Quantum wells (QWs) based on mercury telluride (HgTe) thin films provide a large scale of unusual physical properties starting from an insulator via a two-dimensional Dirac semimetal to a three-dimensional topological insulator. These properties result from the dramatic change of the QW band structure with the HgTe film thickness. Although being a key property, these energy dispersion relations cannot be reflected in experiments due to the lack of appropriate tools. Here we report an experimental and theoretical study of two HgTe quantum wells with inverted energy spectrum in which two-dimensional semimetallic states are realized. Using magneto-optical spectroscopy at sub-THz frequencies we were able to obtain information about electron and hole cyclotron masses at all relevant Fermi level positions and different charge densities. The outcome is also supported by a Shubnikov-de Haas analysis of capacitance measurements, which allows obtaining information about the degeneracy of the active modes. From these data, it is possible to reconstruct electron and hole dispersion relations. Detailed comparative analysis of the energy dispersion relations with theoretical calculations demonstrates a good agreement, reflecting even several subtle features like band splitting, the second conduction band, and the overlaps between the first conduction and first valence band. Our study demonstrates that the cyclotron resonance experiments can be efficiently used to directly obtain the band structures of semimetallic 2D materials.
The experimental setup for contactless conductivity measurements is reported which can easily be operated at frequencies ${10}^{6}$\ensuremath{\le}\ensuremath{\nu}\ensuremath{\le}${10}^{9}$ Hz. The technique is applied to complex conductivity measurements in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ at temperatures from 4 to 300 K. The results are compared with data obtained by conventional techniques.
The behavior of the low-frequency electromagnon in multiferroic ${\text{DyMnO}}_{3}$ has been investigated in external magnetic fields and in a magnetically ordered state. Significant softening of the electromagnon frequency is observed for external magnetic fields parallel to the $a$ axis $(B\ensuremath{\parallel}a)$, revealing a number of similarities to a classical soft-mode behavior known for ferroelectric phase transitions. The softening of the electromagnon yields an increase in the static dielectric permittivity which follows a similar dependence as predicted by the Lyddane-Sachs-Teller relation. Within the geometry $B\ensuremath{\parallel}b$ the increase in the electromagnon intensity does not correspond to the softening of the eigenfrequency. In this case the increase in the static dielectric permittivity seems to be governed by the motion of the domain walls.
Electroactive spin excitations (electromagnons), an analog of the low-frequency quasiferromagnetic antiferromagnetic resonance (AFMR) mode in Fe subsystem, are observed in multiferroic rare-earth ferroborates [specifically in SmFe(BO)] in the frequency range of 40–150 GHz, which are shown to contribute dominantly to the giant static (quasistatic) magnetodielectric effect and which produce two types of dynamic magnetoelectric effects: (a) giant optical activity, which occurs for the wave vector k parallel to the crystallographic -axis, , in a transverse magnetic field and is accompanied by polarization-plane rotation by more than 70 deg mm in the resonance, and (b) directional birefringence and dichroism in a transverse magnetic field -axis, which show up in transmission asymmetry between the forward () and backward () directions, being equivalent to the sign change of the magnetic field, . A theory is developed which explains the observed dynamic magnetoelectric phenomena quantitatively by taking into consideration different symmetries of the tensors of magnetic, magnetoelectric and dielectric susceptibilities for and .
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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ layers provide negative permittivity while negative permeability is achieved via ferromagnetic $(\mathrm{La}\ensuremath{\mathbin:}\mathrm{Sr}){\mathrm{MnO}}_{3}$ 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.
We report on structural, magnetic, dielectric, and thermodynamic properties of (Eu:Y)MnO3 for Y doping levels 0 <= x < 1. This system resembles the multiferroic perovskite manganites RMnO3 (with R= Gd, Dy, Tb) 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 <= 0.1 we find a canted antiferromagnetic and paraelectric groundstate. For higher concentrations x <= 0.3 ferroelectric polarization coexists with the features of a long wavelength incommensurate spiral magnetic phase analogous to the observations in TbMnO3. In the intermediate concentration range around x = 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a canted spiral magnetic structure.
Phase waveplates for THz beam shaping were developed, calculated, and implemented. Developed phase plates could be 3D-printed from commercially available polymers, which makes them affordable and fully customized. These waveplates allow to widely and easily manipulate THz beam shape.
Spin-orbit coupling in thin HgTe quantum wells results in a relativistic-like electron band structure, making it a versatile solid state platform to observe and control nontrivial electrodynamic phenomena. Here we report an observation of universal terahertz (THz) transparency determined by fine-structure constant $\ensuremath{\alpha}\ensuremath{\approx}1/137$ in 6.5-nm-thick HgTe layer, close to the critical thickness separating phases with topologically different electronic band structure. Using THz spectroscopy in a magnetic field we obtain direct evidence of asymmetric spin splitting of the Dirac cone. This particle-hole asymmetry facilitates optical control of edge spin currents in the quantum wells.
