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,...

Hybrid structures synthesized from di erent materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an e ective p-d exchange that is mediated by elliptically polarized phonons. E xchange interactions are the origin for correlated magnetism in condensed matter with multi-faceted behaviour such as ferro-, antiferro-or ferrimagnetism. In magnetic semiconductors (SCs), the exchange occurs between free charge carriers and localized magnetic atoms 1-4 and is determined by their wavefunction overlap. To control this overlap, hybrid structures consisting of a ferromagnetic (FM) layer and a semiconductor quantum well (QW) are appealing objects because they allow wavefunction engineering. Furthermore, the mobility of QW carriers will not be reduced by inclusion of magnetic ions in the same spatial region in these systems.More specifically, for a two-dimensional hole gas (2DHG, the p-system) in a QW the overlap of the hole wavefunction with the magnetic atoms in a nearby ferromagnetic layer (the d-system) is believed to result in p-d exchange interaction 5-8 . This exchange interaction may cause strong coupling between the SC and FM spin systems 9 , through which the ferromagnetism of the unified system, as evidenced by its hysteresis loop, can be tuned. In particular, the 2DHG spin system becomes polarized in the effective magnetic field from the p-d exchange 5,8 . Recently 10 , it was shown that in addition to this equilibrium 2DHG polarization there is an alternative mechanism involving spin-dependent capture of carriers from the SC into the FM. For ferromagnetic (Ga,Mn)As on top of an (In,Ga)As QW, electron capture induces electron spin polarization in the QW, representing a dynamical effect in contrast to the exchange-induced equilibrium polarization.Here we study a different FM/QW hybrid, consisting of a Co layer and a CdTe II-VI semiconductor QW, separated by a nanometrethick barrier. Owing to the negligible hole tunnelling through the barrier, this hybrid combination shows mostly a quasi-equilibrium proximity effect due to p-d exchange interaction between magnetic atoms and holes in the QW. Surprisingly, howev...

We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI3 perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2 K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120 µeV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals.

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...