We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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Electrodynamics of correlated electron materials

Magnetodielectric materials are characterized by a strong coupling of the magnetic and dielectric properties and, in rare cases, simultaneously show both magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto–dielectric coupling and as a consequence fundamentally different spin excitations exist: electro-active magnons (or electromagnons), spin waves that can be excited by a.c. electric fields. Here we provide evidence that these excitations appear in the phase with an incommensurate magnetic structure of the manganese spins. In external magnetic fields this incommensurate structure can be suppressed and the electromagnons wiped out, thereby inducing considerable changes in the index of refraction from d.c. up to terahertz frequencies. Hence, besides adding a creature to the zoo of fundamental excitations, the refractive index can be tuned by moderate magnetic fields, which enables the design of the next generation of optical switches and optoelectronic devices.

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Possible evidence for electromagnons in multiferroic manganites

http://users.df.uba.ar/acha/Lab5/Review_PR400.pdf

Mixed-valence manganites: fundamentals and main properties

We review the first decade of extensive optical studies of ferromagnetic, III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments to the III–V host, which can result in carrier mediated ferromagnetism in these disordered semiconductors. Spectroscopic experiments provide direct access to the strength and nature of the exchange between holes and local moments; the degree of itineracy of the carriers; and the evolution of the states at the Fermi energy with doping. Taken together, the diversity of optical methods reveal that Mn is an unconventional dopant, in that the metal to insulator transition is governed by the strength of the hybridization between Mn and its p-nictogen neighbor. The interplay between the optical, electronic and magnetic properties of III-Mn-V magnetic semiconductors is of fundamental interest and may enable future spin-optoelectronic devices.

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Optical Properties of III-Mn-V Ferromagnetic Semiconductors

Optical properties of III-Mn-V ferromagnetic semiconductors

Review is given on late optical studies on colossal magnetoresistance (CMR) manganites with perovskite or related structures. In La1−x
Sr
 x
 MnO3 which undergoes a ferromagnetic transition, optical conductivity spectrum σ(ω) shows a change from a gap-like feature to metallic but highly diffussive one with increasing spin-polarization. Such a remarkable spectral weight transfer is also observed in a layered manganite, La2−2x
Sr1+2x
Mn2O7 (x = 0.4), with a mid-infrared peak structure due to inter-orbital 
$$(d_{x^2 - y^2 } \to d_{3z^2 - r^2 } )$$

 transition. In Pr1−x
Ca
 x
 MnO3 (x = 0.4) which undergoes the charge ordering phase transition, σ(ω) shows an anisotropic feature reflecting charge/orbital ordering pattern at 10 K. The charge-ordered state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T at 30 K, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV–3 eV).

Y. Okimoto

Papers

Optical Spectroscopy of Perovskite-Type Manganites