J. Cerne

Terahertz Response and Colossal Kerr Rotation from the Surface States of the Topological Insulator Bi2Se3

We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65° and can be explained by a cyclotron resonance effect of the surface states.

MAGNETO-TRANSPORT AND MAGNETO-OPTICAL PROPERTIES OF FERROMAGNETIC (III,Mn)V SEMICONDUCTORS: A REVIEW

Rapid developments in material research of metallic ferromagnetic (III,Mn)V semiconductors over the past few years have brought a much better understanding of these complex materials. We review here some of the main developments and current understanding of the bulk properties of these systems within the metallic regime, focusing principally on the magneto-transport and magneto-optical properties. Although several theoretical approaches are reviewed, the bulk of the review uses the effective Hamiltonian approach, which has proven useful in describing many of these properties namely in (Ga,Mn)As and (In,Mn)As. The model assumes a ferromagnetic coupling between Mn d-shell local moments mediated by holes in the semiconductor valence band.

Electronic structure of ferromagnetic semiconductor Ga1-xMnxAs probed by subgap magneto-optical spectroscopy

We employ Faraday and Kerr effect spectroscopy in the infrared range to investigate the electronic structure of Ga_{1-x}Mn_{x}As near the Fermi energy. The band structure of this archetypical dilute-moment ferromagnetic semiconductor has been a matter of controversy, fueled partly by previous measurements of the unpolarized infrared absorption and their phenomenological impurity-band interpretation. Unlike the unpolarized absorption, the infrared magneto-optical effects we study are intimately related to ferromagnetism, and their interpretation is much more microscopically constrained in terms of the orbital character of the relevant band states. We show that the conventional theory of the disordered valence band with an antiferromatnetic exchange term accounts semiquantitatively for the overall characteristics of the measured infrared magneto-optical spectra.

Terahertz magneto-optical polarization modulation spectroscopy

We report the development of new terahertz (THz) techniques for rapidly measuring the complex Faraday angle in systems with broken time-reversal symmetry. Using the cyclotron resonance of a GaAs two-dimensional electron gas in a magnetic field, we have tested the performance of the techniques. We have made polarization modulation, high sensitivity (<1  mrad) narrowband rotation measurements with a cw optically pumped molecular gas laser, and, by combining the distinct advantages of THz time domain spectroscopy and polarization modulation techniques, we have demonstrated rapid broadband rotation measurements to <5  mrad precision.

Determination of the infrared complex magnetoconductivity tensor in itinerant ferromagnets from Faraday and Kerr measurements

We present measurement and analysis techniques that allow the complete complex magnetoconductivity tensor to be determined from mid-infrared (11-1.6 {micro}m; 100-800 meV) measurements of the complex Faraday ({theta}{sub F}) and Kerr ({theta}{sub K}) angles. Since this approach involves measurement of the geometry (orientation axis and ellipticity of the polarization) of transmitted and reflected light, no absolute transmittance or reflectance measurements are required. Thick-film transmission and reflection equations are used to convert the complex {theta}{sub F} and {theta}{sub K} into the complex longitudinal conductivity {sigma}{sub xx} and the complex transverse (Hall) conductivity {sigma}{sub xy}. {theta}{sub F} and {theta}{sub K} are measured in a Ga{sub 1-x}Mn{sub x}As and SrRuO{sub 3} films. The resulting {sigma}{sub xx} is compared to the values obtained from conventional transmittance and reflectance measurements, as well as the results from Kramers-Kronig analysis of reflectance measurements on similar films.

