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1.
Mathias Steiner Marcus Freitag James C. Tsang Vasili Perebeinos Ageeth A. Bol Antonio V. Failla Phaedon Avouris 《Applied Physics A: Materials Science & Processing》2009,96(2):271-282
We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended
across a trench and partially supported by a SiO2-substrate. By tuning the laser excitation energy across the E
33 excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial
breathing mode (RBM), the D mode and the G mode show strong resonance enhancement of up to three orders of magnitude. In the
supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E
33 excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G band and D band
is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect
density in the supported CNT segments. By comparing the E
33 resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended
CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated
with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving
rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced
by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band
measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements,
however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser
excitation power. 相似文献
2.
In this paper, we introduce a quasi-one-dimensional S = 1 antiferromagnet Heisenberg model, and some physical properties of antiferromagnet L2BaNiO5 without antiferromagnet long-range-order above the Néel temperature are analyzed based on the frame of two-time Green’s function
theory. In a high temperature region, we calculate the correlation functions, and obtain excitation spectrum along Ni chains
and the Haldane gap in this spectrum versus temperature. We find that the short-range correlation still exists at high temperature,
which leads to the existence of Haldane gap in excitation spectrum. The increment of excitation energy in the spectrum along
the Ni chain is found to be induced by the AF interaction between spins of rare-earth and Ni ions. Additionally, we also find
that Haldane gap goes up with temperature increasing. 相似文献
3.
5QMAS experiments on spin-5/2 systems display a low sensitivity compared with their 3QMAS counterparts. Nevertheless, the superior resolution of 5QMAS over 3QMAS makes these experiments a favorable choice for many materials. We report an enhancement scheme for the 5QMAS experiment, using an improved five-quantum excitation pulse scheme combined with a FAM-II conversion pulse. The results are verified experimentally on a polycrystalline sample of gamma-(27)Al(2)O(3), showing an enhancement factor of 2.4 over the simple two-pulse (CW) 5QMAS scheme. Numerical computations of the efficiency parameter epsilon support these results. 相似文献
4.
A superconductor with 4-fermion attraction, considered by Maćkowiak and Tarasewicz is modified by adding to the Hamiltonian
a long-range magnetic interaction V between conduction fermions and localized distinguishable spin 1/2 magnetic impurities. V has the form of a reduced s-d interaction. An upper and lower bound to the system’s free energy density f(H, β) is derived and the two bounds are shown to coalesce in the thermodynamic limit. The resulting mean-field equations for the
gap Δ and a parameter y, characterizing the impurity subsystem are solved and the solution minimizing f is found for various values of magnetic coupling constant g and impurity concentration. The phase diagrams of the system are depicted with five distinct phases: the normal phase, unperturbed
superconducting phase, perturbed superconducting phase with nonzero gap in the excitation spectrum, perturbed gapless superconducting
phase and impurity phase with completely suppressed superconductivity. 相似文献
5.
A model for the blue luminescence of polycrystalline cadmium sulfide and cadmium sulfide single crystals is proposed on the basis of a joint study of the absorption, reflection, emission, and excitation spectra of this luminescence at 77 °K. The blue luminescence arises in the transition of an electron from the conduction band to the 5s2 1S0 ground level of a super-stoichiometric cadmium atom which has trapped a hole from the valence band. The width of the energy gap (2.614 eV) at κ = 0 is estimated on the basis of the oscillatory nature of the excitation spectrum for the blue luminescence, and the effective mass of heavy holes is estimated. 相似文献
6.
L. Mayrhofer M. Grifoni 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(1):43-58
The low energy spectrum of finite size metallic single-walled carbon nanotubes (SWNTs) is determined. Starting from a tight
binding model for the pz electrons, we derive the
low energy Hamiltonian containing all relevant scattering processes
resulting from the Coulomb interaction, including the short
ranged contributions becoming relevant for small diameter tubes. In
combination with the substructure of the underlying honeycomb lattice
the short ranged processes lead to various exchange effects. Using
bosonization the spectrum is determined. We find that the ground state
is formed by a spin 1 triplet, if 4n+2 electrons occupy the
SWNT and the branch mismatch is smaller than the exchange splitting.
