7,038 research outputs found
Stability of antiphase line defects in nanometer-sized boron-nitride cones
We investigate the stability of boron nitride conical sheets of nanometer
size, using first-principles calculations. Our results indicate that cones with
an antiphase boundary (a line defect that contains either B-B or N-N bonds) can
be more stable than those without one. We also find that doping the antiphase
boundaries with carbon can enhance their stability, leading also to the
appearance of localized states in the bandgap. Among the structures we
considered, the one with the smallest formation energy is a cone with a
carbon-modified antiphase boundary that presents a spin splitting of about 0.5
eV at the Fermi level.Comment: 5 two-column pages with 2 figures Accepted for publication in
Physical Review B (vol 70, 15 Nov.
Mott physics, sign structure, ground state wavefunction, and high-Tc superconductivity
In this article I give a pedagogical illustration of why the essential
problem of high-Tc superconductivity in the cuprates is about how an
antiferromagnetically ordered state can be turned into a short-range state by
doping. I will start with half-filling where the antiferromagnetic ground state
is accurately described by the Liang-Doucot-Anderson (LDA) wavefunction. Here
the effect of the Fermi statistics becomes completely irrelevant due to the no
double occupancy constraint. Upon doping, the statistical signs reemerge,
albeit much reduced as compared to the original Fermi statistical signs. By
precisely incorporating this altered statistical sign structure at finite
doping, the LDA ground state can be recast into a short-range antiferromagnetic
state. Superconducting phase coherence arises after the spin correlations
become short-ranged, and the superconducting phase transition is controlled by
spin excitations. I will stress that the pseudogap phenomenon naturally emerges
as a crossover between the antiferromagnetic and superconducting phases. As a
characteristic of non Fermi liquid, the mutual statistical interaction between
the spin and charge degrees of freedom will reach a maximum in a
high-temperature "strange metal phase" of the doped Mott insulator.Comment: 12 pages, 12 figure
Spin-charge separation in the single hole doped Mott antiferromagnet
The motion of a single hole in a Mott antiferromagnet is investigated based
on the t-J model. An exact expression of the energy spectrum is obtained, in
which the irreparable phase string effect [Phys. Rev. Lett. 77, 5102 (1996)] is
explicitly present. By identifying the phase string effect with spin backflow,
we point out that spin-charge separation must exist in such a system: the doped
hole has to decay into a neutral spinon and a spinless holon, together with the
phase string. We show that while the spinon remains coherent, the holon motion
is deterred by the phase string, resulting in its localization in space. We
calculate the electron spectral function which explains the line shape of the
spectral function as well as the ``quasiparticle'' spectrum observed in
angle-resolved photoemission experiments. Other analytic and numerical
approaches are discussed based on the present framework.Comment: 16 pages, 9 figures; references updated; to appear in Phys. Rev.
Multi-subband effect in spin dephasing in semiconductor quantum wells
Multi-subband effect on spin precession and spin dephasing in -type GaAs
quantum wells is studied with electron-electron and electron-phonon scattering
explicitly included. The effects of temperature, well width and applied
electric field (in hot-electron regime) on the spin kinetics are thoroughly
investigated. It is shown that due to the strong inter-subband scattering, the
spin procession and the spin dephasing rate of electrons in different subbands
are almost identical despite the large difference in the D'yakonov-Perel' (DP)
terms of different subbands. It is also shown that for quantum wells with small
well width at temperatures where only the lowest subband is occupied, the spin
dephasing time increases with the temperature as well as the applied in-plane
electric field until the contribution from the second subband is no longer
negligible. For wide quantum wells the spin dephasing time tends to decrease
with the temperature and the electric field.Comment: 6 pages, 4 figures in eps forma
Identification of Urinary Biomarkers for Exercise-Induced Immunosuppression by iTRAQ Proteomics
