Growth Mechanism in Atomic Layer Epitaxy (I) Re-evaporation of Cd and Te from CdTe(111) Surfaces Monitored by Auger Electron Spectroscopy

The mechanism of atomic layer epitaxy (ALE) of cadmium telluride has been studied. Auger electron spectroscopy is used to measure the isothermal re-evaporation rates of elemental Cd and Te deposits on the (111)A and (111)B surfaces of CdTe substrates. The results include an observation that the sticking coefficients of Cd and Te are smaller than unity at the growth temperatures typical of CdTe ALE. After desorption the substrates are left partially covered: 35% by a Cd overlayer on the (111)B surface and 72% by Te on the (111)A surface. The re-evaporation rates of Cd and Te experience a drastic change near the substrate-deposit interface. These rates appear two orders of magnitude smaller than those of bulk-like amorphous Cd and Te solids. The activation energies for reevaporation of the near-interface layer region are estimated to be: 1.5 eV for Te on the (111)A face, 1.0 eV for Te on (111)B and 0.5 eV for Cd on (111)B. It has also been shown that AES can be used to identify the polarity of the CdTe(111) surfaces. The relative difference in peak-to-peak intensity ratios of Cd MNN to Te MNN for (111)A and (111)B is (11 ± 2)%.     

Focusing effect of a graded index photonic crystal lens

We present a new graded-index photonic crystal (GRIN PhC) structure which can be made by laser interference lithography (LIL). It can be applied as miniaturized focusing lens. Numerical simulations are carried out on the structures with different widths, lengths, and modulations. In the qualitative analysis of the focal length, focusing of a GRIN PhC was proved to originate from both the graded sizes of the rods and multimode interference. Multimode interference dominates the focusing effect when a GRIN PhC has a small length or a small difference in the rod sizes. Intensity distributions in different cases were given to support the conclusion.     

Interpretation of a new feature in photoemission from Cu based alloys with polyvalent solutes

In our recent angle resolved photoemission studies of oriented single crystal surface of $\text{Cu}$Al and $\text{Cu}$Ge solid solutions we had found a new structure lying between 4 and 5 eV below the Fermi energy, which could not be related to either the bulk or the surface states in the random alloy. To understand its nature and origin we report and discuss photoemission measurements from Cu films, submonolayer to a monolayer thick, deposited on the Al(111) and $\text{Cu}\text{Al(111)(}\text{3}\text{×}\text{3}\text{R}\text{30°)}$ alloy surfaces and from Al and Zn alloys containing dilute Cu impurities. Some experiments on the (100) surfaces in the various cases were also carried out. On the basis of these results, we suggest that the aforementioned new photoemission feature is characteristic of Cu clusters and isolated Cu impurities in polyvalent impurity-rich environment. These clusters probably lie at the topmost layer of the $\text{Cu}$Al and $\text{Cu}$Ge alloy surfaces.     

UV-photoemission at constant wavevector component k6: Application to copper

Angle-resolved energy distribution curves of photoelectrons obtained at constant wavevector component k₆ have been presented for the surface of Cu(111). The main observed transitions can be interpreted as due to direct interband transitions between the APW initial states and free-electron-like final states which are calculated simply in one-dimension along k_⊥ lines parallel to the surface normal.     

The vibrational spectra of the cyanide ligand revisited: the ν(CN) infrared and Raman spectroscopy of Prussian blue and its analogues

A careful analysis of the Raman spectra of the M′_x[M(CN)₆]_y Prussian blue species has enabled a general model for the interpretation of the ν(CN) vibrational spectra. The spectral patterns are derived from those of the metal ions with local O_h symmetry. Two limiting models are discussed. A ‘localized mode’ model, involving matrix‐isolated species, is in much better accord with the observations than a ‘factor group’ model. The use of the infrared feature as fingerprint of specific M CN M′ units arises from the axis‐specific nature of individual T_1u modes. The interpretation of the A_1g and E_g Raman features is done in terms of localized vibrations, with involvement of additional energy terms from the lattice motions. Copyright © 2011 John Wiley & Sons, Ltd.     