Additionally, we calculate the excitation spectra for the different
charge states and find the lifting of spin-charge separation as well
as the formation of a quasi-continuum at higher excitation energies. 相似文献
7.
Tomoaki Sawabe Koshi Okamura Tomoki Sueyoshi Takashi Miyamoto Kazuhiro Kudo Nobuo Ueno Masakazu Nakamura 《Applied Physics A: Materials Science & Processing》2009,95(1):225-232
Vertical electrical conduction in Au/(polycrystal-line pentacene)/Al diode structures and the influence of the kinetic energy
of incident Au atoms on the conduction property have been comprehensively studied using current–voltage–temperature (I–V–T) measurements, ultraviolet photoelectron spectroscopy (UPS), atomic-force-microscope (AFM) current imaging, etc. In the I–V characteristics, a symmetrical ohmic current component appeared when a low voltage was applied, and a super-linear one appeared
when a high positive voltage was applied to Au. The component in the high-forward-voltage region was concluded to be a thermionic
emission of holes from Au with a 0.23-eV injection barrier, which is the normal hole conduction through the highest occupied
molecular orbital of pentacene. On the other hand, the ohmic component was concluded to be a metal-like electron transport
through high-density gap states at grain boundaries which were induced by the Au penetration into pentacene. UPS and I–V–T measurements clearly indicated the generation of the gap states and the enhancement of their density by the reduction of
Au kinetic energy. For vertical-type devices with polycrystalline organic films, the ohmic conduction through the grain boundary
will increase the leakage current. On the contrary, it possibly enhances the carrier injection in lateral-type transistors
in the case of top-contact configuration. 相似文献
8.
Abdelilah Benyoussef Abdelrhani Boubekri Hamid Ez-Zahraouy 《Journal of magnetism and magnetic materials》1998,190(3):402
The effect of the interlayer and intralayer anisotropies on the energy gap and sublattice magnetization is investigated using the linear spin wave theory for multilayer Heisenberg models with odd number of coupled layers. In the isotropic case, such systems exhibit a long-range order and no energy gap, while, in the anisotropic case, they are still ordered and the energy gap opens for certain anisotropies. 相似文献
9.
Haichao Huang Haibo Wang Jidong Zhang Donghang Yan 《Applied Physics A: Materials Science & Processing》2009,95(1):125-130
P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F16CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface
potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline
facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of
surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F16CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to
be a powerful tool to characterize the local electronic properties of organic semiconductors. 相似文献
10.
Michael E. Lacey Jonathan V. Sweedler Cynthia K. Larive Adrian J. Pipe R. Duncan Farrant 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2001,153(2):215
A capillary NMR flow probe was designed to generate high-resolution 1H NMR spectra at 600 MHz from the cleaved product of individual 160-μm Tentagel combinatorial chemistry beads. By injecting a dissolved sample sandwiched between an immiscible, perfluorinated organic liquid directly into the probe, NMR spectra of the product cleaved from single beads were acquired in just 1 h of spectrometer time without diffusional dilution. Sample handling efficiency on the single bead scale was comparable to that obtained with a bulk sample. Using the relative intensity of the DMSO-d5H versus the analyte signals in a fully relaxed CPMG spectrum, the amount of product cleaved from a single bead was determined to be 540±170 pmol in one of the samples. Following the NMR data collection, the samples were examined with electrospray ionization mass spectrometry to provide additional structural information. By coupling with microliter-volume fluidic capabilities, the capillary flow probe described here will enable multidimensional characterization of single solid-phase resin products in an online manner. 相似文献
11.
