 2 O 8 .

Macroscopic properties of triplon Bose–Einstein condensates

“…Similar findings in a variety of other compounds followed shortly [11][12][13][14][15]. Recently, Bose-Einstein condensation of magnetic excitations was studied theoretically in compounds, where the magnetism is due to the spins of vanadium [16] or chromium [17] ions. These uniform, three dimensional with various degrees of anisotropy and easily tunable systems provide the researchers with new abilities to study the BEC, including the effects of disorder.…”

“…Here we underline that the main difference between the HFP and HFB approximations concerns the anomalous density: neglecting σ as well as a k a p in ( 14), (17), and ( 18) one arrives at the HFP approximation, which can also be obtained in variational perturbation theory [33]. However, the normal, ρ 1 , and anomalous averages, σ, are equally important and neither of them can be neglected without making the theory not self-consistent [34][35][36].…”

Structural and magnetic properties in the quantumS=12dimer system Ba3(Cr<

“…[16][17][18][19][20][21][22][23][24][25][26][27] It provides another test bed to study the effect of impurities in the spin dimer system. For instance, recent work on Ba 3 (Mn 1−x V x ) 2 O 8 powder samples indicates that there is no evidence for a phase transition and the singlet-triplet excitations persist for the V 5+ even if they are replaced at a medium level (x=0.3).…”

Theory of field-induced quantum phase transition in spin dimer system Ba3Cr2O8

“…[4][5][6][7][8][9][10][11] Magnetic Cr 5+ ions have spin S = 1/2 and form couples of triangular lattices along c axis (see Fig. 1).…”

“…The transition at h = h c is of the 3D BEC universality class while that at h = h s is of the first order presumably due to a spin-lattice interaction. 4,5,7 It is assumed, in particular, that exchange interactions between spins from neighboring dimers are symmetric, i.e., interaction of spins 1 (see Fig. 1) from the neighboring dimers ('1-1' interaction for short) is the same as '2-2' interaction, and '1-2' and '2-1' interactions are also equal to each other.…”