| 1821. |
- Suzuki, M. -T, et al.
(författare)
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First-principles theory of multipolar order in neptunium dioxide
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:24, s. 241103-
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Tidskriftsartikel (refereegranskat)abstract
- We provide a first-principles, materials-specific theory of multipolar order and superexchange in NpO2 by means of a noncollinear local-density approximation +U (LDA + U) method. Our calculations offer a precise microscopic description of the triple-q antiferro ordered phase in the absence of any dipolar moment. We find that, while the most common nondipolar degrees of freedom (e.g., electric quadrupoles and magnetic octupoles) are active in the ordered phase, both the usually neglected higher-order multipoles (electric hexadecapoles and magnetic triakontadipoles) have at least an equally significant effect.
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| 1822. |
- Suzuki, Michi-To, et al.
(författare)
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Microscopic theory of the insulating electronic ground states of the actinide dioxides AnO(2) (An= U, Np, Pu, Am, and Cm)
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:19, s. 195146-
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Tidskriftsartikel (refereegranskat)abstract
- The electronic states of the actinide dioxides AnO(2) (with An= U, Np, Pu, Am, and Cm) are investigated employing first-principles calculations within the framework of the local density approximation + U(LDA + U) approach, implemented in a full-potential linearized augmented plane-wave scheme. A systematic analysis of the An-5f states is performed which provides intuitive connections between the electronic structures and the local crystalline fields of the f states in the AnO(2) series. Particularly the mechanisms leading to the experimentally observed insulating ground states are investigated. These are found to be caused by the strong spin-orbit and Coulomb interactions of the 5f orbitals; however, as a result of the different configurations, thismechanismworks in distinctly different ways for each of the AnO(2) compounds. In agreement with experimental observations, the nonmagnetic states of plutonium and curium dioxide are computed to be insulating, whereas those of uranium, neptunium, and americium dioxides require additional symmetry breaking to reproduce the insulator ground states, a condition which is met with magnetic phase transitions. We show that the occupancy of the An-f orbitals is closely connected to each of the appearing insulating mechanisms. We furthermore investigate the detailed constitution of the noncollinear multipolar moments for transverse 3q magnetic ordered states in UO2 and longitudinal 3q high-rank multipolar ordered states in NpO2 and AmO2.
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| 1823. |
- Sychugov, Ilya, et al.
(författare)
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Exciton localization in doped Si nanocrystals from single dot spectroscopy studies
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:7, s. 075311-
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Tidskriftsartikel (refereegranskat)abstract
- The results of low-temperature photoluminescence characterization of single silicon nanocrystals prepared from highly doped silicon-on-insulatorwafers are presented. The effect of B, P, As, and Sb impurities on ensemble as well as individual emission spectra are determined by comparison with the line shapes of undoped nanocrystals. From the statistical analysis of the luminescence spectra, the donor ionization energies for nanocrystals emitting in the range of 1.5-2.0 eV are estimated to be 140-200 meV, while the exciton-impurity binding energy for As- and Sb-doped nanocrystals is found to be about 40-45 meV.
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| 1824. |
- Sychugov, Ilya, et al.
(författare)
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Luminescence blinking of a Si quantum dot in a SiO2 shell
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : The American Physical Society. - 1098-0121 .- 1550-235X. ; 71:11, s. 115331-1-115331-5
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Tidskriftsartikel (refereegranskat)abstract
- The phenomenon of on-off luminescence intermittency - blinking - in silicon nanocrystals was studied using a single-dot microphotoluminescence technique. From recordings of the luminescence intensity trace, on- and off-time distributions were extracted revealing exponential behavior, as expected for systems with blinking of a purely random nature. The corresponding switching rates for on-off and off-on processes exhibit different dependence on the excitation intensity. While the on-off switching rate grows quadratically with the excitation, the inverse process is nearly pumping power independent. Experimental findings are interpreted in terms of a dot "charging" model, where a carrier may become trapped in the surrounding matrix due to thermal and Auger-assisted processes. Observed blinking kinetics appear to be different from that of porous silicon particles.
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| 1825. |
- Sychugov, Ilya, et al.
(författare)
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Photoluminescence measurements of zero-phonon optical transitions in silicon nanocrystals
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:12, s. 125326-
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Tidskriftsartikel (refereegranskat)abstract
- Optical transitions in silicon nanocrystals with different surface passivations were probed at low temperatures on a single-particle level. A type of quasidirect recombination process, different from the quantum-confined exciton transition, is identified. The luminescence spectra have different emission energies, but the contribution of a no-phonon transition is significantly higher than expected from the quantum-confinement model. Its relative strength was found to be temperature dependent, suggesting spatial localization of excitons as a possible origin.
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| 1826. |
- Szamota-Leandersson, Karolina, et al.
