Nonuniversal magnetization at the BEC critical field: Application to the spin dimer compound Ba3Mn2O8

Field-induced spin density wave and spiral phases in a layered antiferromagnet

“…We indeed observe magnetic order consistent with predictions based upon the established Hamiltonian for Ba 3 Mn 2 O 8 , i.e., phase II is an Ising-like amplitude modulated phase and phase I is an incommensurate spiral structure. [11] Low temperature NMR measurements as a function of magnitude and direction of applied magnetic field have found that for H ⊥ c, as in our measurement, the transition between the quantum paramagnet phase and the ordered phase is more Ising-like instead of being defined by the Bose-Einstein quantum critical point for H c. [22] The order parameter we determine for the transition between phase II and the quantum paramagnetic phase agrees with this. Recent large-size cluster mean-field models have determined that the hysteretic response we observe between phase I and phase II may in fact be a general feature of quasi-two-dimensional triangular lattice dimerized antiferromagnets.…”

“…These two phases have been proposed to be due to the existence of a finite on site anisotropy term, D, in the Hamiltonian as well as the existence of significant inter-layer exchange interactions along the c-axis. [22] The focus of this communication is to examine these two phases using neutron diffraction techniques in order to understand their fundamental differences. We find that phase I corresponds to a long range ordered spiral phase with an incommensurate wave-vector.…”

Bose-Einstein condensation in quantum magnets

“…By diagonalizing the resulting quadratic mean-field Hamiltonian it is possible to obtain the low energy excitations and compute different thermodynamic quantities near the field-induced QCP. This procedure has been successfully applied to a long list of quantum magnets including TlCuCl 3 (Misguich and Oshikawa, 2004;Sirker, Weiße, and Sushkov, 2004), BaCuSi 2 O 2 Schmalian and Batista, 2008), DTN (Kohama et al, 2011;, and Ba 3 Mn 2 O 8 (Samulon et al, 2009(Samulon et al, , 2010Suh et al, 2011;Kamiya and Batista, 2013). In general, there are no controlled analytical techniques for solving the problem far away from the dilute limit.…”

Magnons and continua in a magnetized and dimerized spin-12chain

“…[1][2][3][4][5] When antiferromagnetic Heisenberg interactions prevail (J 1 >> J 2 ≡ αJ 1 ), these spin systems can be driven through two quantum critical points, both associated with Bose-Einstein condensation of magnons. [6][7][8][9][10][11][12][13][14][15] Immediately above the lower critical field, µ 0 H c1 ≈ J 1 (1 − α)/(gµ B ), a macroscopic density of magnons gives rise to magnetization. [16][17][18][19] The reverse situation pertains at the upper critical field where magnon condensation below µ 0 H c2 = J 1 (1 + α)/(gµ B ) reduces the magnetic moment per spin below saturation (M < M sat ≡ gµ B /2).…”