Abstract
It has recently been indicated that the hexagonal manganites exhibit Higgs- and Goldstone-like phonon modes that modulate the amplitude and phase of their primary order parameter. Here, we describe a mechanism by which a silent Goldstone-like phonon mode can be coherently excited, which is based on nonlinear coupling to an infrared-active Higgs-like phonon mode. Using a combination of first-principles calculations and phenomenological modeling, we describe the coupled Higgs-Goldstone dynamics in response to the excitation with a terahertz pulse. Besides theoretically demonstrating coherent control of crystallographic Higgs and Goldstone excitations, we show that the previously inaccessible silent phonon modes can be excited coherently with this mechanism.
Highlights
Order parameters are physical observables that are used to quantify the different states of matter
Besides theoretically demonstrating coherent control of crystallographic Higgs and Goldstone excitations, we show that the previously inaccessible silent phonon modes can be excited coherently with this mechanism
Two particular excitations are Higgs and Goldstone modes, which correspond to the modulation of the amplitude and phase of an order parameter that breaks a continuous symmetry
Summary
Structural properties of InMnO3.—InMnO3 crystallizes in the hexagonal manganite structure shown in Fig. 1(a): The potential energy landscape of the lattice displacement forms a buckled Mexican hat that hosts a nonpolar phase at its center, and polar and antipolar phases at the minima and maxima of its brim [48,49,50]; see Fig. 1(b). The primary order parameter corresponds to a tilting of the manganese oxygen bipyramids and a simultaneous buckling of the indium atoms This two-dimensional order parameter couples to a ferroelectric displacement with polarization along the c axis of the crystal. In order to obtain the nonlinear phonon couplings, we calculate the total energy as a function of ion displacements along the normal mode coordinates of the Higgs and Goldstone modes and fit the resulting two-dimensional energy landscape to the potential V in Eq (2). The potential energy of the phonons can be written in a minimal model as
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