Purpose: To identify noninvasive immune biomarkers of exercise-induced immunosuppression using the iTRAQ proteomics technique. Methods: Fifteen healthy males were recruited and subjected to a four-week incremental treadmill running training program. After each week of training, WBC counts and CD4+ and CD8+ lymphocytes were measured to monitor the immune function status. iTRAQ proteomics technology was used to identify differential proteins and their characteristics in urine. Results: Our data showed that the WBC counts, CD4+ lymphocytes, and CD4+/CD8+ ratio decreased by more than 10% after four weeks of training, suggesting exercise-induced immunosuppression. A total of 1854 proteins were identified in urine during the incremental running using the iTRAQ technology. Compared with the urine before training, there were 89, 52, 77, and 148 proteins significantly upregulated and 66, 27, 68, and 114 proteins significantly downregulated after each week, respectively. Among them, four upregulated proteins, SEMG-1, PIP, PDGFRL, and NDPK, increased their abundance with the increased exercise intensity. Bioinformatics analysis indicates that these proteins are involved in stress response and immune function. Conclusion: Four weeks of incremental treadmill running induced immunosuppression in healthy males. By using iTRAQ proteomics, four proteins in the urine, SEMG-1, PIP, PDGFRL, and NDPK, were found to increase incrementally with the increased exercise intensity, which have the potential to be used as noninvasive immune biomarkers of exercise-induced immunosuppression
Mean-Field Description of Phase String Effect in the Model
A mean-field treatment of the phase string effect in the model is
presented. Such a theory is able to unite the antiferromagnetic (AF) phase at
half-filling and metallic phase at finite doping within a single theoretical
framework. We find that the low-temperature occurrence of the AF long range
ordering (AFLRO) at half-filling and superconducting condensation in metallic
phase are all due to Bose condensations of spinons and holons, respectively, on
the top of a spin background described by bosonic resonating-valence-bond (RVB)
pairing. The fact that both spinon and holon here are bosonic objects, as the
result of the phase string effect, represents a crucial difference from the
conventional slave-boson and slave-fermion approaches. This theory also allows
an underdoped metallic regime where the Bose condensation of spinons can still
exist. Even though the AFLRO is gone here, such a regime corresponds to a
microscopic charge inhomogeneity with short-ranged spin ordering. We discuss
some characteristic experimental consequences for those different metallic
regimes. A perspective on broader issues based on the phase string theory is
also discussed.Comment: 18 pages, five figure
Hot-electron effect in spin dephasing in -type GaAs quantum wells
We perform a study of the effect of the high in-plane electric field on the
spin precession and spin dephasing due to the D'yakonov-Perel' mechanism in
-type GaAs (100) quantum wells by constructing and numerically solving the
kinetic Bloch equations. We self-consistently include all of the scattering
such as electron-phonon, electron-non-magnetic impurity as well as the
electron-electron Coulomb scattering in our theory and systematically
investigate how the spin precession and spin dephasing are affected by the high
electric field under various conditions. The hot-electron distribution
functions and the spin correlations are calculated rigorously in our theory. It
is found that the D'yakonov-Perel' term in the electric field provides a
non-vanishing effective magnetic field that alters the spin precession period.
Moreover, spin dephasing is markedly affected by the electric field. The
important contribution of the electron-electron scattering to the spin
dephasing is also discussed.Comment: 11 pages, 11 figures, accepted for publication in Phys. Rev.
Deep-level defects in n-type 6H silicon carbide induced by He implantation
Defects in He-implanted n -type 6H-SiC samples have been studied with deep-level transient spectroscopy. A deep-level defect was identified by an intensity with a logarithmical dependence on the filling pulse width, which is characteristic of dislocation defects. Combined with information extracted from positron-annihilation spectroscopic measurements, this defect was associated with the defect vacancy bound to a dislocation. Defect levels at 0.380.44 eV (E1 E2), 0.50, 0.53, and 0.640.75 eV (Z1 Z2) were also induced by He implantation. Annealing studies on these samples were also performed and the results were compared with those obtained from e- -irradiated (0.3 and 1.7 MeV) and neutron-irradiated n -type 6H-SiC samples. The E1 E2 and the Z1 Z2 signals found in the He-implanted sample are more thermally stable than those found in the electron-irradiated or the neutron-irradiated samples. © 2005 American Institute of Physics.published_or_final_versio
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