Dr. Hidemaro Suwa (Department of Physics, The University of Tokyo)
"Quantum Monte Carlo Analysis of Linearly Dispersing Spinons at Deconfined Quantum-Critical Point"
Most continuous phase transitions are described by the Landau-Ginzburg-Wilson (LGW) paradigm where an effective action
is expanded in powers of an order parameter and its derivatives. For more than a decade, the deconfined quantum
criticality[1] has caught a great deal of attention as a highly non-trivial phase-transition point beyond the LGW
paradigm. The low-energy (or long-length-scale) physics will be described not by the original degrees of freedom
manifest in the model Hamiltonian but by an internal degrees of freedom emerging as fractional excitation. The
existence or the stability of such a deconfined quantum-critical point has been debated for the Neel to the
valence-bond-solid transition in the two-dimensional quantum spin systems, the three-dimensional non-compact
CP$^{N-1}$ action, the loop, and the dimer models. We have studied the excitation energy around the deconfined
quantum-critical point in the two-dimensional quantum spin system, the SU(2) J-Q model, by means of the unbiased
worldline quantum Monte Carlo method. The energy gaps are estimated by the generalized moment method[2] capturing
the asymptotic behavior of the imaginary-time correlation. The transition point is located by the level spectroscopy
using the lowest gaps. We find strong quantitative evidence[3] for deconfined linearly dispersing spinons with
multiple gapless points at ${\bf k}=(0,0)$, $(\pi,0)$, $(0,\pi)$, and $(\pi,\pi)$, as inferred from two-spinon
excitations (S=0 and S=1 states) around these points. We also observe a duality between singlet and triplet
excitations at the critical point and inside the ordered phases, in support of an enhanced symmetry, possibly
SO(5).
[1] T. Senthil, et al., Science 303, 1490 (2004).
[2] H. Suwa and S. Todo, Phys. Rev. Lett. 115, 080601 (2015).
[3] H. Suwa, A. Sen, and A. W. Sandvik, submitted to Phys. Rev. Lett.; arXiv:1607.05110