Anisotropy of Phonon Transition Dipole Moments and Hints of Anisotropy in the Photoconductivity of Single-Crystalline Two-Dimensional Lead-Halide Perovskites

The lattice of hybrid organic-inorganic metal halide perovskites is both soft and ionic. The particular combination gives rise to interesting molecular physics: electrons in these materials are strongly coupled to vibrations and phonons. Analogous to the solvation of ions in water, when the coupling between vibrations and charge carriers is strong enough, it leads to the formation of polarons, where the charge carrier and lattice vibration cannot be disentangled into two separate entities anymore. Here, we study single crystals of butylammonium lead iodide (BAPI), a two-dimensional perovskite structure, with different, but well defined, thicknesses of the inorganic lattice. We measured angle-and-polarization dependent THz transmission through the samples to study the directionality of the transition dipole moments (TDMs) of vibrational modes between 0.3 – 3 THz; the vibrations in the inorganic planes inside the lattice. Next to this, we use optical-pump/THz probe spectroscopy to measure the materials’ photoconductivity along different crystallographic directions. We find signatures of higher photoconductivity in the direction perpendicular to the 1 THz TDMs compared to parallel to it, and discuss possible origins of this phenomenon by comparison to theory.