16,390 research outputs found
Nuclear quantum effects in ab initio dynamics: theory and experiments for lithium imide
Owing to their small mass, hydrogen atoms exhibit strong quantum behavior
even at room temperature. Including these effects in first principles
calculations is challenging, because of the huge computational effort required
by conventional techniques. Here we present the first ab-initio application of
a recently-developed stochastic scheme, which allows to approximate nuclear
quantum effects inexpensively. The proton momentum distribution of lithium
imide, a material of interest for hydrogen storage, was experimentally measured
by inelastic neutron scattering experiments and compared with the outcome of
quantum thermostatted ab initio dynamics. We obtain favorable agreement between
theory and experiments for this purely quantum mechanical property, thereby
demonstrating that it is possible to improve the modelling of complex
hydrogen-containing materials without additional computational effort
Non-adiabatic Kohn-anomaly in a doped graphene monolayer
We compute, from first-principles, the frequency of the E2g, Gamma phonon
(Raman G-band) of graphene, as a function of the charge doping. Calculations
are done using i) the adiabatic Born-Oppenheimer approximation and ii)
time-dependent perturbation theory to explore dynamic effects beyond this
approximation. The two approaches provide very different results. While, the
adiabatic phonon frequency weakly depends on the doping, the dynamic one
rapidly varies because of a Kohn anomaly. The adiabatic approximation is
considered valid in most materials. Here, we show that doped graphene is a
spectacular example where this approximation miserably fails.Comment: 5 pages, 3 figures, Accepted by Phys. Rev. Let
Dark Radiation in LARGE Volume Models
We consider reheating driven by volume modulus decays in the LARGE Volume
Scenario. Such reheating always generates non-zero dark radiation through the
decays to the axion partner, while the only competitive visible sector decays
are Higgs pairs via the Giudice-Masiero term. In the framework of sequestered
models where the cosmological moduli problem is absent, the simplest model with
a shift-symmetric Higgs sector generates 1.56 < N_{eff} - N_{eff,SM} < 1.74.
For more general cases, the known experimental bounds on N_{eff} strongly
constrain the parameters and matter content of the models.Comment: 6 pages; v2. refs update
Sub-ohmic two-level system representation of the Kondo effect
It has been recently shown that the particle-hole symmetric Anderson impurity
model can be mapped onto a slave-spin theory without any need of
additional constraints. Here we prove by means of Numerical Renormalization
Group that the slave-spin behaves in this model like a two-level system coupled
to a sub-ohmic dissipative environment. It follows that the symmetry gets
spontaneously broken at zero temperature, which we find can be identified with
the on-set of Kondo coherence, being the Kondo temperature proportional to the
square of the order parameter. Since the model is numerically solvable, the
results are very enlightening on the role of quantum fluctuations beyond mean
field in the context of slave-boson approaches to correlated electron models,
an issue that has been attracting interest since the 80's. Finally, our results
suggest as a by-product that the paramagnetic metal phase of the Hubbard model
at half-filling, in infinite coordination lattices and at zero temperature, as
described for instance by Dynamical Mean Field Theory, corresponds to a
slave-spin theory with a spontaneous breakdown of a local gauge symmetry.Comment: 4 pages, 5 figure
De Sitter String Vacua from Dilaton-dependent Non-perturbative Effects
We consider a novel scenario for modulus stabilisation in IIB string
compactifications in which the Kahler moduli are stabilised by a general set-up
with two kinds of non-perturbative effects: (i) standard Kahler
moduli-dependent non-perturbative effects from gaugino condensation on
D7-branes or E3-instantons wrapping four-cycles in the geometric regime; (ii)
dilaton-dependent non-perturbative effects from gaugino condensation on
space-time filling D3-branes or E(-1)-instantons at singularities. For the
LARGE Volume Scenario (LVS), the new dilaton-dependent non-perturbative effects
provide a positive definite contribution to the scalar potential that can be
arbitrarily tuned from fluxes to give rise to de Sitter vacua. Contrary to anti
D3-branes at warped throats, this term arises from a manifestly supersymmetric
effective action. In this new scenario the "uplifting" term comes from F-terms
of blow-up modes resolving the singularity of the non-perturbative quiver. We
discuss phenomenological and cosmological implications of this mechanism. This
set-up also allows a realisation of the LVS for manifolds with zero or positive
Euler number.Comment: 22 pages + two appendices, typos correcte
A Note on the Magnitude of the Flux Superpotential
The magnitude of the flux superpotential plays a crucial role in
determining the scales of IIB string compactifications after moduli
stabilisation. It has been argued that values of much less than one
are preferred, and even required for physical and consistency reasons. This
note revisits these arguments. We establish that the coupling (g) of heavy
Kaluza-Klein modes to light states scales as (hence is
suppressed by two third powers of the inverse volume of compactification) and
argue that consistency of the superspace derivative expansion requires , where is the auxiliary field of the light
fields and the ultraviolet cutoff. This gives only a mild constraint on the
flux superpotential, (where V is the volume of the
compactification), which can be easily satisfied for order one values of
. This regime is also statistically favoured and makes the
Bousso-Polchinski mechanism for the vacuum energy hierarchically more
efficient.Comment: 14 page
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