2026/07/16 by Sasank Budaraju, Shi Feng, Josef Willsher +3
#cond-mat.str-el
We investigate the ground-state phase diagram of the J1-J2 Heisenberg model on the triangular lattice under an external Zeeman field H by using the variational Monte Carlo approach. We span a region with 0 ≤ J2/J1 ≤ 0.2 and 0 ≤ H/J1 ≤ 2, to assess the fate of the (putative) spin-liquid phase that has been detected for J2/J1=1/8 at zero magnetic field. Simple variational ansatze are proposed for a few candidate states, and their energetics are compared on large clusters to obtain the phase diagram. For J2/J1 \lesssim 1/6, a continuous transition from a gapless "Y'' phase to a gapped "up-up-down'' phase is obtained, as predicted by spin-wave theory. Most importantly, around J2/J1=1/8, a condensate of monopoles (which are gapless gauge excitations of the spin liquid at H=0) is stabilized in a significant region of the phase diagram, for small Zeeman fields. Here, a finite scalar chirality is present, while no transverse magnetic order is detected. The stability of the monopole phase is confirmed by a field-theory approach that includes a self-consistent random-phase approximation of the low-lying spin fluctuations. The boundary between the monopole and "Y'' phases is also obtained with no free parameters.