10,090 research outputs found
Electronic ground states of Fe and Co as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy
The electronic ground state of the Co diatomic molecular cation
has been assigned experimentally by x-ray absorption and x-ray magnetic
circular dichroism spectroscopy in a cryogenic ion trap. Three candidates,
, , and , for the electronic ground state of Fe
have been identified. These states carry sizable orbital angular momenta that
disagree with theoretical predictions from multireference configuration
interaction and density functional theory. Our results show that the ground
states of neutral and cationic diatomic molecules of transition elements
cannot generally be assumed to be connected by a one-electron process
Design of aircraft turbine fan drive gear transmission system
The following basic types of gear reduction concepts were studied as being feasible power train systems for a low-bypass-ratio, single-spool, geared turbofan engine for general aircraft use: (1) single-stage external-internal reduction, (2) gears (offset shafting), (3) multiple compound idler gear system (concentric shafting), and (4) star gear planetary system with internal ring gear final output member (concentric shafting-counterrotation). In addition, studies were made of taking the accessories drive power off both the high-speed and low-speed shafting, using either face gears or spiral bevel gears. Both antifriction and sleeve-type bearings were considered for the external-internal and star-planet reduction concepts
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
After-School Programs for High School Students: An Evaluation of After School Matters
Evaluates outcomes for teens in Chicago's After School Matters apprenticeship-like program, finding statistically significant benefits on some measures of youth development and reduced problem behaviors but not in job skills or school performance
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
Continuous Time Quantum Monte Carlo method for fermions
We present numerically exact continuous-time Quantum Monte Carlo algorithm
for fermions with a general non-local in space-time interaction. The new
determinantal grand-canonical scheme is based on a stochastic series expansion
for the partition function in the interaction representation. The method is
particularly applicable for multi-band time-dependent correlations since it
does not invoke the Hubbard-Stratonovich transformation. The test calculations
for exactly solvable models as well results for the Green function and for the
time-dependent susceptibility of the multi-band super-symmetric model with a
spin-flip interaction are discussed.Comment: 10 pages, 7 Figure
Phenomenology of the minimal supersymmetric extension of the standard model
We discuss the minimal supersymmetric extension of
the standard model. Gauge couplings unify as in the MSSM, even if the scale of
breaking is as low as order TeV and the model can be
embedded into an SO(10) grand unified theory. The phenomenology of the model
differs in some important aspects from the MSSM, leading potentially to rich
phenomenology at the LHC. It predicts more light Higgs states and the mostly
left CP-even Higgs has a mass reaching easily 125 GeV, with no constraints on
the SUSY spectrum. Right sneutrinos can be the lightest supersymmetric
particle, changing all dark matter constraints on SUSY parameter space. The
model has seven neutralinos and squark/gluino decay chains involve more
complicated cascades than in the MSSM. We also discuss briefly low-energy and
accelerator constraints on the model, where the most important limits come from
recent searches at the LHC and upper limits on lepton flavour violation.Comment: 46 pages, 11 figure
Direct Observation of High-Spin States in Manganese Dimer and Trimer Cations by X-ray Magnetic Circular Dichroism Spectroscopy in an Ion Trap
The electronic structure and magnetic moments of free Mn and Mn
are characterized by x-ray absorption and x-ray magnetic circular
dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron
radiation beamline. Our results show directly that localized magnetic moments
of 5 are created by states at each ionic core,
which are coupled in parallel to form molecular high-spin states via indirect
exchange that is mediated in both cases by a delocalized valence electron in a
singly-occupied derived orbital with an unpaired spin. This leads to total
magnetic moments of 11 for Mn and 16 for Mn, with
no contribution of orbital angular momentum
Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory
We examine whether the Mott transition of a half-filled, two-orbital Hubbard
model with unequal bandwidths occurs simultaneously for both bands or whether
it is a two-stage process in which the orbital with narrower bandwith localizes
first (giving rise to an intermediate `orbital-selective' Mott phase). This
question is addressed using both dynamical mean-field theory, and a
representation of fermion operators in terms of slave quantum spins, followed
by a mean-field approximation (similar in spirit to a Gutzwiller
approximation). In the latter approach, the Mott transition is found to be
orbital-selective for all values of the Coulomb exchange (Hund) coupling J when
the bandwidth ratio is small, and only beyond a critical value of J when the
bandwidth ratio is larger. Dynamical mean-field theory partially confirms these
findings, but the intermediate phase at J=0 is found to differ from a
conventional Mott insulator, with spectral weight extending down to arbitrary
low energy. Finally, the orbital-selective Mott phase is found, at
zero-temperature, to be unstable with respect to an inter-orbital
hybridization, and replaced by a state with a large effective mass (and a low
quasiparticle coherence scale) for the narrower band.Comment: Discussion on the effect of hybridization on the OSMT has been
extende
Light Lepton Number Violating Sneutrinos and the Baryon Number of the Universe
Recent results of neutrino oscillation experiments point to a nonvanishing
neutrino mass. Neutrino mass models favour Majorana-type neutrinos. In such
circumstances it is natural that the supersymmetric counterpart of the
neutrino, the sneutrino, bears also lepton number violating properties. On the
other hand, the fact that the universe exhibits an asymmetry in the baryon and
antibaryon numbers poses constraints on the extent of lepton number violation
in the light sneutrino sector if the electroweak phase transition is second or
weak first order. From the requirement that the Baryon Asymmetry of the
Universe should not be washed out by sneutrino induced lepton number violating
interactions and sphalerons below the critical temperature of the electroweak
phase transition we find that the mass splitting of the light sneutrino mass
states is compatible with the sneutrino Cold Dark Matter hypothesis only for
heavy gauginos and opposite sign gaugino mass parameters.Comment: 13 pages, 4 figure
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