P. Lavallard M. ChamarroJ. Pérez-Conde A.K. BhattacharjeeS.V. Goupalov A.A. Lipovskii 《Solid State Communications》2003,127(6):439-442
Photoluminescence of CdTe nanocrystals (NC) is excited resonantly in the lowest energy absorption peak. The spectrum shows a luminescence line shifted to a lower energy and acoustical and optical phonon replica. The Stokes shift between the luminescence and excitation lines is attributed to the electron-hole exchange energy in the nanocrystal. By tuning the laser line inside the absorption peak, we are able to measure the Stokes shift as a function of the excitation energy. Calculation of the absorption gap and the Stokes shift is done in a tight-binding theory. It allows us to determine the radius R of NC excited at a given wavelength and to compare the experimental and theoretical values of the exchange energy as a function of R. A very good agreement is obtained. The observed size dependence of the acoustical phonon mode energy provides a further confirmation of our analysis. 相似文献
12.
High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective
formation of triplets after excitation. We studied these processes with fs pump–probe spectroscopy. The time behaviour of
transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution.
For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet
state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function.
Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation.
Both processes could be described by a time constant of 62 ps±9 ps.
For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield
increased with higher excitation energy. The kinetics becomes bi-exponential with increasing amplitude of the short time constant
of 2 ps at increasing excitation energy.
The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are – besides the high spin-orbit coupling
factor introduced by the sulphur atom – the almost isoenergetic positions of the S
1 and T
2 states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC
was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state
S
1, ultrafast intersystem crossing takes place.
Received: 6 December 1999 / Published online: 2 August 2000 相似文献
13.
P. Thalmeier A.N. Yaresko 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):495-508
An investigation of the spin excitation spectrum of charge ordered (CO) NaV2O5 is presented. We discuss several different exchange models which may be relevant for this compound, namely in-line and zig-zag
chain models with weak as well as strong inter-chain coupling and also a ladder model and a CO/MV (mixed valent) model. We
put special emphasis on the importance of large additional exchange across the diagonals of V-ladders and the presence of
exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches
may both be interpreted as anisotropy effect as well as acoustic-optic mode splitting in the weakly coupled chain models.
In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter-chain
coupling, as suggested by recent LDA + U results, an additional high energy optical excitation appears and the spin gap is
determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which
agrees very well with recent results obtained by inelastic neutron scattering.
Received 30 April 1999 and Received in final form 5 October 1999 相似文献
14.
B. Renker F. Gompf H. Schober P. Adelmann H. J. Bornemann R. Heid 《Zeitschrift für Physik B Condensed Matter》1993,92(4):451-455
We discuss the results of inelastic neutron time-of-flight spectroscopy on powder samples of C60, K3C60 and Rb3C60. A well structured density of states is obtained from the raw data where the details can be attributed to translational acoustic/optic and librational modes. The intermolecular excitation spectra for all three compounds can be well understood within the frame-work of lattice dynamics of a molecular crystal. We show that a particular sample preparation is necessary to avoid impurity contributions in particular within the gap region between inter- and intramolecular vibrations. There are indications for a weak anomalous temperature dependence of low energy phonons in K3C60 atT
c
. 相似文献
15.
B. Dóra A. Virosztek 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,22(2):167-178
We consider the possibility of formation of an unconventional spin density wave (USDW) in quasi-one-dimensional electronic
systems. In analogy with unconventional superconductivity, we develop a mean field theory of SDW allowing for the momentum
dependent gap Δ() on the Fermi surface. Conditions for the appearance of such a low temperature phase are investigated. The excitation spectrum
and basic thermodynamic properties of the model are found to be very similar to those of d-wave superconductors in spite of the different topology of their Fermi surfaces. Several correlation functions are calculated,
and the frequency dependent conductivity is evaluated for various gap functions. The latter is found to reflect the maximum
gap value, however with no sharp onset for absorbtion.
Received 19 February 2001 相似文献
16.
Structural, optical and electrical properties of cadmium zinc oxide films for light emitting devices
B.K. Sonawane Vrushali Shelke M.P. Bhole D.S. Patil 《Journal of Physics and Chemistry of Solids》2011,72(12):1442-1446
Structural, optical and electrical properties of CdyZn1-yO (y=0–0.1) ternary alloy thin films have been investigated for the films prepared using the spin coating method on the glass substrate. XRD pattern confirmed the nano-size polycrystalline hexagonal wurtzite structure for all the samples. The size of nano-crystals was found to be varied in between 21 and 30 nm. Optical band gap calculated from the absorption coefficient signifies the shift in direct band gap from 3.2 to 2.97 eV with cadmium composition. Through EDAX spectrum, it was confirmed that Cadmium was successfully incorporated into ZnO. SEM studies make clear that even after Cadmium incorporation, the surface morphology of the films remained smooth. The current–voltage characteristics obtained from semiconductor characterization system reveals that resistance of the films was found to be decreased with the increase in cadmium composition. Our investigations lead to the applicability of CdZnO as an active layer in CdZnO/ZnO heterostructure for light emitting devices. 相似文献
17.
