A system of two coupled CdTe quantum dots, one of them containing a single Mn ion, was studied in continuous wave and modulated photoluminescence, photoluminescence excitation, and photon correlation experiments. Optical writing of information in the spin state of the Mn ion has been demonstrated, using orientation of the Mn spin by spin-polarized carriers transferred from the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns were measured, depending on the excitation power. Storage time of the information in the Mn spin was found to be enhanced by application of a static magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple rate equation models were found to describe correctly static and dynamical properties of the system.One of important research directions that may influence the future of information processing, especially of spintronics [1], is focused on physical phenomena occurring in nanoscale-size quantum objects. One of such objects, close to the ultimate limit of information storage miniaturization, is a single Mn atom in a semiconductor quantum dot (QD) [2,3]. After intensive studies of semimagnetic QD containing many magnetic ions [4,5,6,7], single Mn atoms in CdTe [8] and InAs [9] QDs have been observed in photoluminescence (PL) experiments. Many experiments supplied substantial knowledge on physical properties of single Mn atoms, especially in CdTe QDs. In particular, they revealed a strong influence of the position of the Mn atom in the QD, reflecting the symmetry of the system [10]. They demonstrated an efficient optical read-out of the Mn spin state [8]. Furthermore, the dynamics of this state has been studied in photon correlation experiments [11], revealing an important influence of photo-created carriers on Mn spin relaxation. The writing and storing of the information in the Mn spin state has received less attention so far. These issues represent the focus of the present work.In particular, we demonstrate optical writing of information in the spin state of a single Mn ion and we test the stability of this state in the time range up to 0.2 ms.CdTe QDs containing single Mn ions were grown by molecular beam epitaxy. A single layer of self-assembled QDs was deposited in a ZnTe matrix. Manganese was added by briefly opening the Mn effusion cell during the growth of the CdTe layer [12]. The opening time and the Mn flux were adjusted to achieve a large probability of growth of QDs with a single Mn ion in each dot. The selection of single QDs was done by spatial limitation of PL excitation and detection to an area smaller than 0.5 micrometer in diameter, with microscope objective immersed in pumped liquid helium. Continuous wave excitation was used either above the ZnTe barrier gap (at 457 nm) or by a tunable dye laser in the range 570 -600 nm. Well separated photoluminescence lines from individual QDs were observed in the low energy part of the PL spectrum. We were able to select numerous lines showing a PL pattern characteristic for the presence of a...

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

The doping dependence of the superconducting condensate density, n o s , has been studied by muon-spin-rotation for Y 0. The superconducting condensate density, n s , is proportional to the squared amplitude of the superconducting (SC) order parameter (OP), i.e. of the macroscopic wave function which describes the SC charge carriers. It is thus a fundamental parameter whose variation as a function of temperature (T) and of carrier doping provides important information about the

By zero field muon spin rotation we studied the antiferromagnetic correlations in the single layer system La 22x Sr x CuO 4 and the bilayer system Y 12x Ca x Ba 2 Cu 3 O 6 . We observe a common phase diagram as a function of hole doping per plane with two distinct transitions in the magnetic ground state. The first transition marks the border between the 3D antiferromagnetic state and a disordered state with short ranged correlations. The second transition marks a distinct change in the magnetic correlations at the onset of superconductivity. [S0031-9007 (98)05990-0] PACS numbers: 74.25.Ha, 74.62.Dh, 76.75. + i

A degenerate pump-probe technique is used to investigate the non equilibrium carrier dynamics in multi-layer graphene. Two distinctly different dynamics of the carrier relaxation are observed.A fast relaxation (∼ 50 fs) of the carriers after the initial effect of phase space filling followed by a slower relaxation (∼ 4 ps) due to thermalization. Both relaxation processes are less efficient when a magnetic field is applied at low temperatures which is attributed to the suppression of the electron-electron Auger scattering due to the non equidistant Landau level spacing of the Dirac fermions in graphene.

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