4,261 research outputs found
De Rham Cohomology of SO(n) by Supersymmetric Quantum Mechanics
We give an elementary derivation of the de Rham cohomology of SO(n) in terms
of supersymmetric quantum mechanics. Our analysis is based on Witten's Morse
theory. We show reflection symmetries of the theory are useful to select true
vacuums. The number of the selected vacuums will agree with the de Rham
cohomology of SO(n).Comment: 7pages, latex, no figure
de Rham cohomology of SO(n) and some related manifolds by supersymmetric quantum mechanics
We study supersymmetric quantum mechanics on RP_{n},SO(n),G_{2} and U(2) to
examine Witten's Morse theory concretely. We confirm the simple instanton
picture of the de Rham cohomology that has been given in a previous paper. We
use a reflection symmetry of each theory to select the true vacuums. The number
of selected vacuums agrees with the de Rham cohomology for each of the above
manifolds.Comment: 18pages,Late
Quantum integrability of the deformed elliptic Calogero-Moser problem
The integrability of the deformed quantum elliptic Calogero-Moser problem
introduced by Chalykh, Feigin and Veselov is proven. Explicit recursive
formulae for the integrals are found. For integer values of the parameter this
implies the algebraic integrability of the systems.Comment: 23 page
Selective d-state Conduction Blocking in Nickel Nanocontacts
The lowest conductance step for a Ni nanocontact is anomalously small in
comparison with the large expected number of conducting channels. We present
electronic structure calculations for an extremely idealized Ni nanobridge
consisting of just a monatomic nanowire. Our calculations show that no less
than eight single spin bands cross the Fermi level in a nonmagnetic Ni
monatomic wire, dropping marginally to seven in the more stable, fully
ferromagnetic state. However, when we build in the wire a magnetization
reversal, or domain wall, by forcing the net magnetization to be zero, we
suddenly find that d electrons selectively cease to propagate across the wall.
s electron propagation remains, and can account for the small observed
conductance steps.Comment: 9 pages, 4 figures, Surface Science, to appea
Electronic Structures of N-doped Graphene with Native Point Defects
Nitrogen doping in graphene has important implications in graphene-based
devices and catalysts. We have performed the density functional theory
calculations to study the electronic structures of N-doped graphene with
vacancies and Stone-Wales defect. Our results show that monovacancies in
graphene act as hole dopants and that two substitutional N dopants are needed
to compensate for the hole introduced by a monovacancy. On the other hand,
divacancy does not produce any free carriers. Interestingly, a single N dopant
at divacancy acts as an acceptor rather than a donor. The interference between
native point defect and N dopant strongly modifies the role of N doping
regarding the free carrier production in the bulk pi bands. For some of the
defects and N dopant-defect complexes, localized defect pi states are partially
occupied. Discussion on the possibility of spin polarization in such cases is
given. We also present qualitative arguments on the electronic structures based
on the local bond picture. We have analyzed the 1s-related x-ray photoemission
and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales
defect in connection with the experimental ones. We also discuss characteristic
scanning tunneling microscope (STM) images originating from the electronic and
structural modifications by the N dopant-defect complexes. STM imaging for
small negative bias voltage will provide important information about possible
active sites for oxygen reduction reaction.Comment: 40 pages, 2 tables, 16 figures. The analysis of Clar sextets is
added. This version is published on PHYSICAL REVIEW B 87, 165401(2013
Spin-filter tunnel junction with matched Fermi surfaces
Efficient injection of spin-polarized current into a semiconductor is a basic
prerequisite for building semiconductor-based spintronic devices. Here, we use
inelastic electron tunneling spectroscopy to show that the efficiency of
spin-filter-type spin injectors is limited by spin scattering of the tunneling
electrons. By matching the Fermi-surface shapes of the current injection source
and target electrode material, spin injection efficiency can be significantly
increased in epitaxial ferromagnetic insulator tunnel junctions. Our results
demonstrate that not only structural but also Fermi-surface matching is
important to suppress scattering processes in spintronic devices.Comment: 5 pages, 4 figure
Simulations of slow positron production using a low energy electron accelerator
Monte Carlo simulations of slow positron production via energetic electron
interaction with a solid target have been performed. The aim of the simulations
was to determine the expected slow positron beam intensity from a low energy,
high current electron accelerator. By simulating (a) the fast positron
production from a tantalum electron-positron converter and (b) the positron
depth deposition profile in a tungsten moderator, the slow positron production
probability per incident electron was estimated. Normalizing the calculated
result to the measured slow positron yield at the present AIST LINAC the
expected slow positron yield as a function of energy was determined. For an
electron beam energy of 5 MeV (10 MeV) and current 240 A (30 A)
production of a slow positron beam of intensity 5 10 s is
predicted. The simulation also calculates the average energy deposited in the
converter per electron, allowing an estimate of the beam heating at a given
electron energy and current. For low energy, high-current operation the maximum
obtainable positron beam intensity will be limited by this beam heating.Comment: 11 pages, 15 figures, submitted to Review of Scientific Instrument
Application of photoluminescence and electroluminescence techniques to the characterization of intermediate band solar cells
The intermediatebandsolarcell (IBSC) is a photovoltaic device with a theoretical conversion efficiency limit of 63.2%. In recent years many attempts have been made to fabricate an intermediateband material which behaves as the theory states. One characteristic feature of an IBSC is its luminescence spectrum. In this work the temperature dependence of the photoluminescence (PL) and electroluminescence (EL) spectra of InAs/GaAs QD-IBSCs together with their reference cell have been studied. It is shown that EL measurements provide more reliable information about the behaviour of the IB material inside the IBSC structure than PL measurements. At low temperatures, the EL spectra are consistent with the quasi-Fermi level splits described by the IBSC model, whereas at room temperature they are not. This result is in agreement with previously reported analysis of the quantum efficiency of the solarcell
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