Magnetoreflectivity Measurements Research Articles

Enhanced valley splitting in monolayer WSe2 due to magnetic exchange field.

Exploiting the valley degree of freedom to store and manipulate information provides a novel paradigm for future electronics. A monolayer transition-metal dichalcogenide (TMDC) with a broken inversion symmetry possesses two degenerate yet inequivalent valleys, which offers unique opportunities for valley control through the helicity of light. Lifting the valley degeneracy by Zeeman splitting has been demonstrated recently, which may enable valley control by a magnetic field. However, the realized valley splitting is modest (∼0.2 meV T-1). Here we show greatly enhanced valley spitting in monolayer WSe2, utilizing the interfacial magnetic exchange field (MEF) from a ferromagnetic EuS substrate. A valley splitting of 2.5 meV is demonstrated at 1 T by magnetoreflectance measurements and corresponds to an effective exchange field of ∼12 T. Moreover, the splitting follows the magnetization of EuS, a hallmark of the MEF. Utilizing the MEF of a magnetic insulator can induce magnetic order and valley and spin polarization in TMDCs, which may enable valleytronic and quantum-computing applications.

Valley Zeeman Splitting and Valley Polarization of Neutral and Charged Excitons in Monolayer MoTe2 at High Magnetic Fields.

Semiconducting transition metal dichalcogenides (TMDCs) give rise to interesting new phenomena in external magnetic fields, such as valley Zeeman splitting and magnetic-field-induced valley polarization. These effects have been reported for monolayers (MLs) of the transition metal diselenides MoSe2 and WSe2 and, more recently, for disulfides MoS2 and WS2. Here, we present helicity-resolved magneto-photoluminescence and magneto-reflectance contrast measurements for MLs of the telluride member of the semiconducting TMDCs, 2H-MoTe2, in magnetic fields up to 29 T in Faraday geometry. Well-resolved valley Zeeman splittings for the neutral A and B excitons (XA0 and XB0) and the charged exciton X± are observed with effective g-factors of −4.6 ± 0.2, – 3.8 ± 0.6, and −4.5 ± 0.3, respectively. The magnetic field induced valley polarization of XA0 and X± reaches 78% and 36%, respectively, at a magnetic field of 29 T.

Symmetry-Breaking Dynamics in Wurzite Semiconductors

The symmetry of excitons in ZnO was comparatively studied with GaN by using magnetoreflectivity measurements. The magnetic field perpendicular to the c-axis leads to prominent additional peaks, which were assigned to forbidden exciton states at zero field. In addition, the collapse of the exciton symmetry observed by using time-resolved dierential reflectivity spectroscopy was briefly introduced in a facet excitation geometry of strain-free bulk GaN. With a proper choice of polarization configuration between the pump and the probe, we find that ultrafast symmetry breaking of excitons during the energy relaxation from the J = 1/2 to the J = 3/2 states proceeds via an angular momentum flip or a spin flip.

Tight-binding model for carbon from the third-generation LMTO method: A study of transferability

The third-generation LMTO method provides a new wave function basis set in which the energy dependence of the interstitial region and inside muffin-tin (MT) spheres is treated on an equal footing. Within the improved method, basis functions in the interstitial are the screened spherical waves (SSWs) with boundary condition defined in terms of a set of ‘hard’ sphere radii a RL. Energy eigenvalues obtained from the single-particle Schrödinger equation for MT potential is energetically accurate and very useful for predicting a reliable first-principles tight-binding (TB) model of widely different systems. In this study, we investigate a possibility of the new basis sets transferability to different environment which could be crucial for TB applications to very large and complicated systems in realistic materials modelling. For the case of C where the issue of sp2 vssp 3 bonding description is primarily important, we have found that by downfolding the unwanted channels in the basis, the TB electronic structure calculations in both hexagonal graphite and diamond structures are well compared with those obtained from the full LDA schemes if we use the same choice of hard sphere radii,a RL and a fixed, arbitrary energy, εw. Moreover, the choice is robust and transferable to various situations, from different forms of graphite to a wide range of coordination. Using the obtained minimal basis set, we have been investigating the TB Hamiltonian and overlap matrices for different structure types for carbon, in particular we have predicted the on-site and hopping parameters (y1, y2, …, y6) within an orthogonal representation for Slonczewski-Weiss-McClure (S WMcC) model of the Bernal structure. Our theoretical values are in excellent agreement with experimental ones from magnetoreflection measurements of Fermi surfaces for hexagonal graphite.

