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Semiclassical evidence of columnar order in the fully frustrated transverse field Ising model on the square lattice

We investigate the zero-temperature phase diagram of the fully frustrated transverse field Ising model on the square lattice both in the classical limit and in the presence of quantum fluctuations. At the classical level (the limit of infinite spin $S$), we find that upon decreasing the transverse field $Γ$ this model exhibits a phase transition from the fully polarized state into an eight-fold degenerate translational symmetry breaking state. This phase can be identified to correspond to plaquette order in the dimer language and remains the lowest-energy state in the entire range of fields below the critical one, $Γ_c$. The eight-fold degenerate solution which corresponds to columnar order in the dimer language is a saddle point of the classical energy. It is degenerate with the plaquette solution at $Γ=0$ and is only slightly higher in energy in the whole interval $0<Γ<Γ_c$. The effect of quantum fluctuations is investigated in the context of a large S expansion both for the plaquette and columnar structures. For this purpose we employ an approximate method allowing to estimate from above the fluctuation-induced correction to the energy of a configuration which at the classical level is a saddle point of the energy, \textit{not} a local minimum. Although the convergence of the $1/S$ expansion in the $Γ/J\rightarrow 0$ limit remains an open question, harmonic quantum fluctuations show a clear tendency to overcome the energy difference between the two states and to change the classical picture favoring the columnar order over the plaquette one in a wide parameter range.