14,577 research outputs found
Kondo resonance line-shape of magnetic adatoms on decoupling layers
The zero-bias resonance in the dI/dV tunneling spectrum recorded using a
scanning tunneling microscope above a spin-1/2 magnetic adatom (such as Ti)
adsorbed on a decoupling layer on metal surface can be accurately fitted using
the universal spectral function of the Kondo impurity model both at zero field
and at finite external magnetic field. Excellent agreement is found both for
the asymptotic low-energy part and for the high-energy logarithmic tails of the
Kondo resonance. For finite magnetic field, the nonlinear fitting procedure
consists in repeatedly solving the impurity model for different Zeeman energies
in order to obtain accurate spectral functions which are compared with the
experimental dI/dV curves. The experimental results at zero field are
sufficiently restraining to enable an unprecedented reliability in the
determination of the Kondo temperature, while at finite fields the results are
more ambiguous and two different interpretations are proposed
Field-theory calculation of the electric dipole moment of the neutron and paramagnetic atoms
Electric dipole moments (edms) of bound states that arise from the
constituents having edms are studied with field-theoretic techniques. The
systems treated are the neutron and a set of paramagnetic atoms. In the latter
case it is well known that the atomic edm differs greatly from the electron edm
when the internal electric fields of the atom are taken into account. In the
nonrelativistic limit these fields lead to a complete suppression, but for
heavy atoms large enhancement factors are present. A general bound-state field
theory approach applicable to both the neutron and paramagnetic atoms is set
up. It is applied first to the neutron, treating the quarks as moving freely in
a confining spherical well. It is shown that the effect of internal electric
fields is small in this case. The atomic problem is then revisited using
field-theory techniques in place of the usual Hamiltonian methods, and the
atomic enhancement factor is shown to be consistent with previous calculations.
Possible application of bound-state techniques to other sources of the neutron
edm is discussed.Comment: 21 pages, 5 figure
Advanced double layer capacitors
Work was conducted that could lead to a high energy density electrochemical capacitor, completely free of liquid electrolyte. A three-dimensional RuO sub x-ionomer composite structure has been successfully formed and appears to provide an ionomer ionic linkage throughout the composite structure. Capacitance values of approximately 0.6 F/sq cm were obtained compared with 1 F/sq cm when a liquid electrolyte is used with the same configuration
Magnetic excitations in the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2)
Inelastic neutron scattering experiments were performed for the spin-trimer
compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2) in order to study the dynamic magnetic
properties. The observed excitations can be associated with transitions between
the low-lying electronic states of linear Cu-Cu-Cu, Cu-Cu-Ni, and Ni-Cu-Ni
trimers which are the basic constituents of the title compounds. The exchange
interactions within the trimers are well described by the Heisenberg model with
dominant antiferromagnetic nearest-neighbor interactions J. For x=0 we find
JCu-Cu=-4.74(2) meV which is enhanced for x=1 to JCu-Cu=-4.92(6) meV. For x=1
and x=2 we find JCu-Ni=-0.85(10) meV and an axial single-ion anisotropy
parameter DNi=-0.7(1) meV. While the x=0 and x=1 compounds do not exhibit
long-range magnetic ordering down to 1 K, the x=2 compound shows
antiferromagnetic ordering below TN=20 K, which is compatible with the
molecular-field parameter 0.63(12) meV derived by neutron spectroscopy.Comment: 22 pages (double spacing), 1 table, 9 figures, Submitted to Phys.
Rev. B (2007
Slip energy barriers in aluminum and implications for ductile versus brittle behavior
We conisder the brittle versus ductile behavior of aluminum in the framework
of the Peierls-model analysis of dislocation emission from a crack tip. To this
end, we perform first-principles quantum mechanical calculations for the
unstable stacking energy of aluminum along the Shockley partial
slip route. Our calculations are based on density functional theory and the
local density approximation and include full atomic and volume relaxation. We
find that in aluminum J/m. Within the Peierls-model
analysis, this value would predict a brittle solid which poses an interesting
problem since aluminum is typically considered ductile. The resolution may be
given by one of three possibilites: (a) Aluminum is indeed brittle at zero
temperature, and becomes ductile at a finite temperature due to motion of
pre-existing dislocations which relax the stress concentration at the crack
tip. (b) Dislocation emission at the crack tip is itself a thermally activated
process. (c) Aluminum is actually ductile at all temperatures and the
theoretical model employed needs to be significantly improved in order to
resolve the apparent contradiction.Comment: 4 figures (not included; send requests to [email protected]
Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials
Slow crack growth in a model of homogenous brittle elastic material is
described as a thermal activation process where stress fluctuations allow to
overcome a breaking threshold through a series of irreversible steps. We study
the case of a single crack in a flat sheet for which analytical predictions can
be made, and compare them with results from the equivalent problem of a 2D
spring network. Good statistical agreement is obtained for the crack growth
profile and final rupture time. The specific scaling of the energy barrier with
stress intensity factor appears as a consequence of irreversibility. In
addition, the model brings out a characteristic growth length whose physical
meaning could be tested experimentally.Comment: To be published in : Europhysics Letter
Linear Chains of Styrene and Methyl-Styrene Molecules and their Heterojunctions on Silicon: Theory and Experiment
We report on the synthesis, STM imaging and theoretical studies of the
structure, electronic structure and transport properties of linear chains of
styrene and methyl-styrene molecules and their heterojunctions on
hydrogen-terminated dimerized silicon (001) surfaces. The theory presented here
accounts for the essential features of the experimental STM data including the
nature of the corrugation observed along the molecular chains and the
pronounced changes in the contrast between the styrene and methyl-styrene parts
of the molecular chains that are observed as the applied bias is varied. The
observed evolution with applied bias of the STM profiles near the ends of the
molecular chains is also explained. Calculations are also presented of electron
transport along styrene linear chains adsorbed on the silicon surface at
energies in the vicinity of the molecular HOMO and LUMO levels. For short
styrene chains this lateral transport is found to be due primarily to direct
electron transmission from molecule to molecule rather than through the silicon
substrate, especially in the molecular LUMO band. Differences between the
calculated position-dependences of the STM current around a junction of styrene
and methyl-styrene molecular chains under positive and negative tip bias are
related to the nature of lateral electron transmission along the molecular
chains and to the formation in the LUMO band of an electronic state localized
around the heterojunction.Comment: 17 pages plus 11 figures. To appear in Physical Review
EPR studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers
X- and Q-band electron paramagnetic resonance (EPR) study is reported on the
SrTiO3 single crystals doped with 0.5-at.% MnO. EPR spectra originating from
the S = 2 ground state of Mn3+ ions are shown to belong to the three distinct
types of Jahn-Teller centres. The ordering of the oxygen vacancies due to the
reduction treatment of the samples and consequent formation of oxygen vacancy
associated Mn3+ centres are explained in terms of the localized charge
compensation. The EPR spectra of SrTiO3: Mn crystals show the presence of next
nearest neighbor exchange coupled Mn4+ pairs in the directions.Comment: 17 pages, 8 figure
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