Excitonic resonant spin-flip Raman scattering in Cd 1-xMn xTe multilayers

We report the observation of resonant spin-flip Raman scattering due to localized heavy-hole excitons in quasi-bulk Cd 1-xMn xTe multilayers. The exciton- and electron spin-flip energies were compared with excitonic Zeeman splittings obtained from magnetoreflectivity measurements on the same sample. A good agreement was observed between the electron spin splitting values obtained from both experiments. The exciton spin-flip energy was found to be slightly reduced with respect to the Zeeman splitting value. Possible mechanisms of this reduction are discussed.

Magnetization and exchange interactions in Zn 1− xFe xTe diluted magnetic semiconductors

Magnetization and magnetoreflectivity measurements have been performed in diluted Zn 1− x Fe x Te compounds, at T=1.8 K. The magnetization of Fe 2+ ions is quantitatively analyzed in the framework of the crystal field and spin–orbit coupling model and is in excellent agreement with the crystal field and spin–orbit coupling parameters we deduced from previous optical studies ( Δ=−2693 cm −1 and λ=−102.2 cm −1 ). The six Zeeman-like components of the free exciton spectrum are observed, in Faraday or Voigt configuration. The comparison between the energy splittings and the magnetization data yields the values for the conduction and valence exchange parameters: N 0α=0.29±0.09 eV and N 0β=−1.50±0.17 eV.

Symmetry of excitons in GaN

Magnetoreflectivity measurements on GaN are used to resolve the symmetry of three excitonic resonances, arising from the crystal-field and spin-orbit effects on the valence band. The symmetry characteristic for each valence subband is directly seen in the observed Zeeman pattern. The linear magnetic-field effect for all exciton components is well accounted for by introducing only two coupling constants: the electronic g factor for the conduction band and the Luttinger $\ensuremath{\kappa}$ parameter for the valence band. The strength of the electron-hole exchange interaction is determined.

Magnetic and magnetoreflectivity study of quaternary manganese based semimagnetic semiconductors

Magnetisation and magnetoreflectivity of quaternary semimagnetic semiconductors with small Mn mole fraction: cubic Zn 0.1Cd 0.89Mn 0.01Te and wurtzite Cd 0.99Mn 0.01Se 0.3Te 0.7 were studied. Magnetisation measurements, performed by SQUID technique at temperature of T=2 K in the magnetic field up to 5 T exhibited paramagnetic like behaviour. Magnetoreflectivity measurements were also performed in the magnetic field up to 5 T and temperature T=2 K in Faraday and Voigt configuration. Exchange integrals for both materials based on experimentally obtained data were calculated. Results were compared with those for ternary semimagnetic semiconductors.

Characterization of normal and inverted interfaces by the Zeeman effect inCd1−xMnxTe/CdTe/Cd1−yMgyTequantum wells

We present a study of the interface profile in asymmetric ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{1\ensuremath{-}y}{\mathrm{Mg}}_{y}\mathrm{Te}$ quantum wells, with normal (${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ on CdTe) and inverted (CdTe on ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$) interfaces. The Zeeman spectra of the confined exciton states were determined by magnetoreflectivity and Kerr rotation measurements. Using a segregation model and an asymmetric exponential profile, we interpret the Zeeman spectra of quantum wells, and determine the interface widths. For the same concentrations of Mn in the barriers and the same growth conditions, the results can be coherently explained assuming an interface width independent of the quantum-well width and the type of interface (normal or inverted).

Excitonic properties and magnetooptical characterization of [formula omitted] heterostructures

We present an experimental and theoretical study of optical and magneto-optical properties of excitons in strained layer ZnCdSeZnSe superlattices. The optical characterizations of these structures are based on photoluminescence and reflectivity measurements at 2 K. The excitonic transition energies are determined experimentally and are found in agreement with the predictions of the envelope function, taking into account band offsets and exciton binding energies. These binding energies and also the oscillator strengths are calculated using a recent formalism of fractional dimensional space (αD) and compare favourably with variational calculations. Magnetoreflectivity measurements are performed up to 5.5 T. They permit us to calculate the Zeeman splitting of | ± 3 2 , ± 1 2 〉 and | ± 1 2 , ± 1 2 〉 transitions and then, to deduce the corresponding effective g- values g 1 2 and g 1 2 . In our knowledge, our results concerning the calculations of ΔE 1 2 and then g 1 2 for such superlattices are the first ones and permit us to confirm the interlevel mixing in these structures.

Interband magneto-optical studies of resonant polaron coupling in CdTe/Cd1-xMnxTe quantum wells.

