Quantum measurement of an electron in a disordered potential: Delocalization versus measurement voltages

Quantum point contact (QPC), one of the typical mesoscopic transport devices, has been suggested to be an efficient detector for quantum measurement. In the context of two-state charge qubit, our previous studies showed that the QPC's measurement back-action cannot be described by the conventional Lindblad quantum master equation. In this work, we study the measurement problem of a multistate system, say, an electron in disordered potential, subject to the quantum measurement of the mesoscopic detector QPC. The effect of measurement back-action and the detector's readout current are analyzed, where particular attention is focused on some new features and the underlying physics associated with the measurement-induced delocalization versus the measurement voltages.

Bloch oscillation under a bichromatic laser: Dynamical delocalization and localization, persistent terahertz emission, and harmonics generation

A novel AC driving configuration is proposed for biased semiconductor superlattices, in which the THz driving is provided by an intense bichromatic cw laser in the visible light range. The frequency difference between two components of the laser is resonant with the Bloch oscillation. Thus, multi-photon processes mediated by the conduction (valence) band states lead to dynamical delocalization and localization of the valence (conduction) electrons, and to the formation and collapse of quasi-minibands. Thus, driven Bloch oscillators are predicted to generate persistent THz emission and harmonics of the dipole field, which are tolerant of the exciton and the relaxation effects.

Exciton Localization and Delocalization in GaNAs/GaAs Quantum Wells

We have studied exciton localization and delocalization effect in GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) and timeresolved PL measurements. Studied results suggest that, at low temperature and under a conventional CW excitation, measured PL spectra were dominated by localized exciton (LE) emission caused by potential fluctuations in GaNAs layer. However, under short pulse laser excitation, it is different. An extra high-energy PL peak comes out from GaNAs/GaAs QWs and dominates the PL spectra under high excitation and/or at high temperature. By investigation, we have attributed the new PL peak to the recombination of delocalized excitons in QWs. This recombination process competes with the localized exciton emission, which, we believe, constitutes the "S-shaped" temperature-dependent emission shift often reported in ternary nitrides of InGaN and AlGaN in the literature.

First-principles LDA plus U and GGA plus U study of neptunium dioxide

We have performed a systematic first-principles investigation to calculate the electronic structures, mechanical properties, and phonon-dispersion curves of NpO2. The local-density approximation+U and the generalized gradient approximation+U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Np 5f electrons. By choosing the Hubbard U parameter around 4 eV, the orbital occupancy characters of Np 5f and O 2p are in good agreement with recent experiments [A. Seibert, T. Gouder, and F. Huber, J. Nucl. Mater. 389, 470 (2009)]. Comparing to our previous study of ThO2, we note that stronger covalency exists in NpO2 due to the more localization behavior of 5f electrons of Np in line with the localization-delocalization trend exhibited by the actinides series.

Zn2SiO4/ZnO Core/Shell Coaxial Heterostructure Nanobelts Formed by an Epitaxial Growth

Si-doped ZnO can be synthesized on the surface of the early grown Zn2SiO4 nanostructures and form core/ shell coaxial heterostructure nanobelts with an epitaxial orientation relationship. A parallel interface with a periodicity array of edge dislocations and an inclined interface without dislocations can be formed. The visible green emission is predominant in PL spectra due to carrier localization by high density of deep traps from complexes of impurities and defects. Due to band tail localization induced by composition and defect fluctuation, and high density of free-carriers donated by doping, especially the further dissociation of excitons into free-carriers at high excitation intensity, the near-band-edge emission is dominated by the transition of free-electrons to free-holes, and furthermore, exhibits a significant excitation power-dependent red-shift characteristic. Due to the structure relaxation and the thermalization effects, carrier delocalization takes place in deep traps with increasing excitation density. As a result, the green emission passes through a maximum at 0.25I(0) excitation intensity, and the ratio of the violet to green emission increases monotonously as the excitation laser power density increases. The violet and green emission of ZnO nanostructures can be well tuned by a moderate doping and a variation in the excitation density.

Optical properties and exciton localization in GaNas/GaAs

GaNAs/GaAs single quantum wells (SQWs) and dilute GaNAs bulk grown by molecular beam epitaxy(MBE) were studied by photoluminescence (PL), selectively-excited PL, and time-resolved PL. Exciton localization and delocalization were investigated in detail. Under short pulse laser excitation, the delocalization exciton emission was revealed in GaNAs/GaAs SQWs. It exhibits quite different optical properties from N-related localized states. In dilute GaNAs bulk, a transition of alloy band related recombination was observed by measuring the PL dependence on temperature and excitation intensity and time-resolved PL, as well. This alloy-related transition presents intrinsic optical properties. These results are very important for realizing the abnomal features of III-V-N semiconductors.

