M agnetodielectric 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, TbMnO 3 and GdMnO 3 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.Enormous progress has been made in the field of multiferroics and the discovery of new classes of ferroelectromagnets (FEMs) with the simultaneous occurrence of magnetic and polar order [1][2][3][4][5] has triggered a revival 6,7 of this old field of magneto-dielectric effects and the electrodynamics of multiferroic media 8 . As well as promising applications of FEMs in the field of modern electronics, for example, as multiple-state memory devices with mutual magnetic or electric control or as magnetically switchable optical devices, fascinating new problems can be tackled in basic research, such as the search for magneto-dielectric excitations.

We report on structural, magnetic, dielectric, and thermodynamic properties of Eu 1−x Y x MnO 3 for Y doping levels 0 ഛ x Ͻ 1. This system resembles the multiferroic perovskite manganites RMnO 3 ͑with R = Gd, Dy, Tb͒ but without the interference of magnetic contributions of the 4f ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x ഛ 0.1 we find a canted antiferromagnetic and paraelectric ground state. For higher concentrations x ജ 0.3 ferroelectric polarization coexists with the features of a presumably spiral magnetic phase analogous to the observations in TbMnO 3 . In the intermediate concentration range around x Ϸ 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a conelike magnetic structure.

The electrodynamics of topological insulators (TIs) is described by modified Maxwell's equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of α/4π per surface. Here we report on the observation of this so-called topological magnetoelectric effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant α=e2/2hc (in SI units) when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants.

The infrared and Terahertz properties of GdMnO3 have been investigated as function of temperature and magnetic field, with special emphasis on the phase boundary between the incommensurate and the canted antiferromagnetic structures. The heterogeneous incommensurate phase reveals strong magnetodielectric effects, characterized by significant magnetoelectric contributions to the static dielectric permittivity and by the existence of electrically excited magnons (electromagnons). In the commensurate canted antiferromagnetic phase the magnetoelectric contributions to the dielectric constant and electromagnons are suppressed. The corresponding spectral weight is transferred to the lowest lattice vibration demonstrating the strong coupling of phonons with electromagnons.Multiferroic materials with the simultaneous occurrence of magnetism and ferroelectricity, are a hot topic in recent solid-state research. They provide interesting and spectacular physical properties and promise attractive applications [1,2,3]. Multiferroic behavior occurs in a variety of systems originating from very different physical mechanisms, including materials with independent magnetic and ferroelectric subsystems, like some boracites, Aurivillius phases, hexagonal manganites, and the lonepair ferroelectrics with magnetic ions [3]. Recently a new class of multiferroics, namely ferromagnetic sulfospinels with relaxor-like short range ferroelectric (FE) order have been detected [4] with a strong coupling of the electric and magnetic properties al low frequencies. In these spinel compounds the ferromagnetism is induced via strong indirect exchange interaction, but the origin of ferroelectricity remains unclear sofar. Finally, in the perovskite manganites there is robust experimental evidence [5,6] that the onset of helical magnetic order induces spontaneous FE polarization [7,8]. Dzyaloshinskii-Moriya type interactions have been utilized to explain the ferroelectricity which is induced by the helical spin structure [9,10,11]. A similar spin-driven ferroelectricity is believed to be operative in NiAfter having established the ground-state properties of this interesting class of materials, the study of their novel dynamic properties will significantly enhance our knowledge of the magneto-electric (ME) coupling [13]. Magnons are the characteristic excitations of magnetic structures, while soft phonons as inferred by the Lyddane-Sachs-Teller relation condense at canonical ferroelectric phase transitions. It seems clear that soft phonons cannot be relevant excitations in the ferroelectric manganites, as (improper) ferroelectricity is induced by the magnetic order coupled to the lattice. Recently it has been shown that electro-magnons are relevant collective modes in this new class of ferroelectrics [14]. Electromagnons are spin waves that are excited by an ac electric field. By measurements in TbMnO 3 and GdMnO 3 it has been documented that these new excitations exist not only in the magnetic phase characterized by the helical spin structure, but also ...

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.