Using the trion as a monitor we investigate the anisotropy of the single-hole state in epitaxial CdSe/ ZnSe quantum dots. Heavy-light hole mixing caused by a symmetry reduction below D 2d results in elliptical polarization of the optical transitions with a specific axis for each dot defined by strain and shape. In a transverse magnetic field, a quartet of strictly linearly polarized lines appears that reveals the off-diagonal coupling of both electron and hole states. Although induced by the field, the linear polarization is not related to the field orientation, but either along or perpendicular to the dot axis seen at zero field. We find an in-plane hole g factor as large as 0.3 with distinct anisotropic behavior.Semiconductor quantum dots (QDs) are often called "artificial atoms." However, crystal symmetry and specific band structure of the semiconductor make the energy eigenstates utterly different from those of the simple particle-in-a-box problem. 1,2 Detailed knowledge of the energy levels and the corresponding wave functions is of immense fundamental and practical interest. In the present work, the hole eigenstates of epitaxial Stranski-Krastanov (SK) QDs are examined in this respect. We consider the prototype case of a semiconductor with zinc-blende symmetry. Unlike colloidal nanocrystals, SK QDs exhibit a defined quantization axis ͑z͒ given by the growth direction. In the frequent case of [001] growth, the symmetry of the bulk semiconductor implies that x and y direction are equivalent ͑D 2d ͒. That equivalence is generally lifted in a QD due to shape and/or strain. The splitting of the optically allowed exciton into a line doublet caused by the electron-hole exchange interaction is a wellstudied consequence of this symmetry reduction. 3,4 The question of whether the in-plane anisotropy is also manifested on a single-particle level has not been addressed so far. As long as the confinement energy is much smaller than the band gap, the electron with its s-like Bloch function can be safely treated as an isotropic effective-mass particle with spin ±1 / 2. On the other hand, such an approximation fails even qualitatively for the p-type holes of total angular momentum ±3 / 2 connected with the fourfold-degenerate edge of the valence band. In what follows, we demonstrate that the single-hole states of a SK QD are indeed substantially affected and that the anisotropy translates into a specific coupling with a transverse magnetic field. Our experimental concept is based on the trion feature representing the fundamental optical excitation of charged QDs. 5,6 In case of a single negative resident charge, the trion consists of one hole and two electrons. As the total spin of the latter is zero in the singlet ground state, the trion represents a direct monitor of the angular momentum configuration of the hole. 7 The CdSe/ ZnSe QD samples used in this study are grown by molecular-beam epitaxy using a thermal activation procedure. 8,9 The height and diameter of the pure CdSe core are about 2 and 5 -10 nm, respectively,...

A paradoxical behavior of the linear polarization of luminescence has been observed in CdTe͞CdMnTe quantum wells. Although the polarization is induced by a magnetic field, neither the magnitude of the polarization nor the orientation of its plane vary when the field is rotated in the quantum well plane. An analysis shows that this can be accounted for by a low-symmetry perturbation of the crystal lattice that gives rise to a mixing of the valence subbands leading, in turn, to an anisotropy of the in-plane heavy-hole g factor. [S0031-9007(99)08866-3] PACS numbers: 78.66. Hf, 75.50.Pp, In diamond and zinc blende semiconductors, such as Si, GaAs, or CdTe, the valence-band states are fourfold degenerate at the Brillouin zone center G. The top of the valence band consists of the heavy-and light-hole subbands, each twofold degenerate in angular-momentum projection. The heavy-hole subband states are characterized by the angular momentum projections of 63͞2, and the light-hole subband states, by 61͞2.A biaxial strain usually present in quantum-well (QW) structures, as well as the difference between the light and heavy-hole effective masses result in a partial lifting of valence band degeneracy. Unless there is a strong tensile in-plane strain of the QW, the states which are higher in energy are those of the heavy-hole subband, and it is these states that determine properties of the recombination radiation emitted from QWs. The spin-orbit interaction results in a strong anisotropy of the Zeeman splitting of the hole states [1,2]. In an ideal QW having D 2d symmetry, only the longitudinal component of the heavy-hole gfactor tensor g zz is appreciable. The in-plane, or transverse components are determined by the Luttinger parameter q which describes cubic corrections to the spin Hamiltonian and which is small in value.Let us note that the transverse g factor is connected with the spin relaxation efficiency. A vanishing transverse g factor would suppress the most effective channels of the hole spin relaxation, contrary to numerous optical pumping experiments performed on low-dimensional systems which evidence a fairly high efficiency of this process [3]. Smallness of g Ќ in QWs made of diluted magnetic semiconductors, where the anisotropy of the hole exchange field gives rise to an anisotropic spin structure of the magnetic polaron state [4,5] and an unusual spin dynamics [6-9], has even more specific consequences.The present Letter reports observation of a strong dependence of the linear polarization of the photolumi-nescence in (001)-CdTe͞CdMnTe quantum wells on the direction of an in-plane magnetic field. We show that the anisotropy of the hole ground state g factor manifests itself not only as a difference between g zz and small g Ќ ; in fact, the transverse g factor is found to be also essentially anisotropic, g xx fi g yy . This anisotropy may range from moderate, jg xx j . jg yy j, to ultimately strong, g xx 2g yy , depending on the QW width and/or the barrier height. All the results can be understood assuming that th...

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