Hole Spin States Research Articles

Electronic states of PrCoO3: x-ray photoemission spectroscopy andLDA +U density of states studies

Electronic states of PrCoO3 are studied using x-ray photoemission spectroscopy. ThePr 3d5/2 core level and valence band (VB) were recorded using anMg K α source. The core level spectrum shows that the3d5/2 level is split into two components of multiplicity four and two,respectively, due to coupling of the spin states of the hole in3d5/2 with the Pr 4f hole spin state. The observed splitting is 4.5 eV. The VB spectrumis interpreted using density of states (DOS) calculations under LDA andLDA +U. It is noted that LDA is not sufficient to explain the observed VBspectrum. Inclusion of on-site Coulomb correlation for Co 3d electrons inLDA +U calculations gives DOS which is useful in qualitative explanation of the ground state.However, it is necessary to include interactions between Pr 4f electrons to get betteragreement with the experimental VB spectrum. It is seen that the VB consistsof Pr 4f, Co 3d and O 2p states. Pr 4f, Co 3d and O 2p bands are highly mixed,indicating strong hybridization of these three states. The band near the Fermi levelhas about equal contributions from Pr 4f and O 2p states with somewhat smallercontribution from Co 3d states. Thus in the Zaanen, Sawatzky and Allen schemePrCoO3 can be considered as a charge transfer insulator. The charge transfer energyΔ can be obtained using LDA DOS calculations and the Coulomb-exchange energyU′ fromLDA +U. The explicitvalues for PrCoO3 are Δ = 3.9 eV and U′ = 5.5 eV; the crystal field splitting and 3d bandwidth of Co ions are also found to be 2.8 and 1.8 eV,respectively.

Half-skyrmion picture of a single-hole-dopedCuO2plane

Based on the Zhang-Rice singlet picture, it is argued that the half-skyrmion is created by the doped hole in the single hole doped high-T_c cuprates with N'eel ordering. The spin configuration around the Zhang-Rice singlet, which has the form of superposition of the two different d-orbital hole spin states, is studied within the non-linear \sigma model and the CP^1 model. The spin configurations associated with each hole spin state are obtained, and we find that the superposition of these spin configuration turns out to be the half-skyrmion that is characterized by a half of the topological charge. The excitation spectrum of the half-skyrmion is obtained by making use of Lorentz invariance of the effective theory and is qualitatively in good agreement with angle resolved photoemission spectroscopy on the parent compunds. Estimated values of the parameters contained in the excitation spectrum are in good agreement with experimentally obtained values. The half-skyrmion theory suggests a picture for the difference between the hole doped compounds and the electron doped compounds.

Spin-flip relaxation due to phonon macroscopic deformation potential in quantum dots

We have calculated the spin-flip relaxation rates of electrons in a single CdTe spherical quantum dot due to the interaction with acoustical phonons via macroscopic deformation potential, also called the ripple mechanism. The relaxation of spin states is possible considering that the applied magnetic field and the coupling between valence and conduction bands produce admixture, both in the electrons as well as in the hole spin states. We have demonstrated that the inclusion of spin states mixture produces an appreciable probability of spin-preservingand spin-flip transitions. The electronic states are calculated within the multiband k·p approximation and the phonon modes are described as plane-waves. We have studied the dependence of the transition rates with the magnetic field and with the dot size. Our results show a fair compatibility with available experimental data.

Exciton spin manipulation inInAs∕GaAsquantum dots: Exchange interaction and magnetic field effects

We report quantum beat phenomena between different polarization states in InAs quantum dots, observed in time-resolved photoluminescence and transient dichroism measurements. The polarization of the emitted light can be changed from linear to circular by applying a small magnetic field. Following quasiresonant excitation the electron and hole spin states remain stable during the exciton lifetime, independent of the applied magnetic field. We show that the exciton spin coherence is partially preserved in the energy relaxation process. These experiments enable us to measure the anisotropic exchange energy $\ensuremath{\Delta}{E}_{XY}$ and the exciton $g$ factor. We find $\ensuremath{\Delta}{E}_{XY}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ and $g\ensuremath{\simeq}2.5$ for the quantum dot excitons.

MAGNETO-PHOTOLUMINESCENCE STUDY AT A FRACTIONAL QUANTUM HALL REGIME OF CHARGED EXCITONS IN A DILUTE MAGNETIC SEMICONDUCTOR

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1-x Mn x Te / Cd 1-y Mg y Te single quantum wells. In low magnetic fields (the electrons filling factor ν>1), we observed several photoluminescence lines associated with spin-singlet and spin-triplet charged excitons. In high magnetic fields (ν<1), we observed only one of the spin-triplet charged excitons that are called as a bright type due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells, where the photoluminescence intensity increased at a fractional value of [Formula: see text]. We consider that the screening effect plays a crucial role for this behavior in high magnetic fields.

Spin sensitive dynamics of a charged exciton in magnetic fields

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1− x Mn x Te/Cd 1− y Mg y Te single quantum wells. We observed several transitions arising from spin-singlet and spin-triplet charged excitons. The transient photoluminescence data showed characteristic decay time depending on the spin configuration of the charged excitons. The spin-triplet charged excitons of a bright type are known to realize due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells. We have also found that the decay-time of charged excitons depends much on photon energies used for excitation.

