Giant Optical Polarization Rotation Induced by Spin-Orbit Coupling in Polarons
Résumé
We have uncovered a giant gyrotropic magneto-optical response for doped ferromagnetic manganite La 2=3 Ca 1=3 MnO 3 around the near room-temperature paramagnetic-to-ferromagnetic transition. At odds with current wisdom, where this response is usually assumed to be fundamentally fixed by the electronic band structure, we point to the presence of small polarons as the driving force for this unexpected phenomenon. We explain the observed properties by the intricate interplay of mobility, Jahn-Teller effect, and spin-orbit coupling of small polarons. As magnetic polarons are ubiquitously inherent to many strongly correlated systems, our results provide an original, general pathway towards the generation of magnetic-responsive gigantic gyrotropic responses that may open novel avenues for magnetoelectric coupling beyond the conventional modulation of magnetization. Polarons, first conceived by Landau [1], are quasipar-ticles formed by electrons that are bound to lattice deformations. Their physics is underpinned by electron-phonon interactions, which dominate transport in semiconductors, many poor metals, and organics [2]. When the electronic bandwidth W is sufficiently large Fröhlich-large polarons spread over many lattice sites with nearly free-electron propagation and slightly increased effective mass [2–4]. Yet, when the polaron binding energy E b is larger than half-bandwidth, i.e., λ ¼ ð2E b =WÞ > 1, the coupling to the lattice is so strong that electronic states are heavily dressed by phonons and electrons are self-trapped, forming small Holstein polarons [5–7]. In the latter, the transport rather than diffusive proceeds by thermally activated hopping. Interestingly, thermal agitation or phonons are not the only ways to prompt polaron hopping; light provides an additional pathway. In particular, the absorption of photons may deliver the required energy to jump between sites, i.e., approximately E b =2 or half the polaron binding energy. In the presence of a magnetic field breaking time-reversal symmetry, the conductivity tensor acquires asymmetric nondiagonal terms, thus generating gyrotropic responses [8]; as a consequence, light with different circularly polarized states is absorbed differently, inducing rotation and ellipticity in the polarization of light [2,9–11]. Since the strong coupling of Holstein polarons to the lattice implies vibronic states with energy scales close to the eV, gyrotropic responses are expected in the optical range [8]. So far, though, the specific contribution of small polarons to the gyrotropic effect has remained unsolved. Here we disclose a unique gyrotropic response of small polarons in the visible range and uncover that Holstein-like hopping transport coupled to spin-orbit coupling leads to a gigantic magneto-optical response. To demonstrate this outstanding phenomenon, we selected optimally doped ferromagnetic manganites of composition RE 1−x A x MnO 3 (x ≈ 0.3), where RE is a rare earth and A an alkaline element. Since a large part of the polaron binding energy E b can be identified with the Jahn-Teller energy E JT arising from strong electron-phonon interactions, the selected materials are ideal for test-bedding polaron optical responses. In these systems a colossal magnetoresistance (CMR) [12,13] is observed around the paramagnetic-to-ferromagnetic transition. The reason is that electrons are self-trapped in narrow d orbitals of Mn with e g symmetry forming magnetic small polarons that are sharply suppressed below the Curie temperature T C. Two different responses—sketched in Fig. 1—reveal the effect of polar-ons on the optical properties of CMR manganites [14,15]. First, at temperatures below T C a magneto-optical (MO) signal develops, which is proportional to the magnetization and reflects its typical temperature dependence [Fig.