5,326 research outputs found
Full electrical control of Charge and Spin conductance through Interferometry of Edge States in Topological Insulators
We investigate electron interferometry of edge states in Topological
Insulators. We show that, when inter-boundary coupling is induced at two
quantum point contacts of a four terminal setup, both Fabry-P\'erot-like and
Aharonov-Bohm-like loop processes arise. These underlying interference effects
lead to a full electrically controllable system, where the magnitude of charge
and spin linear conductances can be tuned by gate voltages, without applying
magnetic fields. In particular we find that, under appropriate conditions,
inter-boundary coupling can lead to negative values of the conductance.
Furthermore, the setup also allows to selectively generate pure charge or pure
spin currents, by choosing the voltage bias configuration.Comment: 12 pages, 5 figures (expanded discussion section, corrected typos
Tuning and Backreaction in F-term Axion Monodromy Inflation
We continue the development of axion monodromy inflation, focussing in
particular on the backreaction of complex structure moduli. In our setting, the
shift symmetry comes from a partial large complex structure limit of the
underlying type IIB orientifold or F-theory fourfold. The coefficient of the
inflaton term in the superpotential has to be tuned small to avoid conflict
with Kahler moduli stabilisation. To allow such a tuning, this coefficient
necessarily depends on further complex structure moduli. At large values of the
inflaton field, these moduli are then in danger of backreacting too strongly.
To avoid this, further tunings are necessary. In weakly coupled type IIB theory
at the orientifold point, implementing these tunings appears to be difficult if
not impossible. However, fourfolds or models with mobile D7-branes provide
enough structural freedom. We calculate the resulting inflaton potential and
study the feasibility of the overall tuning given the limited freedom of the
flux landscape. Our preliminary investigations suggest that, even imposing all
tuning conditions, the remaining choice of flux vacua can still be large enough
for such models to provide a promising path to large-field inflation in string
theory.Comment: 46 pages, 6 figures; v2: typos removed, references added; v3:
references adde
Spectral properties of a two-orbital Anderson impurity model across a non-Fermi liquid fixed point
We study by NRG the spectral properties of a two-orbital Anderson impurity
model in the presence of an exchange splitting which follows either regular or
inverted Hund's rules. The phase diagram contains a non-Fermi liquid fixed
point separating a screened phase, where conventional Kondo effect occurs, from
an unscreened one, where the exchange-splitting takes care of quenching the
impurity degrees of freedom. On the Kondo screened side close to this fixed
point the impurity density of states shows a narrow Kondo-peak on top of a
broader resonance. This narrow peak transforms in the unscreened phase into a
narrow pseudo-gap inside the broad resonance. Right at the fixed point only the
latter survives. The fixed point is therefore identified by a jump of the
density of states at the chemical potential. We also show that particle-hole
perturbations which simply shift the orbital energies do not wash out the fixed
point, unlike those perturbations which hybridize the two orbitals.
Consequently the density-of-state jump at the chemical potential remains finite
even away from particle-hole symmetry, and the pseudo-gap stays pinned at the
chemical potential, although it is partially filled in. We also discuss the
relevance of these results for lattice models which map onto this Anderson
impurity model in the limit of large lattice-coordination. Upon approaching the
Mott metal-insulator transition, these lattice models necessarily enter a
region with a local criticality which reflects the impurity non-Fermi liquid
fixed point. However, unlike the impurity, the lattice can get rid of the
single-impurity fixed-point instability by spontaneously developing
bulk-coherent symmetry-broken phases, which we identify for different lattice
models.Comment: 43 pages, 11 figures. Minor corrections in the Appendi
Numerical study of a superconductor-insulator transition in a half-filled Hubbard chain with distant transfers
The ground state of a one-dimensional Hubbard model having the next-nearest
neighbor hopping (t') as well as the nearest-neighbor one (t) is numerically
investigated at half-filling. A quantum Monte Carlo result shows a slowly
decaying pairing correlation for a sizeable interaction strength ,
while the system is shown to become insulating for yet larger
from a direct evaluation of the charge gap with the density-matrix
renormalization group method. The results are consistent with Fabrizio's recent
weak-coupling theory which suggests a transition from a superconductor into an
insulator at a finite U.Comment: 4 pages, RevTeX, uses epsf.sty and multicol.st
A Test of the Rational Expectations Hypothesis using data from a Natural Experiment
Data on contestants' choices in Italian Game Show Affari Tuoi are analysed in a way that separates the effect of risk attitude (preferences) from that of beliefs concerning the amount of money that will be offered to contestants in future rounds. The most important issue addressed in the paper is what belief function is actually being used by contestants. The parameters of this function are estimated freely along with the parameters of a choice model. Separate identification of the belief function and preferences is possible by virtue of the fact that at a certain stage of the game, beliefs are not relevant, and risk attitude is the sole determinant of choice. The rational expectations hypothesis is tested by comparing the estimated belief function with the "true" offer function which is estimated using data on offers actually made to contestants. We find that there is a significant difference between these two functions, and hence we reject the rational expectations hypothesis. However, when a simpler "rule-of-thumb" structure is as- sumed for the belief function, we find a correspondence to the function obtained from data on actual offers. Our overall conclusion is that contestants are rational to the extent that they make use of all available relevant information, but are not fully rational because they are not processing the information in an optimal way. The importance of belief-formation is confirmed by the estimation of a mixture model which establishes that the vast majority of contestants are forward-looking as opposed to myopic.Beliefs, Discrete choice models, Method of simulated likelihood, Natural Experiments, rational expectations, risky choice
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