Photoluminescence of small-size semiconductor quantum dots

The results of photoexcited carrier dynamics and optically detected microwave resonance (ODMR) spectroscopy of smallsize InAs/GaAs quantum dots (QDs) are presented. Very long lifetimes (1–10 ns) of photoexcited carriers were observed in this system by time-resolved photoluminescence measurements. This feature could be exploited to develop high-sensitivity infrared photodetectors based on small-size quantum dots. The ODMR spectra of InAs/GaAs QDs were detected via modulation of the total intensity of the QDs emission induced by 95 GHz microwave excitation, and the exciton fine structure was studied. The microwave-induced signal at magnetic field of 1.6 T was attributed to magnetic resonance transitions between spin states of holes confined in the dots. The obtained low field (at 0.18 T) signal is ascribed to cyclotron resonance of the electron in a two-dimensional wetting layer, corresponding to an effective electron mass of 0.053m0.

Spin-Dependent Carrier Confinements and Radiative Recombinations in Zn1-xMnxTe/Zn1-yMgyTe Spin Superlattices

The properties of carrier confinement in Zn 0.950 Mn 0.050 Te/Zn 0.965 Mg 0.035 Te spin superlattices have been studied by transient photoluminescence in magnetic field at various temperatures. The polarization characteristics of asymmetric Zeeman splitting, the increased photoluminescence intensity and the enhanced exciton lifetime are assigned to the evidence of spin superlattices. The type-I optical transition for the electron and hole spin states is realized by the formation of spin superlattices due to the magnetic-field-induced confinement of the band potential. The radiative recombination process dominates at 4.2-30 K in the present spin superlattice structure.

Spin-splitting renormalization in the neutral dense magnetoplasma in InxGa1-xAs/InP quantum wells.

Photoluminescence spectra of the neutral dense (${\mathit{n}}_{\mathrm{eh}}$2\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) magnetoplasma photoexcited in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/InP single quantum wells have been investigated in ${\mathrm{\ensuremath{\sigma}}}^{+}$ and ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{\ensuremath{-}}}$ polarizations. Fine structure of Landau-level transitions has been observed which is caused by the mixture of hole-spin states. A considerable enhancement of electron and hole spin splittings of the zero and first Landau levels arises when the uppermost occupied Landau levels with different spins are unequally filled. An explanation is offered in terms of the exchange inter-Landau-level interactions.

Electron and hole spin lifetimes in Zn1−xMnxSe and Zn1−yFeySe based heterostructures

We have studied carrier spin relaxation in diluted magnetic semiconductor (DMS) epilayer and spin superlattice samples using polarization and magnetic field dependent photoluminescence. The giant Zeeman splitting of the DMS materials permits unambiguous identification of electron and hole excitonic spin relaxation times. We find that the heavy hole spin lifetime τhs is comparable to the radiative recombination time τr, and substantially longer than the electron spin lifetime τes, with τhs/τr≊4 and τes/ τr≊0.06. We attribute these long spin lifetimes to the strain induced splitting of the bulk degenerate light and heavy hole bands, which inhibits mixing of the hole spin states and subsequent heavy hole spin flip relaxation processes. In the regime of weak confinement, the spin relaxation times are independent of the degree of quantum confinement, the number of interfaces, the magnetic ion concentration, and the magnetic species. We conclude that spin flip via exchange scattering between the carriers and magnetic ions is not the dominant excitonic spin relaxation mechanism.

Long hole spin relaxation times in diluted magnetic semiconductor heterostructures

We report the observation of long spin lifetimes for heavy holes in (Zn,Mn)Se and (Zn,Fe)Se based heterostructures. These long spin relaxation times are observed in both simple strained epilayers, as well as in complex spin superlattice structures in which the spin up and spin down carriers occupy alternating layers of the superlattice. In photoluminescence spectra, both the higher energy (+1/2, +3/2) and lower energy (−1/2, −3/2)heavy hole exciton interband transitions are observed, even though it would be energetically favorable for the spin up carriers to first relax to the spin down state before radiative recombination. From the magnetic field dependence of the intensity ratio of these components and a rate equation model, we find that the heavy hole spin lifetime τhs is substantially longer than the electron spin lifetime τes, with τhs /τr ≈ 4 and τes /τr ≈ 0.06, where τr is the radiative recombination time. This is attributed to the strain induced splitting of the heavy and light hole bands, which inhibits mixing of the hole spin states and subsequent dipole-allowed transitions producing fast spin relaxation.

Spin superlattice formation in ZnSe/Zn1-xMnxSe multilayers.

We present magneto-optical evidence for the formation of a magnetic-field-induced spin superlattice in modulated ZnSe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se structures. In the samples studied, the offsets in both the conduction band and the valence band are very small at zero magnetic field. When a magnetic field is applied, the large Zeeman splitting of the ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se band edges overcomes the zero-field offsets and results in the formation of a spin superlattice in which spin states of both electrons and holes are spatially and periodically separated.