Quantum measurement of single electron state by a quantum point contact

Closely related to the quantum information processing in solid states, we study the quantum measurement of single electron state by a mesoscopic charge-sensitive detector, namely the quantum point contact (QPC). We find that the conventional Lindblad-type master equation is not appropriate for describing the underlying measurement dynamics. The treatment developed in this work properly accounts for the energy-exchange between the detector and the measured system, and its role on the detailed-balance relation. A valid description for the QPC measurement dynamics is provided which may have impact on the study of quantum measurement and quantum feedback control in solid states.

Polarity determination for GaN thin films by electron energy-loss spectroscopy

The intensity of the N K edge in electron energy-loss spectra from a GaN thin film shows a pronounced difference when the orientation of the film approaches the (0002) and (000-2) Bragg reflections, along the polar direction. This experimental result can be interpreted by the effect associated with interference between the Bloch waves of the incident electron in the GaN crystal. The theoretical calculations indicate that, at the Bragg condition of g=0002 along the Ga-N bond direction, the thickness-averaged electron current density on the N atom plane is much higher than that at g=000 (2) over bar, with a maximum as the specimen thickness is about 0.4xi(0002) (the two-beam extinction distance). The delocalization effect on the experimental spectra is also discussed. (C) 2002 American Institute of Physics.

Degenerate four-wave-mixing signals from dc- and ac-driven semiconductor superlattice

Within the one-dimensional tight-binding model;rnd chi-3 approximation, we have calculated four-wave-mixing (FWM) signals for a semiconductor superlattice in the presence of both static and high-frequency electric fields. When the exciton effect is negligible, the time-periodic field dynamically delocalizes the otherwise localized Wannier-Stark states, and accordingly quasienergy band structures are formed, and manifest in the FWM spectra as a series of equally separated continua. The width of each continuum is proportional to the joint width of the valence and conduction minibands and is independent of the Wannier-Stark index. The realistic homogeneous broadening blurs the continua into broad peaks, whose line shapes, far from the Lorentzian, vary with the delay time in the FWM spectra. The swinging range of the peaks is just the quasienergy bandwidth. The dynamical delocalization (DDL) also induces significant FWM signals well beyond the excitation energy window. When the Coulomb interaction is taken into account, the unequal spacing between the excitonic Wannier-Stark levels weakens the DDL effect, and the FWM spectrum is transformed into groups of discrete lines. Strikingly, the groups are evenly spaced by the ac field frequency, reflecting the characteristic of the quasienergy states. The homogeneous broadening again smears out the line structures, leading to the excitonic FWM spectra quite similar to those without the exciton effect. However, all these features predicted by the dynamical theory do not appear in a recent experiment [Phys. Rev. Lett. 79, 301 (1997)], in which, by using the static approximation the observed Wannier-Stark ladder with delay-time-dependent spacing in the FWM spectra is attributed to a temporally periodic dipole field, produced by the Bloch oscillation of electrons in real space. The contradiction between the dynamical theory and the experiments is discussed. In addition, our calculation indicates that the dynamical localization coherently enhances the time-integrated FWM signals. The feasibility of using such a technique to study the dynamical localization phenomena is shown. [S0163-1829(99)10607-6].

ELECTRONIC-STRUCTURES OF THE ALKALI-CONTAINING BUCKMINSTERFULLERENES (A-DELTA-C60) (A=LI,NA,K,RB,CS) AND THE HALOGEN-CONTAINING BUCKMINSTERFULLERENES (H-DELTA-C60) (H=F,CL,BR,I)

The geometrical parameters and electronic structures of C60, (A partial derivative C60) (A = Li, Na, K, Rb, Cs) and (H partial derivative C60) (H = F, Cl, Br, I) have been calculated by the EHMO/ASED (atom superposition and electron delocalization) method. When putting a central atom into the C60 cage, the frontier and subfrontier orbitals of (A partial derivative C60) (A = Li, Na, K, Rb, Cs) and (H partial derivative C60) (H = F, Cl) relative to those of C60 undergo little change and thus, from the viewpoint of charge transfer, A (A = Li, Na, K, Rb, Cs) and H (H = F, Cl) are simply electron donors and acceptors for the C60 cage resPeCtively. Br is an electron acceptor but it does influence the frontier and subfrontier MOs for the C60 cage, and although there is no charge transfer between I and the C60 cage, the frontier and subfrontier MOs for the C60 cage are obviously influenced by I. The stabilities DELTAE(X) (DELTAE(X) = (E(X) + E(C60)) - E(x partial derivative C60)) follow the sequence I < Br < None < Cl < F < Li < Na < K < Rb < Cs while the cage radii r follow the inverse sequence. The stability order and the cage radii order have been explained by means of the (exp-6-1) potential.