Magnetoreflectivity measurements of the 1s and 2s exciton energies in a CdTe/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te superlattice have been made in magnetic fields up to 45 T, showing the resonant polaron coupling of electrons to LO phonons. Strong reflectivity features are seen for both the 1s and 2s excitons, which show a strong field-dependent spin splitting due to the dilute magnetic barriers. At ${\mathit{B}}_{\mathit{z}}$=0, the 2s exciton feature is observed lying 18 meV above the 1s state, and is shifted upward in energy by the magnetic fields. No resonant behavior occurs when the 2s state passes through the LO-phonon energy of 21 meV, but at higher fields of around 20 T, the resonances for both spin states (${\mathrm{\ensuremath{\sigma}}}^{\ifmmode\pm\else\textpm\fi{}}$) of the 2s exciton broaden and show a strong anticrossing behavior. These experiments are shown to be in excellent agreement with a theoretical treatment which includes the resonant polaron coupling of the electrons alone. Both experiment and theory demonstrate an extremely strong resonant splitting of the 2s exciton states of approximately 11 meV, which is over 50% of the LO-phonon energy. The dominance of single-particle polaron coupling is attributed to the relative sizes of the polaron (35 \AA{}) and the exciton (50 \AA{}) radius.

Rapid thermal processing of semimagnetic superstructures studied by magnetoreflectivity

Thermally induced interdiffusion in CdTe/CdMnTe quantum wells and a superlattice was investigated by magnetoreflectivity measurements. The structures were grown by the MBE technique and annealed by rapid thermal processing for 15s at 450 centigrade. A blue shift of the exciton states confined in quantum wells as well as a significant enhancement of the Zeeman effect were observed for the annealed samples. The manganese distribution profiles after diffusion process are discussed and it is shown that the simplest model of the interdiffusion of the Mn and Cd ions based on the Fick's law does not explain the observed enhancement of the Zeeman effect.

Magneto-optical studies of strain effects on the excitons in InxGa1-xAs/AlyGa1-yAs strained quantum wells.

Low-temperature (4.2 K) magnetoreflectance measurements have been performed on a series of In x Ga 1-x As/Al y Ga 1-y As strained-layer quantum wells (QW's) with the InAs mole fraction x varying from 0 to 0.2, for three different well widths, and for the magnetic-field range of 0-9 T. The diamagnetic shift data of 1s, 2s, 3s,... excitons are fit to a theoretical model to determine the exciton binding energies and the in-plane reduced effective masses, which are presented as a function of x, for excitons in 80, 120, and 340 A In x Ga 1-x As/Al y Ga 1-y As QW's. In the case of high-barrier QW's, the binding energies and effective masses appear to decrease linearly with increasing x over the range studied

Changes of FIR Reflectivity of HgSe:Cr in a Magnetic Field

Diluted magnetic semiconduction are crystals in which one of the non-magnetic components is substituted in a controlled way by magnetic ions [1]. Currently, the main interest has shifted from manganese based compounds to crystals in which the cations are substituted by other transition metal ions, see for example, see for example [2]. In HgSe:Cr the Cr 2+ ion substitute Hg atoms in the zinc blende type host lattice. HgSe, having an inverted band structure is always n-type. The only possible way of investigating low-energy excitations in such a material is through reflectivity measurements. The considerable number of competing processes such as plasma-, phononand electron-related excitations can make it difficult to identify the origin of observed stuctures. However, for energy levels which can be tuned by an applied magnetic field, magnetospectroscopy can help to clarify the nature of the observed spectral features. The investigated samples had low concentrations of Cr, not exceeding several times 1018 cm-3 . Therefore for these crystals we do not expect to observe effects due to local vibrations and exchange interaction. The magnetoreflectivity measurements were performed in the spectral range 50-700 cm -1 in magnetic fields up to 20 T using a Fourier Transform Spectrometer which was connected via an outcoupling optic to either a superconducting coil or a Bitter magnet [3]. All measurements were done in the Faraday geometry with nonpolarized light at 2 K.

Free carriers in heavily doped polyacetylene

Reflection and magneto-reflection measurements on heavily doped Durham-Graz trans-CH x are reported. The reflection spectra show plasma edges in the visible (with AsF 5-doping) and the near infrared (with iodine-doping) regions. From the magnetic field dependences of the plasma edge values of the effective carrier masses, m ∗ are derived. Within experimental error, m ∗ ∼ 0.15 m 0 for iodine-doped as well as for AsF 5-doped samples. We conclude that the two dopants mainly differ by the concentrations of the carriers induced on the polymer chains.

