8,865 research outputs found
Electronic structure and magnetic properties of few-layer CrGeTe: the key role of nonlocal electron-electron interaction effects
Atomically-thin magnetic crystals have been recently isolated experimentally,
greatly expanding the family of two-dimensional materials. In this Article we
present an extensive comparative analysis of the electronic and magnetic
properties of , based on density functional
theory (DFT). We first show that the often-used approaches fail
in predicting the ground-state properties of this material in both its
monolayer and bilayer forms, and even more spectacularly in its bulk form. In
the latter case, the fundamental gap {\it decreases} by increasing the
Hubbard- parameter, eventually leading to a metallic ground state for
physically relevant values of , in stark contrast with experimental data. On
the contrary, the use of hybrid functionals, which naturally take into account
nonlocal exchange interactions between all orbitals, yields good account of the
available ARPES experimental data. We then calculate all the relevant exchange
couplings (and the magneto-crystalline anisotropy energy) for monolayer,
bilayer, and bulk with a hybrid functional,
with super-cells containing up to atoms, commenting on existing
calculations with much smaller super-cell sizes. In the case of bilayer , we show that two distinct intra-layer
second-neighbor exchange couplings emerge, a result which, to the best of our
knowledge, has not been noticed in the literature.Comment: 13 pages, 6 figures, 3 table
DebtRank: A microscopic foundation for shock propagation
The DebtRank algorithm has been increasingly investigated as a method to
estimate the impact of shocks in financial networks, as it overcomes the
limitations of the traditional default-cascade approaches. Here we formulate a
dynamical "microscopic" theory of instability for financial networks by
iterating balance sheet identities of individual banks and by assuming a simple
rule for the transfer of shocks from borrowers to lenders. By doing so, we
generalise the DebtRank formulation, both providing an interpretation of the
effective dynamics in terms of basic accounting principles and preventing the
underestimation of losses on certain network topologies. Depending on the
structure of the interbank leverage matrix the dynamics is either stable, in
which case the asymptotic state can be computed analytically, or unstable,
meaning that at least one bank will default. We apply this framework to a
dataset of the top listed European banks in the period 2008 - 2013. We find
that network effects can generate an amplification of exogenous shocks of a
factor ranging between three (in normal periods) and six (during the crisis)
when we stress the system with a 0.5% shock on external (i.e. non-interbank)
assets for all banks.Comment: 10 pages, 2 figure
Devil's staircase phase diagram of the fractional quantum Hall effect in the thin-torus limit
After more than three decades the fractional quantum Hall effect still poses
challenges to contemporary physics. Recent experiments point toward a fractal
scenario for the Hall resistivity as a function of the magnetic field. Here, we
consider the so-called thin-torus limit of the Hamiltonian describing
interacting electrons in a strong magnetic field, restricted to the lowest
Landau level, and we show that it can be mapped onto a one-dimensional lattice
gas with repulsive interactions, with the magnetic field playing the role of a
chemical potential. The statistical mechanics of such models leads to interpret
the sequence of Hall plateaux as a fractal phase diagram, whose landscape shows
a qualitative agreement with experiments.Comment: 5 pages main text, 11 pages supplementary, 2 figure
Convergence and pitfalls of density functional perturbation theory phonons calculations from a high-throughput perspective
The diffusion of large databases collecting different kind of material
properties from high-throughput density functional theory calculations has
opened new paths in the study of materials science thanks to data mining and
machine learning techniques. Phonon calculations have already been employed
successfully to predict materials properties and interpret experimental data,
e.g. phase stability, ferroelectricity and Raman spectra, so their availability
for a large set of materials will further increase the analytical and
predictive power at hand. Moving to a larger scale with density functional
perturbation calculations, however, requires the presence of a robust framework
to handle this challenging task. In light of this, we automatized the phonon
calculation and applied the result to the analysis of the convergence trends
for several materials. This allowed to identify and tackle some common problems
emerging in this kind of simulations and to lay out the basis to obtain
reliable phonon band structures from high-throughput calculations, as well as
optimizing the approach to standard phonon simulations
A compact ultranarrow high-power laser system for experiments with 578nm Ytterbium clock transition
In this paper we present the realization of a compact, high-power laser
system able to excite the Ytterbium clock transition at 578 nm. Starting from
an external-cavity laser based on a quantum dot chip at 1156 nm with an
intra-cavity electro-optic modulator, we were able to obtain up to 60 mW of
visible light at 578 nm via frequency doubling. The laser is locked with a 500
kHz bandwidth to a ultra-low-expansion glass cavity stabilized at its zero
coefficient of thermal expansion temperature through an original thermal
insulation and correction system. This laser allowed the observation of the
clock transition in fermionic Yb with a < 50 Hz linewidth over 5
minutes, limited only by a residual frequency drift of some 0.1 Hz/s
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