Measurement of the infrared complex Faraday angle in semiconductors and insulators

We measure the infrared (wavelength λ = 11 − 0.8 μm; energy E = 0.1 − 1.5 eV) Faraday rotation and ellipticity in GaAs, BaF2, LaSrGaO4, LaSrAlO4, and ZnSe. Since these materials are commonly used as substrates and windows in infrared magneto-optical measurements, it is important to measure their Faraday signals for background subtraction. These measurements also provide a rigorous test of the accuracy and sensitivity of our unique magneto-polarimetry system. The light sources used in these measurements consist of gas and semiconductor lasers, which cover 0.1 - 1.3 eV, as well as a custom-modified prism monochromator with a Xe lamp, which allows continuous broadband measurements in the 0.28 - 1.5 eV energy range. The sensitivity of this broadband system is approximately 10 μrad. Our measurements reveal that the Verdet coefficients of these materials are proportional to λ-2, which is expected when probing with photon energies below the band gap. Reproducible ellipticity signals are also seen, which is unexpected since the photon energy is well below the absorption edge of these materials, where no magnetic circular dichroism or magnetic linear birefringence should occur. We suggest that the Faraday ellipticity is produced by the static retardance of the photoelastic modulator and other optical elements such as windows, which convert the polarization rotation produced by the sample into ellipticity. This static retardance is experimentally determined by the ratio of the Faraday rotation and ellipticity signals, which are induced by either applying a magnetic field to a sample or mechanically rotating the polarization of light incident on the photoelastic modulator and/or other optical components.

Systematic study of magnetic linear dichroism and birefringence in (Ga,Mn)As

nd a clear blue shift of the dominant peak at energy exceeding the host material band gap. These results are discussed in the general context of the GaAs host band structure and also within the framework of thek p and mean-eld kinetic-exchange model of the (Ga,Mn)As band structure. We nd a semi-quantitative agreement between experiment and theory and discuss the role of disorder-induced non-direct transitions on magneto-optical properties of (Ga,Mn)As.

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect

The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.

High precision magnetic linear dichroism measurements in (Ga,Mn)As

Investigation of magnetic materials using the first-order magneto-optical Kerr effects (MOKEs) is well established and is frequently used. On the other hand, the utilization of the second-order (or quadratic) magneto-optical (MO) effects for the material research is rather rare. This is due to the small magnitude of quadratic MO signals and the fact that the signals are even in magnetization (i.e., they do not change a sign when the magnetization orientation is reversed), which makes it difficult to separate second-order MO signals from various experimental artifacts. In 2005 a giant quadratic MO effect-magnetic linear dichroism (MLD)-was observed in the ferromagnetic semiconductor (Ga,Mn)As. This discovery not only provided a new experimental tool for the investigation of in-plane magnetization dynamics in (Ga,Mn)As using light at normal incidence, but it also motivated the development of experimental techniques for the measurement of second-order MO effects in general. In this paper we compare four different experimental techniques that can be used to measure MLD and to separate it from experimental artifacts. We show that the most reliable results are obtained when we monitor the polarization of reflected light while the magnetization of the sample is rotated by applying an external magnetic field. Using this technique we measure the MLD spectra of (Ga,Mn)As in a broad spectral range from 0.1 eV to 2.7 eV and we observe that MLD has a magnitude comparable to the polar MOKE signals in this material.

Growth mechanism of largescale MoS2 monolayer by sulfurization of MoO3 film

Monolayer two-dimensional transition metal dichalcogenides (TMDCs) such as MoS2 with broken inversion symmetry possesses two degenerate yet inequivalent valleys that can be selectively excited by circularly polarized light. This unique property renders interesting valley physics. The ability to manipulate valley degrees of freedom with light or external field makes them attractive for optoelectronic and spintronic applications. There is great demand for large area monolayer (ML) TMDCs for certain measurements and device applications. Recent reports on large area ML TDMCs focus on chemical vapor deposition growth. In this work, we report a facile approach to grow largescale continuous ML MoS2 nearly free of overgrowth and voids, by sulfurizing evaporated molybdenum trioxide ultrathin films. Photo conductivity scales with device sizes up to 4.5 mm, suggesting excellent film uniformity. The growth mechanism is found to be vaporization, diffusion, sulfurization and lateral growth, all at local micrometer scale. Our approach provides a new pathway for large-area ML TMDC growth and lithography-free device fabrication.