Optically probing the fine structure of a single Mn atom in an InAs quantum dot

We report on the optical spectroscopy of a single InAs/GaAs quantum dot (QD) doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A^0 whose effective spin J=1 is significantly perturbed by the QD potential and its associated strain field. The spin interaction with photo-carriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of micro-eV, but vanishingly small for electrons.Comment: 5 pages, 3 figure

Remarks on hard Lefschetz conjectures on Chow groups

We propose two conjectures of Hard Lefschetz type on Chow groups and prove them for some special cases. For abelian varieties, we shall show they are equivalent to well-known conjectures of Beauville and Murre.Comment: to appear in Sciences in China, Ser. A Mathematic

IMMUNE STATUS OF MICE TOLERANT OF LIVING CELLS : II. CONTINUOUS PRESENCE AND NATURE OF FACILITATION-ENHANCING ANTIBODIES IN TOLERANT ANIMALS

CBA mice were rendered highly tolerant to A/Jax cells by neonatal intravenous injections of (CBA x A)F1 spleen cells. The high degree of tolerance was ascertained by the absence of circulating antibodies detected in the sera by the usual tests and by the perfect state of A skin grafts during all the experiments. Tolerant sera (sera from tolerant animals) were studied at three periods of tolerance: before skin test grafting, from 2 to 11 wk after grafting, and at time of sacrifice at almost 6 months of age. The tolerant sera were shown to have specific facilitation-enhancing properties promoting the take and growth of A/Jax sarcoma (SaI and /Sa 15091a grafted on normal CBA mice. These properties were present throughout the duration of the experiments, showing that they were not the result of a beginning interruption of tolerance. The tolerant sera, although lacking the usual serological properties (hemagglutination, hemolysis, cytotoxicity, passive cutaneous anaphylaxis) had, however, specific synergistic hemagglutinating properties (increasing the hemagglutinating titer of a reference immune serum). Antibodies giving direct specific hemagglutination could be extracted from spleens of 20% of highly tolerant mice. The tolerant sera were also found to contain more IgG1 and more IgA than normal sera while they contained normal quantities of the complement-fixing immunoglobulins IgG2 and IgM. Fractionation of tolerant sera on DEAE chromatography column confirmed the data concerning immunoglobulin classes and demonstrated direct specific serological activities undetected in unfractionated sera: a weak hemolysis in the most cationic fractions and a weak hemagglutination in the middle fractions. Synergistic hemagglutination, detected in unfractionated serum, was localized in fast anionic fractions containing high IgA concentration, along with facilitation-enhancing activity, thus confirming a link suggested previously between these three properties. The relation between immunological tolerance and facilitating antibodies was discussed in the light of the fact that antibodies, possibly of a particular class continuously present at low dose in the sera of highly tolerant animals, are able to transfer (at least partly) this state of tolerance provided a sensitive test system is utilized

Valley filtering and spatial maps of coupling between silicon donors and quantum dots

Exchange coupling is a key ingredient for spin-based quantum technologies since it can be used to entangle spin qubits and create logical spin qubits. However, the influence of the electronic valley degree of freedom in silicon on exchange interactions is presently the subject of important open questions. Here we investigate the influence of valleys on exchange in a coupled donor/quantum dot system, a basic building block of recently proposed schemes for robust quantum information processing. Using a scanning tunneling microscope tip to position the quantum dot with sub-nm precision, we find a near monotonic exchange characteristic where lattice-aperiodic modulations associated with valley degrees of freedom comprise less than 2~\% of exchange. From this we conclude that intravalley tunneling processes that preserve the donor's $\pm x$ and $\pm y$ valley index are filtered out of the interaction with the $\pm z$ valley quantum dot, and that the $\pm x$ and $\pm y$ intervalley processes where the electron valley index changes are weak. Complemented by tight-binding calculations of exchange versus donor depth, the demonstrated electrostatic tunability of donor/QD exchange can be used to compensate the remaining intravalley $\pm z$ oscillations to realise uniform interactions in an array of highly coherent donor spins.Comment: 6 pages, 4 figures, 6 pages Supplemental Materia

Giant optical anisotropy in a single InAs quantum dot in a very dilute quantum-dot ensemble

We present the experimental evidence of giant optical anisotropy in single InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals a linear polarization ratio with huge fluctuations, from one quantum dot to another, in sign and in magnitude with absolute values up to 82%. Systematic measurements on hundreds of quantum dots coming from two different laboratories demonstrate that the giant optical anisotropy is an intrinsic feature of dilute quantum-dot arrays.Comment: submitted to Applied Physics Letter

Measurement of miniband parameters of a doped superlattice by photoluminescence in high magnetic fields

We have studied a 50/50\AA superlattice of GaAs/Al$_{0.21}$Ga$_{0.79}$As composition, modulation-doped with Si, to produce $n=1.4\times 10^{12}$ cm$^{-2}$ electrons per superlattice period. The modulation-doping was tailored to avoid the formation of Tamm states, and photoluminescence due to interband transitions from extended superlattice states was detected. By studying the effects of a quantizing magnetic field on the superlattice photoluminescence, the miniband energy width, the reduced effective mass of the electron-hole pair, and the band gap renormalization could be deduced.Comment: minor typing errors (minus sign in eq. (5)

A Coboundary Morphism For The Grothendieck Spectral Sequence

Given an abelian category $\mathcal{A}$ with enough injectives we show that a short exact sequence of chain complexes of objects in $\mathcal{A}$ gives rise to a short exact sequence of Cartan-Eilenberg resolutions. Using this we construct coboundary morphisms between Grothendieck spectral sequences associated to objects in a short exact sequence. We show that the coboundary preserves the filtrations associated with the spectral sequences and give an application of these result to filtrations in sheaf cohomology.Comment: 18 page

Webs of Lagrangian Tori in Projective Symplectic Manifolds

For a Lagrangian torus A in a simply-connected projective symplectic manifold M, we prove that M has a hypersurface disjoint from a deformation of A. This implies that a Lagrangian torus in a compact hyperk\"ahler manifold is a fiber of an almost holomorphic Lagrangian fibration, giving an affirmative answer to a question of Beauville's. Our proof employs two different tools: the theory of action-angle variables for algebraically completely integrable Hamiltonian systems and Wielandt's theory of subnormal subgroups.Comment: 18 pages, minor latex problem fixe