Ultrafast Structure Switching through Nonlinear Phononics

Abstract

We describe an ultrafast coherent control of the transient structural distortion arising from nonlinear phononics in ErFeO3. Using density functional theory, we calculate the structural properties as input to an anharmonic phonon model that describes the response of the system to a pulsed optical excitation. We find that the trilinear coupling of two orthogonal infrared-active phonons to a Raman-active phonon causes a transient distortion of the lattice. The direction of the distortion is determined by the polarization of the exciting light, suggesting a route to nonlinear phononic lattice control and switching. Since the occurrence of the coupling is determined by the symmetry of the system we propose that it is a universal feature of orthorhombic and tetragonal perovskites.Over the last decade it has been shown repeatedly that laser excitation of infrared-active phonons is a powerful tool for modifying the properties of materials. This dynamical materials design approach has been used to drive metal-insulator transitions [1,2], to melt orbital order [3,4] and to induce superconductivity or modify superconducting transition temperatures [5,6] in a range of complex oxides. Particularly intriguing is the case in which the laser intensity is so high that the usual harmonic approximation for the lattice dynamics breaks down and anharmonic phonon-phonon interactions become important. Recent experimental and theoretical studies [7,8] have clarified that quadratic-linear cubic coupling of the form Q 2 IR Q R between a driven infraredactive mode, Q IR , and a Raman-active mode, Q R , causes a shift in the equilibrium structure to a nonzero value of the Raman mode normal coordinates. This nonlinear phononic effect has most notably been associated with the observation of coherent transport, an indicator of superconductivity, far above the usual superconducting Curie temperature in underdoped YBaCu 3 O 6+δ [9-11].Here we investigate theoretically a different kind of cubic phononic coupling of the trilinear form Q IR1 Q IR2 Q R , in which two different infrared-active (IR) modes are excited simultaneously and couple anharmonically to a single Raman mode. Our motivation is provided by recent experimental work on the perovskite-structure orthoferrite ErFeO 3 [12], in which two polar modes of similar frequencies with atomic displacement patterns along the inequivalent a and b orthorhombic axes were simultaneously excited. Ref. [12] reported and analyzed the resulting excitation of a magnon; here our focus is on the changes caused by and the implications of the nonlinear phonon dynamics.ErFeO 3 is a distorted perovskite with the orthorhombic P nma structure and the typical G-type antiferromagnetic ordering of the Fe 3+ magnetic moments [13] (Fig. 1). The primitive magnetic unit cell contains 20 atoms, resulting in 60 phonon modes characterised by representations (within the orthorhombic point group mmm) A g , B (1,2,3)g , A u and B (1,2,3)u . Of the polar "u" modes, only B 1u , B 2u and B 3u have dipole moments and are therefo...

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