采用密度泛函理论的B3LYP方法在6-311+G(2DF)水平上研究了电场强度为-003—003 a.u.的外电场对MgO基态分子的几何结构、HOMO能级、LUMO能级、能隙、费米能级、谐振频率和红外光谱强度的影响规律.结果表明,在所加的电场范围内,随着正向电场的增大核间距先减小后增大,在F=002 a.u.时,Re取得最小为017397 nm;分子总能量不断升高,但增大的幅度呈减小的趋势; EH先增大后减小,在
关键词:
MgO
外电场
能隙
红外 相似文献
18.
M. R. Beitins S. T. Boneva V. A. Khitrov L. A. Malov Yu. P. Popov P. T. Prokofjev G. L. Rezvaya L. I. Simonova A. M. Sukhovoj E. V. Vasilieva 《Zeitschrift für Physik A Hadrons and Nuclei》1992,341(2):155-170
A singles-ray spectrum and a spectrum of summed amplitudes of coinciding pulses (SACP) were measured in the186W(n, )187W reaction experiment on the thermal neutron beam. It was for the first time that the data on187W-transitions were obtained in the excitation energy region from 1500–2500 keV. The neutron binding energy was determined to beB
n=5467.25 (4) keV (statistical error only). In result the level scheme of187W was developed in the excitation energy interval 1E
f
< 3.4=" mev,=" which=" contained=" 105=" levels=" with=" about=" 70=" of=" them=" being=" identified=" for=" the=" first=" time.=" the=" experimental=" values=" for=" summed=" intensities=" of=" two-step=" cascades=" were=" established=" to=" exceed=" those=" predicted=" by=" the=" modern=" statistical=" theory=" (by=" 36±6%).=" this=" is=" explained=" by=" a=" considerable=" contribution=" of=" few-quasiparticle=" components=" to=" wave=" functions=" of=" compound=" and=" intermediate="> 相似文献
19.
Nikolaos V. Tsekos Michael Garwood Kmil Ugurbil 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,156(2):187
A new technique is presented for generating myocardial tagging using the signal intensity minima of the transition zones between the bands of 0° and 360° rotations, induced by a tandem of two adiabatic delays alternating with nutations for tailored excitation (DANTE) inversion sequences. With this approach, the underlying matrix corresponds to magnetization that has experienced 0° or 360° rotations. The DANTE sequences were implemented from adiabatic parent pulses for insensitivity of the underlying matrix to B1 inhomogeneity. The performance of the proposed tagging technique is demonstrated theoretically with computer simulations and experimentally on phantom and on the canine heart, using a surface coil for both RF transmission and signal reception. The simulations and the experimental data demonstrated uniform grid contrast and sharp tagging profiles over a twofold variation of the B1 field magnitude. 相似文献
20.
Jian-Jun Jiang Yong-Jun Liu Cui-Hong Yang 《Journal of magnetism and magnetic materials》2009,321(19):3300-3306
We investigate the effect of frustration on spin-wave excitation spectra and the properties of the quasi-one-dimensional Heisenberg chain using a spin-wave-wave analysis, the exact diagonalization method and the density matrix renormalization group method. The results show that frustration can cause the softening of the acoustic excitation spectrum ω3, as well as the hardening of the optical excitation spectrum ω1. As a function of the frustration parameter α, the phase diagram exhibits a ferromagnetic phase, a narrow canted phase and a singlet phase. The results obtained from numerical methods show that the spin gap obviously opens and the tetramer-dimer state dominates the properties of the ground state in the singlet phase. 相似文献