(författare)
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Creation of a metallic channel at the Sn/InAs(111)B surface studied using synchrotron-radiation photoelectron spectroscopy
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:205406
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Tidskriftsartikel (refereegranskat)abstract
- The properties of a Sn-induced two-dimensional electron gas at the As-terminated InAs(111)B(1x1) surface was studied by synchrotron radiation photoelectron spectroscopy. The two-dimensional electron gas reveals itself via a narrow structure at the Fermi level, visible close to normal emission for tin coverage in the range 0.5 to 2 monolayers. Although this electron gas exhibits properties that in several respects resemble those of intrinsic charge accumulation layers on free InAs surfaces, our observations suggest that the present electron gas is much more linked to the Sn adlayer.
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| 1827. |
- Szasz, Krisztian, et al.
(författare)
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Spin and photophysics of carbon-antisite vacancy defect in 4H silicon carbide: A potential quantum bit
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 91:12, s. 121201-
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Tidskriftsartikel (refereegranskat)abstract
- Silicon carbide with engineered point defects is considered as very promising material for the next generation devices, with applications ranging from electronics and photonics to quantum computing. In this context, we investigate the spin physics of the carbon antisite-vacancy pair that in its positive charge state enables a single photon source. We find by hybrid density functional theory and many-body perturbation theory that the neutral defect possesses a high spin ground state in 4H silicon carbide and provide spin-resonance signatures for its experimental identification. Our results indicate the possibility for the coherent manipulation of the electron spin by optical excitation of this defect at telecom wavelengths, and suggest the defect as a candidate for an alternative solid state quantum bit.
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| 1828. |
- Szilva, Attila, et al.
(författare)
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Universal distribution of magnetic anisotropy of impurities in ordered and disordered nanograins
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:13
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Tidskriftsartikel (refereegranskat)abstract
- We examine the distribution of the magnetic anisotropy experienced by a magnetic impurity embedded in a metallic nanograin. As an example of a generic magnetic impurity with a partially filled d shell, we study the case of d(1) impurities embedded into ordered and disordered Au nanograins, described in terms of a realistic band structure. Confinement of the electrons induces a magnetic anisotropy that is large, and can be characterized by five real parameters, coupling to the quadrupolar moments of the spin. In ordered (spherical) nanograins, these parameters exhibit symmetrical structures and reflect the symmetry of the underlying lattice, while for disordered grains they are randomly distributed and, for stronger disorder, their distribution is found to be characterized by random matrix theory. As a result, the probability of having small magnetic anisotropies K-L is suppressed below a characteristic scale Delta(E), which we predict to scale with the number of atoms N as Delta(E) similar to 1/N-3/2. This gives rise to anomalies in the specific heat and the susceptibility at temperatures T similar to Delta(E) and produces distinct structures in the magnetic excitation spectrum of the clusters that should be possible to detect experimentally.
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| 1829. |
- Söderlind, Per, et al.
(författare)
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High-temperature phonon stabilization of γ-uranium from relativistic first-principles theory
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:6, s. 060301-
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Tidskriftsartikel (refereegranskat)abstract
- A microscopic explanation for temperature stabilization of the body-centered cubic (bcc) phase in the actinide metals is proposed. We show that for a prototype actinide, uranium, phonon-phonon interaction promotes bcc gamma-U when heated, even though at low temperatures, it is mechanically a strongly unstable phase. Utilizing the recently developed self-consistent ab initio lattice dynamics (SCAILD) scheme in conjunction with highly accurate and fully relativistic density functional theory we obtain phonon dispersion and density of states that compare well with data acquired from inelastic neutron-scattering experiments. The investigation thus establishes that high-temperature lattice dynamics can be modeled from ab initio theory even for complex materials with substantial electron correlation including the actinides.
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| 1830. |
- Sörman, E., et al.
(författare)
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Silicon vacancy related defect in 4H and 6H SiC
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:4, s. 2613-2620
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Tidskriftsartikel (refereegranskat)abstract
- We report on an irradiation-induced photoluminescence (PL) band in 4H and 6H SiC and the corresponding optically detected magnetic resonance (ODMR) signals from this band. The deep PL band has the same number of no-phonon lines as there are inequivalent sites in the respective polytype. These lines are at 1352 and 1438 meV in the case of 4H and at 1366, 1398, and 1433 meV in the case of 6H. The intensity of the PL lines is reduced after a short anneal at 750░C. ODMR measurements with above-band-gap excitation show that two spin-triplet (S=1) states with a weak axial character are detected via each PL line in these bands. One of these two triplet states can be selectively excited with the excitation energy of the corresponding PL line. These triplet signals can therefore be detected separately and only then can the well documented and characteristic hyperfine interaction of the silicon vacancy in SiC be resolved. Considering the correlation between the irradiation dose and the signal strength, the well established annealing temperature and the characteristic hyperfine pattern, we suggest that this PL band is related to the isolated silicon vacancy in 4H and 6H SiC. The spin state (S=1) implies a charge state of the vacancy with an even number of electrons. By combining the knowledge from complementary electron-spin resonance measurements and theoretical calculations we hold the neutral charge state for the strongest candidate. ⌐2000 The American Physical Society.
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