Spin superlattice formation in ZnSe-based diluted magnetic semiconductor heterostructures

We have observed spin–polarized confinement of both electrons and holes in ZnSe-based single and multiple quantum well structures. The samples were grown by molecular beam epitaxy with 100 Å Zn0.99Fe0.01Se barriers and 100–200 Å ZnSe wells. In these systems, the band offsets are very small, so that the large spin–splitting of the diluted magnetic semiconductor levels dominates the alignment of both the conduction and valence bands, permitting magnetic field-tunable confinement. Magnetoreflectivity measurements show a pronounced asymmetry of the heavy hole spin–splitting, a signature of spin-dependent quantum confinement. The relative phase and near-unity intensity ratio of the ‘‘spin–up’’ and ‘‘spin–down’’ excitons confirm localization of spin–up carriers to the ZnSe wells and spin–down carriers to the ZnFeSe layers, producing a field-tunable ‘‘spin–superlattice.’’ These results will be compared to those obtained for higher magnetic ion concentrations in the barriers.

Magnetoreflectance and magnetization of the Co-based wurtzite-structure diluted magnetic semiconductor Cd1-xCoxSe.

Magnetoreflectance measurements of excitonic interband transitions in the wurtzite-structure compound ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se with various directions of applied magnetic field combined with magnetization measurements, allowed us to determine the ion-carrier exchange parameters for the conduction and valence bands (${\mathit{N}}_{0}$\ensuremath{\alpha}=258\ifmmode\pm\else\textpm\fi{}10 meV, ${\mathit{N}}_{0}$\ensuremath{\beta}=-1883\ifmmode\pm\else\textpm\fi{}30 meV), without making any assumption concerning the band parameters. The magnitude of these exchange parameters, compared with the values previously obtained for ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se, indicates a small variation of the conduction-band exchange parameter, but an important enhancement of the valence-band exchange parameter. This large variation of the hybridization-induced exchange is explained within a generalized Schrieffer-Wolff formula.

Spin separation in diluted magnetic semiconductor quantum well systems (invited)

Heterostructures containing diluted magnetic semiconductor (DMS) layers offer the possibility of magnetically tuning the heterojunction band alignment due to the extraordinarily large spin-splitting of the DMS bands (large effective g-factor). This field-dependent band alignment has significant consequences for spin-dependent carrier confinement as evidenced in magnetooptic or magneto-transport experiments. We have examined two wide-gap DMS quantum well systems in which the band alignment is dominated by the DMS spin-splitting rather than by the more commonly observed effects of differences in bandgap, natural band offset, and strain. Quantum well structures with (Zn,Fe)Se or (Zn,Mn)Se barriers and ZnSe wells have been grown to investigate magnetically tuned, spin-dependent quantum confinement. In these systems, the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. However, the hole confinement is continuously tunable by an external magnetic field applied normal to the layer plane, since the valence band spin-splitting is approximately an order of magnitude larger at modest fields (1 T) than the zero field VB offset. This produces a field-induced spin dependent type I/type II band alignment and a consequent spatial spin separation of the holes. The quantum well structure exhibits a type I ‘‘straddling’’ alignment for the mj=+3/2 level, with the spin ‘‘up’’ holes localized in the ZnSe wells, and a type II ‘‘staggered’’ alignment for the mj=−3/2 level in which the spin ‘‘down’’ holes are localized in the barriers. The observed exciton splittings, intensities and temperature dependence are consistent with this model. In the Zn0.91Mn0.09Se/ZnSe system, a second bound state is observed in the conduction band in the σ− polarization in magnetoreflectivity measurements. A quantitative fit to the data yields a value of approximately 1 meV for the zero field valence band offset. These systems represent the first DMS-based heterostructures in which such field-dependent confinement has been observed.

Ion-carrier electron exchange constants for CdCoSe semimagnetic semiconductor

Magnetoreflectance and magnetization measurements were performed on the recently synthesized semimagnetic semiconductor Cd 0.952Co 0.048Se. The results allowed to obtain for the first time both ion-carrier exchange constants Noα = 279±29 meV and Noβ = -1873±42 meV.

FIR fourier transform spectroscopy in high magnetic fields: Investigations of low energy excitations in dilute magnetic semiconductors

We describe an experimental set-up for magnetoreflection and magnetotransmission measurements by means of fast scan FIR Fourier transform spectroscopy in magnetic fields up to 20 tesla produced by Bitter coils. Various aspects of FIR technique in the special measuring configuration are discussed. The capabilities of the configuration are demonstrated with results obtained from the study of low energy excitations in wide and narrow gap dilute magnetic semiconductors.