Syntheses and Crystal Structures of Fused Thiophenes: [7]Helicene and Double Helicene, a D-2-Symmetric Dimer of 3,3 '-Bis(dithieno[2,3-b:3 ',2 '-d]thiophene)

The efficient synthesis of (TMS)(2)-[7]helicene (rac-3) and double helicene, a D-2-symmetric dimer of 3,3'-bis(dithieno-[2,3-b:3',2'-d]thiophene) (rac-4) was developed. The crystal structures of 3 and 4 show both strong intermolecular pi-pi interactions and S center dot center dot center dot S interactions. UV/vis spectra reveal that both 3 and 4 show significant pi-electron delocalization.

Magnetic and electronic properties of CaCu3Cr4O12 and CaCu3Cr2Sb2O12 by first-principles density functional calculation

The electronic and magnetic properties of CaCu3Cr4O12 and CaCu3Cr2Sb2O12 are investigated by the use of the full-potential linearized augumented plane wave (FPLAPW) method. The calculated results indicate that CaCu3- Cr4O12 is a ferrimagnetic and half-metallic compound, in good agreement with previous theoretical studies. CaCu3- Cr2Sb2O12 is a ferrimagnetic semiconductor with a small gap of 0.136 eV. In both compounds, because Cr4+ 3d (d(2)) and Cr3+ 3d (d(3)) orbitals are less than half filled, the coupling between Cr-Cu is antiferromagnetic, whereas that between Cu-Cu and Cr-Cr is ferromagnetic. The total net spin moment is 5.0 and 3.0 mu(B) for CaCu3Cr4O12 and CaCu3Cr2Sb2O12, respectively. In CaCu3Cr4O12, the 3d electrons of Cr4+ are delocalized, which strengthens the Cr-Cr ferromagnetic coupling. For CaCu3Cr2Sb2O12, the doping of nonmagnetic ion Sb5+ reduces the Cr-Cr ferromagnetic coupling, and the half-filled Cr3+ t(2g) (t(2g)(3)) makes the chromium 3d electrons localized. In addition, the ordering arrangement of the octahedral chromium and antimony ions also prevents the delocalization of electrons. Hence, CaCu3Cr2Sb2O12 shows insulating behavior, in agreement with the experimental observation.

Study on the difference of photochromic and thermochromic mechanism of ethylenediamine trimolybdate

Ethylenediamine trimolybdate ((C2H10N2)[Mo3O10], denoted as ENTMo) shows unusual photochromic and thermochromic properties. The color of the white ENTMo compounds becomes reddish brown gradually under UV irradiation, and changes gradually to blue-black upon annealing. XRD patterns and FT-IR spectra verify that the crystal structure of the colored samples is almost unchanged except distortion. UV-vis diffuse reflectance spectra (DRS) and ESR spectra of the photochromic and the thermochromic samples could confirm that there must exist difference between thermochromic and photochromic mechanism.

Titanium and zirconium complexes with aminoiminophosphorane ligands

A series of titanium and zirconium complexes based on aminoiminophosphorane ligands [Ph2P(Nt-Bu)(NR)](2)MCl2 (4, M = Ti, R = Ph; 5, M = Zr, R = Ph; 6, M = Ti, R = SiMe3; 7, M = Zr, R = SiMe3) have been synthesized by the reaction of the ligands with TiCl4 and ZrCl4. The structure of complex 4 has been determined by X-ray crystallography. The observed very weak interaction between Ti and P suggests partial pi-electron delocalization through both Ti and P. The complexes 4-7 are inactive for ethylene polymerization in the presence of modified methylaluminoxane (MMAO) or i-Bu3Al-Ph3CB(C6F5)(4) under atmospheric pressure, and is probably the result of low monomer ethylene concentration and steric congestion around the central metal.

ESR STUDY OF SUPERHYPERFINE INTERACTION OF I-127 IN [CU(IO5OH)(2)](5-) ION COMPLEX

ESR method has been used to study superhyperfine. interaction of I-127 in [Cu (IO5OH)(2)](5-) ion for Na4KCu(IO5OH)(2) . 12H(2)O single crystal. The main purpose of this paper is to confirm the existence of unpaired electron spin on iodine atom and to find a reasonable explanation for the spin delocalization of CU2+ ions. Based on the ESR parameters of paramagnetic [Cu(IO5OH)(2)](5-) ions, the calculated results show that about 0.77% of the unpaired eletron spin is located on each iodine atom.