A. Pimenov

Negative refraction in ferromagnet-superconductor superlattices.

Negative refraction, which reverses many fundamental aspects of classical optics, can be obtained in systems with negative magnetic permeability and negative dielectric permittivity. This Letter documents an experimental realization of negative refraction at millimeter waves, finite magnetic fields, and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and superconducting thin films. In the present case the superconducting YBa2Cu3O7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO3 layers for frequencies and magnetic fields close to the ferromagnetic resonance. In these superlattices the refractive index can be switched between positive and negative regions using external magnetic field as tuning parameter.

Spin-controlled Mott-Hubbard bands in LaMnO3 probed by optical ellipsometry

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of LaMnO3 in the range 0.5-5.6 eV at temperatures 50<or=T<or=300 K. A pronounced redistribution of spectral weight is found at the Ne el temperature T(N)=140 K. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite d-d transitions, and that LaMnO3 is a Mott-Hubbard insulator.

Low loss and magnetic field-tunable superconducting terahertz metamaterial

Superconducting terahertz (THz) metamaterial (MM) made from niobium (Nb) film has been investigated using a continuous-wave THz spectroscopy. The quality factors of the resonance modes at 0.132 THz and 0.418 THz can be remarkably increased when the working temperature is below the superconducting transition temperature of Nb, indicating that the use of superconducting Nb is a possible way to achieve low loss performance of a THz MM. In addition, the tuning of superconducting THz MM by a magnetic field is also demonstrated, which offers an alternative tuning method apart from the existing electric, optical and thermal tuning methods.

Electromagnons in multiferroic manganites

We review terahertz experiments on magnetoelectric excitations in rare earth multiferroic manganites RMnO3 with R = Gd, Tb, Dy, (Eu:Y). In all these compounds characteristic excitations of the novel type, called electromagnons, have been observed for frequencies 10 cm−1<ν<30 cm−1. From the spectroscopic point of view electromagnons are responsible for the magnetoelectric effects in RMnO3 and, contrary to magnons, are excited only by electric components of the electromagnetic wave. In all compositions the electromagnons appear as a broad Debye-like contribution in the sinusoidally modulated antiferromagnetic phase and transform to well-defined excitations as the magnetic structure becomes spiral. At the lowest temperatures the fine structure of electromagnons is observed, reflecting the increasing complexity of the magnetic structure.

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (T(c)) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data

Terahertz conductivity at the Verwey transition in magnetite

The complex cond. at the (Verwey) metal-insulator transition in Fe3O4 was investigated at terahertz and IR frequencies. In the insulating state, both the dynamic cond. and the dielec. const. reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency n1.simeq.8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielec. const. abruptly decreases and becomes neg. Together with the cond. spectra this indicates the formation of a narrow Drude peak with a characteristic scattering rate of about 5 meV contg. only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral wt. from the hopping process to the free-carrier cond. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy. [on SciFinder (R)]

THz Spectroscopy of Superconductors

Low-energy excitations in solids are among the most fascinating fields in condensed matter physics, albeit one of the most challenging too. Superconductors were probably the first systems extensively studied by probing with electromagnetic radiation in the terahertz (THz) frequency range; and after 50 years, the THz spectroscopy of superconductivity is still the most active field ranging from fundamental questions to commercial applications. Here, we highlight some aspects of the electrodynamics of the superconducting state explored in recent years. The experiments we will concentrate on have been conducted by quasi-optical methods using a continuous wave THz spectrometer, with backward wave oscillators as coherent and tunable radiation sources.

Terahertz quantum Hall effect of Dirac fermions in a topological insulator

Using terahertz spectroscopy in external magnetic fields, we investigate the low-temperature charge dynamics of strained HgTe, a three-dimensional topological insulator. In resonator experiments, we observe quantum Hall oscillations at terahertz frequencies, which offer direct access to the unusual electrodynamic properties of the surface states of topological insulators. The 2D density estimated from the period of the quantum Hall oscillations agrees well with dc transport experiments on the topological surface state. The Dirac character of the surface state is further evidenced by the observation of the characteristic Berry phase in the dependence of the Landau levels on magnetic field.

Room temperature electrically tunable terahertz Faraday effect

We demonstrate electrical control of the room temperature Faraday effect in a 100-nm-thick film of strained HgTe, which is a topological insulator. The terahertz (THz) response of our device is dominated by the Drude response of carriers with high mobility. The electrical control is achieved by gating the carrier density in a static magnetic field, opening new perspectives for applications like high-speed amplitude and phase modulators in the THz frequency range.

Observation of the universal magnetoelectric effect in a 3D topological insulator

The electrodynamics of topological insulators (TIs) is described by modified Maxwells 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.