From SU(2) Gauge Theory to Qubits on the Fuzzy Sphere

We consider a classical pure SU(2) gauge theory, and make an ansatz, which separates the spatio-temporal degrees of freedom from the internal ones. This ansatz is gauge-invariant but not Lorentz invariant. In a limit case of the ansatz, obtained through a contraction map, and corresponding to a vacuum solution, the SU(2) gauge field reduces to an operator, which is the product of the generator of a global U(1) group times a Pauli matrix. We give a geometrical interpretation of the ansatz and of the contraction map in the framework of principal fiber bundles. Then, we identify the internal degrees of freedom of the gauge field with the non-commutative coordinates of the fuzzy sphere in the fundamental representation. In this way we obtain a qubit state.     

Emission studies of InGaN layers and LEDs grown by plasma-assisted MBE

We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S, shows that there is a strong tendency of GaN to form a separate phase as T_S is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 μW at 60 mA drive current.     

Growth of strain-compensated GaInNAs/GaAsP quantum wells for 1.3 μm lasers

Strain-compensated GaInNAs/GaAsP quantum well structures and lasers were grown by gas source molecular beam epitaxy using a RF-plasma nitrogen radical beam source. The optimal growth condition for the quantum well structure was determined based on room-temperature photoluminescence measurements. Effects of rapid thermal annealing (RTA) on the optical properties of GaInNAs/GaAsP quantum well structures as well as laser diodes are examined. It was found to significantly increase the photoluminescence from the quantum wells and reduce the threshold current density of the lasers, due to a removal of N induced nonradiative centers from GaInNAs wells.     

Synthesis and Structure of New Cobalt–Carbonyl Complexes Containing Tellurium Atoms

Co₂(CO)₈ and Te₂O react to form the well known Co₄(CO)₁₀Te₂, Co₄(CO)₁₁Te₂ complexes and the two new cluster complexes CCo₆(CO)₁₂Te₂(1), and CCo₆(CO)₁₀Te₂(Te₃) (2). The structures of 1 and 2 were determined by X-ray analysis, together with the triphenylphosphine derivative of 1, CCo₆(CO)₁₁(PPh₃)Te₂(3), which was analyzed to clarify the disordered structure of the parent compound. Complex 1 is formed by a prismatic cluster of cobalt atoms with a carbon embedded in the cage; two tellurium atoms cap the triangular faces of the prism and each cobalt atom links two terminal carbonyl groups. The complex 2 has a similar prismatic cage CCo₆; two ₄-Te atoms cap two rectangular faces of the prism, while other two Te atoms bridge two edges of the triangular faces and are linked each other through a third Te atom. Electron counting gives for complex 2 92 electrons: the presence of two long Co–Co distances suggests that the two excess electrons are located on Co–Co antibonding orbitals. Crystal data for 1, space group C2/c, a = 12.845(2) Å, b = 13.449(2) Å, c = 13.246(2) Å, = 91.95(2)°, Z = 4, R = 0.097 for 2555 reflections; for 2, space group Pnna, a = 17.219(5) Å, b= 14.969(6) Å, c = 9.178(4) Å, Z = 4,R = 0.037 for 3103 reflections; for 3, space group P2₁/c, a = 9.288(2) Å, b = 14.920(6) Å, c = 26.300(9) Å, = 99.99(2)°, Z = 4, R = 0.037 for 4300 reflections. The vibrational analysis of the complex 1 was performed and most of the (CO), (₆C–Co), (Co–Co) and (Co–Co) modes were assigned. The (Co–Te) modes were interpreted on the basis of the intermolecular coupling, due to the close contact between neighboring clusters in one distinct direction in the crystal.     

Quantum dissipation and neural net dynamics.

Inspired by the dissipative quantum model of brain, we model the states of neural nets in terms of collective modes by the help of the formalism of Quantum Field Theory. We exhibit an explicit neural net model which allows to memorize a sequence of several informations without reciprocal destructive interference, namely we solve the overprinting problem in such a way last registered information does not destroy the ones previously registered. Moreover, the net is able to recall not only the last registered information in the sequence, but also anyone of those previously registered.